TRACE ELEMENT PARTITIONING BETWEEN MINERALS OF THE ALPE ARAMI GARNET PERIDOTITE, SWITZERLAND

Abstract

The internal zones of orogenic belts often bear (ultra)high-pressure peridotites. The P-T paths recorded by their mineral assemblages can give valuable information on orogenic processes. Apart from phase diagrams, the internal chemical structure of individual mineral grains and the partitioning of elements between different mineral phases are the main tools to constrain the P-T evolution of peridotitic rocks. We present the first results of a combined electron microprobe and ion microprobe study on the abundances of major and trace elements in minerals of the garnet peridotite from the classical Alpe Arami locality (Switzerland). Previous studies on the Alpe Arami peridotite have not documented the existence of pronounced chemical zoning of mineral grains (Fig. 1). Furthermore, textural relationships as well as major element compositions indicate the presence of two clinopyroxene (cpx) generations. Type-I cpx (cpx I) forms large porphyroclasts and inclusions in garnet (grt); type-II cpx (cpx II) is the ordinary matrix cpx. Compared with cpx I, cpx II is characterized by lower Al2O3 (1.9 vs 3.0 wt %) and Na2O (1.0 vs 2.0 wt %), but higher CaO (22.0 vs 21.3 wt %). In grt grains Mg, Ca and XMg decrease and Fe and Mn increase from core to rim. Cr shows a more complex, W-shaped zoning pattern (Fig. 1), whereby Cr abundances first decrease from core to rim, but then abruptly increase in the outermost rims (250 mm). Orthopyroxene (opx) exhibits strong zonation, with Al and Cr increasing from core to rim, followed by a decrease only in the outermost rims (100 mm). In cpx, Al, Cr and Na decrease but Mg and Ca increase from core to rim. With respect to Al, Cr and Ca, core compositions in grt and cpx are constant. Stepscan ion microprobe analyses were performed along the same profiles investigated by electron microprobe. Grt shows decreasing abundances of Sc and Ni from core to rim; only a narrow zone of constant composition is displayed in the core. In opx, trace element zoning is only shown by Ni and V. Ni decreases only in the outermost rims while V behaves like Al. In cpx, Sc increases and V decreases (like Al) in the outermost portion of rims. Geothermobarometric calculations on mineral core compositions (chemical plateaus) yield temperatures and pressures of 1150 ± 50°C and 5.9 ± 0.2 GPa derived from the combination of the grt-olivine (ol) Fe-Mg exchange thermometer (O’ Neill and Wood, 1979) with the Al-in-opx barometer (Brey and Köhler, 1990). Applying the Ni-in-grt thermometer (Ryan et al., 1996) yields similar temperatures. Our P-T values are slightly higher than previous estimates (Brenker and Brey, 1997). Lower temperatures of 1050 ± 50°C were obtained using a thermometer based on Ni-Mg partitioning between ol and opx (Podvin, 1988). The twopyroxene thermometer of Brey and Köhler (1990) involving cpx II yields even lower temperatures of 850 ± 50°C (see also Brenker and Brey 1997. Using cpx I, the obtained temperatures are only slightly higher (870 ± 50°C because both cpx generations have nearly similar Ca contents. While the abundances of Li in grt (0.2 to 0.3 ppm), opx (0.2 to 0.4 ppm), cpx I (1 to 3 ppm) and ol (1.0-1.5 pmm) are normal and reflect equilibrium partitioning (Seitz and Woodland, 1999), those of cpx II grains are abnormally high (6 to 16 ppm) indicating disequilibrium with the other phases and suggesting a secondary (metasomatic) nature of cpx II. Considering the zonation patterns and assuming that transition element diffusion is mainly controlled by temperature, we suggest that the first stage of decompression was nearly isothermal (Fig. 2) and slow enough for an increase of Al in opx. Subsequent cooling was a more rapid process because only the outermost opx rims were affected by a decrease of Al. This is supported by the decrease of Ni in opx and of Sc in grt observed in the outermost rims of grains.