Mineralogy and geochemistry of manganese crust from southernmost Mariana Trough: implications to hydrothermal activity

Hydrothermal fluids are important for understanding element cycling in subduction systems. Although thirty-four hydrothermal vent sites have been documented in arc and back-arc regions of the Mariana subduction zone, none was reported in the southernmost part. As a common product of hydrothermal activity, manganese crust is discovered in various tectonic settings, such as mid-ocean ridge, islands arc and intraplate seamounts. Here we are the first to report the mineralogical and geochemical characteristics of the manganese crusts from the Southwest Mariana Rift (SWMR) in the southernmost Mariana, which were collected by TV grabber during R/V Tan Suo Yi Hao TS-09 cruise in 2018. These manganese crusts are thick in 1-2 cm, and locate on the top of volcaniclastic rocks. They show the layered structure under microscope, and mainly constitute of birnessite. These samples have high Mn contents (48.10% and 52.15%), high Mn/Fe values (36.7-62.2), but low Co, Ni, and Cu contents (lower than 100ppm). They also display high concentrations in Li (>800ppm) and Mo (>1000ppm), but low in total REE contents (∑REE of 28.4 and 38.4 ppm), and no positive Ce anomalies. The above-mentioned characteristics suggest a hydrothermal origin for these manganese crusts. The results from LA-ICPMS further show that the inner dark and lower layers have relatively higher concentrations of chalcophile elements (e.g., Ni), which imply the different layers may associate with the various intensity of hydrothermal activity. The bathymetry of sampling site did not indicate any obvious volcanic edifice, but show northeast-southwest normal faults. We therefore propose that the manganese crust discovered might origin from the hydrothermal activity related to normal faults in the active arc-rifting SWMR (Wan et al., 2019).

Reference:

1. Wan et al. (2019), Geochem Geophys Geosyst, 10.1029/2018JB017080.