Geochemistry of southern Pagan Island lavas,Mariana arc: the role of subduction zone processes |
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Authors: | Jared P Marske Aaron J Pietruszka Frank A Trusdell Michael O Garcia |
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Institution: | (1) Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, USA;(2) Department of Geological Sciences, San Diego State University, San Diego, CA 92182, USA;(3) Hawaii Volcano Observatory, United States Geological Survey, Hawaii Volcanoes National Park, Hawaii, HI 96718, USA |
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Abstract: | New major and trace element abundances, and Pb, Sr, and Nd isotopic ratios of Quaternary lavas from two adjacent volcanoes
(South Pagan and the Central Volcanic Region, or CVR) located on Pagan Island allow us to investigate the mantle source (i.e.,
slab components) and melting dynamics within the Mariana intra-oceanic arc. Geologic mapping reveals a pre-caldera (780–9.4 ka)
and post-caldera (<9.4 ka) eruptive stage for South Pagan, whereas the eruptive history of the older CVR is poorly constrained.
Crystal fractionation and magma mixing were important crustal processes for lavas from both volcanoes. Geochemical and isotopic
variations indicate that South Pagan and CVR lavas, and lavas from the northern volcano on the island, Mt. Pagan, originated
from compositionally distinct parental magmas due to variations in slab contributions (sediment and aqueous fluid) to the
mantle wedge and the extent of mantle partial melting. A mixing model based on Pb and Nd isotopic ratios suggests that the
average amount of sediment in the source of CVR (~2.1%) and South Pagan (~1.8%) lavas is slightly higher than Mt. Pagan (~1.4%)
lavas. These estimates span the range of sediment-poor Guguan (~1.3%) and sediment-rich Agrigan (~2.0%) lavas for the Mariana
arc. Melt modeling demonstrates that the saucer-shaped normalized rare earth element (REE) patterns observed in Pagan lavas
can arise from partial melting of a mixed source of depleted mantle and enriched sediment, and do not require amphibole interaction
or fractionation to depress the middle REE abundances of the lavas. The modeled degree of mantle partial melting for Agrigan
(2–5%), Pagan (3–7%), and Guguan (9–15%) lavas correlates with indicators of fluid addition (e.g., Ba/Th). This relationship
suggests that the fluid flux to the mantle wedge is the dominant control on the extent of partial melting beneath Mariana
arc volcanoes. A decrease in the amount of fluid addition (lower Ba/Th) and extent of melting (higher Sm/Yb), and an increase
in the sediment contribution (higher Th/Nb, La/Sm, and Pb isotopic ratios) from Mt. Pagan to South Pagan could reflect systematic
cross-arc or irregular along-arc melting variations. These observations indicate that the length scale of compositional heterogeneity
in the mantle wedge beneath Mariana arc volcanoes is small (~10 km). |
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