Chlorine in mid-ocean ridge magmas: Evidence for assimilation of seawater-influenced components

Abstract

Suites of depleted MORB glasses from the fast-spreading Pacific-Nazca Ridge at 28°S and 32°S and the slow-spreading eastern boundary of the Juan Fernandez microplate were analyzed for chlorine by electron microprobe. Cl contents of primitive MORB are about 20–50 ppm, similar to values reported previously for primitive MORB from the Mid-Atlantic Ridge (MAR). Cl increases steadily with decreasing MgO to 1100 ppm in evolved MORB (FeTi basalts). FeTi basalts can be related to primitive magmas by a maximum of 67% fractional crystallization based on major element modelling. The Cl concentrations in FeTi basalts exceed by a factor of 5 to 10 the amounts that can be generated by fractional crystallization of the primitive magmas. An additional process besides crystallization must be contributing the excess Cl. FeTi basalts also contain more H 2O than can be produced by fractional crystallization of a primitive parent. The H 2O Cl ratio of the hypothetical additional component that is necessary to account for the excess Cl and H 2O in FeTi basalts is 1–6 and rules out direct addition of seawater to the magma chamber. Assimilation of hydrothermally altered wall rocks of the magma chamber most likely provides the extra Cl and H 2O. Selective melting or breakdown of amphibole and incorporation of Cl-rich brine contained in the wall rocks may be important processes. Bulk assimilation is less likely because the Cl content of altered crust is too low to generate the excess Cl unless unrealistically large amounts of assimilation are invoked. A magmatic source for the additional Cl and H 2O cannot be ruled out on geochemical grounds but is physically unrealistic because it requires that large volumes of magma have crystallized and exsolved a Cl-rich vapor phase that has somehow migrated to a small magma chamber. Excess Cl in evolved magmas (i.e., Cl overenrichment) is best developed in evolved MORB from propagating or overlapping spreading centers such as the Galapagos Spreading Center at 85°W and 95°W and the west ridge of the Juan Fernandez microplate. Cl overenrichment has not been observed on slow-spreading ridges including the eastern ridge of the Juan Fernandez microplate, the Southwest Indian Ridge, and the mid-Atlantic Ridge. The existence of high-Cl magmas implies that some of the Cl-rich mineralization observed in deep crustal sections and ophiolites could be due to exsolved magmatic volatiles. The assimilation of hydrothermally altered material could influence the concentration and isotopic ratios of other elements which have low abundances in MORB relative to seawater.