TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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were the types of crustal rocks assimilated, as favored by Ingle [75], assimilation must have occurred prior to plagioclase dominated partial crystallization. Core, intermediate zone, and rim 87 Sr/ 86 SrI variations are subtle, which indicates that the Unit 4 and 10 plagioclase crystals grew from already contaminated parent magmas. This may also suggest that over time the magmatic system became well buffered, which minimized late stage pulses of greater crustal assimilation. Inverted parent magma compositions from both Units 4 and 10 are offset from ranges of trace and REE abundances of their respective whole-rock basalt groups. This indicates either poor choices of partition coefficients and/or that diffusion of trace elements occurred within Unit 4 and 10 plagioclase crystals. Indeed, a greater proportion of crystals from the Unit 10 basalt are diffusively re-equilibrated relative to the Unit 4 crystals. This indicates that Unit 10 crystals were held at magmatic temperatures for greater periods of time. This consistent with the working hypothesis that the majority of the Unit 4 plagioclase phenocrysts bear a close genetic relationship with their host basalt, and the majority of the Unit 10 crystals were stirred up from a cumulate layer or other mush pile. I propose that, much like the Ferrar magmatic mush column and Kerguelen Archipelago examples, the magma chamber system below Elan Bank was a complex system of interconnected dikes and sills (c.f., [36, 100]. I suggest that the abundant plagioclase phenocrysts In the Unit 4 and 10 basalts originated from the shallowest magma chambers within this system where crustal contamination was an insignificant process. Crustal assimilation was most prominent during the initial emplacement of basaltic magma into Elan Bank crust, and variations in the crustal contamination signature over time may be as much linked to re- mobilization and partial resorption of crystal mush material formed directly fol- 219
lowing initial Elan Bank magma emplacement as progressive erosion of crustal wall rock by repeated injection of hot and primitive magmas into Elan Bank magma chambers. 5.2 Recommendations for Future Work Once one embarks upon the type of work detailed in this dissertation it does not take one long to make the realization that igneous rocks (and thus igneous systems) are far more complex than they are commonly modeled. The notion that mantle source characteristics are preserved in magmas from mantle to eruption indeed may be true in some settings, but there is overwhelming evidence that magma chamber systems are complex, ubiquitous, and tend to buffer the compositions erupted magmas (e.g., [100]. Indeed, this level of magma chamber complexity is apparent in the three studies described in this dissertation. The im- portance of bulk-rock compositional studies should not be understated, however an entire new level of insight into the genesis, evolution, and eruption of magmas is on the horizon thanks to technological advances that make it possible conduct accurate and precise compositional studies on the smallest of samples. Innovative implementations of the new and advanced microanalytical techniques will bring about a paradigm shift in igneous petrology and volcanology. With increased use of techniques such as LA-ICP-MS to measure low abun- dance elements within minerals comes an increased need for accurate partition coefficients. More accurate partition coefficients will improve estimates of parent magma compositions inverted from minerals such as plagioclase and will improve the accuracy of trace element diffusion calculations within minerals, which has proven a useful way to estimate magmatic residence time [145]. In plagioclase 220
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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 181 and 182: 168 TABLE 3.4 Continued Sample Zone
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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
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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 and 220: 4.5 Discussion 4.5.0.1 Origin of Ma
Page 221 and 222: 4.5.0.2 Insights from Trace Element
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: over short time spans but may be ea
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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