Other
Scientific paper
Oct 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.119..527s&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 119, no. 4, p. 527-537
Other
15
Composition (Property), Earth Mantle, Geochemistry, Magma, Melts (Crystal Growth), Olivine, Seamounts, Spinel, Aluminum Oxides, Basalt, Calcium Oxides, Earth Crust, Lava, Melting
Scientific paper
Partially crystalline melt inclusions in anorthite phenocrysts from a Galapagos seamount have been rehomogenized in a series of heating experiments in order to accurately determine their initial compositions. They are primitive tholeiites, high in CaO and Al2O3 and low in TiO2 and alkies. We infer that they were entrapped at a temperature close to 1270 C. At this temprature, the trapped melt composition is multiply saturated with olivine, plagioclase and spinel at 1 atmosphere pressure, but it is also near-primary requiring addition of less than 5% olivine plus or minus plagioclase and spinel in order to be in equilibrium with mantle olivine. The inclusions occur in association with, and are potentially parental to, a suite of Mid oceanic ridge basalt (MORE) like pillow lavas which range in MGO content from 10 to 8 wt% and which could have been derived by up to 40% fractionation of olivine plus plagioclase from the inclusion composition. Although the host anorthites are remarkably homogeneous and unzoned and the inclusions themselves are uniform in major element composition, minor element contents vary by a factor of two or more. Unlike many other plagioclase megacrysts, these anorthites must have crystallized under conditions in which the major element compositions were buffered but minor (and presumably trace) elements were free to vary. Such conditions could be achieved by liquid-solid interactions either in the melting regime or, perhaps more likely, in a liquid-crystal mush within the oceanic crust. These melt inclusions are not unique. They appear to belong to a class of primative, high CaO and Al2O3 MORB that occur in association with high-An plagioclase in a variety of oceanic settings, usually where magma supply is low. We propose that such magmas are derived from the shallowest, most depleted part of a mantle melt column, and that they consitute one end member of a spectrum of primary magmas which givves rise to the global array of MORB compositions.
Christie David M.
Coombs Valarie L.
Fisk Martin R.
Nielsen Roger L.
Sinton Chris W.
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