CHEMICAL COMPOSITIONS OF LARGE CLUSTER IDPs

Introduction: We previously determined that ~10 µm interplanetary dust particles (IDPs) collected from the Earth's stratosphere are enriched in many moderately volatile elements by a factor of ~3 over the CI meteorites [1]. However, these IDP measurements provide no direct constraint on the bulk chemical composition of the parent body (or parent bodies) of the IDPs. Collisions are believed to be the major mechanism for dust production by the asteroids, producing dust by surface erosion, cratering and catastrophic disruption. Hypervelocity impact experiments at ~5 km/sec, which is the mean collision velocity in the main belt [2], performed by Flynn and Durda [3] on ordinary chondrite meteorites and the carbonaceous chondrite meteorite Allende show that the 10 µm debris is dominated by matrix material while the debris larger than ~25 µm is dominated by chondrule fragments. Thus, if the IDP parent body is similar in structure to the chondritic meteorites, it is likely that the ~10 µm IDPs over-sample the fine-grained component of the parent body. We have examined the matrix material from the few meteorites that are sufficiently fine-grained to be samples of potential IDP parent bodies. This search has, thus far, not produced a compositional and minera-logical match to either the hydrous or anhydrous IDPs. This result, coupled with our recent mapping of the element distributions, which indicates the enrichment of moderately volatile elements is not due to contamination on their surfaces [4], suggests the IDPs represent a new type of extraterrestrial material. Nonetheless, the meteorite fragmentation results suggest that compositional measurements on 10 µm IDPs only provide a direct constraint on the bulk chemical composition of the IDP parent body if the size-scale of the grains in the parent body is <<10 µm. The stratospheric collections include many non-chondritic, mono-mineralic grains, collected along with the fine-grained chondritic IDPs. Some of these grains, which include volatile-poor olivine and pyroxene as well as calcophile-rich sulfides, have fine-grained, chondritic material (i.e., small bits of typical IDPs) adhering to their surfaces. This indicates that at least some of the non-chondritic grains found on the strato-spheric collectors are fragments from the same parent as the fine-grained IDPs. Thus, the bulk composition of the IDP parent body can only be reconstructed by adding to the fine-grained, chondritic IDPs the correct amount of this non-chondritic material. Qualitatively, the addition of olivines and pyrox-enes will reduce the mean content of many moderately