TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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water altered basalt or materials with a seawater brine component as the source of widespread Cl contamination of OJP basaltic glasses. Additionally, Neal and Davidson [108] invoked seawater altered basalt as the contaminant to the parental magma that produced megacrysts entrained in post-OJP alnite pipes on Malaita, Solomon Islands. However, I note that the assimilation of such a seawater-altered component did not radically elevate the water content of the magma within the different chambers. Magma chambers are more sheet-like near the Earths surface [97]. The maximum vertical extent of interconnected sheet-like chambers would have been where the supply of intruding magma was the greatest (i.e., toward the center of the plateau; Fig. 2.11). The total integrated thickness of the dike and sill system was thus less along the margins of the OJP, which allowed magmas distinct from Kwaimbaita-type to ascend through the mush column and erupt (see Fig. 2.11). Indeed, the greatest compositional diversity is observed along the plateau margins e.g., [46, 91, 93, 94]. Along the thinner margins of the plateau the magma chamber system and overlying rock would have acted as less of a density filter than the thicker central portion of the plateau (Fig. 2.11). I cal- culated average OJP magma densities using the KWare Magma program written by Dr. Ken Wohletz using the glass compositions reported by Michael [103] and Roberge et al. [121] at a temperature of 1180 ◦ C and a pressure of 1 kbar (KWare Magma program is available at http://geont1.lanl.gov/Wohletz/Magma.htm). It would have been possible for more (e.g., Kroenke-type: 2869 ± 3 kg/m 3 ) and less dense (Singgalo-type: 2860 kg/m 3 and Kwaimbaita-type: 2861± 3 kg/m 3 ) magmas to ascend the mush pile and reach the surface along the margins of the plateau where there was less overlying volume of intruded and extruded magma. Indeed, Kroenke basalt flows are found to be stratigraphically above Kwaimbaita 83
asalt at Site 1185 of ODP Leg 192 [91]. Although the magma chamber system may have been thinner along the margins of the plateau, variations in magma res- idence time in the system did not preclude formation and eruption of Kwaimbaita magmas along the OJP margins. Differences, however slight, in the densities of Kroenke, Kwaimbaita, and Singgalo magmas would have lead to different rates and patterns of magma ascent, pooling, and crystallization en route to the surface. 2.7 Summary and Conclusions Anorthite-rich (An80−86) portions of plagioclase crystals from cumulate xenoliths 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). Evidence from this study shows that the role of H2O-rich evolved boundary layer interstitial melts in the formation of An-rich plagioclase was, at best, minor. Par- ent liquid compositions derived from OJP xenolith and phenocryst plagioclase crystals contain a wider compositional record of magma evolution than that revealed by OJP whole-rock basalt data. Cumulate xenoliths and phenocrysts gen- erally 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 con- duits). Crystals and crystalline debris were extensively recycled throughout the OJP magma chamber system. This recycling accounts for the wide compositional 84
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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
Page 45 and 46: Figure 2.2. Hand sample and thin se
Page 47 and 48: 2.3.1 A Cogenetic Relationship Betw
Page 49 and 50: Figure 2.4. Stratigraphic section o
Page 51 and 52: Figure 2.5. Backscatter electron im
Page 53 and 54: studies of Kwaimbaita basalt by San
Page 55 and 56: 42 Crystal Zone An (mol%) TABLE 2.1
Page 57 and 58: 44 Crystal Zone An (mol%) 63R2 phen
Page 63 and 64: along the xenolith margins (e.g., F
Page 65 and 66: are more abundant in phenocrysts fr
Page 67 and 68: 54 Crystal Zone 5 ML-X1 xenolith TA
Page 69 and 70: 56 TABLE 2.2 Continued Crystal Zone
Page 75 and 76: Figure 2.7. Measured major and trac
Page 77 and 78: 2.5.3.2 Trace Elements It is diffic
Page 79 and 80: Figure 2.9. Trace element ratio plo
Page 81 and 82: 2.5.4 Resorption Features in Site 1
Page 83 and 84: should, however, be noted that in t
Page 85 and 86: 2.6.1.2 An-rich OJP Plagioclase: In
Page 87 and 88: equivocally the roles of H2O, press
Page 89 and 90: ent magma requires significant clin
Page 91 and 92: tion driven plumes, or stirring by
Page 93 and 94: Yamashita, [123], and thus did not
Page 95: erally extensive crystal-mush netwo
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 and 140: Unit 14 and 31 magmas accumulated o
Page 141 and 142: sition is similar to parent magmas
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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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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KP has been done via drilling at 11
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and over time crustal wall-rocks be
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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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