Petrogenesis of the new lherzolitic shergottite Grove Mountains 99027: Constraints of petrography,mineral chemistry,and rare earth elements

Abstract— We report petrography, mineral chemistry, and microdistribution of rare earth elements (REE) in a new lherzolitic shergottite, Grove Mountains (GRV) 99027. The textural relationship and REE patterns of minerals suggest precipitation of cumulus olivine and chromite, followed by equilibrium crystallization of a closed system with a bulk composition of the inferred intercumulus melt. Subsolidus equilibrium temperatures of pyroxenes and olivine range from 1100 to 1210 °C, based on a two‐pyroxene thermometry and Ca partitioning between augite and olivine. Oxygen fugacity of the parent magma is 1.5–2.5 (av. 2.0 ± 0.4) log units below the quartz‐fayalite‐magnetite (QFM) buffer at 960–1360 °C, according to the olivine‐orthopyroxene‐chromite barometer. The ilmenite‐chromite barometer and thermometer show much wider ranges of oxygen fugacity (1.0–7.0 log unit below QFM) and temperature (1130–480 °C), suggesting subsolidus equilibration of the oxides at low temperatures, probably due to deep burial of GRV 99027 on Mars. The low oxygen fugacity and LREE depletion of the parent magma of GRV 99027 suggest low contamination by martian crust. Characteristics of GRV 99027 demonstrate similarity of lherzolitic shergottites, suggesting a high possibility of launch pairing or a homogeneous upper mantle of Mars if they were ejected by individual impact events. However, GRV 99027 probably experienced severe post‐shock thermal metamorphism in comparison with other lherzolitic shergottites, based on the re‐crystallization of maskelynite, the homogeneity of minerals, and the low subsolidus equilibrium temperatures between chromite and ilmenite.     

Bulk chemical composition of lherzolitic shergottite Grove Mountains 99027—Constraints on the mantle of Mars

Abstract— We report the concentration of 50 elements, including rare earth elements (REEs) and platinum group elements (PGEs) in bulk samples of the Grove Mountains (GRV) 99027 lherzolitic shergottite. The abundances of REEs are distinctly lower than those of Allan Hills (ALH) A77005 and other lherzolitic shergottites, indicating that GRV 99027 is not paired with them. It may, nevertheless, sample the same igneous unit as the others (Lin et al. 2005b; Wang and Chen 2006). The CI‐normalized elemental pattern of GRV 99027 reveals low (0.004–0.008 × CI) and unfractionated PGEs (except for Pd of 0.018 × CI) without depletion of W. or Ga relative to lithophile element trends. Fractionation between siderophile and lithophile elements become less pronounced with increase of volatility, except for high abundances of Ni and Co. These characteristics are probably representative of the mantle of Mars, which is consistent with previous work that the Martian mantle formed in a deep magma ocean followed by a later accretion of chondritic materials.     

Grove Mountains 020090 enriched lherzolitic shergottite: A two‐stage formation model

Grove Mountains (GRV) 020090 is an enriched lherzolitic shergottite, distinct from other lherzolitic shergottites, except RBT 04262/1. Its characteristics include high abundance of plagioclase (24.2 vol% in the nonpoikilitic area), presence of K‐feldspar, common occurrence of baddeleyite, high FeO contents of olivine (bimodal peaks at Fa 33 mol% and Fa 41 mol%) and low‐Ca pyroxenes (bimodal peaks at Fs 23.8–31.7 mol% and Fs 25.7–33.9 mol%), and significant LREE enrichment of phosphates (500–610 × CI). The bulk composition of GRV 020090 suggests derivation from partial melting of an enriched reservoir. However, the REE patterns of the cores of pigeonite oikocrysts and the olivine chadacrysts are indistinguishable from those of GRV 99027 and other moderately depleted lherzolitic shergottites, and reveal a LREE‐depleted pattern of the primordial parent magma. We propose that the primordial parent magma of GRV 020090 was derived from a moderately depleted Martian upper mantle reservoir, and later the residual melt was contaminated by oxidized and enriched Martian crustal materials as it ascended up to the subsurface. GRV 020090 and RBT 04262/1 may have sampled an igneous unit different from other lherzolitic shergottites.     

Rb‐Sr and Sm‐Nd isotopic systematics of the lherzolitic shergottite GRV 99027

Abstract– Rb‐Sr and Sm‐Nd isotopic analyses of the lherzolitic shergottite Grove Mountains (GRV) 99027 are reported. GRV 99027 yields a Rb‐Sr mineral isochron age of 177 ± 5 (2σ) Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio (I_Sr) of 0.710364 ± 11 (2σ). Due to larger uncertainties of the Sm‐Nd isotopic data, no Sm‐Nd isochron age was obtained for GRV 99027. The ε¹⁴³Nd value is estimated approximately +12.2, assuming an age of 177 Ma. The I_Sr of GRV 99027 is distinguishable from other lherzolitic shergottites, confirming our previous conclusion that it is not paired with them ( Lin et al. 2005 ). The new data of GRV 99027 support the same age of approximately 180 Ma for most lherzolitic shergottites, and fill the small gap of I_Sr between Allan Hills A77005 and Lewis Cliff 88516 ( Borg et al. 2002 ). All available data are consistent with a single igneous source for the intermediate subgroup of lherzolitic shergottites.     

Shock‐induced melting,recrystallization, and exsolution in plagioclase from the Martian lherzolitic shergottite GRV 99027

Abstract— Plagioclase in the Martian lherzolitic shergottite Grove Mountains (GRV) 99027 was shocked, melted, and recrystallized. The recrystallized plagioclase contains lamellae of pyroxene, olivine, and minor ilmenite (<1 μm wide). Both the pyroxene and the olivine inclusions enclosed in plagioclase and grains neighboring the plagioclase were partially melted into plagioclase melt pools. The formation of these lamellar inclusions in plagioclase is attributed to exsolution from recrystallizing melt. Distinct from other Martian meteorites, GRV 99027 contains no maskelynite but does contain recrystallized plagioclase. This shows that the meteorite experienced a slower cooling than maskelynite‐bearing meteorites. We suggest that the parent rock of GRV 99027 could have been embedded in hot rocks, which facilitated a more protracted cooling history.     

Petrogenesis of Grove Mountains 020090: An enriched “lherzolitic” shergottite

Grove Mountains (GRV) 020090 is a “lherzolitic” shergottite found in the Grove Mountains, Antarctica. It exhibits two distinct textures: poikilitic and nonpoikilitic. In poikilitic areas, large pyroxene oikocrysts enclose subhedral olivine and chromite chadacrysts. Pyroxene oikocrysts are zoned from pigeonite cores to augite rims. In nonpoikilitic areas, olivine, pyroxene, and interstitial maskelynite occur as major phases, and minor phases include chromite and merrillite. Compared with typical “lherzolitic” shergottites, GRV 020090 contains a distinctly higher abundance of maskelynite (19 vol%). Olivine and pyroxene are more ferroan (Fa_28–40, En_57–72Fs_24–31Wo_4–14 and En_46–53Fs_17–21Wo_26–35), and maskelynite is more alkali‐rich (Ab_43–65Or_2–7). The major phases, whole‐rock (estimated) and fusion crust of GRV 020090, are relatively enriched in light rare earth elements (LREE), similar to those of the geochemically enriched basaltic shergottites, but distinct from those of LREE‐depleted “lherzolitic” shergottites. Combined with a high oxygen fugacity of log fO₂ = QFM ? 1.41 ± 0.04 (relative to the quartz‐fayalite‐magnetite buffer), it is clear that GRV 020090 sampled from an oxidized and enriched mantle reservoir similar to those of other enriched shergottites. The calculated REE abundances and patterns of the melts in equilibrium with the cores of major phases are parallel to but higher than that of the whole rock, suggesting that GRV 020090 originated from a single parent magma and experienced progressive fractional crystallization in a closed system. The crystallization age recorded by baddeleyite is 192 ± 10 (2σ) Ma, consistent with the young internal isochron ages of enriched shergottites. Baddeleyite dating results further demonstrated that the young ages, rather than ancient ages (>4 Ga), appear to represent the crystallization of Martian surface lava flow. GRV 020090 shares many similarities with Roberts Massif (RBT) 04261/2, the first enriched “lherzolitic” shergottite. Detailed comparisons suggest that these two rocks are petrologically and geochemically closely related, and probably launch paired.     

Petrology and trace element geochemistry of Tissint,the newest shergottite fall

The fall and recovery of the Tissint meteorite in 2011 created a rare opportunity to examine a Martian sample with a known, short residence time on Earth. Tissint is an olivine‐phyric shergottite that accumulated olivine antecrysts within a single magmatic system. Coarse olivine grains with nearly homogeneous cores of Mg# >80 suggest slow re‐equilibration. Many macroscopic features of this sample resemble those of LAR 06319, including the olivine crystal size distribution and the presence of evolved oxide and olivine compositions. Unlike LAR 06319, however, no magmatic hydrous phases were found in the analyzed samples of Tissint. Minor and trace element compositions indicate that the meteorite is the product of closed‐system crystallization from a parent melt derived from a depleted source, with no obvious addition of a LREE‐rich (crustal?) component prior to or during crystallization. The whole‐rock REE pattern is similar to that of intermediate olivine‐phyric shergottite EETA 79001 lithology A, and could also be approximated by a more olivine‐rich version of depleted basaltic shergottite QUE 94201. Magmatic oxygen fugacities are at the low end of the shergottite range, with log fO₂ of QFM‐3.5 to ‐4.0 estimated based on early‐crystallized minerals and QFM‐2.4 estimated based on the Eu in pyroxene oxybarometer. These values are similarly comparable to other depleted shergottites, including SaU 005 and QUE 94201. Tissint occupies a previously unsampled niche in shergottite chemistry: containing olivines with Mg# >80, resembling the enriched olivine‐phyric shergottite LAR 06319 in its crystallization path, and comparable to intermediate olivine‐phyric shergottite EETA 79001A, depleted olivine‐phyric shergottite DaG 476, and depleted basaltic shergottite QUE 94201 in its trace element abundances and oxygen fugacity. The apparent absence of evidence for terrestrial alteration in Tissint (particularly in trace element abundances in the whole‐rock and individual minerals) confirms that exposure to the arid desert environment results in only minimal weathering of samples, provided the exposure times are brief.     

Localized shock melting in lherzolitic shergottite Northwest Africa 1950: Comparison with Allan Hills 77005

Abstract— The lherzolitic Martian meteorite Northwest Africa (NWA) 1950 consists of two distinct zones: 1) low‐Ca pyroxene poikilically enclosing cumulate olivine (Fo_70–75) and chromite, and 2) areas interstitial to the oikocrysts comprised of maskelynite, low‐ and high‐Ca pyroxene, cumulate olivine (Fo_68–71) and chromite. Shock metamorphic effects, most likely associated with ejection from the Martian subsurface by large‐scale impact, include mechanical deformation of host rock olivine and pyroxene, transformation of plagioclase to maskelynite, and localized melting (pockets and veins). These shock effects indicate that NWA 1950 experienced an equilibration shock pressure of 35–45 GPa. Large (millimeter‐size) melt pockets have crystallized magnesian olivine (Fo_78–87) and chromite, embedded in an Fe‐rich, Al‐poor basaltic to picro‐basaltic glass. Within the melt pockets strong thermal gradients (minimum 1 °C/μm) existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites, resulting in gradational textures of olivine and chromite. Dendritic and skeletal olivine, crystallized in the melt pocket center, has a nucleation density (1.0 × 10³ crystals/mm²) that is two orders of magnitude lower than olivine euhedra near the melt margin (1.6 × 10⁵ crystals/mm²). Based on petrography and minor element abundances, melt pocket formation occurred by in situ melting of host rock constituents by shock, as opposed to melt injected into the lherzolitic target. Despite a common origin, NWA 1950 is shocked to a lesser extent compared to Allan Hills (ALH) 77005 (45–55 GPa). Assuming ejection in a single shock event by spallation, this places NWA 1950 near to ALH 77005, but at a shallower depth within the Martian subsurface. Extensive shock melt networks, the interconnectivity between melt pockets, and the ubiquitous presence of highly vesiculated plagioclase glass in ALH 77005 suggests that this meteorite may be transitional between discreet shock melting and bulk rock melting.     

The Antarctic achondrite,Grove Mountains 021663: An olivine‐rich winonaite

Abstract– The Grove Mountains (GRV) 021663 meteorite was collected from the Grove Mountains region of Antarctica. The meteorite is composed primarily of olivine (Fa_5.4), orthopyroxene (Fs_4.7Wo_3.0), chromian diopside (En_53.6Fs_2.4Wo₄₄), troilite, kamacite, and plagioclase (Ab_74.5Or₄An_21.5). Minor phases include schreibersite and K‐feldspar. The meteorite is highly weathered (W3) and weakly shocked (S2). We determine a whole rock oxygen isotopic composition of δ¹⁸O = 7.50‰, δ¹⁷O = 3.52‰. Comparisons of these data with other primitive achondrites have resulted in the reclassification of this meteorite as a member of the winonaite group. The occurrences of troilite, metal, and schreibersite in GRV 021663 indicate that these minerals were once completely molten. Euhedral inclusions of pyroxene within plagioclase further suggest that these may have crystallized from a silicate melt, while the depletion of plagioclase, metal, and troilite indicates that GRV 021663 could represent a residuum following partial melting on its parent asteroid. Trace element distributions in silicate minerals do not, however, confirm this scenario. As with other winonaite meteorites, the formation of GRV 021663 probably relates to brecciation and mixing of heterogeneous lithologies, followed by varying degrees of thermal metamorphism on the parent body asteroid. Peak metamorphic conditions may have resulted in localized partial melting of metal and silicate mineralogies, but our data are not conclusive.     

The basaltic shergottite Northwest Africa 856: Petrology and chemistry

Abstract— We report on the discovery of a new shergottite from South Morocco. This single stone weighing 320 g is referenced as Northwest Africa (NWA) 856 with Djel Ibone as a synonymous name. It is a fresh, fine‐grained basaltic rock consisting mainly of two pyroxenes (total ?68 vol%: 45% pigeonite, En_61‐16Wo_9–22Fs_26–68; 23% augite, En_46‐26Wo_34‐29Fs_21–43) and plagioclase converted to maskelynite (?23 vol%, Ab_43–57Or_1–5An_54‐36). Accessory minerals include merrillite, Cl‐apatite, pyrrhotite, ilmenite, ulvöspinel, silica (stishovite and glass), amorphous K‐feldspar and baddeleyite. Amorphous mixtures of maskelynite and silica occur most commonly as median layers inside maskelynite laths. In addition, melt pockets (?2 vol%) were recognized with relics of maskelynite, pyroxene and both dense silica glass and stishovite occurring as both grains and submicrometer needles. The compositions of the melt pockets are consistent with mixtures of maskelynite and pyroxenes with an average of ?50 vol% maskelynite. The meteorite is highly fractured at all scales. The bulk composition of NWA 856 has been measured for 44 elements. It is an Al‐poor ferroan basaltic rock which strongly resembles Shergotty and Zagami in its major and trace element composition. The nearly flat rare earth element (REE) pattern (La/Lu)_n = 0.9, is similar to that of Shergotty or Zagami and differs significantly from NWA 480, another Moroccan shergottite recently described. According to the U, Ba and Sr abundances, NWA 856 is not significantly weathered. The oxygen isotopes (δ18O = +5.03%, δ17O = +3.09%, and Δ17O = +0.47%) are in agreement with the martian origin of this meteorite. On the basis of grain size, pyroxene zoning and composition, abundance of silica inclusions associated with maskelynite, trace element abundances, REE pattern and oxygen isotopes, pairing with NWA 480 is excluded. The similarity with Shergotty and Zagami is striking. The only significant differences are a larger grain size, a greater abundance of silica and melt pockets, a slightly more restricted range of pyroxene compositions and the absence of significant mesostasis.     

Petrogenesis of basaltic shergottite Northwest Africa 8657: Implications for fO2 correlations and element redistribution during shock melting in shergottites

Northwest Africa (NWA) 8657 is an incompatible trace element-enriched, low-Al basaltic shergottite, similar in texture and chemistry to Shergotty, Zagami, and NWA 5298. It is composed of zoned pyroxene, maskelynite, merrillite, and Ti-oxide minerals with minor apatite, silica, and pyrrhotite. Pyroxene grains are characterized by patchy zoning, with pigeonite or augite cores zoned to Fe-rich pigeonite mantles. The cores have rounded morphologies and irregular margins. Combined with the low Ti/Al of the cores, the morphology and chemistry of the pyroxene grains are consistent with initial crystallization at depth (30–70 km) followed by partial resorption en route to the surface. Enriched rare earth element (REE) equilibrium melt compositions and calculated oxygen fugacities (fO₂) conditions for pigeonite cores indicate that the original parent melts were enriched shergottite magmas that staged in chambers at depth within the Martian crust. NWA 8657 does not represent a liquid but rather entrained a proportion of pyroxene crystals from magma chambers where fractional crystallization was occurring at depth. Variation between fO₂ and bulk-rock (La/Yb)_N of the enriched and intermediate shergottites suggests that oxidation conditions and degree of incompatible element enrichment in the source may not be correlated, as thought previously. Shock melt pockets are characterized by an absence of phosphates and oxide minerals. It is likely that these phases were melted during shock. REEs were redistributed during this process into maskelynite and to a lesser extent the shock melt; however, the overall normalized REE profile of the shock melt is like that of the bulk-rock, but at lower absolute concentrations. Overall, shock melting has had a significant effect on the mineralogy of NWA 8657, especially the distribution of phosphates, which may be significant for geochronological applications of this meteorite and other Martian meteorites with extensive shock melt.     

A new martian meteorite from the Sahara: The shergottite Dar al Gani 489

Abstract— Dar al Gani 489 (DaG 489) is a meteorite fragment of 2146 g found in the Libyan Sahara by a meteorite finder during one of his search campaigns in 1997–98. It is a porphyritic rock with millimetersized olivine crystals (Fo_79–59) set in a fine‐grained groundmass (average grain size 0.1 mm) consisting of pigeonite (En_75–57 Wo_5–15) crystals and interstitial feldspathic glass (An_67–56 Or_0–1). Minor phases include enstatite (En_82–71 Wo_2–4), augite (En_48–52 Wo_29–32), chromite, Ti‐chromite, ilmenite, pyrrhotite, merrillite, and secondary calcite and iron oxides. On the basis of mineralogical, petrographic, bulk chemical, O‐isotopic, and noble gas data, DaG 489 can be classified as a highly shocked martian meteorite (e.g., Fe/Mn_(bulk) = 42.1, Ni/Mg_(bulk) = 0.002; δ¹⁷O = 2.89, δ¹⁸O = 4.98, and Δ¹⁷O = 0.305), belonging to the basaltic shergottite subgroup. The texture and modal composition of DaG 489 are indeed those of basalts; nonetheless, the bulk chemistry, the abundance of large olivine and chromite crystals, and enstatitic pyroxene suggest some relationship with lherzolitic shergottites. As such, DaG 489 is similar to the hybrid shergottite Elephant Moraine (EET) A79001 lithology A; however, there are some relevant differences including a higher olivine content (20 vol%), the lack of orthopyroxene megacrysts, a higher molar Mg/(Mg + Fe)_(molar) = 0.68, and a lower rare earth element content in the bulk sample. Therefore, DaG 489 has the potential of providing us with a further petrogenetic link between the basaltic and lherzolitic shergottites. Noble gases data show that DaG 489 has an ejection age of ～1.3 Ma. This young age lends support to the requirement of several ejection events to produce the current population of shergottites, nakhlites, and chassignites (SNC) meteorites. In terms of texture, mineral and bulk compositions, shock level, and weathering features, DaG 489 is essentially identical to DaG 476, another basaltic shergottite independently found ～25 km due northnortheast of DaG 489. Because DaG 489 also has the same exposure history as DaG 476, it is very likely that both meteorites are fragments of the same fall. In addition to the existing hypotheses on the petrogenesis of the similar EETA79001 lithology A and the identical DaG 476, we propose that DaG 489 could have formed through high‐degree partial melting of a lherzolite‐like material.     

Light rare earth element enrichments in ureilites: A detailed ion microprobe study

Abstract— This paper explores the possible origin of the light rare earth element (LREE) enrichments observed in some ureilites, a question that has both petrogenetic and chronologic implications for this group of achondritic meteorites. Rare earth element and other selected elemental abundances were measured in situ in 14 thin sections representing 11 different ureilites. The spatial microdistributions of REEs in C‐rich matrix areas of the three ureilites with the most striking V‐shaped whole‐rock REE patterns (Kenna, Goalpara, and Novo Urei) were investigated using the ion imaging capability of the ion microprobe. All olivines and clinopyroxenes measured have LREE‐depleted patterns with little variation in REE abundances, despite large differences in their major element compositions from ureilite to ureilite. Furthermore, we searched for but did not find any minor mineral phases that carry LREEs. The only exception is one Ti‐rich area (～20μm) in Lewis Cliff (LEW) 85400 with a major element composition similar to that of titanite; REE abundances in this area are high, ranging from La ? 400 × CI to Lu ? 40 × CI. In contrast, all ion microprobe analyses of C‐rich matrix in Kenna, Goalpara, and Novo Urei revealed large LREE enrichments. In addition, C‐rich matrix areas in the three polymict ureilites, Elephant Moraine (EET) 83309, EET 87720, and North Haig, which have less pronounced V‐shaped whole‐rock REE patterns, show smaller but distinct LREE‐enrichments. The C‐rich matrix in Antarctic ureilites tends to have much lower LREE concentrations than the matrix in non‐Antarctic ureilites. There is no obvious association of the LREEs with other major or minor elements in the C‐rich areas. Ion images further show that the LREE enrichments are homogeneously distributed on a microscale in most C‐rich matrix areas of Kenna, Goalpara, and Novo Urei. These observations suggest that the LREEs in ureilites most probably are absorbed on the surface of fine‐grained amorphous graphite in the C‐rich matrix. It is unlikely that the LREE enrichments are due to shock melts or are the products of metasomatism on the ureilite parent body. We favor LREE introduction by terrestrial contamination.     

The igneous crystallization history of an ancient Martian meteorite from rare earth element microdistributions

Abstract— Rare earth element (REE) and other selected trace and minor element concentrations were measured in individual grains of orthopyroxene, feldspathic glass (of plagioclase composition) and merrillite of the ALH 84001 Martian meteorite. Unlike in other Martian meteorites, phosphate is not the main REE carrier in ALH 84001. The REE pattern of ALH 84001 bulk rock is dependent on the modal abundances of three REE-bearing phases, namely, orthopyroxene, which contains most of the heavy rare earth elements (HREEs); feldspathic glass, which dominates the Eu abundances; and merrillite, which contains the majority of the light rare earth elements (LREEs). Variations in the REE abundances previously observed in different splits of ALH 84001 can easily be explained in terms of small variations in the modal abundances of these three minerals without the need to invoke extensive redistribution of LREEs. At least some orthopyroxenes (i.e., those away from contacts with feldspathic glass) in ALH 84001 appear to have preserved their original REE zonation from igneous fractionation. An estimate of the ALH 84001 parent magma composition from that of the unaltered orthopyroxene “core” (i.e., zoned orthopyroxene with the lowest REE abundances) indicates that it is LREE depleted. This implies that the Martian mantle was already partly depleted within ～100 Ma of solar system formation, which is consistent with rapid accretion and differentiation of Mars. Although equilibration and exchange of REEs between phases (in particular, transport of LREEs into the interstitial phases, feldspathic glass and merrillite) cannot be ruled out, our data suggest that the LREE enrichment in melts “in equilibrium” with these interstitial phases is most likely the result of late-stage infiltration of the cumulate pile by a LREE-enriched melt.     

Rare earth element measurements and mapping of minerals in the Allende CAI, 7R19‐1, by NanoSIMS ion microprobe

We have established analytical procedures for quantitative rare earth element (REE) measurements by NanoSIMS 50L ion microprobe with 2–10 μm spatial resolution. Measurements are performed by multidetection using energy filtering under several static magnetic field settings. Relative sensitivity factors and REE oxide/REE element secondary ion ratios that we determined for the NanoSIMS match values previously determined for other ion microprobes. REE measurements of 100 ppm REE glass standards yielded reproducibility and accuracy of 0.5–2.5% and 5–15%, respectively. REE measurements of minerals of an Allende type‐A CAI, 7R19‐1, were performed using three different methods: spot analysis, line profile, and imaging. These data are in excellent agreement with previous REE measurements of this inclusion by IMS‐3f ion microprobe. The higher spatial resolution NanoSIMS measurements provide additional insight into the formation process of this CAI and offer a promising new tool for analysis of fine‐grained and complexly zoned materials.     

A petrologic and trace element study of Dar al Gani 476 and Dar al Gani 489: Twin meteorites with affinities to basaltic and lherzolitic shergottites

Abstract— We present the results of a combined mineralogic‐petrologic and ion microprobe study of two martian meteorites recently recovered in the Lybian Sahara, Dar al Gani 476 (DaG 476) and Dar al Gani 489 (DaG 489). Having resided in a hot desert environment for an extended time, DaG 476 and DaG 489 were subjected to terrestrial weathering that significantly altered their chemical composition. In particular, analyses of some of the silicates show light rare earth element (LREE)‐enrichment resulting from terrestrial alteration. In situ measurement of trace element abundances in minerals allows us to identify areas unaffected by this contamination and, thereby, to infer the petrogenesis of these meteorites. No significant compositional differences between DaG 476 and DaG 489 were found, supporting the hypothesis that they belong to the same fall. These meteorites have characteristics in common with both basaltic and lherzolitic shergottites, possibly suggesting spatial and petrogenetic associations of these two types of lithologies on Mars. However, the compositions of Fe‐Ti oxides and the size of Eu anomalies in the earliest‐formed pyroxenes indicate that the two Saharan meteorites probably experienced more reducing crystallization conditions than other shergottites (with the exception of Queen Alexandra Range (QUE) 94201). As is the case for other shergottites, trace element microdistributions in minerals of the DaG martian meteorites indicate that closed‐system crystal fractionation from a LREE‐depleted parent magma dominated their crystallization history. Furthermore, rare earth element abundances in the orthopyroxene megacrysts are consistent with their origin as xenocrysts rather than phenocrysts.     

Noble gases and mineralogy of meteorites from China and the Grove Mountains,Antarctica: A 0.05 Ma cosmic ray exposure age of GRV 98004

Abstract— We determined the mineralogical and chemical characteristics and the He, Ne, and Ar isotopic abundances of 2 meteorites that fell in China and of 2 meteorites that were recovered by the 15th Chinese Antarctic Research Expedition. Guangmingshan (H5), Zhuanghe (H5), and Grove Mountain (GRV) 98002 (L5) yield cosmic ray exposure (CRE) ages of 68.7 ± 10.0 Ma, 3.8 ± 0.6 Ma, and 17.0 ± 2.5 Ma, respectively. These ages are within the range typically observed for the respective meteorite types. GRV 98004 (H5) had an extremely short parent body‐Earth transfer time of 0.052 ± 0.008 Ma. Its petrography and mineral chemistry are indistinguishable from other typical H5 chondrites. Only 3 other meteorites exist with similarly low CRE ages: Farmington (L5), Galim (LL6), and ALH 82100 (CM2). We show that several asteroids in Earth‐crossing orbits, or in the main asteroid belt with orbits close to an ejection resonance, are spectrally matching candidates and may represent immediate precursor bodies of meteorites with CRE ages ≤0.1 Ma.     

The formation conditions of enstatite chondrites: Insights from trace element geochemistry of olivine‐bearing chondrules in Sahara 97096 (EH3)

We report in situ LA‐ICP‐MS trace element analyses of silicate phases in olivine‐bearing chondrules in the Sahara 97096 (EH3) enstatite chondrite. Most olivine and enstatite present rare earth element (REE) patterns comparable to their counterparts in type I chondrules in ordinary chondrites. They thus likely share a similar igneous origin, likely under similar redox conditions. The mesostasis however frequently shows negative Eu and/or Yb (and more rarely Sm) anomalies, evidently out of equilibrium with olivine and enstatite. We suggest that this reflects crystallization of oldhamite during a sulfidation event, already inferred by others, during which the mesostasis was molten, where the complementary positive Eu and Yb anomalies exhibited by oldhamite would have possibly arisen due to a divalent state of these elements. Much of this igneous oldhamite would have been expelled from the chondrules, presumably by inertial acceleration or surface tension effects, and would have contributed to the high abundance of opaque nodules found outside them in EH chondrites. In two chondrules, olivine and enstatite exhibit negatively sloped REE patterns, which may be an extreme manifestation of a general phenomenon (possibly linked to near‐liquidus partitioning) underlying the overabundance of light REE observed in most chondrule silicates relative to equilibrium predictions. The silicate phases in one of these two chondrules show complementary Eu, Yb, and Sm anomalies providing direct evidence for the postulated occurrence of the divalent state for these elements at some stage in the formation reservoir of enstatite chondrites. Our work supports the idea that the peculiarities of enstatite chondrites may not require a condensation sequence at high C/O ratios as has long been believed.     

The regulation of carbon dioxide and climate: Gaia or geochemistry

This is a review of the Gaia hypothesis which postulates a condition of planetary homeostasis affecting chemical composition and climate. Some criticisms are answered and a new model is introduced for the long term regulation of the mean surface temperature through the biological control of CO₂ partial pressure.     

The first 800 million years: Environmental models for early earth

Space exploration has changed our views not only on the properties of celestial bodies and the interplanetary medium but also our perspective on the formation of such bodies, including the Earth. However, on few points do the new hetegonic insights approach certainty. More appropriately, they can be said to raise physically meaningful questions and to decrease the degrees of freedom allowable in any given scenario, which necessarily extends into the early history of the planetary crust, ocean and atmosphere. An attempt is made here to outline the basic schemes envisaged in modern studies of the origin of the Earth and the ramification of consequences for its early evolution that currently appear allowable and supported by observation.