Accretion and Oxidation of a Superfast-Spread Axial Melt Lens: TIMS and SIMS Zircon Analyses of the IODP Hole 1256D Gabbros

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IODP Hole 1256D penetrates a complete section of upper oceanic crust spread from the East Pacific Rise at a full rate of up to 220 mm/yr. Here we present U-Pb zircon data from Hole 1256D gabbroic rocks using isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and secondary ion mass spectrometry (SIMS). Three gabbro samples yielded Th-corrected weighted mean 206Pb/238U SIMS dates of 15.03±0.14 Ma, 15.13±0.35 Ma, and 15.23±0.12 Ma. An ID-TIMS Th-corrected weighted mean 206Pb/238U date of 15.191±0.040 Ma from the first of the three samples provides verification and refinement of the timing of magmatism. The U-Pb dates agree with the predicted age of the crust based on marine magnetic anomalies, suggesting the gabbros most likely formed by intrusion and crystallization at or near the ridge axis. Zircons from two of the studied samples have anomalously high Th/Uratios, likely reflecting oxidation of U prior to or during zircon crystallization. The elevated ƒO2 of late stage melts in the axial melt lens may be related to progressive crystallization, assimilation of hydro-thermally altered sheeted dikes and gabbros, and/or contamination of the melt lens by seawater derived saline brines.