November 4, 2013 at 3:41 pm

Grimes Makes Two Presentations on Oceanic Mantle, Crust

Ohio University faculty made an impact with 23 presentations at the Geological Society of America’s 125th Anniversary Annual Meeting & Exposition Oct. 27–30 in Denver.

Dr. Craig Grimes, Takayuki Ushijubo, Reinhard Jozdon, & John Valley presented on INSIGHTS INTO THE ORIGIN OF SILICIC ROCKS IN OPHIOLITES FROM OXYGEN ISOTOPES IN ZIRCON. Grimes is Assistant Professor in the Department of Geological Sciences in the College of Arts & Sciences.

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 SiO– 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.


Ocean drilling near the 15°20’ and Atlantis (30°N) Fracture Zones along the Mid-Atlantic Ridge (MAR) reveals discontinuous gabbroic crust and mantle peridotite exposed on the seafloor. Published core observations, geochronology, and geochemical evidence suggest that these crustal sections were built by protracted intrusion of small melt bodies (10’s of m to cm’s thick). We report ɛHf values from 134 zircons hosted by 17 evolved rocks from 3 cores drilled in these locations in order to further examine mechanisms for igneous construction of the lower oceanic crust and investigate the upper mantle source of the crust-forming magmas.

Hafnium isotope compositions were determined by LA-ICP-MS at the University of Florida. The zircon standard FC-1 was analyzed throughout the session and gave an external precision of ±1.4 (ɛHf; 2 s.d.; n=55). Ocean crustal zircons display a broad range of ɛHf values from 13.3 to 22.1. Zircons within single cores differ by up to 7.4 epsilon units (ɛHf = 13.3 – 20.7, 1.4 km core 1309D, 30°N). Core 1275D (209 m, 15°44’N) yields ɛHf values from 15.4 to 22.1 (n=44). Two samples from core 1270D (57 m, 244 km south of 1275D) have uniform ɛHf values from 13.3 to 16.8 (n=16). These single cores are more variable than published zircon values from the Markov Deep (ɛHf = 18.8 ± 0.3; Kostitsyn et al., 2009) and similar to the range of ɛHf reported for basalts extending ~200 km along this portion of the ridge.

The overall range and downhole distribution of ɛHf in cores 1275D and 1309D are consistent with crustal construction from discreet pulses of magma derived from local heterogeneously depleted mantle sources. It is inferred that small-scale regions of the mantle source preserve varying degrees of isotopic depletion (also indicated by harzburgites on the seafloor near the 15º20’ FZ), resulting from differential melt extraction or refertilization in the geologic past. The ɛHf range arises from segregation and isolation of mantle sources and is dependent on 176Lu/177Hf ratio and time. If it is assumed that heterogeneity developed from an average depleted MORB mantle at the time indicated by the NHRL Pb ‘age’ (~1.8 Ga), a spread in 176Lu/177Hf of the mantle sources of 0.0365 to 0.0435 would be required to develop the observed ɛHf ranges. More recent development of local heterogeneities would imply larger differences in Lu/Hf.


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