MAGNESIUM SULFATE-WATER TO 400 MPA USING A NOVEL PIEZOMETER - DENSITIES, PHASE-EQUILIBRIA, AND PLANETOLOGICAL IMPLICATIONS

Carbonaceous chondrites commonly contain 10-20% water-soluble salts by mass, the products of low-temperature aqueous alteration under oxidiz ing conditions. About 75% (by mass) of chondrite salts typically consi sts of magnesium sulfate hydrates, Conditions similar to those that af fected carbonaceous chondrites may have prevailed within some asteroid s and icy satellites, resulting in the formation of similar salt-rich rock (plus ice). These salts would be important in determining the phy sical and chemical characteristics of cryomagmatic brines. Frozen eute ctic mixtures of MgSO4-rich brines could constitute a large fraction o f the mass and volume of differentiated salty icy satellites, and wide spread volcanic ice plains on some icy satellites may consist of froze n MgSO4-rich brines, The nature of brine magmatism depends in part on phase equilibria and volumetric relations of solid and liquid phases u nder the pertinent conditions of temperature, pressure, and other phys ical parameters. Accordingly, we have investigated densities and phase equilibria in the system MgSO4-H2O under pressures ranging from simil ar to 0.1 MPa to similar to 400 MPa, temperatures from 230 K to 300 K, and compositions up to 22% (by mass) MgSO4 using a novel high-pressur e apparatus, described here for the first time in detail. We have foun d no evidence for a transition of MgSO4 hydrates to high-pressure poly morphs, although we have seen the expected transitions in water ice an d we have found some evidence of a possible new magnesium sulfate hydr ate, The graph of the eutectic melting point vs pressure approximately parallels the melting curve of water ice, except that the freezing-po int depression increases slightly with pressure. Brine flows on icy sa tellites and chondritic asteroids mostly should correspond to eutectic and peritectic compositions (similar to 17 and similar to 21% MgSO4, respectively, if modeled in the pure system H2O-MgSO4; compositions va ry somewhat with pressure). Ice phases I and III, MgSO4 hydrates, and the eutectic solid mixture have large density differentials with respe ct to the eutectic liquid. Because of this and the liquid's low viscos ity, gravitational separation of solids and liquids (fractional meltin g and fractional crystallization) could be very efficient even on low- g satellites and asteroids, Flotation of water ice may cause a tendenc y of brine flows to have salt-depleted, ice-rich surfaces. Brine intru sions should tend to segregate into ice-rich and salt-rich layers. Ice -rich masses in layered intrusions would be highly buoyant in a salt-i ce crust and may be prone to solid-state diapirism. Ice diapirism offe rs 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 coalescen ce of intrusive complexes of ice. The mantles of large salty icy satel lites may consist of hydrated and anhydrous salts and high-pressure ic e phases formed by sinking of these minerals. These processes have ana logs in the formation of the lunar anorthositic highlands crust and ma ntle. (C) 1995 Academic Press, Inc.