64th Annual Meteoritical Society Meeting (2001) 5146.pdf

TRANSMISSION ELECTRON MICROSCOPY OF CARBONATES AND ASSOCIATED MINERALS IN ALH84001: IMPACT-INDUCED DEFORMATION AND CARBONATE DECOMPOSITION. D.J. Barber1 and E.R.D. Scott2. 1 School of Chemical and Life Sciences, University of Greenwich, London SE18 6PF & Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, U.K. ([email protected]), 2 Hawai'i Institute of Geophysics and Planetology, University of Hawai'i at Manoa, Honolulu, HI 96822, U.S.A.

Introduction: We have carried out the first comprehensive transmission electron microscopy study of carbonates and associated minerals in a thin section of the martian meteorite ALH84001 after first mapping their occurrences by optical and scanning electron microscopy. Massive (globular-type),interstitial and fracture-filling carbonates in typical locations have been characterized by imaging, X-ray microanalysis and electron diffraction. The relationships of the carbonates to the orthopyroxene rock-mass, fine-grained magnetite, glasses and iron sulfides have been investigated. Where possible we have characterized the mineral morphologies and microstructures. Here we focus on three previously undescribed or incompletely characterised features. Decomposition of Magnesian Carbonate to Periclase: Magnesite is partially decomposed to periclase, MgO, which occurs as ~1µm-sized patches of fine-grained oxide (grainsize ~3nm), and individual crystals (typically 30-50nm dia.) either enclosed within, or growing into, tiny voids in the carbonate. In magnesite in massive carbonate, the patches of periclase are adjacent to fractures and open porosity, where the effects of incipient lattice breakdown are prevalent and most easily recognised. The nature and locations of the periclase (analogous to the formation of lime when calcite loses CO2) are clear signs of in-situ formation during decomposition of the solid carbonate. These findings reinforce the conclusion that the adjacent more sideritic carbonates and ankerite have also undergone impact-induced decomposition to form magnetite. Decomposition of Ferroan Carbonate to Magnetite: In addition to the magnetite-rich rims on carbonate grains, which are claimed to be partly biogenic [1], magnetite occurs in fractures, in small internal (often faceted) voids in Fe-rich carbonate [2] and within the solid carbonate. The nucleation and growth of magnetite crystals in carbonate fractures and voids show that Fe diffused on sub-micrometer scales during magnetite formation. The rim-type agglomerated magnetite, which also occurs in interstices in the orthopyroxene, is usually intermixed with iron sulfide, much of which appears to be amorphous. We infer that these magnetite crystals were not washed into place by an aqueous solution that dissolved carbonate to form voids [3], but that both the magnetite crystals and the voids are direct results of carbonate decomposition caused by an impact [4,5]. Orthopyroxene Glass and Shocked Orthopyroxene: Microstructures in the orthopyroxene rock-mass show that it was heavily deformed and that it is unrecovered. Fracture-filling and interstitial carbonates are usually intimately associated with glass. The largest grains of glass have plagioclase composition but stringers of glass in fracture-filling carbonate usually have the orthopyroxene composition. In the contiguous orthopyroxene we have found various microstructures indicating shock pressures of >30 GPa including lamellae of diaplectic glass separating small blocks of the silicate, some of which has transformed to clinopyroxene. The distribution of stringers of orthopyroxene glass in carbonate indicates that fracture-filling carbonates crystallized from a hot fluid that contained shock-formed silicate melt inclusions, which had been acquired during injection into orthopyroxene. Conclusions: Our on-going studies suggest that fracture-filling carbonates crystallized as a result of an impact that subjected ALH84001 to a >30GPa shock that deformed and partly transformed orthopyroxene and that carbonates show incipient decomposition, probably induced by the effects of the same impact. Biogenic origins for the majority of magnetite grains are highly improbable, as Bradley et al. [6] argued. References: [1] Thomas-Keprta K.L. et al. (2000) GCA, 64, 4049-4081. [2] Blake D. et al. (1998) LPS XXIX, 1347-1348. [3] McKay D.S. et al. (1996) Science, 273, 924-930. [4] Scott E.R.D. et al. (1997) Nature, 387, 377-379. [5] Brearley A.J. (1998) LPS XXIX, 1451-1452. [6] Bradley J.P. et al.(1998) MAPS, 33, 765-773.