Petrological,petrofabric, and oxygen isotopic study of five ungrouped meteorites related to brachinites

Northwest Africa (NWA) 6112, Miller Range (MIL) 090206 (plus its pairs: MIL 090340 and MIL 090405), and Divnoe are olivine‐rich ungrouped achondrites. We investigated and compared their petrography, mineralogy, and olivine fabrics. We additionally measured the oxygen isotopic compositions of NWA 6112. They show similar petrography, mineralogy, and oxygen isotopic compositions and we concluded that these five meteorites are brachinite clan meteorites. We found that NWA 6112 and Divnoe had a c axis concentration pattern of olivine fabrics using electron backscattered diffraction (EBSD). NWA 6112 and Divnoe are suggested to have been exposed to magmatic melt flows during their crystallization on their parent body. On the other hand, the three MIL meteorites have b axis concentration patterns of olivine fabrics. This indicates that the three MIL meteorites may be cumulates where compaction of olivine grains was dominant. Alternatively, they formed as residues and were exposed to olivine compaction. The presence of two different olivine fabric patterns implies that the parent body(s) of brachinite clan meteorites experienced diverse igneous processes.     

MIL 03443, a dunite from asteroid 4 Vesta: Evidence for its classification and cumulate origin

Abstract– The absence of dunite (>90 vol% olivine) in the howardite, eucrite, and diogenite (HED) meteorite suite, when viewed with respect to spectroscopic and petrologic evidence for olivine on Vesta, is problematic. Herein, we present petrologic, geochemical, and isotopic evidence confirming that Miller Range (MIL) 03443, containing 91 vol% olivine, should be classified with the HED clan rather than with mesosiderites. Similarities in olivine and pyroxene FeO/MnO ratios, mineral compositions, and unusual mineral inclusions between MIL 03443 and the diogenites support their formation on a common parent body. This hypothesis is bolstered by oxygen isotopic and bulk geochemical data. Beyond evidence for its reclassification, we present observations and interpretations that MIL 03443 is probably a crustal cumulate rock like the diogenites, rather than a sample of the Vestan mantle.     

Petrology and geochemistry of Northwest Africa 5480 diogenite and evidence for a basin‐forming event on Vesta

We performed a petrological and geochemical study of an olivine diogenite, Northwest Africa (NWA) 5480. NWA 5480 is a crystalline stone, but shows a heterogeneous texture. Olivine aggregates and grains of olivine and chromite display resorption textures set in a crystalline pyroxene matrix. Large olivine aggregates are penetrated by pyroxene matrix. Flow textures are observed near olivine aggregates. Olivine, chromite, and pyroxene show minor chemical zoning, implying relatively rapid cooling. NWA 5480 contains a significant amount of platinum group elements with chondritic relative proportions. All this evidence supports that NWA 5480 is an impact‐melt breccia from a target composed of olivine and pyroxene‐rich lithologies. Such impact melt would have formed by melting crustal materials, possibly during one of the impacts that formed the South Pole basins on Vesta.     

Lead and Mg isotopic age constraints on the evolution of the HED parent body

The large collection of howardite‐eucrite‐diogenite (HED) meteorites allows us to study the initial magmatic differentiation of a planetesimal. We report Pb‐Pb ages of the unequilibrated North West Africa (NWA) 4215 and Dhofar 700 diogenite meteorites and their mass‐independent ²⁶Mg isotope compositions (²⁶Mg*) to better understand the timing of differentiation and crystallization of their source reservoir(s). NWA 4215 defines a Pb‐Pb age of 4484.5 ± 7.9 Myr and has a ²⁶Mg* excess of +2.3 ± 1.6 ppm whereas Dhofar 700 has a Pb‐Pb age of 4546.4 ± 4.7 Myr and a ²⁶Mg* excess of +25.5 ± 1.9 ppm. We interpret the young age of NWA 4215 as a thermal overprint, but the age of Dhofar 700 is interpreted to represent a primary crystallization age. Combining our new data with published Mg isotope and trace element data suggests that approximately half of the diogenites for which such data are available crystallized within the first 1–2 Myr of our solar system, consistent with a short‐lived, early‐formed magma ocean undergoing convective cooling. The other half of the diogenites, including both NWA 4215 and Dhofar 700, are best explained by their crystallization in slowly cooled isolated magma chambers lasting over at least ~20 Myr.     

Northwest Africa 6232: Visible–near infrared reflectance spectra variability of an olivine diogenite

Visible and near-infrared (VNIR) reflectance is an important spectroscopic technique to identify minerals, and their associations, on planetary body surfaces. Howardites, eucrites, and diogenites (HED) are a class of igneous-like meteorites whose genetic connection with asteroid 4 Vesta has since long been inferred and recently confirmed by Dawn mission results. Pyroxene and olivine are the two major mafic minerals present in HED which can be identified with VNIR reflectance measurements. Thus, studying the compositional variability of those phases and their mixtures by means of laboratory spectroscopic measurements on different diogenitic or eucritic samples is one of the prime methods to better understand the evolution of 4 Vesta's crust. Here, we report the VNIR reflectance spectral analysis of a harzburgitic olivine diogenite, Northwest Africa 6232 (probably paired with Northwest Africa 5480), containing variable amounts of olivine as small grains or aggregates. We found that the olivine diogenite spectral parameters (e.g., band position) of powdered samples and polished slabs are in agreement. Moreover, the olivine diogenite band position shifts from synthetic orthopyroxene in accordance with the presence of olivine and chromite. In particular, the presence of a large olivine clast permits us to determine a linear variation of the band position from synthetic orthopyroxene and olivine, but underestimates the presence of olivine in the olivine diogenite spot.     

Absence of olivine orientation fabric in highly shocked Martian dunite

Shock is often given as the cause for many observations in meteorites due to the assumed previous exposure of most meteorites to at least one impact event that ultimately led to their ejection from their parent body. Here we present electron backscatter diffraction (EBSD) results on a substantially shocked dunitic achondrite, chassignite Northwest Africa (NWA) 8694, and question the general culpability of shock exposure for the formation of preferred orientation fabrics of meteoritic olivine crystals. Despite the ubiquitous presence of substantial shock indicators, the EBSD results for NWA 8694 reveal an absence of preferred orientation of olivine crystals, displaying instead an overall random fabric. We propose that the passage of shock waves through olivine crystals within a solid, crystalline, dunitic rock does not produce an overall preferred orientation, nor is it likely to actively form a whole‐rock random fabric but instead has likely no bearing on the formation of olivine orientation fabrics.     

Challenges in detecting olivine on the surface of 4 Vesta

Identifying and mapping olivine on asteroid 4 Vesta are important components to understanding differentiation on that body, which is one of the objectives of the Dawn mission. Harzburgitic diogenites are the main olivine‐bearing lithology in the howardite‐eucrite‐diogenite (HED) meteorites, a group of samples thought to originate from Vesta. Here, we examine all the Antarctic harzburgites and estimate that, on scales resolvable by Dawn, olivine abundances in putative harzburgite exposures on the surface of Vesta are likely at best in the 10–30% range, but probably lower due to impact mixing. We examine the visible/near‐infrared spectra of two harzburgitic diogenites representative of the 10–30% olivine range and demonstrate that they are spectrally indistinguishable from orthopyroxenitic diogenites, the dominant diogenitic lithology in the HED group. This suggests that the visible/near‐infrared spectrometer onboard Dawn (VIR) will be unable to resolve harzburgites from orthopyroxenites on the surface of Vesta, which may explain the current lack of identification of harzburgitic diogenite on Vesta.     

Reclassification of Hart and Northwest Africa 6047: Criteria for distinguishing between CV and CK3 chondrites

The single parent body model for the CV and CK chondrites (Greenwood et al. 2010 ) was challenged by Dunn et al. ( 2016a ), who argued that magnetite compositions could not be reconciled by a single metamorphic sequence (i.e., CV3 → CK3 → CK4–6). Cr isotopic compositions, which are distinguishable between the CV and CK chondrites, also support two different parent bodies (Yin et al. 2017 ). Despite this, there are many petrographic and mineralogical similarities between the unequilibrated (petrologic type 3) CK chondrites and the CV chondrites (also type 3), which may result in misclassification of samples. Hart and Northwest Africa 6047 (NWA 6047) are an excellent example of this. In this study, we revisit the classification of Hart and NWA 6047 using magnetite compositions, petrography, and compositions of olivine, the most ubiquitous mineral in both CV and CK chondrites. Not only do our results suggest that NWA 6047 and Hart were misclassified, but our assessment of CV and CK3 chondrites has also led to the development of criteria that can be used to distinguish between CV and CK3 chondrites. These criteria include: abundances of Cr₂O₃, TiO₂, NiO, and Al₂O₃ in magnetite; Fa content and NiO abundance of matrix olivine; FeO content of chondrules; and the chondrule:matrix ratio. Classification as a CV chondrite is also supported by the presence of igneous chondrule rims, calcium‐aluminum‐rich inclusions, and an elongated petrofabric. However, none of these petrographic characteristics can be used conclusively to distinguish between CV and CK3 chondrites.     

Characterization of weathering and heterogeneous mineral phase distribution in brachinite Northwest Africa 4872

Terrestrial weathering of hot desert achondrite meteorite finds and heterogeneous phase distributions in meteorites can complicate interpretation of petrological and geochemical information regarding parent‐body processes. For example, understanding the effects of weathering is important for establishing chalcophile and siderophile element distributions within sulfide and metal phases in meteorites. Heterogeneous mineral phase distribution in relatively coarsely grained meteorites can also lead to uncertainties relating to compositional representativeness. Here, we investigate the weathering and high‐density (e.g., sulfide, spinel, Fe‐oxide) phase distribution in sections of ultramafic achondrite meteorite Northwest Africa (NWA) 4872. NWA 4872 is an olivine‐rich brachinite (Fo_{63.6 ± 0.5}) with subsidiary pyroxene (Fs_9.7 ± 0.1Wo_{46.3 ± 0.2}), Cr‐spinel (Cr# = 70.3 ± 1.1), and weathered sulfide and metal. Raman mapping confirms that weathering has redistributed sulfur from primary troilite, resulting in the formation of Fe‐oxide (‐hydroxide) and marcasite (FeS₂). From Raman mapping, NWA 4872 is composed of olivine (89%), Ca‐rich pyroxene (0.4%), and Cr‐spinel (1.1%), with approximately 7% oxidized metal and sulfide and 2.3% marcasite‐dominated sulfide. Microcomputed tomography (micro‐CT) observations reveal high‐density regions, demonstrating heterogeneities in mineral distribution. Precision cutting of the largest high‐density region revealed a single 2 mm Cr‐spinel grain. Despite the weathering in NWA 4872, rare earth element (REE) abundances of pyroxene determined by laser‐ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) indicate negligible modification of these elements in this mineral phase. The REE abundances of mineral grains in NWA 4872 are consistent with formation of the meteorite as the residuum of the partial melting process that occurred on its parent body. LA‐ICP‐MS analyses of sulfide and alteration products demonstrate the mobility of Re and/or Os; however, highly siderophile element (HSE) abundance patterns remain faithful recorders of processes acting on the brachinite parent body(ies). Detailed study of weathering and phase distribution offers a powerful tool for assessing the effects of low‐temperature alteration and for identifying robust evidence for parent‐body processes.     

An experimental study of partial melting and fractional crystallization on the HED parent body

We have performed an experimental and modeling study of the partial melting behavior of the HED parent body and of the fractional crystallization of liquids derived from its mantle. We estimated the mantle composition by assuming chondritic ratios of refractory lithophile elements, adjusting the Mg# and core size to match the density and moment of inertia of Vesta, and the compositions of Mg‐rich olivines found in diogenites. The liquidus of a mantle with Mg# (=100*[Mg/(Mg+Fe)]) 80 is ~1625 °C and, under equilibrium conditions, the melt crystallizes olivine alone until it is joined by orthopyroxene at 1350 °C. We synthesized the melt from our 1350 °C experiment and simulated its fractional crystallization path. Orthopyroxene crystallizes until it is replaced by pigeonite at 1200 °C. Liquids become eucritic and crystal assemblages resemble diogenites below 1250 °C. MELTS correctly predicts the olivine liquidus but overestimates the orthopyroxene liquidus by ~70 °C. Predicted melt compositions are in reasonable agreement with those generated experimentally. We used MELTS to determine that the range of mantle compositions that can produce eucritic liquids and diogenitic solids in a magma ocean model is Mg# 75–80 (with chondritic ratios of refractory elements). A mantle with Mg# ~ 70 can produce eucrites and diogenites through sequential partial melting.     

Olivine‐rich exposures at Bellicia and Arruntia craters on (4) Vesta from Dawn FC

We present an analysis of olivine‐rich exposures at Bellicia and Arruntia craters using Dawn Framing Camera (FC) color data. Our results confirm the existence of olivine‐rich materials at these localities as described by Ammannito et al. ( 2013a ) using Visual Infrared Spectrometer (VIR) data. Analyzing laboratory spectra of various howardite–eucrite–diogenite meteorites, high‐Ca pyroxenes, olivines, and olivine‐orthopyroxene mixtures, we derive three FC spectral band parameters that are indicators of olivine‐rich materials. Combining the three band parameters allows us, for the first time, to reliably identify sites showing modal olivine contents >40%. The olivine‐rich exposures at Bellicia and Arruntia are mapped using higher spatial resolution FC data. The exposures are located on the slopes of outer/inner crater walls, on the floor of Arruntia, in the ejecta, as well as in nearby fresh small impact craters. The spatial extent of the exposures ranges from a few hundred meters to few kilometers. The olivine‐rich exposures are in accordance with both the magma ocean and the serial magmatism model (e.g., Righter and Drake 1997 ; Yamaguchi et al. 1997 ). However, it remains unsolved why the olivine‐rich materials are mainly concentrated in the northern hemisphere (approximately 36–42°N, 46–74°E) and are almost absent in the Rheasilvia basin.     

Unique achondrite Northwest Africa 11042: Exploring the melting and breakup of the L chondrite parent body

Northwest Africa (NWA) 11042 is a heavily shocked achondrite with medium‐grained cumulate textures. Its olivine and pyroxene compositions, oxygen isotopic composition, and chromium isotopic composition are consistent with L chondrites. Sm‐Nd dating of its primary phases shows a crystallization age of 4100 ± 160 Ma. Ar‐Ar dating of its shocked mineral maskelynite reveals an age of 484.0 ± 1.5 Ma. This age coincides roughly with the breakup event of the L chondrite parent body evident in the shock ages of many L chondrites and the terrestrial record of fossil L chondritic chromite. NWA 11042 shows large depletions in siderophile elements (<0.01×CI) suggestive of a complex igneous history involving extraction of a Fe‐Ni‐S liquid on the L chondrite parent body. Due to its relatively young crystallization age, the heat source for such an igneous process is most likely impact. Because its mineralogy, petrology, and O isotopes are similar to the ungrouped achondrite NWA 4284 (this work), the two meteorites are likely paired and derived from the same parent body.     

Northwest Africa 5790: A previously unsampled portion of the upper part of the nakhlite pile

We present a geochemical study of recently discovered Martian meteorite Northwest Africa (NWA) 5790 and use our results to constrain its origin and relationship with the other nakhlites. This nakhlite is a clinopyroxene cumulate composed of phenocrysts of augite, olivine, and rare oxides surrounded by a mesostasis composed of vitrophyric glass, feldspars, oxides, phosphates, and fine‐grained olivines and augite. Petrography, and major and trace element compositions of the phases present are consistent with derivation of NWA 5790 from a parental magma common to all the nakhlites. Olivine cores grew from a distinct, incompatible‐element enriched magma and are surrounded by rims containing augite inclusions that grew from the nakhlite parental liquid, supporting previous arguments for xenocrystic olivine cores in nakhlites. Rare earth element microdistributions suggest derivation of NWA 5790 augites from an evolved, relatively oxidized magma, produced by augite fractionation from the common nakhlite parental liquid. Augite grain shapes and CSD patterns are consistent with rapid cooling and derivation near the top of the nakhlite cumulate pile, but patterns are distinct from other nakhlites thought to have formed near the stratigraphic top. The high mesostasis abundance (~44 vol%) indicates solidification near the top of the nakhlite pile close to locations suggested for nakhlites NWA 817 and Miller Range (MIL) 03346. However, the geochemical and petrographic characteristics of these three samples do not permit their placement in a simple stratigraphic order as would occur in a single lava flow. This lack of simple ordering suggests that the nakhlite lava flow split into multiple sections as would occur during breakouts from a single lava flow. Finally we note that NWA 5790 is unique among currently available nakhlites in having phenocryst abundances low enough to allow it to flow.     

Vestan lithologies mapped by the visual and infrared spectrometer on Dawn

We present global lithological maps of the Vestan surface based on Dawn mission's Visible InfraRed (VIR) Spectrometer acquisitions with a spatial sampling of 200 m. The maps confirm the results obtained with the data set acquired by VIR with a spatial sampling of 700 m, that the reflectance spectra of Vesta's surface are dominated by pyroxene absorptions that can be interpreted within the context of the distribution of howardites, eucrites, and diogenites (HEDs). The maps also partially agree with the ground and Hubble Space Telescope observations: they confirm the background surface being an assemblage of howardite or polymict eucrite, as well as the location of a diogenitic‐rich spot; however, there is no evidence of extended olivine‐rich regions in the equatorial latitudes. Diogenite is revealed on the Rheasilvia basin floor, indicating that material of the lower crust/mantle was exposed. VIR also detected diogenites along the scarp of Matronalia Rupes, and the rims of Severina and a nearby, unnamed crater, and as ejecta of Antonia crater. The diogenite distribution is fully consistent with petrological constraints; although the mapped distribution does not provide unambiguous constraints, it favors the hypothesis of a magma ocean.     

Occurrence and implications of secondary olivine veinlets in lunar highland breccia Northwest Africa 11273

Lunar breccias preserve the records of geologic processes on the Moon. In this study, we report the occurrence, petrography, mineralogy, and geologic significance of the observed secondary olivine veinlets in lunar feldspathic breccia meteorite Northwest Africa (NWA) 11273. Bulk‐rock composition measurements show that this meteorite is geochemically similar to other lunar highland meteorites. In NWA 11273, five clasts are observed to host veinlets that are dominated by interconnecting olivine mineral grains. The host clasts are mainly composed of mafic minerals (i.e., pyroxene and olivine) and probably sourced from a basaltic lithology. The studied olivine veinlets (~5 to 30 μm in width) are distributed within the mafic mineral host, but do not extend into the adjacent plagioclase. Chemically, these olivine veinlets are Fe‐richer (Fo_41.4–51.9), compared with other olivine grains (Fo_54.3–83.1) in lithic clasts and matrix of NWA 11273. By analogy with the secondary olivine veinlets observed in meteorites from asteroid Vesta (howardite–eucrite–diogenite group samples) and lunar mare samples, our study suggests that the newly observed olivine veinlets in NWA 11273 are likely formed by secondary deposition from a lunar fluid, rather than by crystallization from a high‐temperature silicate melt. Such fluid could be sulfur‐ and phosphorous‐poor and likely had an endogenic origin on the Moon. The new occurrence of secondary olivine veinlets in breccia NWA 11273 reveals that the fluid mobility and deposition could be a previously underappreciated geological process on the Moon.     

Trace elements in olivine and the petrogenesis of the intermediate,olivine‐phyric shergottite NWA 10170

Olivine‐phyric shergottites represent primitive basaltic to picritic rocks, spanning a large range of Mg# and olivine abundances. As primitive olivine‐bearing magmas are commonly representative of their mantle source on Earth, understanding the petrology and evolution of olivine‐phyric shergottites is critical in our understanding of Martian mantle compositions. We present data for the olivine‐phyric shergottite Northwest Africa (NWA) 10170 to constrain the petrology with specific implications for magma plumbing‐system dynamics. The calculated oxygen fugacity and bulk‐rock REE concentrations (based on modal abundance) are consistent with a geochemically intermediate classification for NWA 10170, and overall similarity with NWA 6234. In addition, we present trace element data using laser ablation ICP‐MS for coarse‐grained olivine cores, and compare these data with terrestrial and Martian data sets. The olivines in NWA 10170 contain cores with compositions of Fo₇₇ that evolve to rims with composition of Fo₅₈, and are characterized by cores with low Ni contents (400–600 ppm). Nickel is compatible in olivine and such low Ni content for olivine cores in NWA 10170 suggests either early‐stage fractionation and loss of olivine from the magma in a staging chamber at depth, or that Martian magmas have lower Ni than terrestrial magmas. We suggest that both are true in this case. Therefore, the magma does not represent a primary mantle melt, but rather has undergone 10–15% fractionation in a staging chamber prior to extrusion/intrusion at the surface of Mars. This further implies that careful evaluation of not only the Mg# but also the trace element concentrations of olivine needs to be conducted to evaluate pristine mantle melts versus those that have fractionated olivine (±pyroxene and oxide minerals) in staging chambers.     

The origin of eucrites,diogenites, and olivine diogenites: Magma ocean crystallization and shallow magma chamber processes on Vesta

The asteroid 4 Vesta is one of the very few heavenly bodies to have been linked to samples on Earth: the howardite‐eucrite‐diogenite (HED) meteorite suite. This large and diverse suite of meteorites provides a detailed picture of Vesta's igneous and postigneous history. We have used the range of igneous rock types and compositions in the HED suite to test a series of chemical models for solidification processes following peak melting (magma ocean) conditions on Vesta. Fractional crystallization cannot have been a dominant early process in the magma ocean because it leads to excessive Fe‐enrichment in the melt. Models that are dominated by equilibrium crystallization cannot produce orthopyroxene cumulates (diogenites). Our best models invoke 60–70% equilibrium crystallization of a magma ocean followed by continuous extraction of the residual melt into shallow magma chambers. Fractional crystallization in these magma chambers combined with continuous or periodic addition of more melt from the slowly compacting crystal mush (magmatic recharge) can produce all of the igneous HED lithologies (noncumulate and cumulate eucrites, diogenites, dunites, harzburgites, and olivine diogenites). Magmatic recharge can also explain the narrow range in eucrite compositions and the variability of incompatible trace element concentrations in diogenites. We predict an internal structure for Vesta that permits excavation of the HEDs during the formation of the Rheasilvia basin, while remaining consistent with observations from the Dawn mission and most impact models.     

Mixing relations of the howardite‐eucrite‐diogenite suite: A new statistical approach of independent component analysis for the Dawn mission

Dawn has recently revealed that the surface of Vesta is heterogeneously covered by polymictic regoliths represented by mixtures of howardite, eucrite, and diogenite (HED) meteorites. Mixing relations of the HED suite are examined here using a new computational statistical approach of independent component analysis (ICA). We performed eight‐component ICA (Si, Ti, Al, Cr, Fe, Mn, Mg, and Ca) for 209 HED bulk‐rock compositions. The ICA results indicate that the HED bulk‐rock compositions can be reduced into three independent components (IC) and these IC vectors can reasonably explain compositional variation, petrographic observations, and the mixing relations of the HED suite. The IC‐1 vector represents a eucrite variation that extends from cumulate eucrite toward main‐group (MG) and incompatible‐element enriched eucrites. The IC‐2 vector represents a compositional variation of howardites that extends from diogenites to MG‐eucrites, indicating the well‐known two‐component mixing trend of diogenite and eucrite. The IC‐3 vector represents a compositional variation defined by diogenites and olivine‐bearing diogenites, suggesting mixing of olivine and orthopyroxene. Among the three ICs, the diogenite‐eucrite mixing trend IC‐2 is most statistically robust and dominates the compositional variations of the HED suite. Our ICA study further indicates that the combination of only three elements (Mg, Si, and Fe) approximates the eight‐component ICA model, and that the limited number of resolvable γ‐ray spectra obtained by the Dawn mission possibly discriminates olivine lithologies from the olivine‐free regolith breccias on the surface of Vesta.     

Geochemistry of 4 Vesta based on HED meteorites: Prospective study for interpretation of gamma ray and neutron spectra for the Dawn mission

Abstract— Asteroid 4 Vesta, believed to be the parent body of the howardite, eucrite, and diogenite (HED) meteorites, will be investigated by the Dawn orbiting spacecraft. Dawn carries a gamma ray and neutron detector (GRaND) that will measure and map some major‐ and trace‐element abundances. Drawing on HED geochemistry, we propose a mixing model that uses element ratios appropriate for the interpretation of GRaND data. Because the spatial resolution of GRaND is relatively coarse, the analyzed chemical compositions on the surface of Vesta will likely reflect mixing of three endmember components: diogenite, cumulate eucrite, and basaltic eucrite. Reliability of the mixing model is statistically investigated based on published whole‐rock data for HED meteorites. We demonstrate that the mixing model can accurately estimate the abundances of all the GRaND‐analyzed major elements, as well as of minor elements (Na, Cr, and Mn) not analyzed by this instrument. We also show how a similar mixing model can determine the modal abundance of olivine, and we compare estimated and normative olivine data for olivine‐bearing diogenites. By linking the compositions of well‐analyzed HED meteorites with elemental mapping data from GRaND, this study may help constrain the geological context for HED meteorites and provide new insight into the magmatic evolution of Vesta.     

The water content and parental magma of the second chassignite NWA 2737: Clues from trapped melt inclusions in olivine

NWA 2737, the second known chassignite, mainly consists of cumulate olivine crystals of homogeneous composition (Fo = 78.7 ± 0.9). These brown colored olivine grains exhibit two sets of perpendicular planar defects due to shock. Two forms of trapped liquids, interstitial melts and magmatic inclusions, have been examined. Mineral assemblages within the olivine‐hosted magmatic inclusions include low‐Ca pyroxene, augite, kaersutite, fluorapatite, biotite, chromite, sulfide, and feldspathic glass. The reconstructed parental magma composition (A^#) of the NWA 2737 is basaltic and resembles both the experimentally constrained parental melt composition of chassiginites and the Gusev basalt Humphrey, albeit with lower Al contents. A^# also broadly resembles the average of shergottite parent magmas or LAR 06319. However, we suggest that the mantle source for the chassignite parental magmas was distinct from that of the shergottite meteorites, particularly in CaO/Al₂O₃ ratio. In addition, based on the analysis of the volatile contents of kaersutite, we derived a water content of 0.48–0.67 wt% for the parental melt. Finally, our MELTS calculations suggest that moderate pressure (approximately 6.8 kb) came closest to reproducing the crystallized melt‐inclusion assemblages.