The record of mantle heterogeneity preserved in Earth's oceanic crust
Abstract
Earth's oceanic crust is produced by melting of the upper mantle where it upwells beneath mid-ocean ridges, and provides a geographically widespread elemental and isotopic `sample' of Earth's mantle. The chemistry of mid-ocean ridge basalts (MORB), therefore, holds key information on the compositional diversity of the upper mantle, but the problem remains that mixing and reaction during melt ascent acts to homogenise the chemical variations they acquire. Nearly all isotope and elemental data obtained thus far are for measurements of MORB glass, and this represents the final melt to crystallise, evolving in an open system. However, the crystals that are present are often not in equilibrium with their glass host. Melts trapped in these minerals indicate that they crystallised from primitive magmas that possess diverse compositions compared to the glass. Therefore, these melt inclusions preserve information on the true extent of the mantle that sources MORB, but are rarely amenable to precise isotope measurement. An alternative approach is to measure the isotope composition of the primitive minerals themselves. Our new isotope data indicates that these minerals crystallised from melts with significantly different isotope compositions to their glass host, pointing to a mantle source that has experienced extreme melt depletion. These primitive minerals largely crystallised in the lower oceanic crust, and our preliminary data for lower crustal rocks and minerals shows that they preserve a remarkable range of isotope compositions. Taken together, these results indicate that the upper mantle sampled by MORB is extremely heterogeneous, reflecting depletion and enrichment over much of Earth's geological history.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.V51D0384B
- Keywords:
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- 1037 Magma genesis and partial melting;
- GEOCHEMISTRY;
- 3075 Submarine tectonics and volcanism;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGY;
- 8416 Mid-oceanic ridge processes;
- VOLCANOLOGY