Contrasting Garnet Lherzolite Xenolith Suites from the Letšeng Kimberlite Pipes: Inferences for the Northern Lesotho Geotherm

Abstract

The Main and Satellite Pipe kimberlites at Letseng-la-Terae, Lesotho, both contain abundant crustal and mantle xenoliths. In garnet lherzolite xenoliths that are the subject of this paper, textures range from coarse to highly deformed in samples from both pipes, but the most extreme deformation is seen only in Main Pipe samples. Development of reaction coronas around the garnets in all samples indicates retrogression to spinel peridotite facies conditions prior to entrainment in the host kimberlites. Bulk rock compositions of most samples are depleted relative to pristine mantle compositions. Most Main Pipe samples equilibrated at 950–1,000 °C and pressures of 40–45 Kbar, with others extending the temperature range to 820 °C and 1,350 °C. In the temperature range 950–1,000 °C, there is no overlap with Satellite Pipe xenoliths which equilibrated at <800 and >1,305 °C. High-T samples from both pipes lie on the high-T side of the Cretaceous Kalahari geotherm of Rudnick and Nyblade (1999), indicating a perturbed geotherm, similar to that deduced from Thaba Putsoa xenoliths but unlike the non-perturbed geotherm inferred from mantle xenoliths in other Cretaceous Lesotho kimberlites. Hence, xenolith suites from individual kimberlite intrusions each have distinctive thermal characteristics, and there is no consistently perturbed Cretaceous geothermal gradient beneath Lesotho.

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Barry Dawson died suddenly at his home in Edinburgh in late January 2013. He will be especially missed by all of his family, colleagues and diverse friends including those of the kimberlite carbonatite and upper mantle community. Barry was known and loved for his wit, charm and conviviality together with his extensive knowledge of kimberlite geology, poetry, ornithology, and African history. He was a raconteur and a great after-dinner speaker, with a penchant for singing rather rude and amusing songs. Barry was widely regarded as the patriarch of the “kimberlite family”. He was well-respected by university faculty, students and industrial geoscientists. Typically, he never held a grudge against persons who held opposing views. Many of us will really miss staying at the “Braid Farm Road Hotel” enjoying the hospitality and G&Ts so generously provided by our host.

Barry was born in Leeds (UK) and attended Leeds University from which he graduated with a B.Sc. in Geology in 1957. His interest in all things African was developed as graduate student with the Research Institute of African Geology. His work there took him to Basutoland (now Lesotho) to study kimberlite intrusions and culminated in his Ph.D (1960). Parts of his thesis appeared in the seminal paper Basutoland Kimberlites (Geol. Soc. America Bull., 73, 1962). This work was the first modern study of kimberlites and was notable in that Barry introduced the term “fluidization” to kimberlite geology; there are still arguments as to the role of this process in diatreme emplacement! Subsequent, to his graduation Barry, and his late wife Christine, embarked again for Africa to join the Tanganyika Geology Survey. It was here he discovered the amazing sodium carbonate volcanism of Oldoinyo Lengai and earned himself a permanent place in petrological history with the publication of his paper in Nature (1962) on these extraordinary rocks. At the time of his death, some 50 years later, Barry was still studying Oldoinyo Lengai and other Tanzanian volcanoes in the Gregory Rift.

In his academic career, Barry was initially employed at the University of St. Andrews (1964–1978), followed by appointment as Sorby Professor of Geology University of Sheffield, and ultimately at the University of Edinburgh 1988–1997. He retired from active teaching in 1997 and continued an active program of research in his two favourite topics; upper mantle-derived xenoliths, and Recent volcanic rocks of northern Tanzania. In recognition of his research he was awarded the Collins medal of the Mineralogical Society of Great Britain in 2012 and the Clough Medal of the Geological Society of Edinburgh (1999). He was a Fellow of the Royal Society of Edinburgh and the German Academy of Scientists.

Barry’s involvement with kimberlites continued throughout his life. In particular, he was one of the convenors of the 1st International Kimberlite Conference held in Cape Town (1973) with field excursions led by him and Peter Nixon to Lesotho. This conference resulted in world-wide attention being given to kimberlites and upper mantle-derived xenoliths, and to Barry as “the authority” on all things “kimberlitic”. Subsequently, Barry was a convenor of the 2nd Kimberlite Conference held in Santa Fe (1978); an event at which major advances in kimberlite geology were reported; these resulting from studies of the material collected in Lesotho and South Africa. As a consequence of the success of these conferences Barry was instrumental in the formation of the International Kimberlite Conference Advisory Committee and was Chairman of this committee from 1986–1998.

During the last years of his life Barry undertook several expeditions to the Oldoinyo Lengai area, even making the difficult trek to the top of the volcano Kerimasi. As late as December 2012 he was back in Lesotho doing field work. One thing he regretted was that he had not yet climbed all of the Scottish Munros—something he was working on still. His passing is a great loss to us all. To some of us involved in kimberlite and carbonatite research he can never be replaced as he was a mentor and guide throughout our careers. Perhaps Barry was one of the last of the great “gentlemen scientists”?

Roger H. Mitchell

Emeritus Professor of Geology

Lakehead University, Thunder Bay, Ontario

Canada