Willcox, Alistair and Buisman, Iris and Sparks, Steve and Brown, Richard and Manya, S. and Schumacher, J. C. and Tuffen, Hugh (2015) Petrology, geochemistry and low-temperature alteration of extrusive lavas and pyroclastic rocks of the Igwisi Hills kimberlites, Tanzania. Chemical Geology, 405. pp. 82-101. ISSN 0009-2541Full text not available from this repository.
Geochemical data are presented for the kimberlitic Holocene Igwisi Hills volcanoes (IHV), Tanzania, which preserve extra-crater lavas and pyroclastic rocks. Their young age and exceptional preservation enable investigation of kimberlite magma compositions and alteration pathways of kimberlites. The IHV lavas have a variable matrix assemblage dominated by calcite, olivine and a serpentine-like mineral (termed serpentine-X). Minor primary groundmass phases include apatite, phlogopite, monticellite, perovskite and spinel representing late-stage crystalisation. Secondary phases include hydrogarnet, a mixed-layer chlorite-vermiculite-montmorrilonite, minor brucite and low-temperature oxides and clays such as goethite and jamborite. The matrix of pyroclastic rocks is dominated by calcite with fewer groundmass phases. The parental magmas are inferred to have had ~ 21 wt % SiO2, ~ 22 wt % CaO, ~ 23 wt % MgO and Mg# ~ 70. The IHV are classified as calcite kimberlites. The total volatile concentrations of the primary melt are ~ 14 wt %, which predominantly consists of CO2 although the H2O content is also high. Whole-rock geochemical analyses indicate minor crustal contamination, low-temperature alteration and weathering. Pervasive serpentinisation in both lavas and pyroclastic rocks results from low-temperature alteration induced by the circulation of meteoric waters during cooling. Serpentine-X is potentially a new mineral and is richer in Al2O3 and FeO and poorer in SiO2 than published analyses of serpentine minerals. These compositions are attributed to a 1:2 mixture of serpentine and hydrotalcite. We propose that serpentine-X has replaced a reactive, late stage residual silicate glass, the existence of which helps explain the presence of vesicular scoria (similar to glassy basaltic pyroclasts) and viscous kimberlite lavas.
|Journal or Publication Title:||Chemical Geology|
|Uncontrolled Keywords:||Extrusive kimberlite ; Igwisi Hills ; serpentine ; geochemistry ; petrology|
|Departments:||Faculty of Science and Technology > Lancaster Environment Centre|
|Deposited On:||20 Dec 2011 09:36|
|Last Modified:||09 May 2016 01:22|
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