Constance M. Bertka

Mineralogy of the Martian interior up to core‐mantle boundary pressures

In order to determine the mineralogy of the Martian interior along a high-temperature areotherm, multianvil experiments have been performed with a model Martian mantle composition up to 23.5 GPa. The Dreibus and Wanke [19851 Martian mantle composition yields an upper mantle that consists of olivine + clinopyroxene + orthopyroxene + garnet at pressures up to 9 GPa. Above 9 GPa, orthopyroxene is no longer present. The transition zone is marked by the appearance of γ spinel at 13.5 GPa. Up to 15 GPa, clinopyroxene and majorite coexists with β phase and/or γ spinel. By 17 GPa, clinopyroxene is entirely replaced by majorite and the modal abundance of γ spinel increases at the expense of β phase. The dominant assemblage throughout most of the transition zone is γ spinel + majorite. Two experiments completed in the perovskite stability field indicate that the lower mantle consists of Mg-Fe silicate-perovskite, magnesiowustite, and majorite. CaSiO 3 -perovskite is not present in these experiments. Both the presence of a Martian lower mantle, i.e., an Mg-Fe silicate-perovskite bearing zone, and the phase assemblage stable in the Martian lower mantle are very sensitive to the temperature profile of the interior. A low-temperature profile may stabilize stishovite in the lower mantle or it may lead to the absence of the lower mantle because of the higher transition pressure required for forming perovskite at lower temperatures. Regardless of the temperature profile assumed, the Martian upper mantle and transition zone will account for a larger proportion of the planets interior than is the case for the Earths interior because of the smaller size of Mars.

Structure and Density of FeS at High Pressure and High Temperature and the Internal Structure of Mars

In situ x-ray diffraction measurements revealed that FeS, a possible core material for the terrestrial planets, transforms to a hexagonal NiAs superstructure with axial ratio (c/a) close to the ideal close-packing value of 1.63 at high pressure and high temperature. The high-pressure-temperature phase has shorter Fe-Fe distances than the low-pressure phase. Significant shortening of the Fe-Fe distance would lead to metallization of FeS, resulting in fundamental changes in physical properties of FeS at high pressure and temperature. Calculations using the density of the high-pressure-temperature FeS phase indicate that the martian core-mantle boundary occurs within the silicate perovskite stability field.

Density profile of an SNC model Martian interior and the moment-of-inertia factor of Mars

The density profile of an SNC model Martian interior is calculated from the results of previous experimental work that determined the modal mineralogy of the model mantle up to Martian core-mantle boundary pressures. The moment-of-inertia factor of Mars is calculated as a function of core composition and crustal thickness using the SNC model mantle density profile as a constraint. Two sets of calculations are performed. In the first, the bulk composition of the planet is not constrained to a C1 chondrite composition. Assuming a Martian crust density of 2.7–3.0 g/cm3, a crust thickness of 25–150 km, and the core composition proposed by Dreibus and Wanke, Fe 14 wt% S, the calculated moment-of-inertia factor ranges from 0.367 to 0.357. These models include a perovskite-bearing zone in the Martian interior. Considering core compositions ranging from pure Fe to pure FeS changes the moment-of-inertia factor by only ±0.001, but at higher S abundances, core size increases, such that the depth of the core-mantle boundary is shallower than the depth of perovskite stability. In the second set of calculations, the bulk composition of the planet is constrained to a C1 composition requiring a crust thickness of 180–320 km, assuming a crust density of 2.7–3.0 g/cm3. The calculated moment-of-inertia factor is 0.354 and a perovskite-bearing layer is absent from the Martian interior. As it is unlikely that the thickness of the Martian crust is greater than 100 km, the bulk composition of Mars cannot be constrained to a C1 chondrite composition as proposed by the Dreibus and Wanke model (G. Dreibus, H. Wanke, Meteoritics 20 (1985) 367–382). In order to determine if the Martian mantle is more iron-rich than the Earths mantle, we may need not only an improved estimate of the moment-of-inertia factor of Mars, but also tighter constraints on Martian crust thickness and density. The absolute degree of iron-enrichment, however, cannot be specified without also knowing the size of the Martian core. A moment-of-inertia factor of less than 0.342 is not geochemically feasible, because it requires that the mantle of Mars contains no iron.

Anhydrous partial melting of an iron-rich mantle I: subsolidus phase assemblages and partial melting phase relations at 10 to 30 kbar

Anhydrous partial melting experiments, at 10 to 30 kbar from solidus to near liquidus temperature, have been performed on an iron-rich martian mantle composition, DW. The DW subsolidus assemblage from ≤5 kbar to at least 24 kbar is a spinel lherzolite. At 25 kbar garnet is stable at the solidus along with spinel. The clinopyroxene stable on the DW solidus at and above 10 kbar is a pigeonitic clinopyroxene. Pigeonitic clinopyroxene is the first phase to melt out of the spinel lherzolite assemblage at less than 20°C above the solidus. Spinel melts out of the assemblage about 50°C above the solidus followed by a 150° to 200°C temperature interval where melts are in equilibrium with orthopyroxene and olivine. The temperature interval over which pigeonitic clinopyroxene melts out of an iron-rich spinel lherzolite assemblage is smaller than the temperature interval over which augite melts out of an iron-poor spinel lherzolite assemblage. The dominant solidus assemblage in the source regions of the Tharsis plateau, and for a large percentage of the martian mantle, is a spinel lherzolite.

Anhydrous partial melting of an iron-rich mantle II: primary melt compositions at 15 kbar

Primary melt and coexisting mineral compositions, at increasing degrees of partial melting at 15 kbar, were determined for an iron-rich martian mantle composition, DW. The composition of primary melts near the solidus was determined with basalt-peridotite sandwich experiments. In order to evaluate the approach of the liquids to equilibrium with a DW mantle assemblage, experiments were also performed to establish the liquidus mineralogy of the primary melts. Primary melt compositions produced from an iron-rich mantle are more picritic than those produced from an iron-poor mantle. By increasing the iron content of a model mantle composition (decreasing the mg#, where mg# = atomic [Mg/(Mg+Fe2+)*100]), picritic and komatiitic magmas result at lower percentages of melting and at temperatures closer to the solidus than in an iron-poor mantle. Terrestrial iron-rich primitive volcanics may be the partial melting products of iron-rich, mg# ≥80, source regions. The DW partial melting results support the conclusion of previous authors that the parent magmas of the SNC (shergottites, nakhlites, chassignites) meteorites were derived from a source region that had been previously depleted in an aluminous phase.

The role of H 2 O in Martian magmatic systems

Abstract We have estimated the water content of Mars’ interior by using analyzed water contents of kaersutite inclusions from shergottites nakhlites chassignites (SNC) meteorites in conjunction with an experimentally-derived crystal-chemical model of kaersutite amphibole. This model predicts quantitatively the relationships between iron oxidation and hydrogen deficiency in the kaersutite. The H2O content of the magma from which the kaersutites in SNC meteorites could have crystallized is in the 100-1000 ppm range. That H2O content leads to an estimated water content of 1-35 ppm for a Martian mantle that could have been the source rock for such magmas.

Pigeonite at Solidus Temperatures' Implications for Partial Melting

The results of partial melting experiments with iron-rich and iron-poor mantle compositions have been used to determine the pressure at which pigeonite is stable at mantle solidus temperatures. Pigeonite is stable at the solidus at lower pressure in an iron-rich mantle (Mg# 75, ~1.0 GPa, Mg# = atomic (Mg/(Mg+Fe))*100), than in an iron-poor mantle (Mg# 90, >2.0 GPa). The stabilization of pigeonite in a peridotite mantle will result in the absence or low modal abundance of orthopyroxene in the subsolidus assemblage. The melting reaction which takes place at the solidus of a pigeonite-bearing mantle assemblage produces orthopyroxene. Partial melting increases the mode of orthopyroxene in the residue. Orthopyroxene may be absent from igneous eclogites because they are the crystallization product of melts from a pigeonite-bearing The high modal abundance of orthopyroxene in South African cratonic peridotites may be evidence of their origin as the residue of a high pressure, >2.0 GPa, partial melting event.

Constraints on the mineralogy of an iron-rich Martian mantle from high-pressure experiments

The high-pressure mineralogy of a model Martian mantle composition has been determined experimentally up to 24 GPa. Experiments have been performed in the anhydrous multi-component CaOMgOFeOAl2O3SiO2Na2O system with a mantle composition derived from the SNC meteorites. The phase transitions which occur along a model liquid core marstherm have been determined. The importance of magnesiowustite and stishovite in the Martian mantle, two phases generally overlooked in models of Martian mantle mineralogy are also discussed, Magnesiowustite is found to be present in the transition zone of the model mantle. The presence of magnesiowustite is attributed to the shift in olivine, β-phase, γ-spinel and clinopyroxene or majorite towards more Mg-rich compositions as pressure increases. At lower mantle pressures an assemblage of stishovite and magnesiowustite with perovskite and majorite is shown to be consistent with the high iron content of the Martian mantle. These results may have important implications mantle. These results may have important implications for seismic discontinuities in the Martian interior.