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Kaersutite and Kaersutite Eclogite from Kakanui, New Zealand — Water-Excess and Water-Deficient Melting to 30 Kilobars

Merrill, R. B. and Wyllie, P. J. (1975) Kaersutite and Kaersutite Eclogite from Kakanui, New Zealand — Water-Excess and Water-Deficient Melting to 30 Kilobars. Geological Society of America Bulletin, 86 (4). pp. 555-570. ISSN 0016-7606. doi:10.1130/0016-7606(1975)86<555:KAKEFK>2.0.CO;2.

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A natural kaersutite megacryst (compositionally equivalent to olivine nephelinite) and a kaersutite eclogite nodule (equivalent to olivine basanite) from the Mineral Breccia, Kakanui, New Zealand, were reacted in sealed platinum and Pd_(70)Ag_(30) alloy capsules, both with excess water and no additional water present, using half-inch piston-cylinder apparatus. Near-liquidus assemblages include orthopyroxene at pressures greater than about 15 kb in water-rich portions of the olivine-basanite system but not in the olivine-nephelinite system. Reversed high-pressure limits of the amphibole stability fields (excess water) have negative values of dP/dT, which crosses 25 kb at 1075°C and 30 kb at 925°C in the kaersutite system, but which crosses 25 kb at 1025°C and 30 kb at about 775°C in the kaersutite eclogite system. Comparison with experimental results reported elsewhere indicates that amphiboles persist to highest temperatures in basaltic liquids with greatest TiO_2 contents but with lowest Na_2O/(Na_2O + K_2O) ratios and lowest SiO_2 contents. Experimental results suggest that many natural nephelinite and basanite magmas evolve from hydrous picritic parent magmas through deep-seated fractionation of olivine, possibly with clinopyroxene and garnet but excluding orthopyroxene. Although some olivine-rich basanitic liquids may be generated by partial fusion of hydrous mantle peridotite, it is unlikely that orthopyroxene fractionation is important in their subsequent evolution. Experimental observations, together with chemical and petrographic relations, support the following model petrogenetic history for the Kakanui Mineral Breccia: pyrope-rich garnet and omphacitic pyroxene precipitated from ascending hydrous alkali basaltic magma (75 to 85 km, 1200° to 1300°C), then became trapped in deep-seated pockets within Iherzolitic mantle, together with inter-cumulus liquid that precipitated kaersutite on cooling. Resulting kaersutite eclogite assemblages re-equilibrated subsolidus (75 to 85 km, 700° to 800°C) prior to being incorporated into a rapidly ascending hydrous nephelinite magma, which was coprecipitating garnet, clinopyroxene, and probably kaersutite at depths >75 km (1100° to 1200°C). These accidental eclogitic inclusions underwent partial melting during the subsequent rapid ascent, which was terminated by an explosive eruption.

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Additional Information:© 1975 Geological Society of America. Manuscript received by the Society December 26, 1973; Revised manuscript received September 9, 1974. Research was supported by the Earth Sciences Section, National Science Foundation Grant GA-32266X. The National Science Foundation provided general support of the Materials Research Laboratory, and the Fannie and John Hertz Foundation provided a fellowship held by R. B. Merrill. Brian Mason provided the analyzed samples. R. J. Williams, J. R. Holloway, and D. H. Eggler contributed constructive criticisms which were received with appreciation.
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Fannie and John Hertz FoundationUNSPECIFIED
Issue or Number:4
Record Number:CaltechAUTHORS:20160210-082909937
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Official Citation:R. B. MERRILL and P. J. WYLLIE Kaersutite and Kaersutite Eclogite from Kakanui, New Zealand — Water-Excess and Water-Deficient Melting to 30 Kilobars Geological Society of America Bulletin, April, 1975, v. 86, no. 4, p. 555-570, doi:10.1130/0016-7606(1975)86<555:KAKEFK>2.0.CO;2
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:64369
Deposited By: Tony Diaz
Deposited On:10 Feb 2016 19:56
Last Modified:10 Nov 2021 23:30

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