Computer Science
Scientific paper
Oct 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.119..459s&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 119, no. 4, p. 459-475
Computer Science
14
Earth Crust, Earth Mantle, Lead Isotopes, Neodymium, Oxygen 18, Strontium, Volcanoes, Volcanology, Basalt, Geodynamics, Lava, Lithology, Magma
Scientific paper
The Kasuga system in the Mariana Arc consists of three submarine volcanoes, Kasuga 1 (K1) along the magmatic front and Kasuga 2 (K2) and Kasuga 3 (K3) extending into the back arc. K1 and nearby Fukujin are dominated by low- and medium-K tholeiitic andesites and basaltic andesites while K2 and K3 lavas are calc-alkaline and consist of primitive medium- and high-K basalts and absarokites (Mg# = 66-80) and subordinate andesites and dacites. Construction of this complex on the back-arc basin crust that is no older than 45 Ma permits resolution of the sources and processes responsible for the increase in LIL contents in arc volcanic rocks with with distance to the Benioff zone (the K-h relationship). New O, Sr, Nd and Pb isotopic data for fresh lavas from K2 and K3 and Pb isotopic data for K1 are interpreted to better understand the significance of these enrichments. The range in delta O-18 for four basalts (+5.7 to +6.0 parts per thousand) and three clinopyroxene separates (5.23-5.57 parts per thousand) indicates an unmodified mantle source; delta O-18 for three andesites and dacites (6.0-6.2 parts per thousand) is consistant with their evolution by shallow fractionation of basalt. K2 and K3 Sr-87/Sr-86 and epsilon(sub Nd) data range from 0.70327 to 0.70410 and +2.9 to +6.1. These lavas do not show the elevated Sr-87/Sr-86 for a given epsilon(sub Nd) that is characteristic of lavas erupted along a magmatic front; instead they suggest participation of OIB-like mantle. Pb isotopic data also show significant variations, with Pb-206/Pb-204 ranging from 18.77 to 19.13, delta(sub 207)Pb = +5 to +12, and delta(sub 208) = +7 to +30. The trend of the Pb isotopic data and mantle-like Ce/Pb (10-23) and Ba/La (10-27) preclude any significant role for subducted crust or sediments in controlling these enrichments. Instead, the inverse variation of magma production rate and isotopic heterogeneity is interpreted to indicate diminished melting with distance from the magmatic front, such that the smaller volcanoes K2 and K3 are dominated by the lowest temperature-melting fraction of a heterogeneous mantle. Diminished water flux results in preferential melting of OIB-type mantle plums-and attendant enrichments in resultant magmas-with depth to the Benioff zone.
Fryer Patricia
Ito Emi
Jackson Michael C.
Stern Robert J.
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