Martian Basalts Research Articles

Formation of iron mineral fine particles by acidic hydrothermal alteration experiments of synthetic martian basalt

Formation of iron mineral fine particles by acidic hydrothermal alteration experiments of synthetic martian basalt

Developing vanadium valence state oxybarometers (spinel-melt, olivine-melt, spinel-olivine) and V/(Cr+Al) partitioning (spinel-melt) for martian olivine-phyric basalts

A spiked (with REE, V, Sc) martian basalt Yamato 980459 (Y98) composition was used to synthesize olivine, spinel, and pyroxene at 1200 °C at five oxygen fugacities: IW−1, IW, IW+1, IW+2, and QFM. These run products were analyzed by electron microprobe, ion microprobe, and X-ray absorption near-edge spectroscopy to establish four oxybarometers based on vanadium partitioning behavior between the following pairs of phases: V spinel-melt, V/(Cr+Al) spinel-melt, olivine-melt, and spinel-olivine. The results for the spinel-melt, olivine-melt, and V/(Cr+Al) spinel-melt are applicable for the entire oxygen fugacity range while the spinel-olivine oxybarometer is only applicable between IW−1 and IW+1. The oxybarometer based on V partitioning between spinel-olivine is restricted to basalts that crystallized under low oxygen fugacities, some martian, all lunar, as well as samples from 4 Vesta. The true potential and power of the new spinel-olivine oxybarometer is that it does not require samples representative of a melt composition or samples with some remnant of quenched melt present. It just requires that the spinel-olivine pairs were in equilibrium when the partitioning of V occurred. We have applied the V spinel-olivine oxybarometer to the Y98 meteorite as a test of the method.

Recrystallized microbial trace fossils from metamorphosed Permian basalt, southwestern Japan

Microbial trace fossils on terrestrial basalts can be used as an analogue in the search for traces of life on other terrestrial planets. This study reports on microbial trace fossils within Permian greenstones in the Maizuru Terrane, southwest Japan, which is recognized as back-arc basin oceanic crust that consists mainly of metabasalt and metagabbro. The trace fossils have been studied by means of morphology, mineralogy, elemental mapping, and carbon isotope analysis. Although minute original textures of trace fossils are recrystallized in these rocks, Granulohyalichnus vulgaris isp., Tubulohyalichnus spiralis isp., and Tubulohyalichnus annularis isp. were identified. Significant concentration of C within the trace fossils implies these are organic remnants from microbes. The δ13CPDB values <−7‰ of calcite within the greenstones indicates that the bacterial activity took place prior to the formation of calcite veins. The results support that microbial trace fossils within low-grade metamorphic basalt can be reliably identified based on their morphology and chemical composition, as reveled by elemental mapping. In this context, glassy Martian basalt may be the best rock type to investigate in terms of searching for signs of microbial activity on Earth and other planets.

Thermodynamic and mass balance constraints on iron‐bearing phyllosilicate formation and alteration pathways on early Mars

Widespread occurrences of phyllosilicates, predominantly nontronite and Mg saponite, in the Noachian‐aged crust of Mars place constraints on water availability and surface conditions. However, the Noachian atmosphere likely lacked oxidants capable of forming nontronite from the basaltic crust and its origins are thus uncertain. In this study, scenarios for the formation of iron‐bearing phyllosilicates from the weathering and hydrothermal alteration of Martian basalt were investigated using geochemical modeling to establish plausible pathways of nontronite generation and the effect of redox conditions on associated secondary phases. Hydrothermal alteration produced FeII and Mg saponite, serpentine, and zeolites at 100°C and chlorite, prehnite, and talc at 250°C, consistent with phases observed in crater deposits on Mars. Anoxic weathering produced primarily FeII and Mg saponite. High water‐rock ratios produced Al‐clays and amorphous silica and high pCO2 conditions inhibited smectite formation because of carbonate precipitation. Oxic weathering produced primarily nontronite and Mg saponite. Later oxidation of anoxic weathering products also formed nontronite‐Mg saponite assemblages; oxidation products of hydrothermal assemblages included nontronite, Mg saponite, and either Al smectites or residual zeolites or prehnite. These calculations demonstrate that there are multiple plausible routes to form Fe/Mg smectites on early Mars and that an oxidizing atmosphere need not have existed in the Noachian. Decoupling iron oxidation from initial clay formation raises the possibilities that Mars transitioned to conditions favoring widespread iron oxidation after phyllosilicate formation ceased and that residual, unoxidized ferrous iron phyllosilicates still exist today in the Martian subsurface.

Anhydrous liquid line of descent of Yamato‐980459 and evolution of Martian parental magmas

AbstractWe report the results of nominally anhydrous equilibrium and fractional crystallization experiments on a synthetic Yamato‐980459 (Y98) bulk composition at 0.5 GPa. These experiments allow us to test a suggested fractional crystallization model, calculated using MELTS by Symes et al. (), in which a Y98‐like initial liquid yielded a magma closely resembling the bulk composition of QUE 94201. Although the two meteorites cannot be cogenetic owing to their age difference, they are thought to represent bona fide magmatic liquids rather than products of crystal accumulation, as are most Martian basaltic meteorites. Hence, understanding possible petrogenetic links between these types of liquids could be revealing about processes of melting and crystallization that formed the range of Martian basalts. We find that Y98 can, in fact, generate a residual liquid closely resembling QUE, but only after a very different crystallization process, and different degree of crystallization, than that modeled using MELTS. In addition, both the identity and sequence of crystallizing phases are very different between model and experiments. Our fractional crystallization experiments do not produce a QUE‐like liquid, and the crystallizing phases are an even poorer match to the MELTS‐calculated compositions than in the equilibrium runs. However, residual liquids from our experiments define a liquid line of descent that encompasses bulk compositions of parental melts calculated for several Martian basaltic meteorites, suggesting that the known Martian basaltic meteorites had their ultimate origin from the same or very similar source lithologies. These are, in turn, similar to source rocks modeled by previous studies as products of extensive crystallization of an initial Martian magma ocean.

Analysis of Surface Materials by the Curiosity Mars Rover

Analysis of Surface Materials by the Curiosity Mars Rover

Viscous flow behavior of tholeiitic and alkaline Fe-rich martian basalts

The chemical compositions of martian basalts are enriched in iron with respect to terrestrial basalts. Their rheology is poorly known and liquids of this chemical composition have not been experimentally investigated. Here, we determine the viscosity of five synthetic silicate liquids having compositions representative of the diversity of martian volcanic rocks including primary martian mantle melts and alkali basalts. The concentric cylinder method has been employed between 1500°C and the respective liquidus temperatures of these liquids. The viscosity near the glass transition has been derived from calorimetric measurements of the glass transition. Although some glass heterogeneity limits the accuracy of the data near the glass transition, it was nevertheless possible to determine the parameters of the non-Arrhenian temperature-dependence of viscosity over a wide temperature range (1500°C to the glass transition temperature). At superliquidus conditions, the martian basalt viscosities are as low as those of the Fe–Ti-rich lunar basalts, similar to the lowest viscosities recorded for terrestrial ferrobasalts, and 0.5 to 1 order of magnitude lower than terrestrial tholeiitic basalts. Comparison with empirical models reveals that Giordano et al. (2008) offers the best approximation, whereas the model proposed by Hui and Zhang (2007) is inappropriate for the compositions considered.The slightly lower viscosities exhibited by the melts produced by low degree of mantle partial melting versus melts produced at high degree of mantle partial melting (likely corresponding to the early history of Mars), is not deemed sufficient to lead to viscosity variations large enough to produce an overall shift of martian lava flow morphologies over time. Rather, the details of the crystallization sequence (and in particular the ability of some of these magmas to form spinifex texture) is proposed to be a dominant effect on the viscosity during martian lava flow emplacement and may explain the lower range of viscosities (102–104Pas) inferred from lava flow morphology. Further, the differences between the rheological behaviors of tholeiitic vs. trachy-basalts are significant enough to affect their emplacement as intrusive bodies or as effusive lava flows. The upper range of viscosities (106–108Pas) suggested from lava flow morphology is found consistent with the occurrence of alkali basalt documented from in situ analyses and does not necessarily imply the occurrence of basaltic-andesite or andesitic rocks.

Petrogenetic linkages among fO2, isotopic enrichments-depletions and crystallization history in Martian basalts. Evidence from the distribution of phosphorus in olivine megacrysts

An important geochemical characteristic of the shergottites is the large range in initial Sr isotopic ratios and initial εNd values. The depleted olivine-phyric shergottites are commonly reduced (∼IW+1) and exhibit limited variation in fO2 during crystallization, whereas the enriched olivine-phyric shergottites are more oxidized and exhibit a substantial variation in fO2 during crystallization (∼IW+1 (1log unit above the iron-wüstite buffer) to FMQ+0.7 (0.7log units above the fayalite-magnetite-quartz buffer). Using a reduced, depleted shergottite Y980459 (Y98) and an oxidized, enriched shergottite (NWA1183), we reconstructed the crystallization history of the olivine megacrysts using phosphorus zoning and placed trace element variation and indices of oxygen fugacity into the context of that crystallization history. The olivine megacrysts in Y98 and NWA1183 exhibited various episodes of growth defined by the P zoning. Common to both samples were continuous and discontinuous oscillatory chemical zoning in the outer portions of the olivine megacrysts. However, the morphologies of the cores suggest a xenocrystic origin for the core of Y98 and a phenocrystic origin for the core of NWA1183. Phosphorus in the olivine ranges from less than 0.02 to up to 0.70wt.% P2O5. The P in the olivine megacrysts manifests subtle, but real differences, in the P content of different shergottite melts, incompatible element behavior of P during olivine crystallization, and solute trapping during periods of rapid olivine growth. Although P should behave incompatibly in basaltic systems, there is little relationship between the isotopic characteristics and P content in the enriched-depleted array of shergottites. In the olivine, Al is generally correlated with P enrichment and reflects a coupled substitution of P–Al. On the other hand, V is not correlated with P in Y98, but correlated with P in the oscillatory zoned regions of the olivine in NWA1183. The changes in correlation to P may reflect differences in V valance state during olivine crystallization. Within the crystallization histories defined by the P zoning in olivine, the calculated fO2 remains fairly constant at ∼IW+1 for Y98, but changes dramatically for NWA1183 from olivine core to rim (∼IW+1.8 to FMQ+.07). Numerous lines of evidence suggest that the changes in fO2 during olivine crystallization may reflect auto-oxidation that may be at least partially due to degassing of the NWA1183 magma prior to eruption. These observations suggest a limited variation in the fO2 of the Martian mantle (IW to IW+1), that enriched and depleted sources for the shergottites reside in the mantle, and that magmas produced in these sources have different capabilities for changing their redox state during crystallization.

Experimental study of acid‐sulfate alteration of basalt and implications for sulfate deposits on Mars

Acid‐sulfate alteration of basalt by SO2‐bearing volcanic vapors has been proposed as one possible origin for sulfate‐rich deposits on Mars. To better define mineralogical signatures of acid‐sulfate alteration, laboratory experiments were performed to investigate alteration pathways and geochemical processes during reaction of basalt with sulfuric acid. Pyroclastic cinders composed of phenocrysts including plagioclase, olivine, and augite embedded in glass were reacted with sulfuric acid at 145 °C for up to 137 days at a range of fluid : rock ratios. During the experiments, the phenocrysts reacted rapidly to form secondary products, while the glass was unreactive. Major products included amorphous silica, anhydrite, and Fe‐rich natroalunite, along with minor iron oxides/oxyhydroxides (probably hematite) and trace levels of other sulfates. At the lowest fluid : rock ratio, hexahydrite and an unidentified Fe‐silicate phase also occurred as major products. Reaction‐path models indicated that formation of the products required both slow dissolution of glass and kinetic inhibitions to precipitation of a number of minerals including phyllosilicates and other aluminosilicates as well as Al‐ and Fe‐oxides/oxyhydroxides. Similar models performed for Martian basalt compositions predict that the initial stages of acid‐sulfate alteration of pyroclastic deposits on Mars should result in formation of amorphous silica, anhydrite, Fe‐bearing natroalunite, and kieserite, along with relict basaltic glass. In addition, analysis of the experimental products indicates that Fe‐bearing natroalunite produces a Mössbauer spectrum closely resembling that of jarosite, suggesting that it should be considered an alternative to the component in sulfate‐rich bedrocks at Meridiani Planum that has previously been identified as jarosite.

The habitability of vesicles in martian basalt

Sean McMahon, John Parnell Joanna Ponicka, Malcolm Hole and Adrian Boyce argue that cavities in martian volcanic rocks are a good place to look for microbial life on Mars.

Experimental and crystal chemical study of the basalt–eclogite transition in Mars and implications for martian magmatism

This paper represents the start of a campaign to identify possible source regions for pyroxene-phyric “enriched” and “depleted” martian basalts using high pressure studies on martian meteorite compositions that represent liquids or near liquids. Our first experiments focus upon mineralogical and crystal chemical aspects of the basalt–eclogite transformation on Mars. It is anticipated that like Earth, eclogites are not the dominant upper mantle assemblage. However, like Earth they may be important hosts for P, Cl, F, OH, Ti, REE, Sr, Y, high-field-strength elements, Hf, and Zr in the upper mantle. This initial experimental study evaluates the major and trace element crystal chemistry of potential martian eclogite assemblages using a martian melt composition (QUE 94201). This composition is appropriate for this study because it is evolved, so it is pyroxene-phyric, contains abundant phosphate, which is important for storage of REE, and is very well studied. In the high pressure experiments, garnet and omphacitic pyroxene are the dominant phases. The garnet has a compositional range from Al56.9Gr+An24.6Py18.5 to Al41.5Gr+An28.1Py30.4 to Al40.3Gr+An22.6Py37.1 (where Al = almandine, Gr+An = grossular+andradite, and Py = Pyrope) and the pyroxene has jadeite and acmite components of up to 26.0 and 9.9%, respectively. The garnet is enriched in REE over omphacite; this is especially true for the HREE which likely remain sequestered in residual garnet during melting. This observation suggests that garnet may play a key role in producing the relatively flat REE patterns of enriched martian basalts. The prime substitution couple that incorporates REE into garnet is: REE3+X Site+Mg2+Y Site↔R2+X Site+Al3+Y Site, where R2+=Ca, Mn2+, Fe2+, and Mg. This is a very effective couple that accounts for both charge balance and ionic size restrictions in individual sites. The prime substitution couple that incorporates REE into omphacite is: REE3+M2 Site+Na1+M2 Site for 2 Ca2+M2 Site. This is a less effective couple than that for garnet because Na is pulled in two directions in omphacite; it must balance the Al in the M1 site in the jadeite component and it also has to provide a charge deficiency for each REE3+ cation in the M2 site. The potential for the presence of eclogite in the martian mantle, has implications for bulk trace element partitioning differences between eclogite and peridotite assemblages that are closely linked to garnet and pyroxene compositions. Further, the occurrence of eclogites may have implications for the capability of the martian mantle to produce magmas, and the major element composition of those magmas.

CO2 solubility in primitive martian basalts similar to Yamato 980459, the effect of composition on CO2 solubility of basalts, and the evolution of the martian atmosphere

To determine the influence of basalt composition on the CO2 solubility in martian lavas, we investigated experimentally a synthetic melt based on the martian meteorite Yamato 980459 (Y 980459), an olivine-phyric shergottite and a picritic rock (19 wt% MgO) thought to be a near-primary liquid derived from high-temperature (>1540 °C) partial melting of the martian mantle. Experiments were performed in a piston-cylinder apparatus at 1–2 GPa and 1600–1650 °C. CO2 contents in quenched glasses were determined using Fourier transform infrared spectroscopy (FTIR) and range from 0.45–1.26 wt%. Despite large differences in FeO* and MgO contents, the CO2 solubilities in Y 980459 are similar to that in a less primitive synthetic martian basalt based on the Humphrey rock and to a Hawaiian tholeiite. The lack of enhanced solubility in Fe2+- and Mg2+-rich melts is likely owing to the complex structural role of these cations in silicate melts, acting partly as network formers, rather than network modifiers. The small sensitivity of CO2 solubility to compositional variations among martian and tholeiitic basalts means that the experimentally determined solubilities may be applicable to a wide spectrum of martian magmatic products. Using experimentally determined CO2 solubilities of Y 980459 and Humphrey allows the calibration of the thermodynamic parameters governing dissolution of CO2 vapor as carbonate in martian basalts. This relation facilitates calculation of the CO2 dissolved in magmas derived from graphite-saturated martian basalt source regions as a function of P , T , and f O2. The hot conditions in the source of Y 980459, 1540 ± 10 °C, and 1.2 ± 0.5 GPa, are plausible for plume-related magmas forming the giant Tharsis volcanic complex, which accounts for 50% of martian igneous activity since stabilization of the primordial crust. If oxygen fugacity in the sources of hot Tharsis magmatism were equivalent to that at the iron-wustite buffer (IW) or 1 log unit above (IW+1), respectively, then the entire Tharsis event would outgas 30–300 mbars of CO2 to the martian atmosphere, which is far from the 2 bars required to stabilize an equable climate in the late Noachian and early Hesperian epochs. This mismatch could be reconciled if significant martian igneous activity derived from comparatively oxidized mantle sources (i.e., IW+2) similar to those responsible for the nakhlite meteorites.

Origin of water and mantle–crust interactions on Mars inferred from hydrogen isotopes and volatile element abundances of olivine-hosted melt inclusions of primitive shergottites

Volatile elements have influenced the differentiation and eruptive behavior of Martian magmas and played an important role in the evolution of Martian climate and near-surface environments. However, the abundances of volatiles, and in particular the amount of water in the Martian interior, are disputed. A record of volatile reservoirs is contained in primitive Martian basalts (shergottites). Olivine-hosted melt inclusions from a geochemically depleted shergottite (Yamato 980459, representing a very primitive Martian melt) possess undegassed water with a chondritic and Earth-like D/H ratio (δD≤275‰). Based on volatile measurements in these inclusions, the water content of the depleted shergottite mantle is calculated to be 15–47ppm, which is consistent with the dry mantle hypothesis. In contrast to D/H in the depleted shergottite, melt from an enriched shergottite (Larkman Nunatak 06319), which either formed by melting of an enriched mantle or by assimilation of crust, exhibits an extreme δD of ∼5000‰, indicative of a surface reservoir (e.g., the Martian atmosphere or crustal hydrosphere). These data provide strong evidence that the Martian mantle had retained the primordial low-δD component until at least the time of shergottite formation, and that young Martian basalts assimilated old Martian crust.

Physical properties of CaAl2Si2O8–CaMgSi2O6–FeO–Fe2O3 melts: Analogues for extra-terrestrial basalt

The effects of increasing quantities of iron on the viscosity, heat capacity and density of a haplobasaltic base composition (anorthite–diopside 1atm eutectic) were determined. Super-liquidus viscosity and density were measured in air using the concentric cylinder method and double-bob Archimedean method, respectively. Low-temperature viscosities were measured using the micropenetration method for the melts that could be quenched to glasses. The effect of iron oxidation state on viscosity was investigated above the liquidus under reduced fO2 and at the glass transition temperature from quenched samples of varying redox state. Iron significantly decreases the melt viscosity, especially near the glass transition and lowers the activation energy at low temperature. Density increases with addition of iron and the experimental measurements are in good agreement with predictions of existing models. The reduction of Fe3+ to Fe2+ produces a slight viscosity decrease at high temperature but affects properties near the glass transition more strongly. Thus, for iron-rich compositions, the redox state must be taken into account to obtain accurate estimates of the physical and thermodynamic properties, especially at low temperatures. As a result, the iron-bearing anorthite–diopside system approaches the viscous behaviour of terrestrial and extra-terrestrial basaltic compositions and then appears to be good analogue for basaltic systems. At magmatic temperatures, the viscosity difference between common terrestrial basalt and lunar or Martian basalt is estimated to be 0.5 to 1 order of magnitude. Although these results are consistent with inferences drawn from planetary observations on the fluidity of lunar and Martian lava flows, the crystallisation sequence of such systems will need to be investigated to improve interpretation of lava flow morphologies.

A technique for mapping Fe/Mg-rich phyllosilicates on Mars using THEMIS multispectral thermal infrared images

[1] We present a method for mapping exposures of Fe/Mg-rich phyllosilicates on the Martian surface using THEMIS daytime multispectral thermal infrared images. The primary utility of this technique comes from the fact that THEMIS coverage is near-global at 100 m/pixel, so it offers the opportunity to extend and improve upon phyllosilicate maps created using CRISM and/or OMEGA near-infrared data alone. The spectral index formulation used for this mapping consists of a sum of spectral band ratios, weighted in different proportions. An initial formulation of the index was determined using a suite of synthetic spectra containing varying proportions of a Martian basalt spectrum and a laboratory phyllosilicate spectrum. A scene-specific formulation was then optimized by exploiting the correlation between pixels the initial formulation flags in each THEMIS scene as containing phyllosilicates and those identified in overlapping CRISM data. Once the optimal formulation of the index is determined, it is applied to the entire THEMIS scene, extending the map of phyllosilicates beyond the area of overlapping CRISM coverage. The method is illustrated using 12 THEMIS scenes from the Mawrth Vallis and Nili Fossae regions. The new THEMIS-derived maps of Fe/Mg-rich phyllosilicates in these two regions have approximately double the coverage provided by CRISM data. The cross-wavelength and cross-instrument approach we use to derive phyllosilicate indices for THEMIS data could potentially be used to define spectral indices for mapping other compositions of interest on Mars that possess spectral features in more than one spectral region.

A Reduced Organic Carbon Component in Martian Basalts

The source and nature of carbon on Mars have been a subject of intense speculation. We report the results of confocal Raman imaging spectroscopy on 11 martian meteorites, spanning about 4.2 billion years of martian history. Ten of the meteorites contain abiotic macromolecular carbon (MMC) phases detected in association with small oxide grains included within high-temperature minerals. Polycyclic aromatic hydrocarbons were detected along with MMC phases in Dar al Gani 476. The association of organic carbon within magmatic minerals indicates that martian magmas favored precipitation of reduced carbon species during crystallization. The ubiquitous distribution of abiotic organic carbon in martian igneous rocks is important for understanding the martian carbon cycle and has implications for future missions to detect possible past martian life.

Synthesis of a spinifex‐textured basalt as an analog to Gusev crater basalts, Mars

Abstract– Analyses by the Mars Exploration Rover (MER), Spirit, of Martian basalts from Gusev crater show that they are chemically very different from terrestrial basalts, being characterized in particular by high Mg‐ and Fe‐contents. To provide suitable analog basalts for the International Space Analogue Rockstore (ISAR), a collection of analog rocks and minerals for preparing in situ space missions, especially, the upcoming Mars mission MSL‐2011 and the future international Mars‐2018 mission, it is necessary to synthesize Martian basalts. The aim of this study was therefore to synthesize Martian basalt analogs to the Gusev crater basalts, based on the geochemical data from the MER rover Spirit. We present the results of two experiments, one producing a quench‐cooled basalt (&lt;1 h) and one producing a more slowly cooled basalt (1 day). Pyroxene and olivine textures produced in the more slowly cooled basalt were surprisingly similar to spinifex textures in komatiites, a volcanic rock type very common on the early Earth. These kinds of ultramafic rocks and their associated alteration products may have important astrobiological implications when associated with aqueous environments. Such rocks could provide habitats for chemolithotrophic microorganisms, while the glass and phyllosilicate derivatives can fix organic compounds.

Secondary sulfate mineralization and basaltic chemistry of craters of the Moon National Monument, Idaho: Potential martian analog

Craters of the Moon National Monument (COM) basalts offer a reasonable analog to martian basalts, as they have elevated iron concentrations compared to traditional terrestrial analogs. Although secondary sulfate minerals on the evaporitic regions of Mars consist primarily of Mg-, Ca-, and Fe-bearing sulfate minerals, recent orbiter spectroscopic data have suggested Na-sulfate minerals may be present. Secondary minerals in the basaltic caves of COM in southern Idaho are white, efflorescent deposits in small cavities along the cave walls and ceilings and localized mounds on the cave floors. These deposits were examined using X-ray powder diffraction (XRD), X-ray fluorescence spectrometry (XRF), Fourier transform infrared spectrometry (FTIR), and laser desorption Fourier transform ion cyclotron mass spectrometry (LD-FTICRMS). The secondary mineral assemblages were dominated by Na-sulfate minerals (thenardite, mirabilite) with a small fraction of the deposits containing minor concentrations of Na-carbonate minerals. Based on thermodynamic modeling results, formation of the deposits was attributed to leaching of basalt minerals by meteoritic water followed by evaporation of solutions. Such deposits could form under similar conditions in basaltic caves on Mars, making caves an excellent target for astrobiological investigations.

CO 2 solubility in Martian basalts and Martian atmospheric evolution

To understand possible volcanogenic fluxes of CO 2 to the Martian atmosphere, we investigated experimentally carbonate solubility in a synthetic melt based on the Adirondack-class Humphrey basalt at 1–2.5 GPa and 1400–1625 °C. Starting materials included both oxidized and reduced compositions, allowing a test of the effect of iron oxidation state on CO 2 solubility. CO 2 contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and Fe 3+/Fe T was measured by Mössbauer spectroscopy. The CO 2 contents of glasses show no dependence on Fe 3+/Fe T and range from 0.34 to 2.12 wt.%. For Humphrey basalt, analysis of glasses with gravimetrically-determined CO 2 contents allowed calibration of an integrated molar absorptivity of 81,500 ± 1500 L mol −1 cm −2 for the integrated area under the carbonate doublet at 1430 and 1520 cm −1. The experimentally determined CO 2 solubilities allow calibration of the thermodynamic parameters governing dissolution of CO 2 vapor as carbonate in silicate melt, K II , ( Stolper and Holloway, 1988) as follows: ln K II 0 = - 15.42 ± 0.20 , Δ V 0 = 20.85 ± 0.91 cm 3 mol −1, and Δ H 0 = −17.96 ± 10.2 kJ mol −1. This relation, combined with the known thermodynamics of graphite oxidation, facilitates calculation of the CO 2 dissolved in magmas derived from graphite-saturated Martian basalt source regions as a function of P, T, and f O 2 . For the source region for Humphrey, constrained by phase equilibria to be near 1350 °C and 1.2 GPa, the resulting CO 2 contents are 51 ppm at the iron–wüstite buffer (IW), and 510 ppm at one order of magnitude above IW (IW + 1). However, solubilities are expected to be greater for depolymerized partial melts similar to primitive shergottite Yamato 980459 (Y 980459). This, combined with hotter source temperatures (1540 °C and 1.2 GPa) could allow hot plume-like magmas similar to Y 980459 to dissolve 240 ppm CO 2 at IW and 0.24 wt.% of CO 2 at IW + 1. For expected magmatic fluxes over the last 4.5 Ga of Martian history, magmas similar to Humphrey would only produce 0.03 and 0.26 bars from sources at IW and IW + 1, respectively. On the other hand, more primitive magmas like Y 980459 could plausibly produce 0.12 and 1.2 bars at IW and IW + 1, respectively. Thus, if typical Martian volcanic activity was reduced and the melting conditions cool, then degassing of CO 2 to the atmosphere may not be sufficient to create greenhouse conditions required by observations of liquid surface water. However, if a significant fraction of Martian magmas derive from hot and primitive sources, as may have been true during the formation of Tharsis in the late Noachian, that are also slightly oxidized (IW + 1.2), then significant contribution of volcanogenic CO 2 to an early Martian greenhouse is plausible.

The effect of Fe on olivine H2O storage capacity: Consequences for H2O in the martian mantle

To investigate the influence of chemical composition on the behavior of H 2 O in Fe-rich nominally anhydrous minerals, and to determine the difference between H 2 O behavior in the martian and terrestrial mantles, we conducted high-pressure H 2 O storage capacity experiments employing a wide range of olivine compositions. Experiments were conducted with bulk compositions in the system FeO-MgO-SiO 2 -H 2 O with Mg no. [Mg no. = 100 × molar Mg/(Mg+Fe)] ranging between 50 and 100 at 3 GPa in a piston-cylinder and at 6 GPa in a multi-anvil apparatus. Experiments at 3 GPa were conducted at 1200 °C, with f O 2 buffered by the coexistence of Fe and FeO, and at 1300–1500 °C in unbuffered assemblies. Experiments at 6 GPa were conducted at 1200 °C without buffers. Experiments at 1200 °C produced olivine+orthopyroxene+hydrous liquid (liq), and higher T experiments produced olivine+liq. Additionally, we synthesized a suite of 7 olivine standards (Mg no. = 90) for low blank secondary ion mass spectrometry (SIMS) analysis of H in multi-anvil experiments at 3–10 GPa and 1250 °C, resulting in large (200–400 μm) homogeneous crystals with 0.037 to 0.30 wt% H 2 O. Polarized Fourier transform infrared (FTIR) measurements on randomly oriented grains from the synthesis experiments were used to determine principal axis spectra through least-squares regression, and H contents were calculated from the total absorbance in the OH stretching region. Using these olivines as calibrants for SIMS analyses, the H contents of olivines and pyroxenes from the variable Mg no. experiments were measured by counting 16 OH ions. Ignoring any matrix effects owing to variation in Mg no., H contents of olivine and pyroxene increase linearly with decreasing Mg no. At 6 GPa and 1200 °C, olivine H contents increase from 0.05 to 0.13 wt% H 2 O (8360 to 23 900 H/10 6 Si) as olivine Mg no. decreases from 100 to 68, and at 3 GPa and 1200 °C olivine H contents increase from 0.017 to 0.054 wt% (278 to 10 000 H/10 6 Si) as Mg no. decreases from 100 to 55. The partition coefficient for H between pyroxene and olivine, D H opx/ol , decreases from 1.05 at 3 GPa and 1200 °C to 0.61 at 6 GPa and 1200 °C. The storage capacity of Fe-rich olivines with compositions expected in the martian mantle is ~1.5 times greater than those in the terrestrial mantle, suggesting that the geochemical behavior of H 2 O in the mantles of the two planets are quite similar. If 50% of the K 2 O on Mars remains in its mantle (Taylor et al. 2006), then a similar or greater proportion of the H 2 O is also in the mantle. Given accretionary models of the total martian H 2 O budget (Lunine et al. 2003), this suggests concentrations of 100–500 ppm H 2 O in the martian mantle and 0.1–1.9 wt% H 2 O in primary martian basalts.