Isotopic Evidence for the Origin of Boninites and Related Rocks Drilled in the Izu-Bonin (Osagawara) Forearc, Leg 125

Abstract

Twenty-six samples representing the wide range of lithologies (low-and intermediate-Ca boninites and bronzite andesites, high-Ca boninites, basaltic andesites-rhyolites) drilled during Leg 125 at Sites 782 and 786 on the Izu-Bonin outer-arc high have been analyzed for Sr, Nd, and Pb isotopes. Nd-Sr isotope covariations show that most samples follow a trend parallel to a line from Pacific MORB mantle (PMM) to Pacific Volcanogenic sediment (PVS) but displaced slightly toward more radiogenic Sr. Pb isotope covariations show that all the Eocene-Oligocene samples plot along the Northern Hemisphere Reference Line, indicating little or no Pb derived from subducted pelagic sediment in their source. Two young basaltic andesite clasts within sediment do have a pelagic sediment signature but this may have been gained by alteration rather than subduction. In all isotopic projections, the samples form consistent groupings: the tholeiites from Site 782 and Hole 786A plot closest to PMM, the boninites and related rocks from Sites 786B plot closest to PVS, and the boninite lavas from Hole 786A and late boninitic dikes from Hole 786B occupy an intermediate position. Isotope-trace element covariations indicate that these isotopic variations can be explained by a three-component mixing model. One component (A) has the isotopic signature of PMM but is depleted in the more incompatible elements. It is interpreted as representing suboceanic mantle lithosphere. A second component (B) is relatively radiogenic (εNd = ca 4-6; 206 Pb/ 204 Pb = ca 19.0-19.3; εSr = ca -10 to -6)). Its trace element pattern has, among other characteristics, a high Zr/Sm ratio, which distinguishes it from the "normal" fluid components associated with subduction and hotspot activity. There are insufficient data at present to tie down its origin: probably it was either derived from subducted lithosphere or volcanogenic sediment fused in amphibolite facies; or it represents an asthenospheric melt component that has been fractionated by interaction with amphibole-bearing mantle. The third component (C) is characterized by high contents of Sr and high εSr values and is interpreted as a subducted fluid component. The mixing line on a diagram of Zr/Sr against ε Sr suggests that component C may have enriched the lithosphere (component A) before component B. These components may also be present on a regional basis but, if so, may not have had uniform compositions. Only the boninitic series from nearby Chichijima would require an additional, pelagic sediment component. In general, these results are consistent with models of subduction of ridges and young lithosphere during the change from a ridge-transform to subduction geometry at the initiation of subduction in the Western Pacific.

A trapped Philippine Sea plate origin for MORB from the inner slope of the Izu–Bonin trench

“…Their values do fall on a trend that is characteristic of a variety of lavas from the Philippine Sea plate. These include MORB-type basalt from the West Philippine, Parece Vela and Shikoku Basins [20,21], as well as arc-related lavas from the Palau Kyushu ridge [21], the modern arc front [14] and forearc boninites [18,19,22,23]. This characteristic isotopic signature of Philippine Sea plate rocks is the same as that from Indian Ocean spreading ridges and is interpreted to be due to a shared asthenospheric mantle domain [20,24].…”

“…This was followed by Nd puri¢cation from other REE on cation resin using 2-methyllactic acid. [13,18,19], (E) Sumisu Rift [17] and (F) active arc basalts from the Shichito Ridge and Torishima knoll [15,17]. Normalization values are from Taylor and McLennan [39].…”

“…Surprisingly, this odd isotopic signature is not shared by the subaerially exposed Eocene tholeiites and boninites from Hahajima [22] and Chichijima [23], also on the outer arc high. Pearce et al [18] interpreted the Pb isotopic trends of Site 782 and 786 samples to be due to simple mixing between Paci¢c MORB mantle and Paci¢c hotspot component or volcanogenic sediment. Because they plot near the NHRL, they have little or no Pb derived from subducted pelagic sediment in their source.…”

“…West Philippine Basin MORB sampled at Deep Sea Drilling Program Site 447 [20] have REE patterns most closely related to the trench slope samples. [16], forearc basement drilled from ODP Site 786 [18], Site 792 [18] and Site 793 [19] (see Fig. 1), boninite from the islands of Chichijima [23] and Hahajima [22], Plio-Pleistocene Izu^Bonin arc volcanics from Torishima and Miyakejima volcanoes [14] and other Philippine Sea plate submarine lavas including the West Philippine, Parece Vela and Shikoku Basins [20,21] and the Sumisu Rift [40].…”

“…The patterns for each of the samples are strikingly parallel, even for the samples dredged from Myojin forearc seamount. The samples do not have the typical enrichment of largeion lithophile elements (such as Ba) that is so characteristic of island arc magmas, including basalts from the Quaternary Izu^Bonin arc [14,15], boninites from the forearc basement [13,18,19] and Neogene sills from the Mariana and Izu^Bo-nin forearcs [7]. They also do not share REE characteristics with backarc basin basalts of the Sumisu Rift [17] or the Shikoku Basin [5,20].…”