Isotopic and incompatible element constraints on the genesis of island arc volcanics from Cold Bay and Amak Island, Aleutians, and implications for mantle structure

Details Isotopic and incompatible element constraints on the genesis of island arc volcanics from Cold Bay and Amak Island, Aleutians, and implications for mantle structure Isotopic and incompatible element constraints on the genesis of island arc volcanics from Cold Bay and Amak Island, Aleutians, and implications for mantle structure

Nov 1983

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983gecoa..47.2015m&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 47, Issue 11, pp.2015-2030

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74

Scientific paper

Cold Bay and Amak Island, two Quaternary volcanic centers in the eastern Aleutians, are orthogonal relative to the trench and separated by ~50 km. Sr, Nd and Pb isotopic compositions of the calc-alkaline andesite magmas show no sign of contamination from continental crust (average 87 Sr / 86 Sr = 0.70323, 143 Nd / 144 Nd = 0.51301, 206 Pb / 204 Pb = 18.82, 207 Pb / 204 Pb = 15.571). These samples plot within the mantle arrays for Sr-Nd and for Pb and are similar to arcs such as the Marianas and New Britain (Sr-Nd) and Marianas and Tonga (Pb). Incompatible element ratios for the Aleutian andesites (K/Rb ~ 332, K/Cs ~ 10,600, K/Sr ~ 22.4, K/Ba ~ 18.3, Ba/La ~ 60) are within the range reported for arc basalts, despite the difference in degree of fractionation. Average K content, K/Rb, K/Ba and K/Sr are approximately the same for basalts from arcs and from oceanic islands (OIB); K/Cs is a factor of 4 lower and Ba/La almost 3 times higher in arcs. Abundance ratio correlations indicate that arcs are enriched in Cs and depleted in La relative to OIB, with other incompatible element abundances very similar. Histograms of Sr and Nd isotopic compositions for MORB, OIB, and intraoceanic arcs show remarkably similar peaks and distribution patterns for intraoceanic arcs and OIB. A "plum pudding" model for the upper mantle best accommodates a) geochemical coherence of OIB and IAV, b) the existence of mantle plumes at some oceanic islands, and c) the presence of a MORB-type source at back arc spreading centers. In this model, OIB plums are imbedded in a MORB matrix; small degrees of melting generate OIB-type magmas while larger degrees of melting dilute the OIB magma with MORB matrix melts. OIB plums are merely less robust lower mantle plumes ( i.e. , blobs) which are distributed throughout the upper mantle by convection. The existence of at least two types of OIB, as indicated by Sr, Nd, and Pb isotopes, suggests that nuggets of recycled oceanic lithosphère may coexist with lower-mantle plums and that both may be tapped in arcs and intraplate environments.

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