Graphite in the martian meteorite Allan Hills 84001

Abstract

We use confocal Raman imaging spectroscopy and transmission electron microscopy to study the martian meteorite Allan Hills (ALH) 84001, reported to contain mineral assemblages within carbonate globules (carbonate + magnetite), interpreted as potential relict signatures of ancient martian biota. Models for an abiologic origin for these assemblages required the presence of graphite, and this study is the first report of graphite within ALH 84001. The graphite occurs as hollow spheres (nano-onions), filaments, and highly crystalline particles in intimate association with magnetite in the carbonate globules. In addition to supporting an abiologic origin for the carbonate globule assemblages in ALH 84001, this work proves that there is an inventory of reduced-carbon phases on Mars that has not yet been thoroughly investigated.

Evidence for perchlorates and the origin of chlorinated hydrocarbons detected by SAM at the Rocknest aeolian deposit in Gale Crater

“…Indigenous reduced organic carbon, including polycyclic aromatic hydrocarbons (PAHs), have also recently been detected in 12 SNC meteorites and confirm previous observations of isotopically light carbon that has been produced by magmatic processes on Mars [ Grady et al ., ; Steele et al ., ]. Graphite and a refractory macromolecular carbon phase containing PAHs have also been confirmed to be indigenous to the carbonate globules in the Martian meteorite Allan Hills (ALH) 84001 [ Steele et al ., ]. The recent measurements contradict previous arguments that the PAHs in the ALH 84001 carbonates are terrestrial in origin [ Becker et al ., ; Stephan et al ., ] and demonstrate the survival of hydrothermally produced organic compounds over 3.9 Ga on Mars [ Jull et al ., ; McKay et al ., ; Steele et al ., ].…”

Critically testing olivine‐hosted putative martian biosignatures in the Yamato 000593 meteorite—Geobiological implications

“…Indigenous organic carbon has been found in several martian meteorites and a variety of sources have been proposed, including: (i) high‐pressure impact generation (Steele et al, ); (ii) primary igneous, that is, magmatic processes (e.g., Steele, McCubbin, Fries, Golden et al, Steele, McCubbin, Fries, Kater 2012); (iii) electrochemical reduction (e.g., Steele et al, ); (iv) dead martian organisms (McKay et al, ); (v) reactions in rapidly cooling magmatic and, or impact‐generated gases (Zolotov & Shock, ); (vi) subsurface hydrothermal fluids (Lin et al, ; Steele et al, ); and (vii) exogenous delivery to Mars (e.g., Sephton et al, ). On the basis of the distribution and ultrastructure of the organics in Y00593 (see discussion of the TEM‐EELS data in the previous section), we have excluded the first three potential sources.…”

Stable isotope analysis of carbon and nitrogen in angrites

“…However, this process requires an element of shock heating to remobilize the carbonates (Steele et al. ) of which there is little evidence within the angrites. In addition, evidence of neither the siderite precursor nor the pure magnetite product (Brearley ) is present in the angrites.…”

“…An alternate hypothesis is that of nanoscale variation in the oxygen fugacity caused by locally increasing the pressure of CO 2 during carbonate formation (Steele et al. ). This seems the more likely of the two based on the mineralogy found within the angrites and, if true, should result in close coincidence of graphite with carbonate globules.…”

“…It has been proposed that the breakdown of carbonate minerals such as siderite at moderate temperatures (approximately 500°C) and angrite relevant oxygen fugacities can yield graphitic material (French 1971;Brearley 2003). However, this process requires an element of shock heating to remobilize the carbonates (Steele et al 2012) of which there is little evidence within the angrites. In addition, evidence of neither the siderite precursor nor the pure magnetite product (Brearley 2003) is present in the angrites.…”