TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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CHAPTER 5 SUMMARY, CONCLUSIONS, <strong>AND</strong> FUTURE WORK 5.1 Results and Conclusions 5.1.1 The Ontong Java Plateau This research focused upon understanding the physical processes (i.e., magma chamber dynamics) of differentiation that led to the repeated production of the same basalt type (Kwaimbaita basalt) over a Greenland-size geographic area? Plagioclase-rich cumulate xenoliths consisting of An-rich (up to An86) plagioclase crystals from provide insight into these processes beyond what has been learned from whole-rock studies alone. Anorthie-rich sections of cumulate xenolith crystals and phenocrysts in host basalts were formed primarily by crystallization in shallow (low pressure, 2-7 km depth) regions of the OJP magma chamber system from more primitive magmas than those basalts sampled from the OJP that were low in H2O and at relatively high temperature (liquidus temperature near 1200 ◦ C). Evi- dence from this work shows that the role of H2O-rich evolved boundary layer inter- stitial melts in the formation of An-rich plagioclase was, at best, minor. This is in contrast the interpretations of a significant role for water by Sano and Yamashita [123], which was based upon plagioclase major element observations. Parent liquid compositions derived from OJP xenolith and phenocryst plagioclase crystals con- tain a wider compositional record of magma evolution than that revealed by OJP 215
whole-rock basalt data. Cumulate xenoliths and phenocrysts generally are pieces of disrupted solidification fronts. Solidification fronts would have been ubiquitous throughout the OJP magma chamber system. The OJP magma chamber system was composed of interconnected dikes and sills and consisted of regions dominated by liquid (magma chamber interior) and regions dominated by crystal-liquid mush (along magma chamber floors and walls, within dikes and conduits). Crystals and crystalline debris were extensively recycled throughout the OJP magma chamber system. This recycling accounts for the wide compositional spectrum of magmas parental to plagioclase xenolith crystals and phenocrysts with An content out of equilibrium with their Kwaimbaita host basalts. Ironically, this process also accounts for the dominance of a single magma composition erupted on to the OJP, as it represents the homogenized body of magma in the main part of the up- per chamber. The compositions of melts periodically flushed from the interstices of the crystal-mush network were affected by their residence times in the mush. Depending upon the extent of the crystal-rich portions of the magma chamber system, residence time may have had a large effect on overall basalt chemistry. The shallow crustal OJP magma chamber system corresponded with a zone of neutral buoyancy in the 0-7 km depth range, as in other volcanic systems [122]. As OJP volcanism progressed the plateau gained elevation, and the zone of neutral buoyancy gradually migrated upward leading to slow yet pervasive assimilation of overlying seawater altered basalt. The greatest diversity of OJP basalts is along the margins of the plateau where it is thinner. During formation, magmas ascending into the marginal and thinner magma chamber system around the plateau margins were subject to less density filtration, which allowed ascent and eruption of relatively dense Kroenke 216
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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 177 and 178: 164 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
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 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: Magma chambers tend to form at hori
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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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