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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methods
  5. Results
  6. Discussion: Genesis of the Hadley-Apennine KREEP Basalt Province
  7. Conclusions
  8. References
  9. Supporting Information

Abstract– We have studied 27 KREEP basalt fragments in six thin sections of samples collected from four Apollo 15 stations. Based on local geology and regional remote sensing data, these samples represent KREEP basalt lava flows that lie beneath the younger, local Apollo 15 mare basalts and under other mare flows north of the Apollo 15 site. Some of these rocks were deposited at the site as ejecta from the large craters Aristillus and Autolycus. KREEP basalts in this igneous province have a volume of 103–2 × 104 km3. Mineral and bulk compositional data indicate that the erupted magmas had Mg# [100 × molar Mg/(Mg + Fe)] up to 73, corresponding to orthopyroxene-rich interior source regions with Mg# up to 90. Minor element variations in the parent magmas of the KREEP basalts, inferred from compositions of the most magnesian pyroxene and most calcic plagioclase in each sample, indicate small but significant differences in the concentrations of minor elements and Mg#, reflecting variations in the composition of lower crustal or mantle source regions and/or different amounts of partial melting of those source regions.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methods
  5. Results
  6. Discussion: Genesis of the Hadley-Apennine KREEP Basalt Province
  7. Conclusions
  8. References
  9. Supporting Information

The abundance of KREEP basalts in the Apollo 15 regolith suggests that the KREEP basalts underlie the mare basalts at the landing site (e.g., Basu and Bower 1977; Spudis et al. 1988). These lavas are associated with the Imbrian-aged Apennine Bench Formation (ABF). On the basis of their occurrence within young (1.3–2.1 Ga) impact melt breccias and other materials on the slopes of the Apennine Front, the site also contains KREEP basalt fragments that were thrown to the Apollo 15 site by impacts that formed the large craters Aristillus and Autolycus, 150–200 km north of the site (e.g., Ryder et al. 1991), which formed within the ABF. Thus, the Apollo 15 KREEP basalt suite represents samples of a large igneous province on the Moon. We have studied KREEP basalt fragments from breccias and soils at the Apollo 15 landing site. Our goal was to determine the range in magma compositions represented by the samples and to gain insight into their petrogenesis, including the compositional variation of their interior source regions and extent of differentiation experienced by KREEP basalt magmas. The genesis of KREEP basalt magmas may be central to understanding the origin of the Mg-suite of nonmare rocks as they probably formed in magmas similar to KREEP basalts (e.g., Snyder et al. 1995). In this article, we re-examine the geological setting of KREEP basalts at the Apollo 15 site, describe their petrography and mineral chemistry, and evaluate models for their origin.

Samples and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methods
  5. Results
  6. Discussion: Genesis of the Hadley-Apennine KREEP Basalt Province
  7. Conclusions
  8. References
  9. Supporting Information

We made detailed studies of 27 KREEP basalt fragments in six thin sections of samples collected from stations 2, 9A, 6A, and 7 at the Apollo 15 landing site. Sampling locations and characteristics of the samples studied are summarized in Table 1 and locations are shown in Fig. 1. Rock 15205, a regolith breccia, was collected from a 1 m boulder at Station 2 on the slope of Hadley Delta. This glass-coated block was angular and apparently recently emplaced (Spudis and Taylor 2009). The Apollo 15 Field Geology team associated this Station 2 block with a very fresh impact crater located about 300 m north of Station 2 and about one third of the way down the slope of Hadley Rille (Swann et al. 1972). Rock 15528, also a regolith breccia, was collected at Station 9A along the rim of Hadley Rille. The other samples were collected from the Apennine Front. Rock 15405 was removed from a 3 m boulder on the slopes of the Apennine Front. KREEP basalt sample 15434,188 is a fragment from the 4–10 mm size fraction of soil 15434 collected from the northwest rim of Spur crater at Station 7. Sample 15358, an impact melt breccia, is a fragment from a rake sample gathered at Station 7. (Rake sample 15386, the largest KREEP basalt sample, was also collected at Station 7. We provide no new data on the sample, but use published results.) Clast designations are given in supporting information Figs. S1 and S2.

Table 1.   Characteristics of the samples studied.
SamplesaSampling stationSample typeDescriptionLocal or exoticAge
  1. aSpecific samples studied in detail are listed in italics. The number of thin sections examined at the curatorial facility at the Johnson Space Center is listed in brackets. We used published data for the largest KREEP basalt, rake sample 15386.

15205 15205,4 15205,210 [39]2Rock sample from boulderRegolith breccia with abundant KREEP basalt clastsLocalUnknown
15528 15528,49ASmall rock sampleRegolith breccia with abundant KREEP basalt clastsLocalUnknown
15405 15405,12 [15]6ARock sample from boulderImpact melt breccia with abundant KREEP basalt and related clastsExotic (possible ejecta from Aristillus crater)1.3 Gab
15434 15434,1887Rock fragment in soil sampleKREEP basaltExoticUnknown
15358 15358,67Rock fragment in rake sampleImpact melt breccia with abundant KREEP basalt fragmentsExotic (possible ejecta from Autolycus crater)2.1 Gac
image

Figure 1.  Map of the Apollo 15 landing site showing sampling stations. Large numbers indicate locations of the samples reported here, and of well-studied rake sample 15386. Samples 15205 (station 2) and 15528 (station 9A) are probably derived locally, whereas the remaining samples (stations 6A and 7) on the Apennine Front probably represent materials thrown to the site by impacts.

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We used optical and scanning electron microscopy to study the thin sections listed in Table 1, focusing attention on clasts of KREEP basalt. To roughly quantify grain size of each sample, we measured the width of the largest plagioclase crystal in each sample. The plagioclase population forms a continuum, hence the largest plagioclase width is part of a population formed in the lavas and do not represent phenocrysts formed at depth. Modal abundances of matrix glass in four clasts in 15358 were made from backscattered electron images taken at 300× magnification using an image analysis program (Environment for Visualizing Images, ENVI). The glass regions have high Fe/Mg, making them brighter than all phases except ilmenite.

Quantitative analyses were performed using a JEOL 8500 field emission electron microprobe operated at 15 keV, a beam current of 15 nA, and a beam size of 10 μm. The beam size was chosen to obtain good averages of the central regions of pyroxene and plagioclase crystals, and to prevent loss of volatiles from glasses. (Backscattered images and point analyses indicate that the central portions of pyroxene and plagioclase crystals are uniform in composition.) We used well-analyzed mineral and glass standards for the analyses. All potentially volatile elements (K, Na, P) were analyzed first to avoid loss during analysis, which is also mitigated by using a 10 μm beam size. We selected the most magnesian pyroxene and the most calcic plagioclase in the centers of crystals by examining backscattered electron images and analyzing 5–10 points in these areas. For comparing magma compositions, we used the single most magnesian pyroxene or most calcic plagioclase analysis. Uncertainties calculated from peak and background counting rates vary with the amount present, but typical 2-sigma uncertainties in wt% are: Plagioclase: SiO2 (0.2), Al2O3 and Ca (0.1), K2O (0.15), Na2O (0.03), and FeO (0.05); Low-Ca pyroxene: SiO2 (0.2), Al2O3 and Ca (0.05), Cr2O3 (0.04), TiO2 (0.04), FeO (0.2), and MgO (0.6).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methods
  5. Results
  6. Discussion: Genesis of the Hadley-Apennine KREEP Basalt Province
  7. Conclusions
  8. References
  9. Supporting Information

Regional Context of the Apollo 15 KREEP Basalt Suite

KREEP Basalts and the Apennine Bench Formation

Extensive work documented the volcanic origin of numerous small fragments of KREEP basalts returned by the Apollo 15 astronauts (e.g., Dowty et al. 1976; Ryder 1976) and remote sensing data demonstrated that a nearby regional, light plains unit (the Apennine Bench Formation, ABF; Hackman 1966) corresponds in composition to these KREEP basalts (Hawke and Head 1978; Spudis 1978; Spudis and Hawke 1986). It was supposed that the KREEP basalts at the Apollo 15 landing site were derived from this regional unit, either directly underlying the mare basalts near the site (Spudis and Hawke 1986) or possibly delivered to the site as ray material from the Copernican craters Aristillus and Autolycus to the north (Schultz 1986). As the oldest, post-basin nonmare unit in the region, the age of the KREEP basalt created an important constraint on the age of the Imbrium Basin—because the ABF filled (and therefore post-dated) the Imbrium Basin the basin could not have formed any later than 3.89 Ga, the age of the Apollo 15 KREEP basalts. (Throughout this article, we use Rb-Sr ages recalculated using the revised 87Rb decay constant proposed by Begemann et al. [2001].)

The correspondence of Apollo 15 KREEP with the ABF was questioned by Deutsch and Stöffler (1987). On the basis of the ages of certain Apollo 14 impact melts, they thought that the Imbrium Basin must have formed at a younger time (3.82 Ga). Deutsch and Stöffler (1987) suggested instead that the Apennine Bench is a pre-Imbrium Basin “megablock” exposed within the basin rim, largely unmasked by any covering ejecta from the basin. They further argued that because no outcrop of ABF occurred at the Apollo 15 landing site, it was not reasonable to suppose that the mission had returned any samples of it. However, the ABF does crop out near the Apollo 15 landing site (Fig. 2), and breccia 15205 and the high abundance of KREEP basalts in the regolith (Basu and Bower 1977) show that KREEP basalt is abundant at the site.

image

Figure 2.  A) Map of the region surrounding the Apollo 15 landing site, based on Hawke and Head (1978) and Blewett and Hawke (2001). Mare plains are shown in tan, highlands and crater ejecta are white. The Apennine Bench Formation (blue) is volcanic and has a composition similar to KREEP basalts (Dowty et al. 1976; Irving 1977; Spudis et al. 1988; Spudis and Taylor 2009). Aristillus and Autolycus probably delivered samples to the Apollo 15 site, especially those found on the Apennine Front (see Fig. 1). The dashed line is an estimate of the extent of a large igneous province dominated by KREEP basalt and its differentiates. The gray square around the landing site is shown in detail in B. B) Image (left) and a geological sketch map (after Swann 1986) of the Apollo 15 landing site, showing small outcrop of both highlands material (light brown) and Apennine Bench Fm. (blue) embayed by Apollo 15 mare basalt (light green).

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Figure 2 shows the regional and local geology of the Apollo 15 landing site. Mapping of the site by the Apollo 15 Geology Team recognized occurrences of ABF—thinly mantled by mare lava—very close to the landing site, the nearest outcrop being 6 km from the Lunar Module (Swann 1986). Additional evidence suggests that the Hadley-Apennine valley was flooded by mare lavas, which ponded in several places and subsequently subsided (Howard et al. 1972; Swann 1986). During this ponding, lava stood at a slightly higher level than at present and coated surfaces made up of both highlands material and ABF. Subsequent drainage back into the rille left a thin covering of lava on these outcrops in addition to the high lava mark observed by the crew and visible in returned images (Howard et al. 1972; Scott et al. 1972). The evidence suggests that the ABF, thinly mantled near the landing site, underlies the mare basalt throughout the site. This would help explain its ubiquitous presence in the soils, distributed as fragments and small rocks throughout the landing site area.

Breccia 15205: A Stratigraphic Marker for KREEP Basalt Underlying Mare Basalts at the Apollo 15 Landing Site

Yet another line of evidence for this inferred sequence comes from the setting and petrography of 15205, a fragmental or regolith breccia collected from the small (∼1 m) boulder sampled at Station 2 during the first extravehicular activity (EVA-1; Swann et al. 1972). As discussed in more detail in the following section, this rock consists predominantly of KREEP basalts, but with significant amounts of quartz-normative mare basalt and green pyroclastic glass. The rock was collected from a boulder that is likely to have been ejected from the very fresh, rayed crater near Station 2 (Fig. 3; Swann et al. 1972). This young crater occurs just over and inside the lip of Hadley Rille, where the rille makes an abrupt turn to the northwest, away from its eastbound course (Fig. 3). This setting inside the rille wall suggests that the crater formed on a geological contact, between KREEP basalt, green glass, and quartz-normative mare basalt. On the basis of such a population of lithic clasts, Spudis and Taylor (2009) suggested that this small crater straddled the ABF/Apollo 15 mare basalt contact, a ∼500 Myr unconformity on the Moon. The relations within this rock are direct evidence that the KREEP basalts of the ABF directly underlie the mare basalts at Apollo 15; based on crater size and position, it appears that the KREEP basalt/mare basalt contact occurs roughly 30 m below the present mare surface. This estimate is in good agreement with the observed position of layered basalt sequences in the western wall of Hadley Rille north of this location at Station 9 (Howard et al. 1972; Spudis et al. 1988).

image

Figure 3.  Station 2 at the Apollo 15 landing site. The sample stop was just south of a 100 m diameter, fresh rayed crater (arrow in upper right image points to the crater), thought to be the source of the boulder sampled by the astronauts. Samples from this boulder consist of a fragmental breccia containing KREEP basalt clasts, quartz-normative mare basalt, and pyroclastic green glass. This petrologic make-up suggests that the rayed crater may have formed on a geological contact between KREEP basalt and mare units.

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Rock 15205 has been described in detail by Dymek et al. (1974). It is a fragmental breccia (Fig. 4) composed mainly of KREEP basalt lithic fragments and mineral fragments from them. Subordinate amounts of mafic green glasses and quartz-normative mare basalts are also present. Although it contains both impact and volcanic glasses, there is sparse evidence for regolith products: IS/FeO is ∼0 and the rock contains no detectable solar wind gases (McKay et al. 1989). Simon et al. (1986) report the presence of small amounts (<1%) of agglutinates, although we did not identify any unambiguous examples in the thin sections studied. Thus, the rock is an extremely immature regolith breccia. Its fragmental nature and immaturity led McKay et al. (1989) to conclude that the 15205 regolith formed when the flux of relatively large objects was high, extensively fragmenting KREEP basalt flows, but not producing products such as agglutinates that form as the result of micrometeorite bombardment. The period of high bombardment would have been strongest at 3.9 Ga, the age of the KREEP basalts, and declined by 3.3 Ga, the age of the Apollo 15 mare basalts (McKay et al. 1989).

image

Figure 4.  Optical light photomosaic of thin section 15205,210. Rock 15205 is a fragmental breccia composed dominantly of KREEP basalt fragments, with subordinate quartz-normative mare basalts and green pyroclastic glasses. Note the glass coating on the rock (left edge) and veins of yellow glass throughout. A red-brown glass coats the large KREEP basalt fragment in the center. KREEP basalt clasts studied are labeled in Fig. S1 of the online supplement.

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Rock 15205 contains several types of glass (Dymek et al. 1974; Fig. 4). We analyzed the three most prominent types: yellowish glass that coats the rock, yellow glass that cuts across it in irregular veins, and greenish beads and aggregates of beads (many cryptocrystalline). We also analyzed a red-brown glass that coats a 5-mm KREEP basalt clast and occurs as fragments throughout the rock. Results are listed in Table 2, along with published bulk rock analyses. Individual glass types do not vary significantly in composition, consistent with results reported by Dymek et al. (1974). The whole rock composition reflects the petrographic observation that the rock is a mixture of KREEP basalts and a mare component. The mare component consists mostly of green glass and quartz-normative basalt, with small amounts of olivine basalt reported (Dymek et al. 1974; Simon et al. 1986). From a least-squares mixing calculation, Simon et al. (1986) estimate that KREEP basalts make up 73% of the rock and mare basalts make up 15%. Other components in the mix are anorthosite (3%) and Low-K Fra Mauro “basalt” (LKFM) (represented in the mixing calculation by rock 62296, 9%). Surprisingly, the calculation does not indicate the presence of any green glass, perhaps reflecting the heterogeneous distribution of large fragments of welded green glass spheres. Simon et al. (1986) observe small amounts (<1%) of feldspathic basalts and granulitic breccias, but not enough to account for the apparent 9% of LKFM. It seems likely that this is an artifact of the mixing calculation or sample heterogeneity, and that KREEP basalt is higher than the 73% calculated.

Table 2.   Bulk composition of 15205, glasses in 15205, and other Apollo 15 materials.
 Bulk rockGlassesKREEP basaltQNB basalt
  1. 1) Bulk rock from large (2 g) sample (Willis et al. 1972); 2) Bulk rock (McKay et al. 1989); 3) Bulk rock (Simon et al. 1986); 4) Yellow glass veins in 15205 (average of five analyses); 5) Yellow glass coating on 15205 (average of five analyses); 6) Green glass in 15205 (Dymek et al. 1974); 7) Red-brown glass on KREEP basalt clast (average of six analyses); 8) KREEP basalt 15386 (Rhodes and Hubbard 1973); 9) Typical Apollo 15 quartz-normative mare basalt (Chappell and Green 1973).

 123456789
SiO251.049.246.445.650.150.848.0
TiO21.992.061.91.681.780.432.082.231.8
Al2O314.913.214.916.812.77.415.714.89.4
Cr2O30.340.40.340.270.440.550.320.350.48
FeO11.2813.310.99.515.619.610.010.620.2
MnO0.160.20.160.270.160.30
MgO8.239.29.69.010.817.58.98.28.7
CaO9.948.99.810.29.88.39.69.710.40
Na2O0.730.660.780.740.430.150.660.730.32
K2O0.530.520.540.210.020.690.670.06
P2O50.560.530.150.030.580.7
Total99.6698.4498.4999.7798.6698.8799.66

The vesicular glass coating on 15205 (Fig. 4) is similar to the bulk rock in being a mixture of KREEP and mare basalts, but contains a greater percentage of mare basalt than does the bulk rock. This is demonstrated by higher FeO and lower Al2O3 in the glass coating than in the bulk rock (Table 2). Dymek et al. (1974) noted that the glass is compositionally distinct from the local soil at Station 2 and that the glass covers the underside of the Station 2 boulder. They concluded that the glass coating was emplaced before deposition of the boulder at its final resting place at Station 2. The composition indicates that it is a total melt of a lunar soil similar to 15205 in composition, substantiating that 15205 represents a regolith composed chiefly of KREEP basalts with subordinate low-Ti pyroclastic glasses and quartz-normative mare basalts.

The yellow glass veins that cut across the rock are also clearly rich in a KREEP basalt component (high K and P, Table 2), but are distinctive in having elevated Al2O3. In fact, Al2O3 is higher than in pure KREEP basalt and substantially higher than in mare basalt or green glass (Table 2). This suggests the presence of a feldspathic component. As noted above, on the basis of mixing calculations, Simon et al. (1986) report the presence of about 3% anorthosite in the bulk rock. The yellow glass veins contain an even greater percentage (∼10%) of an anorthosite component.

Reddish brown glass coats a KREEP basalt clast 5 mm in diameter (Fig. 4); similar glass occurs in small pieces throughout the rock (Dymek et al. 1974). Its composition is very similar to KREEP basalts (Table 2), although it has a somewhat greater concentration of Al2O3, suggesting the presence of a feldspathic component. The glass coating on the large clast has a sharp boundary and appears to have been emplaced before formation of the breccia. It thus represents the product of impact reworking of the KREEP basalts at the Apollo 15 site. The absence of a mare basalt component in the glass, which would be indicated by higher FeO than the clast has, indicates that the reworking occurred before emplacement of the mare basalts and pyroclastic deposits on top of the KREEP basalt layer. This demonstrates reworking of the KREEP basalt flows during the 500 Ma interval between their emplacement and the emplacement of the Apollo 15 mare basalt flows.

Like 15205, breccia 15528 is a polymict fragmental breccia containing KREEP basalt and mare basalt clasts. The presence in 15205 and 15528 of both mare and nonmare volcanic materials like those present at the Apollo 15 site indicates that they represent local lithologies.

KREEP Basalt from the Region Surrounding the Apollo 15 Landing Site

The Apennine Bench Formation is exposed north and southeast of the Apollo 15 landing site (Fig. 2). As argued above, it is reasonable to suppose that the Apennine Bench lavas underlie the mare flows at the site, as indicated by exposures of the ABF near the site and by the clast population in 15205. Two large rayed craters to the north of the landing site, Autolycus (39 km in diameter) and Aristillus (55 km in diameter) have ejected materials rich in Th, and hence, KREEP-rich (Th data from Lawrence et al. [2007]). Schultz (1986) suggests that these two craters could have delivered more than half of the exotic materials to the Apollo 15 site. Thus, some component of the KREEP basalts and related rocks in the Apollo 15 collection could be ejecta from the two craters. A prime place to sample such ejecta is at Station 4, the South Cluster, a group of secondary craters from one of the two craters (Swann et al. 1972). Stations 6, 6A, and 7 on the Apennine Front (which consists mostly of Imbrium Basin deposits) are all downrange from the South Cluster, so any ricochet from the South Cluster impacts might be found there. Other Apollo 15 sampling stations are less likely to contain non-local materials. Stations 1–3 are not downrange from the South Cluster, and Stations 9 and 9A are on the mare, where the Aristillus/Autolycus ejecta are discontinuous. Station 9A is on the lip of Hadley Rille, where mass wasting into the rille has exposed local mare basalt bedrock, hence removing any secondary ejecta. We have studied a fragment from the 4–10 mm sieve fraction of soil 15434, rake samples 15386 and 15358, and rock 15405 from the stations most likely to contain ejecta from Aristillus and/or Autolychus.

Aristillus as the source of 15405 was suggested previously by Ryder (1976) and Ryder and Martinez (1991), who drew attention to the suite of differentiated rocks inside this impact-melt breccia. This interpretation was supported by Gillis and Jolliff (1999) and Blewett and Hawke (2001) based on remote sensing data. Rock 15405 has an age of 1.29 Ga (Bernatowicz et al. 1978), which probably dates the formation of Aristillus. Dating of fragments in the coarse fines of soil 15434 and impact melt breccia 15358 indicates an age of 2.1 Ga (Ryder et al. 1991), and may date the formation of Autolycus crater. These young ages do not represent the ages of KREEP basalt magmatism, which took place at about 3.9 Ga, the age of KREEP basalt 15386 (Nyquist et al. 1975; Carlson and Lugmair 1979), based on Sm-Nd and Rb-Sr dating. KREEP basalt magmas may even have formed earlier, 4.29 Ga, based on the age of the quartz monzodiorite in breccia 15405, as determined by the Pb-Pb method and SIMS analysis (Meyer et al. 1996). These data indicate that we have a sample suite that represents Apennine Bench Formation directly beneath the Apollo 15 site and from the large region to the north, extending to at least as far as Aristillus (Fig. 2). (No age data have been reported for the KREEP basalt fragments in melt breccias 15405.)

KREEP basalts have also been identified as clasts in Apollo 17 breccia 72275 from Boulder 1, Station 2. Their higher FeO contents and lower concentrations of incompatible elements make them compositionally distinct from the Apollo 15 KREEP basalts (Papike et al. 1998). They are significantly older than Apollo 15 KREEP basalts, with Rb-Sr ages ranging from 3.97 to 4.08 Ga (see summary in Papike et al. [1998]; decay constant from Begemann et al. [2001]). Their rarity makes them difficult to fit into the overall picture of KREEP basalts from the Hadley-Apennine region, so we do not discuss them here.

Petrography of KREEP Basalts

KREEP basalt fragments display a range of textures and grain sizes, but are clearly extrusive basaltic rocks. Most have intergranular/intersertal to subophitic textures (Fig. 5A–C). The finest grained samples tend to be intersertal, although regions can be variolitic. One sample has a large orthopyroxene crystal (Fig. 5C), which was interpreted by Ryder (1987) as a phenocryst. Plagioclase and low-Ca pyroxene occur in approximately equal amounts and are often intergrown, suggesting co-crystallization. Accessory minerals include silica (probably cristobalite), ilmenite, and phosphate phases, with a dark, cryptocrystalline mesostasis. Widths of the largest plagioclase crystals range from 40 to 270 μm and are listed for each clast in Tables 3 and 4. We found no evidence, such as the presence of angular clasts or heterogeneous textural variations, that any of the clasts studied are impact melt products. Indeed, 15434,188 and two clasts in 15205,210 contain phenocrysts of low-Ca pyroxene, indicative of two-stage crystallization characteristic of volcanic rocks, as argued by Ryder (1987).

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Figure 5.  Photomicrographs in plain polarized transmitted light of KREEP basalt clasts. A–C are all at the same scale. A) 15205,210 clast 18, a coarse-grained KREEP basalt sample. White is plagioclase, gray is pyroxene, dark is mesostasis. B) 15205,210 clast 8, a medium-grained KREEP basalt sample. C) 15434,188, a KREEP basalt with a prominent low-Ca pyroxene phenocryst cutting across it from lower left to upper right. This clast was originally described by Ryder (1987). D) Clast 1 in impact melt 15358; the clast is a quenched, partially crystallized KREEP basalt.

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Table 3.   Most magnesian pyroxene measured in Apollo 15 KREEP basalts.
SampleClast no.SiO2TiO2Al2O3Cr2O3FeOMnOMgOCaOTotalMg#aWidth (mm)b
  1. aMolar Mg/(Mg + Fe) × 100.

  2. bWidth of the largest plagioclase in each clast (a measure of rock grain size).

15205–210353.80.582.310.9310.60.228.52.0598.9982.7150
15205–210452.80.552.850.9910.00.1929.51.7698.6184.155
15205–210853.50.642.860.9610.70.1928.31.8899.0082.580
15205–210753.10.561.960.8211.60.1928.51.8498.5081.4115
15205–210553.10.622.380.9111.70.1527.91.9798.6781.050
15205–210953.90.442.140.8110.50.1929.71.2898.9383.4200
15205–2101053.90.512.350.8910.90.2128.71.8599.3682.460
15205–2101153.10.582.270.8611.40.2328.61.8398.9481.7180
15205–2101252.50.803.151.0111.90.1626.72.3398.6080.040
15205–2101454.00.481.750.6911.50.1629.21.3199.0581.9130
15205–2101652.00.803.781.0710.20.1928.12.1398.1983.150
15205–2101853.10.493.451.129.10.1630.21.2298.9085.5200
15205–4153.40.593.641.1410.330.21.41100.7084.0120
15434–188All54.50.392.260.939.60.1531.11.17100.0385.280
15405154.00.622.911.0310.30.1729.71.79100.4983.8100
15405252.70.612.360.9312.30.2127.82.0798.9780.155
15405353.20.582.390.9011.70.2128.41.8299.1981.370
15405453.60.662.750.9611.80.2028.91.51100.4581.445
15405554.30.573.411.0610.20.2030.01.30101.0883.9250
15405852.90.762.270.9214.30.3027.11.55100.0677.1120
15405954.60.683.401.0310.80.2029.51.31101.4683.0250
15358152.90.583.130.9711.128.91.9499.5982.260
15358352.90.552.930.8810.929.31.5399.0082.830
15358252.80.603.531.0310.829.71.3499.7383.140
15358453.80.542.800.9510.929.71.36100.0282.970
15528152.90.633.481.0910.929.11.97100.0582.6100
Table 4.   Compositions (wt%) of most calcic plagioclase measured in Apollo 15 KREEP basalts.
SampleClast no.SiO2Al2O3FeOMgOCaONa2OK2OTotalAb (mol%)W (μm)a
  1. aWidth (μm) of largest plagioclase in each clast.

15205,210347.932.30.330.2616.41.690.1799.0815.7150
15205,210447.832.70.350.3916.51.480.1299.3613.955
15205,210846.732.50.360.3016.81.450.1498.2013.580
15205,210748.633.20.240.2916.71.550.13100.6814.4115
15205,210548.232.30.360.2516.21.690.2399.1815.850
15205,210947.333.60.230.2616.91.380.1099.8212.8200
15205,2101048.332.60.270.2916.21.580.1499.3515.060
15205,2101147.832.80.270.3016.41.600.1399.3214.9180
15205,2101248.231.70.460.3715.81.660.2098.3115.840
15205,2101448.033.40.460.2316.91.370.15100.5312.7130
15205,2101648.431.80.590.3516.01.610.2098.9915.250
15205,2101846.534.60.210.2417.81.100.08100.5010.1200
15434,188All45.135.20.370.1418.30.840.0699.967.780
15405,12147.632.70.310.2916.71.620.1499.3614.9100
15405,12247.532.10.370.3316.51.650.1698.6215.355
15405,12346.333.30.240.2817.31.290.1198.7811.870
15405,12446.133.50.310.1917.41.350.1198.9612.345
15405,12543.935.20.110.1619.00.610.0398.975.5250
15405,12846.233.00.220.2417.11.440.1198.3713.2120
15405,12948.630.81.911.1716.11.280.27100.1112.4250
15358,6147.433.00.480.4016.71.480.1299.6513.860
15358,6246.433.20.330.3017.01.370.1098.6212.740
15358,6345.134.00.290.3917.51.100.0898.5110.230
15358,6446.833.20.310.2816.81.350.0998.8012.770
15528,4147.733.80.280.2817.01.490.1100.7213.6100

A particularly intriguing group of KREEP basalt fragments occurs in impact melt breccia 15358, studied and described by Ryder (1988). The fragments have intersertal to intergranular textures (long plagioclase laths with smaller pyroxene crystals in between them), but instead of containing a dark residual mesostasis typical of other KREEP basalts, these fragments contain yellow glass (Fig. 5D). Our modal analyses of four fragments indicate that the glasses occupy 11–18 vol% of the fragments. (Glass abundances are given in Table 5, along with chemical analyses of the yellow glasses in four fragments.) The fragments we studied in 15358,6 contain small grains of ilmenite and silica (probably cristobalite), indicating that crystallization had proceeded sufficiently far for those phases to have crystallized. Ryder (1988) states that among the fragments he studied those with the most glass do not contain cristobalite or ilmenite.

Table 5.   Compositions (wt%; Mg# in mol%) of residual yellow glasses in partly crystallized KREEP basalts; averages of multiple analyses (indicated by N).
  NaSiO2TiO2Al2O3Cr2O3FeOMnOMgOCaONa2OK2OP2O5TotalMg#Vol% glass
  1. aN is number of 10 μm spots analyzed by electron microprobe.

Ryder yellow glassesb
15243,16Clast50.23.412.10.2813.20.196.79.60.530.911.2098.3147.5
15358,6Clast48.85.710.60.0715.40.192.89.60.181.302.0096.6424.5
15434,19Clast47.35.090.1917.50.234.210.10.51.202.1097.3230.0
15343,21Clast46.45.09.20.1118.10.253.710.90.431.202.7097.9926.7
15358,16Clast46.85.49.40.0720.20.282.910.50.421.302.1099.3720.4
15434,25Clast45.24.77.90.0822.30.292.8100.491.403.3098.4618.3
Our yellow glass analyses in 15358 clasts
15358,6C1947.05.59.30.1218.2 3.110.20.481.102.2497.0523.216.0
15358,6C3844.66.39.80.1017.7 3.710.20.441.042.4396.2526.911.1
15358,6C4744.45.78.90.1119.1 3.610.30.421.002.6396.2425.317.1
15358,6C51047.56.210.80.1315.3 3.69.90.521.131.9297.1229.717.7

The totals are low, in both Ryder’s analyses and ours. Only one of the ten clasts analyzed exceeds 99%, seven are <98%, and three are <97% (Table 5). Rare earth elements might make up the difference, but even assuming that the glasses represent only 10% liquid remaining and that the initial lava had REE abundances the same as KREEP basalt 15382, the total REE would be only about 0.6 wt%. Use of a broad beam and analysis of the most volatile elements in the first round of analyses limited volatile loss and comparison to a basaltic glass standard gave totals close to 100%. Furthermore, our analyses of glasses in 15205, analyzed using the same procedures and standards, have reasonable totals. It appears that some unanalyzed component is present in the glasses in unique sample 15358.

The presence of glass in the KREEP basalt clasts in 15358 suggests quenching before full crystallization. On the other hand, the breccia matrix of 15358, which resembles a rapidly crystallized KREEP basalt, might have heated the enclosed clasts, causing partial melting. Nevertheless, the fine grain size of the matrix and limited reaction between glass and minerals suggest that the mineral compositions were not affected by heating from the breccia impact melt.

Mineral Compositions

Using backscattered electron images as a guide, we searched for the most magnesian low-Ca pyroxene crystals and most calcic plagioclase crystals in each KREEP basalt fragment. Our intent was to assess the number of parent magmas represented by this suite of samples and to search for variations between local and suspected exotic samples. The most magnesian pyroxene and most calcic plagioclase in each sample are given in Tables 3 and 4.

Pyroxene compositions (Fig. 6A–C) scatter in minor element and Mg# [molar Mg/(Fe + Mg), expressed as percent]. Most low-Ca pyroxenes are probably orthorhombic as they have low concentrations of the Wo component (2.2–4.8 mol%), and we verified this optically for several larger crystals. The fragments range in Mg# from 85.5 to 77.1. Concentrations of minor elements vary substantially and much more than expected for simple fractional crystallization of a single magma. Al2O3 and TiO2 vary by a factor of 2 and Cr2O3 varies by 50% (relative). Using our data from clasts where we measured the full range in pyroxene compositions (from cores to rims to groundmass pyroxenes), fractional crystallization is not expected to produce more than about a 10% relative variation at a given Mg#. We show an example in Fig. 6A–C of the full range of pyroxene compositions in one clast (#18) in 15205; its pyroxenes follow the trends for the entire data set, but vary in minor elements much less for a give Mg#. (The full range of Mg# in the clast extends to 20; we show in Fig. 6A–C only the region comparable to that shown by the set of most magnesian pyroxene cores in all samples studied.) Thus, it appears that this suite of KREEP basalt magmas represents a variety of primary magma compositions.

image

Figure 6.  Compositions of the most magnesian pyroxene and most calcic plagioclase crystals in KREEP basalt clasts in breccias or soils at the Apollo 15 landing site; see legend in F. A–C: Minor elements in pyroxene; D–E: minor elements in plagioclase; F: Mg# in pyroxene plotted against Ab content of co-existing plagioclase. All pyroxenes are less than Wo5. Range in minor elements in pyroxene (A–C) and FeO in plagioclase (E) indicate that this suite of KREEP basalt lava flows varies in composition, probably due to a combination of differences in partial melting and subsequent fractional crystallization, and in source region composition. The small symbols in A–C span the range of Mg# in clast 18 in breccia 15205. This crystallization trend for one clast is narrower in minor element variations than the scatter shown by the entire suite of most magnesian pyroxenes. (The full range of Mg# in clast #18 extends to 20; we show only the region comparable to that shown by the set of most magnesian pyroxene cores in all samples studied.)

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The most calcic plagioclase crystals range from 7 to 23 mol% albite (Fig. 6D and 6E). Potassium is strongly correlated with albite concentration, probably reflecting crystal-chemical control. FeO broadly correlates with albite, but the range at a given albite concentration is over a factor of 2 for Ab >15, suggesting, in accord with pyroxene data, a range in lava compositions. Plagioclase and pyroxene compositions do not correlate significantly (Fig. 6F); the entire data set of 26 pairs of plagioclase Ab content and pyroxene Mg# has R2 of 0.2.

The mineral data do not suggest different magma compositions for local (clasts from 15205 to 15528, shaded in Fig. 6) and nonlocal samples of KREEP basalt (the other clasts plotted in Fig. 6).

Discussion: Genesis of the Hadley-Apennine KREEP Basalt Province

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methods
  5. Results
  6. Discussion: Genesis of the Hadley-Apennine KREEP Basalt Province
  7. Conclusions
  8. References
  9. Supporting Information

Extent of the Hadley-Apennine Igneous Province

The Apennine Bench Formation is extensive, surrounding the Apollo 15 landing site (Hackman 1966; Fig. 2). Photogeological studies indicate that the ABF represents lava plains (e.g., Hawke and Head 1978; Spudis 1978; Blewett and Hawke 2001) and orbital measurements show that it has a composition like that of KREEP basalts (Spudis and Hawke 1986). Considering the abundance of KREEP basalts in 15205, and the inferred stratigraphy of the Apollo 15 region (KREEP basalt beneath the maria; Spudis et al. 1988), it is likely that KREEP basalts of the Apennine Bench Formation underlie the mare basalt lava flows of Palus Putredinis. Ejecta from Autolycus and Aristillus have Th concentrations in the range of KREEP basalts (Blewett and Hawke 2001), suggesting that KREEP basalt underlies those craters. The high Th concentrations could reflect the generally high concentrations of incompatible elements in the Procellarum-KREEP Terrane, but the presence of KREEP basalts in impact melt breccias and soils collected on the Apennine Front and at Station 4 (Fig. 1) suggest that basalts were abundant in the target areas for Autolycus and Aristillus.

Our estimate of the minimum extent of the Apennine Bench Formation and associated KREEP basalts is given by the dashed line in Fig. 2, an area of about 105 km2. The thickness of basalts throughout this large area is uncertain, particularly because there is no photogeological evidence for the source vents or number of separate flows from them. On the basis of geological observations, Spudis et al. (1988) estimated that the Apennine Bench underlying the Apollo 15 mare basalts could be as much as 200 m thick; we take 200 m as an upper limit for the mean thickness. Considering the wide area apparently covered by KREEP basalts, it seems likely that the KREEP lavas are at least 10 m thick and probably thicker. These limits imply a total volume of 1 × 103–2 × 104 km3. For comparison, the well-developed Erastothenian flows in Mare Imbrium have a volume of 4 × 104 km3 (Schaber 1973) and Kilauea volcano has a volume of 2 × 104 km3 (Bargar and Jackson 1973).

KREEP Basalt Bulk Chemical Systematics

To put our new mineral compositional data in context, we summarize the chemical variations among Apollo 15 KREEP basalt samples. Bulk compositions have been studied previously (e.g., Rhodes and Hubbard 1973; Basu and Bower 1976; Ryder 1976, 1987; Irving 1977; Lindstrom et al. 1977; Warren et al. 1983; Ryder et al. 1988; Simon et al. 1988; Ryder and Sherman 1989; Ryder and Martinez 1991). We summarize available data on bulk compositions, including compositions of glass in 15358 and related samples (our data and those from Ryder 1988), fractional crystallization products (quartz monzodiorites, QMD, and related), and experimental data (Hess et al. 1978; Rutherford et al. 1980). A problem with bulk data on KREEP basalts is that the samples are all small; the largest sample, 15386, had a mass of only 7.5 g. Thus, analyses have been performed on small chips, potentially leading to large sampling errors. The QMD samples are even coarser grained than the basalts, so have the largest sampling problems. Furthermore, many analyses of basalt and QMD samples are broad-beam microprobe analyses, which sample only one plane of small fragments, hence are subject to large sampling errors, in addition to uncertainties in the microprobe correction procedures for polymineralic materials. Nevertheless, broad chemical variations are discernible and informative.

KREEP basalt samples show increasing P, K, and Ti with decreasing Mg# (Fig. 7). The TiO2 trend has more scatter, consistent with the range of magma compositions inferred from pyroxene data (Fig. 6C), although it is likely that sampling causes some of the scatter. Expected compositional variations in the three elements as KREEP basalts fractionate are illustrated by a natural experiment: the compositions of yellow glasses in the incompletely crystallized samples in 15358 (Table 5). Pyroxene and plagioclase crystals are zoned in these samples, suggesting that the glass compositions reflect fractional crystallization trends. These trends show uniformly increasing concentrations of P2O5, K2O, and TiO2 with decreasing Mg#, hence with crystallization. Although small amounts of ilmenite have crystallized in these incompletely crystallized basalts, the TiO2 concentrations do not record ilmenite crystallization, even though TiO2 reaches 5 wt% in the residual magma.

image

Figure 7.  Bulk rock chemical compositions of KREEP basalt and quartz monzodiorite (QMD) clasts (sources below), yellow glass in quenched rocks from 15358 (Ryder 1988; and our new data), and experimental data that approximate equilibrium liquid lines of descent for 15382 (Hess et al. 1978) and 15386 (Rutherford et al. 1980). KREEP basalt and QMD data from: Rhodes and Hubbard (1973), Basu and Bower (1976), Irving (1977), Lindstrom et al. (1977), Warren et al. (1983), Simon et al. (1988), Ryder et al. (1988), Papike et al. (1998), Ryder and Sherman (1989).

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The liquid line of descent of a magma with the composition of 15386 (Rutherford et al. 1980) peaks at a TiO2 concentration of 4.9 wt% (at 1086 °C), then declines (Fig. 7C), clearly indicating ilmenite crystallization. Magma with the composition of KREEP basalt 15382 reaches a peak in TiO2 at 1080 °C (Hess et al. 1978). Although the experimental data on the liquid line of descent were collected at conditions closer to equilibrium than was the case for the yellow glass mesostases in the quenched basalts, both show generally the same trends of increasing concentrations of incompatible elements with crystallization. The experimental melts reach silicate liquid immiscibility at 1014 °C (15386) to 1035 °C (15382), shown in Fig. 7 by the dashed lines joining two points. One melt is enriched in SiO2 and K2O, the other in P2O5 and TiO2 (and in FeO, although the Fe/Mg ratio is not strongly affected by the immiscibility). These data have implications for the genesis of KREEP differentiates, as discussed below.

The QMD samples, generally considered to be formed by fractional crystallization of KREEP basalt magmas (e.g., Ryder and Martinez 1991), deviate from the general compositional trends (Fig. 7). A portion of the variation may be caused by nonrepresentative sampling, but probably not all. For example, the data show a general increase in K2O and P2O5 with decreasing Mg#, consistent with fractional crystallization. On the other hand, P2O5 is low in some samples and TiO2 is systematically low in the samples. These trends hint at fractionation of phosphate minerals and ilmenite. We discuss the possible causes of these compositional variations below.

Mineral Compositions: Partial Melting and Fractional Crystallization

The cores of orthopyroxene and plagioclase crystals in KREEP basalts record the compositions of the magmas in which they crystallized. The rapid cooling of lava flows prevents significant elemental diffusion. Minor and trace element concentrations (such as Cr, Al, and Ti in pyroxene and Fe in plagioclase) might be affected by lava cooling rates and crystal growth rates, but Watson (1996) argues that this is not likely to happen in naturally occurring basalts. However, Grove and Bence (1977) found correlations between Cr, Ti, and Al concentrations in low-Ca clinopyroxenes and cooling rates in an experimental study of low-Ti quartz-normative mare basalts. These minor elements are subject to mutual substitutions during crystallization, thus complicating assessing the compositions of basalt parent magmas. In general, the substitutions increase with increasing cooling rates, as shown by largest concentrations of all three elements being found in the first pyroxenes to crystallize in the most rapidly cooled experimental samples. Our data (Fig. 6A–C) correspond to the first pyroxene to crystallize in each KREEP basalt clast studied, but we find no correlation between concentrations of Cr, Ti, and Al with plagioclase grain size, a rough monitor of cooling rate (Table 4). Thus, we conclude that our measurements (Fig. 6) of minor elements in pyroxene and plagioclase provide useful information about the parent magmas of each basalt sample.

Our mineral data and published bulk compositions of KREEP basalts suggest a relatively small range in Mg#, a point made previously by Ryder (1988). The Mg# of the most magnesian pyroxene ranges from 85.5 to 77.1 (Fig. 6). We can estimate the corresponding Mg# in the magma by using the orthopyroxene-melt partition coefficient. Using Bédard’s (2007) parameterization of a large database of experimental data, we use a value of 0.28, corresponding to a melt MgO concentration of 8 wt% (the partition coefficient varies with MgO content of the melt). This partition coefficient and the observed range in mineral composition translate to a range in Mg# in the magmas of 62–49. Bulk compositions of KREEP basalt fragments suggest a somewhat greater range in magma Mg# (Fig. 7), from 73.3 to 34.6. However, all but two of the 43 samples plotted are in the range from 73.3 to 50.2, suggesting that the most evolved KREEP basalt fragments might be related to the more fractionated quartz monzodiorites (see next section). Dropping the two samples with the lowest bulk Mg#, we calculate that the most magnesian pyroxenes in these fragments would have Mg# in the range 90–79. The most magnesian pyroxene Mg# is higher than the most magnesian orthopyroxene core we measured (85.5, Fig. 6), but in reasonable agreement considering that the bulk compositional data were not obtained on the same set of KREEP basalt fragments as were our mineral data. The important point is that KREEP basalts have a limited range in Mg#, indicating small amounts of fractional crystallization or small differences in partial melting of mantle source regions. If the variation was caused by fractional crystallization, it almost certainly took place in subsurface magma chambers because plagioclase and pyroxene do not readily separate inside small basaltic magma bodies such as lava flows (Mangan and Marsh 1992). Note that KREEP basalts have similar major element compositions to terrestrial basalts, hence similar initial viscosities, so the analogy to terrestrial basalts is valid. This is completely different from lava flows with a long interval (10–100 °C) of crystallization of pyroxene or pyroxene and olivine, such as mare basalts, most Martian meteorite, and terrestrial highly mafic flows.

Minor elements in plagioclase show clear and logical trends with increasing Ab content (Fig. 6D and 6E). The strong correlation with K2O most likely reflects crystal-chemical control, but it is consistent with fractional crystallization. FeO also increases with Ab content, thus recording the increase in Fe and Na with crystallization. There is a weak correlation between Ab in plagioclase and Mg# in the most magnesian pyroxene (Fig. 6F), consistent with fractional crystallization, but the significant variation in Ab at a given Mg# indicates that the magmas must vary in their initial Mg/Fe, Na/Ca, and the extent to which plagioclase and pyroxene co-crystallized or one preceded the other.

The main characteristic of our mineral data set is the large ranges in minor element concentration for a given Mg# in pyroxene or Ab content of plagioclase (for FeO), well outside the range expected for analytical uncertainties (±0.05 wt%, 2-sigma). Concentrations of these elements are affected by both partial melting and fractional crystallization, but fractionation of a single magma ought to have produced less scatter, as shown by the data for clast #18 in 15205 (Fig. 6A–C). Thus, we suggest that the lack of strong correlation with the fractionation parameters Mg# and albite content indicates differences in source region compositions and percentage of partial melting. Such differences are not necessarily large. All sources might have consisted of orthopyroxene and plagioclase with or without olivine, and variable amounts of ilmenite, phosphates, and other minerals. Small amounts of ilmenite in the source might have caused differences in TiO2; exhaustion of ilmenite during partial melting would then dilute an initially higher TiO2 content to a lower one. A similar argument can be made for Cr2O3, with its variability caused by exhaustion of olivine in the source region as the amount of partial melting increased (Cr partitions strongly into olivine compared to silicate melt).

The factor-of-two variation in Al2O3 in the most magnesian pyroxenes (Fig. 6B) is surprising in light of bulk rock Al2O3 ranging from 15 to 20 wt%, with most values in the range from 15 to 18 wt% (Fig. 7). Orthopyroxene Al2O3 concentrations might reflect crystal growth factors, although Al2O3 does not correlate with grain size. Higher Al2O3 in pyroxene might instead indicate pyroxene crystallization before plagioclase, when Al2O3 in the lava was highest. In turn, this may reflect different proportions of plagioclase and orthopyroxene in the source region. Different amounts of partial melting might have led to total melting of plagioclase in the source, and subsequent decrease in Al2O3 in the magma as melting continued. Whatever the details, the most straightforward explanation for minor element variations in pyroxene and plagioclase is that the source region varied in composition, although not enough to lead to drastically different magma compositions: they are all still KREEP basalts.

Differing source compositions are consistent with the weak correlation between albite in the most calcic plagioclase and the Mg# of the most magnesian pyroxene (Fig. 6F). For a given Mg# in pyroxene, co-existing plagioclase ranges in Ab, typically 4 mol%, but up to 6 mol%. Compositions of a suite of basalts formed by partial melting of a common source region ought to show decreasing Ab with increasing Mg#, as the data suggest, but the scatter also indicates variation in the source composition.

Formation of the KREEP Basalt Differentiates

Ryder (1976), Irving (1977), and Ryder and Martinez (1991) drew attention to extensively fractionated samples from Apollo 15. As discussed in the previous section, the relative compositional variation among KREEP basalts might be caused by differences in the amount of partial melting. On the other hand, the samples loosely classified as quartz monzodiorites (QMD) clearly show that extensive fractional crystallization must have operated: they have much lower Mg# than do the basalts, and in general, elevated incompatible element concentrations (Fig. 7). Here, we use published experimental data (Hess et al. 1978; Rutherford et al. 1980) and yellow glasses in 15358 (Ryder 1988; and our data) to track likely compositional trends for fractional crystallization (Fig. 7).

The yellow residual glasses in KREEP basalt clasts in 15358 represent a closer approach to fractional crystallization than do the close-to-equilibrium experiments reported by Hess et al. (1978) and Rutherford et al. (1980). The yellow glasses show continuous increase in the incompatible elements K and P, not surprising for a crystallizing system. Titanium (TiO2) also increases with crystallization (decreasing Mg#), indicating that no significant ilmenite crystallized. In fact, small amounts of ilmenite are present in the basalt clasts in 15358, indicating ilmenite saturation, but not enough crystallization to lead to a measurable decrease in Ti in the residual melt (hence in the preserved glass). In the experiments conducted with a starting composition similar to 15386, ilmenite crystallizes when the TiO2 concentration in the melt reaches about 5 wt%, it eventually reaches a point of silicate liquid immiscibility when the Mg# is less than about 10. The 15382 composition probably also saturates in ilmenite at roughly 5 wt% TiO2, but reaches immiscibility earlier, when the Mg# is about 25. None of the yellow glasses contain blebs of high-Fe and high-Si immiscible melts as present in the Hess et al. (1978) and Rutherford et al. (1980) experiments.

The quartz monzodiorite does not lie on the trends indicated by the yellow glasses or the experiments (Fig. 7). Ignoring the sampling problems in analyzing small, relatively coarse-grained rock fragments, the QMD compositions are most consistent with fractionation of ilmenite and phosphate minerals, as shown by their low P2O5 contents compared with the yellow glass and experimental trends (Fig. 7). Potassium (K2O) shows both enriched and depleted concentrations, suggesting silicate liquid immiscibility. Lunar samples record immiscibility in action in an Apollo 14 QMD rock fragment (Jolliff et al. 1999) and in an Apollo 12 felsite (Warren et al. 1987) in which phosphate crystallization preceded immiscibility. However, P2O5 and TiO2 ought to be enriched in the Fe-rich melt, not depleted. The depth at which fractionation took place may play a role in the path taken by KREEP basalt magmas: Rutherford et al. (1996) showed that silicate liquid immiscibility is suppressed at 3 kbar (corresponding to a depth of about 60 km). Thus, if the magma bodies that produced the QMDs were deep, liquid immiscibility may not have occurred. Alternatively, the differences in element concentrations could be caused by fractional crystallization, but of different starting magmas, one low in K2O and another higher, hence leading to a range in K2O concentrations of the QMD.

The fundamental point is that the QMD is clearly more evolved than are the KREEP basalts, indicating fractional crystallization. The presence of fine exsolution lamellae in the pyroxenes in all QMD fragments is indicative of formation in a small magma body (Ryder and Martinez 1991). The cooling rate of 0.01 °C/yr determined for Apollo 14 sample 14161,7373 (Jolliff et al. 1999) indicates crystallization in a sill at least 1 km thick. If applicable to Apollo 15 QMD samples, the magmatic system must include km-sized sill-like magma bodies. All QMD pieces have been found in the samples collected on the Apennine Front (impact melt breccia 15405 and soil 15434), indicating that the region excavated by Aristillus and/or Autolycus was the probable site of both KREEP basalt volcanism and subsurface magma bodies.

A Long-Lived Igneous Province or Two Episodes of Magmatism?

Only three Apollo 15 KREEP basalt samples have been dated, 15382, 15386, and 15434,73 (a fragment from coarse-fines soil 15434,4). Rb-Sr ages, updated using a 87Rb decay constant of 1.402 Ga−1 (Begemann et al. 2001), of the three samples are 3.87 ± 0.05 Ga (15382; Papanastassiou and Wasserburg 1976), 3.88 ± 0.05 Ga (15434,73; Nyquist et al. 1975) to 3.91 ± 0.04 Ga (15386; Nyquist et al. 1975). Ryder (1994) suggests that all three fragments date a single event (eruption of basaltic lava), which he estimated as occurring at 3.89 ± 0.02 Ga. This is almost certainly the age of the Apennine Bench Formation, which clearly postdates formation of the Imbrium Basin (Spudis and Hawke 1986; cf. Deutsch and Stöffler 1987). At the other end of the age spectrum, the QMD from 15405 has a U-Pb age of 4.30 Ga (Meyer et al. 1996). Because the QMD formed by fractionation of a KREEP basalt magma, at least some KREEP volcanism must substantially pre-date formation of the Imbrium Basin. The old age for the QMD shows that the epoch of KREEP basalt volcanism was either long or pulsed with one event at 4.3 Ga and another at 3.9 Ga. The old QMD age raises the question of the original pre-Imbrium location of the small magma body (or bodies) in which quartz monzodiorites formed.

Role of the Imbrium and Other Large Impacts

As suggested by Spudis (1978) and evaluated in detail by Ryder (1994), formation of the KREEP basalts that compose the Apennine Bench Formation may have been triggered by the formation of the Imbrium Basin. Ryder (1994) notes that the Apollo 15 KREEP basalts are 3.89 ± 0.02 Ga old (age corrected for a different 87Rb decay constant, as noted above) and that the age of the Imbrium Basin is indistinguishable from that age, although the basalts clearly formed after the basin was created. Ryder (1994) argued that large amounts of partially melted rock existed in the lower crust or upper mantle, kept above the solidus by the high concentrations of the heat-producing elements K, Th, and U present in KREEP. The Imbrium impact thus could have released magma already existing, but too dense or too viscous to migrate through the low-density crust. The presence of melt at depth in the Procellarum-KREEP terrane is supported by the thermal modeling of Wieczorek and Phillips (2000).

Elkins-Tanton et al. (2004) modeled post-impact melting processes and aside from the production of impact melt, identified two main sources of melting. One form is pressure-release melting beneath the basin, which effectively moves the solidus deeper into the Moon, inducing a zone of melting. This effect is more efficient if the material is already near or above the solidus, as Ryder (1994) argued. Pressure-release melting is prompt and indistinguishable in time from formation of the basin. The second melting process results from convection induced by the formation of the basin; this causes pressure-release melting of deep mantle rock as it rises adiabatically. This phase could have lasted a few hundred million years and may have led to eruption of mare basalt magmas.

The KREEP basalts that are possibly related to the formation of the Imbrium Basin are only those present within the Apennine Bench Formation. The age of the QMD in impact melt rock 15405 implies a pre-Imbrium age of 4.3 Ga for that episode of KREEP magmatism. Thus, although not yet dated directly, the KREEP basalt clasts within 15405 may be significantly older than the 3.89 Ga age of the KREEP basalts of the Apennine Bench Formation. Besides showing that KREEP volcanism began early in lunar history, this inferred older episode provides constraints on the formation of the Mg- and alkali-suites of nonmare rocks, assuming that they are related to KREEP magmatism. Mg-suite and alkali-suite rocks, and zircon analyses from breccias, show a range in age from about 4.49 Ga to 3.9 Ga (Nemchin et al. 2008, 2009; Edmunson et al. 2009; Taylor et al. 2009; samples are mostly from the Apollo 14 and 17 sites). The zircon data set shows peaks in the distribution, suggesting specific epochs of increased rates of magmatism. Given the possible relation between emplacement of KREEP basalts of the Apennine Bench Formation and the Imbrium Basin-forming impact, these peaks in magmatic activity similarly might be caused by large impacts, as argued by Nemchin et al. (2008), although episodic volcanism caused by internal processes, such as heating in the mantle and mantle dynamics, cannot be ruled out.

Genesis of the Basalts Composing the Apennine Bench Formation

Both mineral and bulk compositions indicate that the KREEP basalt source region was highly magnesian, with Mg# in the range 80–90. This relation reflects a long-standing paradox of KREEP basalt—magnesian magma rich in incompatible trace elements. The KREEP basalt source regions were probably dominated by orthopyroxene and plagioclase (e.g., Ryder 1987). The high Mg# suggests that the source region for Apollo 15 KREEP basalts consisted of magnesian, low-density mafic minerals formed relatively early during crystallization of the lunar magma ocean, but rose to the upper mantle (possibly the lower crust) by overturn of the mantle as denser minerals richer in Fe sank. Such a magnesian source would produce magnesian magmas poor in trace elements unless it was contaminated by late-stage fractionates rich in incompatible elements (e.g., urKREEP; Warren 1986, 1988). Such contamination, however, could have occurred by sinking of urKREEP and dense, mafic silicates and ilmenite, followed by partial melting (e.g., Shearer et al. 1991). Straightforward sinking of solid masses of rock is unlikely (Elkins-Tanton et al. 2002), so perhaps impacts could enhance the process by initiating overturn or even driving shallow materials deeper into the Moon. On the other hand, a problem with such hybrid sources is that a range of partial melting, as seems to be called for by variations in the compositions of KREEP basalt magmas, would result in fractionation among the REE and of Rb/Sr (e.g., Warren and Wasson 1979; Ryder 1994), which is not observed among the KREEP basalts. An alternative to hybrid mantle sources is the interesting idea (Elardo et al. 2011) that rising diapirs of magnesian early magma ocean cumulates mixed in the solid state with urKREEP and plagioclase-rich crust, then eventually partially melt to form the parent magmas of Mg-suite rocks and KREEP basalts. In the diapir-crust mixing model, large impacts might have aided the mixing of materials to produce KREEP basalt source regions. In both assimilation and diapir-crust mixing, KREEP basalt and Mg-suite magmas form close to the crust-mantle boundary from sources that were dominated by olivine, orthopyroxene, and plagioclase, but probably varied in abundances of minor minerals. The extent to which large impacts played a role in mixing materials to make source regions or in causing pressure-release melting is unknown.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methods
  5. Results
  6. Discussion: Genesis of the Hadley-Apennine KREEP Basalt Province
  7. Conclusions
  8. References
  9. Supporting Information

The data and interpretations discussed above lead us to conclude the following:

  •  Compositions of the most magnesian orthopyroxene crystals and most calcic plagioclase grains indicate that their lower crustal or upper mantle source regions were composed mostly of orthopyroxene and plagioclase, but varied in their abundances of other minerals. The sources had Mg# > 80, perhaps as high as 90.
  •  Mineral and bulk compositional data suggest that some amount of fractional crystallization was involved in producing KREEP basalt magmas. Extensive fractional crystallization in subsurface magma bodies produced differentiates of KREEP basalt magmas, such as the quartz monzodiorites.
  •  The Hadley-Apennine KREEP basalt igneous province was active between 4.3 and 3.89 Ga, but the scarcity of age data does not allow us to determine if the magmatism was concentrated at the extremes of this range or occurred sporadically during this interval.

Acknowledgments— We thank the personnel at the Lunar Curatorial Laboratory at NASA Johnson Space Center for arranging for sample loans and for examination of two hand specimens and tens of thin sections during a visit, and for loan of the thin sections studied in our laboratory. We are also grateful to Eric Hellebrand for assistance, training, and advice on use of the JEOL Superprobe in the Department of Geology and Geophysics at the University of Hawai’i. We thank Paul Warren, Larry Taylor, and Brad Jolliff for useful reviews. This work was supported by NASA Grant NNX08AY88G. The work of PDS was partly supported by NASA Lunar Science Institute contract NNA09DB33A to the Lunar and Planetary Institute (PI David A. Kring). This paper is LPI Contribution No. 1661, SOEST Publication No. 8665, and HIGP Publication No. 1931.

Editorial Handling— Dr. Christine Floss

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  5. Results
  6. Discussion: Genesis of the Hadley-Apennine KREEP Basalt Province
  7. Conclusions
  8. References
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methods
  5. Results
  6. Discussion: Genesis of the Hadley-Apennine KREEP Basalt Province
  7. Conclusions
  8. References
  9. Supporting Information

Fig S1. Optical photomosaics of thin sections 15205,210 and 15205,4 with studied clasts labeled. Mineral compositions are given in Tables 3 and 4.

Fig S2. Optical photomosaics of thin sections 15434,188, 15405,12, 15358,6, and 15528,4 with studied clasts labeled. Mineral compositions are given in Tables 3 and 4. [Correction added after online publication May 17 2012: supporting information was revised.]

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MAPS_1364_sm_FigS1-S2.pdf4812KSupporting info item

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