Evolution, hydrothermal assimilation, and ascent of magma inferred from volatile contents in MORB glasses: An example from thick lava pile at IODP Site 1256

A continuous section through an 811 m-thick lava pile of mid-ocean ridge basalts at Integrated Ocean Drilling Program Site 1256 provides important constraints on the variation with depth of volatile element (H₂O, CO₂, S and Cl) contents in fresh matrix glasses to understand evolution of magma, condition of magma ascent, and degree of hydrothermal assimilation. The lava pile is divided into two groups based on eruption location: off-axis basalts of upper portion (250–534 m beneath seafloor, mbsf) and on-axis basalts of lower portion (534–1061 mbsf). The lava pile is composed of pillow, sheet, and massive lavas that include thin (<2 m) hyaloclastite intervals. Variations of H₂O and major element contents in Site 1256 matrix glasses can be explained by fractional crystallization of one primitive basalt (MgO, 7.7 wt%) at the pressure of a melt lens (top of axial magma chamber). CO₂/Nb and S/Dy in a large majority of fresh matrix glasses are lower than undegassed ratios, and these facts indicate that CO₂ and S losses occurred during differentiation within the melt lens and/or magma transport to the seafloor. Saturation pressures calculated from dissolved CO₂ and H₂O contents give a wide range of pressures from seafloor (~25 MPa) to axial magma chamber (~60 MPa). Many off-axis glasses have saturation pressures of seafloor, but on-axis glasses have high saturation pressures up to axial magma chamber depths. The saturation pressures of off-axis glasses decrease with decreasing depth (i.e., with increasing distance from the axis). These observations indicate that the many off-axis glasses were highly degassed because their source magmas travelled for long time periods from melt lens to off-axis seafloor locations. On the other hand, on-axis glasses were less degassed because magma transport times to on-axis locations were shorter than those for off-axis locations. The most distinctive characteristic for the Site 1256 glasses is their higher Cl/Nb than any other mid-ocean ridge basalt glasses. The strong Cl enrichment is explained by assimilation of hydrothermal brines. Beneath Site 1256, the melt lens was very shallow (<1.2 km) and hydrothermal circulation of high-salinity brines with variable salinities would reach easily to the melt lens.