Origins of magnetite nanocrystals in Martian meteorite ALH84001

摘要

The Martian meteorite ALH84001 preserves evidence of interaction with aqueous fluids while on Mars in the form ofmicroscopic carbonate disks. These carbonate disks are believed to have precipitated 3.9 Ga ago at beginning of the Noachianepoch on Mars during which both the oldest extant Martian surfaces were formed, and perhaps the earliest global oceans.Intimately associated within and throughout these carbonate disks are nanocrystal magnetites (Fe3O4) with unusual chemicaland physical properties, whose origins have become the source of considerable debate. One group of hypotheses argues thatthese magnetites are the product of partial thermal decomposition of the host carbonate. Alternatively, the origins of mag-netite and carbonate may be unrelated; that is, from the perspective of the carbonate the magnetite is allochthonous. Forexample, the magnetites might have already been present in the aqueous fluids from which the carbonates were believed tohave been deposited. We have sought to resolve between these hypotheses through the detailed characterization of the com-positional and structural relationships of the carbonate disks and associated magnetites with the orthopyroxene matrix inwhich they are embedded. Extensive use of focused ion beam milling techniques has been utilized for sample preparation.We then compared our observations with those from experimental thermal decomposition studies of sideritic carbonatesunder a range of plausible geological heating scenarios. We conclude that the vast majority of the nanocrystal magnetites pres-ent in the carbonate disks could not have formed by any of the currently proposed thermal decomposition scenarios. Instead,we find there is considerable evidence in support of an alternative allochthonous origin for the magnetite unrelated to anyshock or thermal processing of the carbonates.