Petrology and geochemistry of the lower ocean crust formed at the East Pacific Rise and exposed at Hess Deep: A synthesis and new results

Abstract

[1] The geochemistry and petrology of the lower oceanic crust record information about the compositions of melts extracted from the mantle, how these melts mix and crystallize, and the role of hydrothermal circulation in this portion of the crust. Unfortunately, lower oceanic crust formed at fast spreading ridges is rarely exposed at the seafloor making it difficult to study these processes. At Hess Deep, crust formed at the East Pacific Rise (EPR) is exposed due to the propagation of the Cocos-Nazca spreading center westward. Here we review our state of knowledge of the petrology of lower crustal material from Hess Deep and document new mineral major and trace element compositions, amphibole-plagioclase thermometry, and plagioclase crystal size distributions. Samples from the deeper parts of the gabbroic sequence contain clinopyroxene that is close to being in trace element equilibrium with erupted basalts but which can contain primitive (moderate Cr, high Mg # ) orthopyroxene and very calcic plagioclase. Because primitive mid-ocean ridge basalts (MORBs) are not saturated with orthopyroxene or very calcic plagioclase this suggests that melts added to the crust have variable compositions and that some may be in major but not trace element equilibrium with shallow depleted mantle. These apparently conflicting data are most readily explained if some of the melt extracted from the mantle is fully aggregated within the mantle but reacts with the shallow mantle during melt extraction. The occurrence of cumulates with these characteristics suggests that melts added to the crust do not all get mixed with normal MORB in the axial magma chamber (AMC), but rather that some melts partially crystallize in isolation within the lower crust. However, evidence that primitive melts fed the AMC, along with steep fabrics in shallow gabbros (from near the base of the dyke complex), provides support for models in which crystallization within the AMC followed by crystal subsidence is also an important process in lower crustal accretion. More evolved bulk compositions of gabbros from the upper than lower parts of the plutonic section are due to greater amounts of reaction with interstitial melt and not because their parental melt had become highly fractionated through the formation of large volumes of cumulates deeper in the crust. Amphibole-plagioclase thermometry confirms previous reports that the initial ingress of fluid occurs at high-temperatures in the shallow gabbros (T ave 713°C) and show that the temperature of amphibole formation was similar in deeper gabbros (T ave 722°C). This thermometry also suggests that fracture and grain boundary permeability for seawater-derived fluids was open over the same temperature interval.Components: 15,685 words, 9 figures, 5 tables.

Strontium isotope constraints on fluid flow in the sheeted dike complex of fast spreading crust: Pervasive fluid flow at Pito Deep

“…[ [Perk et al, 2007;Coogan et al, 2002;Francheteau et al, 1990]. Plagioclase separates were leached in hot 6N HCl for 30 min prior to digestion.…”

Aspects of marine geoscience: a review and thoughts on potential for observing active processes and progress through collaboration between the ocean sciences

“…Calcium diffusion profiles between olivine and clinopyroxene suggest that cooling rates are orders of magnitude smaller in the lower crust than immediately below the magma chamber [68]. Areas where the crust has been exposed by deep rifting have provided deep sections where the relationships between different crustal components can be assessed, for example, where East Pacific Rise crust has been rifted at Hess Deep [69].…”

Hydrothermal spinel, corundum and diaspore in lower oceanic crustal troctolites from the Hess Deep Rift

“…Coogan et al (2002) have pointed out the similarity of temperatures for amphibole formation between upper and lower gabbro sections, and discussed its implications on seawater permeability and cooling of oceanic lower crust at different distances from the ridge axis. The formation of spinel is also localized but is associated with centimeter-to decimeter-scale alteration zones, much wider than the amphibole veins reported.…”