Matrix Olivines in the Metamorphosed Ck Chondrite Nwa 1628: Possible Affinities to Olivines in the Matrices of Oxidized Cv3 Chondrites and Dark In- Clusions

Introduction: The metamorphosed CK chondrites frequently contain evidence of shock darkening of silicates, a feature which has been documented by several authors [1,2]. This darkening is the result of shock mobilization of sulfides and their injection into silicate minerals forming micron to submicron inclusions. Several studies have observed matrix olivines in CK chondrites which appear to be the result of shock darkening. In particular, [2] made persuasive arguments that some olivine grains in the matrix of the Kobe CK4 fall formed by crystallization from shock-produced melts. In these grains, the olivines are vesiculated and contain and inclusion assemblage of magnetite, pentlandite and rare pyroxene and chlorapatite. In this study we have examined the inclusion assemblage in matrix olivines in the metamorphosed CK chondrite, NWA1628, a meteorite which has previously received little attention. This CK chondrite is notable because it contains abundant matrix olivines which are morphologically similar to those in the matrices of Allende-like oxidized CV chondrites, and CV dark inclusions. We have carried out a petrographic study of NWA 1628 using scanning electron microscopy and electron microprobe analysis. In addition, selected regions of several matrix olivines were extracted using the focused ion (FIB) beam lift out technique and were studied by transmission electron microscopy. Petrography. NWA 1628 was classified as a CK chondrite of unspecified petrologic type and shock stage S3-6 [3]. Our new observations confirm that it is a metamorphosed CK chondrite, probably of high petrologic type 3. However, although NWA 1628 has clearly been shocked, the shock level indicated by our sample appears to be lower than S3. Chondrule silicates do not show evidence of darkening and there is little evidence of undulose extinction. Matrix in NWA 1628 constitutes about 60 model % of the meteorite, the remainder being mainly porphyritic Mg-rich chondrules. Chondrule olivines and low-Ca pyroxenes show extensive evidence of metamorphic equilibration. Olivines are highly zoned ranging from Fo100 to Fo63 on their rims, whereas low-Ca pyroxene has developed patchy zoning along contraction cracks and cleavage planes. Opaque nodules in chondrules have been entirely replaced by magnetite and an unidentified Mg, Si-rich phase. Mesostasis is fully recrystallized and commonly consists of augite, albitic feldspar and sometimes anorthite. One fine-grained anorthiterich CAI also occurs in the thin section studied. Matrix mineralogy Matrix consists dominantly of equilibrated fayalitic olivine (Fa37) ranging in morphology from anhedral to elongate with grain sizes that range from <2 microns to 100 microns. The matrix texture is variable from one region to another consisting of densely packed anhedral olivines in some regions to more porous aggregates of platelike olivine grains in others. Ca-rich pyroxene En41Fs10Wo49, albitic plagioclase (An37Ab62Or1-to An17Ab82Or1) and sulfides also occur in the matrix. High resolution FESEM imaging of the olivine grains shows that the y have a dusty appearance and \contain myriad inclusions which range from a few microns down to ~ 100 nm in size (Figure 1). Two distinct types of inclusion