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

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Kaersutite and Kaersutite Eclogite from Kakanui, New Zealand — Water-Excess and Water-Deficient Melting to 30 Kilobars Kaersutite and Kaersutite Eclogite from Kakanui, New Zealand — Water-Excess and Water-Deficient Melting to 30 Kilobars P J WYLLIE j department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637 ABSTRACT ments significantly: dehydration reactions serve to buffer supplies of volatiles, principally water, and of the other "incompatible" A natural kaersutite megacryst (compositionally equivalent to elements (for example, Ti, K) available to participate in silicate olivine nephelinite) and a kaersutite eclogite nodule (equivalent to melting reactions, thereby limiting potential degrees of melting and olivine basanite) from the Mineral Breccia, Kakanui, New Zealand, controlling both quantities and compositions of magmas gener- were reacted in sealed platinum and Pd7()Ag3o alloy capsules, both ated. It is essential to comprehension of igneous and tectonic with excess water and no additional water present, using half-inch mechanisms active at depth, therefore, that hydrous minerals capa- piston-cylinder apparatus. Near-liquidus assemblages include or- ble of participating in reactions within Earth's upper mantle be thopyroxene at pressures greater than about 15 kb in water-rich identified and characterized. portions of the olivine-basanite system but not in the olivine- The world-wide occurrence of calcic amphiboles — generally nephelinite system. Reversed high-pressure limits of the amphibole kaersutite or pargasite as primary constituents of high-pressure, stability fields (excess water) have negative values of dP/dT, which possibly mantle-derived mineral assemblages in alpine-type crosses 25 kb at 1075°C and 30 kb at 925°C in the kaersutite sys- ultramafic bodies, in ophiolite complexes, and in ultramafic tem, but which crosses 25 kb at 1025°C and 30 kb at about 775°C xenoliths in alkali basalts — suggests that these amphiboles are es- in the kaersutite eclogite system. Comparison with experimental sential hydrous phases in subcrustal environments (LaCroix, 1917; results reported elsewhere indicates that amphiboles persist to Wilshire and Binns, 1961; Oxburgh, 1964). Experimental studies highest temperatures in basaltic liquids with greatest Ti02 contents indicate that pargasite can exist at upper mantle temperatures and but with lowest Na20/(Na20 + KaO) ratios and lowest Si02 pressures in hydrous peridotite (Kushiro, 1970; Mysen and contents. Boettcher, 1972, 1973; Green, 1973a) and in hydrous basaltic Experimental results suggest that many natural nephelinite and magmas (Allen and Boettcher, 1971, 1973). Irving and Green basanite magmas evolve from hydrous picritic parent magmas (1972) reported kaersutite to be stable in hydrous nepheline through deep-seated fractionation of olivine, possibly with mugearite up to 23 kb. clinopyroxene and garnet but excluding orthopyroxene. Although This investigation was undertaken to evaluate conditions under some olivine-rich basanitic liquids may be generated by partial fu- which natural kaersutite associated with alkali basalt may form sion of hydrous mantle peridotite, it is unlikely that orthopyroxene and, further, to explore possible roles which these amphiboles may fractionation is important in their subsequent evolution. play during the evolution of magmas within the upper mantle. To Experimental observations, together with chemical and petro- these ends, natural kaersutite has been studied experimentally graphic relations, support the following model petrogenetic history under simulated upper mantle conditions in a system of its own for the Kakanui Mineral Breccia: pyrope-rich garnet and omphacit- composition, as one component of a natural kaersutite-rich garnet ic pyroxene precipitated from ascending hydrous alkali basaltic clinopyroxenite (kaersutite eclogite), and in both of these systems magma (75 to 85 km, 1200° to 1300°C), then became trapped in with excess water present. deep-seated pockets within lherzolitic mantle, together with inter- cumulus liquid that precipitated kaersutite on cooling. Resulting EXPERIMENTAL METHODS kaersutite eclogite assemblages re-equilibrated subsolidus (75 to 85 km, 700° to 800°C) prior to being incorporated into a rapidly as- Starting Materials cending hydrous nephelinite magma, which was coprecipitating garnet, clinopyroxene, and probably kaersutite at depths >75 km The natural kaersutite megacryst (K-l) and the natural kaersutite (1100° to 1200°C). These accidental eclogitic inclusions underwent eclogite nodule (K-l4) used in this study were described by Mason partial melting during the subsequent rapid ascent, which was ter- (1966, 1968b). They were found as inclusions in the Mineral Brec- minated by an explosive eruption. Key words: experimental ig- cia Member of the Deborah Volcanic Formation at Kakanui, New neous petrology, phase equilibria, olivine nephelinite, olivine Zealand. The source locality and Mineral Breccia association have basanite, upper mantle, hydrous magma, volcanic breccia, horn- been discussed by Mason (1966, 1968a, 1968b), Dickey (1968), blende eclogite. and White and others (1972). The breccia consists of nephelinite host basalt, enclosing a suite of lherzolite, pyroxenite, dunite, kaer- INTRODUCTION sutite eclogite, and granulite nodules, as well as megacrysts of kaer- sutite, pyrope-rich garnet, aluminous clinopyroxene, anorthoclase, In theory, hydrous minerals can influence magma genesis and and ilmenite. tectonic activity within lower crustal and upper mantle environ- Mason (1966) reported a wet chemical analysis of the kaersutite1 megacryst (K-l), which he later modified (Mason, 1968a). A third * Present address: Lunar Science Institute, 3303 Nasa Road 1, Houston, Texas chemical analysis (Mason, 1969, written commun.) is compared 77058. with a microprobe analysis of the material used in these experi- Geological Society of America Bulletin, v. 86, p. 55J-J70, 7 figs., April 1975, Doc. no. 50415. 555 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/555/3444124/i0016-7606-86-4-555.pdf by guest on 02 October 2021 556 MERRILL AND WYLLIE ments (Table 1). Trace-element contents (K, Rb, Sr, Ba, and REE) TABLE 1. ANALYSES OF EXPERIMENTAL STARTING MATERIALS of another portion of this same megacryst have been reported by Philpotts and others (1972; their specimen GSFC #21e). Weight + + + Mason (1968b) described the kaersutite eclogite nodule K-14 Per Cent 1 2 3 4 5 6 (Table 1), reporting a modal analysis (vol percent): 62 percent Si02 41.53 40.37 41.20 (41.4) (49.4) (39.4) kaersutite, 18 percent pyrope-rich garnet, 18 percent omphacitic clinopyroxene, and 2 percent ilmenite. Philpotts and others (1972) Ti02 4.46 4.38 3.20 5.1 1.5 0.5 reported trace-element contents of similar kaersutite eclogite AI203 14.35 14.90 16.35 13.7 8.4 20.2 nodules from the Mineral Breccia. Fe203 3.30 2.87 * * Apparatus and Experimental Procedures FeO 10.38* 7.95 11.40 13.9 10.8* 22.8 MnO 0.11 0.09 0.20 The megacryst and nodule samples were ground in acetone to pass a 200-mesh sieve, which discriminated against particles >74 MgO 12.84 12.80 10.93 10.2 10.5 10.3 /xm in dimension; average dimension of particles in starting mix- CaO 10.37 10.30 9.61 9.7 16.4 6.8 tures was <50 /xm. Na20 2.(8) 2.60 2.33 3.6 3.0 Resulting powders were sealed into capsules of platinum or K 2.(2) 2.05 0.86 1.4 Pd70Ag30, either dry or with measured percentages of distilled, 2° deionized water. Capsules which initially contained free water were 0.00 0.14 P2°5 tested for leaks before and after runs by application of heat; those 0.90 0.38 (1.0) which failed to expand were discarded. Capsules were run in H20+ piston-cylinder apparatus (Boyd and England, 1960) with a H2O" 0.04 0.20 Vi-in.-diameter pressure chamber. Materials used within the F 0.15 graphite furnace included talc, Pyrex glass, crushable alumina, and AlSiMg 730. Talc pieces were used alone below 850°C; at Total, less 0 for F 99.05 99.70 99.71 (100.0) (100.0) (100.0) higher temperatures, Pyrex glass was used below the capsule and 100 Mq combinations of the other materials above the capsule. 69 68 57 57 63 44 Mg + Fe The "piston-out" experimental procedures described by Boyd and others (1967) was employed. Calculation of pressures exerted on samples is complicated by several factors (Bell and Williams, * Fe reported as FeO. 1971; Johannes and others, 1971), many of which are poorly un- t Partial analysis. Parentheses designate approximate values, derstood. A. J. Irving (1972, written commun.) calibrated our ap- calculated assuming sum to be 100.0. paratus against the melting curve of NaCl, using differential ther- 1. Microprobe analysis of kaersutite megacryst K-l. Applying the 2 2 3 mal analysis techniques, and obtained brackets [1082° to 1089°C, Fe /(Fe + Fe ) ratio of column 2: Fe20, 3.13, FeO 7.55. Total 14.2 ± 0.3 kb; 1213° to 1220°C, 15.4 ± 0.2 kb], which agree with includes Cr203 0.0(1). Analyst: R. B. Berrill. results Clark (1959) obtained in hydrostatic apparatus. This 2. Kaersutite megacryst K-l. Total Fe calculated as FeO: 10.92. confirms that the nominal "piston-out" pressures reported here re- Mason, personal communication, 1969. quire little correction for friction effects. Chromel-alumel ther- 3. Kaersutite eclogite nodule K-14. Whole rock. Mason, 1968b, mocouples were used up to 850°C, and Pt-PtlORh thermocouples Table 3. were used at higher temperatures. Temperatures are precise within 4. Kaersutite in kaersutite eclogite nodule K-14. Microprobe anal- 5°C and have not been corrected for pressure effects on ther- ysis for Ti, Al, Fe, Mg, Ca, Na, and K reported by Mason, 1968b, Table 4. mocouple emf. Judging from recent studies of these effects for chromel-alumel and for Pt-PtlORh thermocouples (for example, 5. Omphacitic pyroxene in kaersutite eclogite nodule K-14. Micro- probe analysis for Ti, Al, Fe, Mg, Ca, and Na reported by Mason, Getting and Kennedy, 1970), the maximum uncertainty attributa- 1968b, Table 4. ble to pressure effects is ±13°C in experiments at 1000°C (30 kb) 6.
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