Geochemistry of dikes and lavas from the north wall of the Hess Deep Rift

An investigation of ~1-m.y.-old dikes and lavas from the north wall of the Hess Deep Rift (2°15'N, 101°30'W) collected during Alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust. The study area encompasses 25 km of an east-west flow line, representing ~370,000 years of crustal accretion at the East Pacific Rise. Samples analyzed exhibit depleted incompatible trace element abundances and ratios [(La/Sm)N < 1]. Indices of fractionation (MgO), and incompatible element ratios (La/Sm, Nb/Ti) show no systematic trends along flow line. Rather, over short (<4 m) and long (~25 km) distances, significant variations are observed in major and trace element concentrations and ratios. Modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions. These findings, combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift-dominated systems, suggest a more realistic model of the magmatic system underlying the East Pacific Rise relative to the commonly assumed twodimensional model. In this model, melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir. Dikes emanating from these distinct reservoirs transport magma along axis, resulting in interleaved dikes and host lavas with different evolutionary histories. This model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin.

Benthic foraminiferal composition at the inner wall of the Middle America Trench, Costa Rica

The sediments of Deep Sea Drilling Project Site 565 and University of Texas Marine Science Institute Cores IG-24-7-38 to -42 taken on the landward slope of the Middle America Trench exhibit characteristics of material subject to reworking during downslope mass flow. These characteristics include a generally homogeneous texture, lack of sedimentary structures, pervasive presence of a penetrative scaly fabric, and presence of transported benthic foraminifers. Although these features occur throughout the sediments examined, trends in bulk density, porosity, and water content, and abrupt shifts in these index physical properties and in sediment magnetic properties at Site 565 indicate that downslope sediment creep is presently most active in the upper 45 to 50 m of sediment. It cannot be determined whether progressive dewatering of sediment has brought the material at this depth to a plastic limit at which sediment can no longer flow (thus resulting in its accretion to the underlying sediments) or whether this depth represents a surface along which slumping has occurred. We suspect both are true in part, that is, that mass movements and downslope reworking accumulate sediments in a mobile layer of material that is self-limiting in thickness.