Continuously Changing Quartz‐Albite Saturated Melt Compositions to 330 °C With Application to Heat Flow and Geochemistry of the Ocean Crust

Abstract

AbstractMelt compositions in equilibrium with quartz and albite at 0.1 GPa and temperatures from 819 to 330 °C are reported for cold seal experiments in the Na2O‐Al2O3‐SiO2‐H2O system. At 60–70 °C intervals, melt compositions continuously change from rhyolite‐like melts with <10 wt.% H2O at the quartz‐albite eutectic to melts of nearly endmember Na2Si2O5 having >30 wt.% H2O at 330 °C (vapor undersaturated). Melt Na2O increases while SiO2 and Al2O3 decrease down temperature. Experiments performed at T > 600 °C are vapor saturated with <9.9–21 wt.% H2O while those T < 600 °C are vapor undersaturated with 18–37 wt.% H2O. Because of the continuous change in the liquid's composition down temperature, all compositions can be called melts; in reality, their high water contents may also lead to being termed hydrous fluids. These results indicate that igneous processes continue to temperatures >300 °C below the haplogranite solidus. The water‐rich melts at low temperature have low viscosities and densities suggesting that they buoyantly ascend by reactive flow at low melt porosities (<5%). Formation of low‐temperature melt will occur along the sides of gabbro bodies of mid‐ocean ridges, resulting in large‐scale convection that removes heat from the lower crust. This circulation may move heat off axis influencing observed heat flow. Melt reaction processes result in silicic differentiates (plagiogranites) and formation of greenschist metabasalts. This frees calcium from silicate bonding, providing an important sink for CO2 in Earth's carbon cycle. Sr isotopes of the ocean crust change with ocean crust production rates indicating mid‐ocean ridge systems are a major control on Earth's climate over time.