Reference

The petrology of lavas and associated plutonic inclusions of Ascension Island

The petrology of lavas and associated plutonic inclusions of Ascension Island. Harris, Chris (Dep. Geol. Mineral., Oxford OX1 3PR, UK). J. Petrol., 24(4), 424-70 (English) 1983. CODEN: JPTGAD. ISSN: 0022-3530. DOCUMENT TYPE: Journal CA Section: 53 (Mineralogical and Geological Chemistry) The lavas and pyroclastics of Ascension Island contain a suite of plutonic inclusions which range in compn. from olivine-gabbro to peralkaline-granite thus paralleling, but extending beyond, the compositional range of the volcanics. The lavas range from alk. basalt through hawaiite, trachybasalt, trachyandesite, and trachyte to comendite.Several substances with their cas registry numbers 7440-67-7 and 7440-17-7 may be metioned in this study. Major- and minor-element variations within the lavas suggest that crystal fractionation at low pressures is the prime differentiation process; the very presence of cumulate blocks implies that crystal settling. Mixing models for the major elements suggest that feldspar was the principal fractionating phase along with olivine, clinopyroxene, and ilmenite and/or magnetite, in proportions which are in general agreement with phenocryst abundances in the lavas and modal mineralogies of the plutonic inclusions. Although there is continued depletion of total FeO with increasing SiO2, there is evidence that an Fe oxide phase is not continuously fractionating, allowing some short-term FeO enrichment. Chem. analyses used to det. the d. contrast between lavas and possible fractionating minerals show that settling of feldspar as well as pyroxene, olivine, and an oxide phase could have occurred throughout the evolution of the suite. Coexisting magnetite and ilmenite compns. in the gabbroic inclusions and the assemblage aegirine + arfvedsonite ± aenigmatite ± fayalite in the granites suggest both crystd. under moderately reducing conditions. .

Trace-element content of "karst bauxites" and their parent rocks in the Mediterranean belt

Trace-element content of "karst bauxites" and their parent rocks in the Mediterranean belt. Ozlu, N. (Inst. Sci. Terre, Univ. Sci. Technol. Houari Boumedien, Alger, Algeria). Miner. Deposita, 18(3), 469-76 (English) 1983. CODEN: MIDEBE. ISSN: 0026-4598. DOCUMENT TYPE: Journal CA Section: 53 (Mineralogical and Geological Chemistry) A concise but comprehensive study was made of the trace element distribution in the bauxite deposits of the Mediterranean belt. Although the allocthonous and mainly detrital characters of several karst bauxite deposits are well established, their parent rock has remained largely enigmatic. 7440-67-7 and 7440-47-3 are cas registry numbers of chemicals which are used as reagents here. Evidence is available that some chem. elements, notably Zr, Cr and Ga are largely immobile during the weathering and diagenesis, and study of such trace elements in karst bauxites permits estn. of the parent rock lithol. Based on ref. to analyses of bauxites from Mediterranean areas, the deposits studied are aligned from a Cr-rich pole, related to ultramafics, to another pole rich in Ga and Zr contents and genetically related to acidic parent rocks. The results obtained in this way were controlled by application of a trace-element accumulation coeff., which permits to distinction of 4 principal zones within each of which bauxite was derived from parent rocks of different lithol. .