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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 (TLAB), the temperature in the source of OIB (Tp, oib) and the extent of equilibrium during melting (i.e. grain size). TLAB has been constrained to 1200–1300°C and Tp, oib is constrained to be <1400°C. However, we consider the constraints on Tp, 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 TLAB and Tp, 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.
Original language | English |
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Pages (from-to) | 755-790 |
Number of pages | 36 |
Journal | Journal of Petrology |
Volume | 60 |
Issue number | 4 |
DOIs | |
Publication status | Published - Apr 2019 |
Keywords
- disequilibrium melting
- mantle plume
- oceanic lithosphere
- lithosphere–asthenosphere boundary
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Dive into the research topics of 'Chemical disequilibria, lithospheric thickness, and the source of ocean island basalts'. Together they form a unique fingerprint.Projects
- 1 Finished
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Developing a geophysically relevant conduction model for the upper mantle
Clark, S., Afonso, J. C., Jones, A. & MQRES, M.
30/06/16 → 31/12/20
Project: Research