Alteration_Mass_Balance

A polynomial is t to the data array polynomial is of

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Unformatted text preview: sitions • We create a series of plots of the least altered suite of elements against Zr (e.g., SiO2-Zr, K2O-Zr, Cs-Zr). • • A polynomial is fit to the data array. Polynomial is of the form: • Elementprecursor = AZrprecursor2+BZrprecursor+C (A, B, C are coefficients) • e.g.,SiO2precursor = AZrprecursor2+BZrprecursor+C • Zrprecursor from the previous slide is utilized in the equation above for each sample. • We then use the compare the reconstructed composition to the precursor composition to calculate mass change like the single precursor method. Wednesday, 15 August, 12 Calculating Precursor Compositions 80.00 75.00 SiO2 70.00 65.00 Least Altered Precursor Curve SiO2 = =-0.000479*Zr2+(0.28*Zr)+32.72 60.00 55.00 50.00 45.00 50.00 • • • (B) 100.00 150.00 200.00 Zr(P) 250.00 300.00 350.00 For each sample we have calculated a Zrprecursor that is substituted into the equation above to calculate the precursor SiO2 for each sample. This is repeated for each element-Zr plot. We then compare precursor compositions to reconstructed compositions to get mass change: MC = RC-precursor Wednesday, 15 August, 12 MacLean’s (1990) Method • After we calculate mass changes we can construct mass change plots and mass-gain bar graphs. • Mass change plots have the ΔM (wt%) vs. other ΔM (wt%) and are useful in outlining the mineralogical nature of the alteration. • Graphical and does an excellent job of providing fields for altered rocks. Wednesday, 15 August, 12 From Barrett and MacLean (1999) Wednesday, 15 August, 12 From Barrett and MacLean (1999) Wednesday, 15 August, 12 From Barrett and MacLean (1999) Wednesday, 15 August, 12 Stanley and Madeisky’s (1994) Method • Method is based on the usage of Pearce Element Ratio (PER) analysis (Pearce, 1968). • PER analysis uses ratios of elements to a common conserved element to remove the effects of closure or constant sum. • PER also assume that the minerals in a rock control the major element behaviour. • In particular, if a mineral fractionates from a melt it takes out major elements in stochiometric proportions to the mineral. • If this rock is then altered the alteration mineralogy controls the major element distributions in the rock. Wednesday, 15 August, 12 Stanley and Madeisky’s (1994) Method • Closure example: • • • Rock always closes to 100% • In essence we have 80% left in rocks [SiO2 (50/80) + Na2O (20/80) + CaO (10/80)], but must add to 100% • Now just via closure we have apparent gains: We have 20% MgO, 50% SiO2, 10% CaO, 20% Na2O Remove all of the MgO -> all other element % must increase in order for the rock to add up to 100% • • • • Wednesday, 15 August, 12 SiO2 = 50% + (5/8)*20% = 50% + 12.5% = 62.5% Na2O = 20% + (1/4)*20% = 20% + 5% = 25% CaO = 10% + (1/8)*20% = 10% + 2.5% = 12.5% Total = 100% Stanley and Madeisky’s (1994) Method • • PER have plots of X/Z versus Y/Z. • PER converts all oxide data to molar format such that X, Y, Z are in molar format. • X and Y are elements, or combinations of elements, that have coefficients for each element proportional to their abundance in specific minerals. • These coefficients result in m...
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