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: | Text |
| Language: | English |
| Published: |
Oxford University Press
2000
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| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/41/3/411 https://doi.org/10.1093/petrology/41.3.411 |
| Summary: | 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 H 2 O; 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 (47·4–51·1 wt % SiO 2 , 5·65–8·75 wt % MgO) is the broad range of K 2 O (0·1–1·8 wt %) and the strong positive covariation of K 2 O with other incompatible minor and trace elements (e.g. TiO 2 0·97–2·1%; Na 2 O 2·4–4·3%; P 2 O 5 0·08–0·7%; H 2 O 0·25–1·5%; La 4·3–46·6 ppm). The extent of enrichment in incompatible elements in glasses correlates positively with isotopic ratios of Sr ( 87 Sr/ 86 Sr = 0·70255–0·70275) and Pb ( 206 Pb/ 204 Pb = 18·951–19·493; 207 Pb/ 204 Pb = 15·528–15·589; 208 Pb/ 204 Pb = 38·523–38·979), and negatively with Nd ( 143 Nd/ 144 Nd = 0·51310–0·51304). Macquarie Island basaltic glasses are divided into two compositional groups according to their <rm>mg</rm>-number–K 2 O relationships. Near-primitive basaltic glasses (Group I) have the highest <rm>mg</rm>-number (63–69), and high Al 2 O 3 and CaO contents at a given K 2 O content, and carry microphenocrysts of primitive olivine (Fo 86–89·5 ... |
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