Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific)
Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Australia-Pacific plate boundary south of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transfo...
Published in: | Journal of Petrology |
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Main Authors: | , , , , , |
Format: | Article in Journal/Newspaper |
Language: | English |
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OXFORD UNIV PRESS
2000
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Online Access: | https://doi.org/10.1093/petrology/41.3.411 http://ecite.utas.edu.au/20562 |
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institution |
Open Polar |
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eCite UTAS (University of Tasmania) |
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ftunivtasecite |
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English |
topic |
Earth Sciences Geology Igneous and Metamorphic Petrology |
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Earth Sciences Geology Igneous and Metamorphic Petrology Kamenetsky, VS Everard, JL Crawford, AJ Varne, R Eggins, SM Lanyon, R Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific) |
topic_facet |
Earth Sciences Geology Igneous and Metamorphic Petrology |
description |
Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Australia-Pacific plate boundary south of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transform faults linked by short spreading segments, and currently into a right-lateral strike-slip plate boundary. The rocks of Macquarie Island were formed during spreading at this plate boundary in Miocene times, and include intrusive rocks (mantle and cumulate peridotites, gabbros, sheeted dolerite dyke complexes), volcanic rocks (N- to E-MORB pillow lavas, picrites, breccias, hyaloclastites), and associated sediments. A set of Macquarie Island basaltic glasses has been analysed by electron microprobe for major elements, S, Cl and F; by Fourier transform infrared spectroscopy for H2O; by laser ablation-inductively coupled plasma mass spectrometry for trace elements; and by secondary ion mass spectrometry for Sr, Nd and Pb isotopes. An outstanding compositional feature of the data set (474-511 wt % SiO2, 565-875 wt % MgO) is the broad range of K2O (01-18 wt %) and the strong positive covariation of K2O with other incompatible minor and trace elements (e.g. TiO2 097-21%; Na2O 24-43%; P2O5 008-07%; H2O 025-15%; La 43-466 ppm). The extent of enrichment in incompatible elements in glasses correlates positively with isotopic ratios of Sr (87Sr/86Sr = 070255-070275) and Pb (206Pb/204Pb = 18951-19493; 207Pb/204Pb = 15528-15589; 208Pb/204Pb = 38523-38979), and negatively with Nd (143Nd/144Nd = 051310-051304). Macquarie Island basaltic glasses are divided into two compositional groups according to their mg-number-K2O relationships. Near-primitive basaltic glasses (Group I) have the highest mg-number (63-69), and high Al2O3 and CaO contents at a given K2O content, and carry microphenocrysts of primitive olivine (Fo(86-895)). Their bulk compositions are used to calculate primary melt compositions in equilibrium with the most magnesian Macquarie Island olivines (Fo(905)). Fractionated, Group II, basaltic glasses are saturated with olivine + plagioclase clinopyroxene, and have lower mg-number (57-67), and relatively low Al2O3 and CaO contents. Group I glasses define a seriate variation within the compositional spectrum of MORB, and extend the compositional range from N-MORB compositions to enriched compositions that represent a new primitive enriched MORB end-member. Compared with N-MORB, this new end-member is characterized by relatively low contents of MgO, FeO, SiO2 and CaO, coupled with high contents of Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements, and has the most radiogenic Sr and Pb regional isotope composition. These unusual melt compositions could have been generated by low-degree partial melting of an enriched mantle peridotite source, and were erupted without significant mixing with common N-MORB magmas. The mantle in the Macquarie Island region must have been enriched and heterogeneous on a very fine scale. We suggest that the mantle enrichment implicated in this study is more likely to be a regional signature that is shared by the Balleny Islands magmatism than directly related to the hypothetical Balleny plume itself. |
format |
Article in Journal/Newspaper |
author |
Kamenetsky, VS Everard, JL Crawford, AJ Varne, R Eggins, SM Lanyon, R |
author_facet |
Kamenetsky, VS Everard, JL Crawford, AJ Varne, R Eggins, SM Lanyon, R |
author_sort |
Kamenetsky, VS |
title |
Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific) |
title_short |
Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific) |
title_full |
Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific) |
title_fullStr |
Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific) |
title_full_unstemmed |
Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific) |
title_sort |
enriched end-member of primitive morb melts: petrology and geochemistry of glasses from macquarie island (sw pacific) |
publisher |
OXFORD UNIV PRESS |
publishDate |
2000 |
url |
https://doi.org/10.1093/petrology/41.3.411 http://ecite.utas.edu.au/20562 |
geographic |
Balleny Islands New Zealand Pacific |
geographic_facet |
Balleny Islands New Zealand Pacific |
genre |
Balleny Islands Macquarie Island |
genre_facet |
Balleny Islands Macquarie Island |
op_relation |
http://dx.doi.org/10.1093/petrology/41.3.411 Kamenetsky, VS and Everard, JL and Crawford, AJ and Varne, R and Eggins, SM and Lanyon, R, Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific), Journal of Petrology, 41, (3) pp. 411-430. ISSN 0022-3530 (2000) [Refereed Article] http://ecite.utas.edu.au/20562 |
op_doi |
https://doi.org/10.1093/petrology/41.3.411 |
container_title |
Journal of Petrology |
container_volume |
41 |
container_issue |
3 |
container_start_page |
411 |
op_container_end_page |
430 |
_version_ |
1766367962775683072 |
spelling |
ftunivtasecite:oai:ecite.utas.edu.au:20562 2023-05-15T15:37:24+02:00 Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific) Kamenetsky, VS Everard, JL Crawford, AJ Varne, R Eggins, SM Lanyon, R 2000 https://doi.org/10.1093/petrology/41.3.411 http://ecite.utas.edu.au/20562 en eng OXFORD UNIV PRESS http://dx.doi.org/10.1093/petrology/41.3.411 Kamenetsky, VS and Everard, JL and Crawford, AJ and Varne, R and Eggins, SM and Lanyon, R, Enriched end-member of primitive MORB melts: petrology and geochemistry of glasses from Macquarie Island (SW Pacific), Journal of Petrology, 41, (3) pp. 411-430. ISSN 0022-3530 (2000) [Refereed Article] http://ecite.utas.edu.au/20562 Earth Sciences Geology Igneous and Metamorphic Petrology Refereed Article PeerReviewed 2000 ftunivtasecite https://doi.org/10.1093/petrology/41.3.411 2019-12-13T21:02:55Z Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Australia-Pacific plate boundary south of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transform faults linked by short spreading segments, and currently into a right-lateral strike-slip plate boundary. The rocks of Macquarie Island were formed during spreading at this plate boundary in Miocene times, and include intrusive rocks (mantle and cumulate peridotites, gabbros, sheeted dolerite dyke complexes), volcanic rocks (N- to E-MORB pillow lavas, picrites, breccias, hyaloclastites), and associated sediments. A set of Macquarie Island basaltic glasses has been analysed by electron microprobe for major elements, S, Cl and F; by Fourier transform infrared spectroscopy for H2O; by laser ablation-inductively coupled plasma mass spectrometry for trace elements; and by secondary ion mass spectrometry for Sr, Nd and Pb isotopes. An outstanding compositional feature of the data set (474-511 wt % SiO2, 565-875 wt % MgO) is the broad range of K2O (01-18 wt %) and the strong positive covariation of K2O with other incompatible minor and trace elements (e.g. TiO2 097-21%; Na2O 24-43%; P2O5 008-07%; H2O 025-15%; La 43-466 ppm). The extent of enrichment in incompatible elements in glasses correlates positively with isotopic ratios of Sr (87Sr/86Sr = 070255-070275) and Pb (206Pb/204Pb = 18951-19493; 207Pb/204Pb = 15528-15589; 208Pb/204Pb = 38523-38979), and negatively with Nd (143Nd/144Nd = 051310-051304). Macquarie Island basaltic glasses are divided into two compositional groups according to their mg-number-K2O relationships. Near-primitive basaltic glasses (Group I) have the highest mg-number (63-69), and high Al2O3 and CaO contents at a given K2O content, and carry microphenocrysts of primitive olivine (Fo(86-895)). Their bulk compositions are used to calculate primary melt compositions in equilibrium with the most magnesian Macquarie Island olivines (Fo(905)). Fractionated, Group II, basaltic glasses are saturated with olivine + plagioclase clinopyroxene, and have lower mg-number (57-67), and relatively low Al2O3 and CaO contents. Group I glasses define a seriate variation within the compositional spectrum of MORB, and extend the compositional range from N-MORB compositions to enriched compositions that represent a new primitive enriched MORB end-member. Compared with N-MORB, this new end-member is characterized by relatively low contents of MgO, FeO, SiO2 and CaO, coupled with high contents of Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements, and has the most radiogenic Sr and Pb regional isotope composition. These unusual melt compositions could have been generated by low-degree partial melting of an enriched mantle peridotite source, and were erupted without significant mixing with common N-MORB magmas. The mantle in the Macquarie Island region must have been enriched and heterogeneous on a very fine scale. We suggest that the mantle enrichment implicated in this study is more likely to be a regional signature that is shared by the Balleny Islands magmatism than directly related to the hypothetical Balleny plume itself. Article in Journal/Newspaper Balleny Islands Macquarie Island eCite UTAS (University of Tasmania) Balleny Islands New Zealand Pacific Journal of Petrology 41 3 411 430 |