Relative Burial Depths of Nakhlites: an Update

Introduction: Nakhlites are augite-rich cumulate rocks with variable amounts of olivine and groundmass plus minor Fe, Ti oxides [e.g., 1]. Our previous studies revealed that nakhlites showed correlated petrography and mineralogy that could be explained by different locations (burial depths) in a common cooling cumulate pile [e.g., 2]. We so far analyzed six of the seven currently known nakhlites, Nakhla (Nak), Governador Valadares (GV), Lafayette (Laf), NWA817, Y000593 (Y) and MIL03346 (MIL) [e.g., 2,3] and calculated cooling rates of four nakhlites (Nak, GV, Laf, and NWA817) by using chemical zoning of olivine [e.g., 4]. In this abstract, we complete our examination of petrographic and mineralogical variation of all currently known nakhlites by adding petrology and mineralogy of NWA998. We also report results of cooling calculations for Y, MIL and NWA998. Then, we update our model of the nakhlite igneous body in terms of relative burial depth of each sample. Groundmass variation: Although all the nakhlites show a similar unbrecciated cumulate texture, modal abundances of augite, olivine and interstitial groundmass (or mesostasis) show some variations from one sample to another. The groundmass abundance is the most variable. In Nak, GV and Laf, the groundmass abundance is about 78%. Y and NWA998 have a slightly higher abundance of 10%. NWA817 and MIL have much higher abundance of 20-24%. The groundmass texture is generally related to its abundance. The groundmass of Nak, GV and Laf is mainly composed of lathy feldspar grains (~50 μm wide). The Y groundmass is similar to that of Nak, GV and Laf, but plagioclase laths are slightly thinner (~20 μm wide). In NWA998, feldspar is a large blocky crystal (some reaching 500 μm) with clear twinning and one single grain usually constitutes each interstitial groundmass area. In contrast, NWA817 and MIL have glassy mesostasis with few or no feldspar crystals and skeletal titano-magnetite is the most abundant crystalline phase in the mesostasis. Mineral compositional variation: Augite grains in all samples have a nearly identical core composition of En39Fs22Wo39, but MIL augite is slightly higher in Ca content (En38Fs21Wo41) (Fig. 1). The cores are large and constitute most areas of the grains. In all samples, there are thin Fe-rich rims where augite grains abut groundmass or mesostasis. The degree of chemical zoning towards the Fe-rich rims varies from one sample to another (Fig. 1). The interior portions of the Fe-rich rims have a fairly uniform Wo content, but the Wo contents drop at the outer edge of the grain except for MIL pyroxene (Fig. 1). NWA998 has the most Mg-rich edge composition. The edge composition becomes more Fe-rich in the order of Laf, Nak/GV, Y and NWA817. MIL shows a qualitatively different zoning pattern from the others as the Fe-rich edge is zoned to the hedenbergite composition (Fig. 1). We found fine exsolution lamellae (~1 μm) of augite at Fe-rich pigeonite edges except for NWA817 and MIL. In NWA998, some of the lamellae reach ~3 μm wide. Nakhlite pyroxenes also show correlated variations in minor elements, and they will be discussed in a separate abstract [5]. Olivine grains in all samples except Laf and NWA998 show extensive chemical zoning whose degree is clearly related to the zoning patterns in pyroxenes (Fig. 1). MIL and NWA817 have the widest compositional ranges (Fa54-93). Y shows a similar but slightly narrower compositional range of Fa58-85. Nak and GV have even narrower ranges of Fa58-72. In contrast to these samples, Olivines in Laf and NWA998 are quite homogeneous (Fa66-67 and Fa61-62, respectively). Olivines in Nak, GV and Y are also different from other samples in the presence of symplectic exsolution.