Additional Complexities in Nakhlite Pyroxenes: a Progress (?) Report

Introduction: Nakhlites are olivine-bearing clinopyroxene cumulates [e.g., 1]. Based on petrographic characteristics, they may be divided into groups that cooled at different rates and may have been formed at different depths in a single flow [e.g., 2, 3]. The order of cooling rate is Lafayette < Governador Valadares ~ Nakhla < Yamato000593 < NWA817 ~ MIL03346. Nakhlite cumulus pyroxene grains consist of large cores that are nearly homogeneous in major element composition surrounded by thin rims that are zoned to Fe-rich compositions. Detailed study of these pyroxenes is important because they retain a record of the crystallization history of the nakhlite magma. Moreover, because the composition of the nakhlite parent melt cannot be directly determined, “inversion” of the major and minor element composition of the cumulate pyroxene cores can be used to estimate the composition of that melt. Thus it is important to understand the major and minor element zoning in the cumulus pyroxenes. We recently reported complications in the minor element zoning of nakhlite pyroxenes, especially for Al and Cr [4]. This abstract reports additional complications noted since that report. Observations: (1) Although homogeneous in quadrilateral components, most nakhlite pyroxene cores are heterogeneous in Al content. We first reported on this heterogeneity in 1993 [5], but modern elemental mapping techniques have allowed greater insight into this phenomenon. Fig. 1 shows Al maps of two nakhlites MIL 03346 (MIL) and Y-000593 (Y593). They have fundamentally different zoning patterns. MIL is essentially homogeneous in Al, while Y593 has patchy zoning that appears to be crystallographically controlled in some areas, but irregular in others. Such patchy zoning is typical of all but the most rapidly cooled nakhlites. (2) Pyroxene cores of several nakhlites a have a bimodal distribution of Al contents [Fig. 2]. There is a clear bimodal distribution for Y593, Nakhla (Nak), and Governador Valadares (GV), and a hint of one for Lafayette (Laf). NWA817 (NWA) has no obvious bimodal distribution, and MIL is strongly peaked at about 0.9 wt% Al2O3. (3) The distribution of Al contents appears to vary systematically with inferred cooling rate. For MIL and NWA, the most rapidly cooled nakhlites, the peak of the distribution in Al content is around 9%. As cooling rate decreases, the average Al content shifts to lower values, primarily by a “flow” in the population from the high-Al mode to the low-Al mode. (4) Core pyroxenes from most nakhlites contain tiny melt inclusions that are much more abundant in the Al-rich regions than in the Al-poor ones. Figure 3 shows images and an Al map of pyroxenes from Y593, a typical moderately-cooled nakhlite. This figure illustrates the abundant small melt inclusions that are present in the pyroxene cores of most nakhlites. These inclusions have been described previously [e.g., 1, 6, and 7]. They typically contain Si, Al-rich glass plus