This review is intended to summarize the current observations of reduced carbon in Martian meteorites, differentiating between terrestrial contamination and carbon that is indigenous to Mars. Indeed, the identification of Martian organic matter is among the highest priority targets for robotic spacecraft missions in the next decade, including the Mars Science Laboratory and Mars 2020. Organic carbon compounds are essential building blocks of terrestrial life, so the occurrence and origin (biotic or abiotic) of organic compounds on Mars is of great significance; however, not all forms of reduced carbon are conducive to biological systems. This paper discusses the significance of reduced organic carbon (including methane) in Martian geological and astrobiological systems. Specifically, it summarizes current thinking on the nature, sources, and sinks of Martian organic carbon, a key component to Martian habitability. Based on this compilation, reduced organic carbon on Mars, including detections of methane in the Martian atmosphere, is best described through a combination of abiotic organic synthesis on Mars and infall of extraterrestrial carbonaceous material. Although conclusive signs of Martian life have yet to be revealed, we have developed a strategy for life detection on Mars that can be utilized in future life-detection studies.

The timing and mode of deposition for Martian regolith breccia Northwest Africa (NWA) 7034 were determined by combining petrography, shape analysis, and thermochronology. NWA 7034 is composed of igneous, impact, and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact/volcanic glass. The brecciated clasts are likely lithified portions of Martian regolith with some evidence of past hydrothermal activity. Represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4.4Ga. One ancient zircon was hosted by an alkali-rich basalt clast, confirming that alkalic volcanism occurred on Mars very early. NWA 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water. The clast size distribution is similar to terrestrial pyroclastic deposits. We infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption(s) and/or impact event(s), although the ancient ages of igneous components favor mobilization by impact(s). Despite ancient components, the breccia has undergone a single pervasive thermal event at 500-800 degrees C, evident by groundmass texture and concordance of similar to 1.5Ga dates for bulk rock K-Ar, U-Pb in apatite, and U-Pb in metamict zircons. The 1.5Ga age is likely a thermal event that coincides with rainout/breccia lithification. We infer that the episodic process of regolith lithification dominated sedimentary processes during the Amazonian Epoch. The absence of pre-Amazonian high-temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification.

Mixed-habit (octahedral+cuboid) diamonds from the Marange alluvial deposits in the eastern Zimbabwe craton have high nitrogen and hydrogen contents that provide an opportunity to evaluate diamond growth mechanisms and C-N-H-O bearing fluids in the lithospheric keel. Light grey cuboid sectors with hydrogen-containing defects trap abundant dispersed CH4 inclusions (Raman peaks at 2917 cm(-1)) associated with graphite (Raman peaks at 1580 cm(-1)). Clear octahedral sectors are richer in nitrogen and free of any such inclusions. Core to rim co-variations of delta C-13-delta N-15 and N content can be explained by a mixing trend between earlier fluids that are CH4-rich and later fluids that are more CO3- or CO2-rich. Marange diamonds have limited overall delta C-13 variation, but do show fractionation during growth towards higher delta C-13 values. This trend can be explained by diamond precipitation from mixed CH4 and CO2 fluids, where isotopic fractionation occurs as the amount of fluid wanes. Calculated delta N-15 values for diamond source fluids evolving in this manner are between +2.3 and +6.4 parts per thousand. These N isotopic compositions require CH4-rich and CO3-/CO2-rich 'end-member' fluids to have a recycled metasedimentary component perhaps introduced with subduction of eclogite. (C) 2016 Elsevier B.V. All rights reserved.

Ureilite meteorites are partially melted asteroidal-peridotite residues, or more rarely, cumulates that can contain greater than three weight percent carbon. Here we describe an exceptional C-rich lithology, composed of 34 modal % large (up to 0.8 mm long) crystalline graphite grains, in the Antarctic ureilite meteorite Miller Range (MIL) 091004. This C-rich lithology is embedded within a silicate region composed dominantly of granular olivine with lesser quantities of low-Ca pyroxene, and minor FeNi metal, high-Ca pyroxene, spinel, schreibersite and troilite. Petrological evidence indicates that the graphite was added after formation of the silicate region and melt depletion. Associated with graphite is localized reduction of host olivine (Fo(88-89)) to nearly pure forsterite (Fo(99)), which is associated with FeNi metal grains containing up to 11 wt.% Si. The main silicate region is typical of ureilite composition, with highly siderophile element (HSE) abundances similar to 0.3 x chondrite, Os-187/Os-188 of 0.1260-0.1262 and Delta O-17 of -0.81 +/- 0.16 parts per thousand. Mineral trace-element analyses reveal that the rare earth elements (REE) and the HSE are controlled by pyroxene and FeNi metal phases in the meteorite, respectively. Modeling of bulk-rock REE and HSE abundances indicates that the main silicate region experienced similar to 6% silicate and >50% sulfide melt extraction, which is at the lower end of partial melt removal estimated for ureilites. Miller Range 091004 demonstrates heterogeneous distribution of carbon at centimeter scales and a limited range in Mg/(Mg + Fe) compositions of silicate grain cores, despite significant quantities of carbon. These observations demonstrate that silicate rim reduction was a rapid disequilibrium process, and came after silicate and sulfide melt removal in MIL 091004. The petrography and mineral chemistry of MIL 091004 is permissive of the graphite representing late-stage C-rich melt that pervaded silicates, or carbon that acted as a lubricant during anatexis and impact disruption in the parent body. Positive correlation of Pt/Os ratios with olivine core compositions, but a wide range of oxygen isotope compositions, indicates that ureilites formed from a compositionally heterogeneous parent body that experienced variable sulfide and metal melt-loss that is most pronounced in relatively oxidized ureilites with Delta O-17 between similar to 1.5 and similar to 0 parts per thousand. (C) 2016 Elsevier Ltd. All rights reserved.