The Nakhla Parent Magma: Old Problems, New Approaches

Introduction: Nakhlites are olivine-augite cumulates that formed in flows or shallow intrusions of basaltic magma on Mars [1]. There have been at least 6 independent attempts to determine the composition of the Nakhla parent magma, and at least 14 variants have been published [2-12]. Four distinct methods have been used: 1) mass balance; (2) augite/melt element partitioning; 3) melt inclusions in olivine; 4) melt inclusions in augite. Table 1 shows the compositions that we consider the most robust products of each of these methods. One striking difference among them is that the one derived by [4] from melt inclusions in olivine, NK93 (and variants NK93' and NK93''), has much higher K2O and K2O/Na2O than all others. High K2O in the nakhlite parent magma would have significant implications for the depth and composition of the mantle source [3,13,14]. Thus, we are using new approaches to determine whether high K2O is a real property of the melt trapped in olivine, and if so, whether it is also a property of the parent magma. Analytical: Sections NK19 and NK12 were examined with the Zeiss EVO50 XVP scanning electron microscope and Cameca SX-Ultrachron electron microprobe at the University of Massachusetts. Compositional maps were collected with the Ultrachron using an optimized CeB6 cathode at 15kV and 600nA (regulated), with 100ms pixel dwell time and 2 μm step size (600x600 pixels). Elements, acquired lines, and monochromators were: Al Kα (TAP), Ca Kα (LPET), P Kα (VLPET), S Kα (VLPET), Cr Kα (LLIF). Unrepresentative Sampling? NK93 could be inaccurate due to unrepresentative sampling of 3-D objects in 2-D sections. However, Treiman [4] analyzed 13 large (>70 μm diameter) melt inclusions and all of them showed bulk compositions with high K2O. In averaging these compositions, [4] weighted them by inclusion area, which coincidentally gave the greatest weight to the inclusion with the highest K2O. Nevertheless, a recalculated NK93 without weighting (NK93'') still has very high K2O (Table 1). In fact, even if K2O of the average bulk inclusion were reduced to the lowest measured value, NK93 would still have >2× as much K2O as any other proposed parent magma composition. So, we tentatively conclude that the K2O content of NK93 is not erroneously high due to unrepresentative sampling. Table 1. Some proposed Nakhla parent magma compositions.