Abstract
Mantle xenoliths investigated within this study are from Harrat Uwayrid, a mid Miocene to Recent volcanic field in the northwestern part of the Arabian plate. The Red Sea region offers the chance to study the formation of a new embryonic ocean, where continental rifting begins to convert into seafloor spreading. Volcanic activity related to the opening of the Red Sea started about 32 Ma B.P. and continued to recent time. Seafloor spreading in the Red Sea Region approximately commenced at 5 Ma B.P. All investigated xenoliths can be divided into spinel-peridotites (lherzolites, harzburgites, wehrlites) and spinel-pyroxenites (clino- and orthopyroxenites, websterites) and furthermore subdivided into four groups by their XCr [=Cr/(Cr+Al)] content of spinel. Some of these rocks contain additional pargasites in textural equilibrium with the other faces. In addition, most of the xenoliths show also different secondary phases (pargasite, olivine, clinopyroxene, apatite) as result of metasomatic overprints with fluids or melts. One websterite contains Ba-rich minerals like Ba-phlogopite (BaO: 11-13 wt%), Ba-pargasite (BaO: 0.8 wt%) and celsian (BaO: 30-33 wt%). Temperatures and pressures of the samples are in the range of 800 to 1100 °C (2-pyroxene-thermometer; Brey and Kohler 1990) and pressures vary between 10 and 20 kbar (Ca-in-Ol-barometer; Kohler and Brey 1990). Within the thermo barometric data of well-equilibrated xenoliths it was possible to construct a continental model geotherm (heat flow: 80-90 mW/m2) for the mantle beneath Harrat Uwayrid, which is in good accordance to other volcanic fields in this region (McGuire 1987, 1988; Al-Mishwat and Nasir 2004). The elevated heat flow is due to a young heating event of the lithospheric mantle in the north, which has not reached the surface so far. Also it can be suggested that the lithosphere in this region is attenuated. Li, Be and B were studied using the SIMS (secondary ion mass spectrometry, university of Heidelberg). Our results show that the partitioning of Li, Be and B is independent of temperature and pressure. But it appears associated with the kind and degree of metasomatic overprint and xenolith composition. Especially clino- and orthopyroxene of some spllherzolites show a decrease in their Li content from core to rim because of metasomatic reaction with a Li-depleted melt. Be and B display no zoning at all, because of slower diffusion rates. Partitioning of Li, Be and B in cpx vs. ol, opx vs. ol and cpx vs. opx follows a linear trend for all types of xenoliths. Deviation from this trend could be caused by a supply or removal of Li, Be and B without re-equilibration. Li and B are both fluid and melt mobile, Be is nearly immobile in fluids but mobile in melts. Hence fluid-influenced xenoliths are enriched in Li and B, but not in Be. This makes Li, Be and B excellent tracers to detect the kind of metasomatic agents.
Published Version
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