New inductively coupled plasma mass spectrometry (ICP-MS) trace elemen
t data are presented on a suite of are lavas from the northern Mariana
and southern Bonin island arcs. The samples were dredged fi om seamou
nts in the Central Island Province (CIP), the Northern Seamount Provin
ce (NSP) and the Volcano Arc (VA), and they range in composition from
low-K tholeiites to shoshonites. Previous studies on these samples con
cluded that the primary compositional control was two-component mixing
between a fluid-metasomatized mid-ocean ridge basalt (MORB) source an
d an enriched, ocean island basalt (OIB)-like, mantle component, with
subducted sediment material playing a secondary role. However, the new
trace element data suggest that the compositional variations along th
e Mariana are can be better explained by the addition of spatially var
ying subduction components to a spatially varying mantle source. The d
ata suggest that the subduction component in the CIP and VA is dominat
ed by aqueous fluids derived from altered oceanic crust and a pelagic
sediment component, while the subduction component in the NSP is domin
ated by more silicic fluids derived from volcanogenic sediments as wel
l as from pelagic sediment and altered oceanic crust. The mantle wedge
in the CIP and VA is depleted relative to a normal mid-ocean ridge ba
salt source by loss of a small melt fraction, while the mantle wedge i
n the NSP is enriched either by possible gain of a small melt fraction
or addition of a sediment-derived melt. Because the subduction of sea
mounts controls the are and back-are geometries, so the concomitant va
riation between subducted material and mantle composition may be no co
incidence. The high field strength element (HFSE) data indicate a high
degree of melting (similar to 25-30%) throughout the are, similar to
10% of which may be attributed to decompression and similar to 20% to
fluid addition.