The concentration, behavior and storage of H2O in the suboceanic upper mantle: Implications for mantle metasomatism
1988; Elsevier BV; Volume: 52; Issue: 2 Linguagem: Inglês
10.1016/0016-7037(88)90110-x
ISSN1872-9533
Autores Tópico(s)earthquake and tectonic studies
ResumoMid-ocean ridge basalt glasses from the Pacific-Nazca Ridge and the northern Juan de Fuca Ridge were analyzed for H2O by gas chromatography. Incompatible element enriched (IEE) glasses have higher H2O contents than depleted (IED) glasses. H2O increases systematically with decreasing Mg/Mg + Fe2+ within each group. Near-primary IED MORBs have an average of about 800 ppm H2O, while near-primary IEE MORBs (with chondrite normalized Nb/Zr or La/Sm ≈2) have about 2100 ppm H2O. If these basalts formed by 10–20% partial melting then the IED mantle source had 100–180 ppm H2O, while the IEE source had 250–450 ppm H2O. The ratio H2O/(Ce + Nd) is fairly constant at 95 ± 30 for all oceanic basalts from the Pacific. During trace element fractionation in the suboceanic upper mantle, H2O behaves more compatibly than K, Rb, Nb, and Cl, but less compatibly than Sm, Zr and Ti. H2O is contained mostly in amphibole in the shallow upper mantle. At pressures greater than the amphibole stability limit, it is likely that a significant proportion of H2O is contained in a mantle phase which is more refractory than phlogopite at these pressures. The role of H2O in mantle enrichment processes is examined by assuming that an enriched component was added. The modelled concentrations of K, Na, Ti and incompatible trace elements in this component are high relative to H2O, indicating that suboceanic mantle enrichment is caused by silicate melts such as basanites and not by aqueous fluids.
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