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Chemical Disequilibria, Lithospheric Thickness, and the Source of Ocean Island Basalts

Grose, Christopher J. and Afonso, Juan C. (2019) Chemical Disequilibria, Lithospheric Thickness, and the Source of Ocean Island Basalts. Journal of Petrology, 60 (4). pp. 755-790. ISSN 0022-3530. https://resolver.caltech.edu/CaltechAUTHORS:20191114-104517673

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Abstract

We examine REE (Rare-Earth Element) and isotopic (Sr–Hf–Nd–Pb) signatures in OIB (Ocean Island Basalts) as a function of lithospheric thickness and show that the data can be divided into thin- (<12 Ma) and thick-plate (>12 Ma) sub-sets. Comparison to geophysically constrained thermal plate models indicates that the demarcation age (∼12 Ma) corresponds to a lithospheric thickness of about 50 km. Thick-plate OIB show incompatible element and isotopic enrichments, whereas thin-plate lavas show MORB-like or slightly enriched values. We argue that enriched signatures in thick-plate OIB originate from low-degree melting at depths below the dry solidus, while depleted signatures in MORB and thin-plate OIB are indicative of higher-degree melting. We tested quantitative explanations of REE systematics using melting models for homogeneous fertile peridotite. Using experimental partition coefficients for major upper mantle minerals, our equilibrium melting models are not able to explain the data. However, using a new grain-scale disequilibrium melting model for the same homogeneous lithology the data can be explained. Disequilibrium models are able to explain the data by reducing the amount of incompatible element partitioning into low degree melts. To explore new levels of detail in disequilibrium phenomena, we employ the Monte-Carlo Potts model to characterize the textural evolution of a microstructure undergoing coarsening and phase transformation processes simultaneous with the diffusive partitioning of trace elements among solid phases and melt in decompressing mantle. We further employ inverse methods to study the thermochemical properties required for models to explain the OIB data. Both data and theory show that OIB erupted on spreading ridges contain signatures close to MORB values, although E-MORB provides the best fit. This indicates that MORB and OIB are produced by compositionally indistinguishable sources, although the isotopic data indicate that the source is heterogeneous. Also, a posteriori distributions are found for the temperature of the thermomechanical lithosphere-asthenosphere boundary (T_(LAB)), the temperature in the source of OIB (T_(p, oib)) and the extent of equilibrium during melting (i.e. grain size). T_(LAB) has been constrained to 1200–1300°C and T_(p, oib) is constrained to be <1400°C. However, we consider the constraints on T_(p, oib) as a description of all OIB to be provisional, because it is a statistical inference from the global dataset. Exceptional islands or island groups may exist, such as the classical ‘hotspots’ (Hawaii, Reunion, etc) and these islands may originate from hot sources. On the other hand, by the same statistical arguments their origins may be anomalously hydrated or enriched instead. Mean grain size in the source of OIB is about 1–5 mm, although this is also provisional due to a strong dependence on knowledge of partition coefficients, ascent rate and the melting function. We also perform an inversion in which partition coefficients were allowed to vary from their experimental values. In these inversions T_(LAB) and T_(p, oib) are unchanged, but realizations close to equilibrium can be found when partition coefficients differ substantially from their experimental values. We also investigated bulk compositions in the source of OIB constrained by our inverse models. Corrections for crystallization effects provided ambiguous confirmations of previously proposed mantle compositions, with depleted mantle providing the poorest fits. We did not include isotopes in our models, but we briefly evaluate the lithospheric thickness effect on isotopes. Although REE data do not require a lithologically heterogeneous source, isotopes indicate that a minor enriched component disproportionately contributes to thick-plate OIB, but is diluted by high-degree melting in the generation of thin-plate OIB and MORB.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1093/petrology/egz012DOIArticle
ORCID:
AuthorORCID
Grose, Christopher J.0000-0002-8318-1649
Afonso, Juan C.0000-0001-9938-6692
Additional Information:© 2019 The Author(s). Published by Oxford University Press. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Received March 5, 2016; Accepted February 18, 2019; Published: 02 March 2019. C. J. Grose thanks Jeffrey G. Ryan for encouragement to investigate links between geophysical observations, thermodynamics and basalt chemistry. Thanks to Yaoling Niu for discussions on the lithospheric thickness effect in the early stages of this research and for providing his database of major, trace and radiogenic isotopes in OIB. Thanks to Paul Asimow and William L. Griffin for providing comments on early drafts. Yaoling Niu, Hugh O’Neill and two anonymous reviewers are thanked for critical reviews of the submitted manuscript. The editorial handling and commentary of Wendy Bohrson is greatly appreciated. This is contribution 1332 from the Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and 1306 in the GEMOC Key Centre (http://www.gemoc.mq.edu.au). C J. Grose acknowledges support from National Science Foundation (grant number 1826310). The work of J. C. Afonso has been supported by an Australian Research Council Discovery Grant (DP160103502). JCA also acknowledges support from the Research Council of Norway through its Centers of Excellence funding scheme, Project Number 223272.
Group:Seismological Laboratory
Funders:
Funding AgencyGrant Number
NSFOCE-1826310
Australian Research CouncilDP160103502
Research Council of Norway223272
Issue or Number:4
Record Number:CaltechAUTHORS:20191114-104517673
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20191114-104517673
Official Citation:Christopher J Grose, Juan C Afonso, Chemical Disequilibria, Lithospheric Thickness, and the Source of Ocean Island Basalts, Journal of Petrology, Volume 60, Issue 4, April 2019, Pages 755–790, https://doi.org/10.1093/petrology/egz012
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99836
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:14 Nov 2019 21:26
Last Modified:14 Nov 2019 21:26

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