MAGMATIC DIFFERENTIATION OF IO

If Io has been volcanically active through much of its history, it mus t be highly differentiated. We present an initial attempt to quantify the differentiation of the silicate portion of Io. We suggest that, on average, each part of Io has undergone about 400 episodes of partial melting. We employ a widely used thermodynamic model of silicate melts to examine the effect of such repeated differentiation. Despite many caveats, including a grossly oversimplistic model of the differentiati on process, uncertainties in the initial composition of the mantle, an d the failure to model more than four episodes of partial melting, we are able to make some robust conclusions. Io should have a roughly 50 km thick, low density (2600-2900 kg m(-3)), alkali-rich, siliceous cru st composed primarily of feldspars and nepheline. The crustal magmas s hould have relatively low melting temperatures (<1100 degrees C). The bulk of the mantle should be essentially pure forsterite (magnesian ol ivine). It is possible that the denser iron-and calcium-rich materials are segregated into a lower mantle and thus no longer involved in sur face processes. These model predictions are generally consistent with the observations of Io. The enrichment of the crust in alkalis may hel p to explain the composition of the neutral clouds around Io. The fail ure to detect silicates at the surface of Io to date might be due in p art to the difficulty in detecting Fe-poor minerals such as nepheline, feldspars, and forsterite via near-IR spectroscopy. Many hot spot tem peratures are too high for sulfur alone but are in line with silica-ri ch melts. The mountains on Io could be manifestations of large buoyant plutons. The highest temperature lavas may be the result of melts fro m the depleted mantle making their way to the surface from great depth s. (C) 1997 Academic Press.