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Mg-Al Sapphirine- and Ca-Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya 总被引:2,自引:0,他引:2
Basanites of the Chyulu Hills (Kenya Rift) contain mafic MgAland CaAl granulite xenoliths. Their protoliths are interpretedas troctolitic cumulates; however, the original mineral assemblageswere almost completely transformed by subsolidus reactions.MgAl granulites contain the minerals spinel, sapphirine,sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxeneand garnet, whereas CaAl granulites are characterizedby hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase,quartz, clinopyroxene, corundum, and garnet. In the MgAlgranulites, the first generation of orthopyroxene and some spinelmay be of igneous origin. In the CaAl granulites, hibonite(and possibly some spinel) are the earliest, possibly igneous,minerals in the crystallization sequence. Most pyroxene, spineland corundum in MgAl and CaAl granulites formedby subsolidus reactions. The qualitative PT path derivedfrom metamorphic reactions corresponds to subsolidus cooling,probably accompanied, or followed by, compression. Final equilibrationwas achieved at T 600740°C and P <8 kbar, inthe stability field of sillimanite. The early coexistence ofcorundum and pyroxenes (± spinel), as well as the associationof sillimanite and sapphirine with clinopyroxene and the presenceof hibonite, makes both types of granulite rare. The CaAlhibonite-bearing granulites are unique. Both types enlarge thespectrum of known CaAlMg-rich granulites worldwide. KEY WORDS: granulite xenoliths; corundum; sapphirine; hibonite; Kenya Rift 相似文献
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Experimental and analytical procedures devised for measurement of rare earth element (REE) abundances using a secondary ion
mass spectrometer (ion microprobe) are described. This approach is more versatile than the conventional techniques such as
neutron activation analysis and isotope dilution mass spectrometry by virtue of its high spatial resolution that allows determination
of REE abundances in small domains (10-20 micron) within individual mineral phases. The ion microprobe measurements are performed
at a low mass-resolving power adopting the energy-filtering technique (Zinner and Crozaz 1986) for removal and suppression
of unresolved complex molecular interferences in the REE masses of interest. Synthetic standards are used for determining
various instrument specific parameters needed in the data deconvolution procedure adopted for obtaining REE abundances. Results
obtained from analysis of standards show that our ion microprobe may be used for determining REE abundances down to ppm range
with uncertainties of ∼ 10 to 15%. Abundances of rare earth and several other refractory trace elements in a set of early
solar system objects isolated from two primitive carbonaceous chondrites were determined using the procedures devised by us.
The results suggest that some of these objects could be high temperature nebular condensates, while others are products of
melting and recrystallization of precursor nebular solids in a high temperature environment. 相似文献
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