Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones

Using a new high-resolution dataset, this study presents evidence for short length scale 18O/16O heterogeneity in the mantle source region of young (age ≲12 ka bp) Icelandic basalts. The dataset comprises secondary ion mass spectrometry determinations of 18O/16O in single compositional zones of plag...

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Published in:Journal of Petrology
Main Authors: Winpenny, B., Maclennan, J.
Format: Article in Journal/Newspaper
Language:English
Published: Oxford Journals, Oxford University Press 2014
Subjects:
Online Access:http://eprints.esc.cam.ac.uk/3248/
http://eprints.esc.cam.ac.uk/3248/1/Winpenny%20and%20Maclennan%20-%202014%20-%20Short%20Length%20Scale%20Oxygen%20Isotope%20Heterogeneity%20in.pdf
http://eprints.esc.cam.ac.uk/3248/4/Electronic_Appendix_3.pdf
http://eprints.esc.cam.ac.uk/3248/3/Electronic_Appendix_5.pdf
http://eprints.esc.cam.ac.uk/3248/2/Supplementary_Figure_S1.pdf
http://eprints.esc.cam.ac.uk/3248/5/Electronic_Appendix_1.xls
http://eprints.esc.cam.ac.uk/3248/6/Electronic_Appendix_2.xls
http://eprints.esc.cam.ac.uk/3248/7/Electronic_Appendix_4.xls
http://petrology.oxfordjournals.org/content/55/12/2537
https://doi.org/10.1093/petrology/egu066
id ftucambridgeesc:oai:eprints.esc.cam.ac.uk:3248
record_format openpolar
institution Open Polar
collection University of Cambridge, Department of Earth Sciences: ESC Publications
op_collection_id ftucambridgeesc
language English
topic 05 - Petrology - Igneous
Metamorphic and Volcanic Studies
spellingShingle 05 - Petrology - Igneous
Metamorphic and Volcanic Studies
Winpenny, B.
Maclennan, J.
Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones
topic_facet 05 - Petrology - Igneous
Metamorphic and Volcanic Studies
description Using a new high-resolution dataset, this study presents evidence for short length scale 18O/16O heterogeneity in the mantle source region of young (age ≲12 ka bp) Icelandic basalts. The dataset comprises secondary ion mass spectrometry determinations of 18O/16O in single compositional zones of plagioclase crystals from the primitive Borgarhraun flow in northern Iceland, along with trace and major element data from the same zones. The presence of mantle under Iceland with δ18O below typical mid-ocean ridge basalt (MORB) values of ∼5·5 ± 0·3‰ (VSMOW) has previously been disputed, because variability in δ18O in many Icelandic basalts is also known to be caused by the interaction of basaltic melts with crustal lithologies that have been altered by low-δ18O meteoric water. Primitive basalt flows, such as Borgarhraun, and their macrocrysts are the most likely candidates to retain a mantle δ18O signature. However, the role of crustal processes in generating the low δ18O in olivine crystals from these flows has not unequivocally been ruled out. By making intra-crystal analyses in Borgarhraun plagioclase it has been possible in this study to obtain a detailed record of the chemical and isotopic compositions of the melts that crystallized the plagioclase zones. The variability observed in trace element compositions of the early crystallized anorthitic plagioclase zones (80·9–89·4 mol % anorthite) is firstly shown to arise from melt compositional variability, and equilibrium melt concentrations of Sr, La and Y are then calculated from the crystal concentrations of these elements using carefully selected partition coefficients. The ranges of incompatible trace element ratios (La/Y, Sr/Y) in these equilibrium melts reflect a range of compositions of fractional mantle melts, a result that is in agreement with previous proposals for the cause of variability in trace element indices of Borgarhraun olivine-hosted melt inclusions and clinopyroxene compositional zones. Correlations observed between La/Y and Sr/Y in the melts in equilibrium with the Borgarhraun plagioclase zones and the δ18O of these zones therefore support the hypothesis that the mantle under Iceland is heterogeneous in 18O/16O. Such correlations have not previously been observed in intra-crystal data from Iceland, and provide strong evidence that mantle material with abnormally low δ18O may exist in the form of readily fusible heterogeneities alongside ambient mantle with MORB-like δ18O (≈+5·5‰) on a length scale of <100 km. The lowest δ18O of plagioclase that is attributed to a mantle origin in this study is 4·5 ± 0·4‰, equating to a melt equivalent value of 4·3 ± 0·5‰ or an olivine equivalent value of 3·8 ± 0·5‰.
format Article in Journal/Newspaper
author Winpenny, B.
Maclennan, J.
author_facet Winpenny, B.
Maclennan, J.
author_sort Winpenny, B.
title Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones
title_short Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones
title_full Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones
title_fullStr Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones
title_full_unstemmed Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones
title_sort short length scale oxygen isotope heterogeneity in the icelandic mantle: evidence from plagioclase compositional zones
publisher Oxford Journals, Oxford University Press
publishDate 2014
url http://eprints.esc.cam.ac.uk/3248/
http://eprints.esc.cam.ac.uk/3248/1/Winpenny%20and%20Maclennan%20-%202014%20-%20Short%20Length%20Scale%20Oxygen%20Isotope%20Heterogeneity%20in.pdf
http://eprints.esc.cam.ac.uk/3248/4/Electronic_Appendix_3.pdf
http://eprints.esc.cam.ac.uk/3248/3/Electronic_Appendix_5.pdf
http://eprints.esc.cam.ac.uk/3248/2/Supplementary_Figure_S1.pdf
http://eprints.esc.cam.ac.uk/3248/5/Electronic_Appendix_1.xls
http://eprints.esc.cam.ac.uk/3248/6/Electronic_Appendix_2.xls
http://eprints.esc.cam.ac.uk/3248/7/Electronic_Appendix_4.xls
http://petrology.oxfordjournals.org/content/55/12/2537
https://doi.org/10.1093/petrology/egu066
long_lat ENVELOPE(-17.022,-17.022,65.810,65.810)
geographic Borgarhraun
geographic_facet Borgarhraun
genre Iceland
genre_facet Iceland
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http://eprints.esc.cam.ac.uk/3248/4/Electronic_Appendix_3.pdf
http://eprints.esc.cam.ac.uk/3248/3/Electronic_Appendix_5.pdf
http://eprints.esc.cam.ac.uk/3248/2/Supplementary_Figure_S1.pdf
http://eprints.esc.cam.ac.uk/3248/5/Electronic_Appendix_1.xls
http://eprints.esc.cam.ac.uk/3248/6/Electronic_Appendix_2.xls
http://eprints.esc.cam.ac.uk/3248/7/Electronic_Appendix_4.xls
Winpenny, B. and Maclennan, J. (2014) Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones. Journal of Petrology, 55 (12). pp. 2537-2566. ISSN Print ISSN 0022-3530 - Online ISSN 1460-2415 DOI https://doi.org/10.1093/petrology/egu066 <https://doi.org/10.1093/petrology/egu066>
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spelling ftucambridgeesc:oai:eprints.esc.cam.ac.uk:3248 2023-05-15T16:47:53+02:00 Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones Winpenny, B. Maclennan, J. 2014-12 text http://eprints.esc.cam.ac.uk/3248/ http://eprints.esc.cam.ac.uk/3248/1/Winpenny%20and%20Maclennan%20-%202014%20-%20Short%20Length%20Scale%20Oxygen%20Isotope%20Heterogeneity%20in.pdf http://eprints.esc.cam.ac.uk/3248/4/Electronic_Appendix_3.pdf http://eprints.esc.cam.ac.uk/3248/3/Electronic_Appendix_5.pdf http://eprints.esc.cam.ac.uk/3248/2/Supplementary_Figure_S1.pdf http://eprints.esc.cam.ac.uk/3248/5/Electronic_Appendix_1.xls http://eprints.esc.cam.ac.uk/3248/6/Electronic_Appendix_2.xls http://eprints.esc.cam.ac.uk/3248/7/Electronic_Appendix_4.xls http://petrology.oxfordjournals.org/content/55/12/2537 https://doi.org/10.1093/petrology/egu066 en eng Oxford Journals, Oxford University Press http://eprints.esc.cam.ac.uk/3248/1/Winpenny%20and%20Maclennan%20-%202014%20-%20Short%20Length%20Scale%20Oxygen%20Isotope%20Heterogeneity%20in.pdf http://eprints.esc.cam.ac.uk/3248/4/Electronic_Appendix_3.pdf http://eprints.esc.cam.ac.uk/3248/3/Electronic_Appendix_5.pdf http://eprints.esc.cam.ac.uk/3248/2/Supplementary_Figure_S1.pdf http://eprints.esc.cam.ac.uk/3248/5/Electronic_Appendix_1.xls http://eprints.esc.cam.ac.uk/3248/6/Electronic_Appendix_2.xls http://eprints.esc.cam.ac.uk/3248/7/Electronic_Appendix_4.xls Winpenny, B. and Maclennan, J. (2014) Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones. Journal of Petrology, 55 (12). pp. 2537-2566. ISSN Print ISSN 0022-3530 - Online ISSN 1460-2415 DOI https://doi.org/10.1093/petrology/egu066 <https://doi.org/10.1093/petrology/egu066> cc_by_nd cc_by CC-BY-ND CC-BY 05 - Petrology - Igneous Metamorphic and Volcanic Studies Article PeerReviewed 2014 ftucambridgeesc https://doi.org/10.1093/petrology/egu066 2020-08-27T18:09:36Z Using a new high-resolution dataset, this study presents evidence for short length scale 18O/16O heterogeneity in the mantle source region of young (age ≲12 ka bp) Icelandic basalts. The dataset comprises secondary ion mass spectrometry determinations of 18O/16O in single compositional zones of plagioclase crystals from the primitive Borgarhraun flow in northern Iceland, along with trace and major element data from the same zones. The presence of mantle under Iceland with δ18O below typical mid-ocean ridge basalt (MORB) values of ∼5·5 ± 0·3‰ (VSMOW) has previously been disputed, because variability in δ18O in many Icelandic basalts is also known to be caused by the interaction of basaltic melts with crustal lithologies that have been altered by low-δ18O meteoric water. Primitive basalt flows, such as Borgarhraun, and their macrocrysts are the most likely candidates to retain a mantle δ18O signature. However, the role of crustal processes in generating the low δ18O in olivine crystals from these flows has not unequivocally been ruled out. By making intra-crystal analyses in Borgarhraun plagioclase it has been possible in this study to obtain a detailed record of the chemical and isotopic compositions of the melts that crystallized the plagioclase zones. The variability observed in trace element compositions of the early crystallized anorthitic plagioclase zones (80·9–89·4 mol % anorthite) is firstly shown to arise from melt compositional variability, and equilibrium melt concentrations of Sr, La and Y are then calculated from the crystal concentrations of these elements using carefully selected partition coefficients. The ranges of incompatible trace element ratios (La/Y, Sr/Y) in these equilibrium melts reflect a range of compositions of fractional mantle melts, a result that is in agreement with previous proposals for the cause of variability in trace element indices of Borgarhraun olivine-hosted melt inclusions and clinopyroxene compositional zones. Correlations observed between La/Y and Sr/Y in the melts in equilibrium with the Borgarhraun plagioclase zones and the δ18O of these zones therefore support the hypothesis that the mantle under Iceland is heterogeneous in 18O/16O. Such correlations have not previously been observed in intra-crystal data from Iceland, and provide strong evidence that mantle material with abnormally low δ18O may exist in the form of readily fusible heterogeneities alongside ambient mantle with MORB-like δ18O (≈+5·5‰) on a length scale of <100 km. The lowest δ18O of plagioclase that is attributed to a mantle origin in this study is 4·5 ± 0·4‰, equating to a melt equivalent value of 4·3 ± 0·5‰ or an olivine equivalent value of 3·8 ± 0·5‰. Article in Journal/Newspaper Iceland University of Cambridge, Department of Earth Sciences: ESC Publications Borgarhraun ENVELOPE(-17.022,-17.022,65.810,65.810) Journal of Petrology 55 12 2537 2566