Feldspar in type 4–6 ordinary chondrites: Metamorphic processing on the H and LL chondrite parent bodies

Authors


Corresponding author. E-mail: rjones@unm.edu

Abstract

Abstract– We have carried out a study of feldspar compositions in a suite of H and LL ordinary chondrites, of petrologic types 4, 5, and 6, in order to examine the process of recrystallization and equilibration of feldspar as the degree of metamorphism increases. In the H chondrites, there is little variation in feldspar compositions among the petrologic types, suggesting that homogenization of chondrule mesostasis, from which feldspar is presumed to have crystallized, occurred before feldspar crystallization began. The LL chondrites we studied are more complex. In Bjurböle (L/LL4), plagioclase in individual relict chondrules has distinct compositions, with a range of An/Ab ratios and low Or contents. This heterogeneity is most likely attributable to original compositional heterogeneity among chondrule mesostases: localized recrystallization of mesostasis must have occurred before diffusional equilibration took place. In Tuxtuac (LL5), the An/Ab ratio of plagioclase is more homogeneous, and plagioclase includes a significant Or component. In addition, we observe what appears to be exsolution of K-feldspar from albitic host grains. In Saint Séverin (LL6), the An/Ab ratio of plagioclase is homogeneous, but plagioclase compositions show a range of Or contents, corresponding to a patchy distribution of K in individual feldspar grains. The observations in these LL chondrites are difficult to interpret with a simple model of progressive equilibration with increasing petrologic type. We suggest that the current criteria for assigning petrologic types are poorly defined: it is possible that the assigned petrologic types of these chondrites do not correlate with their peak temperatures. We propose that feldspar compositions might record conditions during the heating stage of metamorphism, and that the early stages of metamorphism may have occurred in the presence of fluids, rather than under the dry conditions that are commonly assumed.

Introduction

Ordinary chondrites (OCs) account for more than 85% of all meteorites in the meteorite collection (e.g., Krot et al. 2005). Most OCs show the effect of an extended period of heating experienced on their parent bodies, which took place shortly after accretion (e.g., Huss et al. 2006). Thermal metamorphism is responsible for major changes in mineral compositions in the OCs, causing an increasing degree of recrystallization and chemical equilibrium with increasing petrologic type, defined on a scale of petrologic types 3 to 6 (Van Schmus and Wood 1967; Brearley and Jones 1998; Weisberg et al. 2006). Determination of the peak temperatures and conditions of metamorphism recorded by the OCs can lead to an understanding of the early geologic history of chondrite parent bodies (e.g., Miyamoto et al. 1981; Bennett and McSween 1996; Trieloff et al. 2003; Bouvier et al. 2007).

Unfortunately, metamorphic conditions are hard to determine accurately, and our existing classification scheme for the effects of thermal metamorphism is potentially flawed because of the problems associated with defining peak temperatures. The peak temperatures for petrologic types 4, 5, and 6 OCs are currently not very well defined, with a general range of 500–800 °C for types 4 and 5 and 800–1000 °C for type 6 (Scott and Krot 2005; Huss et al. 2006). One of the most quantitative parameters in the petrologic type classification scheme is the degree of equilibration of olivine and pyroxene compositions, which occurs due to interdiffusion of Fe and Mg. In type 3 chondrites, silicates are heterogeneous. As thermal metamorphism progresses, olivine becomes equilibrated by petrologic type 4 and pyroxene is equilibrated by petrologic type 5. The later equilibration of pyroxene is due to slower diffusion rates of Fe-Mg and Ca in pyroxene than in olivine. As olivine and pyroxene are equilibrated before petrologic type 6, there are no comparable quantifiable criteria for classification of a type 6 chondrite. The current classification scheme uses poorly defined feldspar grain size criteria (the presence of abundant feldspar grains >50 μm in size), and a subjective description of chondrules as having “poorly defined” outlines, to identify a chondrite as being petrologic type 6.

In most petrologic type 4–6 OCs, feldspar is the third most abundant silicate mineral after olivine and pyroxene, representing approximately 10 wt% (Van Schmus and Ribbe 1968). During thermal metamorphism, plagioclase crystallizes due to the devitrification of chondrule glass, and plagioclase grain size coarsens with increasing petrologic type (Van Schmus and Wood 1967). As diffusion rates of major elements in feldspar are generally slower than those of olivine and pyroxene (e.g., Grove et al. 1983), feldspar compositions should equilibrate after both olivine and pyroxene in the metamorphic sequence. The goal of this study is to examine changes in feldspar compositions in petrologic type 4 to 6 OCs, to assess whether compositional changes in feldspar may provide better criteria for interpreting metamorphic conditions in the higher petrologic types.

In addition to compositional equilibration via diffusion, feldspars have the ability to record many phenomena that could help interpret parent body processes. For example, exsolution of alkali feldspars is one of the most common examples of exsolution in silicate minerals. The presence of two feldspars as exsolution lamellae allows for determination of an equilibration temperature which can be used to place temperature constraints on metamorphic processes (Brady and Yund 1983; Wen and Nekvasil 1994). Feldspar also has the potential to record interactions with fluids: for example, calcic feldspar is able to undergo albitization by means of diagenesis (e.g., Ramseyer et al. 1992). Order/disorder phenomena in feldspars provide a geothermometer that allows for the determination of crystallization temperatures: Nakamuta and Motomura (1999) applied this geothermometer to the OCs.

Little is currently known about feldspar compositions in OCs, so it is not known whether they show changes through the metamorphic sequence. Analyses collated by Brearley and Jones (1998) show some variation in type 4–6 OCs. Van Schmus and Ribbe (1968) showed that plagioclase compositions are generally homogeneous among H6 and L6 chondrites, but are slightly more variable among LL6 chondrites, probably because of varying bulk chondrite Na/K ratios. Compositional variation within individual type 4, 5, and 6 chondrites has been documented by Nagahara (1980).

Type 3 OC chondrule mesostases have widely varying compositions from chondrule to chondrule (Brearley and Jones 1998; Grossman and Brearley 2005). Hence, it is possible that plagioclase heterogeneity in type 6 chondrites is the result of crystallization from heterogeneous glass in type 3 chondrites, and that heterogeneity in type 6 chondrites was inherited from the varying mesostasis compositions. Our study was designed to test this hypothesis by examining compositions of feldspar in recognizable chondrules in chondrites of petrologic types 4, 5, and 6 from the H and LL chondrite groups. As shock and weathering can affect plagioclase compositions (as discussed in more detail below), we selected chondrites that showed minimal shock and weathering effects, with shock stages S1–S2 and weathering effects W0–W3.

Samples and Analytical Methods

We studied thin sections of eight different OCs, including one H4, two H5, two H6, one L/LL4, one LL5, and one LL6 (Table 1). The chondrites were selected on the basis of their low shock and weathering effects reported in the literature. Five of the chondrites are falls. All of these thin sections were available in the University of New Mexico meteorite collection.

Table 1.   Ordinary chondrites studied.
ChondriteThin sectionPetrologic typeShock stageWeathering grade
AvanhandavaUNM 88H4S2W0 (fall)
Oro GrandeUNM 55H5S1W3
RichardtonUNM 384H5S2W0 (fall)
EstacadoUNM 609H6S1W0
Nazareth(e)UNM 871H6S2W3
BjurböleUNM 146L/LL4S1W0 (fall)
TuxtuacUNM 627LL5S2W0 (fall)
Saint SéverinUNM 693LL6S2W0 (fall)

Analytical techniques used include scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), and secondary ion mass spectrometry (SIMS). All the instruments used are at the Microbeam Analysis Facility of the Department of Earth and Planetary Sciences/Institute of Meteoritics at the University of New Mexico.

Individual chondrules were first identified in thin sections using optical microscopy. The JEOL 5800LV SEM was used to survey the chondrites further, by using backscattered electron (BSE) imaging to identify chondrules suitable for analysis, i.e., chondrules that contained feldspar grains >10 μm across. Operating conditions for the SEM were a 20 keV accelerating voltage and condenser aperture size of 30 μm.

The JEOL 8200 electron microprobe was used for quantitative wavelength dispersive (WDS) analyses of feldspar grains. Operating conditions for the electron microprobe analyses were an accelerating voltage of 15 keV, beam current of 20 nA, and a 5 or 10 μm electron beam diameter. Appropriate mineral standards were used for calibration. We conducted tests to investigate the rate of Na volatilization during the feldspar analyses. We ran experiments on feldspar grains in a chondrule for 2 min for different beam diameters (0, 5, and 10 μm) using a chart recorder, which keeps track of the counts received at the spectrometer on which the Na is analyzed. For a focused beam, the Na X-ray counts fall significantly after about 20 s. However, for 5 μm and 10 μm beam diameters, there are no changes in counts over the entire 2 min interval, indicating that either beam diameter would be appropriate at these conditions to analyze the feldspars. In order to minimize any volatilization, we analyzed Na first in the analysis sequence. The peak count time was 20 s, and the background count times were 10 s, giving a total count time of 40 s, which is well under the 2 min interval we tested.

X-ray maps of feldspar grains and whole chondrules were also obtained using the EPMA. Operating conditions were a focused beam, a 20 nA beam current, and dwell times of 30 μs per pixel, with pixel sizes of 0.5 μm for smaller maps and 1 μm for larger maps. Most of the individual point analyses on the chondrules were performed before collecting X-ray maps in order to ensure the analyses were not compromised by the mapping, in particular by volatilization of Na.

Modal abundances of mineral phases were determined using both single element and combined X-ray maps of Na, K, Al, Mg, Fe, and Ca. The combined X-ray maps of Fe, Mg, and Ca allowed us to identify olivine, pyroxene, diopside, and metals. The combination X-ray maps of Na, K, and Al allowed for feldspar to be identified easily. After the maps were combined, they were imported into Adobe Photoshop© where each mineral phase identified was given a specific color and layer. The layers were combined, and the histogram function was used to determine the percentage of each mineral present in the chondrules.

The Cameca 4f (Cameca, France, Paris) secondary ion mass spectrometer (SIMS) was used to measure trace element (Ba and Sr) concentrations in selected plagioclase grains. The operating conditions included an O ion source, a primary ion beam accelerating voltage of 10keV, beam current of 15nA, and a voltage offset of −105 v. The counting times were 2 s for background, 4 s for 30Si, and 10 s for both 138Ba and 88Sr. The standards used were a combination of glass (NIST 610), two plagioclase standards (Moore County and UMASS PG-721), and clinopyroxene (KAUG).

Results

The purpose of this study is to examine compositional variations in feldspar that is located in chondrules of type 4, 5, and 6 OCs. We therefore needed to select readily identifiable relict chondrules that had large, inclusion-free feldspar grains at least 10 μm in diameter. The chondrules we examined included barred, radiating, and porphyritic textures. Appropriate feldspar grains were easiest to locate in porphyritic chondrules (mostly porphyritic olivine/pyroxene–POP chondrules). Large feldspar grains were also found in some barred chondrules, but this was not as common. In the type 5 and 6 chondrites, suitable feldspar grains were abundant and easy to find. In the type 4 chondrites, it was harder to find relict chondrules with large enough feldspar grains: many chondrules have mesostasis that consists of fine-grained crystallites. We studied 7–10 chondrules in each chondrite.

In type 4 chondrites, the largest feldspar grains observed in the chondrules we studied were 91 μm (H4) and 72 μm (L/LL4) across. Grain sizes increased with petrologic type. In the type 5 chondrites, the largest feldspar grain sizes in the chondrules we studied were 112 μm (H5) and 140 μm (LL5), and in the type 6 chondrites, 331 μm (H6) and 322 μm (LL6).

H Chondrites

In all the relict chondrules studied in H chondrites, plagioclase grains occur interstitial to olivine and pyroxene in porphyritic, radiating, and barred chondrules. In general, in petrologic types 4, 5, and 6, plagioclase grains appear homogeneous in BSE images and do not exhibit any core to rim zoning or exsolution lamellae (Fig. 1). However, one chondrule in Avanhandva (H4), chondrule 5, shows a more complex texture (Fig. 2). The plagioclase grains have a dark rim <1 μm in thickness, which compositionally appears to be glass (Fig. 2d). The rim compositions were analyzed on the SEM using the qualitative EDS detector with a focused electron beam.

Figure 1.

 BSE images of feldspar-bearing chondrules in H chondrites, showing typical occurrences of feldspar grains in type 4, 5, and 6 H chondrites, which do not appear to have any compositional zoning. a, b) Avanhandava (H4), chondrule 2. The area indicated in (a) is shown in (b). c, d) Richardton (H5), chondrule 3. The area indicated in (c) is shown in (d). Olv = olivine; pyx = pyroxene; fsp = feldspar. White phases are a mixture of Fe, Ni metal and sulfides.

Figure 2.

 BSE images and EDS X-ray spectra of Avanhandava (H4), chondrule 5. The area indicated in (a) is shown at higher magnification in (b). Olv = olivine; pyx = pyroxene; fsp = feldspar. Feldspar grains appear homogeneous, but have narrow (<1 μm), dark rims. EDS spectra for c) feldspar and d) a rim show that the rim is likely a glass. The rims on all the grains in the chondrule have similar compositions.

Individual analyses of plagioclase grains from the H chondrites are plotted on a ternary diagram in Fig. 3. Analyses are from relict chondrules with feldspar grain sizes >10 μm in diameter. The data include 6–8 analyses from 7–10 chondrules in each chondrite. In Fig. 3, all the analyses cluster together with a slightly larger spread in the An/Ab ratio in type 5 chondrites. A few analyses in type 5 and 6 H chondrites are slightly more K-rich. These compositions are associated with a patchy heterogeneity in K distribution that is only visible in X-ray maps (Kovach and Jones 2007a, 2007b). Mean compositions for each chondrule are summarized in Table 2. There is little variation in feldspar compositions among chondrules. The mean compositions of all the individual analyses from all chondrules in each petrologic type are as follows: type 4, An12Ab83Or5; type 5, An11Ab83Or6; and type 6, An12Ab82Or6. These overall mean compositions are very similar to each other, and agree with previous work compiled by Brearley and Jones (1998), who gave an overall mean of An12Ab82Or6 for the H4–6 chondrite suite, identical to that given by Van Schmus and Ribbe (1968) for all available H6 chondrite data. For all plagioclase analyses, MgO contents are low, mostly <0.1 wt%, whereas FeO contents are generally higher, between 0.3 and 1 wt%.

Figure 3.

 Compositions of feldspar grains in relict chondrules in H chondrites. Symbols are as follows: triangles: type 4 (Avanhandava); circles: type 5 (Richardton and Oro Grande); crosses: type 6 (Nazareth (e) and Estacado). Data include 6–8 analyses from 7–10 chondrules in each chondrite. There is little spread in feldspar compositions in the type 4 and 6 chondrites; however, feldspar in the type 5 chondrites is slightly more heterogeneous.

Table 2.   Mean feldspar compositions for each chondrule: H chondrites.
ChondruleSiO2Al2O3FeOMgOCaONa2OK2OTotalCationsaAnAbOrnb
  1. n.d. = not detected.

  2. Oxides in wt%; feldspar endmembers in mole%.

  3. aTotal cations on the basis of 8 oxygen atoms

  4. bNumber of analyses in each chondrule

  5. cAverage of all analyses in each chondrite.

Avanhandava (H4)
 Ch-166.321.80.400.012.5610.040.86101.95.0211.783.54.76
 Ch-266.221.70.250.012.549.831.09101.75.0111.882.26.07
 Ch-366.421.50.540.072.4510.020.92101.95.0211.383.65.15
 Ch-566.221.60.520.112.569.841.05101.85.0211.882.45.85
 Ch-665.821.50.690.402.569.770.99101.65.0212.082.65.56
 Ch-766.121.70.470.012.5810.070.89101.95.0211.883.34.87
 Ch-865.821.80.320.012.759.880.98101.55.0212.682.05.36
Averagec11.982.85.342
Oro Grande (H5)
 Ch-264.821.71.130.052.669.761.06101.25.0412.381.85.84
 Ch-365.921.70.780.102.589.971.01102.05.0311.882.75.518
 Ch-866.221.80.450.032.5210.040.99102.15.0211.583.15.417
 Ch-966.121.50.600.182.489.951.08101.95.0211.482.75.914
 Ch-1065.521.60.620.272.719.791.10101.65.0312.581.56.015
 Ch-1165.821.90.480.102.789.950.81101.85.0212.882.84.52
 Ch-1266.121.50.530.482.689.811.03102.15.0212.482.05.712
 Ch-1365.19.81.060.262.48101.2921.511.05.0311.682.65.912
Averagec11.982.45.794
Richardton (H5)
 Ch-365.421.60.580.022.4110.160.77100.95.0311.184.74.211
 Ch-566.421.10.470.141.9910.241.04101.35.029.185.25.712
 Ch-765.721.40.510.012.249.931.24101.05.0210.382.96.84
 Ch-865.321.70.720.022.4810.220.72101.25.0311.484.73.91
 Ch-1065.721.01.250.832.009.771.35101.95.049.483.17.48
Averagec10.084.35.736
Estacado (H6)
 Ch-166.021.70.170.012.709.740.99101.45.0012.682.05.510
 Ch-266.321.90.18n.d.2.749.721.03101.85.0012.781.65.79
 Ch-366.021.80.340.072.699.671.05101.65.0112.681.65.812
 Ch-466.221.90.230.012.719.701.02101.85.0012.681.75.715
 Ch-566.221.90.210.012.729.701.09101.85.0112.681.46.014
 Ch-666.222.00.240.012.729.740.99101.85.0012.681.95.516
 Ch-766.221.80.240.012.699.671.03101.75.0012.681.75.719
Averagec12.881.65.695
Nazareth (e) (H6)
 Ch-164.821.20.600.052.589.821.05100.15.0311.982.35.86
 Ch-264.821.20.490.072.539.910.98100.05.0311.782.95.48
 Ch-465.121.30.640.082.499.801.10100.55.0311.582.46.19
 Ch-565.321.40.540.042.539.970.99100.85.0311.683.05.413
 Ch-664.921.10.730.062.539.951.01100.35.0311.682.85.513
 Ch-764.521.10.910.042.319.851.2499.95.0410.782.56.86
 Ch-864.621.00.780.072.449.820.9999.75.0311.483.15.59
 Ch-965.221.20.620.042.459.891.11100.55.0311.382.66.111
 Ch-1064.621.30.630.032.479.980.99100.05.0411.483.25.48
 Ch-1165.321.40.550.032.4510.040.92100.75.0311.383.65.110
Averagec11.582.95.793

Modal analysis was carried out on selected chondrules from the type 4, 5, and 6 H chondrites, to assess the distribution and relative abundances of feldspar and diopside (Fig. 4, Table 3). The chondrules selected for modal analysis are representative of chondrules found in each chondrite. The goal of the modal analysis was to see if diopside was progressively crystallizing from glass with feldspar. For the chondrules examined, mesostasis is abundant in petrologic type 4 (Figs. 4a and 4b, Table 3), and absent in types 5 and 6. In the type 5 and 6 H chondrites, chondrule mesostasis appears to have crystallized predominantly to plagioclase, which is typically much more abundant than diopside (Figs. 4c–f, Table 3).

Figure 4.

 Mineral phase distributions and BSE images of chondrules from a, b) type 4, c, d) type 5, and e, f) type 6 H chondrites. Mesostasis is a significant component of the type 4 chondrule but is absent in the type 5 and 6 chondrules. Mesostasis crystallizes predominantly as plagioclase, with diopside a minor component. Modal analyses obtained from these images are given in Table 3.

Table 3.   Mineral modes in selected chondrules (vol%).
ChondriteAvanhandavaRichardtonNazareth (e)BjurböleTuxtuacSaint Séverin
  1. aFigure where phase distribution map is illustrated.

  2. bTotal abundance of metal + sulfide + apatite.

Pet. typeH4H5H6L/LL4LL5LL6
ChondruleCh-6Ch-3Ch-10Ch-8Ch-7Ch-5
Figurea4b4d4f11b11d11f
Olivine355159413162
Low-Ca Pyx382814304711
Feldspar81614121613
Diopside6236514
Met/Sul/Apb12100.4<0.10.3
Mesostasis1100110.10

LL Chondrites

In the LL chondrites, plagioclase occurs interstitial to olivine and pyroxene in chondrules with a variety of textural types. In Bjurböle (L/LL4), feldspar grains in several chondrules have homogeneous compositions. However, in four out of eight chondrules studied, we observed albite-rich rims, typically a few micrometers wide, on anorthite-rich cores (Fig. 5). There is a sharp boundary between the cores and rims. In the example illustrated in Fig. 5, Bjurböle chondrule 5, separate patches of albitic feldspar up to 13 μm across are also present. In this chondrule, the composition of the anorthite-rich cores of grains is An73Ab25Or1, and the albitic feldspar is An6Ab85Or9.

Figure 5.

 BSE images of Bjurböle (L/LL4), chondrule 5. Plagioclase grains consist of lighter gray anorthite-rich cores (An73Ab25Or1) surrounded by darker gray albite-rich rims (An6Ab85Or9), which are a few μm in thickness. Several larger patches of albite-rich feldspar also occur. Areas indicated in (a) are enlarged in (b) and (c). Olv = olivine; pyx = pyroxene; fsp = feldspar.

In the type 5 LL chondrite, Tuxtuac, plagioclase grains are generally homogeneous and there is no evidence of albite-rich zones like those observed in Bjurböle. However, we observed several examples of more complex zoning, including what appears to be exsolution of potassium-rich feldspar from albite (Figs. 6 and 7). In BSE images (Figs. 6 and 7a), light gray exsolution lamellae of potassic feldspar are found within darker gray albitic grains. In chondrule 10 (Figs. 6a, 6b, and 7), coarser patches of potassic feldspar (An3Ab18Or79) occur in association with albitic plagioclase (An9Ab89Or2), in addition to thin potassium-rich lamellae. Figure 7 shows elemental X-ray maps of a region containing the two feldspars in chondrule 10. The K X-ray map (Fig. 7b) indicates that the cores of the K-rich regions are more potassic than the rim.

Figure 6.

 BSE images of chondrules in Tuxtuac (LL5), showing apparent exsolution lamellae in plagioclase grains: a, b) Chondrule 1; c, d) Chondrule 10. Black boxes in (a) and (c) indicate areas enlarged in (b) and (d). Fine lamellae of K-rich feldspar (light gray) are apparent in albite-rich feldspar grains (dark gray) in both chondrules. Elemental X-ray maps of the region in the white box labeled X are shown in Fig. 7.

Figure 7.

 a) BSE image and elemental X-ray maps for b) K, c) Na, and d) Ca of a region of Tuxtuac (LL5), chondrule 10 (see Fig. 6c), showing two feldspars. a) In the BSE image, the darker gray phase is albite-rich (An9Ab89Or2), and the lighter gray regions and thin lamellae are potassium-rich (An3Ab18Or79).

No zoning or exsolution lamellae are present in any of the chondrules studied in the LL6 chondrite, Saint Séverin. All the plagioclase grains appear homogeneous in BSE images (e.g., Fig. 8a and 8b). However, in K X-ray maps, it is evident the feldspar is heterogeneous and shows patchy zoning (Fig. 8c). Almost all of the feldspar grains analyzed in the type 6 chondrites exhibit similar heterogeneity, which is visible via X-ray maps and apparent in compositional data (see below).

Figure 8.

 Saint Séverin (LL6), chondrule 1. a, b) BSE images show no compositional variation in plagioclase grains. c) However, in the elemental X-ray map for K, feldspar grains show significant heterogeneity in K. This heterogeneity is typical of most feldspar grains in Saint Séverin. There are no weathering veins present in the grain, which could be a potential cause of heterogeneity. Olv = olivine; fsp = feldspar.

Individual feldspar analyses from the type 4, 5, and 6 LL chondrites studied are plotted on ternary diagrams in Figs. 9 and 10. Analyses are from relict chondrules with feldspar grain sizes >10 μm in diameter. The data include 6–8 analyses from 7–10 chondrules in each chondrite. Mean compositions for each chondrule are summarized in Table 4. The mean compositions of all the analyses from each petrologic type for LL chondrites are as follows: type 4, An42Ab56Or2; type 5, An9Ab80Or11; and type 6, An10Ab84Or6. The mean composition for type 6 is very similar to the compilation by Brearley and Jones (1998), who gave an overall mean of An10Ab86Or4 for all available LL4–6 chondrite data, identical to that given by Van Schmus and Ribbe (1968) for LL6 chondrites. However, the overall means do not accurately represent the trends present in each chondrite: each chondrite has a very different range of feldspar compositions.

Figure 9.

 Compositions of feldspar grains in relict chondrules in LL chondrites. Symbols are as follows: gray triangles: type 4 (Bjurböle); white squares: type 5 (Tuxtuac); dark gray crosses: type 6 (Saint Séverin). Data include 6–8 analyses from 7–10 chondrules in each chondrite. Plagioclase compositions in Bjurböle (type 4) show a wide spread along the An-Ab join. In Tuxtuac (type 5), there is a range of compositions in An-Ab, but also an Ab-Or mixing trend. Plagioclase in Saint Séverin (type 6) is more homogeneous but shows a slight trend toward K-rich compositions.

Figure 10.

 Feldspar compositions in Bjurböle L/LL4 chondrite, which shows chondrule to chondrule heterogeneity. Each symbol represents an individual chondrule. Feldspar in each chondrule analyzed lies on a different segment of the An-Ab join.

Table 4.   Mean feldspar compositions for each chondrule: LL chondrites.
ChondruleSiO2Al2O3FeOMgOCaONa2OK2OTotalCationsaAnAbOrnb
  1. Oxides in wt%; feldspar endmembers in mole%.

  2. aTotal cations on the basis of 8 oxygen atoms.

  3. bNumber of analyses in each chondrule.

  4. cAverage of all analyses in each chondrite.

Bjurböle (L/LL4)
 Ch-157.727.20.540.049.426.630.12101.75.0243.755.70.78
 Ch-252.930.70.580.1613.834.090.17102.45.0264.834.30.98
 Ch-365.618.41.081.451.759.591.7299.65.058.182.29.72
 Ch-467.920.50.320.011.1111.000.71101.65.025.191.03.96
 Ch-555.927.11.240.6910.295.700.41101.35.0449.048.72.26
 Ch-653.728.01.091.0511.964.940.13100.95.0556.642.70.73
 Ch-757.326.71.230.178.766.750.18101.15.0441.557.51.06
Averagec41.756.32.039
Tuxtuac (LL5)
 Ch-166.320.30.940.541.459.921.44100.95.036.985.08.19
 Ch-266.621.10.490.021.9510.150.88101.25.019.186.04.97
 Ch-364.922.40.360.023.349.490.59101.15.0015.880.93.36
 Ch-565.222.10.510.023.039.700.73101.35.0114.281.84.04
 Ch-766.520.10.360.161.418.443.85100.95.016.671.521.99
 Ch-866.521.30.330.012.0510.060.86101.15.009.685.64.88
 Ch-965.321.40.770.142.449.740.84100.65.0111.683.74.78
 Ch-1066.820.30.390.061.248.693.64101.25.015.873.420.712
Averagec9.180.310.663
Saint Séverin (LL6)
 Ch-165.821.20.610.022.199.841.47101.25.0310.181.88.119
 Ch-265.921.30.640.022.219.941.28101.35.0310.282.87.011
 Ch-365.821.30.540.012.2010.150.75100.85.0210.385.64.210
 Ch-466.321.40.200.012.2210.310.71101.15.0210.285.93.911
 Ch-566.021.20.680.172.2110.440.66101.45.0310.186.33.611
 Ch-666.421.40.620.012.1910.500.68101.85.0310.086.43.79
 Ch-766.021.30.480.062.2510.071.11101.25.0210.383.66.18
Averagec10.184.35.579

Mean MgO contents of plagioclase in Bjurböle chondrules are variable, up to 1.5 wt%, (Table 4). By contrast, plagioclase in the LL5 and LL6 chondrites has lower MgO contents, with mean compositions for individual chondrules generally <0.1 wt%. FeO contents in all three LL chondrites are in the range 0.3 to 1.2 wt%, comparable with compositions of plagioclase in H chondrites (Table 2).

In the L/LL4 chondrite Bjurböle, plagioclase compositions show a wide range of An/Ab ratios with little to no potassium content. However, individual chondrules show more limited compositional ranges: mean An contents range from An5 to An65 (Table 4). These chondrule means include data for albite-rich regions such as those illustrated in Fig. 5. The compositions of anorthitic cores vary from An5 to An85, and show a narrow range within individual chondrules (Fig. 10).

Feldspar in Tuxtuac (LL5) is generally albitic, but also shows two compositional trends: i) a range of compositions that trends along the An-Ab join; and ii) a range of compositions that trends toward orthoclase (Fig. 9). The latter includes analyses from chondrules in which K-rich exsolution from albite is observed. Intermediate compositions are attributable to beam overlap between albitic and potassium-rich endmembers.

Plagioclase analyses from Saint Séverin (LL6) are albitic and more homogeneous in An/Ab ratio than those in Bjurböle and Tuxtuac. They show a slight trend toward potassium-rich compositions, but we have not observed two endmember feldspars as seen in Tuxtuac. The feldspar compositional ranges found from chondrule to chondrule in Saint Séverin are very similar (Table 4).

Modal analysis was carried out on selected chondrules from the type 4, 5, and 6 LL chondrites in order to examine differences in modal mineralogy between the different petrologic types (Fig. 11, Table 3). The chondrules selected for modal analysis are representative of chondrules found in each chondrite. Mesostasis is abundant in petrologic type 4, and absent or in very low abundance in types 5 and 6. In the type 5 and 6 LL chondrites, a significant amount of coarse-grained diopside has crystallized, along with plagioclase. Diopside occurs primarily as rims on low Ca-pyroxene grains and is significantly coarser in Tuxtuac (LL5) than it is in Bjurböle (L/LL4). In Saint Séverin (LL6), chondrule 5, diopside and feldspar are both coarse-grained.

Figure 11.

 Mineral phase distributions and BSE images of chondrules from a, b) type 4, c, d) type 5, and e, f) type 6 LL chondrites. Mesostasis crystallized into feldspar and diopside in the type 4, 5, and 6 chondrites. The diopside primarily forms rims along the edge of pyroxene grains in the type 4 and 5 chondrites.

For each of the three LL chondrites, we measured concentrations of the trace elements Ba and Sr on selected feldspar grains using SIMS (Table 5). Major element analyses of each grain were obtained prior to SIMS analysis, using EPMA. Twelve points were analyzed in each chondrite (4 chondrules, 3 points each). Figure 12 shows plots of Ba and Sr versus An and versus Or. There is a wide range of both Ba and Sr abundances in Bjurböle, and the range of both Ba and Sr contents decreases as the petrologic type increases (although there is an outlier in Tuxtuac (chondrule 10, analysis 1) with high Ba (899 ppm) and Sr (355 ppm) that is not included in the figure). Neither Ba nor Sr abundance is strongly correlated with An or Or content, although there is possibly a weak negative correlation between An content and Ba abundance.

Table 5.   Ba and Sr abundances in selected feldspar grains, LL chondrites.
ChondruleAnalysis numberAn mole%Ab mole%Or mole%Ba ppmSr ppm
Bjurböle (L/LL4)
 Ch-1346.453.00.6134154
444.954.30.882218
843.056.30.687186
 Ch-2285.014.80.219156
611.885.13.2114122
886.113.80.127173
 Ch-414.592.03.58591
45.290.44.415680
66.089.64.46367
 Ch-5173.026.60.514088
221.976.02.1121119
35.685.68.8217176
Tuxtuac (LL5)
 Ch-134.880.414.887101
55.585.19.471105
66.686.76.810096
 Ch-714.714.680.7121150
55.485.88.86892
65.062.832.266102
 Ch-8110.184.35.682167
59.885.54.778140
79.885.34.95797
 Ch-1013.517.579.0899355
65.186.98.08668
105.175.819.1279162
Saint Séverin (LL6)
 Ch-1110.478.611.080126
810.277.512.389142
1810.076.014.067136
 Ch-2210.582.66.993139
710.184.85.276130
1110.682.07.361103
 Ch-4110.185.94.069116
410.185.54.488147
910.184.85.077142
 Ch-5110.385.93.75699
210.087.12.960104
89.787.23.15085
Figure 12.

 Trace element analyses of feldspar grains in LL chondrites, including Ba abundances versus a) An content and b) Or content and Sr abundances versus c) An content and d) Or content. As petrologic type increases, the ranges in Ba and Sr contents decrease. For Bjurböle (L/LL4) (squares), different gray scales represent individual chondrules. For the type 5 and 6 chondrites, individual chondrules are not distinguished. One outlier with high Ba and Sr (Tuxtuac chondrule 10, analysis 1) has been omitted. The arrow indicates the An and Or position where the outlier would plot.

Discussion

Thermal metamorphism in the OCs results in changes in chondrite textures and mineral compositions, summarized by the classification scheme of petrologic types 3 to 6 (Van Schmus and Wood 1967). In a type 4 chondrite, olivine has equilibrated, and in a type 5 chondrite, pyroxene has equilibrated. The goal of this study is to examine changes in feldspar compositions in type 4 to 6 OCs to try and examine whether feldspar might be a useful mineral to distinguish relative peak temperatures of OCs of high petrologic types (5 and 6). A better understanding of the thermal histories of these chondrites would lead to improved models for the early geological evolution of OC parent bodies.

Models for Metamorphism on the OC Parent Body

Two heating models have been proposed for thermal metamorphism of OCs: the onion shell model (Miyamoto et al. 1981; Trieloff et al. 2003; Bouvier et al. 2007) and the rubble pile model (Scott and Rajan 1981). In the onion shell model, type 3 chondrites are found on the outer edge of the parent body while higher degrees of thermal metamorphism (higher peak temperatures and longer heating durations) are associated with increased depth. Models for onion shell thermal evolution of chondrite bodies, using decay of the short-lived radionuclide 26Al as an internal heat source, give parent body radii of tens of kilometers (Miyamoto et al. 1981; Bouvier et al. 2007). According to these models, the overall cooling rate for type 6 chondrites is a few tens of degrees per million years, and at the center of the parent body, the total duration of thermal metamorphism is predicted to be ∼25 Myr. A prediction of this model is that higher petrologic types should show lower cooling rates than low petrologic types. The problem with the onion shell model is that chondrite cooling rates do not have an inverse correlation with petrologic type (Taylor et al. 1987; Rubin 1995).

The rubble pile model was developed to explain the absence in correlation between cooling rates and petrologic type. Scott and Rajan (1981) proposed that peak temperatures occurred in small planetesimals before accretion into a larger parent body. The planetesimals were then assembled into asteroid sized bodies in which the metallographic cooling rates were controlled by burial depth. However, this model only works for highly insulating planetesimals that will not shatter during impact (Grimm 1985).

An alternative heating mechanism for OC parent bodies is that of induction heating (Herbert and Sonnett 1979; Herbert 1989; Shimazu and Terasawa 1995). This process results from interactions of the parent body with solar flares in the T Tauri phase of the Sun, which would have lasted a few million years (Ciesla and Charnley 2006). Herbert (1989) showed that peak temperatures within planetesimals are a function of both size and distance from the Sun. For example, (partial) melting temperatures could be reached on parent bodies with diameters between 1 and 50 km at distances up to 2.5 AU, if the induction epoch lasted 104 yr, and on parent bodies with diameters between 10 and 500 km in diameter, at distances up to 2.8 AU, if the induction epoch lasted 107 yr.

The peak temperatures for petrologic types 4, 5, and 6 OCs are not well defined, with a general range of 500–800 °C for types 4–5 and 800–1000 °C for type 6 (Scott and Krot 2005). A recent study performed by Kessel et al. (2007), based on the equilibration temperature of olivine and spinel, found closure temperatures, and thus the minimum peak temperatures, to range between 586–777 °C. Use of pyroxene thermometers to determine metamorphic temperatures is problematic because coexisting orthopyroxenes and clinopyroxenes are not in equilibrium (McSween and Patchen 1989). Several other geothermometers have been applied to OCs. These include the distribution of Ni between olivine and diopside (Curtis and Schmitt 1979), and the distribution of 18O in olivine, plagioclase, and orthopyroxene (Onuma et al. 1972). Nakamuta and Motomura (1999) applied a sodic plagioclase geothermometer, based on the structural state of feldspar. Because diffusion of the framework cations is slow, the plagioclase thermometer is assumed to record the temperature of plagioclase crystallization, independent of any subsequent heating or cooling, and thus has the potential to provide information about low temperature events in OCs (Nakamuta and Motomura 1999).

Nakamuta and Motomura (1999) applied the sodic plagioclase geothermometer to plagioclase grains separated from type 6 OCs. They assumed that plagioclase crystallized at peak metamorphic temperatures, and determined peak temperatures for type 6 OCs to be 725–742 °C (H), 808–820 °C (L), and 800 °C (LL). Our study of plagioclase compositions in OCs allows us to investigate the assumptions of the plagioclase geothermometer, namely that plagioclase growth occurs at the peak temperature, and that plagioclase does not recrystallize during subsequent metamorphism.

Processes That Can Affect Feldspar Compositions

Thermal metamorphism is not the only process that can affect feldspar compositions. Aqueous alteration, the effects of which have been observed in very low petrologic type OCs (Hutchison et al. 1987; Alexander et al. 1989; Grossman et al. 2000, 2002; Brearley 2006) could potentially affect feldspar compositions. Other tertiary factors, such as shock and weathering, can also complicate the overall history of chondrites.

Shock metamorphism is very common in OCs and can cause plagioclase compositions to be reset (e.g., Rubin 1992). Ostertag and Stöffler (1982) found that postshock annealing in meteorites with feldspars shocked to >22–26 GPa (S4), resulted in a drastically increased diffusivity of Si and Al along with K, Na, and Ca. Feldspars that experienced shock levels below this did not change their distribution of Al and Si due to postshock annealing (Ostertag and Stöffler 1982).

Terrestrial weathering can also have an effect on plagioclase compositions. The interaction of silicates with the terrestrial environment allows for the leaching of Mg, Na, K, Ca, and Rb, resulting in formation of evaporates and clay minerals. Thus, weathering of OCs can cause the compositions of plagioclase to become compromised due to the loss of Na, K, and Ca (Bland et al. 2006).

In order to study the response of plagioclase to metamorphism, we selected chondrites with minimal shock or weathering effects to ensure the plagioclase compositions we looked at were primary and unaltered. Chondrites examined in this study had shock levels S1 to S2 and weathering grades W0 to W3 (Table 1).

Feldspar Grain Size

The current classification scheme for OCs primarily uses feldspar grain size as a criterion to distinguish between type 5 and type 6 chondrites. According to Huss et al. (2006), a type 4 has feldspar grains <2 μm in size, a type 5 contains mostly microcrystalline (2–10 μm) grains, and a type 6 contains abundant feldspar grains >50 μm in size. By contrast, Krot et al. (2005) give criteria of <2 μm grains for type 4, 2–50 μm grains for type 5, and >50 μm grains for type 6 OCs. These grain size criteria are derived from the study of Van Schmus and Wood (1967), and are not based on quantitative studies of grain size distribution. Although the criteria are generally applicable, and provide a broad framework for classification, there is necessarily considerable subjectivity in the way they are applied, which limits the value of grain size as a classification parameter. It is clear from our study that grain size criteria cannot be taken too literally: we observed plagioclase grains >50 μm in diameter in H and LL type 4 chondrites, as well as grains up to 112 μm in H5 and 140 μm in LL5 chondrites. A more detailed statistical analysis of feldspar grain size distribution would no doubt provide better classification parameters and a more quantitative means to distinguish between type 5 and 6 chondrites.

Plagioclase Compositions in Type 4 to 6 H Chondrites

In general, compositions and textures of plagioclase in the H chondrites are straightforward. There is no significant heterogeneity in the type 4 chondrite we studied, and there is little difference in feldspar composition from type 4 to 5 to 6 (Fig. 3). Our hypothesis was that we should see heterogeneous feldspar compositions in low petrologic types, if feldspar crystallized from heterogeneous mesostasis glass, followed by progressive equilibration with increasing petrologic type. If this model is correct, initially heterogeneous feldspar in H chondrites would have had to equilibrate during heating to the peak temperatures experienced by a type 4 chondrite. We consider this to be unlikely, because diffusion of major elements in plagioclase is slower than in pyroxene and pyroxene is not equilibrated in type 4 chondrites. We discuss diffusion in feldspar in more detail below.

It is clear that the original mesostasis compositions were not all the same in each chondrule (e.g., Brearley and Jones 1998; Grossman and Brearley 2005). Hence, we have to conclude that any initial heterogeneity in mesostasis glass compositions from chondrule to chondrule was erased before plagioclase crystallized, through diffusional exchange. Although we have little evidence for the presence of a fluid in these chondrites, it is possible that diffusion at low temperatures, which would have probably been dominated by grain-boundary diffusion rather than bulk diffusion, might have been enhanced by the presence of fluids. Mineral phase mapping (Fig. 4) and modal abundance analysis shows that in the H chondrites, the mesostasis glass appears to mainly crystallize into feldspar. This observation supports the suggestion that chondrule mesostasis compositions were homogenized prior to feldspar crystallization.

Whatever the equilibration mechanism, the similar feldspar compositions found in type 4 to 6 H chondrites preclude using compositional parameters to differentiate among types 4, 5, and 6 chondrites.

In Avanhandava (H4), chondrule 5, albitic feldspar grains have thin, glassy rims <1 μm in diameter that surround the entire grain (Fig. 2). One interpretation of this texture is that the glassy rims could be the last remnants of glassy mesostasis that did not crystallize to feldspar. Alternatively, the rims could have formed during a reheating event after the feldspar grains crystallized, during which incipient melting could have occurred along plagioclase-pyroxene grain boundaries. A rough estimate of the necessary temperature for melting was obtained by calculating the liquidus temperature of a typical mesostasis composition from the LL3.0 chondrite, Semarkona (Jones 1990), using MELTS (Ghiorso and Sack 1995; Asimow and Ghiorso 1998). A value of 1180 °C was obtained. This temperature is above the Fe–Ni–S eutectic, 950 °C. As we do not see any evidence for melting of the Fe–Ni–S eutectic, we conclude that if melting did occur, it must have been brief. A possible process that could have produced melting is shock. However, Avanhandava is classified as having an S2 shock level, and we have not observed any other chondrules in Avanhandava which contain this texture. If the rims represent a shock-induced melt, we suggest that the shock event took place in a different location, before this chondrule was incorporated into its current host material.

Plagioclase Compositions in Type 4 to 6 LL Chondrites

In contrast with the H chondrites, the LL chondrites show major differences in feldspar compositions between the different petrologic types (Figs. 9 and 10). Plagioclase compositions in Bjurböle (L/LL4) show a wide spread along the An-Ab join, while Tuxtuac (LL5) shows a smaller range in An-Ab as well as an Ab-Or mixing trend that appears to be controlled by potassium feldspar exsolution from albite. Saint Séverin (LL6) is more homogeneous in An/Ab ratio, but shows a slight trend toward K-rich compositions. Another significant difference between the H and LL chondrites is that diopside appears to be much more abundant in the recrystallized mesostasis in the LL chondrites than in the H chondrites (compare Figs. 4 and 11). In the LL chondrules (Fig. 11), diopside coarsens progressively with increasing petrologic type.

Bjurböle (L/LL4)

In Bjurböle, plagioclase compositions from individual chondrules lie on separate segments of the An-Ab join (Fig. 10). This compositional heterogeneity is most likely inherited from the original chondrule-to-chondrule mesostasis heterogeneity found in type 3 chondrites (Brearley and Jones 1998). Within each chondrule, there is a limited range of compositions with An contents varying by ∼10 mol%, aside from the albite-rich rims seen in some chondrules. This indicates that the original glass composition in each individual chondrule did not vary much.

Plagioclase grains in four out of seven chondrules analyzed in Bjurböle consist of an anorthite-rich core (e.g., An73Ab25Or1) overgrown with an albite-rich rim (e.g., An6Ab85Or9) (Fig. 5). Although this compositional variation is reminiscent of normal igneous zoning, there is an abrupt change in composition from the An-rich core to the Ab-rich rim, i.e., the zoning is discontinuous. This observation rules out models for the rims forming from a melt, for example as a result of igneous crystallization from the chondrule melt, before the chondrule reached the parent body. Another argument against this origin is that this texture is not observed in type 3 chondrites, such as Semarkona LL3.0 (e.g., Jones 1990).

A melt could also have been produced as a result of shock, which could have melted and recrystallized mesostasis and/or existing plagioclase grains. This would require a postshock temperature just above the solidus, so that only the microcrystalline mesostasis interstitial to plagioclase grains melted. However, we do not observe any other shock features in the chondrules in which the rims occur. As an alternative possible heat source for melting, we suggest that induction heating from a solar flare could have caused a secondary heating event, in the form of a relatively short “spike” superimposed on the slower heating and cooling track resulting from 26Al decay.

We also considered the possibility that the zoning could be due to albitization, the replacement of plagioclase by almost pure albite. This process takes place in a terrestrial environment through burial diagenesis, the dissolution and precipitation of authigneic phases (e.g., Boles 1982; Ramseyer et al. 1992; Hirt et al. 1993). Albitization occurs at low temperatures (75–160 °C) and low pressures (21–65 MPa), and these parameters are well within conditions that might be expected to occur on a chondrite parent body. Aqueous alteration has been observed in very primitive type 3.0 chondrites, such as Semarkona LL3.0 and Bishunpur (Hutchison et al. 1987; Alexander et al. 1989). Albitization results in the formation of clay minerals, as given by the following equation (Boles 1982):

image(1)

Clay minerals are not observed in type 4 to 6 OCs, which would appear to rule out this process. However, if albitization took place prior to the chondrite reaching its peak metamorphic temperature, it is possible that any clay minerals present would have undergone later dehydration reactions and would no longer be present.

Tuxtuac (LL5)

Plagioclase grains in several chondrules in Tuxtuac show evidence for what appears to be exsolution (Figs. 6 and 7). Exsolution would most likely have occurred during cooling. Exsolution in alkali feldspar only requires partitioning of Na and K, and occurs relatively easily because diffusion rates of Na and K in alkali feldspar are quite high (Brady and Yund 1983). Equilibrium on the sub-solidus solvus may be used as a geothermometer. We used the SOLVCALC program (Wen and Nekvasil 1994) to try to determine the temperature recorded by the exsolved feldspar pair, assuming they are at equilibrium on the solvus. The temperature determined would not be the peak temperature, and is dependent on cooling rate, but an equilibrium temperature would put a minimum value on the peak metamorphic temperature.

We applied SOLVCALC to the two feldspars in Tuxtuac chondrule 10 that show the most extreme Ab-rich and Or-rich compositions: An9Ab89Or2 and An3Ab18Or79. The Or-rich composition is the composition of a 10-μm-wide region of K-rich plagioclase (Fig. 7), because it was not possible to measure the composition of the very thin individual potassic lamellae in albite. The pressure selected was 0.001Kb. According to the SOLVCALC program, these feldspars equilibrated at 757 °C, which is consistent with the accepted temperature range for type 5 chondrites. However, the tie-lines at this temperature and pressure are not a close match to the Tuxtuac feldspar pair, and the compositions do not lie exactly on the solvus (Fig. 13a). This suggests we do not have an equilibrium pair of feldspar compositions. The SOLVCALC tie-lines at 500 °C are a much better match (Fig. 13b). It appears that the SOLVCALC program puts most emphasis on fitting the K-rich endmembers to get an appropriate temperature. The albite-rich endmember is less sensitive to small differences in measured compositions. Overall, the results of applying SOLVCALC are somewhat ambiguous: both of these temperatures (500 °C and 757 °C) are acceptable for the minimum temperature of metamorphism in a type 5 OC.

Figure 13.

 Results of SOLVCALC program (Wen and Nekvasil 1994) calculation showing calculations for the two feldspar compositions found in Tuxtuac, chondrule 10. a) The equilibrium temperature determined by the program for this feldspar pair is 757 °C. However, the location of the solvus and the orientation of the tie-lines at this temperature are not a good match to the measured pair. b) Feldspar solvus and tie-lines at 500 °C are a closer match to the measured tie-line.

Saint Séverin (LL6)

Plagioclase in Saint Séverin is generally homogeneous, but compositions show a trend toward potassic values (Fig. 9). This trend is different than that seen in Tuxtuac, and we do not observe two feldspars. Instead, X-ray maps of feldspar grains in Saint Séverin show patchy distributions of potassium (Fig. 8). Similar heterogeneity is seen in the K X-ray maps of feldspar grains in H5 and H6 chondrites, but to a lesser extent, and this likely explains the small spread in feldspar compositions toward K-rich compositions in the H chondrites (Fig. 3). Van Schmus and Ribbe (1968) described a similar patchy distribution of K in plagioclase of LL6 chondrites. They suggested that it could be attributable to partial ordering of Na and K cations during slow cooling of the chondrite, but we consider that such an effect would occur on the atomic scale, rather than on the scale of micrometers that we observe. We considered that a possible explanation for the patchiness could be that it is attributable to terrestrial weathering, for example in our preliminary study of the H chondrite Nazareth (e) (Kovach and Jones 2007a, 2007b). However, K heterogeneity is seen in grains without weathering veins present, such as Saint Séverin (Fig. 8), which is a fall, so this appears unlikely. Another possible explanation for the observed heterogeneity is that it is inherited from original heterogeneity in alkali elements in chondrule mesostases (e.g., Grossman and Brearley 2005): the initial feldspar grains could have crystallized with heterogeneous distributions of K and Na, and subsequent thermal processing might not have been sufficient to result in equilibration by diffusion. However, we do not see the same Na-K compositional trend in the type 4 or 5 LL chondrites. If plagioclase in the type 6 chondrite is derived from material similar to the type 4 or 5, this explanation is not feasible.

There is a possibility that shock caused the plagioclase compositions to be reset. Saint Séverin is classified as having a low shock level, S2 (Rubin 2004). Also, a specific indicator of low shock levels for plagioclase is that it shows a low degree of maskelynitization (Van Schmus and Ribbe 1968). However, Ashworth and Barber (1977) and Ashworth (1980) showed that Saint Séverin underwent an early, strong shock event, prior to parent body metamorphism, and Leroux et al. (1996) showed that it also underwent a mild late-stage shock when it was released from its parent body. This shock and thermal history is consistent with observations of chromite/plagioclase assemblages, chromite veinlets, and silicate darkening in Saint Séverin, which are all indicators of high shock levels (S3 and above) that were not erased by subsequent metamorphism (Rubin 2004). Clearly, Saint Séverin has experienced a complex shock and thermal history. The effect of shock on plagioclase compositions is considered to produce a compositional spread in the An/Ab ratio (Rubin 1992), rather than in the Ab/Or ratio, because of melting and recrystallization of plagioclase. However, Chen and El Goresy (2000) have also shown that alkali cations can be redistributed in maskelynite at high shock levels. Hence, the patchy distribution of K in plagioclase in Saint Séverin could be attributed to an early moderate to strong shock event, although compositional heterogeneity resulting from this event could have been modified to some extent by cation diffusion during subsequent metamorphism.

Although there is little evidence for the activity of fluids in the chondrites we have studied, we may also consider the possibility that the patchy heterogeneity seen in feldspar is the result of leaching by a fluid on the chondrite parent body. Two H chondrite regolith breccias, Zag and Monahans (1998), contain halite, which is considered to have precipitated from an aqueous brine. Two possible origins for the brines are indigenous fluids from within the asteroid or exogenous fluids, e.g., from a comet (Zolensky et al. 1999): the presence of fluids likely postdated metamorphism (Zolensky et al. 1999; Rubin et al. 2002). We are planning further work, including TEM analysis, to examine feldspar microstructures and investigate this possibility.

LL Chondrites: Overall Trends

Feldspar in the LL chondrites appears to show compositional evolution toward equilibrated compositions with increasing petrologic type. This is generally consistent with our hypothesis that plagioclase crystallizing initially from chondrule mesostasis was heterogeneous, reflecting primary compositional differences among chondrules, and that thermal metamorphism resulted in equilibration of plagioclase compositions throughout the chondrite. However, there are some important problems for this simple model, including an apparent lack of correlation of peak temperature and cooling rate, and predicted time scales for diffusional equilibration that exceed reasonable models for thermal metamorphism.

Exsolution lamellae are observed in the LL5 chondrite Tuxtuac, whereas we did not observe exsolution in the LL6 chondrite Saint Séverin. As plagioclase compositions in the two chondrites are essentially very similar (Fig. 9), this difference suggests that Tuxtuac records slower cooling rates than Saint Séverin. This is consistent with previous studies that have shown that cooling rates are not correlated with petrologic type (Taylor et al. 1987; Rubin 1995). The “rubble pile” model (Scott and Rajan 1981) attempts to explain this observation by arguing that cooling rate peak temperatures occurred in small planetesimals before accretion into a larger parent body. However, we suggest an alternative interpretation. As feldspar grain size is not sufficiently well quantified to use as a criterion to distinguish between type 5 and type 6 chondrites (see above), it is possible that the classification of Tuxtuac as a type 5 and Saint Séverin as a type 6 bears little relationship to their relative peak temperatures. If this is the case, Tuxtuac may have reached a higher peak temperature than Saint Séverin, and may have experienced slower cooling as a direct function of burial depth.

Our model for OC metamorphism is that we see progressive equilibration from type 4 to 5 to 6. However, diffusion rates in feldspar are too slow to explain the trend from feldspar that has a wide range of An/Ab ratios in LL4, to a more homogeneous An/Ab ratio in LL5 and LL6 chondrites (Fig. 9). Diffusion rates for major elements in plagioclase have been determined by Grove et al. (1983) and Cherniak (2003): Cherniak (2003) showed that the rate-limiting process is silicon diffusion. As an example, for plagioclase of composition An93, the diffusion coefficient for Si at 900 °C is 7.43 × 10−28 m2 s−1 (Cherniak 2003). Diffusion is several orders of magnitude faster in plagioclase with more intermediate compositions, An23–67. Using the relationship x2 = Dt (where x = diffusion distance, D = diffusion coefficient, and t = time), we determined that the times required to homogenize plagioclase grains of intermediate composition that are 10 μm in size at a constant temperature of 900 °C or 800 °C are 3 Myr and 144 Myr, respectively. However, most of the feldspar grains are larger than this. For a grain 50 μm in size, homogenization requires 72 Myr at a constant temperature of 900 °C and 3.6 Gyr at 800 °C. This is clearly problematic: peak temperatures of even 800 °C would have to be sustained for times much longer than those predicted by models for metamorphism of chondritic parent bodies (Miyamoto et al. 1981; Bennett and McSween 1996).

Diffusion of Ba and Sr in plagioclase is significantly faster than CaAl/NaSi diffusion (Cherniak 2002). We observe a general decrease in the range of Ba and Sr contents in plagioclase from the type 4 LL chondrite to the type 5 and 6 LL chondrites (Fig. 12). Cherniak (2002) showed that at 800 °C, diffusion of Ba and Sr over a distance of 100 μm will take 1 Myr and 1 kyr, respectively. Thus, it is possible that Ba and Sr record progressive equilibration in feldspar with increasing petrologic type.

An additional problem for explaining feldspar in the OCs by progressive equilibration is that plagioclase in the type 5 and 6 LL chondrites contains a significant Or component, whereas plagioclase in Bjurböle with An >40 mole% has very low K contents (Figs. 9 and 10). It would appear that if the type 5 was derived from the type 4, addition of K would be necessary during the equilibration process. Although we have not carried out a mass balance calculation, it also may be necessary to add Na to the system to produce the predominantly albitic compositions in the chondrites in which feldspar is equilibrated.

Based on our observations of three LL chondrites, it appears to be unlikely that either the type 5 or the type 6 chondrite is derived from the type 4 chondrite. One way to reconcile our observations is to suggest that crystallization and equilibration of feldspar largely takes place during prograde metamorphic heating, rather than at peak metamorphic temperatures: the degree of equilibration observed may be a function of heating rate. In this model, the type 4 chondrite would have been heated rapidly, preserving heterogeneity among crystallizing feldspars, and the type 6 chondrite would have been heated more slowly. Hence, observations about the degree of equilibration of a chondrite may be unrelated to peak metamorphic temperatures. This model would explain the lack of correlation between petrologic type and cooling rate. Above, we discussed how the H chondrites show evidence for equilibration of mesostasis compositions before feldspar crystallized. If fluids were involved at an early stage, as we speculate, the differences in degree of equilibration among the LL chondrites could possibly be attributed to the presence or absence of fluids during metamorphic heating. This putative fluid may have been a source of alkalis. Without a reliable quantitative indicator of peak metamorphic temperature for the OCs, we should use the current petrologic type classification scheme with caution if we wish to make inferences about the thermal history of OC parent bodies.

Summary and Conclusions

Our survey of feldspars in H and LL chondrites of petrologic types 4 to 6 allows us to draw several important conclusions. First, feldspar grain size is not currently a reliable criterion for classification of petrologic types 5 and 6 OCs. Although the mean grain size of plagioclase clearly increases with petrologic type, grain size variation should be described more quantitatively if it is to be a useful classification parameter. Because of this, if the petrologic type of a chondrite has been assigned on the basis of the size of observed plagioclase grains, its petrologic type may bear little relationship to its peak metamorphic temperature.

For the H chondrites, feldspar compositions are similar in petrologic types 4, 5, and 6, and we do not observe progressive equilibration of feldspar compositions with increasing petrologic type. Based on diffusion calculations, it is unlikely that equilibration of initially heterogeneous feldspar compositions that may occur in type 3 chondrites will occur before a type 4 is reached. We suggest that equilibration of chondrule mesostases took place before feldspar crystallized. This process may have been facilitated by the presence of fluids.

The three LL chondrites we studied show significant differences in feldspar compositions: Bjurböle (L/LL4) shows a wide spread of compositions with varying An/Ab ratios, with significant heterogeneity among chondrules that is apparently attributable to original heterogeneity in mesostasis; feldspar in Tuxtuac (LL5) shows a smaller range in the An/Ab ratio as well as an Ab/Or mixing trend that appears to be controlled by potassium feldspar exsolution from albite; and Saint Séverin (LL6) contains more homogeneous plagioclase, with a slight trend toward more K-rich compositions that is related to a patchy distribution of K observable in X-ray maps. A simple model for progressive equilibration of feldspar from petrologic type 4 to 5 to 6 does not hold for the LL chondrites: it is hard to explain the LL5 or LL6 as an equilibrated version of the LL4 because diffusion of framework cations in plagioclase is very slow. In addition, exsolution present in the LL5 but not the LL6 indicates slower cooling in the LL5. This is inconsistent with a simple thermal model in which cooling rate is inversely correlated with peak temperature. We suggest that peak temperatures are poorly constrained by the existing classification scheme.

Several of our observations lead us to speculate that feldspars may have crystallized during prograde metamorphic heating, and that fluids may have been present at the time of feldspar crystallization. We also speculate that fluids may have played a role in redistributing alkali elements in feldspar grains after crystallization. These possibilities need to be tested further by more detailed microstructural studies of feldspar, and by examining a wider suite of OCs.

Acknowledgments

Acknowledgments— The authors wish to thank A. J. Brearley and C. Shearer for extensive discussions, P. Burger and C. Shearer for assistance with the SIMS analyses, and M. Spilde for assistance with electron microprobe analyses. We thank Ed Scott, H. C. Connolly Jr., and an anonymous reviewer for review comments that improved the manuscript. This work was supported by NASA grant NNG06GF73G (R. Jones, P. I.).

Editorial Handling––Dr. Edward Scott

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