This paper reports the igneous petrology and geochemistry of harzburgite-tectonites, crosscutting dunites, and local gabbroic segregations drilled from a tectonically exposed outcrop of shallow East Pacific Rise mantle at Ocean Drilling Program Site 895 near Hess Deep. Melting of the Hess Deep mantle section, as for most abyssal peridotites, occurred in the four-phase field olivine-enstatite-diopside-spinel limited by the diopside-out phase boundary. The Site 895 peridotites are clinopyroxene-poor harzburgites at the most depleted end of the range for abyssal peridotites. Unlike similar highly depleted peridotites from the Atlantic and Indian Oceans, however, these are not spatially associated with a mantle hot spot. Abyssal basalts from the walls of Hess Deep near Site 895, believed to have erupted at the East Pacific Rise, are N-type mid-ocean-ridge basalt (MORB). These indicate that the Site 895 peridotites are residues of melting of an N-type MORB source. Like ridge basalts near hot spots, however, the Hess Deep basalts, and Hole 894G gabbros nearby, require crystallization from very low sodium primary liquids consistent with the very depleted Hess Deep residual mantle peridotites.
Given seismic data that indicate a somewhat thin East Pacific Rise crust in the region, and an absence of any chemical or physical evidence for a mantle hot spot, we suggest that the high degree of depletion of the Hess Deep peridotites and the inferred primary basalt compositions, reflect moderate degrees of melting of a refractory mantle section poor in basaltic components under a normal geothermal gradient. This region of the East Pacific Rise, then, does not fit the proposed global correlation of Klein and Langmuir (1987) between ridge basalt and crustal thickness. Rather, it implies that crustal thickness at ocean ridges may be as much a function of variations of initial mantle composition as initial mantle temperature (e.g., Dick et al., 1984).
Primary melts transported through the Site 895 mantle section, as inferred from the geochemistry of gabbroic segregations in dunites crosscutting the harzburgite tectonites, were fully aggregated MORB. They were not, therefore, in equilibrium with the shallow mantle since clinopyroxenes in the Hess Deep harzburgites have trace element compositions in equilibrium with highly refractory melts previously only reported as inclusions in phenocrysts in Atlantic MORBs. This supports, and may require, highly focused flow and aggregation of melt through the underlying mantle-melting-column. The crystallization sequence of MORB in the Hess Deep shallow mantle section is the same as for normal abyssal tholeiites. However, high-magnesium diopside and high-calcium plagioclase (up to An99), present in some of the gabbroic segregations, are believed to be the product of late-stage reaction between melt and the harzburgite tectonite to produce high calcium and alumina melts. Similar sporadic xenocrysts widely distributed in MORBs might then be explained by mixing of MORB with stagnated highly reacted melts in shallow mantle conduits.
The dike-like form and compound nature of some gabbroic segregations in the dunites crosscutting the residual mantle harzburgite tectonites at Site 895 demonstrate that late-stage melt transport was, at least in part, fracture controlled. The segregations themselves, and small amounts of late-magmatic-textured intergranular clinopyroxene impregnating the harzburgite tectonites, also demonstrate that magma mass was decreasing as melt was transported through the transition zone in the mantle immediately beneath the crust. This requires that the mantle temperature there is depressed below the mantle adiabat due to conductive heat loss. The presence of this conductive lid is a significant factor in the evolution of MORB and impacts estimates of the composition of the oceanic crust. Such a lid limits the extent of melting of the upwelling mantle, traps late melt fractions generated at the top of the mantle melting column, and creates a zone where melt-rock reaction produces small, but apparently significant, quantities of high alumina and calcic MORB magmas.
Date of initial receipt: 3 August 1994
Date of acceptance: 1 July 1995
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