Ying D. Sheikh1, 1Department of Geological Sciences, University of Florida, Gainesville, FL, 32611 ([email protected])

Introduction: The Tenham meteorite belongs to the L6 group of ordinary chondrites which are characterized by their blurred chondrule outlines formed as a result of extensive thermal metamorphism [1]. The Tenham meteorite is classified in the S5-S6 shock stage due to the presence of high-pressure mineral phases and shock melt veins [2,3]. The aim of this paper was to analyze the composition and texture of the Tenham meteorite chondrules, altered as a result of secondary and tertiary processes such as thermal metamorphism and shock metamorphism respectively, with the goal of understanding how these processes alter the primitive chondritic parent body during both its accretion and differentiation phases of formation.

Fig. 1. 5.4-gram slice of Tenham meteorite.

Analyzing this chondrule under the SEM The Barred Olivine texture chondrule revealed a porphyritic texture, as there were This was the largest chondrule present in the analyzed under Energy-dispersive X-ray small anhedral to subhedral crystals of Tenham meteorite. Analyzing this chondrule spectroscopy (EDX) revealed a low atomic olivine and pyroxene present in this under the SEM revealed a glassy, percentage for both iron and calcium, and a chondrule. Also present was a large porphyritic texture. The grains of olivine and higher than normal atomic percentage for inclusion containing a phosphate mineral pyroxene present were anhedral to both sodium and aluminum, suggesting an present in it, possibly merrillite. An iron- subhedral. Also present in the chondrule enrichment of the surrounding feldspathic nickel mineral is present to the right of the were scattered, small grains of chrome. This matrix component, possibly due to phosphate inclusion, either Taenite or chondrule appears to have been homogenization of the chondrule with the Kamacite. This chondrule appears to have homogenized with the surrounding surrounding matrix as a result of extensive been homogenized with the surrounding component as per the EDX data, and I would thermal metamorphism [5]. This chondrule component as per the EDX data, and I would classify this chondrule as a Type 1 chondrule would be classified as a Type 1 chondrule classify this chondrule as a Type 1 chondrule (Fig 5). [4] (Fig 3). (Fig 4).

Fig. 3. Barred Olivine texture (BO) chondrule in Tenham meteorite under SEM. Fig. 4. Porphyritic texture chondrule in Tenham meteorite under SEM. Fig. 5.Porphyritic/Glassy texture chondrule in Tenham meteorite under SEM.

Conclusion: Analyzing the 5.4-gram sample of the Tenham meteorite under a Scanning Electron Microscope (SEM) revealed a very low volume of chondrules present (Fig 1, Fig 2). The effects of both thermal and shock metamorphism in the alteration of these chondrules are evident. The blurring and homogenization of the chondrules as a result of extensive thermal metamorphism combined with the alteration of certain minerals in the chondrules to their respective high-pressure mineral phases and the presence of shock melt veins as a result of shock metamorphism have all but destroyed the existing chondrule structures in the meteorite. Due to the fact that these chondrules are all Type 1 chondrules, and that anhydrous Merrillite is present inside of them, it is possible to say that these chondrules could have originated in a part of the solar nebula containing small amounts of oxidizing conditions present. Having been heavily altered during analysis, it is not a guarantee though that these chondrules originated from there. Fig. 2. Tenham meteorite under SEM.

References: [1] Van Schmus, W. R., and J. A. Wood (1967), A chemical-petrologic classification for the chondritic meteorites, Geo-chim. Cosmochim. Acta, 31, 747-765. [2] Stöffler, D., K. Keil, and E. R. D. Scott (1991), Shock metamorphism of ordinary chondrites, Geo-chim. Cosmochim. Acta, 55, 3845-3867. [3] Langenhorst, F., P. Joreau, and J. C. Doukhan (1995), Thermal and shock metamorphism of the Tenham chondrite: A TEM examination, Geochim. Cosmochim. Acta, 59, 1835-1845. [4] Jones R. H., Grossman J. N., and Rubin A. E. (2005), Chemical, mineralogical, and isotopic properties of chondrules: Clues to their origin. In Chondrules and the protoplanetary disk, edited by Krot A. N., Scott E. R. D., and Reipurth B. Astronomical Society of the Pacific Conference Series 341:251-285. [5] Weisberg, M. K. (1987), Barred olivine chondrules in ordinary chondrites, J. Geophys. Res., 92(B4), E663-E678.