Reassessing the thermal history of martian meteorite Shergotty and Apollo mare basalt 15555 using kinetic isotope fractionation of zoned minerals
Résumé
Elemental abundance and isotopic fractionation profiles across zoned minerals from a martian meteorite (Shergotty) and from a lunar olivine-normative mare basalt (Apollo 15555) were used to place constraints on the thermal evolution of their host rocks. The isotopic measurements were used to determine the extent to which diffusion was responsible for, or modified, the zoning. The key concept is that mineral zoning that is the result of diffusion, or that was significantly affected by diffusion, will have an associated diagnostic isotopic fractionation that can quantify the extent of mass transfer by diffusion. Once the extent of diffusion was determined, the mineral zoning was used to constrain the thermal history. An isotopic and chemical profile measured across a large zoned pigeonite grain from Shergotty showed no significant isotopic fractionation of either magnesium or lithium, which is evidence that the chemical zoning was dominantly the result of crystallization from an evolving melt and that the crystallization must have taken place at a sufficiently fast rate that there was not time for any significant mass transfer by diffusion. Model calculations for the evolution of the fast-diffusing lithium showed that this would have required a cooling at a rate of about ∼150 °C/h or more. Measurable isotopic fractionation across a zoned olivine grain from lunar mare basalt 15555 indicated that the chemical zoning was mainly due to crystallization that was modified by a small but quantifiable amount of diffusion. The results of a diffusion calculation that was able to account for the amplitude and spatial scale of the isotopic fractionation across the olivine grain yielded an estimate of 0.2 °C/h for the cooling rate of 15555. The results of an earlier study of zoned augite and olivine grains from martian nakhlite meteorite NWA 817 were reviewed for comparison with the results from Shergotty. The isotopic fractionations near the edges of grains from NWA 817 showed that, in contrast to Shergotty, the lithium zoning in augite and of magnesium in olivine was due entirely to diffusion. The isotopic fractionation data across zoned minerals from the martian meteorites and from the lunar basalt were key for documenting and quantifying the extent of mass transfer by diffusion, which was a crucial step for validating the use of diffusion modeling to estimate their cooling rates.