Constraints on the water,chlorine, and fluorine content of the Martian mantle

Previous estimates of the volatile contents of Martian basalts, and hence their source regions, ranged from nearly volatile‐free through estimates similar to those found in terrestrial subduction zones. Here, we use the bulk chemistry of Martian meteorites, along with Martian apatite and amphibole chemistry, to constrain the volatile contents of the Martian interior. Our estimates show that the volatile content of the source region for the Martian meteorites is similar to the terrestrial Mid‐Ocean‐Ridge Mantle source. Chlorine is enriched compared with the depleted terrestrial mantle but is similar to the terrestrial enriched source region; fluorine is similar to the terrestrial primitive mantle; and water is consistent with the terrestrial mantle. Our results show that Martian magmas were not volatile saturated; had water/chlorine and water/fluorine ratios ~0.4–18; and are most similar, in terms of volatiles, to terrestrial MORBs. Presumably, there are variations in volatile content in the Martian interior as suggested by apatite compositions, but more bulk chemical data, especially for fluorine and water, are required to investigate these variations. Finally, the Noachian Martian interior, as exemplified by surface basalts and NWA 7034, may have had higher volatile contents.     

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.     

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.     

The chlorine isotopic composition of Martian meteorites 1: Chlorine isotope composition of Martian mantle and crustal reservoirs and their interactions

The Martian meteorites record a wide diversity of environments, processes, and ages. Much work has been done to decipher potential mantle sources for Martian magmas and their interactions with crustal and surface environments. Chlorine isotopes provide a unique opportunity to assess interactions between Martian mantle‐derived magmas and the crust. We have measured the Cl‐isotopic composition of 17 samples that span the range of known ages, Martian environments, and mantle reservoirs. The ³⁷Cl of the Martian mantle, as represented by the olivine‐phyric shergottites, NWA 2737 (chassignite), and Shergotty (basaltic shergottite), has a low value of approximately ?3.8‰. This value is lower than that of all other planetary bodies measured thus far. The Martian crust, as represented by regolith breccia NWA 7034, is variably enriched in the heavy isotope of Cl. This enrichment is reflective of preferential loss of ³⁵Cl to space. Most basaltic shergottites (less Shergotty), nakhlites, Chassigny, and Allan Hills 84001 lie on a continuum between the Martian mantle and crust. This intermediate range is explained by mechanical mixing through impact, fluid interaction, and assimilation‐fractional crystallization.     

Heterogeneous distribution of H2O in the Martian interior: Implications for the abundance of H2O in depleted and enriched mantle sources

We conducted a petrologic study of apatite within 12 Martian meteorites, including 11 shergottites and one basaltic regolith breccia. These data were combined with previously published data to gain a better understanding of the abundance and distribution of volatiles in the Martian interior. Apatites in individual Martian meteorites span a wide range of compositions, indicating they did not form by equilibrium crystallization. In fact, the intrasample variation in apatite is best described by either fractional crystallization or crustal contamination with a Cl‐rich crustal component. We determined that most Martian meteorites investigated here have been affected by crustal contamination and hence cannot be used to estimate volatile abundances of the Martian mantle. Using the subset of samples that did not exhibit crustal contamination, we determined that the enriched shergottite source has 36–73 ppm H₂O and the depleted source has 14–23 ppm H₂O. This result is consistent with other observed geochemical differences between enriched and depleted shergottites and supports the idea that there are at least two geochemically distinct reservoirs in the Martian mantle. We also estimated the H₂O, Cl, and F content of the Martian crust using known crust‐mantle distributions for incompatible lithophile elements. We determined that the bulk Martian crust has ~1410 ppm H₂O, 450 ppm Cl, and 106 ppm F, and Cl and H₂O are preferentially distributed toward the Martian surface. The estimate of crustal H₂O results in a global equivalent surface layer (GEL) of ~229 m, which can account for at least some of the surface features on Mars attributed to flowing water and may be sufficient to support the past presence of a shallow sea on Mars' surface.     

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.     

Highly siderophile element (HSE) abundances in the mantle of Mars are due to core formation at high pressure and temperature

Highly siderophile elements (HSEs) can be used to understand accretion and core formation in differentiated bodies, due to their strong affinity for FeNi metal and sulfides. Coupling experimental studies of metal–silicate partitioning with analyses of HSE contents of Martian meteorites can thus offer important constraints on the early history of Mars. Here, we report new metal–silicate partitioning data for the PGEs and Au and Re across a wide range of pressure and temperature space, with three series designed to complement existing experimental data sets for HSE. The first series examines temperature effects for D(HSE) in two metallic liquid compositions—C‐bearing and C‐free. The second series examines temperature effects for D(Re) in FeO‐bearing silicate melts and FeNi‐rich alloys. The third series presents the first systematic study of high pressure and temperature effects for D(Au). We then combine our data with previously published partitioning data to derive predictive expressions for metal–silicate partitioning of the HSE, which are subsequently used to calculate HSE concentrations of the Martian mantle during continuous accretion of Mars. Our results show that at midmantle depths in an early magma ocean (equivalent to approximately 14 GPa, 2100 °C), the HSE contents of the silicate fraction are similar to those observed in the Martian meteorite suite. This is in concert with previous studies on moderately siderophile elements. We then consider model calculations that examine the role of melting, fractional crystallization, and sulfide saturation/undersaturation in establishing the range of HSE contents in Martian meteorites derived from melting of the postcore formation mantle. The core formation modeling indicates that the HSE contents can be established by metal–silicate equilibrium early in the history of Mars, thus obviating the need for a late veneer for HSE, and by extension volatile siderophile elements, or volatiles in general.     

The composition and thickness of the crust of Mars estimated from rare earth elements and neodymium-isotopic compositions of Martian meteorites

Abstract— Isotopic and trace element compositions of Martian meteorites show that early differentiation of Mars produced complementary crustal and mantle reservoirs that were sampled by later magmatic events. This paper describes a mass balance model that estimates the rare earth element (REE) content and thickness of the crust of Mars from the compositions of shergottites. The diverse REE and Nd isotopic compositions of shergottites are most easily explained by variable addition of light rare earth element (LREE)–enriched crust to basaltic magmas derived from LREE-depleted mantle source regions. Antarctic shergottites EET 79001, ALH 77005, LEW 88516, and QUE 94201 all have strongly LREE-depleted patterns and positive initial ¹⁴³Nd isotopic compositions, which is consistent with the generation of these magmas from depleted mantle sources and little or no interaction with enriched crust. In contrast, Shergotty and Zagami have negative initial ¹⁴³Nd isotopic compositions and less pronounced depletions of the LREE, which have been explained by incorporation of enriched crustal components into mantle-derived magmas (Jones, 1989; Longhi, 1991; Borg et al., 1997). The mass balance model presented here derives the REE composition of the crustal component in Shergotty by assuming it represents a mixture between a mantle-derived magma similar in composition to EET 79001A and a LREE-enriched crustal component. The amount of crust in Shergotty is constrained by mixing relations based on Nd-isotopic compositions, which allows the REE pattern of the crustal component to be calculated by mass balance. The effectiveness of this model is demonstrated by the successful recovery of important characteristics of the Earth's continental crust from terrestrial Columbia River basalts. Self-consistent results for Nd-isotopic compositions and REE abundances are obtained if Shergotty contains ～10–30% of LREE-enriched crust with >10 ppm Nd. This crustal component would have moderately enriched LREE (Sm/Nd = 0.25–0.27; ¹⁴⁷Sm/¹⁴⁴Nd = 0.15–0.17; La/Yb = 2.7–3.8), relatively unfractionated heavy rare earth elements (HREE), and no Eu anomaly. Crust with these characteristics can be produced from a primitive lherzolitic Martian mantle by modest amounts (2–8%) of partial melting, and it would have a globally averaged thickness of <45 km, which is consistent with geophysical estimates. Mars may serve as a laboratory to investigate planetary differentiation by extraction of a primary basaltic crust.     

Impact craters in the northern hemisphere of Mars: Layered ejecta and central pit characteristics

Abstract— Mars Global Surveyor (MGS) and Mars Odyssey data are being used to revise the Catalog of Large Martian Impact Craters. Analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of Mars. This report focuses on the ejecta morphologies and central pit characteristics of these craters. The results indicate that single‐layer ejecta (SLE) morphology is most consistent with impact into an ice‐rich target. Double‐layer ejecta (DLE) and multiple‐layer ejecta (MLE) craters also likely form in volatile‐rich materials, but the interaction of the ejecta curtain and target‐produced vapor with the thin Martian atmosphere may be responsible for the large runout distances of these ejecta. Pancake craters appear to be a modified form of double‐layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions. Pedestal craters are proposed to form in an icerich mantle deposited during high obliquity periods from which the ice has subsequently sublimated. Central pits likely form by the release of vapor produced by impact into ice‐soil mixed targets. Therefore, results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the Martian northern hemisphere.     

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.     

The mantle of Mars: Some possible geological implications of its high density

The density of the Martian mantle is estimated to be about 3.55 g/cm³ (Reasenberg, 1977). Model mineral assemblages for the Martian mantle (at 30 kbar) were calculated using a modified CIPW norm scheme by adding FeO to model terrestrial mantle compositions. The density of the resulting mineral assemblages vary with increasing FeO content. With pyrolite starting compositions for the terrestrial mantle, the resulting model Martian mantle with density of 3.55 g/cm³ is not garnet-lherzolite like the Earth; rather it is an assemblage properly called oxide-garnet wehrlite: oxide (periclase-wüstite) 2%; garnet 11%; olivine 73%; clinopyroxene 12%; with no orthopyroxene. Partial melting of such an assemblage wouldyield iron-rich, ultrabasic lavas, with extremely low viscosities. Specifically, model partial melts, assuming production from the quaternary eutectic (inferred to be near: op₇ g_{42 cpx43} ox₈) yields an ultrabasic (SiO₂, 41 to 44%) picritic alkali-basaltic melt (norm composition ne 2.5, plag 32, or 2.4, di 20, ol 37, mt 4.4 and ilm, tr), with a computed viscosity of about 12 P at 1200°C. This model for the composition of the Martian surface lavas (derived from geophysical data and petrologic arguments) is in remarkable agreement with a recently published model by Maderazzo and Huguenin (1977) (derived from reflection spectroscopy, experimental and theoretical models for weathering in the Martian environment). The result also appears to be consistent with recent interpretations (Rasool and Le Sergeant, 1977) of Viking atmospheric chemistry results, namely that the Martian crust is potassium poor. There are a number of geological implications which follow, including (1) superfluid lavas may account for some flood and erosional features observed on Mars; (2) the XRF inorganic chemistry experiment on Vikings 1 and 2 (Baird, 1976) indeed may be measuring compositions approaching primary lavas, contrary to current interpretations which favor a rather mature (weathered) soil; (3) ultrabasic (ferrokimberlitic) ash might be a major constituent of the Martian soil, especially if cosmological models concerning the incorporation of a much volatile material within the early accreting Mars are correct—a matter of current debate; (4) a number of mineral assemblages not previously considered are possible in the Martian mantle depending principally on the activity of volatile substances, (S, O, C, H); it is possible that some very unusual magmas are produced on partial melting; and (5) some ferro-granite melts might be produced by liquid immiscibility.     

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.     

Characterization of mesostasis regions in lunar basalts: Understanding late‐stage melt evolution and its influence on apatite formation

Recent studies geared toward understanding the volatile abundances of the lunar interior have focused on the volatile‐bearing accessory mineral apatite. Translating measurements of volatile abundances in lunar apatite into the volatile inventory of the silicate melts from which they crystallized, and ultimately of the mantle source regions of lunar magmas, however, has proved more difficult than initially thought. In this contribution, we report a detailed characterization of mesostasis regions in four Apollo mare basalts (10044, 12064, 15058, and 70035) in order to ascertain the compositions of the melts from which apatite crystallized. The texture, modal mineralogy, and reconstructed bulk composition of these mesostasis regions vary greatly within and between samples. There is no clear relationship between bulk‐rock basaltic composition and that of bulk‐mesostasis regions, indicating that bulk‐rock composition may have little influence on mesostasis compositions. The development of individual melt pockets, combined with the occurrence of silicate liquid immiscibility, exerts greater control on the composition and texture of mesostasis regions. In general, the reconstructed late‐stage lunar melts have roughly andesitic to dacitic compositions with low alkali contents, displaying much higher SiO₂ abundances than the bulk compositions of their host magmatic rocks. Relevant partition coefficients for apatite‐melt volatile partitioning under lunar conditions should, therefore, be derived from experiments conducted using intermediate compositions instead of compositions representing mare basalts.     

Oxygen fugacity in the Martian mantle controlled by carbon: New constraints from the nakhlite MIL 03346

Abstract— Pyroxene structural data, along with analyses of titanomagnetite, fayalite and mesostasis of the new nakhlite Miller Range (MIL) 03346, define equilibration near 1 bar, 1100 °C, and oxygen fugacity near the FMQ buffer. There is a clear progression of oxygen fugacity (fO₂) in Martian meteorites from reduced Allan Hills (ALH) 84001 to intermediate shergottites to oxidized nakhlites. This trend can be explained by polybaric graphite‐CO‐CO₂ equilibria in the Martian mantle. Shergottites would have formed at pressures between 1.2 and 3.0 GPa, and nakhlite parent liquids formed at pressures >3.0 GPa, consistent with geochemical and petrologic data for the shergottites and nahklites. Carbon buffering in the Martian mantle could be responsible for variation in fO₂ in Martian meteorites (rather than assimilation or crustal interaction), as well as C‐H‐O fluids that could be the source of ˜30 ppb CH₄ detected by recent spacecraft missions. The conundrum of an oxidized current mantle and basalts, but reduced early mantle during core‐mantle equilibrium exists for both the Earth and Mars. A polybaric buffering role for graphite can explain this discrepancy for Mars, and thus it may not be necessary to have an oxidation mechanism like the dissociation of MgFe‐perovskite to account for the oxidized terrestrial mantle.     

Determination of volatile concentrations in fluorapatite of Martian shergottite NWA 2975 by combining synchrotron FTIR,Raman spectroscopy,EMPA, and TEM,and inferences on the volatile budget of the apatite host‐magma

We combined the focused ion beam sample preparation technique with polarized synchrotron‐based FTIR (Fourier transform infrared) spectroscopy, laser‐Raman spectroscopy, electron microprobe analysis (EMPA), and transmission electron microscope (TEM) analysis to identify and quantify structurally bound OH, F, Cl, and CO₃ groups in fluorapatite from the Northwest Africa 2975 (NWA 2975) shergottite. In this study, the first FTIR spectra of the OH‐stretching region from a Martian apatite are presented that show characteristic OH‐bands of a F‐rich, hydroxyl‐bearing apatite. Depending on the method of apatite‐formula calculation and whether charge balance is assumed or not, the FTIR‐based quantification of the incorporated OH, expressed as wt% H₂O, is in variably good agreement with the H₂O concentration calculated from electron microprobe data. EMP analyses yielded between 0.35 and 0.54 wt% H₂O, and IR data yielded an average H₂O content of 0.31 ± 0.03 wt%, consistent with the lower range determined from EMP analyses. The TEM observations implied that the volatiles budget of fluorapatite is magmatic. The water content and the relative volatile ratios calculated for the NWA 2975 magma are similar to those established for other enriched or intermediate shergottites. It is difficult to define the source of enrichment: either Martian wet mantle or crustal assimilation. Comparing the environment of parental magma generation for NWA 2975 with the terrestrial mantle in terms of water content, it displays a composition intermediate between enriched and depleted MORB.     

Moderately and slightly siderophile element constraints on the depth and extent of melting in early Mars

The thermal history of Mars during accretion and differentiation is important for understanding some fundamental aspects of its evolution such as crust formation, mantle geochemistry, chronology, volatile loss and interior degassing, and atmospheric development. In light of data from new Martian meteorites and exploration rovers, we have made a new estimate of Martian mantle siderophile element depletions. New high pressure and temperature metal–silicate experimental partitioning data and expressions are also available. Using these new constraints, we consider the conditions under which the Martian mantle may have equilibrated with metallic liquid. The resulting conditions that best satisfy six siderophile elements—Ni, Co, W, Mo, P, and Ga—and are consistent with the solidus and liquidus of the Martian mantle phase diagram are a pressure of 14 ± 3 GPa and temperature of 2100 ± 200 K. The Martian mantle depletions of Cr and V are also consistent with metal–silicate equilibration in this pressure and temperature range if deep mantle silicate phases are also taken into account. The results are not consistent with either metal–silicate equilibrium at the surface or at the current‐day Martian core–mantle boundary. Recent measurements and modeling have concluded that deep (～17 GPa or 1350 km) mantle melting is required to explain isotopic data for Martian meteorites and the nature of differentiation into core, mantle, and crust. This is in general agreement with our estimates of the conditions of Martian core formation based on siderophile elements that result in an intermediate depth magma ocean scenario for metal–silicate equilibrium.     

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.     

Effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on Mars

Abstract— Impact cratering is an important geological process on Mars and the nature of Martian impact craters may provide important information as to the volatile content of the Martian crust. Terrestrial impact structures currently provide the only ground‐truth data as to the role of volatiles and an atmosphere on the impact‐cratering process. Recent advancements, based on studies of several well‐preserved terrestrial craters, have been made regarding the role and effect of volatiles on the impact‐cratering process. Combined field and laboratory studies reveal that impact melting is much more common in volatile‐rich targets than previously thought, so impact‐melt rocks, melt‐bearing breccias, and glasses should be common on Mars. Consideration of the terrestrial impact‐cratering record suggests that it is the presence or absence of subsurface volatiles and not the presence of an atmosphere that largely controls ejecta emplacement on Mars. Furthermore, recent studies at the Haughton and Ries impact structures reveal that there are two discrete episodes of ejecta deposition during the formation of complex impact craters that provide a mechanism for generating multiple layers of ejecta. It is apparent that the relative abundance of volatiles in the near‐surface region outside a transient cavity and in the target rocks within the transient cavity play a key role in controlling the amount of fluidization of Martian ejecta deposits. This study shows the value of using terrestrial analogues, in addition to observational data from robotic orbiters and landers, laboratory experiments, and numerical modeling to explore the Martian impact‐cratering record.     

Crystallization experiments on a Gusev Adirondack basalt composition

Abstract— Until recently, the SNC meteorites represented the only source of information about the chemistry and petrology of the Martian surface and mantle. The Mars Exploration Rovers have now analyzed rocks on the Martian surface, giving additional insight into the petrology and geochemistry of the planet. The Adirondack basalts, analyzed by the MER Spirit in Gusev crater, are olivine‐phyric basaltic rocks which have been suggested to represent liquids, and might therefore provide new insights into the chemistry of the Martian mantle. Experiments have been conducted on a synthetic Humphrey composition at upper mantle and crustal conditions to investigate whether this composition might represent a primary mantle‐derived melt. The Humphrey composition is multiply saturated at 12.5 kbar and 1375 °C with olivine and pigeonite; a primary anhydrous melt derived from a “chondritic” mantle would be expected to be saturated in orthopyroxene, not pigeonite. In addition, the olivine and pigeonite present at the multiple saturation are too ferroan to have been from a Martian mantle as is understood now. Therefore, it seems likely that the Humphrey composition does not represent a primary anhydrous melt from the Martian mantle, but was affected by mineral/melt fractionations at lower (crustal) pressures.     

History of Martian volatiles: Implications for organic synthesis

A theoretical reconstruction of the history of Martian volatiles indicates that Mars probably possessed a substantial reducing atmosphere at the outset of its history and that its present tenous and more oxidized atmosphere is the result of extensive chemical evolution. As a consequence, it is probable that Martian atmospheric chemical conditions, now hostile with respect to abiotic organic synthesis in the gas phase, were initially favorable. Evidence indicating the chronology and degradational history of Martian surface features, surface mineralogy, bulk volatile content, internal mass distribution, and thermal history suggests that Mars catastrophically developed a substantial reducing atmosphere as the result of rapid accretion. This atmosphere probably persisted—despite the direct and indirect effects of hydrogen escape—for a geologically short time interval during, and immediately following, Martian accretion. That was the only portion of Martian history when the atmospheric environment could have been chemically suited for organic synthesis in the gas phase. Subsequent gradual degrassing of the Martian interior throughout Martian history could not sustain a reducing atmosphere due to the low intensity of planet-wide orogenic activity and the short atmospheric mean residence time of hydrogen on Mars. During the post-accretion history of Mars, the combined effects of planetary hydrogen escape, solar-wind sweeping, and reincorporation of volatiles into the Martian surface produced and maintained the present atmosphere.