OS22C-07
AUV mapping and targeted ROV sampling on the Alarcon Rise

Tuesday, 15 December 2015: 11:50
3018 (Moscone West)
David A Clague1, David W Caress1, Brian M Dreyer2, Jennifer Brophy Paduan1, Lonny Lundsten1, Julie Bowles3, Paterno Castillo4, Rigoberto G. France5, Ryan A Portner6, Ronald M Spelz5 and Robert A Zierenberg7, (1)Monterey Bay Aquarium Research Institute, Watsonville, CA, United States, (2)University of California Santa Cruz, Santa Cruz, CA, United States, (3)University of Wisconsin Milwaukee, Geosciences, Milwaukee, WI, United States, (4)University of California San Diego, La Jolla, CA, United States, (5)Universidad Autonoma de Baja California, Ensenada, Mexico, (6)Brown University, Providence, RI, United States, (7)University of California Davis, Earth and Planetary Sciences, Davis, CA, United States
Abstract:
Alarcon Rise, the northernmost bare-rock East Pacific Rise segment, and its intersections with the adjacent Pescadero and Tamayo Transforms were mapped at 10-cm vertical and 1-m lateral resolution using an AUV. The ~50 km long ridge segment is the first completely mapped at such high-resolution. Using the AUV base maps, targeted sampling during 15 ROV dives and 29 wax-tip cores in 2012 and 2015 recovered 322 precisely located glassy lavas.

Melts are most primitive (MgO>8.5%) near the shallowest part of the segment where a chemically heterogeneous (7.96-8.73% MgO) sheet flow erupted from a 9 km-long fissure, ~1/3 from the southwest end. Four active black-smoker hydrothermal fields were discovered here using the maps. Inactive fields occur to the north. Lavas are N- to T-MORB with an off-axis E-MORB near mid-segment. A cross-axis transect to the northwest shows similar lava chemistry to 3.4 km off-axis.

Extensive flows with glass MgO of 5.3-7.3% and 6.7-7.5% occur on the Tamayo and Pescadero Transforms, respectively. The flows have variable sediment cover indicating a wide age range. The transforms have sediment domes likely uplifted and deformed by sills. Most domes are surrounded by younger lava flows. Magmatic activity on the transforms indicates they are transtensional.

An unusually rough faulted terrain on the northeastern end of the segment occurs near the northwestern edge of the neovolcanic zone. Sampling during 5 ROV dives recovered 110 glassy samples of rhyolite, dacite, andesite, and basaltic andesite from the 8 km-long fault-bounded region with basalt recovered along the southeastern part of the zone. The samples form an almost continuous sequence with glass compositions from 50-77% SiO2 and 8-0.05% MgO and include the first rhyolitic lavas from the global submarine ridge system. Isotopic data preclude significant continental crustal contamination, indicating the evolved rocks formed by fractional crystallization and magma mixing of mantle derived melts.

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