Alteration_Mass_Balance

Z is a conserved element one that doesnt take part in

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Unformatted text preview: inerals having specific slopes (control lines) on PER diagrams; if data lie upon these lines reflect that they have fractionated this mineral, or contain a specific alteration mineral – see below. Z is a conserved element, one that doesn’t take part in a the process, be that alteration (immobile, incompatible) or fractional crystallization (i.e., Zr, Ti, Al). Wednesday, 15 August, 12 Stanley and Madeisky’s (1994) Method • Fractionation of forsterite example: • • Foresteritic olivine (Fo) = Mg2SiO4 • Thus on PER diagram we’d plot Si/Z (x-axis) vs. Mg/Z (y-axis) – and the slope for the Fo control line would be 2. • • To make the slope 1 we could plot Si/Z – 0.5Mg/Z If Fo fractionates from a melt for every 1 Si atom we’ll fractionate 2 Mg atoms. Now compare analytical data to this to test if Fo fractionation occurred • • Wednesday, 15 August, 12 if data lies on the line then yes if off the line – no and Si-Mg must be controlled by some other process. 0.5Mg/Z Fractionating Forsterite Have to little Mg or too much Si to be xtallizing Fo Si/Z Wednesday, 15 August, 12 Stanley and Madeisky’s (1994) Method • Consider feldspar fractionation, and subsequent alteration to either chlorite or muscovite. • • • K-Feldspar – (Na,K)AlSi3O8 Muscovite - KAl2(AlSi3O10)(F, OH)2 Chlorite - (Fe, Mg, Al)6(Si, Al)4O10(OH)8 • Assumption is that Al is common to all minerals and not readily transferred during alteration. • Plotted Al/Z - Na+K/Z plot. • • • K-feldspar - 1 (Na+K) per Al –> m = 1 Muscovite – 1 (Na+K) per 3 Al -> m = 1/3 Chlorite – 0 (Na+K) per 6 Al -> m = 0/6 = 0 Wednesday, 15 August, 12 From Stanley and Madeisky (1994) Wednesday, 15 August, 12 Stanley and Madeisky’s (1994) Method • • • Although quantitative it isn’t very intuitive to most users. • • Results are graphically easy to see. Requires large amount of numerical manipulation to plot diagrams. Must at first define fractionating assemblages and then the associated alteration minerals that would form from them. Is gaining significant ground as a research and exploration tool, especially in Australia and in the orogenic Au exploration world. Wednesday, 15 August, 12 Other Tools • • Mineralogy or normative mineralogy plots. • Then created vectors to specific minerals proportional to a type of alteration. Macdonald et al. (1996) used plots of Al2O3-Na2O+K2O+CaO – MgO and compared fresh and altered rocks relative to the bulk composition of specific mineral phases. Wednesday, 15 August, 12 From Macdonald et al. (1996) Wednesday, 15 August, 12 Other Tools • Contoured plots of elements, or element ratios, or combinations of elements sensitive to alteration. • • • • Na2O, K2O, Al2O3/Na2O, MgO, etc. Cu, Zn, Pb, Tl, Sb, etc. Alteration indexes. Normative mineralogy. Wednesday, 15 August, 12 From Galley et al. (1994) Wednesday, 15 August, 12 From Date et al. (1983) Wednesday, 15 August, 12 Other Tools • Volatile element halos...
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