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Plank, T.A., 1993

Mantle melting and crustal recycling in subduction zones

Bibliographic Reference

Plank, T.A., 1993, Mantle melting and crustal recycling in subduction zones: New York City, New York, Columbia University, Ph.D. dissertation, 463 p.


Major element data for basalts from ~100 arc volcanoes are examined in order to test a model whereby the mantle melts to varying extents beneath different arcs. Because the downgoing plate is at a fairly constant depth (~120 km) beneath arc volcanoes worldwide, the total length of the mantle column that is available for adiabatic melting is largely dependent on the thickness of the arc crust. Chemical parameters that reflect the degree of mantle melting might then correlate with crustal thickness. Major element data for arc basalts are corrected for the effects of differentiation by calculating values at 6% MgO. Na6 and Ca6 correlate strongly with the thickness of the arc crust: basalts erupted onto thick crust are rich in Na and poor in Ca. These characteristics are typical of low degree mantle melts. Thus, where the crust is thick, the mantle melting column is short, and the mantle melts to a small extent, producing high Na and low Ca melts. Mantle melting variations can also explain the variations in some trace elements (e.g., Sc, Ce, Zr) but others (e.g., Ba, K, Sr) may reflect subducted sediment instead. In order to test the sediment subduction model, sediment fluxes into trenches are estimated. Over 250 new chemical analyses are reported for marine sediments from DSDP/ODP Sites 765, 595, 596 and 183. Based on evaluation of these reference sites, relationships are found between the geochemical and lithologic variations in sediments. This lithologic approach is used to calculate the bulk composition for 8 trench sections (Java, Tonga, Aleutians, Antilles, Guatemala, Mexico, Vanuatu and Marianas). The sediment input fluxes correlate well with the associated arc enrichments in K, Rb, Cs, Sr, Ba, U and Th for the 8 arc/trenches pairs examined. The subducted input and volcanic output fluxes can be balanced if the entire oceanic crust (sediment + basalt) loses elements to the arc. The results of this flux balance are that the sediment loses roughly 10 - 30% of all the elements to the arc, while the basalt loses more variable amounts (0%, 2.5%, 7-15% and 30% of its Th, U, alkalis and Ba, respectively). The relatively constant proportions from the sediment suggest that the transport phase to the mantle wedge is a sediment melt, while the variable proportions from the basalt suggest an aqueous fluid. In this model, elements are recycled in subduction zones via sediment melting and basalt dehydration.

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