1¾«ķ9::;;;C:\WORD55\STANDARD.DFVEPFX<@åŌÅ×V::;µMajor and trace element contents and oxygen isotope abundances of refractory inclusions in micrometeorites Gero Kurat1, Peter Hoppe2, Cecile Engrand1, Jrgen Walter3, and Michel Maurette4 1Naturhistorisches Museum, Postfach 417, A-1014 Vienna, Austria, 2Physikalisches Institut, Universit„t Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland, 3............, 4CSNSM, Bat. 104, F-...Campus Orsay, France submitted to GCA, May 1996 ABSTRACT INTRODUCTION Micrometeorites represent the bulk of the extraterrestrial matter (interplanetary dust) accreted by the Earth today (e.g., Love and Brownlee, 1994). Because of their large size (median diameter 220 ęm) they cannot be collected in the terrestrial atmosphere like the smaller particles of the interplanetary dust (the stratospheric interplanetary dust particles which can be collected by airplanes and balloons -see Brownlee, 1985, for a summary). However, micrometeorites have successfully been recovered from the polar ice caps with the best preserved samples having been collected from the Antarctic blue ice (e.g., Maurette et al., 1991). The chemical and mineralogical compositions of micrometeorites are similar to those of the rather rare CM- and CR-type carbonaceous chondrites (e.g., Kurat et al., 1994a), but they differ from the latter in so many ways that they must be considered a matter of their own and a solar system matter which is not represented by meteorites or constituents thereof. However, their similarity to CM/CR carbonaceous chondrites suggests that they should also contain Ca-Al-rich inclusions (CAIs). The types and amounts of such inclusions could not safely be predicted, but could, of course, be in the range known from CM/CR chondrites. Indeed, CAIs were found in micrometeorites in about the right abundance and of the appropriate type as to support the existence of a relationship between micrometeorites and CM/CR chondrites. Here we report on our discoveries of CAIs from a total sample of about 700 micrometeorites. Not all CAIs found were large enough to allow application of all methods available. However, some were large enough to allow determination of major, minor, and trace element contents, mineral compositions, and the isotopic composition of oxygen. The data were collected over several years and some preliminary results were presented at a few conferences (Kurat et al., 1994b,c; Hoppe et al., 1995). Here we present the final data together with a summary on our search for CAIs in micrometeorites and a comparison of our data with those collected from micrometeorites and stratospheric interplanetary dust particles by others. SAMPLES AND METHODS Dust particles have been recovered from Antarctic blue ice near the French station ....d'Urville at Cap-Prudhomme during three(?) seasons on 1988, 1991, and 1995. Micrometeorites were selected from the 100-400 ęm size-fraction under the microscope according to their optical appearance, dark and angular (apparently unmelted), mounted in epoxy, and polished. This way ...polished mounts were produces containing about 700(?) micrometeorites of all types identified, i.e., unmelted phyllosilicate-rich, unmelted coarse-grained crystalline, and partially melted, scoriaceous micrometeorites (for type description see Kurat et al., 1994a). The mounts were investigated by optical microscopy and scanning electron microscopy and analyzed with an analytical scanning electron microscope (JEOL 6400 with Kevex light element detector and Voyager 2100) and an electron microprobe X-ray analyser (ARL-SEMQ), operated at 15 kV and 0.1 and 1.5 nA, respectively. Energy dispersive analyses were done standardless, the electron microprobe analyses were made against mineral standards and by applying the appropriate corrections. Of the seven refractory mineral or inclusions found three were large enough for investigation by secondary ion mass spectrometry utilizing the CAMECA ... of the Physikalisches Institut of the University of Berne, Switzerland. Trace element contents were detemined by applying a modified procedure developed by Zinner and Crozaz (1986), using a 1-5 nA O- primary beam (~ 10-20 ęm diameter), positive secondary ions, and energy filtering to suppress interferences from complex molecular ions. The oxygen isotope analyses were made with a 10-50 pA Cs+ primary beam (~ 3-5 ęm diameter) and negative secondary ions. RESULTS Of the ~ 700 micrometeorites investigated 7 contained refractory minerals or inclusions and 1 consisted intirely of a refractory inclusion. Seven of these inclusions have been analyzed by us as permitted by the mineral's grain size. The eighth sample was found in our sample by Scott Messenger, Washington University, St. Louis, and is a tiny hibonite included in a highly foamy scoriaceous micrometeorite (particle 94/19-4??) which is to be analyzed in the near future. Sample description Figure 1 shows scanning electron images of the CAI-bearing micrometeorites found. They are shown in order of decreasing importance or size. The desciption below follows that order: MM 94/19-5 almost entirely consist of a large CAI (~ 110 ęm in length) which is made of a porous but sintered aggregate of Mg-Al spinel with abundant intergranular perovkites, some of which are partially or totally transformed into armalcolite. Voids make up a high proportion of the core. That is incompletely surrounded by a diopside mantle (up to 10 ęm thick) which in turn is covered by a scoriaceous, melted former phyllosilicate matrix. MM 92/15-23 contains several ameboid, spinel-rich CAIs embedded into a thermometamorphosed former phyllosilicate matrix. The CAIs consist of Mg-Al spinel aggregates with mostly open grain boundaries. At these grain boundaries there are present a few tiny (~ 1 ęm) perovskites. The aggregates are covered by a mantle of Fe-Al-rich phyllosilicate which in turn is covered by a dicontinous crust of Ti-bearing augite. The phyllosilicate matrix is dehydrated and dense, but not molten, and contains also a large (~ 15 ęm) Cr spinel. MM 94/4-36 contains two porous, ameboid spinel-rich CAIs which consist of well-sintered spinel, a few small perovskites, and abundant voids enveloped by a dicontinuos mantle (up to 10 ęm thick) of Fe-rich phyllosilicates. The presence of two more Fe-rich phyllosilicates indicates that some more CAIs are present outside the polished plane. The matrix consists of thermally altered, but not melted, phyllosilicate. MM 94/10-29 is a highly vesicular scoriaceous micrometeorite with abundant relictic Mg-Al spinel which contains a few perovskites and ilmenites MM 91/2-21 is a scoriaceous micrometeorite which contains an ameboid spinel-rich CAI (~ 50 ęm in length) with a large central void and a thermally altered , fragmentary mantle of phyllosilicate. This micrometeorite also contains a large aggregate of Mg-rich low-Ca pyroxene. MM 94/1-28 is a scoriaceous micrometeorite containing a small (~ 15x8 ęm) relictic Mg-Al spinel with some small ilmenite grains attached. MM 91/2-43ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ܀œ’’t_’’amķkÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ  €ī’’y*Tš_’’amķkÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ   õ my ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ"*02.09.9602.08.96mÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ