The earth's mantle is degassed along mid-ocean ridges, while rehydration and possibly recarbonaton occurs at subduction zones. These processes and the speciation of C-H-O fluids in the mantle are related to the oxidation state of mantle peridotite. Peridotite xenoliths from continental localities exhibit an oxygen fugacity (fo(2)) range from -1.5 to +1.5 log units relative to the FMQ (fayalite-magnetite-quartz) buffer. The lowest values are from zones of continental extension. Highly oxidized xenoliths (fo(2) greater than FMQ) come from regions of recent or acive subduction (for example, Ichinomegata, Japan), are commonly amphibole-bearing, and show trace element and isotopic evidence of fluid-rock interaction. Peridotites from ocean ridges are reduced and have an averae fo(2) of about -0.9 log units relative to FMQ, virtually coincident with values obtained from mid-ocean ridge basalt (MORB) glasses. These data are further evidence of the genetic link between MORB liquids and residual peridotite and indicate that the asthenosphere, although reducing, has CO(2) and H(2)O as its major fluid species. Incorporation of oxidized material from subduction zones into the continental lithosphere produces xenoliths that have both asthenospheric and subduction signatures. Fluids in the lithosphere are also dominated by CO(2) and H(2)O, and native C is generally unstable. Although the occurrence of native C (diamond) in deep-seated garnetiferous xenoliths and kimberlites does not require reducing conditions, calculations indicate that high Fe(3+) contents are stabilized in the garnet structure and that fo(2) deareases with increasing depth.
Simultaneous outbreaks of S. sonnei infections occurred in September 1983 at two day care centers in Seattle, Washington. At both centers, there were high rates of diarrheal illness in the children (22/80 and 11/23, respectively), in staff members, and in family members of the ill children. The smaller center was temporarily closed, but the larger one remained open. Convalescent children and staff whose diarrhea had resolved and who were receiving appropriate antimicrobials were allowed to return to the larger center before negative cultures were obtained and were separated in an isolation room. No culture-proven cases of
IntroductionThe growing use of day care (attended by 11 million
Volcano, a Quaternary volcanic centre of the eastern Aleutian Arc, produces predominantly andesiies and dacites of low-to medium-K calc-alkaline composition. Mineralogical and major element characteristics of representative lavas suggest that magmatic evolution has been influenced by both crystal fractionation and magma-mixing processes. However, incompatible trace element variations (e.g. K/Rb) indicate that these evolved lavas have been contaminated by the mafic arc crust of the underlying Talkeetna accreted terrane. The limited range of isotope compositions also supports the assimilation of nonradiogenic mafic crust (e.g. S7 Sr/ S6 Sr = 0 7032-0-7034; m Nd/ I44 Nd = 0-51301-0-5130). In addition, Pb-isotope compositions parallel the North Pacific mean oceanic trend (2(k Pb/ 2M Pb = 18-3-18-8; 2o/ Pb/ 2O4 Pb = 15-5-15-6; 208 Pb/ 204 Pb-38-2-38-3) and do not require a subducted sediment component in the source. Relatively high (Ba/La) # (0-79-18-10) and B/Be (14-5) ratios do, however, suggest a metasomatic fluid component derived from the dehydration of the subducting plate. The thickened continental crust (~35 km) of the eastern Aleutian Arc prevents the ascent of basaltic melts, which fractionate and assimilate at various depths to produce andesitic magmas. These andesites evolve towards more silicic compositions by fractional crystallization. The absence of evidence for a large high-level crustal magma chamber implies that the magmatic system beneath the volcano is young and at an immature stage of evolution.
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