Computer Science
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999e%26psl.171..367g&link_type=abstract
Earth and Planetary Science Letters, Volume 171, Issue 3, p. 367-381.
Computer Science
12
Scientific paper
Olivine-normalized ≤2.0 Ma magnesian basalts erupted close to the volcanic axis of the Cascadia subduction system exhibit arc-parallel compositional variations compatible with a northward decrease in slab-derived components in the underlying mantle wedge. Inferred decreases in slab input correlate strongly with a systematic decrease in age of the oceanic crust along the convergent margin. Geochemical trends are most pronounced in southwestern British Columbia and northern Washington basalts, where the Garibaldi belt strikes obliquely to the trend of isochrons on the subducted plate. From southern Washington to northern California, High Cascades arc segments trend nearly parallel to subducted plate isochrons, and basalts show more subdued arc-parallel variations. These observations support a model in which temperature and depth of melting in the mantle wedge beneath the arc are influenced by the age of the subducted plate. Where the arc is underlain by relatively young and hot oceanic crust, only a minor amount of heating during subduction is required before dehydration reactions begin in the subducted plate. The slab loses much of its volatiles trenchward of the arc, and the volatile budget in the mantle wedge beneath the arc is relatively low. In the Garibaldi belt, which overlies very young oceanic crust, this produces progressively more alkalic basalts to the north. The High Cascades overlies older oceanic lithosphere which was cooler at the time of subduction. As a result, dehydration reactions are delayed and a greater amount of volatiles are released beneath the arc. This results in lower melt temperatures and higher degrees of melting, producing predominantly low-K tholeiite and LILE-enriched HFSE-depleted calc-alkaline basalt magmas.
Green Nathan L.
Harry Dennis L.
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