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Title:Petrogenesis of natrocarbonatite at Ol Doinyo Lengai, East Africa—evidence from Fe and U isotope variations
Author(s):Zhou, Zhenhao
Advisor(s):Lundstrom, Craig Campell
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Ol Doinyo Lengai
Isotope variations
Fluid percolation
Abstract:Ol Doinyo Lengai (ODL), Tanzania, is the only active carbonatite volcano on earth. Cyclical activity that consists of quiescent natrocarbonatite lava flow, explosive silicate eruption and dormancy has been observed throughout the 20th century at ODL. From 2007 to 2008, ODL explosively erupted coexisting natrocarbonatites and nephelinites. Numerous studies have been aimed at understanding how ODL natrocarbonatite forms. Liquid immiscibility is a favored hypothesis although condensate fluid separation is an alternative model. However, the exact mechanism that forms the ODL natrocarbonatite remains unresolved. We carried out Fe and U isotope analyses among a variety of ODL samples. Our sample set includes natrocarbonatite that erupted in 2005, 2 comingled tephras (mixture of natrocarbonatite and nephelinite) and a sequence of 8 nephelinite tephras that erupted in 2007-2008; as well as magnetites separated from 2005 natrocarbontite; Ti-andradites and clinopyroxenes that were separated from one of the nephelinite tephras. Our results show a lighter Fe isotope composition of natrocarbonatite (𝛿56Fe of -0.08‰ relative to IRMM-14) compared to nephelinite tephras (-0.06 to 0.20 ‰ relative to IRMM-14). Magnetites yield heavier Fe isotope composition (0.03‰) than natrocarbonatite; Ti-andradite has the heaviest Fe isotope composition among all analyzed samples due to its enrichment in Fe3+. U isotope results show that natrocarbonatite, has a 𝛿238U of -0.20 ‰ (relative to CRM-112A), being isotopically heavier than 8 nephelinite tephras (range from -0.32 to -0.41 ‰, relative to CRM-112A). Fe isotope results may reflect the interaction between an upward-moving carbonate-rich melt with previous emplaced crystal mush zone, during which Fe2+ preferentially enters the carbonate-rich melt and leaves Fe3+ in the remaining crystal rich mush zone. The carbonate-rich melt degases water as pressure drops causing the observed dry natrocarbonatite lava on the surface. U isotope results may be explained also as the role of melt interaction with the mush zone, with the soluble, 238U rich U(VI) dissolved in the carbonate-rich melt ultimately causing the observed heavier U isotope composition in natrocarbonatite relative to the nephelinite.
Issue Date:2017-04-25
Rights Information:Copyright 2017 Zhenhao Zhou
Date Available in IDEALS:2017-08-10
Date Deposited:2017-05

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