Making oceanic plagiogranite bodies by thermal migration: evidence from the Agros transect, Troodos ophiolite, Cyprus

The mechanism by which plagiogranite bodies form within the ocean crust remains enigmatic. Plagiogranites are anomalously high in their silica content relative to basalt, and their petrogenesis has been primarily attributed to either extreme (>90%) fractional crystallization of a basaltic melt (Haase et al., 2006) or to partial melting and assimilation of hydrated oceanic crust (Koepke et al., 2004). Neither mechanism, however, provides a consistent explanation for the compositional and isotopic variations that are observed among plagiogranites. Laboratory experiments show that temperature gradients can lead to compositional differentiation and pronounced stable isotope fractionation (Huang et al., 2009, Lacks et al., 2012) as a function of temperature difference alone with the cold end becoming silicic in composition and isotopically heavy. This process, thermal migration, whereby elements diffuse in a manner driven by mineral-melt equilibrium within a temperature gradient, should also be apparent as isotopic gradients in a natural transect of differentiated rocks. A spreading center melt lens, with perhaps a near order of magnitude difference between the temperatures of freshly injected on-axis mafic melts and the hydrothermally cooled off-axis crust during crustal formation, provides one locality to test this hypothesis. Here, I suggest that plagiogranites, commonly found at the sheeted dike-gabbro boundary of most ophiolites, represent highly differentiated compositions produced in a steady state temperature gradient of a spreading center melt lens via thermal migration. For this study, twelve samples were collected along a spatial transect from the uppermost plutonic gabbros, through a plagiogranite pod, and up to the basal sheeted dikes from the eastern side of the Troodos ophiolite, Cyprus (location near Agros). Major and trace element analysis and x-ray mapping were used to characterize the samples. Fe and Si isotopic ratios were measured by HR-MC-ICPMS. Results show both isotopic systems become heavier with increasing whole rock SiO2 wt. % and with increasing distance from gabbros toward the sheeted dike contact. This observation is consistent with the prediction of isotopic zoning due to a temperature gradient effect. Models of Fe isotopic fractionation by fractional crystallization are not consistent with observed isotopic changes. Plagiogranites could thus reflect a common layer occurring at the sheeted dike-gabbro contact reflecting this melt lens process. This differentiation process may also provide a mechanism for creating silicic crust from a mafic magma setting in the early earth