Jiddat al Harasis 556: A howardite impact melt breccia with an H chondrite component

Authors


*Corresponding author. E-mail: emilie.janots@ujf-grenoble.fr

Abstract

Abstract– A petrographic and geochemical study was undertaken to characterize Jiddat al Harasis (JaH) 556, a howardite find from the Sultanate of Oman. JaH 556 is a polymict impact melt breccia containing highly shocked clasts, including mosaicized olivine and recrystallized plagioclase, set in a finely recrystallized vesicular matrix (grain diameter <5–10 μm). Plagioclase (An76–92) and clinopyroxene (En48–62Wo7–15) are associated with orthopyroxene and olivine clasts like in a howardite. JaH 556 oxygen isotope data indicate that it has an anomalous bulk-rock composition as howardite, resulting from a mixture between HED material and at least one second reservoir characterized by a higher Δ17O. The bulk meteorite has a composition consistent with howardites, but it is enriched in siderophile elements (Ni = 3940 and Co = 159 ppm) arguing for a chondritic material as second reservoir. This is independently confirmed by the occurrence of chondrule relics composed of olivine (Fo56–80), orthopyroxene (En79Wo2), and plagioclase (An61–66). Based on oxygen isotopic signature, siderophile composition, and chondrule core Mg number (Fo80 and En79Wo2), it is proposed that JaH 556 is a howardite containing approximately 20% H chondrite material. This percentage is high compared with that observed petrographically, likely because chondritic material dissolved in the impact melt. This conclusion is supported by the observed reaction of orthopyroxene to olivine, which is consistent with a re-equilibration in a Si-undersaturated melt. JaH 556’s unique composition enlarges the spectrum of howardite-analogs to be expected on the surface of 4 Vesta. Our data demonstrate that oxygen isotopic anomalies can be produced by a mixture of indigenous and impactor materials and must be interpreted with extreme caution within the HED group.

Introduction

The achondrite Jiddat al Harasis (JaH) 556 was discovered during a systematic search for meteorites in the Sultanate of Oman in 2008 (in the Meteoritical Society database). Petrological observations suggest that this meteorite belongs to the howardite, eucrite, and diogenite (HED) group. HEDs represent between 2 and 3% of all meteorites so far collected on a worldwide basis and, with the exception of lunar rocks, are the largest suite of extraterrestrial crustal igneous rocks (Mittlefehldt et al. 1998; Barrat et al. 2003). JaH 556 is a heterogeneous polymict breccia containing both eucritic and diogenitic clasts, suggesting it is a howardite. Amongst the HED group, howardites correspond to fragmental or regolith breccias composed of mineral and/or lithic clasts set in a fine-grained matrix, primarily consisting of a mixture of eucritic and diogenitic material (Wahl 1952; Mason 1983). These polymict breccias are thought to form via surface processes, such as impacts, that combined eucrite, diogenite, and additional nonindigenous clasts (Wilkening 1973; Bunch 1975; Chou et al. 1976; Labotka and Papike 1980; Fuhrman and Papike 1981; Laul and Gosselin 1990; Zolensky et al. 1996; Pun et al. 1998; Lorenz et al. 2007). Chondritic components in howardites were first identified in the early 1970s (Wilkening 1973) and have since been found in up to 20 different HED achondrites with compositions including CM2 and CR2 (Pun et al. 1998; Gounelle et al. 2003; Buchanan et al. 2009). Although chondritic contribution is commonly at levels of only a few percent, three Antarctic finds have high proportions of chondrite clasts (Herrin et al. 2011), reaching up to 60% in the howardite PRA 04401. The chondritic clasts led some scientists to conclude that these inclusions may in fact sample the impactor(s) which produced the howardites. On the other hand, exotic clasts found in howardites are also commonly viewed as having a similar mode of formation to micrometeorites presently falling on the surface of the Earth (Lorenz et al. 2007).

While eucrites and diogenites provide information about the differentiation and subsequent magmatic processes on the HED parent body, howardites inform us about the surface of the HED parent body (Delaney et al. 1983). Howardites, eucrites, and diogenites are generally thought to originate from 4 Vesta, which is one of the largest differentiated asteroids identified to date (Binzel and Xu 1993). This genetic link is based on spectral comparisons between 4 Vesta and HED meteorites (McCord et al. 1970; Pieters et al. 2005), and also the identification of an efficient delivery mechanism from 4 Vesta to Earth (Binzel and Xu 1993). Documenting new howardite samples and understanding their petrogenesis is crucial to the accurate interpretation of spectroscopic analyses of 4 Vesta’s surface and the calibration of instruments during the Dawn mission (McSween et al. 2010).

Recently, some eucrites were reclassified as ungrouped basaltic achondrites because their oxygen isotopic compositions do not match the HED group (Scott et al. 2009). It is inferred that the anomalous eucrites are probably derived from distinct Vesta-like parent bodies. Bland et al. (2009) proposed that the orbital properties and the oxygen isotopic composition of the Bunburra Rockhole fall belong to a V-type asteroid other than Vesta. Preliminary oxygen isotopic data for JaH 556 indicated that it had an anomalous isotopic oxygen composition. In this study, the petrology and geochemistry of JaH 556 are investigated to elucidate its origin and to discuss why its bulk-rock oxygen isotopic composition deviates from reported HED values. We will show that JaH 556 documents a unique type of howardite, in which the isotopic composition is affected by a significant contribution of an ordinary chondrite impactor.

Analytical Methods

Petrographic and quantitative mineral analysis and clast classification were completed on a polished thin section of JaH 556 using a JEOL JXA 8900 EMP (Institut für Mineralogie, Universität Münster). Beam conditions were fixed at 15 kV, 20 nA, and 1 μm spot size. A ϕ(ρz) matrix correction procedure was applied (Pouchou and Pichoir 1984). Clasts with a diameter of ≥30 μm were arbitrarily selected for quantitative analysis. Smaller clasts are considered as part of the matrix. Point counting using optical microscopy techniques was hampered by the small clast size and presence of shock features. Even in backscattered mode (BSE), it was difficult to distinguish clinopyroxene clasts from the matrix. Thus, clast abundances were estimated from X-ray maps of the studied thin section. Elemental imaging was obtained with an Eagle III micro-XRF (ISTerre, University of Grenoble) using a 30 μm-sized beam, at 20 kV and 250 μA. The entire thin section was mapped in two successive acquisitions using dwell times of 200 and 400 ms. Step size was adjusted to fit the beam diameter (approximately 30 μm).

Oxygen isotope analysis was carried out at the Open University using an infrared laser-assisted fluorination system (Miller et al. 1999). All analyses were obtained on powdered samples (0.5–2 mg) taken from larger homogenized aliquots of generally greater than 50 mg. After fluorination, the O2 released was purified by passing it through two cryogenic nitrogen traps and over a bed of heated KBr. O2 was analyzed using a MAT 253 dual inlet mass spectrometer. Analytical precision (1σ), based on replicate analyses of international (NBS-28 quartz, UWG-2 garnet) and internal standards, is approximately ±0.04‰ for δ17O; ±0.08‰ for δ18O; ±0.024‰ for Δ17O (Miller et al. 1999). The precision (1σ) quoted for individual fraction from JaH 556 is based on replicate analyses (Table 1).

Table 1.   Oxygen isotopic composition of JaH 556 matrix and clast.a
SampleNδ17O‰δ18O‰Δ17O‰
  1. aUntreated; all other samples were leached in EATG.

  2. bAverage of all matrix samples.

Bulk samples
 JaH 556 bulka23.150.006.200.01−0.090.00
 JaH 556 bulk 22.320.004.650.02−0.110.00
Matrix samples
 JaH 556 matrix32.190.024.440.06−0.140.01
 JaH 556 matrix low magnetic22.740.115.490.23−0.130.01
 JaH 556 matrix medium magnetic22.470.034.970.00−0.130.02
 JaH 556 matrix high magnetic22.530.055.020.08−0.100.01
 JaH 556 matrix averageb92.450.234.920.42−0.130.02
Clast samples
 JaH 556 large clast CO-171.890.064.050.14−0.230.01
 JaH 556 clast CO-222.100.074.460.13−0.240.00
 JaH 556 clast CO-312.24 4.72 −0.23 

Isotopic analyses were performed on bulk samples of the meteorite, as well as various groundmass and clast fractions (Table 1). As initial oxygen isotope analysis indicated significant levels of terrestrial alteration, all analyses, apart from a single bulk sample, were performed on material that had been leached in a solution of ethanolamine thioglycollate (EATG) prior to analysis. EATG is known to be efficient at removing terrestrial alteration products, such as iron-hydroxides, -carbonates, and -sulfates from a rock (Cornish and Doyle 1984; Martins et al. 2007).

Oxygen isotopic analyses are reported in standard δ notation, where δ18O has been calculated as: δ18O = [(18O/16Osample/18O/16Oref) − 1] × 1000 (‰) and similarly for δ17O using the 17O /16O ratio. Δ17O, which represents the deviation from the terrestrial fractionation line, has been calculated using the linearized format of Miller (2002):

image(1)

where λ = 0.5247, which was determined using 47 terrestrial whole-rock and mineral separate samples (Miller et al. 1999; Miller 2002).

Whole-rock geochemical analyses were obtained from an aliquot of the powdered sample (1.56 g of aliquot) by a combination of inductively coupled plasma–optical emission spectroscopy (ICP-OES) and inductively coupled plasma–mass spectrometry (ICP-MS) at Activation Laboratories Ltd., Ancaster, Ontario, Canada (code 4 Lithoresearch).

Sample Description and Mineral Identification

JaH 556 is a single, porous, 2–3 cm-sized stone weighing 36.65 g. It is a polymict breccia with an orange-brown groundmass containing fragments and oblong vesicles (Fig. 1a). Abundant Fe-oxihydroxides and near-complete lack of Fe metal indicate pervasive terrestrial weathering. Optical microscopy showed that JaH 556 contains abundant mineral and lithic clasts set in a cryptocrystalline matrix, grayish in transmitted light (Fig. 1b). The clast size ranges from 30 to 500 μm with the largest clast (Clast 1) reaching 4 mm (Figs. 1 and 2). The majority of mineral and lithic clasts are composed of plagioclase, clinopyroxene, and orthopyroxene. Olivine clasts are scarce. Hollow vesicles and those filled with white/orange material have a maximum length of 5 mm.

Figure 1.

 a) Slab of JaH 556 with millimeter scale. Clasts occur in an orange-brown matrix containing abundant pores; (b–e) are transmitted light optical microscope images. b) Mineral clasts and vesicles occurring in a finely crystallized matrix. Plane light and crossed polarizers. While plagioclase is always limpid in plane light, ferromagnesian silicates are commonly brownish; c) recrystallized polycrystalline and fibrous plagioclase set in a fine-grained matrix (crossed polarizers); d) orthopyroxene (right) and recrystallized plagioclase (left) clasts (crossed polarizers). The orthopyroxene shows planar fractures and a rim of clinopyroxene, which is clearly visible due to its high birefringence; e) diogenite clast with coarse orthopyroxene grains and a protuberance of mosaicized olivine.

Figure 2.

 XRF element distribution and transmitted light images of the studied thin section. Scale bar represents 5 mm.

Plagioclase, orthopyroxene, clinopyroxene, olivine, chromite, calcium sulfates, and Fe-oxihydroxide were identified on X-ray maps (Fig. 2). Plagioclase abundance is indicated by elevated Al concentrations. Pyroxene clasts have very low Al contents and they are more abundant than the plagioclase clasts. Orthopyroxene has higher Mg and lower Ca contents than clinopyroxene. The large Clast 1 is mainly composed of orthopyroxene. In the pyroxene clast fraction <1 mm clinopyroxene is more abundant than orthopyroxene. Olivine crystals are rare and difficult to distinguish when using the XRF imaging. Olivine is best seen at the edge of Clast 1, where it has higher Fe content than Clast 1. The presence of chromite is inferred from the high concentrations of Cr (map not shown). Fe-oxihydroxides are highlighted by a high concentration of Fe and Co and absence of Si. Ni is pervasively distributed, but appears to be decoupled from Fe and Co and does not correlate with any other element. Sulfur concentrations significantly increase toward the edge of the sample and correlate well with Ca. This relationship indicates that the S-rich domains are Ca-sulfates, apparently formed during the meteorite alteration in the desert. All phases were independently confirmed by optical microscopy.

Chemical Composition of JaH 556

Oxygen Isotope Analyses

Two bulk fractions of JaH 556 were analyzed for oxygen isotopes, one untreated and the other leached in EATG (Table 1; Fig. 3). The untreated fraction (δ17O = 3.15‰, δ18O = 6.20, Δ17O = −0.09‰) is strongly shifted to significantly higher δ18O values, and only slightly higher Δ17O values, compared with the EATG leached bulk fraction (δ17O = 2.32‰, δ18O = 4.65‰, Δ17O = −0.11‰). The displacement of hot desert finds to higher δ18O values (and slightly higher Δ17O) compared with more pristine samples (i.e., falls, or in the present case the EATG leached fraction) has been well documented in other meteorite groups, including carbonaceous (Greenwood and Franchi 2004) and ordinary chondrites (Bland et al. 2000). Similar effects were also observed in finds from the Oman desert (Martins et al. 2007; Janots et al. 2011). The relationship between the untreated and EATG-treated bulk samples seen in Fig. 3b is therefore consistent with the petrographic observations indicating that JaH 556 has experienced significant levels of terrestrial alteration.

Figure 3.

 a) Plot of δ18O versus Δ17O showing average H, L, and LL ordinary chondrite values (Clayton et al. 1991) in relation to bulk value for JaH 556. Other data and symbols as in Fig. 3b. b) Plot of δ18O versus Δ17O showing the oxygen isotope composition of the JaH 556 bulk-rock, the treated bulk-rock, the treated groundmass (all black squares) and clast (black diamonds) in relation to HEDs (gray circles) and angrites (open squares). HED and angrite data: Greenwood et al. (2005). TFL: terrestrial fractionation line, AFL: angrite fractionation line, EFL: eucrite fractionation line.

The oxygen isotope composition of the leached bulk sample falls outside the fields of any of the main meteorite groups, being intermediate between HED and angrite values (AFL; Fig. 3b).

The oxygen isotope composition of the JaH 556 matrix (groundmass with clast fraction) was determined on four fractions, three of which were separated on the basis of their magnetic properties. All these four matrix samples were treated with EATG. These different fractions show no clear systematic differences in their oxygen isotope compositions (Table 1). The average matrix value shown in Fig. 3b plots close to the leached bulk value. However, it is important to note that on a relatively small, very heterogeneous meteorite such as JaH 556 it is difficult to obtain a truly representative bulk sample.

Three clasts from JaH 556 were sampled for oxygen isotope analysis. A total of seven replicate analyses were undertaken on the large orthopyroxene-rich clast “Clast 1” (Figs. 1, 2, 8a, and 8b). The average analysis of Clast 1 (labeled CO-1 in Fig. 3a) plots at the high δ18O end of the HED array of Greenwood et al. (2005). With respect to its Δ17O value, CO-1 (−0.23 ± 0.01) is indistinguishable from other HED meteorites (average HED Δ17O value = −0.24 ± 0.01 (1σ); Greenwood et al. 2005). The two other clasts sampled for oxygen isotope analysis, CO-2 and CO-3, have somewhat higher δ18O values than those measured by Greenwood et al. (2005), and in the case of CO-3 it is also slightly beyond the range measured by Wiechert et al. (2004). However, both samples, CO-2 and CO-3, have Δ17O values within error of the average HED value of −0.24 ± 0.01 (1σ; Greenwood et al. 2005). In view of the heavily altered nature of JaH 556, the relatively high δ18O values measured in CO-1 and CO-2 may indicate that not all traces of terrestrial weathering were removed by EATG treatment.

Figure 8.

 Electron backscattered images of the lithic clasts. a) Clast 1 with a brighter rim composed of clinopyroxene; b) Clast 1 with olivine protuberance; in (a) and (b) Clast 1 limit is marked by a white line; c) zoom out of the rectangle in Fig. 8b. It shows that the contact between clast1 and matrix consists of clinopyroxene and microcrystalline olivine; d) spherical Clast 2 with barred olivine intercalated with orthopyroxene, and plagioclase. Clast 2 rim is enriched in Fe in comparison to the core; e) Clast 3 displaying olivine rim enclosing an assemblage of plagioclase and orthopyroxene.

In summary, the oxygen isotope data indicates that there are at least two isotopic components present in JaH 556, one with a composition given by the HED clasts and a second with a higher Δ17O value.

Major and Trace Elements

The JaH 556 whole-rock composition is characteristic of howardites with major and trace element abundances falling within the appropriate compositional fields (Table 2). MgO, CaO, Al2O3, Ti, and Sc abundances are within the range generally found in howardites (Fig. 4b), and intermediate between eucrites and diogenites. JaH 556, however, is deficient in SiO2 (43.38 wt%) in comparison to reported HED material (Fig. 4a). The FeO and MnO abundances also diverge from HED values with slightly higher FeO and slightly lower MnO than reported HED compositions (Fig. 4a). As a consequence, the Fe/Mn ratio (approximately 41.8; Table 2) is higher than those found in HED material (approximately 33–34; Warren et al. 2009).

Table 2.   Comparison of whole-rock major elements in JaH 556 with averaged values of the HED and H meteorites taken from Warren et al. (2009) and Wasson and Kallemeyn (1988).
Oxide
wt%
JaH 556Detection
limit
 Diogenite
(29)
Eucrite
(35)
Howardite
(40)
H chondrite
  1. aFor simplicity, Fe concentrations assume that all Fe is divalent.

  2. bLOI = Loss on ignition.

SiO243.40.01 52.8 ± 1.749.6 ± 0.850.3 ± 1.036.2
Al2O37.450.01 1.13 ± 0.6912.5 ± 1.6 7.8 ± 1.92.14
FeOa19.80.01 16.5 ± 2.218.0 ± 2.017.6 ± 1.035.4
MnO0.4670.001 0.49 ± 0.110.54 ± 0.060.51 ± 0.030.300
MgO14.10.01 26.5 ± 2.3 7.8 ± 2.015.5 ± 2.923.2
CaO6.940.01 1.38 ± 0.66 9.9 ± 0.8 6.4 ± 1.41.75
Na2O0.4000.01 0.03 ± 0.030.39 ± 0.110.26 ± 0.070.816
K2O0.0300.01     
TiO20.4030.001     
P2O50.0700.01     
LOIb2.09      
Total97.4      
        
Fe/Mn41.8  33.232.934.1117
Figure 4.

 Whole-rock major and trace elements in JaH 556 compared with other meteorites from the HED group (data from Warren et al. 2009). a) Graphs showing the SiO2 and FeO/MnO variations. b) Comparison of averages of oxide and trace element. Graph clearly shows the similarity in composition between our sample and the average values for howardite meteorites except for Ti. c) Plot of Ni versus Co demonstrates uniquely high Co and Ni values in JaH 556.

The JaH 556 trace element composition shows enrichment in incompatible elements relative to CI, including the rare earth elements (REE, Table 3). The flat chondrite-normalized REE pattern (Fig. 5b), with values being approximately 6–7*CI, is consistent with howardite REE compositions: Concentrations of the two siderophile elements Co (159 ppm) and Ni (3940 ppm) are the highest ever documented in an HED meteorite (Fig. 4c). Ni content was independently confirmed by XRF measurements (about 3460 ppm). In addition, U, Sr, and Ba concentrations are 10–100*CI (Fig. 5a; Table 3). The enrichments in U, Sr, and Ba are comparable to those observed in other <50 kyr old meteorites found in hot deserts (Al-Kathiri et al. 2005; Barrat et al. 1999). The enrichment in U, Sr, and Ba was even identified by comparing samples of the diogenite Tatahouine collected directly after the fall with samples collected 63 yr later (Barrat et al. 1999). This study clearly showed that Sr, Rb, and U may be enriched in the meteorite by hot desert alteration over a very short time scale.

Table 3.   Comparison between selected trace elements in JaH 556 and averaged HED and H chondrite values from Warren et al. (2009) and Wasson and Kallemeyn (1988), respectively.
Element (ppm)Detection
limit (ppm)
JaH 556Diogenite
(29)
Eucrite
(35)
Howardite
(40)
H chondrite
Sc121 14.8 ± 5.628.5 ± 3.722.4 ± 2.812.5
V572  131 ± 61  74 ± 16  98 ± 1574
Sr22907  1.8 ± 2.4  74 ± 16  44 ± 1510.0
Ba3313    
Cr202900    
Co1159 20.6 ± 7.4 6.9 ± 2.823.8 ± 14.2810
Ni203940   37 ± 36 2.6 ± 2.9 277 ± 48116,000
Cu1020    
Zn3030    
Ga12 0.20 ± 0.10 1.3 ± 0.4 1.0 ± 0.76.0
Ge0.52.8    
Rb12    
Y0.59.8    
Zr129    
Nb0.21.6    
Sb0.26.8    
La0.051.54 0.09 ± 0.12 2.5 ± 1.61.68 ± 0.540.295
Ce0.053.97    
Pr0.010.67    
Nd0.053.25    
Sm0.010.95 0.07 ± 0.081.50 ± 0.871.01 ± 0.300.185
Eu0.0050.3650.020 ± 0.0340.60 ± 0.170.36 ± 0.080.073
Gd0.011.45    
Tb0.010.28    
Dy0.011.87    
Ho0.010.39    
Er0.011.14    
Tm0.0050.163    
Yb0.011.02    
Lu0.0020.1620.027 ± 0.0230.23 ± 0.100.16 ± 0.040.031
Hf0.10.8    
Ta0.010.1    
Pb510    
Th0.050.19    
U0.010.21    
Figure 5.

 CI-normalized plots of a) selected incompatible elements and b) REE pattern of JaH 556.

Petrology of JaH 556

Mineral composition has been carefully acquired for different textural position in the matrix and clasts. Selected representative EMP analyses are given for the different mineral populations in Tables 4–6.

Table 4.   Selected EMP analyses of the plagioclase from the matrix (Ma), mineral clasts (MCl), clast 2 (Cl2), and clast 3 (Cl3).
Analysis No.71108225110111159160253
PositionmamamaMClMClCl2Cl2Cl3
SiO243.6245.6946.3545.2444.9451.6752.5953.14
TiO20.050.020.010.170.000.030.060.02
Cr2O30.020.020.030.010.000.090.070.04
Al2O334.2234.4534.4634.0434.7929.6329.2228.75
FeO0.850.670.700.630.300.770.760.84
MnO0.020.040.000.000.030.020.030.02
NiO0.020.060.080.000.000.000.010.16
MgO0.070.070.200.120.050.200.220.25
CaO18.3017.7717.5018.1618.5213.4412.9012.27
Na2O1.001.231.311.070.893.533.924.00
K2O0.060.110.050.080.070.430.380.47
Total98.23100.12100.6999.5299.5999.82100.1699.96
Ab9.011.111.99.68.031.434.736.1
An90.788.387.889.991.666.163.161.1
Or0.40.70.30.50.42.52.22.8
Table 5.   Selected EMP analyses of olivine from the matrix (Ma), from mineral clasts (MCl), clast 1 (Cl1), clast 2 (Cl2), and clast 3 (Cl3).
Analysis No.921761352811342806566152155163168246254
PositionMaMaMClMClMClMClCl1Cl1Cl2Cl2Cl2Cl2Cl3Cl3
SiO234.8635.1436.3435.7738.1539.0534.9334.9039.1839.3735.7235.8734.9235.77
TiO20.020.030.000.060.010.020.000.000.130.000.030.050.120.05
Cr2O30.080.100.170.090.060.100.170.120.490.300.100.200.080.10
Al2O30.020.010.030.020.010.020.010.000.250.100.020.030.060.01
FeO40.2739.8231.2136.1317.9717.5135.7634.6519.2018.3436.7436.7337.3737.03
MnO0.790.830.650.710.500.490.650.690.370.400.760.690.770.77
NiO0.050.070.000.000.000.070.060.000.000.030.000.020.150.01
MgO25.5424.5631.9027.8743.0542.6929.0330.5240.6442.2327.3726.9825.5026.76
CaO0.330.270.220.250.050.140.250.220.240.120.210.240.320.32
Na2O0.010.030.020.030.000.000.000.000.020.010.000.010.000.00
K2O0.000.030.000.000.000.000.000.000.000.000.030.000.000.00
Total101.98100.88100.53100.9399.81100.08100.86101.09100.51100.90100.98100.8199.29100.82
Fo53.152.464.657.981.081.359.161.179.180.457.056.754.956.3
Fa46.947.635.442.119.018.740.938.920.919.643.043.345.143.7
Table 6.   Selected EMP analyses of pyroxene from the matrix (Ma), from mineral clasts (MCl), clast 1 (Cl1), clast 2 (Cl2), and clast 3 (Cl3).
Analysis No.86873153212983099235360151156258260
PositionMaMaMClMClMClMClCl1Cl1Cl1Cl1Cl2Cl2Cl3Cl3
SiO251.1651.1551.0152.3153.6153.8652.6853.8351.5551.7252.3353.9553.3151.80
TiO20.230.300.420.180.120.050.150.040.380.210.160.210.150.30
Cr2O30.640.650.460.520.490.460.420.410.450.540.841.090.420.85
Al2O31.151.321.040.880.720.760.990.421.010.942.132.270.841.87
FeO20.1020.9622.8121.5917.9815.0018.2115.3421.6920.7219.7512.0019.4220.64
MnO0.630.710.660.800.670.520.600.590.770.650.670.470.640.66
NiO0.000.060.040.060.010.000.050.030.000.000.090.030.000.03
MgO21.0120.1719.0819.8424.7827.3124.8127.2120.8220.9721.0728.3922.2421.36
CaO3.874.213.953.871.390.991.401.073.433.293.401.162.063.23
Na2O0.010.080.020.030.000.020.020.060.020.010.020.020.020.00
K2O0.010.000.020.030.030.010.040.000.000.000.000.040.000.00
Total98.8199.6199.51100.1199.8098.9899.3898.99100.1299.04100.4499.6299.09100.74
Wo7.98.78.28.02.82.02.82.17.06.87.12.34.36.6
En59.957.755.057.169.175.068.974.458.760.060.979.064.360.6
Fs32.233.636.934.928.123.128.323.534.333.232.018.731.532.8

Matrix

The finely crystallized matrix has an igneous texture. It is composed of subeuhedral plagioclase surrounded by subeuhedral and interstitial clinopyroxene (Fig. 6a). Matrix plagioclase occurs as approximately 10 μm-sized crystals and smaller lath-shaped crystals (Fig. 6a). Plagioclase shows a significant compositional spread (An76–92; Fig. 7a), with most of the values lying between An83–92. In BSE images, matrix pyroxene displays zoning with a darker core progressively evolving toward a brighter rim enriched in Ca (Fig. 6a). Orthopyroxene was not found in the matrix. All pyroxene compositions fall within the pigeonite range (En48–62Wo6–15; Fig. 7b). The matrix also contains abundant Fe-oxihydroxide, and minor olivine and spinel (chromite). Mineral assemblage and compositions are consistent with eucritic material, with the exception of the olivine occurrence. Olivines are especially numerous in the area surrounding the orthopyroxene clasts (Figs. 6b and 8b), where they occur as homogeneous and subeuhedral grains (Fo50–53, Fig. 7c), 5 to 25 μm in size. Their morphology suggests that they formed coevally with the matrix crystallization.

Figure 6.

 Backscatter electron images of mineral clasts and matrix. a) Matrix composed of plagioclase (dark gray) and oxide grains, surrounded by zoned clinopyroxene (gray); b) subeuhedral olivine (ol) at vicinity of the diogenite clast (D-clast); c) flow-shaped polycrystalline plagioclase clast (pl) containing abundant oxide inclusions; d) polycrystalline plagioclase clasts with olivine inclusion; e) clinopyroxene clast; f) zoned pyroxene grains with an orthopyroxene core (Opx) and a clinopyroxene rim (Cpx). Note two sets of perpendicularly oriented fractures; g) zoned olivine showing a Mg-rich Ol 1 core (dark gray) and Fe-enriched Ol 2 rim (bright gray); h) polycrystalline olivine enclosing orthopyroxene (Opx) relics.

Figure 7.

 EMP analyses of a) anorthite contents versus K (atoms per formula unit—afu) in plagioclase; b) XFe versus Ca (afu) in pyroxene; c) Fe (afu) versus Mn (afu) in olivine. The different symbols correspond to distinct textural positions mentioned in the text.

Eucrite Clasts

Eucritic material is represented by clinopyroxene and plagioclase clasts (Fig. 2). Plagioclase clast size is up to 500 μm. Most of the plagioclase clasts are polycrystalline consisting of fine-grained and oriented needles/plates (Figs. 1c and 1d), and can display flow-deformed shapes (Figs. 6c and 6d). They commonly contain Fe-phases and occasionally pyroxene and olivine (Fo57). Iron-phases are commonly aggregated and aligned within the polycrystalline plagioclase aggregates (Fig. 6c). Occasionally, the plagioclase contains melt inclusions. Recrystallized and deformed plagioclase has a composition (An76–92) overlapping that of the matrix (Fig. 5a). A second, less common group of plagioclase clasts is characteristically undeformed and corresponds to single homogeneous grains with no shock features. However, this second plagioclase group has a similar composition (An83–92) as the deformed plagioclase clasts. This compositional range is typical of eucritic material.

Clinopyroxene clasts (clinopyroxene) are clearly visible in optical microscopy (Fig. 1b) with high birefringence in crossed polarizers. In plain light, they are commonly masked by an orange-brownish clouding, similar to those described in the Kapoeta howardite (Pun et al. 1998). Regarding the shock state, clasts present mosaic texture indicating high shock levels. In BSE, these clinopyroxene clasts (En46–57Wo5–15, Fig. 7b) are easily confused with the intergranular clinopyroxene matrix having a similar composition. Only the largest clinopyroxene clasts (>100 μm) are clearly observable in BSE. Most clinopyroxene grain boundaries are sharp. However, in some instances, the clast boundaries are unclear and embayed by the matrix (Fig. 6e). In terms of composition, clinopyroxene clasts are homogeneous, but frequently contain abundant inclusions that are probably responsible for the brownish clinopyroxene clouding.

Diogenite Clasts

Clast 1 is the largest fragment in the sample (about 4 mm). It is a polycrystalline clast with a “subeuhedral shape” (Figs. 1, 2, and 8), which suggests that it was originally a single grain that was later brecciated by impact. It is composed of coarse-grained (up to 1 mm) orthopyroxene homogeneous in composition (En68–75Wo2–3, Fig. 7b) and minor chromite (up to 150 μm), Fe-sulfide (<5 μm, not analyzed), and interstitial olivine. At the periphery of Clast 1, olivine occurs as a millimetric protuberance (Fig. 8b) clearly visible on the Fe X-ray maps (Fig. 2). This olivine lobe is polycrystalline with homogeneous Fo59–63 composition (Fig. 7c). Observations in transmitted light with crossed polarizers reveal a mosaic texture (Fig. 1e). At the contact with the matrix (Fig. 8c), the olivine lobe is decomposed to finely crystallized grains (about 1 μm). Within Clast 1, olivine is also observed interstitially along orthopyroxene grain boundaries (Fig. 8a). This olivine has a submicrometric and porous texture and is associated with chromite, Fe-oxihydroxides, and Fe-sulfide. Quantitative analyses of interstitial olivine were compromised by the submicrometric crystal size, but one accurate EMP analysis yields a composition close to the polycrystalline olivine aggregate (Fo64; Fig. 7c). At the contact with the matrix, Clast 1 possesses a μm-sized rim which appears brighter in BSE mode (Figs. 8a and 8b). The rim consists mainly of clinopyroxene (En51–64Wo6–11; Fig. 7b), but it occasionally contains very small olivine inclusions. The mineralogical composition of Clast 1 unambiguously indicates that Clast 1 is a diogenitic clast.

Other orthopyroxene clasts dispersed in the matrix are similar in composition and morphology to the diogenitic Clast 1, which suggests that all orthopyroxene clasts represent diogenitic material. Orthopyroxene clasts are systematically zoned in BSE mode with dark cores and a brighter, <30 μm wide rim (Fig. 6f). The core composition corresponds to orthopyroxene (En68–76Wo2–3; Fig. 7b) and the rim falls within the clinopyroxene range of pigeonite, with a composition enriched in CaO and FeO analogous to matrix or clinopyroxene clasts (En52–60Wo8–10, Fig. 7b). The rim is also visible when using the optical microscope (Fig. 1d): low-birefringence orthopyroxene core with decorated planar fractures is surrounded by a rim of clinopyroxene with high birefringence under crossed polarizers. Locally, orthopyroxene clasts were also partly replaced by olivine (Fig. 6h). There, an orthopyroxene core is embayed by olivine, with a composition (Fo52) similar to that of the matrix. Olivine textural relations with the matrix suggest that they crystallized coevally.

Chondritic Clasts

Two lithic clasts (Clasts 2 & 3) are interpreted to be chondrite relics. Unfortunately, both were too small to be separated for oxygen isotope analyses. Clast 2 is round, approximately 200 μm in diameter, zoned, and partly altered (Fig. 8d). It has a spherical morphology with elongated crystals in the core and a zoned rim, which is typical of barred chondrule texture. The elongated crystals are olivine (Fo77–80) separated by finely crystallized orthopyroxene (En79Wo2) and clinopyroxene (En60Wo7), plagioclase (An62–66), and Fe-oxihydroxide (Fig. 7). Mineral compositions in Clast 2 are very distinct from the matrix and the eucritic and diogenetic materials (Fig. 7), but are consistent with H ordinary chondrite. Toward the rim, olivine composition evolves progressively toward more Fe-rich composition (Fo56–74). Clast 3 is a polycrystalline aggregate, approximately 150 μm in size, for which the limits are defined by a narrow olivine rim enclosing a fine assemblage of orthopyroxene and plagioclase (Fig. 8e). As in Clast 2, plagioclase (An61) is lower in Ca compared with eucritic material (Fig. 7a). Orthopyroxene compositions (En68Wo3) plot within the diogenitic field. Olivine composition (Fo50–57) is similar to that of olivine grains in the matrix. Mineral assemblages and compositions of Clasts 2 and 3 are not typical of HED sources (Mittlefehldt et al. 1998; Shearer et al. 2010). Based on the resemblance of their morphology with that of chondrules and their assemblages with Mg-rich olivine associated with An∼60–65, it is proposed that Clasts 2 & 3 correspond to chondritic material. The occurrence of well-crystallized plagioclase in both clasts indicates that they derived from an equilibrated chondrite.

A small number of olivine clasts were also recognized using electron microprobe (Fig. 6g). They are generally below 50 μm. Most of the olivine clasts are (sub)euhedral and zoned (Figs. 6g and 7c). Planar fractures were the only deformation features identified. In BSE mode, olivine presents a dark core (Ol 1) surrounded by a brighter, 10 μm-sized rim (Ol 2), reflecting Mg variations (Fig. 7c): the core has higher Mg# (Fo73–81) than the rim (Fo56–64). The Mg# in Ol 1 is analogous to that of Clast 2, and higher than in the diogenitic Clast 1. Thus, olivine mineral clasts are attributed to chondritic material. Similarities between the compositions of Ol 2 and matrix olivine suggest that the zoning in olivine clasts developed coevally with matrix crystallization.

Vesicles and Terrestrial Weathering Signatures

Abundant roundish and elongated vesicles reaching 5 mm in length are distributed randomly throughout the meteorite matrix. Elongation orientation is not clear (Figs. 1 and 2), but it appears often subparallel to the largest clasts (e.g., around Clast 1 in Fig. 2). Vesicles are often partly filled with desert alteration products such as anhydrite, celestite, Fe-oxihydroxides, and some hydrous secondary silicates. Amongst them, the sulfates are the most abundant as seen on the X-ray maps (Fig. 2), which show that these sulfates are preferentially located at the border of the sample. Oxidation of sulfides and formation of secondary sulfates is a common feature in meteorites found in the hot desert of Oman (Al-Kathiri et al. 2005; Janots et al. 2011). Among these secondary sulfates, celestite and barite are common (Al-Kathiri et al. 2005).

Classification and Petrogenesis of JAH 556—A New Howardite

JaH 556 is a heterogeneous polymict breccia composed of a mixture of HED and chondritic material. The presence of diogenite clasts is confirmed by mineral composition and geochemical data. Petrologically, JaH 556 can be classified as howardite (diogenite >10 vol%; Delaney et al. 1983). As with other howardites (e.g., Kapoeta; Pun et al. 1998), it is composed of variably sized fragments of eucrite, diogenite, and chondrite embedded in a finely recrystallized cryptocrystalline matrix (grain size typically <5 μm) composed of subeuhedral plagioclase and clinopyroxene. Howardite classification is confirmed by the whole-rock major composition that falls within the howarditic range of compositions (Table 2; Figs. 4 and 5).

JaH 556, however, shows some differences in comparison to other howardites in terms of oxygen isotopic compositions, siderophile abundances, and mineralogy. These unique characteristics may be mainly explained by the admixture of an ordinary chondrite component.

A Significant H Chondrite Component in JaH 556

The occurrence of exotic clasts, especially chondrites, in howardites has long been recognized. Most of these clasts are attributed to CM2 (about 80%) and CR2 (about 20%, Zolensky et al. 1996) chondrites. In JaH 556, the presence of chondrules (Clast 2) confirms that it is partly contaminated by chondritic material, but the possibility of a significant CM or CR contribution is ruled out by the bulk-rock isotopic oxygen composition which requires a second reservoir with higher Δ17O, most likely ordinary chondrite (Fig. 3b). Based on the δ18O values of the H, L, and LL groups (Clayton et al. 1991), the best fit to the EATG washed bulk composition of JaH 556 is obtained by invoking an impactor with a composition similar to the L group (Fig. 3b). Using the average HED Δ17O value of −0.239‰ (Greenwood et al. 2005) and the average Δ17O value of +1.06‰ for L chondrites (Clayton et al. 1991), this would indicate that the bulk JaH 556 oxygen isotope value represents a mix of 10% L chondrite and 90% HED.

However, an H chondrite impactor is compatible with the mineral composition of the chondrule (Clast 2) that is composed of elongated olivine intercalated with finely grained orthopyroxene and plagioclase (barred olivine chondrule). Inside the chondrule, mineral composition changes toward the rim as it re-equilibrated with the matrix. Thus, core compositions are most probably representative of primary chondritic compositions. An H chondrite impactor is consistent with the core olivine composition in the chondritic clasts (Fo80 and En79Wo2), which corresponds to that seen in equilibrated H chondrites (Jones 1998). The high δ18O value of the JaH 556 meteorite may be accounted for by incomplete removal of the alteration products that shifted the δ18O toward high values (Fig. 3b and oxygen isotopic compositions of hot desert finds; Bland et al. 2000; Greenwood and Franchi 2004; Martins et al. 2007; Janots et al. 2011).

Additional precision on the nature of the chondritic component is given by the siderophile element composition of JaH 556. To date, the siderophile whole-rock composition of JaH 556 is unique. In JaH 556, Ni and Co concentrations are the highest documented in any howardite (Fig. 4c), with a Ni/Co ratio approximately 25. On average, eucrites and diogenites contain only a 0.1 to 200 ppm Ni and 5 to 100 ppm Co (Warren et al. 2009), while higher siderophile concentrations are found in howardites (average of 503 ± 183 ppm of Ni; Warren et al. 2009). Although some rare eucritic meteorites can contain relatively high abundances of Ni and Co (Buchanan et al. 1993), due to abundant Fe/Ni metal grains in the matrix (Duke 1965), high siderophile concentrations are usually attributed to exogenous contamination. Taking the Ni content of the L chondrite impactor to be 12,000 ppm (Wasson and Kallemeyn 1988), and the Howardite target rock to be 270 ppm (Warren et al. 2009), then a 10% L chondrite, 90% HED mix will give a value of 1550 ppm, too low to account for the measured value of 3940 ppm Ni in JaH 556. Considering an H chondrite impactor with an average Δ17O = 0.73‰ (Clayton et al. 1991), JaH 556 would correspond to a 13% H chondrite, 87% HED mix. Given an H chondrite Ni content of 16,000 ppm (Wasson and Kallemeyn 1988) this mix would result in a Ni content for bulk JaH 556 of 2320 ppm. The lowest Δ17O value measured in an H chondrite is that for the H3 find Willaroy (Δ17O = 0.40‰; Clayton et al. 1991); a value confirmed by our measurements of this meteorite. This value would indicate a 20% H chondrite—80% HED mix for bulk JaH 556. Using the H chondrite Ni content of 16,000 ppm would give a predicted Ni content for bulk JaH 556 of 3420 ppm, which is relatively close to the measured value of 3940 ppm (3460 ppm measured by XRF).

On the one hand, the unique nature of the JaH 556 meteorite suggests that ordinary chondrite admixtures may be rare at the HED parent body surface. Alternatively, the scarcity of such howardites could be explained by a collection bias. The observed meteorite record indicates that meteorites attributed to 4 Vesta record an important contribution of carbonaceous chondrite. Carbonaceous chondrite contribution is commonly due to micrometeorites in 4 Vesta, as also observed at the Earth’s surface (Lorenz et al. 2007). If compared with the terrestrial record of observed meteorite fall statistics and meteorite finds in desert areas (e.g., Bevan et al. 1998; Zolensky 1998), exogenic contribution of H and L chondrite with millimetric or larger size on HED asteroids should statistically be more frequent than carbonaceous chondrites.

Impact Melting and Clast Re-Equilibration

Most meteorites from the HED group display textural features typical of thermal metamorphism and complex impact events (Takeda 1991), the chronological sequence of which has been summarized in Metzler et al. (1995). The first stage involves the primary crystallization of the magmatic rocks. The second stage is characterized by recrystallization during slow cooling after a reheating phase. The last stage corresponds to multiple periods of impact brecciation and recrystallization due to thermal metamorphism. Although the question of the heat source is still in debate, impact alone appears insufficient to supply the heat required for the thermal metamorphism of the HEDs (Keil 2000).

The textural observations in JaH 556 indicate that melting took place (Figs. 1 and 6) during at least one major impact event (Stöffler et al. 1991). JaH 556 contains highly shocked clasts (mosaicized olivine and orthopyroxene displaying sets of decorated planar fratures) in a fine and vesicular matrix, which is interpreted to result from recrystallization of an impact melt. Other original shock features include the flow-deformed shape and polycrystalline structure of plagioclase containing abundant inclusions. These features are reminiscent of plagioclase described in the Martian shergottites GRV 99027 (Wang and Chen 2006) where the plagioclase texture is interpreted to be the result of shock-induced melting, recrystallization, and exsolution. Based on the plagioclase texture, Wang and Chen (2006) proposed that GRV 99027 experienced a slower cooling than samples with maskelynite. Similar slow cooling can be envisaged in JaH 556, where the silicates are completely recrystallized with no remaining relics of impact melt pockets.

In JaH 556, olivine and orthopyroxene clasts are commonly zoned with decreasing Mg# toward the matrix. Locally, orthopyroxene reacts to form clinopyroxene and olivine. The low SiO2 bulk composition may explain why orthopyroxene is absent in the microcrystalline matrix and olivine is found instead. It is noteworthy that orthopyroxene replacement by olivine is common in terrestrial peridotite xenoliths and ophiolites (e.g., Braun and Kelemen 2002) and is caused by reaction with a SiO2-undersaturated melt. This was demonstrated experimentally by Shaw et al. (1998), who were able to reproduce similar replacements when orthopyroxene was placed in contact with an SiO2-undersaturated melt.

In JaH 556, the thermal increase due to impact and thermal annealing provoked plagioclase melting and possibly caused partial dissolution of the eucritic, diogenitic, and chondritic material. It is highly probable that SiO2-undersaturated melt was produced by partial dissolution of chondritic material in the impact melt and that it was this dissolution reaction that generated numerous olivine grains in the matrix. This also explains why so little chondritic material was observed petrographically. This mechanism precludes the idea that chondrite clasts were added to the HED after the impact melting. However, it is unclear whether chondritic material was assembled before, or during the impact that caused the melting. In the case of other howardite samples, it is thought that the carbonaceous chondrite material is incorporated in the regolith before its lithification and communition (Pun et al. 1998). In JaH 556, the chondritic contribution is much higher than that seen in other howardites, which suggests that the H chondrite material is derived from a major impactor. From our sample alone, it is impossible to estimate the size of the original H chondrite impactor and so to conclude whether or not it caused the major shock features and thermal history observed in JaH 556.

Summary and Perspective

JaH 556 is a unique howardite specimen with a bulk-rock oxygen isotopic signature diverging from the HED field due to a significant contribution (about 20 wt%) of ordinary chondrite impactor material that may have caused JaH 556 to melt and recrystallize.

In the available meteorite collection, basaltic meteorites are mainly attributed to 4 Vesta and there is only little evidence of differentiated asteroids. Based on the oxygen isotopic composition, Scott et al. (2009) and Yamaguchi et al. (2002) proposed that several HED meteorites with anomalous oxygen isotope characteristics are derived from other asteroid bodies than 4 Vesta. It is inferred that anomalous eucrites may be derived from distinct Vesta-like parent bodies, although the possibility of contributions from impactors is not ruled out for Ibitira and NWA 1280. In JaH 556, the oxygen isotopic deviation is unambiguously attributed to an exogenic source by the convergence of oxygen isotope data, high siderophile element concentrations, and petrographic identification of chondrule relics. Petrographically, chondritic material is underrepresented (<5% of the clasts). Olivine (re)-crystallization suggests that original chondritic material partly dissolved in the impact melt. This means that the exogenic component cannot be ruled out although not visible in other anomalous HEDs. Extreme caution is therefore recommended before attributing slight isotopic deviations to an anomalous eucrite with an origin from a V-type asteroid. Based on the results of this study, it is probably a good idea to investigate in more detail the oxygen isotope composition of clasts in other anomalous HEDs.

The complex JaH 556 meteorite enhances our understanding of the lithological diversity present on the surface of the asteroid 4 Vesta, which is currently being studied by the NASA Dawn spacecraft. The unique nature of this sample suggests that it may be unrepresentative of the HED parent body surface. Alternatively, the high proportion of ordinary chondrite found on Earth’s surface suggests that rocks similar to JaH 556 are locally expected at the surface of 4 Vesta.

Acknowledgments—  This study was partially financed through Swiss National Foundation research grants 107681 and 119937. Ing. Salim Omar Al Ibrahim, Dr. Ali Al Rajhi and Dr. Salim Al Busaidi, Directorate General of Minerals, Ministry of Commerce and Industry, Muscat, are thanked for their support during the project. We appreciated the constructive reviews of one anonymous reviewer and T. Mikouchi, and of the editor, M. Zolensky. We also thank J. Berndt-Gerdes for his help at the electron microprobe.

Editorial Handling—  Dr. Michael Zolensky

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