Evidence for chemically heterogeneous Arctic mantle beneath the Gakkel Ridge
Date
2015-12-07
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Publisher
Elsevier
Abstract
Ultraslow spreading at mid-ocean ridges limits melting due to on-axis conductive cooling, leading to the prediction that
peridotites from these ridges are relatively fertile. To test this, we examined abyssal peridotites from the Gakkel Ridge, the
slowest spreading ridge in the global ocean ridge system. Major and trace element concentrations in pyroxene and olivine minerals
are reported for 14 dredged abyssal peridotite samples from the Sparsely Magmatic (SMZ) and Eastern Volcanic (EVZ)
Zones. We observe large compositional variations among peridotites from the same dredge and among dredges in close proximity
to each other. Modeling of lherzolite trace element compositions indicates varying degrees of non-modal fractional mantle
melting, whereas most harzburgite samples require open-system melting involving interaction with a percolating melt. All
peridotite chemistry suggests significant melting that would generate a thick crust, which is inconsistent with geophysical
observations at Gakkel Ridge. The refractory harzburgites and thin overlying oceanic crust are best explained by low
present-day melting of a previously melted heterogeneous mantle. Observed peridotite compositional variations and evidence
for melt infiltration demonstrates that fertile mantle components are present and co-existing with infertile mantle components.
Melt generated in the Gakkel mantle becomes trapped on short length-scales, which produces selective enrichments in very
incompatible rare earth elements. Melt migration and extraction may be significantly controlled by the thick lithosphere
induced by cooling at such slow spreading rates. We propose the heterogeneous mantle that exists beneath Gakkel Ridge
is the consequence of ancient melting, combined with subsequent melt percolation and entrapment.
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D’Errico, Megan E., Jessica M. Warren, and Marguerite Godard. "Evidence for chemically heterogeneous Arctic mantle beneath the Gakkel Ridge." Geochimica et Cosmochimica Acta 174 (2016): 291-312.