11:15 AM - 11:30 AM
[U03-09] Initial findings of post-cruise research on IODP Expedition 352 hard-rock cores I: Petrology and geochronology
Keywords:Subduction initiation, Basalt, Boninite, Ogasawara
All lavas from Sites U1439 and U1442 are boninitic with considerable enrichments in fluid-soluble elements, presumably from the newly subducting Pacific plate. Preliminary 40Ar/39Ar ages of the boninites range from ~52-50 Ma from sites U1439 and U1442, respectively. The lowermost cores of Hole U1439C consist of fine-grained dolerites with rare chilled margins, suggesting they could represent the feeder system for the overlying lavas and tephras.1 However, trace element concentrations in clinopyroxene phenocrysts from this basal section suggest that they represent a compositionally transitional unit between boninite and FAB. The sheet flows, pillow lavas, and hyaloclastites that make up the majority of cores from these sites are low-Si boninites. The oldest boninite age from Site U1439 indicates that boninites began erupting within the same time frame as FAB. This age is significantly older than the ages of boninites found on the nearby Ogasawara Ridge2, which lengthens the duration of boninite magma generation in the nascent IBM system to at least 7 million years. High-Si boninites cap the stratigraphy at both sites. One high-Si boninite from Site U1442 was dated at ~50 Ma, indicating that volcanism persisted at our boninite sites for ~2 million years. Overall, REE concentrations in boninites generally decrease up-section, and change from LREE-depleted to LREE-enriched, reflecting an increase in subduction flux with time. Zr/Sm ratios increase up section, which we attribute to Zr mobility. Major element data imply melting depths shallowed over time. These data are consistent with initial production of basaltic crust during rapid sea-floor spreading immediately after subduction initiation at about 52 Ma with minimal involvement of fluids from the subducting Pacific plate. Within about 105 years, fluids from subducting Pacific lithosphere became involved in mantle melting to generate first low-, then high-Si boninites. The persistence of boninitic volcanism at Sites U1439 and U1442 suggests that the dynamics of magma genesis transitioned from sea-floor spreading towards central-vent volcanism about the time boninites began erupting.
1Reagan et al. (2017) Intl. Geol. Rev., doi:10.1080/00206814.2016.1276482.
2Ishuzuka et al. (2006) Earth and Planet. Sci. Lett., doi:10.1016/j.epsl.2006.08.007.