TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

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1.5 Crystal Stratigraphy - Microanalysis of Crystal Populations Advances in microsampling and technology have made it possible to chemically and in some cases isotopically dissect igneous rocks, mineral by mineral (e.g., [22]). Detailed rim-to-rim isotopic, major, and trace element studies (i.e., crystal stratigraphy) have commonly documented phenocryst-host rock disequilibrium (e.g., [39, 40, 134, 135, 143]). Crystal stratigraphy studies have also provided insights into the microenvironments surrounding crystals as they grow or where growth has been disrupted (e.g., [59, 60]). Processes affecting magmatic crystal growth are revealed on a crystal-by-crystal basis using this method, however its power requires a statistically relevant number of crystals be analyzed. Analysis of too few crystals provides only a limited, local-scale history of the rock. As an example of this potential limitation, consider a basaltic magma body that has experienced numerous magma mixing events consisting of liquid ± crystal exchange. Lavas produced from the system will carry crystals from some or all of the mixing events (e.g., [59, 139]). Like whole-rock analysis, documenting the histories of too few crystals from the basalt will not adequately characterize all of the processes (in this case mixing events) responsible for the final texture and composition of the rock. This potential limitation is minimized via integration of CSDs and crystal stratigraphy studies, whereby CSDs are used to identify crystal populations with related nucleation and growth histories that then become targets for microanalysis. 1.5.1 Plagioclase as a Target for Microanalysis of Basaltic Igneous Rocks In this study plagioclase feldspar is a frequent target for major and trace element microanalysis. As noted by Bindeman et al. [8], plagioclase is ideal for 15
investigating magmatic evolution using crystal stratigraphy because 1) it is a common igneous mineral in mafic through felsic systems; 2) it appears early as a liquidus phase in basaltic systems and is stable on the liquidus for a relatively long period of time because of its capability for solid solution; 3) minerals that crystallize before plagioclase (e.g., olivine) do not significantly fractionate trace elements so plagioclase compositions adequately reflect the composition of the parental magma; 4) plagioclase contains measurable quantities of trace elements from different geochemical groups (e.g., large ion lithophile elements - LILE and rare earth elements - REE); 5) plagioclase has a highly polymerized crystal struc- ture relative to olivine or clinopyroxene that leads to very slow diffusivities of major and trace cations so magmatic-induced zonations are commonly preserved (e.g., [13, 24–28, 56–58]). Plagioclase therefore commonly records events in the evolution of a magma body that are evident as optical zonation or resorption features. Such features have been previously studied through experimental work (e.g., [77, 87, 107, 136]) and numerical modeling (e.g., [1, 86, 90]). The simple fact that Sr is compatible in plagioclase makes it an ideal candidate mineral for intra-mineral measurements of 87 Sr/ 86 Sr ratios via microdrilling, micro Sr extraction and analysis by TIMS [38]. Micro Sr isotope studies have proven to be fruitful at elucidating important processes during open system shallow magma evolution, as 87 Sr/ 86 Sr ratios are not heavily influenced by closed system magmatic processes and partial crystallization (e.g., [39]). 16
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: distribution of crystal sizes (e.g.
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
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
Page 83 and 84:
should, however, be noted that in t
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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
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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
Page 103 and 104:
Figure 3.2. Stratigraphy of basalts
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[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
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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
Page 135 and 136:
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 167 and 168:
Page 169 and 170:
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Page 175 and 176:
Page 177 and 178:
Page 179 and 180:
Page 181 and 182:
Page 183 and 184:
Page 185 and 186:
Page 187 and 188:
KP has been done via drilling at 11
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
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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
Page 237 and 238:
22. B. Charlier, J. Davidson, O. Ba
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46. J. Fitton and M. Godard, Origin
Page 241 and 242:
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
Page 247 and 248:
129. J. Tarduno, W. Sliter, L. Kroe
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