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Effects of source heterogeneity on segmentation and melt production at mid-ocean ridges

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New trace element abundances and isotope compositions (Sr, Nd, Hf and Pb) of mid-ocean ridge basalts from 5.5-19 degrees N East Pacific Rise show systematic variations along the ridge axis that coincide with ridge discontinuities. Two adjacent ridge segments are different in both isotopic and trace element systematics and thus source. The compositional variability along the northern EPR can be explained by three components: a depleted component, an enriched one and a component similar to recycled gabbro. Our high-resolution sampling indicates that on a segment-scale two “local” components are needed to explain the chemical variability within the individual ridge segments and two adjacent segments have, at most, one “local” component in common. These segment-scale “local” components are a combination of the three end members. The coincidence of distinct source compositions in adjacent ridge segments indicates that source composition influences segmentation. Ultra depleted mantle is an often ignored component of the asthenosphere because, by virtue of its nature, it is hard to recognize. Our isotope studies of abyssal peridotite show that an ultra depleted component is present and sometimes abundant. The Hf-Nd isotope variations in MORB also indicate the presence of an ultra depleted component. Such component will yield little or no melt during ascent under a ridge and thus reduce overall melt productivity and allows models where even for thin crust melting starts deep, i.e. in the garnet stability field. Our data also allows calculation of the model age of depletion of a abyssal peridotite. Lu-Hf and Sm-Nd model ages on abyssal peridotites are all relatively young (up to 2.4Ga) and are consistent with multiple depletion and enrichment events.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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