O, Sr, Nd and Pb isotopic composition of the Kasuga Cross-Chain in the Mariana Arc: A new perspective on the K-h relationship

Robert J. Stern, Michael C. Jackson, Patricia Fryer, Emi Ito

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Abstract

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 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 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 δ18O for four basalts (+5.7 to +6.0‰) and three clinopyroxene separates (5.23-5.57‰) indicates an unmodified mantle source; δ18O for three andesites and dacites (6.0-6.2‰) is consistent with their evolution by shallow fractionation of basalt. K2 and K3 87Sr 86Sr and ε{lunate}Nd data range from 0.70327 to 0.70410 and +2.9 to +6.1. These lavas do not show the elevated 87Sr 86Sr for a given ε{lunate}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 206Pb 204Pb ranging from 18.77 to 19.13, Δ207Pb = +5 to +12, and Δ208Pb = +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.

Original languageEnglish (US)
Pages (from-to)459-475
Number of pages17
JournalEarth and Planetary Science Letters
Volume119
Issue number4
DOIs
StatePublished - Oct 1993

Bibliographical note

Funding Information:
We appreciate comments on early drafts of the manuscript by J. Gill and T. Sisson, and thank the referees (J. Davidson, A. Hochstaedter and Anonymous) for their scrutiny. E. Ito acknowledges NSF OCE8515887 and thanks J. Xia for help with mineral separations. M. Jackson and P. Fryer acknowledge NSF OCE8410596. R. Stern acknowledges NSF OCE8918481 and thanks P.-N. Lin and R. Gribble for analytical assistance. This is UTD Programs in Geosciences contribution 748, and University of Hawaii Planetary Geosciences publication 729 and SOEST publication 3326.

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