Iron Isotope Fractionation in Zoned Carbonates from Alh84001

Introduction: The chemically and mineralogically zoned carbonates within the oldest martian meteorite, ALH84001 (a coarse grained orthopyroxenite) have been the subject of intense study [1-9] since McKay et al [3] suggested that they may contain evidence for life. Whilst these studies have revealed a convoluted history, there remains no consensus on the mechanism and temperature of carbonate formation. If we are to determine whether conditions on Mars were once favourable to life or that life may even have existed on the surface then the formation temperature of these secondary minerals must be constrained. Previous studies have focused on oxygen and carbon isotope analysis [6,9-12] which appear to confirm a low temperature origin whilst others [2,13,14] have cited evidence for rapid crystallization, lack of co-existing hydrous minerals and carbonate geo-thermometry to indicate a high temperature origin. The question remains: were the carbonates formed by low temperature evaporation/precipitation and therefore able to support life or was high temperature rapid crystallisation/alteration responsible and therefore less favourable to life existing? The aim of this study is to constrain the carbonate formation temperature from the iron isotope composition of the zoned carbonates relative to martian bulk silicate isotopes. ALH84001 bulk silicate δFe isotope value have been reported as 0.027±0.051 by Poitrasson et al [15]. Samples and preparation: The zoned carbonates were removed from a pristine fragment of ALH84001. The fragment is approximately 6.0 x 5.5mm in size and, as shown in figure 1, there are numerous areas of orange-stained zoned carbonates on the surface of the fragment.