Temporal and spatial variability in the composition of lavas exposed along the Western Blanco Transform Fault

Abstract

The northern scarp of the Western Blanco Transform (BT) Fault Zone provides a “tectonic window” into crust generated at an intermediate‐rate spreading center, exposing a ∼2000 m vertical section of lavas and dikes. The lava unit was sampled by submersible during the Blancovin dive program in 1995, recovering a total of 61 samples over vertical distances of ∼1000 m and a lateral extent of ∼13 km. Major element analyses of 40 whole rock samples exhibit typical tholeiitic fractionation trends of increasing FeO*, Na2O, and TiO2 and decreasing Al2O3 and CaO with decreasing MgO. The lava suite shows a considerable range in extent of crystallization, including primitive samples (Mg# 64) and evolved FeTi basalts (FeO > 12%; TiO2 > 2%). On the basis of rare earth element and trace element data, all of the lavas are incompatible‐element depleted normal mid‐ocean ridge basalts (N‐MORB; La/SmN < 1). The geochemical systematics suggest that the lavas were derived from a slightly heterogeneous mantle source, and crystallization occurred in a magmatic regime of relatively low magma flux and/or high cooling rate, consistent with magmatic processes occurring along the present‐day southern Cleft Segment. The BT scarp reveals the oceanic crust in two‐dimensional space, allowing us to explore temporal and spatial relationships in the horizontal and vertical directions. As a whole, the data do not appear to form regular spatial trends; rather, primitive lavas tend to cluster shallower and toward the center of the study area, while more evolved lavas are present deeper and toward the west and east. Considered within a model for construction of the upper crust, these findings suggest that the upper lavas along the BT scarp may have been emplaced off‐axis, either by extensive off‐axis flow or off‐axis eruption, while the lower lavas represent axial flows that have subsided with time. A calculation based on an isochron model for construction of the upper crust suggests that the Cleft Segment requires at least ∼50 kyr to build the lower extrusive section, consistent to first order with independent estimates for the construction of intermediate‐spreading rate crust.