Paper No. 1
Presentation Time: 9:00 AM


GRIMES, Craig B., Department of Geological Sciences, Ohio University, 316 Clippinger Laboratories, Athens, OH 45701, USHIKUBO, Takayuki, Department of Geoscience, University of Wisconsin, 1215 West Dayton Street, Madison, WI 53706, KOZDON, Reinhard, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706 and VALLEY, John W., Department of Geoscience, University of Wisconsin, Madison, WI 53706,

Felsic rocks are a minor constituent of oceanic crust volumetrically, but they are frequent targets for the application of U-Pb zircon geochronology in ophiolites and modern ocean crust. The formation of oceanic trondhjemite has been debated for decades, being attributed primarily to 1) extreme fractional crystallization of a mantle melt, or 2) partial melting of hydrated mafic crust. Recent support for the latter comes from field evidence and melting experiments; primary magmatic oxygen isotope ratios of these felsic rocks are well-suited to evaluate this hypothesis. To constrain the magmatic δ18O of silicic rocks in oceanic crust, we measured the δ18O of 202 zircons from 22 samples of tonalite and trondhjemite collected at 8 different ophiolites using Secondary Ion Mass Spectrometry (SIMS). In situ measurements on zircon were performed using a 10-micron spot size, with an average external precision of 0.24‰ (2 s.d.). Host rocks are characterized by high SiO2 (57.5-78 wt. %), low K2O (<1 wt. %) and TiO2 (<1.3 wt. %), and define a flat trend on SiO2 vs La plots. Measured δ18O(zircon) values from all sample locations range from 3.9 - 5.6‰, extending ~1‰ below values expected for zircon in equilibrium with mantle. Eleven rocks from the northern Oman Ophiolite, sampled both from km-scale intrusive complexes and cm-scale segregations near the well-exposed dike-gabbro transition zone, yield the lowest δ18O(zircon) with rock-average values of 4.3-5.0‰. The low-δ18O(zircon) values are best explained by remelting of crust previously altered by seawater-derived fluids at temperatures above ~300°C. An origin involving hydrous partial melting is supported by the low TiO2 and flat SiO2 - La trend, which are inconsistent with closed-system differentiation of MORB by fractional crystallization. Experimental studies indicate that reheating to ~850-900°C is required for hydrous melting of diabase and gabbro. The dike-gabbro transition zone, characteristic of crust formed at intermediate and fast spreading rates, represents the fossilized zone between the roof of an active magma chamber and a vigorously convecting hydrothermal system in the overlying crust. Such a horizon is ideal for promoting the formation of evolved melts by hydrous partial melting, and also serves as a site of assimilation/contamination of erupted melts.