PETROLOGY AND GEOCHEMISTRY OF ANTARCTIC MICROMETEORITES

The petrology and geochemistry of twenty-three chondritic dust particl es with masses of 1-47 mu g (sizes 100-400 mu m) were recovered from b lue ice near Cap Prudhomme, Antarctica, and studied by INAA, ASEM, EMP A, and optical microscopy. Sample selection criteria were irregular sh ape and (for a subsample) black color, with the aim of studying as man y unmelted micrometeorites (MMs) as possible. Of thirteen unmelted MMs , six were phyllosilicate-dominated MMs, and seven were coarse-grained crystalline MMs consisting mainly of olivine and pyroxene. The remain ing ten particles were largely melted and consisted of a foamy melt wi th variable amounts of relic phases (scoriaceous MMs). Thus, of the bl ack particles selected, an astonishing portion, 40% (by number), consi sted of largely unmelted MMs. Although unmelted, most phyllosilicate M Ms have been thermally metamorphosed to a degree that most of the phyl losilicates were destroyed, but not melted. The original preterrestria l mineralogy is occasionally preserved and consists of serpentine-like phyllosilicates with variable amounts of cronstedtite, tochilinite-li ke oxides, olivine, and pyroxene. The crystalline MMs consist of olivi ne, low-Ca pyroxene, tochilinite-like oxides, and occasional Ni-poor m etal. Relies in scoriaceous MMs consist of the same phases. Mineral co mpositions and the coexistence of phyllosilicates with anhydrous phase s are typical of CM and CR-type carbonaceous chondrites. However, the olivine/pyroxene ratio (similar to 1) and the lack of carbonates, sulf ates, and of very Fe-poor, refractory element-rich olivines and pyroxe nes sets the MMs apart from CM and CR chondrites. The bulk chemistry o f the phyllosilicate MMs is similar to that of CM chondrites. However, several elements are either depleted (Ca, Ni, S, less commonly Na, Mg , and Mn) or enriched (K, Fe, As, Br, Rb, Sb, and Au) in MMs as compar ed to CM chondrites. Similar depletions and enrichments are also found in the scoriaceous MMs. We suggest that the depletions are probably d ue to terrestrial teaching of sulfates and carbonates from unmelted MM s. The overabundance of some elements may also be due to processes act ing during atmospheric passage such as the recondensation of meteoric vapors in the high atmosphere. Most MMs are coated by magnetite of pla ty or octahedral habit, which is rich in Mg, Al, Si, Mn, and Ni. We in terpret the magnetites to be products of recondensation processes in t he high (>90 km) atmosphere, which are, therefore, probably the first refractory aerominerals identified.