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

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.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.This work was supported by the US National Science Foundation (OCE 02- 22154 to E.K. and J.K. and OCE 9400623 to M.T.)

Composition of dikes and lavas from the Pito Deep Rift

The northwest trending walls of the Pito Deep Rift (PDR), a tectonic window in the southeast Pacific, expose in situ oceanic crust generated ?3 Ma at the superfast spreading southern East Pacific Rise (SEPR). Whole rock analyses were performed on over 200 samples of dikes and lavas recovered from two ~8 km**2 study areas. Most of the PDR samples are incompatible-element-depleted normal mid-ocean ridge basalts (NMORB; (La/Sm)N < 1.0) that show typical tholeiitic fractionation trends. Correlated variations in Pb isotope ratios, rare earth element patterns, and ratios of incompatible elements (e.g., (Ce/Yb)N) are best explained by mixing curves between two enriched and one depleted mantle sources. Pb isotope compositions of most PDR NMORB are offset from SEPR data toward higher values of 207Pb/204Pb, suggesting that an enriched component of the mantle was present in this region in the past ?3 Ma but is not evident today. Overall, the PDR crust is highly variable in composition over long and short spatial scales, demonstrating that chemically distinct lavas and dikes can be emplaced within the same segment over short timescales. However, the limited spatial distribution of high 206Pb/204Pb samples and the occurrence of relatively homogeneous MgO compositions (ranging <2.5 wt %) within a few of the individual dive transects (over distances of ~1 km) suggests that the mantle source composition evolved and magmatic temperatures persisted over timescales of tens of thousands of years. The high degree of chemical variability between pairs of adjacent dikes is interpreted as evidence for along-axis transport of magma from chemically distinct portions of the melt lens. Our findings suggest that lateral dike propagation occurs to a significant degree at superfast spreading centers