Iron Meteorites

A growing number of data acquired on iron meteorites over the past decennia are in conflict with the widely entertained model of iron meteorite genesis: formation by separation of a metal melt from molten chondritic matter (smelting) and subsequent fractional crystallization. Two cases are distinguished: (1) "igneous" irons are the product of planetary fractionation and crystallization of metal in planetary cores and (2) "non-igneous" irons that formed from metal melts separated from silicates in local impact melt pools at the surface of planetesimals. All models create irons by crystallization from a melt. However, there is growing evidence that these models need to be replaced. We want to find some possible answers to a few open questions, such as: Where are the planetary mantle rocks? About 2/3 of differentiated planets consist of peridotites (like Earth), but our meteorite collection do not contain a single piece. Silicate, sulfide and graphite inclusions carry messages from the very early solar nebula: isotopic inhomogeneities and daughters of now extinct short-lived radionuclides. Silicates carry messages indicating formation by condensation from a vapor fractionated solar nebula gas (abundance anomalies in Eu, Sm, Yb). Metal appears to be the oldest matter known to us: it has a deficit in the radionuclide 182 W, documenting the presence of the now extinct 182 Hf during their formation. Metal is chemically inhomogeneous in some irons and has widely varying apparent cooling rates - also within a given chemical group - a situation incompatible with a planetesimal core origin. Our preliminary studies on metal, schreibersite, graphite and silicates of a few iron meteorites produced data that are incompatible with the current genetic models for irons. We found additional evidence for isotopic and chemical disequilibria between phases in irons. Indications for a primitive origin directly in the solar nebula are accumulating - not only in our data. It is our goal to produce a model that can accommodate all observations made up to now and possibly also future ones. In order to achieve that goal, we need to study a small selection of critical samples in very detail, including major, minor and trace element and isotope distributions between phases. A very modest financial support shall ensure mobility of a few researchers, which is an important pre-requisite for a best- possible-quality data acquisition.

Glasses in meteorites have been observed already by SORBY (1864) and TSCHERMAK (1875, 1883) and their formation was - according to the standard knowledge of that time - attributed to a sort of volcanic genesis. Of special interest are glass inclusions in the silicate mineral olivine which in turn mainly occurs in chondrules - being the characteristic constituent of most chondritic meteorites. However, for many decades these glasses were left aside by most meteorite researchers, as they were considered of being trapped remnants of a bulk parent liquid from which the olivine and its host chondrule formed. During several years we have accumulated data and observations of glasses in chondrites, some achondrites and iron meteorites that could not be reconciled with igneous or impact processes, the currently preferred models for the genesis of meteorites. The formulation of a new model (The Primary Liquid Condensation Model) was finalised within this project. Our results revealed a clear relationship between chondritic constituents and achondritic rocks and suggest that liquids (the glass precursor) could have played an important role in the formation of meteorite constituents. The former presence of these liquids has also been confirmed in the Tucson ungrouped iron meteorite. Detailed chemical and petrographic studies of all phases (minerals and glasses) indicate that they have a nebular rather than an igneous origin and give support to a chondritic connection as suggested by PRINZ et al. (1987). Apparently, metal in Tuscon seems to have precipitated before and throughout silicate formation and aggregation, and some of the silicate products were trapped by the metal and became isolated. As in this way, products of all evolutionary steps were preserved, we were able to define a sequence of formation for the major components of Tucson. In addition, we gave new evidences for a variety of possible processes that allowed us to propose an alternative model for the formation of this particular meteorite and its constituents. Tucson could be the most metal-rich and volatile-element-poor member of the CR (carbonaceous chondrite of Renazzo type) clan, reinforcing the relationships between chondritic and achondritic rocks.