TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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1.1 Introduction CHAPTER 1 INTRODUCTION Mantle source characterization is vital for understanding the origins of mag- matism on planetary bodies within our solar system and to achieve a more com- plete understanding of Earth’s geologic history. To accurately characterize mantle source regions it is necessary to understand the extent to which magma composition changes between partial melting of the mantle and eruption at the surface. The notion that source characteristics are preserved transparently in primitive magmas from mantle source to eruption can be misleading. Indeed, O’Hara and Herzberg [113] suggested that truly primitive basaltic magmas rarely escape to the Earth’s surface, since the crust acts as a cool density filter to primitive magmas ascending from hotter mantle source regions. Ponding, cooling, and partial crystallization is favored as ascending magmas reach horizons of neutral buoyancy within the shallow crust, which commonly occurs at depths of 4-8 km [97, 122]. Magma differentiation within the shallow crust may involve a combination of partial crystallization (i.e., fractional, equilibrium, or in-situ), assimilation, and/or magma mixing. A wide range of studies (e.g., [16, 38, 113]) have highlighted the significance of shallow magma evolution in basaltic to rhyolitic magmatic systems. Magma evolution in the shallow crust is complex. The details of this evolution are seldom completely born out by whole-rock geochemistry. There has 1
thus been debate within the volcanology and petrology community regarding the utility of whole-rock data alone for deconvolution of complex differentiation processes. Whole-rock compositions are the sum of all of the processes and compositional end-members that affected a given hybrid magma, which is what extrusive igneous rocks commonly represent (e.g., [37, 98]). Textural and microanalytical approaches differ from whole-rock geochemical approaches in essence because they provide a way to dissect crystal populations to reveal their chemical evolution as well as physical details of their nucleation and growth (e.g., [39, 104]). Retrieval of these types of information are vital for understanding crustal magma evolution. During crustal processing of magmas, crystal populations may be recycled between geochemical reservoirs such as end-members involved in magma mixing, assimilated country rock, or from variably evolved zones of a solidifying magma body [23]. Compositional data retrieved from distinct crystal populations and individual zoned crystals thus provide insights into the timing and chemistry of magma evolution, which helps to link changes in magma composition with physical processes. Indeed, the physical manner in which magmas solidify, and hence evolve, remains a fundamental problem in igneous petrology (e.g., [70, 98]). Individual crystals and related crystal populations (e.g., phenocrysts, micro- lites, and/or xenocrysts) act as physical vessels to carry compositional and tempo- ral information about magma evolution (e.g.,[37]). Charlier et al. [23] presented an example that highlights the the wealth of petrogenetic information that can be recovered using a microanalytical approach to understand magmatic processes. In their examination of U-Th and U-Pb systematics of zircon crystals in Quaternary age rhyolites from Taupo volcano, New Zealand, Charlier et al. [23] reported nu- merous zircon crystals with cores inherited from the country rock (a zircon-bearing 2
Page 1 and 2: TEXTURAL AND MICROANALYSIS OF IGNEO
Page 3 and 4: TEXTURAL AND MICROANALYSIS OF IGNEO
Page 5 and 6: DEDICATION This work is dedicated t
Page 7 and 8: CHAPTER 2: MAGMA EVOLUTION REVEALED
Page 9 and 10: 3.6.3.1 Crystal Population Origins
Page 11 and 12: FIGURES 1.1 Magma Chambers . . . .
Page 13: TABLES 2.1 MAJOR AND TRACE ELEMENT
Page 17 and 18: Cambrian schist) including a crysta
Page 19 and 20: explosive eruption vs. mild effusiv
Page 21 and 22: tiation (e.g., [84, 98]; Fig. 1.3).
Page 23 and 24: defined as having crystallinities b
Page 25 and 26: 12 Figure 1.4: Large Igneous Provin
Page 27 and 28: distribution of crystal sizes (e.g.
Page 29 and 30: investigating magmatic evolution us
Page 31 and 32: length dispersive spectroscopy(WDS)
Page 33 and 34: microdrilling, a pre-cleaned square
Page 35 and 36: 1.7.2 Detroit Seamount, Part of the
Page 37 and 38: lations. Reprocessing of intrusive
Page 39 and 40: nucleated homogenously and grew thr
Page 41 and 42: With accurate partition coefficient
Page 43 and 44: uniform composition of large volume
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
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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
Page 109 and 110:
Figure 3.3. A cross polarized light
Page 111 and 112:
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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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
Page 245 and 246:
100 of Geophysical Monograph, pages
Page 247 and 248:
129. J. Tarduno, W. Sliter, L. Kroe
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