SPEAKER1: Nicole Neira, MS candidate, UCSB

TITLE: Results of First Seafloor SF6 Gas Tracer Experiment: Fluid Flow Direction and Velocity in 3.5 Ma Crust on Juan de Fuca Ridge Flank

ABSTRACT1:

A tracer injection experiment was performed on 3.5 M.y. old seafloor oriented N20°E, sub-parallel to the Endeavor Segment of the Juan de Fuca Ridge, 100 km to the west. A mixture of tracers was injected in Hole 1362B in 2010, during IODP Expedition 327, as part of a 24-hour pumping experiment. Fluid samples were subsequently collected from this hole and three additional holes (1026B, 1362A, and 1301A), located 300 to 500 m away. The site is blanketed by a 300-600 m thick layer of Pleistocene turbidite sediments and the array of boreholes cuts across a hypothesized S-N flow of hydrothermal fluid recharging at the southern exposed outcrop, Baby Bare, to the northern exposed outcrop, Mama Bare, situated north of the aligned holes. The focus of this talk is the analysis of sulfur hexafluoride (SF6), an ideal gas tracer that is nonreactive and almost entirely anthropogenic in its source. SF6 was injected at a concentration of 0.0192 mol/min, with fluid pumping rate of 6.7 L/s for 20.2 h, resulting in a mean concentration of 47.6 µM and 23.3 mol of SF6 being added to crustal fluids. Borehole fluid samples were collected in copper coils using osmotic pumps attached to the wellheads of several long-term, subseafloor observatories (CORKs). These samples were recovered from the seafloor using a remotely-operated vehicle in 2011 and 2013. Analyses of samplers recovered in 2011 indicate the peak arrival of SF6 in Hole 1301A, 550 m south of the injection Hole 1362B, ~304 days after injection. This suggests that the mean lateral transport of gas, at the center of tracer patch, occurred at ~1.81 m/day. SF6 data reflect perturbations caused by IODP CORK operations, but we think we can still see through perturbations to assess fluid flow direction on the 3.5 Ma Juan de Fuca site. Results from this study will provide insights of the hydrothermal aquifer system in the ocean crust by constraining hydrologic properties (flow velocity and direction) at ridge flank settings. Availability of such quantities is required to perform mass balance calculations on the global geochemical budget.  

SPEAKER2: Tyson McKinney, MS candidate, UCSB

TITLE: Evaluating mechanisms for rare earth phosphate mineralization in Proterozoic gneiss, Music Valley, California

ABSTRACT1:

Monazite and xenotime occur in ore-grade concentrations within a Proterozoic gneiss in the Music Valley region (MVR) of southern California. High concentrations of the rare earth phosphates occur in close proximity to a contact between the host gneiss and diorite bodies, the latter of which are cut by felsic pegmatite veins that appear to have been generated by partial melting of the host gneiss. Monazite and xenotime morphologies range from anhedral to subhedral and are typically restricted to folia of biotite within the gneiss. Electron microprobe imaging of the minerals commonly reveals a complex texture in which xenotime and uranothorite appear to have dissolved and reprecipitated from the original monazite and vice-versa (monazite and uranothorite within original xenotime), suggesting a later metasomatic event affected the ore bodies. Unfortunately, the presence of this texture inhibits use of the monazite-xenotime geothermometer of Gratz and Heinrich (1997). At several locations within the ore body, a breakdown reaction involving monazite, calcic-plagioclase, and biotite to form apatite and allanite is observed, also suggesting late-stage metasomatism. Preliminary U-Th/Pb geochronology of monazite and xenotime constrains rare earth phosphate mineralization to approximately 1570-1715 Ma for monazite and 1390-1785 Ma for xenotime. Preliminary U-Th/Pb geochronology of zircon produces ages ranging from 1595-1790 Ma for the Pinto Gneiss and 1475-1750 Ma for the Gold Park Diorite. One sample of diorite also contains a population of concordant ages from 1370-1410 Ma. Th/U ratios of the zircons suggest that the younger age (ca. 1400 Ma) represents the age of crystallization of the diorite while the older range of ages represent inherited zircons from the Pinto Gneiss. The relatively narrow range of zircon ages and fairly uniform Th/U ratios of the Pinto Gneiss suggest that the protolith is igneous in origin. Monazite and xenotime in the Pinto Gneiss are interpreted to have crystallized with the protolith due to the similar range of ages recorded by these phases. Zircons from pegmatite veins that cross-cut the Gold Park Diorite produce a concordant age of ~166 Ma and also contain a detrital signature from the Pinto Gneiss and Gold Park Diorite. The concordant age of ~166 Ma is interpreted to represent crystallization of the pegmatite veins, potentially related to Jurassic magmatism in the region. Future work will focus on further constraining the ages of the Pinto Gneiss, Gold Park Diorite and rare earth phosphates and investigating the monazite breakdown reaction and the generation of the dissolution-reprecipitation texture seen in the rare earth phosphates of the MVR.