Transmission electron microscopy of minerals in the martian meteorite Allan Hills 84001

Abstract

Abstract-We have studied carbonate and associated oxides and glasses in a demountable section of Allan Hills 84001 (ALH 84001) using optical, scanning, and transmission electron microscopy (TEM) to elucidate their origins and the shock history of the rock. Massive, fracture-zone, and fracture-filling carbonates in typical locations were characterized by TEM, X-ray microanalysis, and electron diffraction in a comprehensive study that preserved textural and spatial relationships. Orthopyroxene is highly deformed, fractured, partially comminuted, and essentially unrecovered. Lamellae of diaplectic glass and other features indicate shock pressures >30 GPa. Bridging acicular crystals and foamy glass at contacts of orthopyroxene fragments indicate localized melting and vaporization of orthopyroxene. Carbonate crystals are >5 mm in size, untwinned, and very largely exhibit the R3c calcite structure. Evidence of plastic deformation is generally found mildly only in fracture-zone and fracture-filling carbonates, even adjacent to highly deformed orthopyroxene, and appears to have been caused by low-stress effects including differential shrinkage. High dislocation densities like those observed in moderately shocked calcite are absent. Carbonate contains impactderived glasses of plagioclase, silica, and orthopyroxene composition indicating brief localized impact heating. Stringers and lenses of orthopyroxene glass in fracture-filling carbonate imply flow of carbonates and crystallization during an impact. Periclase (MgO) occurs in magnesite as 30-50 nm crystals adjacent to voids and negative crystals and as ~1 mm patches of 3 nm crystals showing weak preferred orientation consistent with (111) MgO //(0001) carb , as observed in the thermal decomposition of CaCO 3 to CaO. Magnetite crystals that are epitaxially oriented at voids, negative crystals, and microfractures clearly formed in situ. Fully embedded, faceted magnetites are topotactically oriented, in general with (111) mag //(0001) carb , so that their oxygen layers are aligned. In optically opaque rims, magnetites are more irregularly shaped and, except for the smallest crystals, poorly aligned. All magnetite and periclase crystals probably formed by exsolution from slightly non-stoichiometric, CO 2 -poor carbonate following impact-induced thermal decomposition. Any magnetites that existed in the rock before shock heating could not have preserved evidence for biogenic activity.

Comprehensive imaging and Raman spectroscopy of carbonate globules from Martian meteorite ALH 84001 and a terrestrial analogue from Svalbard

“…Concomitant to the shock event, Brearley (2003) proposed that during this heating event, the meteorite was experiencing considerable thermal heterogeneity, a factor expanded upon by Treiman (2003). This possibly explains the observations of Corrigan and Harvey (2004) that high-temperature interactions between carbonate and glass were not apparent in the samples they studied, as opposed to other studies (e.g., Barber and Scott 2003). However, Brearley (2003) based his speculation on voids around magnetite whose presence was due to formation of the magnetite from siderite.…”

“…It has also been suggested that the outer rim of dual rim globules was formed by thermal decomposition of siderite during a shock event to leave magnesite and magnetite (Barber and Scott 2003;Barber et al 2001;Scott et al 2002;Scott et al 1998;Treiman 2003;Golden et al 2001). Brearley (2003) speculated that fracturing of the magnetite rim matrix boundary is evidence of a volume decrease associated with magnetite formation from siderite during a shock event.…”

Signatures in magnetites formed by (Ca,Mg,Fe)CO3 thermal decomposition: Terrestrial and extraterrestrial implications

“…In the case of magnetites of the ALH84001 Martian meteorite, our results suggest that aggregates of anhedral, fine-grained magnetites observed by Barber and Scott (2003), typically near fracture zones, are consistent with the partial decomposition of ALH84001 carbonates, since these authors show a crystallographic orientation with the surrounding carbonate similar to that observed in the present study. This thermal decomposition probably occurred at a temperature not intense enough to fully decompose the carbonate and to coarsen the magnetites to produce larger euhedral crystals.…”

Magnetism, Iron Minerals, and Life on Mars

“…Indeed, detailed discussions can be found in various recent papers, most of them concluding an inorganic origin (Barber and Scott, 2003;Brearley, 2003;Treiman, 2003;Golden et al, 2004), whereas McKay et al (2003 have brought renewed support for the biogenic interpretation. Various papers related specifically to magnetism of ALH84001 have been published (Kirschvink et al, 1997;Weiss et al, 2000Weiss et al, , 2002Weiss et al, , 2004Antretter et al, 2003;Rochette et al, 2005).…”