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H2O Abundance in Depleted to Moderately Enriched Mid-ocean Ridge Magmas; Part I: Incompatible Behaviour, Implications for Mantle Storage, and Origin of Regional Variations |
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| Authors: | DANYUSHEVSKY LEONID V; EGGINS STEPHEN M; FALLOON TREVOR J; CHRISTIE DAVID M |
| Institution: | 1SCHOOL OF EARTH SCIENCES, UNIVERSITY OF TASMANIA, GPO BOX 252-79, HOBART, TAS. 7001, AUSTRALIA 2ARC KEY CENTRE FOR GEOCHEMICAL EVOLUTION AND METALLOGENY OF THE CONTINENTS (GEMOC), DEPARTMENT OF GEOLOGY; AND RESEARCH SCHOOL OF EARTH SCIENCES; AUSTRALIAN NATIONAL UNIVERSITY, GPO BOX 4, CANBERRA, A.C.T. 2601, AUSTRALIA 3COLLEGE OF OCEANIC AND ATMOSPHERIC SCIENCES, 104 OCEAN ADMINISTRATION BUILDING, OREGON STATE UNIVERSITY, CORVALLIS, OR 97331-5503, USA |
| Abstract: | The H2O contents and trace-element abundances are presentedfor two well-studied suites of mid-ocean ridge basalt (MORB)glasses from the Northern East Pacific Rise (EPR, 9–11°N)and the South East Indian Ridge (SEIR, 127–129°E).Exactly the same region of the glass samples has been analysedfor these components using microbeam techniques. Our data allowexamination of the fine details of H2O geochemical behaviourduring MORB genesis. We demonstrate that relative H2O contentsi.e. H2O/(another incompatible element)] vary systematicallywith increasing (La/Sm)N in MORB glasses from both the EPR andSEIR. This indicates that H2O behaves like other incompatible(in peridotite mineralogies) elements during MORB petrogenesis,and is primarily controlled by solid–melt partitioning.However, the relative H2O contents of MORB glasses from theSEIR are higher than in glasses from the EPR at a given (La/Sm)N,demonstrating global variations in the H2O contents of MORB.Despite regional differences in relative H2O contents, the incompatiblebehaviour of H2O is similar in both studied regions. The relativeincompatibility of H2O varies systematically with increasing(La/Sm)N: in depleted MORB, H2O is similar to La whereas inEMORB, H2O is similar to Ce. Similar patterns of varying relativeincompatibility (to REE) are displayed by Zr, Hf, and P. Ourdata are best explained if H2O is stored in the mantle in thesame phase with LREE (clinopyroxene?) at sub-solidus. Regionalvariations in relative H2O contents in EMORB that have moreradiogenic Sr, Nd and Pb isotopes might be explained by differencesin the nature of enriched components recycled via subductionprocesses. However, when EMORB have the same radiogenic isotopecompositions as NMORB within a segment, relative H2O contentsin EMORB probably reflect local processes that lead to enrichmentin incompatible elements. Regional differences in relative H2Ocontents of NMORB may reflect either initial variations in theEarth’s mantle or inhomogeneities left after formationof the continental crust. KEY WORDS: glass; geochemistry; H2O; MORB; petrology |
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