Petrography,mineral chemistry,and crystallization history of olivine‐phyric shergottite NWA 6234: A new melt composition

Knowledge of Martian igneous and mantle compositions is crucial for understanding Mars' mantle evolution, including early differentiation, mantle convection, and the chemical alteration at the surface. Primitive magmas provide the most direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The new Martian meteorite Northwest Africa (NWA) 6234 is an olivine‐phyric shergottite. Its most magnesian olivine cores (Fo₇₈) are in Mg‐Fe equilibrium with a magma of the bulk rock composition, suggesting that it represents a melt composition. Thermochemical calculations show that NWA 6234 not only represents a melt composition but is a primitive melt derived from an approximately Fo₈₀ mantle. Thus, NWA 6234 is similar to NWA 5789 and Y 980459 in the sense that all three are olivine‐phyric shergottites and represent primitive magma compositions. However, NWA 6234 is of special significance because it represents the first olivine‐phyric shergottite from a primitive ferroan magma. On the basis of Al/Ti ratio of pyroxenes in NWA 6234, the minor components in olivine and merrillite, and phosphorus zoning of olivine, we infer that the rock crystallized completely at pressures consistent with conditions in Mars' upper crust. The textural intergrowths of the two phosphates (merrillite and apatite) indicate that at a very last stage of crystallization, merrillite reacted with an OH‐Cl‐F‐rich melt to form apatite. As this meteorite crystallized completely at depth and never erupted, it is likely that its apatite compositions represent snapshots of the volatile ratios of the source region without being affected by degassing processes, which contain high OH‐F content.     

Geochemistry of intermediate olivine‐phyric shergottite Northwest Africa 6234, with similarities to basaltic shergottite Northwest Africa 480 and olivine‐phyric shergottite Northwest Africa 2990

Abstract— The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)‐phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk‐rock major‐ and trace‐element abundances (including Li), abundances of highly siderophile elements, Re‐Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe/Mn, Al/Ti, and Na/Al. The Li concentration and δ⁷Li value of NWA 6234 are similar to that of basaltic shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La‐Nd) compared with the heavy REE (Sm‐Lu), but not as extreme as the known “depleted” shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically “intermediate” in incompatible elements. The only other basaltic or ol‐phyric shergottite with a similar “intermediate” character is the basaltic shergottite NWA 480. Rhenium‐osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.     

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.     

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.     

Two‐stage polybaric formation of the new enriched,pyroxene‐oikocrystic,lherzolitic shergottite,NWA 7397

Northwest Africa (NWA) 7397 is a newly discovered, enriched, lherzolitic shergottite, the third described example of this group. This meteorite consists of two distinct textural lithologies (1) poikilitic—comprised of zoned pyroxene oikocrysts, with chadacrysts of chromite and olivine, and (2) nonpoikilitic—comprised of olivine, low‐Ca and high‐Ca pyroxene, maskelynite, and minor abundances of merrillite, spinel, ilmenite, and pyrrhotite. The constant Ti/Al ratios of pyroxene oikocrysts suggests initial crystallization of the poikilitic lithology at depth (equivalent to pressures of approximately 10 kbar), followed by crystallization of the nonpoikilitic lithology at shallower levels. Oxygen fugacity conditions become more oxidizing during crystallization ranging from fO₂ conditions of approximately QFM‐2 to QFM‐0.7. Magma calculated to be in equilibrium with the major rock‐forming minerals is LREE‐enriched relative to depleted or intermediate shergottites and has flat overall profiles. Therefore, we suggest that the parental magma for NWA 7397 had sampled an enriched, oxidized, Martian geochemical source, similar to that of other enriched basaltic and olivine‐phyric shergottites. We present a polybaric formation model for the lherzolitic shergottite NWA 7397, to account for the petrologic constraints. Three successive stages in the development of NWA 7397 are discussed (1) formation of a REE‐enriched parental magma from a distinct Martian mantle reservoir; (2) magma ponding and development of a staging chamber concomitant with initial crystallization of the poikilitic lithology; and (3) magma ascent to the near surface, with entrainment of cumulates from the staging chamber and subsequent crystallization of the nonpoikilitic lithology en route to the surface.     

Magmatic history and parental melt composition of olivine‐phyric shergottite LAR 06319: Importance of magmatic degassing and olivine antecrysts in Martian magmatism

Several olivine‐phyric shergottites contain enough olivine that they could conceivably represent the products of closed‐system crystallization of primary melts derived from partial melting of the Martian mantle. Larkman Nunatak (LAR) 06319 has been suggested to represent a close approach to a Martian primary liquid composition based on approximate equilibrium between its olivine and groundmass. To better understand the olivine–melt relationship and the evolution of this meteorite, we report the results of new petrographic and chemical analyses. We find that olivine megacryst cores are generally not in equilibrium with the groundmass, but rather have been homogenized by diffusion to Mg# 72. We have identified two unique grain types: an olivine glomerocryst and an olivine grain preserving a primary magmatic boundary that constrains the time scale of eruption to be on the order of hours. We also report the presence of trace oxide phases and phosphate compositions that suggest that the melt contained approximately 1.1% H₂O and lost volatiles during cooling, also associated with an increase in oxygen fugacity upon degassing. We additionally report in situ rare earth element measurements of the various mineral phases in LAR 06319. Based on these reported trace element abundances, we estimate the oxygen fugacity in the LAR 06319 parent melt early in its crystallization sequence (i.e., at the time of crystallization of the low‐Ca and high‐Ca pyroxenes), the rare earth element composition of the parent melt, and those of melts in equilibrium with later formed phases. We suggest that LAR 06319 represents the product of closed‐system crystallization within a shallow magma chamber, with additional olivine accumulated from a cumulate pile. We infer that the olivine megacrysts are antecrysts, derived from a single magma chamber, but not directly related to the host magma, and suggest that mixing of antecrysts within magma chambers may be a common process in Martian magmatic systems.     

Primitive olivine‐phyric shergottite NWA 5789: Petrography,mineral chemistry,and cooling history imply a magma similar to Yamato‐980459

Knowledge of Martian igneous basaltic compositions is crucial for constraining mantle evolution, including early differentiation and mantle convection. Primitive magmas provide direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The recently discovered Martian meteorite Northwest Africa (NWA) 5789 is an olivine‐phyric shergottite. NWA 5789 has special significance among the Martian meteorites because it appears to represent one of the most magnesian Martian magmas known, other than Yamato (Y) 980459. Its most magnesian olivine cores (Fo₈₅) are in Mg‐Fe equilibrium with a magma of the bulk rock composition, suggesting that the bulk represents a magma composition. Based on the Al/Ti ratio of its pyroxenes, we infer that the rock began to crystallize at a high pressure consistent with conditions in Mars’ lower crust/upper mantle. It continued and completed its crystallization closer to the surface, where cooling was rapid and produced a mesostasis of radiating sprays of plagioclase and pyroxene. The mineralogy, petrology, mineral chemistry, and bulk rock composition of NWA 5789 are very similar to those of Y‐980459. The similarities between the two meteorites suggest that NWA 5789 (like Y‐980459) represents a primitive, mantle‐derived magma composition. They also suggest the possibility that NWA 5789 and Y‐980459 formed in the same lava flow. However, based on the mineralogy and texture of its mesostasis, NWA 5789 must have cooled more slowly than Y‐980459. NWA 5789 will help elucidate the igneous geology and geochemistry of Mars.     

Petrology and geochemistry of olivine‐phyric shergottites LAR 12095 and LAR 12240: Implications for their petrogenetic history on Mars

Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine‐phyric shergottite finds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine‐phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine megacrysts have Mg# ~70, whereas megacryst rims and groundmass olivines typically have Mg# ~58–60). The rare earth element (REE) microdistributions in the various phases (olivine, low‐ and high‐Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low‐Ca pyroxene, which is one of the earliest formed REE‐bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last‐formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed‐system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.     

Spinels and oxygen fugacity in olivine‐phyric and lherzolitic shergottites

Abstract— We examine the occurrences, textures, and compositional patterns of spinels in the olivine‐phyric shergottites Sayh al Uhaymir (SaU) 005, lithology A of Elephant Moraine A79001 (EET‐A), Dhofar 019, and Northwest Africa (NWA) 1110, as well as the Iherzolitic shergottite Allan Hills (ALH) A77005, in order to identify spinel‐olivine‐pyroxene assemblages for the determination of oxygen fugacity (using the oxybarometer of Wood [1991]) at several stages of crystallization. In all of these basaltic martian rocks, chromite was the earliest phase and crystallized along a trend of strict Cr‐Al variation. Spinel (chromite) crystallization was terminated by the appearance of pyroxene but resumed later with the appearance of ulvöspinel. Ulvöspinel formed overgrowths on early chromites (except those shielded as inclusions in olivine or pyroxene), retaining the evidence of the spinel stability gap in the form of a sharp core/rim boundary (except in ALH A77005, where subsolidus reequilibration diffused this boundary). Secondary effects seen in chromites include reaction with melt before ulvöspinel overgrowth, reaction with melt inclusions, reaction with olivine hosts (in ALH A77005), and exsolution of ulvöspinel or ilmenite. All chromites experienced subsolidus Fe/Mg reequilibration. Spinel‐olivine‐pyroxene assemblages representing the earliest stages of crystallization in each rock essentially consist of the highest‐Cr#, lowest‐fe# chromites not showing secondary effects plus the most magnesian olivine and equilibrium low‐Ca pyroxene. Assemblages representing the onset of ulvöspinel crystallization consist of the lowest‐Ti ulvöspinel, the most magnesian olivine in which ulvöspinel occurs as inclusions, and equilibrium low‐Ca pyroxene. The results show that, for early crystallization conditions, oxygen fugacity (fO₂) increases from SaU 005 and Dhofar 019 (?QFM ‐3.8), to EET‐A (QFM ‐2.8) and ALH A77005 (QFM ‐2.6), to NWA 1110 (QFM ‐1.7). Estimates for later conditions indicate that in SaU 005 and Dhofar 019 oxidation state did not change during crystallization. In EET‐A, there was an increase in fO₂ that may have been due to mixing of reduced material with a more oxidized magma. In NWA 1110, there was a dramatic increase, indicating a non‐buffered system, possibly related to its high oxidation state. Differences in fO₂ among shergottites are not primarily due to igneous fractionation but, rather, to derivation from (and possibly mixing of) different reservoirs.     

Martian meteorite Tissint records unique petrogenesis among the depleted shergottites

Tissint, a new unaltered piece of Martian volcanic materials, is the most silica‐poor and Mg‐Fe‐rich igneous rock among the “depleted” olivine‐phyric shergottites. Fe‐Mg zoning of olivine suggests equilibrium growth (<0.1 °C h^?1) in the range of Fo_80–56 and olivine overgrowth (Fo_55–18) through a process of rapid disequilibrium (~1.0–5.0 °C h^?1). The spatially extended (up to 600 μm) flat‐top Fe‐Mg profiles of olivine indicates that the early‐stage cooling rate of Tissint was slower than the other shergottites. The chemically metastable outer rim of olivine (55) consists of oscillatory phosphorus zoning at the impact‐induced melt domains and grew rapidly compared to the early to intermediate‐stage crystallization of the Tissint bulk. High‐Ca pyroxene to low‐Ca pyroxene and high‐Ca pyroxene to plagioclase ratios of Tissint are more comparable to the enriched basaltic and enriched olivine‐phyric shergottites. Dominance of augite over plagioclase induced augite to control the Ca‐buffer in the residual melt suppressing the plagioclase crystallization, which also caused a profound effect on the Al‐content in the late‐crystallized pyroxenes. Mineral chemical stability, phase‐assemblage saturation, and pressure–temperature path of evolution indicates that the parent magma entered the solidus and left the liquidus field at a depth of 40–80 km in the upper mantle. Petrogenesis of Tissint appears to be similar to LAR 06319, an enriched olivine‐phyric shergottite, during the early to intermediate stage of crystallization. A severe shock‐induced deformation resulted in remelting (10–15 vol%), recrystallization (most Fe‐rich phases), and exhumation of Tissint in a time scale of 1–8 yr. Tissint possesses some distinct characteristics, e.g., impact‐induced melting and deformation, forming phosphorus‐rich recrystallization rims of olivine, and shock‐induced melt domains without relative enrichment of LREEs compared to the bulk; and shared characteristics, e.g., modal composition and magmatic evolution with the enriched basaltic shergottites, evidently reflecting unique mantle source in comparison to the clan of the depleted members.     

The variability of ruthenium in chromite from chassignite and olivine‐phyric shergottite meteorites: New insights into the behavior of PGE and sulfur in Martian magmatic systems

The Martian meteorites comprise mantle‐derived mafic to ultramafic rocks that formed in shallow intrusions and/or lava flows. This study reports the first in situ platinum‐group element data on chromite and ulvöspinel from a series of dunitic chassignites and olivine‐phyric shergottites, determined using laser‐ablation ICP‐MS. As recent studies have shown that Ru has strongly contrasting affinities for coexisting sulfide and spinel phases, the precise in situ analysis of this element in spinel can provide important insights into the sulfide saturation history of Martian mantle‐derived melts. The new data reveal distinctive differences between the two meteorite groups. Chromite from the chassignites Northwest Africa 2737 (NWA 2737) and Chassigny contained detectable concentrations of Ru (up to ~160 ppb Ru) in solid solution, whereas chromite and ulvöspinel from the olivine‐phyric shergottites Yamato‐980459 (Y‐980459), Tissint, and Dhofar 019 displayed Ru concentrations consistently below detection limit (<42 ppb). The relatively elevated Ru signatures of chromite from the chassignites suggest a Ru‐rich (~1–4 ppb) parental melt for this meteorite group, which presumably did not experience segregation of immiscible sulfide liquids over the interval of mantle melting, melt ascent, and chromite crystallization. The relatively Ru‐depleted signature of chromite and ulvöspinel from the olivine‐phyric shergottites may be the consequence of relatively lower Ru contents (<1 ppb) in the parental melts, and/or the presence of sulfides during the crystallization of the spinel phases. The results of this study illustrate the significance of platinum‐group element in situ analysis on spinel phases to decipher the sulfide saturation history of magmatic systems.     

Northwest Africa 10414, a pigeonite cumulate shergottite

Northwest Africa (NWA) 10414 is an unusual shergottite with a cumulate texture. It contains 73% coarse prismatic pigeonite, plus 18% interstitial maskelynite, 2% Si‐rich mesostasis, 2% merrillite, and minor chromite‐ulvöspinel. It contains no olivine, and only ~3% augite. Phase compositions are pigeonite (En_68‐43Fs_27‐48Wo_5‐15) and maskelynite An_~54‐36, more sodic than most maskelynite in shergottites. Chromite‐ulvöspinel composition plots between the earliest and most fractionated spinel‐group minerals in olivine‐phyric shergottites. NWA 10414 mineralogically resembles the contact facies between Elephant Moraine 79001 lithologic units A and B, with abundant pigeonite phenocrysts, though it is coarser grained. Its most Mg‐rich pigeonite also has a similar composition to the earliest crystallized pyroxenes in several other shergottites, including Shergotty. The Shergotty intercumulus liquid composition crystallizes pigeonite with a similar composition range to NWA 10414 pigeonite, using PETROLOG. Olivine‐phyric shergottite NWA 6234, with a pure magma composition, produces an even better match to this pigeonite composition range, after olivine crystallization. These observations suggest that after the accumulation of olivine from an olivine‐phyric shergottite magma, the daughter liquid could precipitate pigeonite locally to form this pigeonite cumulate, before the crystallization of overlying liquid as a normal basaltic shergottite.     

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.     

Effect of chlorine on near‐liquidus crystallization of olivine‐phyric shergottite NWA 6234 at 1 GPa: Implication for volatile‐induced melting of the Martian mantle

Martian magmas are thought to be rich in chlorine compared with their terrestrial counterparts. Here, we experimentally investigate the effect of chlorine on liquidus depression and near‐liquidus crystallization of olivine‐phyric shergottite NWA 6234 and compare these results with previous experimental results on the effect of chlorine on near‐liquidus crystallization of the surface basalts Humphrey and Fastball. Previous experimental results showed that the change in liquidus temperature is dependent on the bulk composition of the basalt. The effect of chlorine on liquidus depression is greater for lower SiO₂ and higher Al₂O₃ magmas than for higher SiO₂ and lower Al₂O₃ magmas. The bulk composition for this study has lower Al₂O₃ and higher FeO contents than previous work; therefore, we provide additional constraints on the effect of the bulk composition on the influence of chlorine on near‐liquidus crystallization. High pressure and temperature crystallization experiments were performed at 1 GPa on a synthetic basalt, of the bulk composition of NWA 6234, with 0–4 wt% Cl added to the sample as AgCl. The results are consistent with previous notions that with increasing wt% Cl in the melt, the crystallization temperature decreases. Importantly, our results have a liquidus depression ?T (°C) from added chlorine that is consistent with the difference in bulk composition and suggest a dependence on both the bulk Al₂O₃ and FeO content. Our results suggest that the addition of chlorine to the Martian mantle may lower magma genesis temperatures and potentially aid in the petrogenesis of Martian magmas.     

Experimental petrology of the Tissint meteorite: Redox estimates,crystallization curves,and evaluation of petrogenetic models

Tissint is an olivine‐phyric shergottite from an incompatible element depleted Martian mantle source. Oxythermobarometry applied to Tissint mineral phases demonstrates that the Tissint magma underwent an increase in oxygen fugacity, from ~3.5 log units below the quarz‐fayalite‐magnetite (QFM ) buffer during the early stages of crystallization, to QFM ?1.4 during the latter stages. This is the first time that such an oxidation event has been observed in a depleted shergottite. The reason for the oxidation event is unclear; however, calculations using the MELTS thermodynamic model suggest that auto‐oxidation is insufficient to cause more than ~1 log unit of oxidation, and therefore an external oxidation mechanism—such as oxidation by degassing—is required. If volatiles are responsible for the oxidation, then it indicates that volatiles are not exclusively tied to the enriched Martian mantle reservoir. A series of experiments using the Tissint parental magma were carried out under fixed (isothermal) or variable (cooling rate) temperature control, and at either reducing (QFM ?3.2) or oxidizing (QFM ?1) redox conditions. The observed liquid line of descent supports a potential genetic relationship between basaltic shergottites and olivine‐phyric shergottites. A peritectic relation where olivine is resorbed to form pyroxene is favored by increased oxygen fugacity; if oxidation during crystallization is more common than presently believed, it may explain why olivine is typically anhedral in olivine‐phyric shergottites. Results from a cooling‐rate experiment in which the oxygen fugacity was changed during the latter stages of crystallization resulted in olivine with a Cr compositional profile consistent with oxidized isothermal experiments, despite forming primarily under reducing conditions. A similar profile is observed in Tissint olivines, consistent with its redox history. Our results provide insights into the potential influence of oxidation events on the compositional zoning of minor or trace elements in olivine in olivine‐phyric basalts.     

Experimental petrology of the basaltic shergottite Yamato‐980459: Implications for the thermal structure of the Martian mantle

Abstract— The Martian meteorite Yamato (Y‐) 980459 is an olivine‐phyric shergottite. It has a very primitive character and may be a primary melt of the Martian mantle. We have conducted crystallization experiments on a synthetic Y‐980459 composition at Martian upper mantle conditions in order to test the primary mantle melt hypothesis. Results of these experiments indicate that the cores of the olivine megacrysts in Y‐980459 are in equilibrium with a melt of bulk rock composition, suggesting that these megacrysts are in fact phenocrysts that grew from a magma of the bulk rock composition. Multiple saturation of the melt with olivine and a low‐calcium pyroxene occurs at approximately 12 ± 0.5 kbar and 1540 ± 10°C, suggesting that the meteorite represents a primary melt that separated from its mantle source at a depth of ?100 km. Several lines of evidence suggest that the Y‐980459 source underwent extensive melting prior to and/or during the magmatic event that produced the Y‐980459 parent magma. When factored into convective models of the Martian interior, the high temperature indicated for the upper Martian mantle and possibly high melt fraction for the Y‐980459 magmatic event suggests a significantly higher temperature at the core‐mantle boundary than previously estimated.     

Petrogenetic linkages among Martian basalts: Implications based on trace element chemistry of olivine

Abstract— The shergottites exhibit a range of major and trace element compositions, crystallization ages, and initial Sr, Nd, Hf, and Pb isotopic compositions. To constrain the physical mechanisms by which shergottites obtain their compositional characteristics, we examined the major and trace element record preserved in olivine in the more primitive shergottites. Based on such characteristics as the Mg#, V zoning, calculated D_Ni,Co, the olivine in Y‐980459 are most likely phenocrysts. Many of these same characteristics indicate that the olivines in other shergottites are not in equilibrium with the adjacent melt. However, in most cases they are not xenocrystic, but additions of olivine from the same basaltic system. Elephant Moraine (EET) A79001 may be an exception with the olivine data suggesting that it is xenocrystic. In this case, the olivine crystallized from a reduced and LREE‐depleted melt and was incorporated into an oxidized and enriched basalt. Vanadium and CaO in olivine appear to record the appearance of spinel and pyroxene on the liquidus of most of the shergottites. Most of the olivine shergottites represent basalts produced by melting of reduced (IW to IW + 1), depleted mantle sources. Olivine data indicate that many of the primary melts derived from this source had similar Ni, Co, and Mn. Shergottites such as Northwest Africa (NWA) 1110/1068 and perhaps Roberts Massif (RBT) 04261 that appear to be derived from more enriched sources have distinctly different olivine. In the case of NWA 1110/1068, the olivine data suggests that the enriched component was added to system prior to olivine crystallization.     

Chemical layering in the upper mantle of Mars: Evidence from olivine‐hosted melt inclusions in Tissint

Melting of Martian mantle, formation, and evolution of primary magma from the depleted mantle were previously modeled from experimental petrology and geochemical studies of Martian meteorites. Based on in situ major and trace element study of a range of olivine‐hosted melt inclusions in various stages of crystallization of Tissint, a depleted olivine–phyric shergottite, we further constrain different stages of depletion and enrichment in the depleted mantle source of the shergottite suite. Two types of melt inclusions were petrographically recognized. Type I melt inclusions occur in the megacrystic olivine core (Fo_76‐70), while type II melt inclusions are hosted by the outer mantle of the olivine (Fo_66‐55). REE‐plot indicates type I melt inclusions, which are unique because they represent the most depleted trace element data from the parent magmas of all the depleted shergottites, are an order of magnitude depleted compared to the type II melt inclusions. The absolute REE content of type II displays parallel trend but somewhat lower value than the Tissint whole‐rock. Model calculations indicate two‐stage mantle melting events followed by enrichment through mixing with a hypothetical residual melt from solidifying magma ocean. This resulted in ~10 times enrichment of incompatible trace elements from parent magma stage to the remaining melt after 45% crystallization, simulating the whole‐rock of Tissint. We rule out any assimilation due to crustal recycling into the upper mantle, as proposed by a recent study. Rather, we propose the presence of Al, Ca, Na, P, and REE‐rich layer at the shallower upper mantle above the depleted mantle source region during the geologic evolution of Mars.     

Geochemical diversity of shergottite basalts: Mixing and fractionation,and their relation to Mars surface basalts

The chemical compositions of shergottite meteorites, basaltic rocks from Mars, provide a broad view of the origins and differentiation of these Martian magmas. The shergottite basalts are subdivided based on their Al contents: high‐Al basalts (Al > 5% wt) are distinct from low‐Al basalts and olivine‐phyric basalts (both with Al < 4.5% wt). Abundance ratios of highly incompatible elements (e.g., Th, La) are comparable in all the shergottites. Abundances of less incompatible elements (e.g., Ti, Lu, Hf) in olivine‐phyric and low‐Al basalts correlate well with each other, but the element abundance ratios are not constant; this suggests mixing between components, both depleted and enriched. High‐Al shergottites deviate from these trends consistent with silicate mineral fractionation. The “depleted” component is similar to the Yamato‐980459 magma; approximately, 67% crystal fractionation of this magma would yield a melt with trace element abundances like QUE 94201. The “enriched” component is like the parent magma for NWA 1068; approximately, 30% crystal fractionation from it would yield a melt with trace element abundances like the Los Angeles shergottite. This component mixing is consistent with radiogenic isotope and oxygen fugacity data. These mixing relations are consistent with the compositions of many of the Gusev crater basalts analyzed on Mars by the Spirit rover (although with only a few elements to compare). Other Mars basalts fall off the mixing relations (e.g., Wishstone at Gusev, Gale crater rocks). Their compositions imply that basalt source areas in Mars include significant complexities that are not present in the source areas for the shergottite basalts.     

Crystallization kinetics of olivine‐phyric shergottites

Crystal size distribution (CSD) and spatial distribution pattern (SDP) analyses are applied to the early crystallizing phases, olivine and pyroxene, in olivine‐phyric shergottites (Elephant moraine [EET] 79001A, Dar al Gani [DaG] 476, and dhofar [Dho] 019) from each sampling locality inferred from Mars ejection ages. Trace element zonation patterns (P and Cr) in olivine are also used to characterize the crystallization history of these Martian basalts. Previously reported 2‐D CSDs for these meteorites are re‐evaluated using a newer stereographically corrected methodology. Kinks in the olivine CSD plots suggest several populations that crystallized under different conditions. CSDs for pyroxene in DaG 476 and EET 79001A reveal single populations that grew under steady‐state conditions; pyroxenes in Dho 019 were too intergrown for CSD analysis. Magma chamber residence times of several days for small grains to several months for olivine megacrysts are calculated using the CSD slopes and growth rates inferred from previous experimental data. Phosphorus imaging in olivines in DaG 476 and Dho 019 indicate rapid growth of skeletal, sector‐zoned, or patchy cores, probably in response to delayed nucleation, followed by slow growth, and finally rapid dendritic growth with back‐filling to form oscillatory zoning in rims. SPD analyses indicate that olivine and pyroxene crystals grew or accumulated in clusters rather than as randomly distributed grains. These data reveal complex solidification histories for Martian basalts, and are generally consistent with the formation at depth of olivine megacryst cores, which were entrained in ascending magmas that crystallized pyroxenes, small olivines, and oscillatory rims on megacrysts.