Olivine and Carbonate Globules in Alh84001: a Terrestrial Analog, and Implications for Water on Mars

Carbonate globules in ALH84001 are associated with small olivine grains [1] – an unexpected finding because the olivines equilibrated at high T while the carbonate is chemically zoned and unequilibrated. A possible explanation comes from a terrestrial analog on Spitsbergen (Norway) [2], where some carbonate globules grew in cavities left by aqueous dissolution of olivine. For ALH84001, the same process may have acted, with larger olivines dissolved out and smaller ones shielded inside orthopyroxene. Carbonate would have been deposited in holes where the olivine had been. Later shocks crushed remaining void space, and mobilized feldspathic glass around the carbonates. Introduction: The ALH84001 Martian meteorite is an orthopyroxenite, important for data it can provide about ancient Mars and its water. ALH84001 crystallized from basalt magma at ~4.55 Ga [3], was deformed, and had Mg-Fe-(Ca) carbonate deposited in it at ~4.0 Ga [3-5]. The carbonates formed at low temperature, 0-150°C [5]; possible environments include evaporite [6,7], hydrothermal [2], lacustrine [8], marine [9], and periglacial [10]. ALH84001 Carbonates and Olivine: ALH84001 contains chemically zoned masses of carbonate minerals in the forms of hemispherical globules (Fig. 1), pancakes, slabs, and lacework. Globules were first interpreted as replacing feldspar or feldspar glass [4,11-13]. However, their shapes are explained better as aqueous deposits in fluid-filled cavities [2,14; see 15]. Remaining open space would have been crushed closed during a shock event and filled by mobile feldspathic melts [4,5]. Pancakes and slabs were deposited in open fractures [16]. Fig. 1. ALH84001 carbonate globules. Crushed hemispheres in feldspathic glass and orthopyroxene (sect. ,142). Small grains of olivine are scattered in the abundant orthopyroxene, but only within ~100 μm of carbonate globules [1]. This association of carbonate and olivine implies that they have related origins. But the carbonate globules are strongly zoned (Fig. 1) while the olivine grains are unzoned and equilibrated with surrounding orthopyroxene at T>800°C [1,11]. How can low-temperature carbonate globules be associated with high-temperature olivine? Shearer et al. [1] suggest that cation diffusion in carbonate is slow, and that olivine could have by reaction between orthopyroxene and magnesite from the globules, MgCO3 + MgSiO3 = Mg2SiO4 + CO2, during metamorphism. However, cations in carbonate minerals diffuse rapidly enough that the ALH84001 globules could not have experienced significant times above ~450°C [17]. Studies of a terrestrial analog to ALH84001 provide a possible solution to this dilemma. Fig 2a. Basalt sample 01SV08 from Sverrefjell, Norway; thin section, plane light. Tan is basaltic glass, vesicles to bottom and right. White area, upper right to lower left, is a cavity in the shape of olivine, with some relict olivine at lower left (Fig. 2b). Black areas are bubbles filled with crud. A Terrestrial Analog: The best (and best documented) terrestrial analogs for the ALH84001 carbonate globules are in Quaternary basaltic volcanos of northern Spitsbergen Island, Norway [2]. There, chemically zoned masses of Fe-Mg-(Ca) carbonate mineral are found in pyroclastics, in pillow lavas, and in xenoliths of mantle and crustal rocks. The chemical compositions of the carbonates span a wide range, including that of ALH84001 [2]. As in ALH84001, Lunar and Planetary Science XXXVI (2005) 1107.pdf