DOI: 10.1016/j.epsl.2017.11.014
Scopus记录号: 2-s2.0-85035127787
论文题名: Titanates of the lindsleyite–mathiasite (LIMA) group reveal isotope disequilibrium associated with metasomatism in the mantle beneath Kimberley (South Africa)
作者: Giuliani A. ; Woodhead J.D. ; Phillips D. ; Maas R. ; Davies G.R. ; Griffin W.L.
刊名: Earth and Planetary Science Letters
ISSN: 0012821X
出版年: 2018
卷: 482 起始页码: 253
结束页码: 264
语种: 英语
英文关键词: Kimberley
; LIMA
; mantle xenoliths
; metasomatism
; Sr isotope disequilibrium
Scopus关键词: Diffusion in liquids
; Diffusion in solids
; Isotopes
; Mica
; Mineralogy
; Minerals
; Silicates
; Strontium
; Temperature
; Titanium
; Volcanic rocks
; Kimberley
; LIMA
; Mantle xenoliths
; metasomatism
; Sr isotopes
; Transport properties
; crystal chemistry
; crystallization
; disequilibrium
; heterogeneity
; isotopic composition
; metasomatism
; strontium isotope
; titanate
; xenolith
; Kimberley [Northern Cape]
; Northern Cape
; South Africa
英文摘要: Radiogenic isotope variations unrelated to radiogenic ingrowth are common between minerals found in metasomatised mantle xenoliths entrained in kimberlite, basalts and related magmas. As the metasomatic minerals are assumed to have been in isotopic equilibrium originally, such variations are typically attributed to contamination by the magma host and/or interaction with mantle fluids during or before xenolith transport to surface. However, the increasing evidence of metasomatism by multiple, compositionally distinct fluids permeating the lithospheric mantle, coeval with specific magmatic events, suggests that isotopic disequilibrium might be a consequence of discrete, though complex, metasomatic events. Here we provide clear evidence of elemental and Sr isotope heterogeneity between coeval Ti-rich LIMA (lindsleyite–mathiasite) minerals at the time of their formation in the mantle. LIMA minerals occur in close textural association with clinopyroxene and phlogopite in low-temperature (∼800–900 °C), strongly metasomatised mantle xenoliths from the ∼84 Ma Bultfontein kimberlite (South Africa). Previous U/Pb dating of the LIMA phases was used to argue that each xenolith recorded a single event of LIMA crystallisation at ∼180–190 Ma, coeval with the emplacement of Karoo magmas. SEM imaging reveals that up to four types of LIMA phases coexist in each xenolith, and occasionally in a single LIMA grain. Major element and in situ Sr isotope analyses of the different LIMA types show that each phase has a distinct elemental composition and initial 87Sr/86Sr ratio (e.g., 0.7068–0.7086 and 0.7115–0.7129 for two LIMA types in a single xenolith; 0.7053-0.7131 across the entire sample suite). These combined age and isotopic constraints require that multiple fluids metasomatised these rocks at broadly the same time (i.e. within a few thousands to millions of years), and produced similar mineralogical features. Elemental and isotopic variations between different LIMA types could be due to interaction between one (or more) Karoo-related Ti-rich silicate melts and previously metasomatised, phlogopite-rich lithospheric mantle. This study demonstrates that mantle metasomatic assemblages seemingly generated in a single event may instead result from the infiltration of broadly coeval fluids with variable compositions. This in turn implies that the isotopic variations recorded in mantle rocks may be an inherent feature of metasomatism, and that hot fluids infiltrating a rock do not necessarily cause equilibration at the cm scale, as has been assumed previously. Simple modelling of solid-state diffusion in mantle minerals shows that isotopic disequilibrium may be preserved for up to hundreds of Myr at mantle lithosphere temperatures (≤1100–1200 °C), unless subsequently affected by transient heating and/or fluid infiltration events. Radiogenic isotope disequilibrium associated with mantle metasomatism may therefore be a common feature of mantle xenoliths. © 2017 Elsevier B.V. Citation statistics:
资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/110131
Appears in Collections: 影响、适应和脆弱性 气候变化事实与影响
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作者单位: KiDs (Kimberlites and Diamonds), School of Earth Sciences, The University of Melbourne, Parkville, Victoria, 3010, Australia; ARC Centre of Excellence for Core to Crust Fluid Systems, GEMOC, Department of Earth and Planetary Sciences, Macquarie University, North Ryde, NSW 2109, Australia; Department of Earth Sciences, VU Amsterdam, De Boelelaan 1085, Amsterdam, HV 1081, Netherlands; Melbourne Isotope Geochemistry, School of Earth Sciences, The University of Melbourne, Parkville, Victoria, 3010, Australia
Recommended Citation:
Giuliani A.,Woodhead J.D.,Phillips D.,et al. Titanates of the lindsleyite–mathiasite (LIMA) group reveal isotope disequilibrium associated with metasomatism in the mantle beneath Kimberley (South Africa)[J]. Earth and Planetary Science Letters,2018-01-01,482