USING IRON AND SILICON ISOTOPE RATIOS AS TOOLS TO UNDERSTAND PLAGIOGRANITE AND RHYODACITE PETROGENESIS AT MID OCEAN RIDGES

The origin of silicic rocks in the mid ocean ridge (MOR) environment remains an enigma. Whether these rocks reflect fractional crystallization and/or partial melting of altered crust or some combination has been widely debated. Both plagiogranites in ophiolites and erupted rhyodacites provide evidence of differentiation to 75 wt. % SiO2. Yet, whether dacitic and rhyolitic lavas erupting in MOR environments are equivalent to plagiogranites is unknown. Here we compare the geochemistry of plutonic and volcanic suites, including Fe isotope ratios, to shed light on the origin of these silicic rocks.

Within the eastern Troodos Ophiolite (Cyprus) we collected 14 samples from a 700m transect from gabbros through to plagiogranites. Silica content ranges from (~45% in the gabbros to 75 wt.% in the most evolved plagiogranites. Notably, iron and silicon isotope ratios (d56Fe and d30Si) increase (become heavier) with increasing, up-section distance from the gabbros. This spatial pattern is consistent with the prediction of thermal diffusion occurring within a temperature gradient in a MOR magma lens. These isotope ratios also increase with SiO2. Similarly, δ⁵⁶Fe and δ³⁰Si within 3 silicic MOR systems (Galapagos Spreading Center, Juan de Fuca-Blanco ridge-transform intersection [RTI], and 9°N EPR overlapping spreading center [OSC]) exhibit the same relationship with heavier ratios associated with increasing silica content, suggesting a similar petrogenesis. The silicic eruptives may be a result of the melting of these large plagiogranite pods under an actively erupting spreading center. Given the ubiquitous occurrence of plagiogranite at the gabbro-sheeted dike contact in Troodos, we suggest that a plagiogranite layer may exist in nearly all oceanic crust, (although rarely observed) and is only mobilized/melted within particular tectonic settings such as RTIs and OSCs. Understanding the process forming these silicic rocks could be crucial to understanding proto-continental crust formation.