CompuServe Messages

Where does this claim of fractional-distillation in magmas come from? Distillation involves vaporization of the "fractionated" material and that doesn't happen in earth temperature processes. Partial melting and fractional CRYSTALLIZATION are processes that take place in the earth within or beneath the crust. In partial melting, the least strongly bonded atoms (ions) are dislodged from crystals at temperatures near melting and when enough of these loose ions have collected together to move as a body, you have a magma. One foundation of isotope geochemistry is that these magma bodies evolve in a homogeneous fashion. Fractional crystallization selectively extracts the constituents that form the highest melting point minerals and magma (the remaining liquid) composition changes over time (remember Bowen?). This is well known in volcanoes that have erupted sequences of lavas over periods of time, the rocks fall into compositional series like basalt to andesite to trachyte. Isotope ratios of Strontium and Neodymium in these lavas indicate concentrations of parental Rubidium and Samarium in the source rocks from which the magma was derived identifying them as mantle or crust or mixed. Even if it is theoretically possible for the heavy isotopes (masses ranging from 86 to 238) to fractionate, there are vast numbers of isotope studies that are internally consistent so the effect is well below experimental error. Although isotopic ratios can indicate age, they are mostly used as indicators of petrologic process. Potassium-Argon is used to determine the ages of igneous rocks that can be assumed to have crystallized with no 40-Argon or with a small amount of argon entrapped from the atmosphere. Any 40-Argon found over this background comes from the decay of 40 Potassium in the rock and the relative amounts indicate the time since cooling (if none has been lost). This works amazingly well with most volcanic rocks. Dan Lynch, formerly of U of Arizona's Isotope Geochemistry Lab.