White Paper Title: 
Crustal controls on magma reservoirs at Endeavour

The Endeavour ISS is blessed with a larger than usual diversity of mantle-derived geochemical differences that allow its >250 submersible-collected basalts to be grouped into chemo-stratigraphic units that did not pave the entire width of the axial valley. These units represent separate differentiation-related ‘magma batches’, each reflecting separate mantle melting and crustal storage episodes that can be used to address crustal controls on magma reservoirs and the nature of mantle sources and melting. They show the following ten features. First, the greatest diversity of basalt types and, therefore, the least mixing between magma matches, occurs along the western margin of the axial valley where the melt lens terminates and where the largest hydrothermal vents are concentrated. Hence, both melts and fluids rise along faults there. Second, there is a first-order along-strike change in basalt composition where the melt lens is offset vertically south of Mothra. Third, the isotopic composition of Pb in hydrothermal systems has along-strike variations that may reflect those in the basalts which would indicate that the hydrothermal cells are at most a few km long. Fourth, all basalt types are most mafic (>7.5% Mg) at the northern end of the longest melt lens where the axial valley is shallowest. This seems to be the primary site of melt focusing. Fifth, basalts from at least several separate magma batches erupted in the axial graben within the last 104 years. Sixth, young basalts are common up to 2 km off-axis where they differ in composition and eruption style from the axial valley, and differ on one flank versus the other. Compositions on the east flank, which is partly underlain by the current melt lens, are the most uniform at Endeavour (7.0±0.1% MgO) and grade to somewhat more differentiated (lower temperature) compositions on both sides of the flank. Compositions on the far west flank are the least affected by the enriched mantle now tapped in the axial valley. The asymmetry between the flanks precludes their surface basalts from having formed in the axial valley and then separated by spreading. Seventh, many of the chemo-stratigraphic units show evidence of open system behavior (e.g., mixing between magma batches); e.g., variations in the ratios of incompatible trace elements and isotopes that exceed those caused by fractional crystallization or analytical error. Eighth, those units closest to closed systems have major element variations and maximum CO2 contents consistent with differentiation at depths near the current melt lens (2-3 km). Ninth, all basalts erupted in the axial valley or its flanks have the same distinctive enrichment in Th, Nb, and Pb isotopes as in the West Valley segment ~ 40 km north, whereas those erupted >3 km beyond the flanks do not. Heckle Seamounts also lack this enrichment. Therefore, although the Endeavour segment migrated over an enriched mantle domain within the last few hundred Ka, this does not directly explain its thicker crust and inflated topography. Finally, because the 230Th and 231Pa excesses are higher at Endeavour than elsewhere along the Juan de Fuca Ridge including Axial Seamount, the rate of mantle upwelling driving Endeavour magmatism is slower, not faster than elsewhere despite the greater melt productivity.