TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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magmas and slightly less dense Singgalo magmas (see Fig. 2.1 for magma types and distribution across the OJP). Slight variations in paths of magma ascent, depths of magma pooling and crystallization, as well as rates and amounts of assimilation of seawater altered basalt would have affected the isotopic and incompatible trace element characteristics of known OJP basalts. I suggest that these physical factors may account for both the homogeneity of basalts in the central portion of the OJP and the relative heterogeneity of basalts around the plateau margin. 5.1.2 Detroit Seamount, Emperor Seamount Chain A key objective of this study was to elucidate the roles that shallow magmatic processes played during the petrogenesis of depleted hotspot basalts at Detroit Seamount when the Hawaiian hotspot was near a mid-ocean ridge [34]. I employed an integrated approach to understand the physical growth histories and chemical provenance of plagioclase crystals in four pillow basalts from Detroit Seamount. This was done by using plagioclase crystal size distributions to identify unique crystal populations and compositional microanalysis to better under stand their origins. I suggest that the Detroit Seamount magma chamber system was complex and consisted of extensive mush zones and multiple interconnected chambers. There is an undeniable role for plagioclase fractionation and accumulation in the petrogenesis of Detroit Seamount tholeiitic basalts. Evidence of mixing between relatively depleted and enriched end-member magmas recorded in the hybrid Unit 14 basalt do not contradict existing models seeking to explain the origin of the depleted component in Detroit Seamount basalts. Unit 14 magma mixing does suggest, however, that subtle variations in partial melting or source compositions do occur 217
over short time spans but may be easily overlooked in whole-rock compositional studies. I suggest this variation is consistent with subtle variations in partial melting of a heterogeneous mantle source that consisted of relatively depleted and relatively enriched components (c.f., [50, 72, 119]) Subtle variations in partial melting are consistent with enhanced mantle flow and complex mantle dynamics in the near ridge hotspot environment as discussed by Pearce [114]. The nature of the depleted component in Detroit Seamount basalts is prominent in tholeiitic basalts from Site 884, and perhaps most prominent in the cm-size plagioclase phenocrysts. These large crystals have low Sr parent magmas suggesting they were less evolved than their host basalt. A fruitful continuation of this research would be to conduct Sr and Pb isotope studies on these large crystals by in-situ microdrilling (e.g., [39]) or plagioclase separation (e.g., [16]). 5.1.3 Elan Bank, Kerguelen Plateau The main objective of this study was to elucidate the timing and dynamics of crustal contamination of a basaltic magma. This was done via a compositional mi- croanalytical dissection of plagioclase phenocrysts in two Elan Bank basaltic lavas - one contaminated with crust (Unit 10), and the other relatively uncontaminated (Unit 4; as reported by [75]). Plagioclase crystals from Units 4 and 10 exhibit narrow ranges of 87 Sr/ 86 SrI that overlap their respective Site 1137 upper and lower group basalts reported by Ingle et al. [75], which indicates that crustal assimilation was not an ongoing process during plagioclase dominated partial crystallization. Crustal rocks (garnet biotite gneiss clasts) recovered during drilling at Site 1137 are plausible crustal assimilants and have distinctly radiogenic 87 Sr/ 86 SrI (> 0.784) [74]. If these in fact 218
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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
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should, however, be noted that in t
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2.6.1.2 An-rich OJP Plagioclase: In
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equivocally the roles of H2O, press
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ent magma requires significant clin
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tion driven plumes, or stirring by
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Yamashita, [123], and thus did not
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erally extensive crystal-mush netwo
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asalt at Site 1185 of ODP Leg 192 [
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CHAPTER 3 SHALLOW MAGMA EVOLUTION D
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magma compositions [23, 39, 98]. A
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Figure 3.2. Stratigraphy of basalts
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[118]. No volcaniclastic rocks were
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magmatic crystallization. Using the
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Figure 3.3. A cross polarized light
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3.4.5 Major and Trace Element Data
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3.4.6.1 Partition coefficients for
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3.5.1.2 Site 884 CSDs I measured pl
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Figure 3.5. Plagioclase CSDs of ODP
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106 TABLE 3.2 CSD RESULTS Sample OD
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ln (n) [mm -4 ] ln (n) [mm -4 ] 10
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ulation A crystals are unzoned (An6
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Figure 3.8. a) The majority of Site
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3.5.4.3 Site 1203 Unit 31 and Site
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3.5.5 Trace Elements 3.5.5.1 Measur
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Unit 31 > Site 884 Unit 8 (Table 3.
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3.6 Discussion 3.6.1 Evidence of Cr
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3.6.1.1 Site 1203 Alkalic Basalts a
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etween the different crystal popula
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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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Page 179 and 180: 166 TABLE 3.4 Continued Sample Zone
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Page 189 and 190: and over time crustal wall-rocks be
Page 191 and 192: flow [33]. I selected a single samp
Page 193 and 194: 4.3.4 Major and Trace Element Data
Page 195 and 196: B) C) parafilm splash guard A) tape
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Page 199 and 200: 5 mm 5 mm Figure 4.4. Cross polariz
Page 201 and 202: 4.4.2 Electron Probe Microanalysis
Page 203 and 204: An (mol %) An (mol %) An (mol %) An
Page 205 and 206: study exhibit their highest An at t
Page 207 and 208: 4.4.3 LA-ICP-MS - Trace Elements An
Page 209 and 210: 87 86 Sr/ SrI = 0.705722(8) Ba/Sr =
Page 211 and 212: and Rb (Fig. 4.11d,e). 4.4.3.3 Yttr
Page 213 and 214: Sr (ppm) Y (ppm) La (ppm) Ce (ppm)
Page 215 and 216: Unit 4 plagioclase rims plagioclase
Page 217 and 218: 204 TABLE 4.4 UNIT 4 PLAGIOCLASE PA
Page 219 and 220: 4.5 Discussion 4.5.0.1 Origin of Ma
Page 221 and 222: 4.5.0.2 Insights from Trace Element
Page 223 and 224: that other factors influenced the a
Page 225 and 226: C A/B / D A/B C A/B / D A/B Unit 4
Page 227 and 228: Magma chambers tend to form at hori
Page 229: whole-rock basalt data. Cumulate xe
Page 233 and 234: lowing initial Elan Bank magma empl
Page 235 and 236: BIBLIOGRAPHY 1. C. Allegre, A. Prov
Page 237 and 238: 22. B. Charlier, J. Davidson, O. Ba
Page 239 and 240: 46. J. Fitton and M. Godard, Origin
Page 241 and 242: 68. M. Higgins, Measurement of crys
Page 243 and 244: 89. J. Longhi, D. Walker and J. Hay
Page 245 and 246: 100 of Geophysical Monograph, pages
Page 247 and 248: 129. J. Tarduno, W. Sliter, L. Kroe
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