Identification and implications of off-axis lava flows around the East Pacific Rise

Abstract:

Off-axis eruptions at ocean ridges provide critical information with respect to underlying crustal plumbing and mantle melting systems. A detailed study of basaltic glass samples around the East Pacific Rise from 12°00′ to 12°30′N provides geological evidence for the existence of off-axis eruptions with a distinctive chemical composition. This composition has not been found along the axis of the EPR from 8° to 14°N except at a ridge-transform intersection but has been recovered in numerous locations that were farther than 1 km off axis. These off-axis normal mid-ocean ridge basalts, or OA-NMORB, are distinguished by low Na2O, Sr, and Al2O3 and are unusually depleted in incompatible elements. Moderately enriched off-axis transitional mid-ocean ridge basalts (OA-TMORB) with the same compositional tendencies can also be identified. Comparison of EPR axis lavas and the OA type suggests that they come from the same range of (unmelted) mantle source compositions but that the source of the OA magmas was depleted in incompatible trace elements by removal of a small- degree partial melt. This would be consistent with the OA type as an EPR pooled melt that is missing the low-degree melt fraction from deep in the melting regime, which provides a reasonable physical model for their formation. In this case, off-axis magmas do not represent the same range of chemical variation as magmas delivered to the axial magma system. The OA-NMORB are similar to depleted lavas from near-EPR seamounts. Other seamount lavas with depleted trace elements have TMORB-like major elements, and may be classified as OA-TMORB. The similarity between seamount lavas and the lavas erupted off axis close to the EPR suggests that the two are manifestations of the same phenomenon. We suggest that seamount-type volcanism effectively starts within 1–2 km of the axis. This is within the range where lavas derived from the axial plumbing system may also erupt. Therefore there is a narrow zone where young lava flows from the axial plumbing system and from the off-axis systems may overlap. Lavas erupted off axis may ultimately cover 20% of the seafloor around the EPR, which is substantially more than previous estimates that were based primarily on morphological studies.

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