Magmatic processes at mid-ocean ridges: Evidence for high-pressure crystallization and crustal assimilation

This dissertation examines magmatic processes along two mid-ocean ridge segments: the Galapagos Spreading Center (GSC) and the Western Volcanic Zone (WVZ) in Iceland. Observations from these study areas illustrate the importance of variable mantle melting, melt transport and storage conditions, and the potential role of crustal assimilation processes in creating geochemical variability in mid-ocean ridge basalts (MORB).;Some MORB from the GSC exhibit higher Al contents (>16.0 wt. % Al 2O3 at >8.5 wt. % MgO) than expected for normal MORB evolution. Thermodynamic modeling of glass and mineral chemistry shows that high-pressure (0.3--0.4 GPa) crystallization can account for the unusually high Al and low Si contents of these glasses. These samples are similar to moderately rare high-Al MORB from other slow and intermediate spreading ridges and close to fracture zones elsewhere, suggesting that crystal fractionation in the upper mantle is an important process at these settings.;In Iceland's WVZ, two eruptive units---one lava shield (Lambahraun) and one fissure eruption (Thjofahraun)---were erupted ∼4000 yrs B.P. with eruptive centers separated by only ∼25 km. Thjofahraun, which erupted ∼1 km3 of pahoehoe and 'a'a lava from a 9-km long fissure, exhibits geochemical variations consistent with evolution by low-pressure fractionation and eruption from a shallow magma chamber. In contrast, Lambahraun erupted >7 km3 of low effusion-rate pahoehoe, shows evidence of plagioclase accumulation in select samples, and exhibits enrichments in CaO and highly incompatible trace elements that increase with increasing magmatic differentiation. Geochemical modeling of Lambahraun's major and trace element variations indicates the observed correlations between incompatible element concentration, increasing differentiation and time during the eruption can be related by concurrent wallrock assimilation and crystallization during melt migration (MAFC) through the crust.;We explore the conditions under which this melt-wallrock interaction plays a role in the formation of MORB using a 2-d numerical model of heat exchange between a magma conduit and its surrounding wallrock. Erupted crystal fractions and anatectic melt proportions are dependent on conduit geometry and magma ascent rate, with long-lived, low-effusion eruptions resulting in sufficient wallrock partial melting to produce observable geochemical effects in erupted basalts.