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 language | English |
|---|---|
| Journal | Geochimica et Cosmochimica Acta |
| DOIs | |
| Publication status | E-pub ahead of print - 4 Apr 2019 |
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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 journal › Article
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
UR - http://www.scopus.com/inward/record.url?scp=85064264120&partnerID=8YFLogxK
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 -