TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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crystals (glomerocrysts: Fig. 4.4a). Despite the presence of glomerocrysts, most Unit 4 plagioclase crystals are present as individual euhedral crystals. I suggest that the normal zoned plagioclase phenocrysts bear a close genetic relationship with the Unit 4 host basalt, whereas the reverse zoned crystals likely come from disaggregated glomerocrysts that crystallized earlier and under different condi- tions. The mixture of normal and reverse zoned crystals indicates that plagioclase crystals were recycled throughout the magmatic system during the petrogenesis of the Unit 4 basalt, which is significant in that the plagioclase crystals provide a greater temporal record of magma evolution. The majority of the plagioclase phenocrysts in the Unit 10 basalt examined in this study exhibit reverse zonation, which indicates they may have initially grown from a more evolved and/or crustally contaminated magma then exposed to a more primitive magma just before or during eruption. A significant fraction of the large plagioclase phenocrysts in the Unit 10 basalt sample examined in this study are present within glomerocrysts (Fig. 4.4b). Individual Unit 10 plagioclase crystals not present in glomerocrysts are generally sub-rounded and are probably from disaggregated glomerocrysts. I suggest that a large amount of mushy plagioclase dominated crystal debris was entrained prior to and during eruption of the Unit 10 basalt and is the primary source of plagioclase phenocrysts in this basalt. Trace element and Sr isotope data provide a greater insight into these processes and whether early formed Unit 4 and 10 crystals provide insight into temporal variations in crustal assimilation. 207
4.5.0.2 Insights from Trace Elements and Inverted Parent Magma Compositions Plagioclase parent magmas from Units 4 and 10 overlap whole-rock fields defined by their respective Site 1137 upper and lower basalt groups in Ba/Sr vs Rb space (Fig. 4.14). Whole-rock compositions tend to represent an integration of all of the subsurface processes that were active during basalt petrogenesis, and a number of studies have noted greater records of processes such as crustal assimilation and exposure to primitive magmas within individual plagioclase crystals [37, 135]. Although major element and zoning data indicate that the plagioclase phenocrysts contain greater temporal records of magma evolution relative to the whole-rock chemistry, this overlap in figure 4.14a indicates that peak crustal con- tamination present in Units 4 and 10 pre-dated plagioclase crystallization. Plagioclase parent magmas trend to lower La/Sm relative to their respective host basalt groups (Fig. 4.14c), and inverted parent magma trace element and REE abundances are generally lower than those of Site 1137 upper and lower group basalts (Table 4.4 and Table 4.5). Low trace element abundances of plagioclase parent magmas may indicate that plagioclase crystals grew from magmas that were more depleted than their host basalts. This however seems unlikely given the regular occurrence of reversely zoned plagioclase crystals in each basalt and the overlap in figure 4.14a. An alternative origin for the low trace element abundances of plagioclase parent magmas is the use of inaccurate partition coefficients and/or trace element diffusion. It is difficult to gauge the accuracy of calculated partition coefficients, but Bindeman et al. [7] and Blundy and Wood [10] pointed out that An content is a dominant factor that affects cation substitution in plagioclase and temperature has a minor affect. Although An variations and crystallization temperatures are accounted for in partition coefficient calculations, it is possible 208
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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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ent magma requires significant clin
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tion driven plumes, or stirring by
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Yamashita, [123], and thus did not
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erally extensive crystal-mush netwo
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asalt at Site 1185 of ODP Leg 192 [
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CHAPTER 3 SHALLOW MAGMA EVOLUTION D
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magma compositions [23, 39, 98]. A
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Figure 3.2. Stratigraphy of basalts
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[118]. No volcaniclastic rocks were
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magmatic crystallization. Using the
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Figure 3.3. A cross polarized light
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3.4.5 Major and Trace Element Data
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3.4.6.1 Partition coefficients for
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3.5.1.2 Site 884 CSDs I measured pl
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Figure 3.5. Plagioclase CSDs of ODP
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106 TABLE 3.2 CSD RESULTS Sample OD
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ln (n) [mm -4 ] ln (n) [mm -4 ] 10
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ulation A crystals are unzoned (An6
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Figure 3.8. a) The majority of Site
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3.5.4.3 Site 1203 Unit 31 and Site
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3.5.5 Trace Elements 3.5.5.1 Measur
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Unit 31 > Site 884 Unit 8 (Table 3.
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3.6 Discussion 3.6.1 Evidence of Cr
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3.6.1.1 Site 1203 Alkalic Basalts a
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etween the different crystal popula
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Unit 14 and 31 magmas accumulated o
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sition is similar to parent magmas
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Sr - low La/Ce population A parent
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The Unit 31 plagioclase CSD is kink
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was dominated by late stage clinopy
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pressure, and growth of An-rich pla
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138 TABLE 3.3 PLAGIOCLASE PHENOCRYS
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140 TABLE 3.3 Continued Sample Zone
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Page 169 and 170: 156 TABLE 3.3 Continued Sample Zone
Page 187 and 188: KP has been done via drilling at 11
Page 189 and 190: and over time crustal wall-rocks be
Page 191 and 192: flow [33]. I selected a single samp
Page 193 and 194: 4.3.4 Major and Trace Element Data
Page 195 and 196: B) C) parafilm splash guard A) tape
Page 197 and 198: of these elements are the better co
Page 199 and 200: 5 mm 5 mm Figure 4.4. Cross polariz
Page 201 and 202: 4.4.2 Electron Probe Microanalysis
Page 203 and 204: An (mol %) An (mol %) An (mol %) An
Page 205 and 206: study exhibit their highest An at t
Page 207 and 208: 4.4.3 LA-ICP-MS - Trace Elements An
Page 209 and 210: 87 86 Sr/ SrI = 0.705722(8) Ba/Sr =
Page 211 and 212: and Rb (Fig. 4.11d,e). 4.4.3.3 Yttr
Page 213 and 214: Sr (ppm) Y (ppm) La (ppm) Ce (ppm)
Page 215 and 216: Unit 4 plagioclase rims plagioclase
Page 217 and 218: 204 TABLE 4.4 UNIT 4 PLAGIOCLASE PA
Page 219: 4.5 Discussion 4.5.0.1 Origin of Ma
Page 223 and 224: that other factors influenced the a
Page 225 and 226: C A/B / D A/B C A/B / D A/B Unit 4
Page 227 and 228: Magma chambers tend to form at hori
Page 229 and 230: whole-rock basalt data. Cumulate xe
Page 231 and 232: over short time spans but may be ea
Page 233 and 234: lowing initial Elan Bank magma empl
Page 235 and 236: BIBLIOGRAPHY 1. C. Allegre, A. Prov
Page 237 and 238: 22. B. Charlier, J. Davidson, O. Ba
Page 239 and 240: 46. J. Fitton and M. Godard, Origin
Page 241 and 242: 68. M. Higgins, Measurement of crys
Page 243 and 244: 89. J. Longhi, D. Walker and J. Hay
Page 245 and 246: 100 of Geophysical Monograph, pages
Page 247 and 248: 129. J. Tarduno, W. Sliter, L. Kroe
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