The impact of subduction initiation on regional to global tectonics and the compositions of major Earth reservoirs are topics of vigorous ongoing research. Here, pristine glasses extracted from ~51.9 Myr old basalts and younger boninites that erupted in the Izu-Bonin Mariana forearc immediately after subduction initiation were analyzed by microbeam techniques, with goals of characterizing the mantle sources and the conditions under which melting occurred to produce nascent arc crust. Forearc basalts (FAB) have relatively differentiated major element compositions. Thus, to determine melting conditions and source compositions, primitive melt compositions were restored through an inferred crystallization history based on melt liquidus associations. Subsequent modeling indicates that they were generated at high temperatures and low pressures relative to a mid ocean ridge basalt (MORB). Incompatible trace element compositions of FAB show that they are similar to MORB in that they were generated largely by decompression melting. Differences in several trace element ratios between MORB and FAB indicate that the mantle sources for FAB were unusually depleted. Differences between FAB sub-units indicate a range of petrogenetic histories. Upper FAB sub-units are weakly enriched in fluid-mobile elements which may indicate that fluids from the subducting Pacific plate contributed to melting. Boninites are separated into high and low silica types based on preexisting whole rock analyses. Glasses separated from these boninites are highly differentiated and thus classify as high-Mg andesites rather than boninites on MgO-SiO2-TiO2 diagrams. These glasses are also enriched in a suite of fluid mobile elements indicating that they are products of flux melting of the mantle involving fluids and melts from the subducting plate. Olivine calcium concentrations are consistent with hydrous parental boninite melts. Aluminum partitioning between olivine and hosted spinel inclusions constrains the temperatures of initial crystallization between 1170 and 1330 degrees Celsius. The change from decompression melting which generated forearc basalts to flux melting which generated high silica boninites illustrates an evolution of the subduction system over the course of the initiation process. Based on trace element ratio plots, mixing relationships between upper forearc basalts and highly enriched fluids probably released by the nascent subducting slab suggest that both decompression melting and fluid fluxing operated to produce low silica boninite during subduction initiation. This melt composition progressively becomes dominated by fluid flux melts with additional components derived from the slab to make high silica boninite. These late volcanic rocks record melting of a highly depleted mantle source. The fact that heavy rare earth element concentrations become increasingly depleted from FAB to low silica boninite to high silica boninite indicates that the mantle source changed in composition over time. The progressive decrease suggests that the initial mantle source for FAB remained the mantle source for the duration of subduction initiation related magmatism.

Two suites of intermediate-felsic plutonic rocks were recovered by dredges RD63 and RD64 (R/V KK81-06-26) from the northern wall of the Mariana trench near Guam, which is located in the southern part of the Izu-Bonin-Mariana (IBM) island arc system. The locations of the dredges are significant as the area contains volcanic rocks (forearc basalts and boninites) that have been pivotal in explaining processes that occur when one lithospheric plate initially begins to subduct beneath another. The plutonic rocks have been classified based on petrologic and geochemical analyses, which provides insight to their origin and evolution in context of the surrounding Mariana trench. Based on whole rock geochemistry, these rocks (SiO2: 49-78 wt%) have island arc trace element signatures (Ba, Sr, Rb enrichment, Nb-Ta negative anomalies, U/Th enrichment), consistent with the adjacent IBM volcanics. Depletion of rare earth elements (REEs) relative to primitive mantle and excess Zr and Hf compared to the middle REEs indicate that the source of the plutonic rocks is similar to boninites and transitional boninites. Early IBM volcanic rocks define isotopic fields (Sr, Pb, Nd and Hf-isotopes) that represent different aspects of the subduction process (e.g., sediment influence, mantle provenance). The southern Mariana plutonic rocks overlap these fields, but show a clear distinction between RD63 and RD64. Modeling of the REEs, Zr and Hf shows that the plutonic suites formed via melting of boninite crust or by crystallization from a boninite-like magma rather than other sources that are found in the IBM system. The data presented support the hypothesis that the plutonic rocks from RD63 and RD64 are products of subduction initiation and are likely pieces of middle crust in the forearc exposed at the surface by faulting and serpentine mudvolcanoes. Their existence shows that intermediate-felsic crust may form very early in the history of an intra-oceanic island arc system. Plutonic rocks with similar formation histories may exist in obducted suprasubduction zone ophiolites and would be evidence that felsic-intermediate forearc plutonics are eventually accreted to the continents.

Ce travail repose sur (1) une étude de terrain des cumulats lités de l'ophiolite d'Oman, notamment dans plusieurs massifs où cette unité restait largement inexplorée, (2) une étude pétrographique et géochimique (microsonde électronique et LA-ICP-MS) d'environ 700 échantillons et (3) une simulation en laboratoire de la cristallisation de magmas boninitiques en présence d'eau. La cinquantaine de faciès pétrographiques définie, gabbroïques et ultrabasiques, est indispensable pour décrire rigoureusement la croûte profonde omanaise. Cette variété lithologique est révélatrice de la très grande variabilité des paramètres intensifs (P, T°, PH2O ...) et extensifs (composition) lors de la cristallisation des magmas au sein des chambres magmatiques à l'aplomb d'un centre d'expansion océanique. L'orthopyroxène précoce dans les cumulats primitifs (XMg > 80 %) est beaucoup plus commun que ce qui était proposé jusqu'à présent, ce qui remet en cause certaines idées reçues concernant l'origine de l'ophiolite. Je démontre que cette variété lithologique, l'abondance d'orthopyroxène, et d'autres caractéristiques pétrographiques et géochimiques des cumulats d'Oman s'expliquent par le mélange, dans des proportions variables, de deux principaux magmas : un d'affinité tholéiitique issu de la fusion d'une source asthénosphérique " N-MORB ", et un d'affinité andésitique à boninitique issu de la fusion hydratée d'une source lithosphérique déprimée. Ces deux magmas ont circulé dans des chenaux mantelliques (" filons ") avant d'alimenter les chambres. Le rôle de " mélangeur " des chambres magmatiques est ainsi confirmé. La contribution relative de ces deux magmas évolue au cours de l'ascension de diapirs asthénosphériques dans la lithosphère. Des variations abruptes de la pression partielle d'eau sont également indispensables pour rendre compte de l'ensemble de mes observations. Je montre aussi que les failles syn-accrétions, dont l'importance en Oman restait largement sousévaluée, sont un vecteur principal de la pénétration de fluides hydrothermaux jusque dans la croûte profonde en cours de cristallisation. Mon étude me permet également de mieux contraindre l'origine, encore très discutée, des intrusions " wehrlitiques " et de la zone de transition dunitique.