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Mid-Ocean Ridge and Upper Mantle Processes

Noble Gas Geochemistry Lab Figure 1

Click image to enlarge. Map showing the variation of 3He/4He (R/RA) along the global mid-ocean ridge system. Values above 10 RA are characteristic of regions affected by nearby mantle hotspots, such as Iceland (northern Mid-Atlantic Ridge), Shona and Bouvet (southern Mid-Atlantic Ridge), Afar (Red Sea) and Amsterdam/St. Paul (Southeast Indian Ridge). Data from PetDB database and Graham (unpublished).

Mid-ocean ridge basalts (MORBs) form by decompression partial melting as ascending mantle beneath spreading ridges reaches its solidus temperature. Noble gases in MORBs are generally accepted to be a representative sampling of the noble gas state in the convecting upper mantle. The study of noble gases in such igneous rocks has been central to understanding chemical heterogeneity and convection in the Earth's mantle.

The magnitude and length scales of isotopic variations along ridges are related to mixing and melting in Earth's upper mantle, and so have been especially informative for geodynamics. MORBs show a relatively narrow range in 3He/4He ratio. The He isotope composition of a sample is conventionally reported as R/RA, where R is the measured ratio and RA is the ratio in air. MORBs have a median 3He/4He of 8.1 to 8.2 RA and the values are identical for each ocean basin. In contrast, ocean island basalts are much more variable, and extend to values that are higher than the MORB average by more than a factor of 5. The difference in 3He/4He between MORBs and OIBs has often been taken as evidence for two distinct mantle source regions, with the ocean island (hotspot) source being deeper and less degassed, although this viewpoint has occasionally been challenged. The fact that the MORB range is small in comparison to the global range that includes ocean islands implies that convective mixing in the upper mantle is relatively rapid. Nevertheless, the variability in MORBs is significant. This variability results from (1) input of high 3He/4He material from deeper in the mantle via hotspot-ridge interaction (see figure), (2) radiogenic ingrowth of 4He in the upper mantle, (3) the presence of recycled (subducted) crust and lithosphere that is enriched in U and Th, and (4) partial melting of upper mantle that is chemically (mineralogically) heterogeneous. Noble gas isotope variations along mid-ocean ridges provide important clues about these processes.

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