Search

The Martian surface is covered by a fine-layer of oxidized dust responsible for its red color in the visible spectral range ( Bibring et al., 2006; Morris et al., 2006 ). In the near infrared, the strongest spectral feature is located between 2.6 and 3.6 μm and is ubiquitously observed on the planet ( Jouglet et al., 2007; Milliken et al., 2007 ). Although this absorption has been studied for many decades, its exact attribution and its geological and climatic implications remain debated. We present new lines of evidence from laboratory experiments, orbital and landed missions data, and characterization of the unique Martian meteorite NWA 7533, all converging toward the prominent role of hydroxylated ferric minerals. Martian breccias (so-called “Black Beauty” meteorite NWA7034 and its paired stones NWA7533 and NWA 7455) are unique pieces of the Martian surface that display abundant evidence of aqueous alteration that occurred on their parent planet ( Agee et al., 2013 ). These dark stones are also unique in the fact that they arose from a near surface level in the Noachian southern hemisphere ( Humayun et al., 2013 ). We used IR spectroscopy, Fe-XANES and petrography to identify the mineral hosts of hydrogen in NWA 7533 and compare them with observations of the Martian surface and results of laboratory experiments. The spectrum of NWA 7533 does not show mafic mineral absorptions, making its definite identification difficult through NIR remote sensing mapping. However, its spectra are virtually consistent with a large fraction of the Martian highlands. Abundant NWA 7034/7533 (and paired samples) lithologies might abound on Mars and might play a role in the dust production mechanism. [ABSTRACT FROM AUTHOR]

Widespread igneous activity, showing striking mineralogical, petrographical and chemical commonalities with terrestrial komatiites and ferropicrites, intensely affected, reshaped and buried the primary Martian crust. This study evaluates whether the igneous activity on Mars may have led to the formation of orthomagmatic Ni–Cu±(PGE) sulphide mineralisation similar to that associated with terrestrial komatiites and ferropicrites. Particular focus is laid on two different components of the Martian Ni–Cu±(PGE) sulphide mineral system: 1) the potential metal and sulphur fertility of mantle sources and derived melts, and 2) the physicochemical processes that enable sulphide supersaturation and batch segregation of metal-rich sulphide liquids. We show that potentially metal-rich Martian mantle melts likely reach sulphide saturation within 30 wt.% crystal fractionation. This value is comparable to that calculated for the mineralised ferropicrites at Pechenga, Russia. However, the majority of known world-class Ni–Cu±(PGE) sulphide deposits associated with terrestrial komatiites and ferropicrites originated due to the assimilation of crustal sulphur-rich substrate, thus promoting the attainment of sulphide supersaturation and batch segregation of metal-rich sulphide liquids during early stages of magma evolution. Given the high sulphur inventory of Martian crustal reservoirs, ranging from sulphide bearing magmatic rocks to sulphate-rich soils, regoliths and sedimentary deposits, it is likely that mantle-derived melts assimilated significant amounts of crustal sulphur during ascent and emplacement. It is proposed that channelled lava flows, which potentially emplaced and incised into sulphur-rich crustal lithologies, may have led to the formation of orthomagmatic Ni–Cu±(PGE) sulphide mineralisation on Mars. [ABSTRACT FROM AUTHOR]

Abstract: The formation of diamond through metasomatic events, from volatiles-enriched fluids/melts brought into pre-existing mantle rocks, has been suggested from a series of independent studies. However, the link between these hypothetical volatile-rich fluids and deep-seated mineral inclusions (mostly silicate and sulphides) entrapped in diamonds remains unclear, yet the relationship between these two species may provide the key to our understanding of diamond crystallization. In order to address the relationship between the origin and formation of diamonds and their mineral inclusions, we carried out the first coupled stable isotopic study (δ 13C, δ 15N and multiple S-isotopes as δ 34S, δ 33S, Δ33S) of diamonds containing sulphide inclusions, using samples from the Jwaneng kimberlite as our model system. Sulphides extracted from the present collection of 55 diamonds belong to either eclogitic (E-type, 52 diamonds) or peridotitic (P-type, 3 diamonds) paragenetic suites, as attested by their Cr and Ni content (Cr<0.03 wt.% for E-types and Cr>0.18 wt.% and Ni>14 wt.% for P-types). Sulphur isotopic compositions have been measured in-situ by multicollector secondary ion mass spectrometry and reveal that peridotitic sulphide inclusions in diamonds (n =4) lie on the terrestrial fractionation line whereas Mass Independent sulphur isotope Fractionations (MIF) are preserved inside eclogitic sulphides inclusions (−0.5‰<Δ33S<+0.9‰, n =33). Such large MIF values, extended here to the negative values recorded in Archean sediments, are thought to be produced through photochemical reactions in the Archean atmosphere, which implies that sulphides inclusions contain an Archean sedimentary sulphur component that was transferred to the Earth''s mantle. In contrast to this isotopic dichotomy between E-type and P-type sulphides, the geochemical characteristics of their host diamonds (δ 13C, δ 15N, N-content, N-speciation) are largely homogeneous throughout the population, and differ from silicate-bearing diamonds previously studied in the same locality. Sulphide-bearing diamonds appear to define a distinct population, which might reflect a different crystallization process characterized by carbon and nitrogen isotopic compositions falling within the range of unfractionated mantle values. The lack of a distinct isotopic signature of recycled sedimentary nitrogen and carbon in sulphide-bearing diamonds leads us to infer that sulphide-bearing Jwaneng diamonds were not derived from the same chemical reservoir as their inclusions. One possible formation mechanism that is compatible with our observations is diamond crystallization from a mantle-derived carbon-bearing fluid associated with pre-existing sedimentary sulphide minerals. This model has important implications for the interpretation of diamond ages obtained from sulphide inclusions using Re–Os systematics. Here we suggest that the incorporation of a significant amount of mantle-related sulphur during a diamond growth event into the original sulphide could lead to a re-equilibration of the Re–Os isotope composition in any inclusion, and accordingly, account for some scatter on Re–Os isochrons. [Copyright &y& Elsevier]

Some mantle-derived Kerguelen harzburgite and dunite xenoliths have bulk-rock and mineral trace element compositions that provide evidence of carbonatitic metasomatism similar to that described in some continental and other oceanic settings. Rare xenoliths contain carbonates that are highly enriched in rare earth elements (REE), interpreted to be quenched, evolved carbonatitic melts. One amphibole-bearing dunite mantle wall-rock containing carbonates in small interstitial pockets (100–500 μm across) has been studied in detail. Mg-bearing calcite (MgO: <1.4 wt.%, XCa 0.96) with unusually high REE abundances and strong light REE (LREE) enrichment occurs in the pockets and is sometimes associated with euhedral carbonates (dolomite and Mg-free calcite), mafic silicate glass (low in alkalis) and with fine grains of spinel, sulfides and magnesio–wüstite concentrated near the boundaries of the carbonate pockets. The unusual metasomatic mineral assemblage, together with the microstructural features and chemical composition of carbonates (with trace element contents similar to those of common carbonatite magmas), suggests that the pockets of Mg-bearing calcite represent quenched carbonate melts rather than crystal cumulates from carbonate-rich melts. The associated mafic silicate glass could represent the immiscible silicate fraction of an evolved fluid produced by the dissolution–percolation of the original carbonate melt in the dunitic matrix and subsequent unmixing as the xenoliths ascended to the surface. Clinopyroxene formed during the percolation event and is therefore inferred to be in chemical equilibrium with the carbonate melt. This allowed calculation of clinopyroxene/carbonate melt partition coefficients for a large set of trace elements at relatively low pressure (1 GPa). As a result, a significant pressure control on REE partitioning between carbonate melt and silicate minerals was observed. This study provides further evidence for the occurrence of carbonate melts and demonstrates that these melts can be preserved in hot oceanic uppermost mantle. [Copyright &y& Elsevier]

North West Africa (NWA 817) is a single stone of 104 g found in the Sahara (Morocco) by meteorite hunters in November 2000. The meteorite is an unbrecciated, medium-grained olivine-bearing clinopyroxenite with a cumulate texture. It consists of zoned euhedral subcalcic augite (Wo42En38–22Fs20–36), olivine spanning a wide range of compositions (from Fa56 in the core to Fa86) with rare magmatic inclusions and an intercumulus mesostasis made of Fe-bearing albitic plagioclase, Si-rich glass, Ti-magnetite with unusual skeletal growth morphologies containing ilmenite exsolutions, acicular pyroxene, olivine and cristobalite. Trace minerals are sulfide droplets and Cl-apatite. Mineral modes (in vol%) are augite 69%, olivine 10%, mesostasis 20% and Fe–Ti oxides 1%. Pervasive alteration produced a reddish clay mineral (hydrous ferrous silicate) in both olivine crystals and the mesostasis. The major element composition of NWA 817 is very similar to that of the other nakhlites: high FeO, MgO and CaO concentrations reflect the abundance of cumulus augite and olivine. Key element ratios such as FeO/MnO (=37), Na/Al (=0.40), K/La (=449), Ga/Al (=3.9×10−4) and oxygen isotopic composition (Δ17O=+0.37‰) are clear evidence for a Martian origin. The incompatible trace element pattern as in Nakhla displays a strong light rare earth element enrichment relative to chondrite (Lan/Ybn=4.89). However, when compared to the other nakhlites, NWA 817 has specific features: (1) a higher modal proportion of mesostasis; (2) quench textures of Ti-magnetite and Fe-rich clinopyroxene; (3) more Mg-rich olivine core compositions whereas the augite core composition is identical for all nakhlites; (4) a stronger Fe enrichment toward crystal rims of these cumulus minerals. The intercumulus minerals (Ti-magnetite with skeletal growth morphology, acicular chains of clinopyroxene and Fe3+-rich feldspar) indicate rapid crystallization in response to a high degree of undercooling at the end of the sample story. [Copyright &y& Elsevier]