HR: 15:30h
AN: V33F-07    [Abstracts]
TI: Fluoride Melts: Occurrence, Origin and Implications for Element Transfer Processes in Subduction Zones
AU: * Klemme, S
EM: sklemme@min.uni-heidelberg.de
AF: Dept of Mineralogy University of Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AB: Hitherto unknown fluoride melts were found in metasomatized mantle xenoliths from New Zealand. These fluoride melts were only preserved in rock fragments that were carefully polished using non-hydrous polishing liquids. The protogranular spinel wehrlites consist of mm-sized olivine, clinopyroxene, amphibole, accessory minerals as apatite and spinel and, on grain boundaries and in melt pockets on triple junctions, silicate and fluoride glasses. Fluoride glasses occur as veinlets and as thin films on grain boundaries, as well as in melt pockets on triple junctions. The fluoride glass is transparent, slightly yellowish and sometimes contains small secondary clinopyroxenes, and only rarely sulfide blebs or fluid inclusions. The fluoride melts are interpreted to be derived immiscibly from a precursor silicate melt and the most spectacular textural evidence for liquid immiscibility is found in one of the xenoliths. Minerals and melts in the xenoliths were analysed for major and trace elements using electron microprobe and Laser Ablation ICPMS. Trace elements are effectively partitioned between immiscible fluoride and silicate melts. For example, separation of immiscible silicate and fluoride melts fractionates light REE from heavier REE or HFSE from REE. In many cases, silicate glasses found within mantle xenoliths are products of infiltration of the host magma into the mantle xenoliths during ascend. This is, however, not the case here as comparison of the major and trace element composition of the host lava with the silicate glass indicates. The major element composition of the immiscible silicate glasses is characterised by extreme enrichment in Mg and Al, which places them close to high-Mg magmas that are commonly found in subduction zones. The genetic link to subduction zones is further substantiated by extremely low high-field strength element concentrations (e.g., Ti, Zr, Hf) that are characteristic for magmas observed in subduction zone settings.
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting