TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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are thus inconsistent with a typical Hawaii-like emplacement. Huang et al. [72] suggested Site 884 lavas were erupted closest to the spreading ridge axis and that the proximity of the Hawaiian hotspot to the ridge at ∼ 81 Ma brought about greater flow of plume mantle toward the ridge axis prior to mantle partial melting [72, 119]. They suggest Site 1203 lavas were erupted away from the ridge axis through hotspot-related lithosphere [72]. It cannot be ruled out that Sites 1203 and 884 sampled separate shield volcanoes (see discussion in [72]). Huang et al. [72] suggested that all of the Site 1203 lavas came from single volcano, where melts were segregated at lower mean pressure than Hawaiian volcanoes but higher mean pressure than Site 884 melts. The sequence of rocks recovered from Site 1203 represent a more diverse record of eruptive styles relative to cored section from Site 884, as well as several distinct periods of non-deposition or erosion (between Units 5 and 6, Units 23 and 24, and Units 27 and 28) [128] (Fig. 3.2). 3.3.3 Sampling Strategy A large proportion of the basalts from Site 1203 and Site 884 contain high volumes of plagioclase phenocrysts [118, 128]. Regelous et al. [119] noted that this has a diluting effect on whole-rock compositions. Given the size and volume of the phenocrysts in the numerous plagioclase phyric basalts from Sites 1203 and 884, it is apparent that whole-rock compositions of these lavas may be best characterized as hybrids rather than true liquid compositions. Perhaps of greater significance is that the plagioclase crystals, which have clearly influenced bulk-rock compositions ([119]), may hold clues to otherwise inaccesible facets of magma evolution. I measured a total of 14 plagioclase CSDs and two olivine CSDs on 12 separate lava flows from Site 1203 to examine temporal changes in the physical history of 93
magmatic crystallization. Using the CSD results I selected three Site 1203 pillow lavas for microanalytical studies to examine crystal provenance. I measured six plagioclase CSDs on two separate plagioclase megaphyric pillow basalts from Site 884 to examine the origins and histories of the cm size plagioclase phenocrysts in these flows and to examine CSD variations within single flows. 3.4 Analytical Methods 3.4.1 Sample Preparation Basalt samples were prepared as polished thin sections using a microdiamond slurry and final grit size of 0.5 µm. Prior to analytical work the slides were rinsed in ethanol in an ultrasonic bath to remove remnants of the polishing compounds followed by three 30 minute rinses in ultrapure water in an ultrasonic bath. Sam- ples were carbon coated for EPMA work. After completion of EPMA work the carbon coats were removed with 3 µm and 1 µm diamond polishing clothes under ethanol followed by three 30 minutes rinses in ultrapure water in an ultrasonic bath. The surface of each slide was inspected under a reflected light optical microscope to confirm that all vestiges of the carbon coat had been removed prior to LA-ICP-MS work. 3.4.2 Crystal Size Distribution (CSD) Measurement Digital image mosaics of entire petrographic thin sections were captured in cross polarized light using an automated microscope stage system manufactured by Prior Scientific Instruments, Cambridge, UK. Photographs of each scanned area were printed on photograph paper and overlain with tracing paper. Minerals of interest (plagioclase and/or olivine) were outlined by hand using a pen with a 0.1 94
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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
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 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: [118]. No volcaniclastic rocks were
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
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 155 and 156: 142 TABLE 3.3 Continued Sample Zone
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144 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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