MAPPING THE FORMATION OF THE LOWER OCEANIC CRUST IN FOUR DIMENSIONS: HIGH-PRECISION U-PB DATING FROM IODP HOLE U1473A, ATLANTIS BANK, SOUTHWEST INDIAN RIDGE

U-Pb zircon dating of oceanic crust provides unprecedented insights into the timescales and processes of crustal accretion. The first dating of zircons from an active ridge was performed by Mattinson in the late 1980s (Stakes et al., 1991); however, little further work was done prior to a series of SIMS studies starting in 2004 (John et al., 2004). Ongoing SIMS work has been complemented by high-precision ID-TIMS dating, with weighted mean uncertainties as low as ± 5–10 ka, providing detailed constraints on the timing of magmatic processes. Results include the determination of accurate cooling rates, the recognition of asymmetric spreading of oceanic core complexes, and mapping of the distribution and complexity of magmatism during accretion. The emergent field of U-Pb zircon dating of mid-ocean ridge samples is an exciting area of current and future research for (I)ODP drilling.

Here we present new ID-TIMS zircon data from Atlantis Bank on the SW Indian Ridge. Atlantis Bank is interpreted as the footwall to an oceanic detachment fault, exposing lower-crustal plutonic rocks at the seafloor. Three (I)ODP holes drilled into the surface of Atlantis Bank include ODP Hole 735B (1508 mbsf), ODP Hole 1105A (158 mbsf) and IODP Hole U1473A (809 mbsf). Our previous research demonstrated that samples from Hole 735B (n = 16) become progressively younger with depth, with Th-corrected weighted mean ²⁰⁶Pb/²³⁸U dates ranging from 12.116 ± 0.030 Ma at the top of the hole to 11.918 ± 0.012 Ma at the base. Hole U1473A is offset from Hole 735B by 1.9 km in the north-south spreading direction. New analyses from the length of U1473A (n = 20) show a down-hole trend similar to Hole 735B, with weighted mean dates from 11.9097 ± 0.0059 Ma (top) to 11.8669 ± 0.0057 Ma (base). Long-term spreading rates of 14.1 km/Ma predict an offset of 135 ka; however, the rocks from Hole U1473A are on average just 55 ka younger than Hole 735. The data suggest that the rate of crustal accretion was decoupled from tectonic spreading, reflecting expansion and contraction of the magma chamber, linked to magma flux. The similar age versus depth trends for both holes may approximate the solidus (~850°C) isotherm, and hence map the shape of the magma chamber. The combined data from Atlantis Bank provide the first four-dimensional record of the formation of the intrusive oceanic crust.