Role of a Cl‐bearing flux in the origin of depleted ocean floor magmas

Abstract

Plagioclase‐hosted melt inclusions from ocean floor lavas are characterized by great diversity in their minor and trace element compositions. The incompatible element contents of the inclusions range from enriched to ultradepleted. Ultradepleted inclusions in lavas sampled from areas such as the Galapagos Platform and the Endeavour Segment of the Juan de Fuca Ridge have Ti/Zr values as high as 3000. Such high Ti/Zr (low Ti and much lower Zr, as little as >2 ppm) suggest a source material that was melted past the point of the exhaustion of clinopyroxene (harzburgite melting). Nevertheless, these same inclusions have (La/Sm)n > 0.25 (although with low La and Sm concentrations), low K2O (0.01–0.04 wt%), and high Cl (up to 1600 ppm). In addition, examination of melt inclusion data from over 30 locations worldwide shows that there is a correlation between the level of enrichment of the host lava and the trend in K2O versus Cl described by the inclusion population. Melt inclusions from enriched lavas are characterized by relatively low Cl contents at a given K2O and no high Cl inclusions. A wide range of Cl contents and uniformly low K2O characterizes inclusions from depleted lavas. Transitional lavas exhibit either intermediate slopes or two separate trends. In contrast, other incompatible elements, such as P, Ti, and the high field strength elements (HFSE), behave coherently. The paradox presented by high Cl and (La/Sm)n together with low HFSE and K2O in melt inclusion populations from depleted lavas, coupled with uniformly high incompatible element contents in enriched lava inclusions, is inconsistent with their derivation by variable degrees of melting. Correlation of Ti/Zr with Cl in the ultradepleted inclusions, plus the absence of high Cl contents in inclusions from enriched lavas supports the contention that this signal is a mantle phenomenon and not the result of alteration at or near the seafloor. The close association of ultradepleted and normal mid‐ocean ridge basalt (NMORB) melt inclusions, sometimes in the same phenocryst, together with the existence of a complete array of melt compositions (i.e., not two distinct populations), demonstrates that the magma types were part of the same magma production episode. This combination of characteristics exhibited by melt inclusions may be produced by the interaction of a harzburgite source with a Cl, light rare earth element–bearing fluid derived from deep hydrothermal circulation. Fluctuation of magma supply may allow the upper mantle to be periodically cooled and altered by hydrothermal fluids, then undergo melting during resurgence of the magma supply. The ultradepleted component produced by that fluxed harzburgite may comprise as much as 5–10% of the array of magmas that make up NMORB magmas. If this is true, it has important implications for the thermal and mass budget and perhaps the rheology of the upper mantle and lower crust.