TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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calculated melt concentrations for Sr and Ba (within their analytical uncertainty). Likewise, Bindeman et al. [7] showed that variations of ∼150 ◦ C produce < 10% differences for particular partition coefficients, which were often within error of the respective D values. Blundy and Wood [11] presented an alternative model for quantitative predic- tion of D values based upon thermodynamic principles and lattice strain theory (i.e., [15]). Specifically, they noted the dominant roles of cation size, lattice site size, and elasticity of the lattice site related to cation substitution in minerals. The elastic response of a lattice site to cation substitution is measured by Young’s Modulus (E), and the value of E plag is dependent upon plagioclase An content and the charge of the substituent cation [11, 12]. Equation 4.2 (Equation 2 of Blundy and Wood [11]) is the basis of their predictive D model that I apply to plagioclase, Di = D z+ o ∗ exp −4πEplagNA ro 2 (r2 o − r2 i ) + 1 3 (r3 i − r3 o) RT (4.2) where Di of a trace element (i) is a function of the strain compensated partition coefficient for the divalent (D 2+ 0 ) or trivalent (D3+ 0 ) substituent cation, Young’s Modulus for the plagioclase M lattice site (E plag), NA (Avogadro’s Number), the radius of the plagioclase M site (ro), the radius of the substituent cation (ri), the gas constant (R), and crystallization temperature (T). For a more thorough discussion of this model see Blundy and Wood [11, 12]. 4.3.6.1 Partition coefficients for Elan Bank Plagioclase Crystals I used equation 4.2 and the parameterizations of Blundy and Wood [11, 12] to calculate D values for Rb, Sr, Y, Ba, La, Ce, Pr, Nd, and Sm, as partitioning 183
of these elements are the better constrained through experimental studies (see Tables 4.5 and 4.5). Estimates of Sc DSc are unrealistically low (< 0.001) using the Blundy and Wood [11] method, which is most likely due to inadequate estimation of the strain-free partition coefficient. The partitioning behavior of Fe and Eu are strongly affected by oxygen fugacity f O2, and due to f O2 constraints I did not attempt to estimate DFe or DEu values. Magnesium is known to substitute in more than one plagioclase cation site, and based upon this complex partitioning I did not calculate DMg [7]. Blundy and Wood [11] noted that predictive approaches are inaccurate for Pb, which has a lone pair of electrons. I assumed a crystallization temperature of 1150 ◦ C to calculate D values. The validity of this assumption was examined using the MELTS program [2, 55] to determine the temperature at which plagioclase arrives on the liquidus during crystallization of a typical Elan Bank basalt sample. I input four upper group and lower group Site 1137 whole-rock basalt compositions with 4.4 wt% MgO, 5.9 wt% MgO, 6.0 wt% MgO, and 7.3 wt% MgO reported by Shipboard Scientific Party [33] into the MELTS program. During equilibrium crystallization of the lowest MgO basalt, plagioclase appeared on the liquidus at 1080 ◦ C and at at 1180 ◦ C from the highest MgO sample. The use of 1080 ◦ C vs. 1180 ◦ C in D calculations yields < 10% differences in trace element concentrations of parent liquids, which is within the analytical error for my analyses. The effects of pressure on plagioclase D values are not well documented for most elements, and where they were documented for DSr by Vander Auwera et al. [137] they were negligible. I used a crystallization pressure of 0.001 kbar for D calculations. I assume the strain compensated (i.e., strain free) partition coefficient D 2+ 0 in plagioclase is best approximated by DCa (c.f., [12]), and to estimate DCa I used equation 4.1 and the “a” and “b” constants reported 184
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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
Page 145 and 146: The Unit 31 plagioclase CSD is kink
Page 147 and 148: was dominated by late stage clinopy
Page 149 and 150: pressure, and growth of An-rich pla
Page 151 and 152: 138 TABLE 3.3 PLAGIOCLASE PHENOCRYS
Page 153 and 154: 140 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: B) C) parafilm splash guard A) tape
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 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
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129. J. Tarduno, W. Sliter, L. Kroe
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