Temporal and spatial variability in the composition of lavas exposed along the Western Blanco Transform Fault
Pollock, Meagen A.
Klein, Emily M.
Karson, Jeffrey A.
Tivey, Maurice A.
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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 elements 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%). Based on 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 ka to build the lower extrusive section, consistent to first order with independent estimates for the construction of intermediate-spreading rate crust.
Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 6 (2005): Q11009, doi:10.1029/2005GC001026.