In situ trace element and Sm-Nd isotope analysis of accessory minerals in an Eoarchean tonalitic gneiss from Greenland: Implications for Hf and Nd isotope decoupling in Earth's ancient rocks

Johannes Hammerli, Anthony I.S. Kemp, Martin J. Whitehouse

Research output: Contribution to journalArticle

Abstract

Some of the Earth's oldest preserved continental crustal rocks, from southern West Greenland, contain contradictory radiogenic isotope signatures, whereby Hf isotope ratios are chondritic but Nd isotope ratios are distinctly super-chondritic. Models to explain this discrepancy are speculative and variously invoke deep magma ocean crystallisation, Hf[sbnd]Nd decoupling in subduction zones, or metamorphic disturbance during younger thermal events. Determining the cause of this discrepancy is essential for understanding Eoarchean crust-mantle differentiation. We employ, for the first time, micro-analysis of REE-rich accessory minerals to shed light on the Nd isotope evolution of a key tonalitic gneiss sample from southern West Greenland that displays the apparent Hf[sbnd]Nd isotope decoupling. The results show that the Sm-Nd isotope system was homogenized during a metamorphic event at ca. 2690 Ma. We suggest that metamorphic reactions involving consumption and re-crystallisation of REE-bearing phases were accompanied by LREE element mobility and the loss of unradiogenic Nd, shifting the bulk rock composition to a more radiogenic Nd isotope value. Our study provides the first direct evidence that the anomalous Nd isotope signatures in some Eoarchean gneisses are artefacts of the disturbance of the Sm-Nd isotope system, and not due to extensive differentiation of the bulk silicate Earth by magma ocean crystallisation or continental crust formation.

Original languageEnglish
Pages (from-to)394-405
Number of pages12
JournalChemical Geology
Volume524
DOIs
Publication statusPublished - 5 Oct 2019

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accessory mineral
Accessories
Trace Elements
Isotopes
gneiss
Minerals
Earth (planet)
Rocks
trace element
isotope
rock
Crystallization
rare earth element
crystallization
Bearings (structural)
magma
element mobility
in situ
analysis
disturbance

Cite this

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title = "In situ trace element and Sm-Nd isotope analysis of accessory minerals in an Eoarchean tonalitic gneiss from Greenland: Implications for Hf and Nd isotope decoupling in Earth's ancient rocks",
abstract = "Some of the Earth's oldest preserved continental crustal rocks, from southern West Greenland, contain contradictory radiogenic isotope signatures, whereby Hf isotope ratios are chondritic but Nd isotope ratios are distinctly super-chondritic. Models to explain this discrepancy are speculative and variously invoke deep magma ocean crystallisation, Hf[sbnd]Nd decoupling in subduction zones, or metamorphic disturbance during younger thermal events. Determining the cause of this discrepancy is essential for understanding Eoarchean crust-mantle differentiation. We employ, for the first time, micro-analysis of REE-rich accessory minerals to shed light on the Nd isotope evolution of a key tonalitic gneiss sample from southern West Greenland that displays the apparent Hf[sbnd]Nd isotope decoupling. The results show that the Sm-Nd isotope system was homogenized during a metamorphic event at ca. 2690 Ma. We suggest that metamorphic reactions involving consumption and re-crystallisation of REE-bearing phases were accompanied by LREE element mobility and the loss of unradiogenic Nd, shifting the bulk rock composition to a more radiogenic Nd isotope value. Our study provides the first direct evidence that the anomalous Nd isotope signatures in some Eoarchean gneisses are artefacts of the disturbance of the Sm-Nd isotope system, and not due to extensive differentiation of the bulk silicate Earth by magma ocean crystallisation or continental crust formation.",
keywords = "Differentiation of the silicate Earth, In situ Nd isotope analysis, Mechanism of early continental crustal growth, Metamorphosed Eoarchean igneous rocks of southern West Greenland, Nd and Hf isotope decoupling, Nd Hf isotope paradox, Nd isotope resetting of Eoarchean rocks, Nd isotope signatures of Eoarchean gneisses from southern West Greenland, Open system metamorphism of Archean gneisses",
author = "Johannes Hammerli and Kemp, {Anthony I.S.} and Whitehouse, {Martin J.}",
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doi = "10.1016/j.chemgeo.2019.06.025",
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journal = "Chemical Geology",
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TY - JOUR

T1 - In situ trace element and Sm-Nd isotope analysis of accessory minerals in an Eoarchean tonalitic gneiss from Greenland

T2 - Implications for Hf and Nd isotope decoupling in Earth's ancient rocks

AU - Hammerli, Johannes

AU - Kemp, Anthony I.S.

AU - Whitehouse, Martin J.

PY - 2019/10/5

Y1 - 2019/10/5

N2 - Some of the Earth's oldest preserved continental crustal rocks, from southern West Greenland, contain contradictory radiogenic isotope signatures, whereby Hf isotope ratios are chondritic but Nd isotope ratios are distinctly super-chondritic. Models to explain this discrepancy are speculative and variously invoke deep magma ocean crystallisation, Hf[sbnd]Nd decoupling in subduction zones, or metamorphic disturbance during younger thermal events. Determining the cause of this discrepancy is essential for understanding Eoarchean crust-mantle differentiation. We employ, for the first time, micro-analysis of REE-rich accessory minerals to shed light on the Nd isotope evolution of a key tonalitic gneiss sample from southern West Greenland that displays the apparent Hf[sbnd]Nd isotope decoupling. The results show that the Sm-Nd isotope system was homogenized during a metamorphic event at ca. 2690 Ma. We suggest that metamorphic reactions involving consumption and re-crystallisation of REE-bearing phases were accompanied by LREE element mobility and the loss of unradiogenic Nd, shifting the bulk rock composition to a more radiogenic Nd isotope value. Our study provides the first direct evidence that the anomalous Nd isotope signatures in some Eoarchean gneisses are artefacts of the disturbance of the Sm-Nd isotope system, and not due to extensive differentiation of the bulk silicate Earth by magma ocean crystallisation or continental crust formation.

AB - Some of the Earth's oldest preserved continental crustal rocks, from southern West Greenland, contain contradictory radiogenic isotope signatures, whereby Hf isotope ratios are chondritic but Nd isotope ratios are distinctly super-chondritic. Models to explain this discrepancy are speculative and variously invoke deep magma ocean crystallisation, Hf[sbnd]Nd decoupling in subduction zones, or metamorphic disturbance during younger thermal events. Determining the cause of this discrepancy is essential for understanding Eoarchean crust-mantle differentiation. We employ, for the first time, micro-analysis of REE-rich accessory minerals to shed light on the Nd isotope evolution of a key tonalitic gneiss sample from southern West Greenland that displays the apparent Hf[sbnd]Nd isotope decoupling. The results show that the Sm-Nd isotope system was homogenized during a metamorphic event at ca. 2690 Ma. We suggest that metamorphic reactions involving consumption and re-crystallisation of REE-bearing phases were accompanied by LREE element mobility and the loss of unradiogenic Nd, shifting the bulk rock composition to a more radiogenic Nd isotope value. Our study provides the first direct evidence that the anomalous Nd isotope signatures in some Eoarchean gneisses are artefacts of the disturbance of the Sm-Nd isotope system, and not due to extensive differentiation of the bulk silicate Earth by magma ocean crystallisation or continental crust formation.

KW - Differentiation of the silicate Earth

KW - In situ Nd isotope analysis

KW - Mechanism of early continental crustal growth

KW - Metamorphosed Eoarchean igneous rocks of southern West Greenland

KW - Nd and Hf isotope decoupling

KW - Nd Hf isotope paradox

KW - Nd isotope resetting of Eoarchean rocks

KW - Nd isotope signatures of Eoarchean gneisses from southern West Greenland

KW - Open system metamorphism of Archean gneisses

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U2 - 10.1016/j.chemgeo.2019.06.025

DO - 10.1016/j.chemgeo.2019.06.025

M3 - Article

VL - 524

SP - 394

EP - 405

JO - Chemical Geology

JF - Chemical Geology

SN - 0009-2541

ER -