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The Geochemistry and Petrology of the Rodrigues Ridge (Western Indian Ocean)

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The Geochemistry and Petrology of the Rodrigues Ridge (Western Indian Ocean) The Geochemistry and Petrology of the Rodrigues Ridge (Western Indian Ocean). By Susan H. Mellor BSc. MSc. F.G.S. A thesis submitted in partial fulfilment of the requirements of the University of Greenwich for a Doctorate in Philosophy December 1998 \ Gravity Anomaly Map of the West Central Indian Ocean This false colour map covers 56° 00' E - 74° 00' E and 6° 00' S - 24° 00' S. False colour scale (red) +90 - -90 mgal (purple), colour bands are 15 mgal wide. Green - yellow boundary = 0 mgal. The map clearly shows the fracture zones of the Central Indian Ridge (CIR), the Mascarene Plateau (MP), Seychelles Bank (SB) and the Rodrigues Ridge(RR). Source: Scharroo R., No Date. Gravity Anomaly Atlas of the Indian Ocean [on line] Netherlands: DEOS. Available from: http://dutlru8.1r.tudelft.nl/img/indian.map?232,205.p.l5. [Accessed 21 March 1997]. The Geochemistry, Petrology and Petrogenesis of the Rodrigues Ridge (Western Indian Ocean). by Susan H. Mellor BSc. MSc. F.G.S. Abstract The Rodrigues Ridge is a linear east-west trending volcanic ridge, located between 18°S and 20°S in the western Indian Ocean. The trend of the Rodrigues Ridge is contrary to the ocean floor fabric of the underlying crust, which formed on the Central Indian Ridge (CIR) between ca. 48 My and 10 My. Dating of dredged basalts from the Rodrigues Ridge showed them to be 8-10 My, with no systematic variations with longitude. All samples recovered from the Rodrigues Ridge were olivine and plagioclase phyric with traces of chrome spinel. Only the most western site contained any phenocryst clinopyroxene. Phenocryst olivine was in the range Fo88.79 and plagioclase was in the range An?,^. Where present the clinopyroxene was titaniferous (2.0-3.7% TiO2) in the range Ca48.52Mg30.19Fe19.33. The Rodrigues Ridge lavas are transitional alkali basalts which display systematic geochemical variations with longitude. Most notably the trace elements Ba, Nb, Rb, Sr, Th, Y, Zr and the LREE increase from east to west, while Sc decreases. These variations may be described as ranging from depleted (in incompatible elements) MORB-like compositions in the east, to enriched OIB-like compositions in the west. This is manifested by at least a three-fold increase in the concentration of the incompatible elements. In accordance with the model presented by Ellam (1992) it is proposed that this behaviour is controlled by the depth to the base of the lithosphere, which acts as an upper limit to melting. In this model the upper limit on the melting is much deeper under the older thicker lithosphere than it is under the young lithosphere. Thus melt composition will increasingly be influenced by the presence of residual garnet under older ocean crust. Even in the absence of residual garnet clinopyroxene in the upper mantle may be capable of retaining the HREE and some trace elements such as Y and Zr. Furthermore shallow mantle is more likely to have been subjected to one or more previous melting episodes beneath spreading ridges, leading to melts depleted in incompatible elements being derived from beneath young ocean crust. Rayleigh-type modelling for most sites along the Rodrigues Ridge produced only a poor correlation with the observed data. At several sites, notably RR3, some incompatible elements (eg.. Zr, Y, REE) show buffered or decreasing trends with decreasing MgO. Although the causes of this behaviour remain ambiguous, it is possible that these melts have not experienced significant high-level fractionation, and this unusual behaviour has its origin in melt-rock reactions within the upper mantle. At these depths the partition coefficients for Zr, Y and the mid-HREE in clinopyroxene have been shown to be greater than unity (Blundy et al., 1998 and Vannucci et al., 1998). Thus the reaction between the melt and coexisting clinopyroxene within the upper mantle, in conjunction with the crystallisation of olivine, may explain how these buffered and declining trends (with declining MgO) have developed. To characterise their isotopic signature a subset of samples were analysed for Sr, Nd and Pb. Like the trace elements, the isotopes display clear linear trends with longitude. On paired isotope plots the Rodrigues Ridge lavas form similar linear trends between samples from the CIR (the Marie Celeste Fracture Zone) and the Reunion hotspot (notably Mauritius), suggesting that they are products of mixing between the mantle sources of the CIR and Reunion. It is proposed that the upper mantle has been passively contaminated by the Reunion plume (with circa 87Sr/*6Sr = 0.7042, 143Nd/144Nd = 0.51284, ^Pb/^Pb = 18.788, ^Ph/^Pb = 15.585 and a"Pb/204Pb =38.849). At shallow levels, immediately below the lithosphere, the upper mantle is made up predominantly of a depleted MORB- like source, with circa 87Sr/"6Sr = 0.7031, 143Nd/144Nd = 0.51305, MePb/204Pb = 18.354, 207pb/204pb = 15.517 j^ pb/204pb =38 214, while at greater depths more Reunion plume material is available. The observed linear array, on paired isotope plots, reflects the differing contributions made by these deep and shallow sources to the Rodrigues Ridge lavas. It is proposed that the Rodrigues Ridge was formed due to a build up of stress, possibly resulting from its proximity to the Ridge-Ridge-Ridge triple junction (see Patriat &S6goufin 1988), causing the rigid African plate to rupture parallel to the principal stress direction. This would have resulted in decompression melting in the upper mantle, so facilitating rapid but short-lived volcanism. Although volcanism has ceased, there is still elevated heat-flow within this area, suggesting that the upper mantle is still anomalously hot (von Herzen &Vacquier 1966). Isostatic readjustment could account for the magmatic reactivation at 1.5 My which formed Rodrigues island. 11 Acknowledgements I would like to express my gratitude towards the following individuals for the help they have provided without which this work could not have been completed; Dr. lan Flatten, Dr. Matthew Thirlwall, Dr. Dodie James, Prof. Brian Upton, Dr. Lindsay Parsons, The Captain and crew of the RRS. Charles Darwin, Mrs. Mildred Hall, Mr. Barry Saunderson, Mr. Pat Brown, Mr. John Maddocks, Mr. & Mrs. Donald Draper. I would particularly like to thank the library staff at the University of Greenwich especially Mr. Nick Dobson and my supervisors Prof. Alistair Baxter and Dr. Andy Bussell. Lastly I would like to thank my husband Mr. Andrew Mellor for his patience while I have been working on this project. ill Contents Abstract P- i Acknowledgements P- "i Disclaimer P- iv Contents P v List of Figures p. ix List of Tables p. xvii List of Plates p. xx List of Appendices p. xx Abbreviations Used in this Work p. xxi Chapter 1 Introduction p. 1 1.1 Introduction p. 1 1.2 The Evolution of the Indian Ocean p. 3 1.3 The Physiography of the Rodrigues Ridge p. 4 1.4 Rodrigues Island p. 8 1.5 The Mascarene Plateau p. 10 1.6 Reunion and Mauritius p. 13 1.7 The Central Indian Ridge p. 23 1.8 Previous Studies of the Rodrigues Ridge p. 32 19 Outline of Present Work p. 35 Chapter 2 Bathymetry and Seismic Sections p. 36 2.1 Introduction p. 36 2.2 Bathymetry p. 36 2.3 Seismic Sections p. 38 Line 1 p. 38 Line 2 p. 41 Line 3 p. 41 Line 4 p. 43 Line 5 p. 43 Line 6 p. 43 Line 7 p. 43 Line 8 p. 46 Line 9 p. 46 Line 10 p. 46 2.4 Location of Dredge Sites P-48 2.5 Discussion p. 49 Chapter 3 Mineralogy P- 51 3.1 Introduction p. 51 3.2 Mineralogy P- 52 3.3 Phenocrysts P- 56 Olivine P- 56 Plagioclase P- 56 Chrome Spinel p. 65 3.4 Chemical Zoning P- 65 3.5 Matrix Mineralogy P- 72 3.6 Vesicles and Alteration p. 73 3.7 Discussion P- 75 Chapter 4 Geochemistry p. 82 4.1 Introduction p. 82 4.2 Geochemistry p. 82 4.3 General Characteristics p. 82 4.4 Alteration p. 102 4.5 Site Characteristics p. 108 RR2 p. 108 RR3 p. 115 RR4 p. 120 RR8 p. 125 RR9c p. 130 RR10 p. 136 RR12d p. 141 4.6 The C-M-A-S Quaternary System p. 146 4.7 Discussion p. 154 Along Ridge Variation p. 154 Chapters Radiogenic Isotopes p. 158 5.1 Introduction p. 158 5.2 Isotope Geochemistry p. 158 5.3 "WSr p. 158 Alteration p. 160 5.4 143Nd/I44Nd p. 160 VI 5.5 Pb Isotope Ratios p. 163 5.6 Radiogenic Isotopes-Discussion p. 165 5.7 Conclusions p. 175 Chapter 6 Petrogenesis p. 176 6.1 Introduction p. 176 6.2 Major Element Modelling p. 177 RR2 p. 177 RR3 p. 180 RR4 p. 181 RR8 p. 182 RR9c p. 183 RR10 p. 184 RR12d p. 184 6.3 Conclusion p. 185 6.4 Trace Element Modelling p. 185 RR2 p. 187 High MgO Group p. 190 Low MgO Group p. 191 RR3 p. 192 RR4 p. 197 RR8 p. 199 RR9c p. 200 RR12d p. 202 6.5 Conclusion p. 202 6.6 Incompatible Element Ratios p. 204 6.7 Isotope-Trace Element Ratios p. 213 6.8 Melting Models p. 220 6.9 Rodrigues p. 228 6.10 Conclusion p. 229 Chapter 7 Conclusion p. 232 7.1 Introduction p. 232 7.2 The Petrology, Geochemistry and Petrogenesis p. 232 7.3 The Relationship between the Lavas Recovered from Different Sites p. 234 7.4 The Temporal and Spatial Relationship Between the Rodrigues Ridge and the Surrounding Area p.
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