Abstract:
Tectonic windows are faulted escarpments that expose extensive sections of <em>in situ</em> oceanic crust, providing valuable opportunities to examine upper crustal architecture from a perspective unmatched by other approaches. Recent investigations of tectonic windows by submersible (<em>Alvin, Nautile</em>) and remotely-operated vehicle (<em>Jason II</em>) 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.
