Cyclic fractionation processes beneath the Shatsky Rise oceanic plateau

Our understanding of the magma ascent, storage and differentiation beneath large igneous provinces (LIPs) is still under debate. Mainly two competing models are explaining the formation of LIPs: 1) the arrival of a deep sourced mantle plume in the lithosphere, and 2) decompression melting of unusually fusible mantle beneath fast-spreading ridges. The Shatsky Rise oceanic plateau is a unique opportunity to discuss this problem because its tectonic evolution along the Pacific-Farallon-Izanagi triple junction is well known. We examined samples recovered during IODP Expedition 324 at the Tamu Massif, one of three massifs of the Shatsky Rise. In general, the Shatsky Rise glasses are tholeiitic basalts containing 5 to 8.7 wt% MgO. Compared to MORB, the glass compositions are high in FeO (Fe8: 10.3) and low in SiO₂ and Na₂O (Na8: 1.9), indicating high amounts of partial melting at great depth. The CaO/Al₂O₃ ratios in the Tamu Massif evolved glasses are higher respectively to MORB indicating low pressure cotectic fractionation. In general, the basaltic glasses from Shatsky Rise show differentiation trends similar to those of other oceanic plateaus, as e.g. Ontong Java and Kerguelen. Pressures and temperatures of magma storage were estimated for 40 naturally quenched glasses (5-6.5 wt% MgO) along a drilled profile of 150 m length. Based on the glass major element compositions (electron microprobe) and glass H₂O contents (infrared spectroscopy), we simulated the conditions of multiple saturation (Liq+Ol+Pl+Cpx) using the COMAGMAT program. The calculated pressures range from 240 MPa to 1 atm and the calculated temperatures range from 1150 to 1100 °C, indicating polybaric crystallization in a shallow magma reservoir. As a result, along the drilled profile, we identified two complete cycles of magma injection in the reservoir with subsequent fractionation and eruption followed by a time of magmatic inactivity. The most magnesian samples of the Shatsky Rise were drilled at Ori Massif. In contrast to the evolved Tamu magmas, they demonstrate fractionation at deeper levels, with pressures reaching 650 MPa and indicating a multilevel magma storage system. A complementary experimental study using three synthetic compositions, representing different stages of magma differentiation beneath the Shatsky Rise (8.6 wt% MgO, 8.0 wt% MgO, 6.4 wt% MgO), supports the calculated magma storage conditions and shows that low pressures and low H₂O concentrations led to the high CaO/Al₂O₃ ratios. The natural differentiation trend is best reproduced by experiments at 200 MPa and almost anhydrous conditions (<0.2 wt% H₂O). In addition, the experimental results also support the assumption that magma storage and fractionation occurred in different levels in the crust, because the natural liquid lines of descent cannot be reproduced via isobaric fractional crystallization.