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Carbonaceons chondrites commonly contain 10-20% water-soluble salts by mass, the products of low-temperature aqueous alteration under oxidizing conditions. About 75% (by mass) of chondrite salts typically consists of magnesium sulfate hydrates. Conditions similar to those that affected carbonaceous chondrites may have prevailed within some asteroids and icy satellites, resulting in the formation of similar salt-rich rock (plus ice). These salts would be important in determining the physical and chemical characteristics of cryomagmatic brines. Frozen eutectic mixtures of MgSO4-rich brines could constitute a large fraction of the mass and volume of differentiated salty icy satellites, and widespread volcanic ice plains on some icy satellites may consist of frozen MgSO4-rich brines. The nature of brine magmatism depends in part on phase equilibria and volumetric relations of solid and liquid phases under the pertinent conditions of temperature, pressure, and other physical parameters. Accordingly, we have investigated densities and phase equilibria in the system MgSO4-H2O under pressures ranging from ∼0.1 MPa to ∼400 MPa, temperatures from 230 K to 300 K, and compositions up to 22% (by mass) MgSO4 using a novel high-pressure apparatus, described here for the first time in detail. We have found no evidence for a transition of MgSO4 hydrates to high-pressure polymorphs, although we have seen the expected transitions in water ice and we have found some evidence of a possible new magnesium sulfate hydrate. The graph of the eutectic melting point vs pressure approximately parallels the melting curve of water ice, except that the freezing-point depression increases slightly with pressure. Brine flows on icy satellites and chondritic asteroids mostly should correspond to eutectic and peritectic compositions (∼17 and ∼21% MgSO4, respectively, if modeled in the pure system H2O-MgSO4; compositions vary somewhat with pressure). Ice phases I and III, MgSO4 hydrates, and the eutectic solid mixture have large density differentials with respect to the eutectic liquid. Because of this and the liquid's low viscosity, gravitational separation of solids and liquids (fractional melting and fractional crystallization) could be very efficient even on low-g satellites and asteroids. Flotation of water ice may cause a tendency of brine flows to have salt-depleted, ice-rich surfaces. Brine intrusions should tend to segregate into ice-rich and salt-rich layers. Ice-rich masses in layered intrusions would be highly buoyant in a salt-ice crust and may be prone to solid-state diapirism. Ice diapirism offers an alternative explanation for palimpsests observed on Ganymede and viscous flows seen on Ganymede, Ariel, and Miranda. Large-scale brine magmatism may cause global compositional stratification of crusts and mantles. The ice crusts on some salty icy satellites may have formed in part by flotation of ice grains in brine magma bodies and coalescence of intrusive complexes of ice. The mantles of large salty icy satellites may consist of hydrated and anhydrous salts and high-pressure ice phases formed by sinking of these minerals. These processes have analogs in the formation of the lunar anorthositic highlands crust and mantle. |
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