Influence of magma supply and spreading rate on crustal magma bodies and emplacement of the extrusive layer: Insights from the East Pacific Rise at lat 16°N

Abstract

Seismic reflection data from the East Pacific Rise at lat 16°N, which is spreading at the high end of intermediate rates, suggest that the depths at which axial magma chambers reside do not vary smoothly as a function of spreading rate. Rather, magma-chamber depths form two distinct populations, each associated with a distinct axial morphology and with an abrupt transition occurring within the intermediate-spreading-rate range. Our data (1) show that the melt lens at high-intermediate-spreading ridges lies at a shallow level similar to lens depths at fasterspreading ridges, and (2) provide further support for the spreading-rate invariance of ridges with axial highs noted in other ridge properties. The axial morphology of the two ridge segments within the study area differs markedly, and a large contrast in magma supply is inferred. The ridge segment with greater magma supply is associated with a broader and more continuous melt lens, a wider region over which the extrusive crust accumulates, and a thicker extrusive layer off-axis where supply to the ridge segment appears to be centered. However, on-axis, the extrusive layer is thinnest where magma supply is robust and a shallower melt lens is observed, consistent with a model in which magma pressure controls the thickness of the extrusive layer accumulated above the magma lens.

Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N: Implications for ridge crest plumbing and decadal changes in magma chamber compositions

“…Also, E-MORB and D-MORB from the Siqueiros Transform (8°19′N-8°22′N) Figure 7) (Waters et al, submitted manuscript, 2010). These observations together are consistent with a model of discontinuous melt lenses separated by discrete breaks that conform to observed variations in ridge axis morphology [Carbotte et al, 1998[Carbotte et al, , 2000; Fundis et al, 2008]. Together these observations suggest that the central region of the eruption is centered over what has been the most magmatically active site in the area for some time, implying that the AMC and mush zone here experience a high integrated melt input flux.…”

Site 1256

“…Our selection of 100-300 m of lava flows comes from a number of lines of evidence. Multichannel seismic experiments on the EPR estimate Layer 2A thicknesses of ~300 m in the near-axis region (Hooft et al, 1996;Carbotte et al, 1997a), although the geological nature of this seismic layer remains poorly understood.…”

Drilling to Gabbro in Intact Ocean Crust

“…The depth to the reflectors decreases as spreading rate increases ( Fig. 1) (15,16); it is controlled by the rate of magma supply from below and hydrothermal cooling by seawater from above (7). Melt lenses are hypothesized to play a critical role in the formation of the lower oceanic crust.…”