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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., SiO2Zr, K2OZr, CsZr). •
• 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 elementZr plot.
We then compare precursor compositions to reconstructed
compositions to get mass change: MC = RCprecursor Wednesday, 15 August, 12 MacLean’s (1990) Method
• After we calculate mass changes we can construct mass change
plots and massgain 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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 Fall '12
 DrPiercey

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