Diverse Sr isotope signatures preserved in mid-oceanic-ridge basalt plagioclase

Abstract

At mid-ocean ridges, a variety of crustal processes overprint mantle-derived melts and can obscure original mantle compositions. To address the nature of this crustal filter, we report 87 Sr/ 86 Sr ratios from plagioclase phenocrysts and host glasses in mid-oceanic-ridge basalts from the Juan de Fuca Ridge, Blanco Transform Zone, and the Southwest Indian Ridge. Microdrilled isotopic analyses reveal significant inter- and intracrystalline disequilibria within individual samples. These disequilibria suggest that a range of isotopically distinct melt components contribute to individual plagioclase crystals and to the magmas that transport them to the surface. Low Cl/K values both in the host glass and in plagioclase-hosted melt inclusions largely rule out incorporation of seawater-derived material as an explanation for differences in 87 Sr/ 86 Sr. Instead, the observed heterogeneity implies derivation of magmas from isotopically diverse mantle sources. Importantly, the range of Sr isotope values preserved in a single sample is similar to the range of compositions seen at the ridge segment scale. Unlike analyses of host glass compositions, which are the result of extensive crustal processing, isotopic analyses of phenocryst phases record fine-scale aggregation of these distinct mantle-derived melts and are thus an important and underutilized tool in interpreting the nature of the mid-oceanic-ridge basalts.