The oxygen isotope composition of Karoo and Etendeka picrites: High δ18O mantle or crustal contamination?
- Harris, Chris, Le Roux, Petrus, Cochrane, Ryan, Martin, Laure, Duncan, Andrew R, Marsh, Julian S, Le Roex, A P
- Authors: Harris, Chris , Le Roux, Petrus , Cochrane, Ryan , Martin, Laure , Duncan, Andrew R , Marsh, Julian S , Le Roex, A P
- Date: 2015
- Language: English
- Type: article , text
- Identifier: http://hdl.handle.net/10962/60774 , vital:27829 , https://link.springer.com/article/10.1007/s00410-015-1164-1
- Description: Olivine and orthopyroxene phenocrysts from picrite and picrate basalt lavas and dykes (Mg# 64-80) from the Tuli and Mwanezi (Nuanetsi) regions of the ~180 Ma Karoo Large Igneous province (LIP) have δ18O values that range from 6.0 to 6.7 ‰ (Fig. 1), suggesting that they crystallized from magmas having δ18O values about 1 to 1.5 ‰ higher than expected in an entirely mantle-derived magma. Olivines from picrite and picrite basalt dykes from the 135 Ma Etendeka LIP of Namibia and Karoo-age picrite dykes from Dronning Maud Land, Antarctica do not have such elevated δ18O values. The Etendeka picrites show good correlations between δ18O value and Sr-, Nd- and Pb-isotope ratios that are consistent with previously proposed models of crustal contamination (e.g. Thompson et al., 2007). Explanations for the high δ18O values in Tuli/Mwenezi picrites are limited to (i) alteration, (ii) crustal contamination, and (iii) derivation from mantle with an abnormally high δ18O. The lack of variation in olivine and orthopyroxene δ18O values, together with the lack of correlation between mineral and whole-rock δ18O values are not consistent with alteration being the cause of high δ18O values. The high δ18O values in selected olivine cores have been confirmed by SIMS, and aggressive cleaning of crystals with HF makes no difference to the δ18O value obtained. Average εNd and εSr values of -8 and +16, and high concentrations of incompatible elements such as K are typical of picrites from the Mwanezi (Nuanetsi) region, which have been explained by a variety of models that range from crustal contamination to derivation from the ‘enriched’ mantle lithosphere. The primitive character of the magmas combined with the lack of correlation between δ18O values and radiogenic isotope composition and MgO content or Mg# are inconsistent with crustal contamination, and lend weight to arguments in favour of an 18O-enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition. Although elevated initial Sr isotope ratios, εNd values of -8, and δ18O values about 1 ‰ higher than expected for mantle-derived magma are also a feature of the Bushveld mafic and ultramafic magmas, it is unlikely that a long-lived 18O-enriched mantle source would have survived for nearly 2 Ga. Incorporation of crustal material into the mantle by subduction or delamination of the lower crust are the most likely mechanisms for enriching the mantle in 18O.
- Full Text:
- Date Issued: 2015
- Authors: Harris, Chris , Le Roux, Petrus , Cochrane, Ryan , Martin, Laure , Duncan, Andrew R , Marsh, Julian S , Le Roex, A P
- Date: 2015
- Language: English
- Type: article , text
- Identifier: http://hdl.handle.net/10962/60774 , vital:27829 , https://link.springer.com/article/10.1007/s00410-015-1164-1
- Description: Olivine and orthopyroxene phenocrysts from picrite and picrate basalt lavas and dykes (Mg# 64-80) from the Tuli and Mwanezi (Nuanetsi) regions of the ~180 Ma Karoo Large Igneous province (LIP) have δ18O values that range from 6.0 to 6.7 ‰ (Fig. 1), suggesting that they crystallized from magmas having δ18O values about 1 to 1.5 ‰ higher than expected in an entirely mantle-derived magma. Olivines from picrite and picrite basalt dykes from the 135 Ma Etendeka LIP of Namibia and Karoo-age picrite dykes from Dronning Maud Land, Antarctica do not have such elevated δ18O values. The Etendeka picrites show good correlations between δ18O value and Sr-, Nd- and Pb-isotope ratios that are consistent with previously proposed models of crustal contamination (e.g. Thompson et al., 2007). Explanations for the high δ18O values in Tuli/Mwenezi picrites are limited to (i) alteration, (ii) crustal contamination, and (iii) derivation from mantle with an abnormally high δ18O. The lack of variation in olivine and orthopyroxene δ18O values, together with the lack of correlation between mineral and whole-rock δ18O values are not consistent with alteration being the cause of high δ18O values. The high δ18O values in selected olivine cores have been confirmed by SIMS, and aggressive cleaning of crystals with HF makes no difference to the δ18O value obtained. Average εNd and εSr values of -8 and +16, and high concentrations of incompatible elements such as K are typical of picrites from the Mwanezi (Nuanetsi) region, which have been explained by a variety of models that range from crustal contamination to derivation from the ‘enriched’ mantle lithosphere. The primitive character of the magmas combined with the lack of correlation between δ18O values and radiogenic isotope composition and MgO content or Mg# are inconsistent with crustal contamination, and lend weight to arguments in favour of an 18O-enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition. Although elevated initial Sr isotope ratios, εNd values of -8, and δ18O values about 1 ‰ higher than expected for mantle-derived magma are also a feature of the Bushveld mafic and ultramafic magmas, it is unlikely that a long-lived 18O-enriched mantle source would have survived for nearly 2 Ga. Incorporation of crustal material into the mantle by subduction or delamination of the lower crust are the most likely mechanisms for enriching the mantle in 18O.
- Full Text:
- Date Issued: 2015
Petrology of the alkaline core of the Messum igneous complex, Namibia: evidence for the progressively decreasing effect of crustal contamination
- Harris, Chris, Marsh, Julian S, Milner, Simon C
- Authors: Harris, Chris , Marsh, Julian S , Milner, Simon C
- Date: 1999
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/149746 , vital:38880 , https://doi.org/10.1093/petroj/40.9.1377
- Description: The Messum complex of NW Namibia, a part of the Paraná–Etendeka volcanic province, consists of a dominantly felsic central core, surrounded by older gabbros. The igneous rocks of the core can be divided, in order of decreasing age, into (1) a sub-alkaline suite, (2) an outer quartz syenite suite, and (3) an inner silica-undersaturated suite dominated by nepheline syenite. Compositional differences within the quartz syenite suite can be explained by fractional crystallization, but Sr- and O-isotope data indicate that these rocks contain a significant crustal component.
- Full Text: false
- Date Issued: 1999
- Authors: Harris, Chris , Marsh, Julian S , Milner, Simon C
- Date: 1999
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/149746 , vital:38880 , https://doi.org/10.1093/petroj/40.9.1377
- Description: The Messum complex of NW Namibia, a part of the Paraná–Etendeka volcanic province, consists of a dominantly felsic central core, surrounded by older gabbros. The igneous rocks of the core can be divided, in order of decreasing age, into (1) a sub-alkaline suite, (2) an outer quartz syenite suite, and (3) an inner silica-undersaturated suite dominated by nepheline syenite. Compositional differences within the quartz syenite suite can be explained by fractional crystallization, but Sr- and O-isotope data indicate that these rocks contain a significant crustal component.
- Full Text: false
- Date Issued: 1999
The petrogenesis of the Kirwan Basalts of Dronning Maud Land, Antarctica
- Harris, Chris, Marsh, Julian S, Duncan, Andrew R, Erlank, Anthony J
- Authors: Harris, Chris , Marsh, Julian S , Duncan, Andrew R , Erlank, Anthony J
- Date: 1990
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/145492 , vital:38443 , https://doi.org/10.1093/petrology/31.2.341
- Description: The 420 m thick sequence of Kirwan basalt crops out along the southernmost 50 km of the Kirwanveggen Escarpment (74°S, 6°W). There is little variation in major element chemistry of these basalts (SiO2 49·3–51·6 wt.%; MgO 5·1–6·6 wt.%), but the concentrations of certain incompatible elements (e.g., Zr) vary by factors of approximately two or more.
- Full Text: false
- Date Issued: 1990
- Authors: Harris, Chris , Marsh, Julian S , Duncan, Andrew R , Erlank, Anthony J
- Date: 1990
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/145492 , vital:38443 , https://doi.org/10.1093/petrology/31.2.341
- Description: The 420 m thick sequence of Kirwan basalt crops out along the southernmost 50 km of the Kirwanveggen Escarpment (74°S, 6°W). There is little variation in major element chemistry of these basalts (SiO2 49·3–51·6 wt.%; MgO 5·1–6·6 wt.%), but the concentrations of certain incompatible elements (e.g., Zr) vary by factors of approximately two or more.
- Full Text: false
- Date Issued: 1990
The petrogenesis of the Kirwan Basalts of Dronning Maud Land, Antarctica
- Harris, Chris, Marsh, Julian S, Duncan, Andrew R, Erlank, Anthony J
- Authors: Harris, Chris , Marsh, Julian S , Duncan, Andrew R , Erlank, Anthony J
- Date: 1990
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/145472 , vital:38441 , https://doi.org/10.1093/petrology/31.2.341
- Description: The 420 m thick sequence of Kirwan basalt crops out along the southernmost 50 km of the Kirwanveggen Escarpment (74°S, 6°W). There is little variation in major element chemistry of these basalts (SiO2 49·3–51·6 wt.%; MgO 5·1–6·6 wt.%), but the concentrations of certain incompatible elements (e.g., Zr) vary by factors of approximately two or more.
- Full Text: false
- Date Issued: 1990
- Authors: Harris, Chris , Marsh, Julian S , Duncan, Andrew R , Erlank, Anthony J
- Date: 1990
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/145472 , vital:38441 , https://doi.org/10.1093/petrology/31.2.341
- Description: The 420 m thick sequence of Kirwan basalt crops out along the southernmost 50 km of the Kirwanveggen Escarpment (74°S, 6°W). There is little variation in major element chemistry of these basalts (SiO2 49·3–51·6 wt.%; MgO 5·1–6·6 wt.%), but the concentrations of certain incompatible elements (e.g., Zr) vary by factors of approximately two or more.
- Full Text: false
- Date Issued: 1990
Oxygen isotope geochemistry of the Mesozoic volcanics of the Etendeka Formation, Namibia
- Harris, Chris, Smith, H Stuart, Milner, Simon C, Erlank, Anthony J, Duncan, Andrew R, Marsh, Julian S, Ikin, Nicholas P
- Authors: Harris, Chris , Smith, H Stuart , Milner, Simon C , Erlank, Anthony J , Duncan, Andrew R , Marsh, Julian S , Ikin, Nicholas P
- Date: 1989
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/143272 , vital:38219 , https://doi.org/10.1007/BF00371087
- Description: The Etendeka Formation volcanics consist of a bimodal association of basalts and quartz latites. Forty three new whole rock oxygen isotope analyses are reported for all the major magma types. All the rocks except a minor suite of dolerites have higher δ18O values than normal mantle. The basic rocks (average of 29=8.8‰) have significantly different δ18O to the acid rocks (average of 10=14.4‰). These data are apparently consistent with previously published petrogenetic models, which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived.
- Full Text: false
- Date Issued: 1989
- Authors: Harris, Chris , Smith, H Stuart , Milner, Simon C , Erlank, Anthony J , Duncan, Andrew R , Marsh, Julian S , Ikin, Nicholas P
- Date: 1989
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/143272 , vital:38219 , https://doi.org/10.1007/BF00371087
- Description: The Etendeka Formation volcanics consist of a bimodal association of basalts and quartz latites. Forty three new whole rock oxygen isotope analyses are reported for all the major magma types. All the rocks except a minor suite of dolerites have higher δ18O values than normal mantle. The basic rocks (average of 29=8.8‰) have significantly different δ18O to the acid rocks (average of 10=14.4‰). These data are apparently consistent with previously published petrogenetic models, which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived.
- Full Text: false
- Date Issued: 1989
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