Petrology, geochemistry and low-temperature alteration of lavas and pyroclastic rocks of the kimberlitic Igwisi Hills volcanoes, Tanzania A. Willcox a, ⁎, I. Buisman a,1 , R.S.J. Sparks a , R.J. Brown b , S. Manya c , J.C. Schumacher a , H. Tuffen d a Department of Earth Sciences, Wills Memorial Building, University of Bristol, Queens Road, Bristol, BS8 1RJ, UK b Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE, UK c Department of Geology, University of Dar es Salaam, P.O. Box 35052, Dar es Salaam, Tanzania d Environmental Science Department, Lancaster University, Lancaster University, LA1 4YQ, UK abstract article info Article history: Received 28 October 2014 Received in revised form 27 March 2015 Accepted 4 April 2015 Available online 28 April 2015 Editor: David R. Hilton Keywords: Extrusive kimberlite Igwisi Hills Serpentine Geochemistry Petrology Geochemical data are presented for the kimberlitic Holocene Igwisi Hills volcanoes (IHV), Tanzania, which pre- serve 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.% SiO 2 , ~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 CO 2 although the H 2 O 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 Al 2 O 3 and FeO and poorer in SiO 2 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. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Kimberlites are volatile-rich ultrabasic magmas, which ascend rap- idly from depths greater than 150 km and bring mantle derived magma and mantle xenoliths to the surface (Dawson, 1980; Mitchell, 1986; Sparks, 2013). Kimberlites attract considerable attention, not only because of their economic importance as hosts to diamonds, but also as deep magmas sampling the subcontinental lithospheric mantle (Gurney et al., 2005; Coe et al., 2008). Kimberlite volcanism is generally confined to cratons older than 2.4 Ga (Dawson, 1980; Mitchell, 1986; Gurney et al., 2005) and has occurred in pulses throughout the Protero- zoic and Phanerozoic, but has been rare since the Middle Eocene (ca.40 Ma, Kjarsgaard, 2007). The old age of most kimberlites means that their surface deposits have generally been eroded and only subsur- face structures are preserved, mostly in downward tapering volcanic conduits (or diatremes), maar craters or dikes (Dawson, 1980; Mitchell, 1986; Field and Scott-Smith, 1999). Extra-crater pyroclastic kimberlites have been observed in drill cores (e.g. Forte a la Corne, Canada, Zonneveld et al., 2004) and in a tuff ring in the Kasami kimber- lite pipe, Mali (Hawthorne, 1975). Until recently the youngest known kimberlites were the 32.3 ± 2.2 Ma Kundelungu plateau pipes, Demo- cratic Republic of Congo (Batumike et al., 2008). However, cosmogenic 3 He dating of olivine crystals in the kimberlitic lavas from the Igwisi Hills volcanoes (IHV), Tanzania, confirms that they are Quaternary (10 ± 3 ka, Brown et al., 2012). Kimberlites commonly have porphyritic textures, consisting of megacrysts, macrocrysts and phenocrysts that are predominantly oliv- ine, dispersed in a fine-grained matrix. Mineral phases in kimberlite comprise complex mixtures of primary magmatic minerals, xenocrysts, and alteration minerals. The megacryst, macrocryst, and phenocrysts assemblage is typically dominated by forsteritic olivine with lesser amounts of phlogopite, ilmenite, and pyrope-rich garnet (Mitchell, 1986). Primary igneous groundmass minerals include phlogopite, spinel, perovskite, monticellite, calcite and apatite, while matrix min- erals of secondary or equivocal origin include serpentine, carbonates, Chemical Geology 405 (2015) 82–101 ⁎ Corresponding author. E-mail address: ally.willcox@gmail.com (A. Willcox). 1 Now at: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK. http://dx.doi.org/10.1016/j.chemgeo.2015.04.012 0009-2541/© 2015 Elsevier B.V. All rights reserved. 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