The hydrothermal consequences of MORB compositional variation.

Mantle derived basalts along the entirety of the Earth's Mid-Ocean Ridge (MOR) spreading centers are continuously altered by seawater, allowing the hydrosphere to subsume energy and exchange mass with the deep, slowly cooling Earth. Compositional heterogeneities inherent to these basalts—the result of innumerable geophysical and geochemical processes in the mantel and crust—generate spatial variation in the equilibrium states to which these water-rock environments cascades towards. This results in a unique distribution of precipitate assemblages, hydrothermal fluid chemistries, and energetic landscapes among ecosystems rooted within and above the seafloor. In this work we define these equilibrium states for the full complement of basalt compositional heterogeneity present today over all appropriate temperatures and extents of reaction. We also calculate the out-of-equilibrium mixes generated when derived vent fluids mix with seawater, the very process that supports ecosystems the dark. Facilitated by novel, high throughout code, this effort has yielded a high-resolution compositional database which can be mapped back onto all ridge systems. By resolving chemical and energetic consequences of basalt-seawater interaction to sub-ridge scales, we can distinguish alteration features that are globally homogenous from those that are locally heterogenous. For example the collective alteration behavior suggests that quartz may be homogeneously distributed for nearly all ridges while anhydrite is not, that MORB CaO is a better predictor of metabolically significant energy sources such as H₂ and methane than FeO, and that unique subsets of MORB generate fluids that are well suited to seemingly unrelated metabolism, such as how MORB's high in MgO + CaO and low in FeO + Na₂O recovered the central MAR produce fluid mixes which can yield increased energy return to sulfide oxidizers down to lower mixing temperatures when compared with all other locations on Earth. These are just a few of the myriad chemical features to be unveiled within this data set, allowing correlations to be established and causation further investigated between mantle processes producing MORB diversity, the alteration of that MORB by seawater, and its eventual use by ecosystems.