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Isotopes

# 4 given that the scheelite is cogenetic with the gold

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Unformatted text preview: scheelites. Are there one or two gold formation events? 2) Plot both isochrons on the same diagram. What do you notice about the slopes of the different isochrons? How does this relate to the age of the samples? 3) Using the initial ratios of the two scheelite generations calculate the eNdt value for the scheelite (remember you must recalculate CHUR @ time t as well). Which vein set has a more radiogenic signature? What type of source do you think the Nd in the scheelite comes from? 4) Given that the scheelite is cogenetic with the gold in these veins and assuming that they come from the same source what does this say about the origin of the gold? Sm Nd Decay Equation and Isochron Age Calculations Sm ­Nd system is based on the decay of 147Sm to 143Nd from the following equation: (143Nd/144Nd)m = (143Nd/144Nd)o + (147Sm/143Nd)m(e t 1) (1) where: (143Nd/144Nd)m = measured 143Nd/144Nd ratio of the sample (143Nd/144)o = initial 143Nd/144Nd ratio of the sample (or source) when the rock formed (147Sm/143Nd)m = measured 147Sm/144Nd ratio of the sample = decay constant (6.54 x10 ­12 yr ­1) t = age in Ma. This is an equation of the form y = mx + b, by obtaining the slope of this line we can calculate the age of the sample/rock by using: m = e t –1 (2) and rearranging this we have: t = ln(m+1) / λ (3) Since we are often dealing with multiple datapoints for an isochron we must calculate the “least squares” regression line for the multiple datapoints (i.e., a best ­ fit line for the data points). The slope of this line can then be substituted into equation (2) to obtain the age of the sample. The slope of the least squares regression (m) and the y ­intercept (b) are given by: n ∑ XY − ∑ X ∑ Y m= ( 2 4) n ∑ X 2 − (∑ X ) ∑Y − m ∑ X (5) b= € n Where: X = 147Sm/144Nd (measured), € Y = 143Nd/144Nd (measured), n = number of data points. λ λ λ Epsilon...
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