Final Exam Essay

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Final Exam (Fall 2009)
1. From the paper, the supplementary data sheet, and the long list of thermobarometers (Table 15-1 of Spear) that I handed out in class, what are the reactions that were used to determine the P and T of the eclogites? Give the reactions in terms of the pure endmembers. (hint: there are three kinds of reactions)

The reactions that were used to determine the P and T of the eclogites in the paper are Exchange thermometer: Garnet-clinopyroxene (because omphacite belongs to clinopyroxene) Fe3Al2Si3O12+3CaMgSi2O6=Mg3Al2Si3O12+3CaFeSi2O6 And Net transfer equilibria: pyroxene-plagioclase-quartz that is jadeite-albite-quartz in this paper Na[AlSi3O8]=NaAlSi2O6+SiO2

2. The authors say that the eclogites (high pressure metabasites, i.e., metamorphic equivalents of basalts) have trace element abundances similar to MORB. (a) What does a typical rare earth element (REE) pattern of MORB look like (sketch it or provide a plot) ? (b) As a general question, how do the abundances of the REE in MORB compare to chondrites? (c) Why are they higher or lower? (d) How do the abundances of the REE compare to each other (normalized to chondites)? (e) Why are the relative abundances of the REE like that? (i.e., why does the pattern have that shape?) (These are general questions; not specific to the paper.)

a) The typical REE pattern of MORB normalized by chondrite is showing in the following figure.
b) It is obviously that the abundances of the REE in MORB compare to chodrites are higher.
c) Because the REEs are incompatible elements, they are concentrated in the melt when the forming of the core and mantle. However, chondrites represent the bulk earth and MORB represents the mantle or upper mantle, so the abundances of the REE are higher in MORB than in chondrites.
d) In general, the abundances of LREE are lower than the HREE, and the abundances of REE keep on increasing from La to Lu.
e) Because LREEs are more incompatible than HREEs, which means that in melt LREEs are more concentrated than HREEs. After the forming of the crust, the LREEs in mantle are lower than the HREEs, and MORB represents the mantle or upper mantle. Thus the pattern of the REE in MORB has this shape.
[pic]
3. (a) What dating method was used to determine the age of the eclogites? (Be as specific as possible.) (b) How do we know that this date is reliable or has geologic significance? (As opposed to, say, a conventional K-Ar date.) (c) What mineral was dated and how did the authors "prove" that that mineral formed during formation of the eclogite? (Hint: What is Fig. 3C all about?) (d) Is the age they obtained an igneous crystallization age, a metamorphic crystallization age, or a cooling age? Why?

a) The dating method used to determine the age of the eclogites is U-Pb zircon dating. The U-Pb isotopes were measured using the UCLA Cameca IMS 1270 ion microprobe in the polished thin sections. The age significance of U/Pb was determined by assuming that measured Pb was a simple mixture between a single concordant radiogenic endmember and common Pb. Then the zircons analyzed gave a 238U/206Pb age. b) First the standard error 2σ and MSWD show the data is reliable, then we know that the age data fits the concordia line. c) Zircon was dated in this paper. The authors analyzed the REEs of the dated zircon and adjacent garnet, and the chondrite-normalized REE diagram exhibits no significant negative Eu and similarity to alpine eclogites, which can prove that the zircon formed during formation of the eclogites. d) It is a metamorphic crystallization age. First, the dated mineral is zircon which has very high closure temperature, thus the age should be a crystallization age not a cooling
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