THE MAGMATIC ARCHITECTURE OF TANEY SEAMOUNT-A, NORTHEAST PACIFIC OCEAN

The Taney Seamounts are a linear NW-SE trending, near mid-ocean ridge chain consisting of five volcanoes located on the Pacific plate 300 km west of San Francisco, California. Taney Seamount-A, the largest and oldest in the chain, has four well-defined calderas. These calderas have clear cross-cutting relationships, creating a relative chronology. The caldera walls and intracaldera pillow mounds were sampled systematically by a remotely operated vehicle (ROV) to obtain stratigraphically-controlled samples.

The geochemistry of the seamount varies from more differentiated to more primitive with time (6.2 - 8.6 wt. % MgO), suggesting that the sub-caldera reservoir is open and undergoes periodic collapse, replenishment, crystallization, and eruption. The erupted lavas vary from typical peridotite-derived N-MORB compositions to those with an apparent residual garnet signature. The youngest and least differentiated lavas entrained a crystal cargo of high-anorthite plagioclase (An_80-90) with melt inclusion volatile saturation pressures indicating entrapment at the lower crust or upper mantle (8-10 km below the sea floor). Melt inclusions exhibit high Al₂O₃, low SiO₂, positive Sr and Eu anomalies, negative Zr and Nb anomalies, and (Ba/Nb)_N> 1 when normalized to primitive mantle.

We utilize geochemical and thermodynamic modeling to demonstrate that decompression melting of a MORB mantle peridotite re-fertilized by garnet pyroxenite partial melts can reproduce the garnet signature observed in the Taney-A edifice lavas. In addition, we demonstrate that episodic partial melting, magma mixing, and recrystallization of lower-crustal plagioclase-rich cumulates at the Moho are recorded in melt inclusions that carry a plagioclase cumulate signal. Hence the magmatic architecture of Taney Seamount-A is characterized by the melting of a mixed lithology mantle, melt-rock interaction in the upper mantle to lower oceanic crust, and open system evolution in a sub-caldera magma reservoir.