Die intra- und interkristalline Verteilung der Spurenelemente Phosphor, Scandium, Cobalt und Zink in Orogenen Ultramafititen

Abstract

Spinel and garnet-bearing orogenic ultramafic rocks from the Alps and Norway were investigated for their intracrystalline distribution and intercrystalline partitioning of the trace elements P, Sc, Co and Zn, using secondary ion mass spectrometry (SIMS). For this purpose, high resolution profiles through entire mineral grains were analyzed. Major and minor element concentrations were determined by electron probe micro analysis (EPMA) inter alia to estimate the documented pressures and temperatures. This study proves that the investigation of orogenic ultramafic rocks must take into account the intracrystalline distribution of the elements as well. Otherwise the zonings or inhomogeneities of the minerals in orogenic ultramafic rocks would lead to significantly deviating pressure and temperature estimates and, consequently, to wrong reconstructions of their evolution. The diffusion rate of the trace element P in minerals of orogenic ultramafic rocks is so low that despite of the slow moving processes these rocks had undergone during orogenesis, commonly no state of equilibration for P was reached. Even if the cores of bigger mineral grains show homogeneous intracrystalline P-distributions, no established geothermobaro-meter exists to estimate the pressures and temperatures of this equilibrium. Therefore, P is not suited for the geothermobarometry of orogenic ultramafic rocks. Sc also exhibits a slow diffusion rate, but compared to P it diffuses much faster. This often leads to a conservation of older Sc-equilibrations in the mineral cores, while their rims already responded to the changed pressure and temperature values. Since some Sc partition coefficients show significant dependencies on temperature but only minor dependencies on pressure, this trace element could be used as a geothermometer for older events in the history of an orogenic ultramafic rock. For the trace element Co it could be demonstrated that its diffusion in the minerals of ultramafic rocks runs faster than the already fast diffusing Mg or Fe2+. In addition, Co-partitioning between the primary minerals of orogenic ultramafic rocks seems to be clearly controlled by temperature. An effect by pressure is virtually absent in most cases. On the basis of these properties, Co is ideal for geothermobarometry of orogenic ultramafic rocks because it can reflect the recent temperature changes better than most existing geothermometers. Zn behaves in many ways like Co, but the partitioning of Zn between the minerals is not as dependent on temperature as in the case of Co.