Melt-rock interaction at the Moho: Evidence from crystal cargo in lavas from near-ridge seamounts

Abstract:

The Taney Seamounts are a NW-SE trending linear, 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, is defined by four well-exposed calderas which reveal previously infilled lavas. The calderas can be differentiated in time by their 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, a unique aspect of this study.

Lava geochemistry 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, shallow crystallization, and eruption. The primitive replenishing magmas entrain a crystal cargo of high-anorthite plagioclase (An_80-90) with melt inclusion volatile saturation pressures (CO₂ - H₂O) indicating entrapment at the lower crust or upper mantle (8-12 km b.s.f). Melt inclusions exhibit positive Sr and Eu anomalies (e.g., Sr_PM / [Ce_PMNd_PM]^1/2 ), negative Zr and Nb anomalies, and [Ba/Nb]_PM >1 when normalized to primitive mantle. In comparison, the host lavas exhibit positive Sr anomalies but no concurrent Eu, Zr, and Nb anomalies and [Ba/Nb]_PM <1. We propose that episodic partial melting and recrystallization of lower-crustal cumulates at the Moho result in melt inclusions with a plagioclase cumulate signal. Later percolating melts undergo diffusion with and entrain recrystallized plagioclase cumulates resulting in the positive Sr signal (but no Eu, Zr, Nb anomalies, and [Ba/Nb]_PM <1). Geochemistry of the host lava and melt inclusions and crystal textures imply that melt-rock interaction is an important process in oceanic magmatic systems.