Mantle-like oxygen isotopes in kimberlites determined by in situ SIMS analyses of zoned olivine

Andrea Giuliani, Laure A.J. Martin, Ashton Soltys, William L. Griffin

Research output: Contribution to journalArticle

2 Citations (Scopus)

Abstract

Kimberlites are the deepest melts produced on Earth that are erupted at the surface and can therefore provide unique insights into the composition and evolution of the mantle. Radiogenic isotopes provide ambiguous evidence for the occurrence of recycled crustal material in kimberlite sources. Oxygen isotopes can fractionate significantly only in the shallow crust, and thus represent a powerful tracer of subducted material in the sources of kimberlite. To constrain the oxygen isotope composition of kimberlite melts, we have examined olivine grains in eleven Cretaceous to Eocene archetypal kimberlites from southern Africa, Lac de Gras (Canada) and Alto Paranaiba (Brazil), which exhibit radiogenic isotope evidence for recycled crustal material in their sources including highly radiogenic Pb isotopes and Nd-Hf isotope compositions deviating below the mantle array. Olivine grains are commonly zoned between a mantle-derived xenocrystic core and one or more magmatic overgrowths, i.e. occasional internal zones, ubiquitous rims and rare rinds (moving outward from the core). The oxygen isotope composition of different olivine zones was determined in situ within separated olivine grains by secondary ion mass spectrometry (SIMS) after point selection using back-scattered electron (BSE) images combined with major and minor element analyses. With the exception of a few cores, the δ 18 O values of different olivine zones do not deviate from typical mantle olivine values of 5.18 ± 0.28‰ (Mattey et al., 1994). There are no correlations between oxygen isotopes and major/minor element compositions for internal zones and rims from individual localities or in the entire dataset. This indicates that the oxygen isotope composition of kimberlite melts is not affected by melt differentiation to the point of olivine rim crystallisation. However, olivine rinds from the Koala kimberlite (Canada) display an inverse correlation between δ 18 O and Mn-Ca concentrations, with δ 18 O values extending below the mantle range, which is probably due to carbonate fractionation, CO 2 degassing and/or assimilation of serpentine-rich material after kimberlite emplacement in the upper crust. The mantle-like δ 18 O composition of olivine internal zones and rims suggests that assimilation of mantle material and liberation of a CO 2 -rich phase during ascent in the mantle do not significantly modify the original δ 18 O signature of kimberlite melts. Modelling of oxygen isotope fractionation shows that up to 15 wt% of CO 2 can be lost by kimberlites en route to the upper crust. Our results combined with mass balance calculations indicate that only a limited amount (<5–10 wt%) of recycled crustal material could occur in the source of kimberlites from southern Africa, Lac de Gras and Alto Paranaiba, or that the recycled material had an oxygen isotope composition similar to the mantle.

Original languageEnglish
JournalGeochimica et Cosmochimica Acta
DOIs
Publication statusE-pub ahead of print - 4 Apr 2019

Fingerprint

Oxygen Isotopes
Secondary ion mass spectrometry
oxygen isotope
kimberlite
olivine
mass spectrometry
mantle
ion
Chemical analysis
Isotopes
melt
Carbon Monoxide
isotope
Fractionation
upper crust
fractionation
in situ
Degassing
Carbonates
degassing

Cite this

@article{a2f6b08daa65444294908b0b39e7b0b5,
title = "Mantle-like oxygen isotopes in kimberlites determined by in situ SIMS analyses of zoned olivine",
abstract = "Kimberlites are the deepest melts produced on Earth that are erupted at the surface and can therefore provide unique insights into the composition and evolution of the mantle. Radiogenic isotopes provide ambiguous evidence for the occurrence of recycled crustal material in kimberlite sources. Oxygen isotopes can fractionate significantly only in the shallow crust, and thus represent a powerful tracer of subducted material in the sources of kimberlite. To constrain the oxygen isotope composition of kimberlite melts, we have examined olivine grains in eleven Cretaceous to Eocene archetypal kimberlites from southern Africa, Lac de Gras (Canada) and Alto Paranaiba (Brazil), which exhibit radiogenic isotope evidence for recycled crustal material in their sources including highly radiogenic Pb isotopes and Nd-Hf isotope compositions deviating below the mantle array. Olivine grains are commonly zoned between a mantle-derived xenocrystic core and one or more magmatic overgrowths, i.e. occasional internal zones, ubiquitous rims and rare rinds (moving outward from the core). The oxygen isotope composition of different olivine zones was determined in situ within separated olivine grains by secondary ion mass spectrometry (SIMS) after point selection using back-scattered electron (BSE) images combined with major and minor element analyses. With the exception of a few cores, the δ 18 O values of different olivine zones do not deviate from typical mantle olivine values of 5.18 ± 0.28‰ (Mattey et al., 1994). There are no correlations between oxygen isotopes and major/minor element compositions for internal zones and rims from individual localities or in the entire dataset. This indicates that the oxygen isotope composition of kimberlite melts is not affected by melt differentiation to the point of olivine rim crystallisation. However, olivine rinds from the Koala kimberlite (Canada) display an inverse correlation between δ 18 O and Mn-Ca concentrations, with δ 18 O values extending below the mantle range, which is probably due to carbonate fractionation, CO 2 degassing and/or assimilation of serpentine-rich material after kimberlite emplacement in the upper crust. The mantle-like δ 18 O composition of olivine internal zones and rims suggests that assimilation of mantle material and liberation of a CO 2 -rich phase during ascent in the mantle do not significantly modify the original δ 18 O signature of kimberlite melts. Modelling of oxygen isotope fractionation shows that up to 15 wt{\%} of CO 2 can be lost by kimberlites en route to the upper crust. Our results combined with mass balance calculations indicate that only a limited amount (<5–10 wt{\%}) of recycled crustal material could occur in the source of kimberlites from southern Africa, Lac de Gras and Alto Paranaiba, or that the recycled material had an oxygen isotope composition similar to the mantle.",
keywords = "Chemical geodynamics, CO loss, Kimberlite, Olivine, Oxygen isotopes, SIMS",
author = "Andrea Giuliani and Martin, {Laure A.J.} and Ashton Soltys and Griffin, {William L.}",
year = "2019",
month = "4",
day = "4",
doi = "10.1016/j.gca.2019.03.032",
language = "English",
journal = "Geochimica et Cosmochimica Acta",
issn = "0016-7037",
publisher = "Pergamon",

}

Mantle-like oxygen isotopes in kimberlites determined by in situ SIMS analyses of zoned olivine. / Giuliani, Andrea; Martin, Laure A.J.; Soltys, Ashton; Griffin, William L.

In: Geochimica et Cosmochimica Acta, 04.04.2019.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Mantle-like oxygen isotopes in kimberlites determined by in situ SIMS analyses of zoned olivine

AU - Giuliani, Andrea

AU - Martin, Laure A.J.

AU - Soltys, Ashton

AU - Griffin, William L.

PY - 2019/4/4

Y1 - 2019/4/4

N2 - Kimberlites are the deepest melts produced on Earth that are erupted at the surface and can therefore provide unique insights into the composition and evolution of the mantle. Radiogenic isotopes provide ambiguous evidence for the occurrence of recycled crustal material in kimberlite sources. Oxygen isotopes can fractionate significantly only in the shallow crust, and thus represent a powerful tracer of subducted material in the sources of kimberlite. To constrain the oxygen isotope composition of kimberlite melts, we have examined olivine grains in eleven Cretaceous to Eocene archetypal kimberlites from southern Africa, Lac de Gras (Canada) and Alto Paranaiba (Brazil), which exhibit radiogenic isotope evidence for recycled crustal material in their sources including highly radiogenic Pb isotopes and Nd-Hf isotope compositions deviating below the mantle array. Olivine grains are commonly zoned between a mantle-derived xenocrystic core and one or more magmatic overgrowths, i.e. occasional internal zones, ubiquitous rims and rare rinds (moving outward from the core). The oxygen isotope composition of different olivine zones was determined in situ within separated olivine grains by secondary ion mass spectrometry (SIMS) after point selection using back-scattered electron (BSE) images combined with major and minor element analyses. With the exception of a few cores, the δ 18 O values of different olivine zones do not deviate from typical mantle olivine values of 5.18 ± 0.28‰ (Mattey et al., 1994). There are no correlations between oxygen isotopes and major/minor element compositions for internal zones and rims from individual localities or in the entire dataset. This indicates that the oxygen isotope composition of kimberlite melts is not affected by melt differentiation to the point of olivine rim crystallisation. However, olivine rinds from the Koala kimberlite (Canada) display an inverse correlation between δ 18 O and Mn-Ca concentrations, with δ 18 O values extending below the mantle range, which is probably due to carbonate fractionation, CO 2 degassing and/or assimilation of serpentine-rich material after kimberlite emplacement in the upper crust. The mantle-like δ 18 O composition of olivine internal zones and rims suggests that assimilation of mantle material and liberation of a CO 2 -rich phase during ascent in the mantle do not significantly modify the original δ 18 O signature of kimberlite melts. Modelling of oxygen isotope fractionation shows that up to 15 wt% of CO 2 can be lost by kimberlites en route to the upper crust. Our results combined with mass balance calculations indicate that only a limited amount (<5–10 wt%) of recycled crustal material could occur in the source of kimberlites from southern Africa, Lac de Gras and Alto Paranaiba, or that the recycled material had an oxygen isotope composition similar to the mantle.

AB - Kimberlites are the deepest melts produced on Earth that are erupted at the surface and can therefore provide unique insights into the composition and evolution of the mantle. Radiogenic isotopes provide ambiguous evidence for the occurrence of recycled crustal material in kimberlite sources. Oxygen isotopes can fractionate significantly only in the shallow crust, and thus represent a powerful tracer of subducted material in the sources of kimberlite. To constrain the oxygen isotope composition of kimberlite melts, we have examined olivine grains in eleven Cretaceous to Eocene archetypal kimberlites from southern Africa, Lac de Gras (Canada) and Alto Paranaiba (Brazil), which exhibit radiogenic isotope evidence for recycled crustal material in their sources including highly radiogenic Pb isotopes and Nd-Hf isotope compositions deviating below the mantle array. Olivine grains are commonly zoned between a mantle-derived xenocrystic core and one or more magmatic overgrowths, i.e. occasional internal zones, ubiquitous rims and rare rinds (moving outward from the core). The oxygen isotope composition of different olivine zones was determined in situ within separated olivine grains by secondary ion mass spectrometry (SIMS) after point selection using back-scattered electron (BSE) images combined with major and minor element analyses. With the exception of a few cores, the δ 18 O values of different olivine zones do not deviate from typical mantle olivine values of 5.18 ± 0.28‰ (Mattey et al., 1994). There are no correlations between oxygen isotopes and major/minor element compositions for internal zones and rims from individual localities or in the entire dataset. This indicates that the oxygen isotope composition of kimberlite melts is not affected by melt differentiation to the point of olivine rim crystallisation. However, olivine rinds from the Koala kimberlite (Canada) display an inverse correlation between δ 18 O and Mn-Ca concentrations, with δ 18 O values extending below the mantle range, which is probably due to carbonate fractionation, CO 2 degassing and/or assimilation of serpentine-rich material after kimberlite emplacement in the upper crust. The mantle-like δ 18 O composition of olivine internal zones and rims suggests that assimilation of mantle material and liberation of a CO 2 -rich phase during ascent in the mantle do not significantly modify the original δ 18 O signature of kimberlite melts. Modelling of oxygen isotope fractionation shows that up to 15 wt% of CO 2 can be lost by kimberlites en route to the upper crust. Our results combined with mass balance calculations indicate that only a limited amount (<5–10 wt%) of recycled crustal material could occur in the source of kimberlites from southern Africa, Lac de Gras and Alto Paranaiba, or that the recycled material had an oxygen isotope composition similar to the mantle.

KW - Chemical geodynamics

KW - CO loss

KW - Kimberlite

KW - Olivine

KW - Oxygen isotopes

KW - SIMS

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U2 - 10.1016/j.gca.2019.03.032

DO - 10.1016/j.gca.2019.03.032

M3 - Article

JO - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

SN - 0016-7037

ER -