Petrology and Mineral Chemistry of Crust and Mantle Fragments in an Archean Ophiolite from the North China Craton

Herzberg

EAR-0228592

Theories on the FeO content of the Archean mantle are difficult to test because direct rock samples that were melted to produce basaltic crust have never been identified and described. Uncertainties in mantle FeO content have profound consequences on understanding mantle dynamics because Fe is much heavier than all other rock-forming elements, and even a modest iron-enrichment must be compensated by large temperature increases in order to maintain neutral buoyancy. Therefore, the recent discovery of a complete ophiolite with tectonized mantle peridotite from the North China Craton (Kusky et al., 2001; Science 292, 1142-1145) is of monumental importance because it has created an unprecedented opportunity for constraining the FeO content of the Archean mantle.

Unfortunately, the mantle samples from the North China Craton have been metamorphosed, and the extent to which the original Archean geochemistry has been modified is not presently known. We plan on evaluating whether it is feasible to extract from these samples the original mantle FeO content in a 1-year exploratory pilot project. We will use the JEOL-8600 Superprobe with Advanced Microbeam upgrade at Rutgers University to obtain chemistry data on metamorphic and possible relect igneous minerals. Microimage software will be used to obtain modal analyses of 1-4 million points per thin section, and this will permit a mass balance calculation to be made of whole rock major element geochemistry. Mineral and whole rock chemistry obtained at Rutgers University will be crosschecked with whole rock major element geochemistry obtained by collaborator A. Polat at Windsor University (Canada) using XRFS. These data will be compared with the composition of mantle peridotite from abyssal peridotites and ophiolites of Phanerozoic ages. Any differences in composition will be evaluated in terms of possible: 1) different igneous T-P histories, 2) metamorphric alteration effects, or 3) secular variations in mantle composition.