HR: 0800h
AN: V51A-0335    [Abstracts]
TI: Gallium in Feldspar Minerals - an Underutilized Source of Petrogenetic Information
AU: * Renno, A D
EM: Axel.Renno@mineral.tu-freiberg.de
AF: Freiberg University Institute of Mineralogy, Brennhausgasse 14, Freiberg, D-09596, Germany
AB: The trace element gallium is routinely analyzed in all kind of rocks. Regardless of this fact it is rarely used as a petrogenetic indicator element. Arguably the best known exception is the discrimination of A-type granites according to the Ga/Al value. Interpreting these data requires our understanding of how Ga is distributed between coexisting crystal phases and liquids. Because of the great importance of the feldspar minerals for the geochemical evolution of Ga a two- step method for Ga analysis of feldspar minerals using the electron microprobe was developed. The first step includes a screening for increased Ga-values during the routine feldspar analyses. A threshold of 100 ppm Ga was used to define 'Ga-rich feldspars'. These feldspars were analyzed for Ga using a special routine with optimized conditions for trace element analysis. To minimize the well known effects of diffusion of alkaline metals and to maintain the high spatial resolution an accelerating voltage of 15 kV was used. In order to improve precision and the lower detection limit long peak counting times of 360 - 600 seconds were chosen. The background curvature and possible interferences of both Ga-Kα and Ga-Lα lines were studied using standards very poor in gallium. The influence of the beam current on the stability of different feldspar types was tested. In order to ensure the integrity of the sample and the compliance of the analytical task an optimal ratio of beam current to beam diameter has to be chosen for every analysis. Using optimal conditions with low spatial resolution a lower detection limit of 33 ppm was reached. Using this two-step method we found unexpected high Ga values in feldspars of different rock types. Characteristic examples are albite with a Ga-Ca - ratio of > 1 from albite granites in the Central Eastern Desert (Egypt) and zoned plagioclase from mafic microgranular enclaves in granites formed by magma mixing (Karkonosze Mountains in Poland) with Ga-contents greater than 1000 ppm.
DE: 1036 Magma chamber processes (3618)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 1094 Instruments and techniques
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting