CHEMICAL AND PHYSICAL PROPERTIES OF CORE FROM SERPENTINITE MUD VOLCANOES, NORTHERN MARIANA SUBDUCTION ZONE

We obtained 12 core samples for physical and chemical characterization from three serpentinite mud volcanoes (Yinazao, Asùt Tesoru, and Fantangisña) on the forearc of the Mariana subduction system, where the Pacific Plate descends beneath the Philippine Sea Plate. The core was collected during International Ocean Discovery Program Expedition 366 of 2016-2017. The materials comprising the mud volcanoes have risen diapirically along normal fault systems in the forearc that may extend down to the subducting slab; thus, examination of the core will provide insights on the properties and behavior of the megathrust region. We have completed physical-property measurements on the samples and have recently initiated petrographic examinations. Ten of the samples are thoroughly serpentinized ultramafic rocks that consist of the low-temperature serpentine varieties lizardite and chrysotile, magnetite and, in most cases, brucite. The other two come from cored intervals into Fantangisña mud volcano that contain materials derived from subducted oceanic crust of the Pacific Plate, and clasts with relict sedimentary and volcanic textures have been identified in their thin sections. Whole-rock compositions of the two samples, obtained by X-ray fluorescence techniques, are consistent with the inclusion of a substantial crustal component. Nearly all trace and rare-earth element concentrations are substantially higher in those two samples, and they also have higher Al, Ca, Na, Ti, and P contents and lower Mg, Cr, and Ni contents than the serpentinite cores. As a result, their mineral assemblages differ from those of the serpentinites in that they are rich in the Mg-smectite clay minerals saponite and corrensite, with ~7–11 wt% Al₂O₃, and they also contain trace amounts of hornblende, titanite, ilmenite and apatite. The mineralogy of the core samples, in turn, affects their physical properties. The smectite clay-rich cores have lower frictional strengths (µ < 0.2) than the serpentinites (µ = 0.2-0.4), and their measured permeabilities at 5 MPa effective pressure are also somewhat lower, at 9.6 x 10^-20 and 5.0 x 10^-19 m², compared to 8.9 x 10^-19 to 3.9 x 10^-18m² for the serpentinite cores. Future work will focus further on geochemical, mineralogical, and textural comparisons of the clay- and serpentine-rich samples.