Geochemistry of Dikes and Lavas from Tectonic Windows

Abstract

Tectonic windows are faulted escarpments that expose extensive sections of in situ oceanic crust, providing valuable opportunities to examine upper crustal architecture from a perspective unmatched by other approaches. Recent investigations of tectonic windows by submersible (Alvin, Nautile) and remotely-operated vehicle (Jason II) have recovered an unprecedented suite of dikes and lavas. We focus on compositions of dikes and lavas from intermediate- and super-fast rate crust exposed, respectively, in the Western Blanco Transform (BT) fault and the Pito Deep Rift (PD), to better understand accretionary processes at mid-ocean ridges. In the BT, the upper lavas are generally more primitive than the lower lavas, supporting geophysical and geological studies that suggest off-axis volcanism plays an important role in constructing the upper crust at intermediate-rate spreading centers. The wide range in lava compositions exposed along the BT scarp also lends caution to studies that rely on surface lavas to determine the evolution of sub-axial magmatic conditions.The PD suite allows us to examine accretionary processes over an impressive temporal range, including long-term (millions of years) changes in mantle composition and medium-rate (100s of ka) changes in magmatic regime. Compositions of adjacent dikes reveal that the ocean crust is heterogeneous on short time (<10>ka) and spatial (meters) scales, reflecting along-axis transport of magma from chemically heterogeneous portions of the melt lens. High compositional variability was also observed in adjacent dikes from Hess Deep (HD), a tectonic window into fast-rate crust, suggesting that lateral dike intrusion occurs at all mid-ocean ridges. PD lavas are offset to lower density compositions compared to dikes, an observation previously made in HD, but made here for the first time in other dike-lava populations, suggesting that buoyancy plays a major role in partitioning magma between dikes and lavas. A model for intrusion of a single dike shows that crustal density, magma pressure, and tectonic stress affect the intensity of density-based magma partitioning in a systematic way that can be related to compositions of dike-lava populations.