CompuServe Thread

Jon I have a different process that may explain(some/many/all) deposits veins and ore intrusions. Well after the formation of the earth the molten materials stratified into layers of materials based on densities. These are just approximate guesses but the layers were in solid and liquid layers stratified by densities. Under the crust which is 8 miles thick give or take there are layers quartz/granite, Basalt/feldspar/granite, then the metals, then the core. The crust under went major catastrophes or one big where the entire crust was shattered vertically and horizontally throughout. These cracks would open up to great depths at times and would be flooded with materials from these layers. As the cracks worked back and forth different amounts of mixing would occur and the materials would be injected into cracks and spaces as well as pushed up towards the surface at times blasted out the top of the crack as a mixture of gases that would condense and rain down on the land. A simplistic view of a fantastic mechanism that could explain all the combinations of materials we find on the earth. You are in it much deeper than I am but this is a thought I have had wondered how it would run in your mental simulator. Kamron

Hi, Jon– I would expect _slightly_ older datings from the outer edges of an igneous intrusion than in the center, but not because of some isotopic fractionation. The outer regions should generally be the first to cool and solidify, being exposed to a nice heat sink in the form of surrounding rocks. Are these people claiming greater radiodate differences than this? One method of separating heavy water from water is to distill it (keeping only the last water to condense) over and over and over and over… ad nauseum. The amount of water discarded relative to the product is awesome and the efficiency is appalling. Distillation can distinguish isotopes, but it is an extremely weak distinction, and would be even weaker for the heavier atoms used in radiodating. D2O and H2O differ in weight by a "hefty" ratio of about 20/18, where K40 and K39 would be a quarter as distinguishable by weight (40/39). It surely cannot be much compared to the basic isotope ratios that radiodating depends on, unless there were reason to expect many repetitions of the distillation. This distillation effect would have to alter ratios of stable isotopes just as easily if the weight difference is similar. If I see constant (within the intrusion) ratios of stable isotopes, I see no reason to imagine radioisotope ratios were variable. If I read an aged chart of nuclides correctly, original K40/K41 anomalies should be accompanied by much stronger K39/K41 anomalies (both appear to be stable isotopes). –Doug

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.