Fayalite- Spinel + Stishovite in Shocked Umbarger L6 Chondrite

Lunar and Planetary Science XXXIII (2002) 1859.pdf

FAYALITE- SPINEL + STISHOVITE IN SHOCKED UMBARGER L6 CHONDRITE. Z. Xie and T. G. Sharp; Department of Geological Sciences, Arizona State University, Tempe, AZ 85287, USA; E-mail: [email protected], [email protected]

Introduction structured plagioclase (10 to 50µm) represent olivine Fe2SiO4-spinel, the spinel-structured polymorph of and plagioclase that was transformed during shock. fayalite, was synthesized by Ringwood in 1958 at 600 The melt-vein matrix consists of fine-grained silicates ºC at 38 kbars [1]. Although ringwoodite, Mg-rich and metal-troilite blebs, representing immiscible sili- (MgFe)2SiO4-spinel, is believed to be a major mineral cate and metal-sulfide liquids that crystallized at high of Earth’s transition zone [1,2], and commonly found pressure. Our previous TEM work revealed a series of in shocked chondritic meteorites, natural Fe2SiO4- high-pressure phases in the melt vein matrix including spinel has not been previously reported. Here we de- akimotoite, ringwoodite and augite [12]. scribe the natural occurrence of Fe2SiO4-spinel in a Fe2SiO4-spinel, which occurs along with stishovite shock-induced melt vein in the Umbarger L6 Chon- in glassy FeO-SiO2 rich zones of a shock-induced melt drite. vein, was identified by electron diffraction and EDS High-pressure minerals are common in highly analysis. The Fe2SiO4-spinel grains are irregular in shocked (S6) L6 chondrites, occurring within or adja- shape with sizes from 100 to 400 nm. Identification of cent to shock-induced melt veins. The most common of the spinel structure is based on two zone axis SAED these minerals are ringwoodite, majorite, and wadsley- patterns, <0-11> and <-112> zones (Fig. 1). The sym- ite [3-5], whereas akimotoite, magnesiowüstite, metry of the diffraction patterns is consistent with the perovskite, hollandite-structured plagioclase are less face-centered cell of spinel (Fd-3m). The measured d- common [6-8]. High-pressure minerals in shock- spacing are systematically larger than those calculated induced melt veins provide a record of high-pressure from synthetic fayalite spinel [1], resulting in a lattice and temperature conditions during impact events on constant (8.52Å) that is 3.5% larger. However, the ra- chondrite parent bodies. They also provide natural ex- tios of d-spacings and interplaner angles are consistent amples of high-pressure minerals that are expected to with the spinel structure. EDS data from these grains make up Earth's transition zone (410 to 660 km depth) show that they have olivine stoichiometry and lower mantle. (Mg,Fe)2SiO4 with small amounts of Al, Ti and Cr. The Fe/(Fe+Mg) ratio ranges from 0.62 to 0.99. Method and sample Fe2SiO4-spinel occurs with stishovite, a high- Transmission electron microscopy (TEM) is an im- pressure polymorph SiO2, surrounded by SiO2-rich portant tool for studying shock-induced melt veins be- glass and a dissordered Fe-rich phyllosilicate. EDS cause most of the crystals are extremely fine graned shows that the glass is nearly pure SiO2 and phyllosili- (sub-µm to several µm). TEM imaging techniques were cate is rich in Fe with small amount of Na, Mg, Al, S, employed to characterize the micro-texture of the veins K, Ti, Cr, and Ni. The phyllosilicate shows characteris- and the microstructures of the vein minerals. Mineral tic smectic features, consisiting of curved crystals sepa- phases were identified on the basis of selected area rated by voids. High-resolution TEM images show the electron diffraction (SAED) and quantitative EDS mi- phyllosilicate has a basal layer spacing of ~10 Å, con- croanalyses. sistent with smectite that has dehydrated in the vacuum The Umbarger chondrite was found in 1954 as a environment of the electron microscope. single stone with its fusion crust intact [9]. This mete- orite is highly oxidized, containing numerous Fe- Discussion oxide-rich veins. Petrographic classification was given Fe2SiO4-rich spinel and stishovite are surrounded as L3 or L6 [9]. The shock metamorphism in umbarger by SiO2-rich glass and Fe-rich phyllosilicate in an was previously classified as shock stage S4, based on overall FeO-SiO2-rich zone, suggesting that the spinel deformation features of olivines [10]. However, our crystallized from FeO-SiO2-rich melt within a melt discovery of ringwoodite in melt pockets and the pres- pocket. Because shock-induced melt is typically chon- ence of maskelynite and normal plagioclase glass indi- dritic in composition, the FeO-SiO2-rich melt implies a cate a shock stage of S6 [11]. FeO-SiO2-rich procurer to shock melting. FeO-SiO2-rich melt zones may be related to FeO- Results SiO2-rich alteration zones that occur at the edge of melt Shock-induced melt veins range in width from 0.3 pockets or near the melt pockets (Fig. 2). EMPA analy- to 0.7 mm and consist of two lithologies. Rounded ses of the alteration zones indicate similar FeO-SiO2- polycrystalline grains of ringwoodite and hollandite- rich compositions, but with low totals, suggesting that Lunar and Planetary Science XXXIII (2002) 1859.pdf

Fe2SiO4-RICH SPINEL IN SHOCKED UMBARGER, Z. Xie and T. G. Sharp

the altered material is H2O rich. From the texture of FeO-SiO2-rich alteration zones in host rock, it is not clear that alteration proceeded shock. However, the FeO-SiO2-rich zone that forms the edge of the melt pocket in Figure 2 suggests that the shock melting oc- curred adjacent to a preexisting alteration vein that formed the FeO-SiO2-rich edge. Melting of chondritic and FeO-SiO2-rich material in this melt pocket could produce a FeO-SiO2-rich melt at the edge of the melt pocket. The diffuse boundary between the chondric melt pocket and the FeO-SiO2-edge is consistant with limited mixing of shock-induced melt. The FeO-SiO2-rich alteration of Umbarger may be similar to fayalite-bearing material in the matrices and dark inclusions of CV3 and other unequilibrated chondrites (ordinary and carbonaceous) [13]. The fayalite most commonly occurs as rims, veins and halos in and around chondrule silicates. The FeO-SiO2-rich alteration zones need to be characterized by TEM to deter- mine the nature of the alteration and to confirm that this material was melted during shock to produce the

FeO-SiO2-rich melt and fayalite spinel. Fig. 1 a) TEM image showing a Fe2SiO4-spinel (Sp) The presence of Fe2SiO4-rich spinel plus stishovite grain surrounded by phyllosilicates (Ps). b) SAED pat- in the melt vein consists with our previous interpreta- tern of <0-11> zone from spinel grain. c) SAED pat- tion: metastable crystallization of supercooled melt tern of <-112> zone from same grain. during rapid decompression [12]. Mg2SiO4-spinel and stishovite are stable in the pressure range from 15 GPa to 25 GPa in the enstatite-forsterite system [14]. Fe2SiO4-spinel plus stishovite should be stable at lower pressures [1].

Conclusion Fayalite-spinel ( Fe /(Fe+Mg) up to 0.99) occurs naturally in shocked umbarger L3, L6 chondrite. SAED data indicate Fe2SiO4 spinel with unit cell 3.5% larger than that of synthetic Fe2SiO4-spinel. Fe2SiO4-rich spinel and stishovite crystallized from FeO-SiO2-rich melts that formed by melting FeO-SiO2-rich alteration zones.

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