INSIGHTS INTO THE ORIGIN OF SILICIC ROCKS IN OPHIOLITES FROM OXYGEN ISOTOPES IN ZIRCON

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.