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Geochemical Characterization of Rocks and Glasses from Dykes in Selected Sites in Kaua’I, Hawai’I: Implications on the Loa and Kea Trend

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Geochemical Characterization of Rocks and Glasses from Dykes in Selected Sites in Kaua’I, Hawai’I: Implications on the Loa and Kea Trend GEOCHEMICAL CHARACTERIZATION OF ROCKS AND GLASSES FROM DYKES IN SELECTED SITES IN KAUA’I, HAWAI’I: IMPLICATIONS ON THE LOA AND KEA TREND by SRI BUDHI UTAMI A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF BACHELOR OF SCIENCE (HONOURS) in THE FACULTY OF SCIENCE (GEOLOGICAL SCIENCES) This thesis conforms to the required standard ……………………………………… Supervisor THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) MARCH 2013 © Sri Budhi Utami, 2013 ABSTRACT Petrological and geochemical characterization (major element, trace element and Pb isotopes) is done on 11 rock and 13 glass samples from dykes in selected sites in the island of Kaua’i (4.0-5.1 Ma). Petrography of hand sample and thin section indicate dykes are composed of tholeiitic basalt and picritic basalt. Major element analyses show all dykes are tholeiitic with consistent enrichment in major elements consistent with the mineralogy, with significant overlap in glass and rock composition in each location. Tholeiitic sample signature suggests dyke magma may be from the shield-building stage of Kaua’i. C1 Chondrite-normalized REE plots indicate dykes are consistent with geochemical signature typical of tholeiitic ocean island basalts. Binary trace element plots represent complex melting processes within the Hawaiian mantle plume resulting from partial melting of a depleted peridotite lithology source. Primitive mantle-normalized extended trace element diagram show typical composition of mantle-plume derived ocean island basalt with primitive mantle source, with enrichment in LIL and HFS elements. Pb isotopes composition show relatively low 206Pb/204Pb for a given 208Pb/204Pb and indicate dyke samples plotted on the Loa trend field with significant overlap with Kea trend lavas, and straddles the Loa-Kea trend boundary line. Comparison with Kaua’i rejuvenated lavas indicate dykes have higher concentrations of radiogenic 206Pb/204Pb and 208Pb/204Pb, and there is significant overlap between dykes and shield stage Kaua’i lavas indicating that overall, Kaua’i shield stage lava sampled transitional to Loa trend magmatism. ii TABLE OF CONTENTS ABSTRACT ............................................................................................................................... i TABLE OF CONTENTS ......................................................................................................... iii TABLE OF FIGURES .............................................................................................................. v LIST OF TABLES ................................................................................................................... vi ACKNOWLEDGEMENTS .................................................................................................... vii 1. INTRODUCTION AND SCOPE OF STUDY ..................................................................... 1 1.1 Mantle Plumes.............................................................................................................................. 1 1.2 Geochemistry and the Hawaiian Islands ...................................................................................... 2 1.3 Scope of Study ............................................................................................................................. 4 2. GEOLOGICAL SETTING ................................................................................................... 5 2.1. Geology of the Hawaiian Islands ................................................................................................ 5 2.2. The Loa and Kea Geochemical trends ........................................................................................ 9 2.3. Geology of Kaua’i ..................................................................................................................... 11 3. METHODOLOGY ............................................................................................................. 13 3.1 Sample Collection ...................................................................................................................... 13 3.2 Whole Rock Petrography ........................................................................................................... 13 3.3 Thin Section Preparation ............................................................................................................ 13 3.4 Sample Preparation .................................................................................................................... 13 3.4.1 Sample Cleaning ................................................................................................................. 14 3.4.2 Sample Cutting ................................................................................................................... 14 3.4.3 Sample Crushing ................................................................................................................. 14 3.4.4 Sample Pulverization .......................................................................................................... 14 3.5 Major Element Analyses ............................................................................................................ 14 3.6 Trace Element Analyses ............................................................................................................. 15 3.6.1 Sample Weighing................................................................................................................ 15 3.6.2 Digestion ............................................................................................................................. 15 3.6.3 Dilution ............................................................................................................................... 16 3.6.4 Trace Element Analyses by High Resolution Inductively Coupled Plasma Mass Spectrometer (HR ICP-MS) ........................................................................................................ 16 3.7 Sr, Nd, Hf and Pb Isotopic Compositions .................................................................................. 17 3.7.1 Sample Selection ................................................................................................................ 17 3.7.2 Sample Weighing and Loading........................................................................................... 17 3.7.3 Leaching ............................................................................................................................. 17 3.7.4 Digestion ............................................................................................................................. 18 3.7.5 Column Chemistry .............................................................................................................. 18 3.7.6 Pb Isotopic Analyses by MC-ICP-MS ................................................................................ 18 iii 4. RESULTS ........................................................................................................................... 32 4.1 Petrography ................................................................................................................................ 32 4.2 Major Element............................................................................................................................ 34 4.2.1 Silica Content Plots............................................................................................................. 34 4.2.2 MgO Content Plots ............................................................................................................. 39 4.3 Trace Elements ........................................................................................................................... 42 4.3.1 Rare Earth Elements (REE) Plot ......................................................................................... 42 4.3.2 Extended Trace Element Diagrams .................................................................................... 44 4.3.3 Nb/Y vs Zr/Y Plot ............................................................................................................... 46 4.3.4 MgO content vs. Trace Element ......................................................................................... 48 4.3.5 Binary Trace Elements ....................................................................................................... 49 4.4 Pb Isotopic Composition ............................................................................................................ 51 5. DISCUSSION ..................................................................................................................... 55 5.1. Spatial Variability in Major and Trace Element Compositions of Dykes ................................. 55 5.2. Trace Element Variations of Dykes: Implications on Mineral Chemistry ................................ 61 5.3. Dyke Pb Isotopic Composition and the Loa Geochemical trend .............................................. 63 6. CONCLUSION ................................................................................................................... 66 7. REFERENCES ................................................................................................................... 68 APPENDIX ............................................................................................................................. 76 1. Rock Samples ..............................................................................................................................
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