TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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a limited picture of CSD variation across this thick flow, but CSD fanning of co-magmatic lavas may indicate progressive deeper sampling of the solidification fronts lining the magma chamber [146]. The deeper portions of solidification fronts lining the magma chamber contain larger and older crystals, and as this region is increasingly scoured by passing magma en-route to the surface the resulting lava will have a flatter CSD slope (e.g., [146]). In the case of this single thick flow the steeper CSD from the bottom of the flow is representative of the earliest and lowest crystallinity magma emptied from the magma chamber, and as evacuation of the magma chamber progressed and fed the flow, deeper portions of the solidification fronts lining the chamber were eroded leading to a shallower sloped CSD near the top of the flow (i.e., the terminus of magma chamber evacuation). I conclude that flow sorting of plagioclase was a ubiquitous process during the formation of Detroit Seamount and that measurement of one plagioclase CSD per flow is inadequate for making robust interpretations based solely upon CSD results. Crystal size distribution results do, however, provide insight into when and where crystal accumulation or magma mixing most heavily influenced bulk magma composition. Based upon the fact that everywhere I looked for evidence of crystal sorting I found convincing evidence of this process, I suggest that sorting was not limited to plagioclase. Indeed, the greater density of olivine relative to basaltic melt and plagioclase may have accentuated olivine sorting and sorting due to both flow sorting and gravitational settling. Indeed Tarduno et al. [128] noted olivine cumulate picritic basalts at Site 1203 121
3.6.1.1 Site 1203 Alkalic Basalts and Tholeiitic Sheet Flows Each of the alkalic lavas I examined are plagioclase phyric [128]. Close in- spection of the single alkalic basalt with a curved CSD, Unit 19, reveals that the upward curvature is largely due to a slope change at the smallest L (i.e., smallest crystal sizes), which is consistent with a rapid change in cooling related to eruption [96] (Fig. 3.4h). With the overall linearity of Unit 19 CSD considered, each of the alkalic basalts I examined can be taken as having a linear CSDs. This observation suggests an insignificant role for plagioclase removal or accumulation during the petrogenesis of Site 1203 alkalic basalts. Submarine tholeiitic sheet lavas have the lowest overall modal abundances of plagioclase as a group and all have approximately linear CSDs (Figs. 3.4d,f,j). The Unit 11 and 16 tholeiitic sheet lavas each contain > 30 % modal abundance olivine and linear olivine CSDs. If I assume similar olivine and plagioclase growth rates, then the similarity of olivine and plagioclase CSD slopes indicates that they had similar residence times in magma chambers where crystallization was dominated by olivine (Figs. 3.4d,f). The single tholeiitic sheet flow recovered below the alkalic to tholeiitic transition (Unit 24) is devoid of large phenocrysts and has a steeper CSD slope than the upper sheet flows (Units 11 and 16), which indicates a short magmatic residence time and minimal partial crystallization. 3.6.1.2 Site 1203 Pillow Basalts Each tholeiitic pillow basalt I examined except Site 1203 Unit 1 has a curved or kinked plagioclase CSD (Figs. 3.4, 3.5), which indicates that removal or accumulation of plagioclase was a significant process during the petrogenesis of Detroit Seamount basalts and was most prevalent during the formation of pillow basalts. 122
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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
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 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: 3.6 Discussion 3.6.1 Evidence of Cr
Page 137 and 138: etween the different crystal popula
Page 139 and 140: Unit 14 and 31 magmas accumulated o
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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
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172 TABLE 3.4 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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