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

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 (Tave 713°C) and show that the temperature of amphibole formation was similar in deeper gabbros (Tave 722°C). This thermometry also suggests that fracture and grain boundary permeability for seawater‐derived fluids was open over the same temperature interval.