Chromium-Bearing Spinel Group Minerals in Micrometeorites

62nd Annual Meteoritical Society Meeting 5183.pdf

CHROMIUM-BEARING SPINEL GROUP MINERALS IN MICROMETEORITES. M. J. Genge and M. M. Grady, Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK. E-mail: [email protected].

Introduction: Micrometeorites (MMs) are that 1.2 fraction of the extraterrestrial dust flux that survive UOC 1a atmospheric entry and are recovered from the Earth's ) l

surface. Micrometeorites are grouped into fine-grained A 0.8

+ CC particles, dominated by phyllosilicates, and coarse- r 0.6 C (

grained particles, dominated by anhydrous silicates / r 0.4

and glass [1, 2]. Fine-grained particles strongly re- C EOC semble the matrices of the carbonaceous chondrites [1, 0.2

2, 3]. Coarse-grained particles frequently have mag- 0 matic textures and thus resemble fragments of chon- 00.20.40.60.811.2 drules, however, because the compositions of their 2+ 2+ Fe/(Mg+Fe) anhydrous silicates are not entirely diagnostic of 1

source their relations to meteorites are uncertain [4]. ) r 0.9

We present data on the occurrence and mineralogy of C 0.8b + chromium-bearing spinel group minerals found in l 0.7 coarse-grained unmelted micrometeorites (cgMMs) A 0.6 + B + 0.5R and in cosmic spherules (CSs) to better constrain 3 O e 0.4M

their affinities with meteorites. F

( 0.3 /

The occurrence of Cr-spinels in MMs: Chro- + 3 0.2Chondrites mium-bearing spinel group minerals are relatively e 0.1 F common in cgMMs and CSs occurring in ~5% of par- 0 ticles, however, they have been little studied due to 00.20.40.60.811.2 their small crystal sizes. Fe2+/(Mg+Fe2+) Coarse-grained MMs. Four coarse-grained particles containing chromium-rich spinel were studied, Figure 1. The compositions of Cr-spinels in cgMMs two have porphyritic textures, one is poikolitic and (solid circles) and CSs (open circles) compared with one granular (see [4]). Chromium spinels in the por- those in chondrites and MORB [5, 6] phyritic and poikolitic cgMMs have euhedral- subhedral habits suggesting they crystallised from a chromites with lower Fe3+/(Fe3++Al+Cr) ratios than melt. The spinels in the granular particle are anhedral those reported from type II chondrules from carbona- and appear to be fragments. ceous chondrites but within the range of OC chro- Cosmic Spherules. Chromium-bearing spinels mites. In contrast chromium-bearing magnetites from were discovered in 12 porphyritic CSs (EO spherules). CSs, including all of those found with cruciform hab- The spinels are contained in the glassy mesostasis and its, have significantly higher Fe3+/(Fe3++Al+Cr) ratios vary from euhedral quench crystals, some with cruci- (and Fe3+/Fe2+ ratios) than observed in chondrites. form shapes, to anhedral. These are similar ratios to spinels found in mid-ocean The compositions of Cr-spinels: Two groups of ridge basalts. Presumably these Cr-spinels crystallised chromium-bearing spinel occur, (1) chromites in the atmosphere from liquids generated by entry FeCr2O4 and (2) Cr-bearing magnetite/maghemite heating under relatively oxidising conditions. Some of Fe3O4. The chromites in general show a wider range the chromites found in CSs, however, have low of Al and Mg contents than the magnetites (up to ~15 Fe3+/Fe2+ similar to those in chondrites and are proba- and 11 mole% respectively). The Cr-spinels found in bly relicts that survived melting. coarse-grained micrometeorites are all chromites Conclusions: Chromium-bearing spinels found in whereas those in CSs include Cr-bearing magnetite. cgMMs have similar compositions to those found in Comparisons with chondrites: The compositions CC and OC type II chondrules. Those found in CSs of chromites in the cgMM and in two of the CSs fall are either relicts, similar to those in chondrites, or are in the range of those from carbonaceous and ordinary formed by crystallisation in the atmosphere at higher chondrites (Fig. 1). Two coarse-grained particles have fO2. 62nd Annual Meteoritical Society Meeting 5183.pdf

Cr-SPINELS IN MICROMETEORITES: M. J. Genge and M. M. Grady

References: [1] Kurat G. et al., (1994) GCA 58, 3879. [2] Genge et al., (1997) GCA 61, 5149. [3] Engrand C. & Maurette M. M. (1998) MAPS 33, 565. [4] Genge & Grady (1998) MAPS 33, A57. [5] Johnson C. A. & Prinz M., (1991) GCA 55, 893. [6] Roeder P. L. (1994) Canadian Min. 32, 729.