TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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spot basalts at DSM. Keller et al. [79] suggested that the rising Hawaiian plume entrained MORB-source upper mantle to an extent that partial melting of the MORB source component overwhelmed the hot spot source signature in DSM magmas. Regelous et al. [119] favored an alternative model, where a depleted component inherent to the Hawaiian plume was more apparent in DSM magmas due to greater partial melting under a thinner lid of lithosphere near the spreading center. Huang et al. [72] noted the significance of fractionation and accumulation of olivine and plagioclase, and thus an implied of signifigance low pressure partial crystallization (e.g., [130] during the evolution of DSM magmas in the crust. If one is then to relate DSM basalt chemistry to the nature of a mantle source region or some mantle process, it is vital to understand the extent to which bulk DSM magma compositions were influenced during their transit through the crust. Once the extent of magma evolution in the shallow crust is constrained, understanding the roles of mantle processes becomes less convoluted. 3.1.1 Magma Evolution in the Crustal Magma Chambers The crust acts as a cool density filter to ascending hot mantle derived magmas creating a tendency for rising magmas to stall [113]. Crustal magma chambers are sites of extensive partial crystallization (fractional, equilibrium, or in-situ), assimilation, and/or magma mixing, each of which are fundamental processes of magma evolution. Over the lifetime of magmatism a magma chamber may be filled with a variety of magma types including but not limited to more primitive, more evolved, or end-member magmas involved in mixing. Crystals exchanged during transfer, movement, or filling of magma or crystals grown after these events are physical vessels that carry compositional information about otherwise inaccessible 87
magma compositions [23, 39, 98]. A sensible approach then for constraining the processes of shallow magma evolution is to decipher the textural and compositional record of these processes recorded in individual crystals e.g., [37]. In this work I employ an integrated textural and microanalytical approach to constrain shallow magma evolution during the formation of Detroit Seamount by 1) identification of crystal populations with related physical growth histories and 2) understanding the provenance of these crystals. Textural and microanalytical studies focused on plagioclase have been used to elucidate basaltic to silicic magma evolution processes (e.g., [5, 6, 16, 39, 135]). Plagioclase contains mea- surable quantities of select incompatible trace elements, including the light rare earth elements (LREE). When preceded on the liquidus only by olivine, which does not appreciably fractionate the REE or other incompatible trace elements [5], plagioclase crystals carry valuable compositional clues about primitive magma compositions. In this study I use major, minor, and trace element abundances measured in plagioclase by electron probe microanalysis (EPMA) and laser abla- tion ICP-MS (LA-ICP-MS) to invert chemical compositions of parental (equilibrium) magmas. Accurate partition coefficient data are critical for this inversion. Plagioclase partition coefficients must be applied carefully because anorthite (An = 100*[Ca/(Ca+Na)]) content has a dominant influence on cation substitution [7, 10]. Combined with accurate partition coefficients, inferred parent magma compositions provide insight into magma evolution beyond what is revealed by whole-rock data alone. 88
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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
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 and 96: erally extensive crystal-mush netwo
Page 97 and 98: asalt at Site 1185 of ODP Leg 192 [
Page 99: CHAPTER 3 SHALLOW MAGMA EVOLUTION D
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
Page 147 and 148: was dominated by late stage clinopy
Page 149 and 150: 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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