TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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1.6 Analytical Methods 1.6.1 Quantitative Textural Analysis: Crystal Size Distribution Measurement - CSD 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 mm diameter tip (which also corresponds to the smallest measurable crystal size). The tracings were digitized via high resolution scanning with a flatbed scanner. Crystal intersection areas were shaded gray, detected, and measured according to a best fit ellipse routine using the freeware program UTHSCSA Im- ageTool (http://ddsdx.uthscsa.edu/dig/itdesc.html). The best fit ellipse major and minor axis results were input into the spreadsheet program CSDslice written by Morgan and Jerram [106] to estimate the three-dimensional crystal habit. As a thin section through an igneous rock will be a random 2D representation, the plagioclase crystals will be represented as a variety of cross sections. The crystal major axis data, the estimated 3D crystal habit, rock fabric, total area measured, and an estimate of crystal roundness were input into the program CSDcorrections version 1.37 written by Michael Higgins [68] to covert two-dimensional CSD data to true three-dimensional CSDs as outlined by Marsh [96]. 1.6.2 Major Element Microanalysis: Electron Probe Microanalysis - EPMA The Electron Probe Microanalysis (EPMA) and SEM methods outlined in this section are general descriptions of the methods used for quantitative wave- 17
length dispersive spectroscopy(WDS) and collection of compositionally contrasted backscatter electron images throughout all of the work presented in this disser- tation. Deviations from these general methods are noted with accompanying ra- tionale for each change. Backscatter electron images and major element analyses were performed using a JEOL JXA-8600 Superprobe electron microprobe at the University of Notre Dame. Backscatter electron images were collected using a 1 µm beam, an accelerating voltage of 20 kV, and a probe current of 25-50 nA. Microprobe analyses were performed using a 10 µm defocused beam, accelerating voltage of 15 kV, a probe current of 20 nA, 15 second on-peak counting time, and two background measurements per peak. Sodium was measured first to minimize loss via volatilization. Microprobe data were corrected for matrix effects using a ZAF correction routine. Data points near Fe-rich phases such as melt inclu- sions and alteration-filled fractures were discarded. I utilized variety of mineral calibration standards for mineral analyses and glass standards for melt inclusion analyses, and I generally attempted to match mineral standards with the mineral understudy (e.g., orthoclase and sanidine as Al, Na, K, and Si standards for analysis of plagioclase). I routinely measured major elements in mineral standards as unknowns during analytical sessions to monitor calibration and data quality. 1.6.3 Trace Element Microanalysis: Laser Ablation Inductively Coupled Plasma Mass Spectrometry - LA-ICP-MS The LA-ICP-MS method described in this section is a general method descrip- tion, and deviations from this general routine in terms of elements and instrument parameters are noted. Scandium, Ti V, Rb, Sr, Y, Ba, La, Ce, Nd, Sm, Eu, and Pb were measured in plagioclase crystals using a New Wave UP-213 UV laser 18
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 and 14: TABLES 2.1 MAJOR AND TRACE ELEMENT
Page 15 and 16: thus been debate within the volcano
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: investigating magmatic evolution us
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
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
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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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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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