Shoshonitic volcanism in the Northern Mariana Arc: 2. Large‐ion lithophile and rare earth element abundances: Evidence for the source of incompatible element enrichments in intraoceanic arcs

Abstract

The Mariana‐Volcano‐Izu arc system extends 2400 km north to south and is an outstanding example of an intraoceanic magmatic arc. In spite of this, the system is poorly known because most of it is submarine. Volcanism is entirely submarine in the northernmost Mariana arc, between 20°40′ and 24°N. This is the Northern Seamount Province (NSP) and was the focus of a detailed marine geologic and geochemical study, with additional data drawn from adjacent arc segments to the north (Volcano arc) and south (Central Island Province (CIP) of the Mariana arc). Samples from 24 submarine volcanoes and three islands were analyzed for concentrations of K, Rb, Sr, Ba, and the rare earth elements (REE). These data show strong variations along the arc, being relatively depleted in the more mature, tholeiitic and low‐K calc‐alkaline volcanoes of the Volcano arc and the Mariana CIP, containing on average 6100 ppm K, 300 ppm Sr, 200 ppm Ba, and 6 ppm La. All of the NSP is enriched in large ion lithophile (LIL) and light rare earth elements (REE), particularly the northern half (26,000 ppm K, 700 ppm Sr, 900 ppm Ba, 47 ppm La); these lavas have strong shoshonitic affinities. These enrichments do not result from fractional crystallization of CIP‐type melts. The source responsible for these enrichments shares some features in common with Mariana CIP and Volcano arc sources: K/Rb and K/Ba in particular are similar (∼500 and ∼30, respectively). However, Ba/La, Sr/Nd, and (Ce/Yb)n change drastically, with Ba/La and Sr/Nd decreasing to mantle values with increasing LIL and LREE enrichment. The origin of the LIL and LREE enrichments in the NSP shoshonites does not result from variations in the behavior or composition of the subducted lithosphere. Melting occurs exclusively within the mantle wedge, and forward modeling of the REE patterns for all Mariana and Volcano arc lavas indicates that melt generation occurs within the stability field of spinel lherzolite, probably within the upper 40–50 km of the subarc asthenosphere. Lavas from the large volcanoes of the Mariana CIP and Volcano arc result from 10–20% melting of spinel lherzolite, followed by varying amounts of low‐P fractional crystallization. Inferences based on REE forward models that the NSP shoshonites manifest very low (1%) degrees of partial melting of LIL‐ and LREE‐enriched spinel lherzolite are inconsistent with observed similar concentrations in tholeiites and shoshonites of high field strength cations such as TiO2 and Yb. Some of this inconsistency can be explained as resulting from source or melt mixing, with the NSP shoshonites being derived from a LIL‐ and LREE‐enriched source or melt, with Ba/La and La/Yb indistinguishable from that of ocean island basalts (OIB), while Mariana CIP and Volcano arc melts are derived from a depleted mild‐ocean ridge basalt‐like mantle that has been recharged with K, Rb, Sr, and Ba by hydrous fluids. These variations are interpreted as reflecting the evolution of the subarc asthenosphere, with a depletion in time resulting from the continuous extraction of basaltic melts.