Hydrogen and oxygen isotope geochemistry of Ascension Island lavas and granites; variation with crystal fractionation and interaction with sea water

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Authors:Sheppard, Simon M. F.; Harris, C.
Author Affiliations:Primary:
Cent. Rech. Petrogr. Geochim., F-54501 Vandoeuvre-les-Nancy, France
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 91(1), p.74-81. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1985
Note:In English. 58 refs.; illus. incl. 2 tables
Summary:Lavas and pyroclastics on Ascension Island contain plutonic blocks that include fluid-inclusion-bearing peralkaline granite. The 18O/16O ratios, F and Cl were analysed on whole-rocks and/or minerals for lavas and granites. Whole-rock 18O/16O ratios of fresh alkali-basalt, hawaiite, trachyandesite, trachyte and comendite range 6.0-6.9%, with 18O tending to increase with increase in SiO2. The delta 18O values of the granites are 0.0-0.3per mille, depleted in 18O relative to the comendites. The O-isotopic trend of the lavas is consistent with a crystal fractionation model. Fresh igneous rocks with delta 18O values >7per mille involve processes in addition to crystal fractionation of a basaltic magma. The D/H ratios and Cl contents of the H2O-poor comenditic obsidians represent undegassed primary magmatic values. The H-isotope compositions and low H2O and Cl contents of the granites are consistent with major degassing of an H2O-saturated magma derived from interaction of sea- (or possibly meteoric) water with the H2O-undersaturated comenditic melt. It is proposed that, associated with caldera subsidence and stoping, water was sucked in around the residual magma before the system had time to be sealed. The magma consumed this H2O at a P of approx 1.5 kbar. The granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites. Selected chemical and isotopic compositions of 24 samples from 11 localities are presented. [P.Br.]
Subjects:Alkali granites; D/H; Deuterium; Differentiation; Fractional crystallization; Geochemistry; Granites; Hydrogen; Igneous rocks; Isotopes; Lava; Magmas; O-18/O-16; Oxygen; Petrology; Plutonic rocks; Sea water; Stable isotopes; Ascension Island; Atlantic Ocean Islands; Ratios
Abstract Numbers:86M/1995
Record ID:1987017658
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from Geoline, Bundesanstalt fur Geowissenschaften und Rohstoffe
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