Using precious metal probes to quantify mid-ocean ridge magmatic processes

Abstract

Basalts from the East Pacific Rise (EPR), Siqueiros transform zone and Mid-Atlantic Ridge (MAR) area have been analyzed for the platinum-group elements (PGE) and a wide range of incompatible elements. The low PGE content of the most primitive mid-ocean ridge basalts (MORB) suggests that they leave the mantle sulfide-saturated but become sulfide under-saturated, as a consequence of decompression, during their ascent from the mantle to the MOR magma chamber. Because the pressure drop is small, the ascending magma enters the magma chamber slightly sulfide under-saturated and requires only a small amount of fractional crystallization to return it to sulfide-saturation. Sulfide-saturation is marked by the Pd content of the melt falling by over an order of magnitude at ca. 9.5 wt.% MgO. However, once the magma has become sulfide-saturated, it shows no evidence of a further decline in Pd with decreasing MgO as the system evolves. This must result from regular replenishment of the underlying axial magma chamber by fresh batches of primitive magma. Most replenishments occur when the MgO content of the resident melt in the magma chamber is between ∼9 to 6.5 wt.% MgO. We combine the tight constraints imposed by highly compatible Pd abundances, with those imposed by strongly incompatible elements, to produce the first model that successfully accounts for the variations of both classes of elements in open system mid-ocean ridges magma chambers for normal-type MORB (N-MORB). We show that the lack of decline in the Pd content of sulfide-saturated mid-ocean ridge basalts, with decreasing MgO, requires frequent small replenishments (between 1% and 4%) of similar magnitude, rather than large initial inputs that systematically decline from near 100% to near zero during the life of an individual ridge system.Assuming the spreading rate of ∼110 mm/yr for the EPR at 9°N, and 14 yr between the 1991–1992 and 2005–2006 eruptions, the calculated volume of erupted and intruded magma is 2.3 × 103 m3/m of ridge length. If the expelled melt represents 0.5% of the accessible magma in the chamber, as suggested by the Pd modeling, the volume of the magma accessible during tapping is 4.6 × 105 m3/m of ridge length.