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Trace element analyses (first-series transition elements, Ti, Rb, Sr, Zr, Y, Nb, and REE) were carried out on whole rocks and minerals from 10 peridotite samples from both Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc using a combination of XRF, ID-MS, ICP-MS, and ion microprobe. The concentrations of incompatible trace elements are generally low, reflecting the highly residual nature of the peridotites and their low clinopyroxene content (<2%). Chondrite-normalized REE patterns show extreme U shapes with (La/Sm) n ratios in the range of 5.03-250.0 and (Sm/Yb) n ratios in the range of 0.05-0.25; several samples show possible small positive Eu anomalies. LREE enrichment is common to both seamounts, although the peridotites from Conical Seamount have higher (La/Ce) n ratios on extended chondrite-normalized plots, in which both REEs and other trace elements are organized according to their incompatibility with respect to a harzburgitic mantle. Comparison with abyssal peridotite patterns suggests that the LREEs, Rb, Nb, Sr, Sm, and Eu are all enriched in the Leg 125 peridotites, but Ti and the HREEs exhibit no obvious enrichment. The peridotites also give positive anomalies for Zr and Sr relative to their neighboring REEs. Covariation diagrams based on clinopyroxene data show that Ti and the HREEs plot on an extension of an abyssal peridotite trend to more residual compositions. However, the LREEs, Rb, Sr, Sm, and Eu are displaced off this trend toward higher values, suggesting that these elements were introduced during an enrichment event. The axis of dispersion on these plots further suggests that enrichment took place during or after melting and thus was not a characteristic of the lithosphere before subduction.Compared with boninites sampled from the Izu-Bonin-Mariana forearc, the peridotites are significantly more enriched in LREEs. Modeling of the melting process indicates that if they represent the most depleted residues of the melting events that generated forearc boninites they must have experienced subsolidus enrichment in these elements, as well as in Rb, Sr, Zr, Nb, Sm, and Eu. The lack of any correlation with the degree of serpentinization suggests that low-temperature fluids were not the prime cause of enrichment. The enrichment in the high-field-strength elements also suggests that at least some of this enrichment may have involved melts rather than aqueous fluids. Moreover, the presence of the hydrous minerals magnesio-hornblende and tremolite and the common resorption of orthopyroxene indicate that this high-temperature peridotite-fluid interaction may have taken place in a water-rich environment in the forearc following the melting event that produced the boninites. The peridotites from Leg 125 may therefore contain a record of an important flux of elements into the mantle wedge during the initial formation of forearc lithosphere. Ophiolitic peridotites with these characteristics have not yet been reported, perhaps because the precise equiva...

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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.

During the Pleistocene and Holocene, alkaline and tholeiitic magmas were erupted in the Zuni‐Bandera volcanic field (ZBVF) on the western flanks of the Rio Grande Rift, New Mexico. While most of the alkaline basalts are magnesian (i.e., >8.0% MgO), the tholeiitic basalts show evidence of fractionation of olivine and clinopyroxene. The ZBVF lavas have intraplate chemistry similar to that of alkaline and tholeiitic basalts in other continental and oceanic environments, and Sr and Nd isotopes range from depleted compositions similar to mid‐ocean ridge basalts to enriched compositions similar to ocean islands like Kerguelen. Slightly higher Th/Ta and Ba/Nb ratios in some of the ZBVF tholeiitic basalts correlate with isotopic ratios, and this may indicate minor involvement of a Th rich component (i.e., crust). Alternatively, such characteristics may merely reflect intraplate processes if one considers that the isotopic variation observed in the ZBVF is not that different from what is observed in Hawaiian basalts. Systematic changes in elemental and isotopic ratios with the degree of partial melting (Zr/Y) reveal that the geochemistry of the ZBVF magmas is dominated by two components: (1) a depleted mantle component which produced an enriched alkaline magma as a small degree melt (e.g., Ta/Yb=6.0; {La/Yb}N=60; Zr/Y=19; 87Sr/86Sr=0.703); and (2) an enriched mantle component which produced tholeiitic magmas as a larger degree melt (e.g., Ta/Yb <0.5;{La/Yb}N < 15.0; Zr/Y=3–4; 87Sr/86Sr = 0.706). While the depleted component is interpreted to be MORB asthenosphere the enriched component is more problematical and may reside in plume‐contaminated asthenosphere or stratified lithosphere. The lower lithosphere beneath the Proterozoic crust of the western USA appears to be chemically stratified and comprises a depleted peridotite protolith stabilized in the Proterozoic that may have been subsequently enriched due to interaction with fluids related to (1) suprasubduction processes during Proterozoic‐Phanerozoic accretion, (2) upwelling plumes (Rayton‐Clayton) in the Phanerozoic, and (3) asthenospheric upwelling in the late Phanerozoic. Alternatively plume‐contaminated asthenosphere provides a possible source for the enriched intraplate aspect of the ZBVF lavas.