Petrology of Mantle Xenoliths from Harrat al Kishb: The Mantle beneath Western Saudi Arabia

Tertiary to Recent continental rifting and sea floor spreadingformed the Red Sea. Mantle xenoliths from the Saudi ArabianRed Sea margin provide an opportunity to study the mantle beneaththe flanks of this young ocean basin. The Harrat al Kishb mantlexenolith suite consists of Cr-diopside group spinel harzburgiteand lherzolite mantle wall rock, and a variety of pyroxenitesproduced by crystallization from mafic magmas within the mantle.The pyroxenites include two texturally distinct varieties ofCr-diopside group spinel websterites, and Al-augite group spinelpyroxenite, garnet-spinel websterite, and garnet-bearing spinelclinopyroxenite. All Harrat al Kishb xenoliths are deformedto some degree and many are recrystallized. Mineral exsolutionand zoning textures indicate reequilibration to decreasing temperatureconditions. Several xenoliths provide evidence for metasomaticprocesses in the mantle beneath western Saudi Arabia. Estimates of peridotite temperatures are 900–980?C withpressure bracketed between 13 and 19 kb. Al-augite spinel pyroxenitesyield temperatures of 1050–1070?C. Garnet-spinel websteritesyield temperatures and pressures in the range 1000–1030?C,13.8–16.5 kb. These P-T estimates show that mantle temperatures are elevatedwell above those predicted by low surface heat flow measurements.Mantle heating associated with rifting is young enough thatsurface heat flow has not yet equilibrated. The xenolith dataare consistent with a model of asthenosphere upwelling beneaththe Red Sea rift. Comparison of xenolith data with existingseismic refraction data reveals a coherent picture of the compositionof the western Saudi Arabian lithosphere.     

Dolomite Mountains and the origin of the dolomite rock of which they mainly consist: historical developments and new perspectives

Beginning in the late 18th Century, the Dolomite Mountains in Northern Italy have been the location for major sedimentological developments, from the discovery of the mineral dolomite to the formulation of the coral-reef hypothesis to explain the origin of the massive dolomite structures that define the splendid scenery of the region. Further, the Dolomite Mountains have inspired voluminous research into the origin of dolomite, questioning whether dolomite is a primary precipitate or a secondary replacement product. Recently, with the recognition that microbes can mediate dolomite precipitation, a new geomicrobiological approach, combining the study of modern natural environments with bacterial culture experiments, is now being used to calibrate or interpret microbial evidence derived from the dolomite rock record. This three-pronged methodology applied to the study of dolomite formation holds great promise for future research into the 'Dolomite Problem' and provides a new impetus to revisit the Dolomite Mountains in the 21st Century.