TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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3.6.3 Crustal Magma Evolution: Insights from Trace Elements 3.6.3.1 Crystal Population Origins I use Y as a proxy for the heavy REE (HREE), because Y 3+ has a ionic ra- dius intermediate between Dy 3+ and Ho 3+ , and REE heavier than Eu are highly incompatible in plagioclase and thus at or below LA-ICP-MS analytical detection limits. Ratios of LREE such as La or Ce to Y provide insight into OIB (high La/Y) vs. MORB (low La/Y) source affinities (see Figs. 3.13, 3.12). Trivalent La and Ce 3+ have the largest ionic radii of the REE and are the two least com- patible REE in a crystallizing basaltic assemblage (e.g., plagioclase + olivine ± clinopyroxene). Lanthanum and Ce are difficult to fractionate from one another by partial crystallization, and in this sense they provide insight into mantle source affinity. If Unit 3 population A and B crystals indeed share a common parent magma, as indicated by their major element similarities, then parent magmas of these populations should have similar La/Ce and La/Y ratios. Parent magmas of Unit 3 population A and B crystal cores and Unit 3 whole- rock data reported by Huang et al. [72] have broadly similar Sr abundances, La/Ce and La/Y ratios (Figs. 3.12a, 3.13), which supports my initial hypothesis that population A and B crystals had compositionally similar parent magmas. The extension of Unit 3 population B parent magmas to slightly higher Sr abundance can be attributed to crystallization of these older crystals from less fractionated magmas with a source shared with the population A crystals. The gentle upward curvature of the plagioclase CSD from the top of Unit 3 supports this notion, as it provides evidence of minor accumulation of larger (i.e., older) crystals. Accumu- lation and partial resorption of crystalline debris (i.e., large rounded population B crystals) was volumetrically minor, such that the resultant bulk-rock compo- 127
sition is similar to parent magmas of large and small crystals. There are three population A crystal core parent magmas that have notably higher La/Y ratios (but similar La/Ce to other population A and B parent magmas) that plot outside Site 1203 tholeiitic basalt field and within a field defined by Mauna Kea shield stage basalts (Fig. 3.13). Elevated La/Y in a residual magma can be generated in a region where partial crystallization was dominated by clinopyroxene due to the greater compatibility of Y relative to La in clinopyroxene. Based upon trace element data alone it is difficult to determine whether these magmas represent Hawaii-like end-member magma compositions or whether they came from some isolated environment where clinopyroxene crystallization dominated. I explore the latter possibility in the next section. Unit 14 population A parent magmas consistently range to lower La/Ce, lower La/Y, and lower Sr, which is consistent with derivation from a more depleted source than population B magmas (Figs. 3.12b, 3.13). This compositional divi- sion between population A and B parent magmas is not surprising considering the kinked nature of the Unit 14 plagioclase CSD relative to the gentle upward curvature of the Unit 3 CSD. If differences in trace element abundances between Unit 14 population A and B parent magmas were strictly related to partial crystallization, the lower Sr abundances of parent magmas of later formed crystals would be expected but not coupled with lower La/Ce and La/Y. I do point out, however, that while the Sr abundance in the low La/Ce - low La/Y population A parent magmas is slightly greater than EPR N-MORB and Garrett Transform- type basalts that were discussed by Huang et al. [72], the low La/Ce and La/Y ratios are consistent with a relatively depleted mantle source. On a plot of La/Ce vs. Sr, Unit 14 whole-rock compositions plot between low 128
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TEXTURAL AND MICROANALYSIS OF IGNEO
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TEXTURAL AND MICROANALYSIS OF IGNEO
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DEDICATION This work is dedicated t
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CHAPTER 2: MAGMA EVOLUTION REVEALED
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3.6.3.1 Crystal Population Origins
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FIGURES 1.1 Magma Chambers . . . .
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TABLES 2.1 MAJOR AND TRACE ELEMENT
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thus been debate within the volcano
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Cambrian schist) including a crysta
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explosive eruption vs. mild effusiv
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tiation (e.g., [84, 98]; Fig. 1.3).
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defined as having crystallinities b
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12 Figure 1.4: Large Igneous Provin
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distribution of crystal sizes (e.g.
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investigating magmatic evolution us
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length dispersive spectroscopy(WDS)
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microdrilling, a pre-cleaned square
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1.7.2 Detroit Seamount, Part of the
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lations. Reprocessing of intrusive
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nucleated homogenously and grew thr
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With accurate partition coefficient
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uniform composition of large volume
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Figure 2.2. Hand sample and thin se
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2.3.1 A Cogenetic Relationship Betw
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Figure 2.4. Stratigraphic section o
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Figure 2.5. Backscatter electron im
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studies of Kwaimbaita basalt by San
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42 Crystal Zone An (mol%) TABLE 2.1
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44 Crystal Zone An (mol%) 63R2 phen
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along the xenolith margins (e.g., F
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are more abundant in phenocrysts fr
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54 Crystal Zone 5 ML-X1 xenolith TA
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56 TABLE 2.2 Continued Crystal Zone
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Figure 2.7. Measured major and trac
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2.5.3.2 Trace Elements It is diffic
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Figure 2.9. Trace element ratio plo
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2.5.4 Resorption Features in Site 1
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should, however, be noted that in t
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2.6.1.2 An-rich OJP Plagioclase: In
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equivocally the roles of H2O, press
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Page 93 and 94: Yamashita, [123], and thus did not
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Page 99 and 100: CHAPTER 3 SHALLOW MAGMA EVOLUTION D
Page 101 and 102: magma compositions [23, 39, 98]. A
Page 103 and 104: Figure 3.2. Stratigraphy of basalts
Page 105 and 106: [118]. No volcaniclastic rocks were
Page 107 and 108: magmatic crystallization. Using the
Page 109 and 110: Figure 3.3. A cross polarized light
Page 111 and 112: 3.4.5 Major and Trace Element Data
Page 113 and 114: 3.4.6.1 Partition coefficients for
Page 115 and 116: 3.5.1.2 Site 884 CSDs I measured pl
Page 117 and 118: Figure 3.5. Plagioclase CSDs of ODP
Page 119 and 120: 106 TABLE 3.2 CSD RESULTS Sample OD
Page 121 and 122: ln (n) [mm -4 ] ln (n) [mm -4 ] 10
Page 123 and 124: ulation A crystals are unzoned (An6
Page 125 and 126: Figure 3.8. a) The majority of Site
Page 127 and 128: 3.5.4.3 Site 1203 Unit 31 and Site
Page 129 and 130: 3.5.5 Trace Elements 3.5.5.1 Measur
Page 131 and 132: Unit 31 > Site 884 Unit 8 (Table 3.
Page 133 and 134: 3.6 Discussion 3.6.1 Evidence of Cr
Page 135 and 136: 3.6.1.1 Site 1203 Alkalic Basalts a
Page 137 and 138: etween the different crystal popula
Page 139: Unit 14 and 31 magmas accumulated o
Page 143 and 144: Sr - low La/Ce population A parent
Page 145 and 146: The Unit 31 plagioclase CSD is kink
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Page 149 and 150: pressure, and growth of An-rich pla
Page 151 and 152: 138 TABLE 3.3 PLAGIOCLASE PHENOCRYS
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flow [33]. I selected a single samp
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4.3.4 Major and Trace Element Data
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B) C) parafilm splash guard A) tape
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of these elements are the better co
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5 mm 5 mm Figure 4.4. Cross polariz
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4.4.2 Electron Probe Microanalysis
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An (mol %) An (mol %) An (mol %) An
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study exhibit their highest An at t
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4.4.3 LA-ICP-MS - Trace Elements An
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87 86 Sr/ SrI = 0.705722(8) Ba/Sr =
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and Rb (Fig. 4.11d,e). 4.4.3.3 Yttr
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Sr (ppm) Y (ppm) La (ppm) Ce (ppm)
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Unit 4 plagioclase rims plagioclase
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204 TABLE 4.4 UNIT 4 PLAGIOCLASE PA
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4.5 Discussion 4.5.0.1 Origin of Ma
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4.5.0.2 Insights from Trace Element
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that other factors influenced the a
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C A/B / D A/B C A/B / D A/B Unit 4
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Magma chambers tend to form at hori
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whole-rock basalt data. Cumulate xe
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over short time spans but may be ea
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lowing initial Elan Bank magma empl
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BIBLIOGRAPHY 1. C. Allegre, A. Prov
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22. B. Charlier, J. Davidson, O. Ba
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46. J. Fitton and M. Godard, Origin
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68. M. Higgins, Measurement of crys
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89. J. Longhi, D. Walker and J. Hay
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100 of Geophysical Monograph, pages
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129. J. Tarduno, W. Sliter, L. Kroe
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