High- & Low-δ18O magma: Comparative study of crustal and mantle plagiogranites from the Oman ophiolite

Abstract:

Plagiogranite (PLGT) from the crust and mantle sections of the Oman ophiolite preserve widely varied δ¹⁸O values that monitor different processes occurring during ophiolite construction. Mantle-like δ¹⁸O values are expected if MORB fractionation played a dominant role in PLGT genesis. Magmatic values (monitored here by zircon) shifted away from the mantle-like range indicate open system processes which include partial melting of hydrothermally-altered crust or influx of subduction-related, sediment-derived melt. Zircon (zrn) and quartz (qtz) from twenty-four new samples of PLGT from the crustal and mantle sections of the Oman ophiolite were analyzed for δ¹⁸O. Rock-averaged δ¹⁸O from the sheeted dikes (zrn: 4.3-4.5‰, qtz: 6.7-6.9‰) and dike-gabbro transition (zrn: 3.9-4.8‰, qtz: 4.7-7.7‰) are mostly below values in magmatic equilibrium with MORB (zrn = 5.2±0.5‰, qtz = 7.0-7.5‰). δ¹⁸O for PLGT in the gabbro section (zrn: 4.8-5.1‰, qtz: 7.7-8.3‰) are mostly mantle-like. Quartz is generally found to be more variable than coexisting zrn and likely experienced some sub-solidus exchange. When organized into a relative structural position, δ¹⁸O_zrn values typically increase with depth. The lowest δ¹⁸O_zrn are observed near the dike-gabbro transition and are consistent with petrogenesis involving hydrous partial melting of mafic crust previously hydrothermally-altered at high-T. The return to nominally mantle-equilibrated δ¹⁸O_zrn deeper in the gabbro section may reflect decreasing seawater-signatures of fluids penetrating to depth, lower water/rock ratios, or extreme fractional crystallization. Crustal PLGT thus predate the development of high δ¹⁸O signatures in the upper oceanic crust as it cools and experiences low temperature hydrothermal alteration.

Mantle PLGT intrusions (1-3 m thick) from the Haylayn block extend to considerably higher rock-averaged δ¹⁸O values (zrn: 5.1-15.4‰, qtz: 7.0-18.5‰). Individual rocks (5 samples) were uniform in δ¹⁸O, and coexisting minerals yield calculated T = 640-700°C. Taken with recently published whole rock radiogenic isotope studies, the high-δ¹⁸O values require that some mantle plagiogranite formed from deep-seated magmas with a high-δ¹⁸O sediment component reflecting an influx of subduction-related melt.