Mid-Ocean Ridge Serpentinite In The Puerto Rico Trench: From Seafloor Spreading To Subduction (Vol 58, Pg 1729, 2017)

In this study we investigate the origin and alteration history of serpentinites in two dredge hauls from the North Wall of the Puerto Rico Trench, where the North American plate subducts beneath the Greater Antilles island arc. Faulting of the North American plate at the North Wall of the Puerto Rico Trench exposes oceanic basement in a nearly spreading-parallel direction, offering a unique perspective on crustal accretion and alteration processes in the Cretaceous Quiet Zone. U-Pb age dating of porous zircon in an altered mafic vein in serpentinite from 20 degrees 00'00"N, 66 degrees 31'57"W yields an age of 114.8 Ma, suggesting that it originated at the Cretaceous Mid-Atlantic Ridge, c. 2200 km east of its current location. After localized high-temperature (>380 degrees C) hydrothermal alteration to chlorite, antigorite, talc and tremolite, peridotites underwent pervasive serpentinization at lower temperatures (similar to 310-240 degrees C). In addition to lizardite-rich serpentinite, which is prevalent in dredge D10, dredge D2 contains serpentinite dominated by antigorite. Pervasive serpentinization was followed by extensive talc alteration of several antigorite-rich serpentinites from dredge D2, which was accompanied by Fe loss. As the newly formed crust moved off-axis, open-system alteration at low temperatures by seawater (i.e. weathering) modified the mineralogy, chemical composition, and physical properties of the serpentinite. Where present, brucite dissolved and magnetite oxidized to hematite and goethite, which caused a decrease in magnetic susceptibility and an increase in porosity. Weathering led to complete oxidation of several lizardite-serpentinites in dredge D10, whereas talc-altered antigorite serpentinites in dredge D2 were less affected by oxidation. Rare earth element concentrations of lizardite-rich serpentinite in dredge D10 decreased steadily with increasing Fe(III)/Fe-tot, reflecting simultaneous leaching and oxidation by seawater. These results have implications for our understanding of several processes, including hydrothermal alteration of peridotite at the Cretaceous Mid-Atlantic Ridge, mass transfer and redox reactions between serpentinite and seawater during off-axis weathering, and the introduction of volatiles, oxidizing agents, and antigorite beneath the Greater Antilles Arc.