TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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primitive magma compositions recorded in OJP plagioclase are not fully met in a homogenous magma body and require more confined environments such as a crystal mush layers e.g., [84, 98, 123]. 2.6.2 OJP Magma Chambers, Mush Layers, and Solidification Fronts Sano and Yamashita [123] suggested zoned plagioclase, clinopyroxene, and olivine crystals in Kwaimbaita basalts formed as they moved between crystal mush boundary layers (solidification fronts) and the main magma body. Bound- ary mush layers are rheological elements of multiply saturated solidification fronts defined as having crystallinities between 25% and 50%-55% [98]. Crystallization within the mush zone produces interstitial melts that are evolved relative to the magma in the chamber interior, and continued crystallization traps these evolved melts within the solidification front [84, 98]. Mineral phases with compositions out of equilibrium with main magma body grow within the mush zone of the solidification front where they are thermally and mechanically insulated from the hotter magma chamber interior [84]. Early formed crystals that potentially con- tain chemical signatures of relatively primitive magmas may also exist in the rigid crust of the solidification front near the solidus. Solidification fronts thicken over time due to crystal nucleation and growth but may also thicken by capturing crystals carried within new batches of magma. Capture of crystals by the advanc- ing solidification front is greatest along the magma chamber floor due to crystal settling, and the thickness of these cumulates in any given magma chamber may grow to be quite large if repeated transit and storage of crystal carrying magmas occurs from other magma chambers [98]. Crystal debris may be mobilized from mushy cumulate layers and solidification fronts by erosion, density and convec- 77
tion driven plumes, or stirring by new magma input [83, 98]. In the case of the OJP magma chamber system, magma in the chamber interior(s) was the most eruptible, had Kwaimbaita or Kroenke basalt-like composition(s), and made up the most significant compositional and textural fractions of basalts erupted to the surface. Disruption of solidification fronts released evolved material (liquid + crystals) to mix with the main body of magma e.g., [98]. Mixing small fractions of evolved material from disrupted solidification fronts into the main magma body can lead to changes in basalt chemistry (Marsh, 1996). However, if the geometry of the OJP magma chamber system was such that inputs from disrupted solidification fronts were small relative to the size of the magma body, the input of more evolved material would have had little affect on the bulk magma chemistry. Evidence for this process is thus best preserved by allochthonous crystals e.g., [37, 98], much like the An65−73 phenocrysts with evolved parent magma compositions and the An80−86 xenolith crystals with primitive magma compositions, which are all out of equilibrium with the host Kwaimbaita basalt. Parent magma trace element compositions of An65−73 phenocryst zones, which trend to much more evolved compositions than whole-rock data, suggest some of these crystals grew deep within solidification fronts that were eroded and mixed into the main magma body. Crystals with similarly evolved compositions may also form in dikes and conduits within the magma chamber system that become readily congested. In these regions geometry and magma supply rate allow solidification fronts to prop- agate inward and meet filling the space with a crystal mush that has a network of interstitial spaces filled with variably evolved melts [98]. Within the interstices of the crystal mush significant melt differentiation can occur where the mush is infrequently flushed with new (primitive) melt. I suggest the xenolith crystals and 78
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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
Page 39 and 40: nucleated homogenously and grew thr
Page 41 and 42: With accurate partition coefficient
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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: ent magma requires significant clin
Page 93 and 94: Yamashita, [123], and thus did not
Page 95 and 96: erally extensive crystal-mush netwo
Page 97 and 98: asalt at Site 1185 of ODP Leg 192 [
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
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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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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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