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 intercumulus 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 kaersutite, 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

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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. Pyrope-almandine garnet in kaersutite eclogite nodule K-14. and less in experiments at lower temperatures or pressures. Microprobe analysis for Ti, Al, Fe, Mg and Ca reported by Mason, Platinum and palladium-silver capsules are permeable to hy- 1968b, Table 4. drogen at high temperatures; this affects the fugacities of hydrogen, of oxygen, and of water in reacting systems. These fugacities could the high temperatures required. An accepted consequence was that not be buffered during the long-duration runs required to approach Mg/Fe ratios in experimental charges increased at rates variable equilibrium because of the small dimensions of furnace assemblies. with temperature and liquid content during runs. In a 6 hr experiment at 20 kb, 1050°C, an initial mixture of Ni and Chemical analyses were performed with an ARL-EMX-SM elec- NiO (1:1 by weight) sealed with water in a platinum capsule was tron microprobe, following procedures described by Smith (1965) converted entirely to Ni, which indicates that the unbuffered f02 and by Rucklidge and others (1971). Selected run products were was below the Ni/NiO buffer. Allen and others (1972) found that impregnated with epoxy within their capsules, then ground to ex- fo, in a similar unbuffered furnace assembly is close to the Ni/NiO pose charges for identification and analysis of phases. Raw micro- buffer at 11 kb, 1050°C. probe data were reduced with a modified version of the EMPADR We observed considerable loss of iron from charges to platinum correction program (Rucklidge, 1967). capsules in experiments at near-liquidus temperatures (Merrill and Wyllie, 1973). Despite this, we used platinum as the principal cap- Identification of Run Products sule material because it is the only known material that can be sealed satisfactorily against the escape of volatiles (except H2) at Run products were analyzed using standard optical and x-ray powder diffraction techniques for identification of phases. 'According to Leake's (1968) nomenclature, the megacryst K-l is "titaniferous Major minerals in the fine-grained, complex experimental run pargasite," 0.01 Ti atom per half-unit cell short of being classed as "kaersutite." Such a distinction is without significance for this investigation, so the term "kaersutite" is products were readily identifiable, but it was difficult to be certain employed without prejudice. of detecting all minor phases and of distinguishing between stable,

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metastable, and quench phases. The uncertainties warrant caution rapidly during the quench, or by compositional changes within the in application of experimental results to interpretations of phase experimental system (for example, due to leaching, oxidation, or equilibria in natural rock systems. loss of Fe to capsules). Attempts to circumvent these problems have Deeply pleochroic prisms of primary amphibole often are involved comparison of results obtained with different run dura- rimmed by slightly pleochroic to colorless amphibole, either new tions, with different starting materials (minerals, silicate gels, growths, reaction rims, or possibly even quench products (Green, glasses — clear or seeded with mineral grains), or with different run 1973a). Small garnets, clinopyroxenes, and needlelike rutiles com- procedures. monly are enclosed within amphibole grains, the rutiles occasion- In duplicate experiments with the kaersutite eclogite with excess ally forming oriented patterns reminiscent of exsolution textures. water, no significant differences were detected between assem- Clinopyroxene crystals are zoned in many runs because of in- blages produced in 5 hr and 24 hr at 1000°C, 20 kb, or in 5.5 hr complete recrystallization of minerals present in the starting as- and 11 hr at 1050°C, 20 kb. In duplicate water-deficient experi- semblage. Garnets commonly are anhedral, rarely euhedral. ments, however, similar phases were present after experiments last- Olivine was identified in near-liquidus experiments through its ing 1 hr and 11.5 hr at 1150°C, 20 kb, but breakdown reactions of optical properties and was verified with x-ray powder diffraction hornblende and garnet had progressed much further in the longer photographs. In experiments at lower temperatures, where olivine run. Significant compositional differences exist between cores and forms less euhedral crystals and can easily be confused with am- margins of garnets and clinopyroxenes produced in short duration, phibole, primary reliance was on x-ray techniques. near-liquidus water-deficient experiments with the kaersutite Orthopyroxene possesses two distinctive crystal habits: (1) short ecologite composition (Tables 4 and 5). Kinetic problems were stubby prisms, with length to width ratios 4 or 5 to 1. Crystals hav- even more severe at lower temperatures. ing this form, found only in water-excess experiments with the Garnet does not nucleate readily in glassy starting materials and kaersutite eclogite, appear to have grown from the liquid in associ- tends to persist metastably in crystalline starting mixtures. The ation with other stable crystalline phases. (2) Long euhedral prisms problem is further complicated in natural systems by solid-solution terminated by domes (Qopx) are found closely associated only with relationships among garnets and coexisting mineral phases (Ring- glass and (quench?) rutiles. This form, of which the largest crystal wood and Green, 1966; Green and Ringwood, 1967a, 1967b, observed was 0.25 mm long by 0.02 mm cross section, apparently 1972; Cohen and others, 1967). Experiments with the kaersutite crystallized from liquid that had become separated from other eclogite nodule define a boundary (garnet-out) limiting the P-T crystals. Stringers and blebs of glass enclosed in Qopx crystals indi- field within which garnets initially present persist (stably or meta- cate that the crystals grew rapidly, possibly during the quench pro- stably) in run products. Experiments with the kaersutite megacryst, cess. Qopx was observed among run products of water excess but however, define low-pressure P-T limits (garnet-in) of successful not among products of water-deficient experiments in both the nucleation and growth of garnet as a breakdown product of am- kaersutite eclogite and kaersutite megacryst systems. phibole in an initially garnet-free assemblage. The equilibrium Phlogopite is present among run products in the water-excess low-pressure stability limit for garnet in a given bulk composition kaersutite megacryst system. The most abundant form is a sub- must lie between the garnet-in and garnet-out boundaries. hedral pseudohexagonal cleavage fragment, which may include Garnet nucleation and metastability difficulties also affect ex- small clinopyroxene crystals and which has been found associated perimental determinations of stability relations of amphibole, with glass, clinopyroxene, garnet, and quench crystals. Phlogopite whose high-pressure breakdown products include garnet (Lambert having this habit was interpreted to have grown stably during runs. and Wyllie, 1968, 1972; Essene and others, 1970; Allen and Thin white micas (muscovite?) with undulatory extinction are others, 1972). Essene and others (1970) reported that in experi- ubiquitous and appear to have crystallized at capsule margins as ments with mixtures of "amphibolites" and "eclogites," "am- vapor phase deposits, probably during the quench process. phibole was found to be unstable in the region where it had previ- Liquids quenched to glass and crystalline phases (Q). Amphibole ously been synthesized, and it did not perceptibly grow at the ex- and clinopyroxene quench crystals typically are acicular clusters, pense of the garnet and pyroxene until the pressure had been low- commonly nucleated on crystals of the stable assemblage. Other ered 5-10 kb below the original synthesis limit" (Essene and large, well-formed prisms enclose blebs of glass similar to those in others, 1970, p. 380). The kaersutite eclogite starting material con- Qopx, which presumably became trapped during periods of rapid tains both the calcic amphibole and its principal high-pressure growth, possibly during the quench. In water-rich charges, near- breakdown products, which reduces potential nucleation liquidus and super-liquidus liquids quenched almost entirely to difficulties. crystalline phases. Amphibole stability limits were checked by performing two- Run products quenched from vapor in our experiments include stage reversal experiments (designated "R" in Tables 2 and 3), in aqueous fluid and various solids, including thin plates and fibrous which starting materials first were reacted under conditions known aggregates of mica (muscovite?) and clear glass (spherules, shards, to be outside the amphibole stability field and then were brought to and rims around stable crystals) with refractive index <1.5. Minor conditions just within the amphibole field as determined from one- quantities of anhedral quartz, which were observed in charges stage reaction experiments. Presence of amphibole crystals among quenched at all pressures from temperatures between 900° and products of a two-stage run was accepted as evidence that the sec- 1300°C, commonly are associated with glass quenched from vapor. ond set of experimental conditions was within the amphibole sta- Quartz melts completely within 30°C of the water-saturated sol- bility field. Green (1973a) discussed the additional problem of dis- idus in quartz eclogite (740°C at 30 kb; Lambert and Wyllie, tinguishing between stable amphibole and amphibole quenched 1972). It is extremely unlikely that quartz persists stably to higher from liquid. temperatures in silica-undersaturated eclogites. The quartz presumably precipitated from silica-rich vapor during the quench. EXPERIMENTAL RESULTS

Checks on Equilibrium The results of runs are listed in Tables 2 and 3, and run points are plotted in P-T projections in Figures 1 through 4. Phase bound- Interpretations of quenched run products are complicated by in- aries shown in these figures delimit stability fields for minerals orig- complete reactions, by metastable persistence of phases, by failure inally present in the crystalline starting materials and fields within of stable phases to nucleate, by phases that nucleate and grow which new minerals nucleate and grow during runs. These results

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30 • too out o -O - m O -O aj 3 20 in a> to a> co c/Ï ou

800 1000 1200 1400 -liquidus - Temperature °C Figure 1. Experiments with kaersutite megacryst (olivine nephelinite) plus excess H20. Light solid lines, field boundaries. Water-saturated liq- 800 1000 1200 1400 uidus contour (heavy solid line) is extrapolated below 10 kb to meet anhydrous olivine nephelinite liquidus contour at 1 atm (Bultitude and Green, 1971). Heavy dashed line, estimated from Yoder and Tilley (1962) Temperature °C and from Haygood and others (1971). Experiments are plotted with circles: Figure 2. Experiments with kaersutite megacryst (olivine nephelinite)

solid circles, runs whose products include crystals; open circles, crystal-free under H20-deficient (vapor-absent) conditions: 0.9 wt percent H20 ini- run products. Abbreviations as in Table 2. tially bound in kaersutite. Liquidus contour is extrapolated to meet water- saturated contour (from Fig. 1) near 0.8 kb, estimated from solubility data are used to determine approximate distributions of primary or (Kadik, 1971; Green, 1973c). Solidus from Figure 1. Phlogopite observed in near-liquidus minerals on the water-undersaturated liquidus sur- runs at 30 kb below 1200°C. Abbreviations as in Table 2. Symbols as in faces for the two compositions (Figs. 5 and 6). Figure 1.

Kaersutite with Excess Water Kaersutite with No Water Added

Results are shown in Figure 1. The solidus was not determined This starting mixture contains 0.9 wt percent water, structurally but was assumed to be similar to that reported for alkali basalts bound in amphibole. Results are shown in Figure 2. (Yoder and Tilley, 1962; Haygood and others, 1971; Helz, 1973). Garnets nucleated only in runs at 30 kb, but the apparent dis- At pressures below 20 kb, kaersutite melts incongruently near the placement of the "ga-in" curve to higher pressures between Figures solidus to produce an amphibole of different composition and 1 and 2 probably reflects only nucleation difficulties (Irving, 1971). minor amounts (5 percent) of liquid, sphene, and rutile. At higher Many amphiboles at 30 kb, 1100° and 1150°C, enclose acicular temperatures, clinopyroxene appears simultaneously with the first rutile crystals intersecting each other at amphibole cleavage angles. production of significant amounts (20 to 30 percent) of liquid. Conceivably these are quench amphibole crystals (note queries for From a similar relationship at 5 kb for tholeiite, Holloway and runs 58 and 64 in Table 2). Apparently stable amphiboles, free of Burnham (1972) inferred the amphibole decomposition reaction to rutile inclusions, are abundant at 1000° and 1050°C at 30 kb, be (by weight) 100 amphibole = 23 clinopyroxene + 77 liquid. temperatures well above the amphibole field boundary observed Sphene reacts with liquid, probably to form clinopyroxene and with excess water. Two-stage reversal runs from the liquid field are rutile, at temperatures just above the "cpx-in" curve. Abundant consistent with the one-stage reaction runs, indicating that the am- stable rutiles and thin, acicular quench rutiles in glass indicate that phibole field extends to 1200°C at 20 kb, about 100°C higher than liquids were saturated with Ti02 to temperatures very near the with excess water. Minor phlogopite occurs in runs at 30 kb but liquidus. Minor phlogopite (<1 vol percent) appears just above not at the liquidus. "sph-out" and is present up to the liquidus. At 20 kb, 1100° and 1150°C, quenched charges consisted of 85 The water-saturated liquidus is extrapolated to meet the anhy- to 90 vol percent amphibole, <5 percent clinopyroxene, and 10 to drous nephelinite liquidus at 1 atm (Bultitude and Green, 1971). 12 percent liquid. At 1200°C, amphibole decreased to 5 percent Amphibole, abundant on the liquidus at 20 kb, probably is a stable and liquid increased to 50 percent, with clinopyroxene and minor liquidus phase between about 13 and 23 kb. Pyrope-rich garnet olivine forming the balance. Similarly, at 10 kb, amphibole disap- nucleates and grows above the "ga-in" boundary, which was not peared and liquid increased from 35 percent to 90 percent between closely bracketed. A minimum location in terms of pressure is 1100°C and 1200°C. shown in Figure 1. Above about 20 kb, kaersutite breaks down at near-solidus Kaersutite Eclogite with Excess Water temperatures to an assemblage of amphibole, clinopyroxene, garnet ± rutile ± sphene ± phlogopite ± liquid. A vapor phase may Results of runs in platinum capsules are shown in Figure 3. In exist in subsolidus regions or. depending on relative amounts of experiments below the "cpx-in" boundary at 10 and 15 kb, am- K20 and H20 liberated by the dehydration reaction, all water may phibole grew at the expense of clinopyroxenes and garnets present be incorporated into phlogopite and sphene under water-deficient, in the starting material. At 10 kb, 750°C, amphibole increased vapor-absent conditions. from its original 62 percent to about 85 percent, clinopyroxene

Koersutite Eclogite,0.4% H20J

o o — o 30 -O

Z3 to foTH 10" 600 800 1000 1200 1400

Temperature °C -liquidus Figure 3. Experiments with kaersutite eclogite nodule (olivine basanite)

plus excess H20. Liquidus contour is extrapolated to meet anhydrous alkali olivine basalt liquidus contour (Green and Ringwood, 1967b). Solidus es- 800 1000 1200 1400 timated from Voder and Tilley (1962) and from Haygood and others (1971). Abbreviations as in Table 3. Symbols as in Figure 1. Temperature °C Figure 4. Experiments with kaersutite eclogite nodule (olivine basanite)

disappeared, and 5 to 15 percent of liquid formed, but garnet failed under H20-deficient (vapor-absent) conditions: 0.4 wt percent HaO ini- to disappear completely even after 70 hr. Liquid amounted to 39 to tially bound in kaersutite. Liquidus contour is extrapolated to meet water- 40 percent at 10 kb, 850°C, and at temperatures above "cpx-in," saturated contour (from Fig. 3) near 0.4 kb, estimated from solubility data amphibole dissolved incongruently, leaving a different amphibole (Kadik, 1971; Green, 1973c). Solidus from Figure 3. Abbreviations as in and clinopyroxene as dominant minerals. Table 3. Symbols as in Figure 1. Two-stage reversal runs at 20 and 25 kb are consistent with one-stage reaction runs for the position of the amphibole phase With excess water in Pt capsules, garnet disappears in 5.5 hr at boundary. Garnet did not nucleate in the 25 kb reversal run at 1050°C, orthopyroxene is present between 950° and 1100°C, and 1000°C. the liquidus is at 1110°C (Fig. 3). In Pd-Ag capsules, garnet remains At near-liquidus temperatures, the "ga-out" curve in Figure 3 abundant after 4 hr at 1075°C, no orthopyroxene is produced, and agrees well with the "ga-in" curve in Figure 1. At lower tempera- the liquidus is at 1085°C (Table 3). For the water-deficient sample tures, "ga-out" extends to considerably lower pressures than "ga- in Pt capsules, the liquidus at 20 kb is about 1215°C (Fig. 4), but in in." The equilibrium low-pressure boundaries of garnet stability Pd-Ag capsules, it lies between 1250°C and 1300°C (Table 3); this fields in these silica-undersaturated compositions lie between the difference opposes that found under water-excess conditions. We two observed curves, which leaves a large uncertainty in their posi- conclude that it is inadvisable to use multiple capsule materials in tions at temperatures below 1000°C. experimental studies of iron-bearing silicate phase relations, but as The absence of phlogopite is tentatively attributed to the lower yet we can express no preference for results obtained in one mate- potash content of the eclogite (0.86 percent) relative to the kaersu- rial over those obtained in the other. tite (2.05 percent) starting materials. Kaersutite Eclogite: Microprobe Kaersutite Eclogite with No Water Added Analyses of Run Products

This starting mixture contains 0.4 wt percent water structurally Tables 4 and 5 list analyses of garnets and clinopyroxenes in the bound in amphibole. Results of runs in platinum capsules are original rock and in experimental runs quenched from 1250° and shown in Figure 4. Amphibole persists to slightly higher tempera- 1100°C at 25 kb. These results appear to have been little affected tures than in Figure 3 with excess water but is not stable on the by proximity of analyzed materials to surrounding glass and liquidus. Results of two-stage reversal runs are consistent with quench crystals, but some analytical uncertainty was unavoidable those of one-stage reaction runs in locating the amphibole phase due to the small grain size. boundary at 20 and 25 kb. Compositions of the pyrope-rich garnets change little between Orthopyroxene was not found in any runs conducted under 1250° and 1100°C in 0.25-hr runs or between 0.25-hr and 2-hr these conditions nor in runs with as much as 4 percent water runs at 1100°C. Garnets in experimental run products contain present in the system. about 2 wt percent less FeO than those in the original starting material. Kaersutite Eclogite in There are significant differences among compositions of ompha- Palladium-Silver Capsules cite in the starting material and clinopyroxenes in the run products. At 1250°C there is little change in FeO content of clinopyroxene in

A series of runs in Pd70Ag30 capsules was completed at 20 kb for 0.25 hr, but the effect of progressive iron loss from the liquid and comparison with results (Figs. 3 and 4) obtained in Pt capsules. consequent readjustment of mineral compositions is shown (analy- There are significant differences in the positions of phase bound- ses 3 and 4 in Table 5) for 2-hr runs at 1100°C. The central aries determined in experiments using the two capsule materials. portion of a crystal remnant from starting material contains about

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TABLE 2. EXPERIMENTS WITH THE KAERSUTITE MEGACRiTST (K-l) IN PLATINUM CAPSULES

Pressure HO Temp. Duration phases observed among run products kbar Added, °c hours Weight Hb Cpx Ga 01 Ru Sph Ph Per Cent

44 10 0 1300 1.3 ______+ 0 39 10 0 1250 1.2 - tr _ tr _ _ + 0 32 10 0 1200 3.5 - + + _ _ _ + 0 45 10 0 1150 3.0 + 7 _ + _ _ _ + 0 35 10 0 1100 2.5 + _ tr _ _ _ + 0 25 10 33 1200 2.2 - _ - - _ _ + Q 57 10 32 1175 2.0 - _ - - - _ - + Q 42 10 24 1150 2.0 _ 7 - •3 _ _ - + Q 15 10 19 1100 4.5 - + - + + _ + + Q.Q2 16 10 13 1050 5.0 + + - + - + + O/Q/Qz 29 10 24 900 16.5 + _ - - + + - + Q,Qz 24 10 19 700 44.0 + _ - - + + _ tr 71 15 0 1275 1.0 - 7 _ tr - _ - + 68 15 32 1175 2.0 - _ - - _ - - + Q 69 15 33 1150 2.0 _ _ - - _ - _ + Q 74 15 32 1125 1.0 ? _ - - _ + Q 78 15 27 1050 18.0 + + _ + + _ + + Q 10 15 37 950 3.2 + + - - + + _ + Q 2 15 13 800 22.2 + _ - - + tr _ + 14 15 25 725 67.0 + + - - + + _ tr 4R 30 15 850 3.0 {+ + + - + _ + tr) 15 15 725 19.0 + - tr - + + tr tr Q 59 20 0 1300 1.0 ------_ + 0,Q 43 20 0 1250 2.0 - + - + - - _ + 0,Q 33 20 0 1200 4.0 + + _ p - _ - + 0 ,Q 65R 20 0 1300 1.0 (- - _ _ _ _ _ + ) 20 0 1200 3.0 + + _ tr _ _ _ + 0,Q 62 20 0 1150 7.0 + + - - - _ - + O.Q 40 20 0 1100 2.0 + tr _ - + + ? + Q 66R 20 0 1300 1.0 _ _ - _ _ _ + O) 20 0 1100 3.0 + + - - + _ + + 0,Q 36 20 33 1250 1.1 - _ _ _ _ _ + Q 46 20 29 1200 2.0 - ______+ Q 26 20 33 1150 2.5 - ? - - _ - •5 + Q.Qz 47 20 31 1100 3.0 + + - + - + + Q 41 20 35 1050 3.3 + + - - + - + + Q 23 20 29 950 6.0 + + - - + - + + Qz 22 20 17 850 20.0 + _ - - + + - + 20 20 23 750 22.0 + _ - - + + - tr Qopx 19 20 21 650 44.0 + - - - + - - 7 Qopx 31 24 0 850 21.0 + _ ------0 1 24 12 850 19.0 + + + - + - + + 73 25 0 1350 1.0 - _ - - - - - + Q 72 25 0 1300 1.2 - + - - - - - + Q 76 25 0 1150 5.0 + + - - + - - + 0 67 25 0 1100 4.0 + + _ _ + _ _ + 0 70 25 34 1275 1.2 - - _ _ - - - + Q,Qz 75 30 0 1400 0.5 - _ _ _ - - - + Q 61 30 0 1350 1.2 ------+ 0/ Q 53 30 0 1300 1.1 - + + - - - - + 0,Q 64 30 0 1150 14.0 7 + + - + - + + 0 ,Q 58 30 0 1100 13.0 7 + + - + - + + 0,Q 63 30 0 1050 13.0 + + + - + - + + 0,Q 54 30 0 1000 3.0 + + + - + - + + 56 30 0 1000 18.0 + + + - + - + + 0 52 30 38 1350 1.0 ------+ O.Q 49 30 42 1300 1.0 ------+ Q 79 30 30 1265 2.0 - - - _ - _ - + Q 80 30 30 1215 2.0 ------+ Q 81 30 30 1165 2.0 - + + - - - ? + Q 27 30 31 1150 2.5 - 4- + - - - + + 0,Q,0z 21 30 36 1050 5.8 - + + - + - + + Q,Qz 6 30 33 950 3.5 - + + - + - + + Q,Qz 48 30 34 900 20.0 + + + - + - + + 0,Q,Qz 18 30 20 850 18.0 + + + - + - + 7 17 30 16 800 19.0 + + + - + - + 7 9 30 34 700 22.5 + + + - + - + 7 Qopx

"R" denotes two-stage run; parentheses enclose assemblage presumed to have been stable at run conditions durinq the first stage; "tr", trace; "?", possible; "+", present; "-", absent. "Hb", amphibole; "Cpx"; clinopyroxene; "Ph", phlogopite; "Ga", garnet; "01", olivine; "Ru", rutile' "Sph", sphene; "Gl", glass quenched from liquid; "0", opaques; "Q", phases formed during quench, not stable at run conditions: airphibole, clinopyroxene, rutile, mica; "Qz", quartz crystallized from vapor phase; "Qopx", quench orthopyroxene.

1 percent less FeO than the original omphacite, but in recrystallized clinopyroxenes than in garnets. True equilibrium between experi- pyroxene, the FeO has decreased from 10.8 percent to 6.6 wt mental charge and platinum capsule requires that iron initially in percent. the charge be transferred almost completely to the platinum (Mer- These results illustrate the conflict between the need to keep runs rill and Wyllie, 1973). as short as possible in order to minimize iron loss and the need for runs to be of sufficient duration to permit a reasonable approach to WATER-UNDERSATURATED LIQUIDUS SURFACES equilibrium. As iron migrates from the silicate liquid to the enclosing platinum, Mg/(Mg + Fe) increases in coexisting crystals. Every silicate mineral or rock has a water-undersaturated liq- Analytical results indicate that readjustments occur more rapidly in uidus surface in P-T-XH20 space, extending from the 0 percent H20

1 1 1 1 1 Weight % water excess iq 5 o.4 0 1 / / / ' 1 go\ / - 100 30 100 /cpx + go// / o - /+ opx/,9°+cp* y e O - hb + cpx + ga -O -O + opx E

LU ccLLJ CC 20 — hb+cpx + opx- ¡211 CPX / in Q. ZD CO LU CO ' I 1' CL LU Q CO hb + cpx---' / 1 / LU cc LU */ / - Q 50 / I / 50 Q_ CC hb+ ol + cpx- / Q. i ol + cpx J 10 1 ! 1 /\ OLIVINE " BASANITE - \°\ 1 LIQUIDUS SURFACE " >U , 1 1 0 1 0 1200 1000 1200 1400 1600 Temperature TEMPERATURE °C Figure 5. Near-liquidus phase relations in olivine nephelinite as func- Figure 6. Near-liquidus phase relations in olivine basanite as functions

tions of H20 content. Light dashed lines, contours for water content: 5 wt of H20 content. Contours for 5 and 10 wt percent H20 are interpolated. percent H20 contour is interpolated. Light solid lines, liquidus stability field Composite of Figures 3 and 4. Anhydrous relations based upon experi- boundaries. Composite of Figures 1 and 2. Anhydrous relations based on ments with synthetic alkali olivine basalt glass (Green and Ringwood, experiments with synthetic olivine nephelinite glass (Bultitude and Green, 1967b). Abbreviations as in Table 3. 1968, 1971). Abbreviations as in Tabic 2.

liquidus for the anhydrous composition to the water-saturated liq- The two starting materials are compared in Table 3; the glass is uidus curve (Figs. 1 and 3). The water content corresponding to the richer than the kaersutite in normative nepheline and olivine. latter curve increases from essentially 0 percent at 1 kb to values of Boundaries for liquidus phases and phase assemblages are drawn about 27 wt percent at 30 kb (Green, 1973c; Stern and Wyllie, through points determined experimentally on the liquidus contours 1973). The shape of such a surface can be illustrated with contours (interpolated for the 5 percent H20 contour). Future investigations, for constant water contents determined experimentally. which overcome the experimental uncertainties reviewed above, Figures 2 and 4 (for low water contents) show one or two miner- may shift the positions of some phase boundaries but are unlikely als coexisting at the liquidus, and Figures 1 and 3 (water-saturated) to alter the general pattern of liquidus phase relations significantly. show two to five minerals at the liquidus. Each of these minerals Note the addition of a small overlap of garnet and amphibole phase could have a separate phase boundary, with only one of them being fields at the water-saturated liquidus boundary, not shown in Fig- the true liquidus phase, but we were unable to distinguish among ure 1.

them. Therefore, in the following discussion, we group "near- The addition of only 1 wt percent H20 to olivine nephelinite re- liquidus minerals" together as "liquidus minerals" (Figs. 1 through duces liquidus temperatures by about 100°C at 20 kb and by more 4). Experimental results are combined with other data in Figures 5 than 150°C at 30 kb, with larger amounts effecting smaller temper- and 6 to illustrate the distribution of liquidus minerals on water- ature reductions. Addition of H20 also stabilizes amphibole and undersaturated liquidus surfaces, projected from P-T-XH20 space. phlogopite on the liquidus. The liquidus field for primary am- The H20 contours join the water-saturated liquidus boundaries at phibole exists only for H20 contents of more than about 2 wt per- points estimated from the solubility of HaO in basic magmas cent at temperatures below about 1200°C and for pressures bet- (Kadik, 1971; Green, 1973c). ween 12 and 24 kb. Small amounts of phlogopite occur on the li- In discussing liquidus mineralogies, magmatic terms for bulk quidus with excess water but not with 0.9 percent H20. Allen and compositions are more appropriate than specific rock names of Boettcher (1973) did not find phlogopite coexisting with am- starting materials. The kaersutite megacryst is equivalent composi- phibole in their water-excess experiments with an olivine nepheli- tionally to olivine nephelinite basalt, and the kaersutite eclogite nite (Table 6). The field boundary shown for phlogopite (Fig. 5) is nodule corresponds to olivine basanite. Bultitude and Green (1971) schematic, based upon results from synthetic systems (Modreski have compared chemical variations among 13 silica- and Boettcher, 1973). undersaturated rocks ranging from basanite through olivine nephelinite to olivine-melilite nephelinite, and they discussed pet- Olivine Basanite (Kaersutite Eclogite Composition) rogeneses in contexts of partial fusion of a periodotite mantle and deep-seated fractionation of basic magmas. Table 6 compares pub- Figure 6 is a projection of the water-undersaturated liquidus sur- lished analyses of similar materials. face for olivine basanite, in part schematic, based on Figures 3 and 4 and on the dry liquidus determined for synthetic alkali olivine Olivine Nephelinite (Kaersutite Composition) basalt glass (Green and Ringwood, 1967b). The two starting materials are compared in Table 6. Major-element chemistry of the Figure 5 is a projection, in part schematic, of the water- glass matches that of the kaersutite eclogite more closely than other undersaturated liquidus surface for olivine nephelinite based on the basanitic materials studied experimentally, despite a significant dif- water-saturated liquidus in Figure 1, the 0.9 wt percent H20 liq- ference in normative nepheline/albite ratios. Boundaries for liquidus contour in Figure 2, and the dry liquidus determined for a uidus phases and phase assemblages were drawn to connect equiv- synthetic olivine nephelinite glass by Bultitude and Green (1971). alent points on the contours.

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TABLE 3. EXPERIMENTS WITH THE KAERSUTITE ECLOGITE NODULE (K-14)

Run No. Pressure Temp. Duration Phases observed among run products kbar aáded. °C hours Hb Cpx Opx Ga 01 Ru Sph Weight G1 Others Per Cent

Runs in platinum capsules

169 10 0 1250 2.5 ______+ 171 10 0 1200 2.0 - ? _ - + _ _ + 0 174 10 0 1150 2.5 - + _ - + _ _ + 0 194 10 0 1100 14.0 - + _ - + _ _ + 0 221 10 0 1050 20.0 + + - - + + _ Q 160 10 25 1200 2.0 - - - - _ _ _ + 0, Q 196 10 22 1150 2.0 - - _ - - _ + Q 236 10 26 1125 2.0 - - - - - _ _ + Q 154 10 20 1100 4.0 - + - - + _ _ + Q 217 10 26 1050 12.5 + + - - + _ _ + Q 189 10 29 950 20.0 + + - - - + _ + 0,Q 213 10 20 900 18.5 + tr - - - + + + 0 155 10 14 850 17.0 + - - - - + + + 0 264 10 25 800 72.0 + - - - - + + + 0 280 10 25 750 70.0 + - - + - + + + 181 10 14 700 20.0 + tr - + - _ + + 199 15 0 1250 2.0 - - - - - _ - + 0,Q 281 15 0 1225 2.0 - - - - - _ - + 0,Q 206 15 0 1200 2.2 - 7 - - + - - + 0,Q,Qz 205 15 0 1150 3.0 + + - - + - - + Q 220 15 0 1100 5.5 + + - - + + - + Q 299 15 0 1050 23.5 + + - - + + - + Q 306 15 0 1000 19.5 + + - - - + - + 0,Q,Qz 304 15 0 950 22.0 + + - - - + - + Q 197 15 29 1200 2.5 ------+ Q 215 15 30 1150 2.0 ------+ Q 218 15 30 1100 3.0 + + - - - - + Q 222 15 25 1050 3.0 + + - - - + - + Q 230 15 26 1000 6.0 + + - - - + - + Q/Qz 298 15 24 950 17.0 + + - - - + - + 0,Q 279 15 24 900 48.0 + + - - - + - + Q,Qopx 242 15 19 850 21.0 + tr - + - + + + Q 265 15 24 800 44.0 + - - + - + + + Q, Qopx 261 15 28 800 30.0 + - - + - + + + Q.Qopx 276 15 22 7 50 41.5 + - - + - tr + tr Qopx 165 20 0 1300 1.0 ------+ O/Q 170 20 0 1250 2.0 ------+ Q 302 20 0 1200 11.0 - + - - - - - + O/Q 178 20 0 1150 11.5 tr + - tr - - - + Q 183 20 0 1150 1.0 + + - + - - - + 0 184R 20 0 1300 0.5 ( ------+ ) 20 0 1150 1.0 + + - - - - - + 0,Q 201 20 0 1100 12.0 + + - + - + - + O/Q 307 20 0 850 72.0 + + - + - + - 4- Q 313 20 2 1150 5.0 + + - - - - - + 308 20 8 1150 5.0 ------+ Q 161 20 33 1200 2.0 ------+ Q 297 20 27 1150 2.0 ------+ Q 278 20 28 1100 7.0 tr + + - - - - + Q 195 20 25 1050 5.5 + + + - - + - + O/Q 262 20 27 s 1050 11.0 + + tr - - + - + 0,Q 283R 20 26 1150 3.0 ( ------+ ) 20 26 1050 14.5 + + - - - + - + Q 232 20 25 1000 24.0 + + + + - + - + Q/Qz 226 20 24 1000 5.0 + + + + - + - 4- 0,Q 209 20 22 950 13.0 + + + + - + - + Q 231 20 — 30 900 24.0 + + - - - 4- - 4- 0 151 20 20 850 20.0 + + - + - + 4- 270 20 22 800 43.0 + + - + - + + + Qopx 182 20 21 750 25.0 + + - + - + 7 tr 303 20 25 600 62.5 + + - + - tr - - O/Qopx 208 25 0 1400 0.5 ------4- Q 207 25 0 1350 1.0 ------+ 0,Q 200 25 0 1300 2.5 ------4- O/Q 216 25 0 1300 2.0 ------+ 0,Q 229 25 0 1275 2.0 ------+ 0,Q 203 25 0 1250 2.0 - + - + - - - + O/Q 300 25 0 1150 7.0 - + - + - tr - + O/Q

The olivine basanite liquidus surface differs considerably from periment with 4 percent H20 at 20 kb, 1150°C, just below the liq- the olivine nephelinite liquidus surface in shape. Water-saturated uidus, produced amphibole + clinopyroxene, thereby fixing the liquidus temperatures in the two compositions are similar above 10 configuration of the phase boundaries near 20 kb (Fig. 6). The large kb, but 0.4 percent HzO depresses the basanite liquidus to lower field depicted for primary orthopyroxene, expanding with increas- temperatures than does 0.9 percent H20 in the olivine nephelinite. ing pressure above about 17 kb, was not established experimen- The dry liquidus adopted for the basanite model is consistently tally. The field is drawn this way because orthopyroxene occurs lower in temperature than the dry nephelinite liquidus. The values through a wide temperature interval at and below the liquidus with of 0.4 and 0.9 wt percent H20 for the experimentally determined liq- excess water but was not found with only 0.4 percent H20 present. uidus contours (Figs. 2 and 4) are taken from chemical analyses of Green (1973b) reviewed previous experimental data on olivine the starting materials (Table 1); if these should ever be revised, then tholeiite, alkali basalt, basanite, and olivine nephelinite composi- labels on the contours should be revised accordingly. tions, which indicate that an orthopyroxene liquidus field expands Successive liquidus assemblages for low water contents with in- into nepheline-normative compositions with low water contents at creasing pressure are similar to those on the nephelinite liquidus, pressures above 10 kb. In a diagram corresponding to Figure 6, with the addition of a field for primary garnet near 30 kb. An ex- Green summarized unpublished data for liquidus phases on the

TABLE 3 {continued)

Run no. Pressure H 0 Temp Duration Phases observed among run products kbar added, °C hours «eight Hb Cpx Opx Ga 01 Ru Sph Gl Others Per Cent

211 25 0 1100 17. 0 + _ + _ + _ + 0,Q 285 25 0 1050 19. 0 + 4- - + - + + 0,Q 301R 25 0 1150 7. 2 ( - + - + - + _ + ) 25 0 1050 38. 0 + + - + - + - + 0 ,Q,Qz 295 25 0 900 41. 0 + + - + - + - + Q 238 25 36 1250 2., 0 ------_ + 0,Q 202 25 30 1200 2., 0 ------_ + Q 210 25 32 1150 2.. 0 ------_ + Q 224 25 31 1100 2.. 0 + Cr + - tr _ + Q 227 25 33 1050 3.. 0 - + + + - + _ + 0,Q,Qopx ? + - - 239 25 28 1000 6., 0 + + + + OtQ,Qz,Qopx 246R 25 31 1250 2.. 0 ( » - - - - - _ + ) 25 31 1000 20., 0 + + + - - + _ + 0,Q 244 25 30 950 22.. 0 4- + + + - + - + Qopx 237 25 27 900 18.. 0 + + + + - + _ + 243 25 24 850 23.. 0 + + ? + - + - + 0,Q 273 25 23 800 48.. 0 + + - + - + - + 0,Q 267 25 22 750 68.. 0 + + - + - + - ? Q, Qopx 186 30 0 1450 0.. 5 ------+ 0,Q 179 30 0 1400 0.. 5 ------+ Q 168 30 0 1350 1.. 0 - - - - - _ _ + Q 166 30 0 1300 1. 5 - + - + - - + O.Q.Qz 175 30 0 1150 3., 5 - + - + _ + _ + Q 172 30 0 1100 19., 0 - + _ + _ + _ + - 193 30 0 1050 19.. 0 + _ + _ + _ + Qz 287 30 0 900 42.. 0 - + _ + _ + _ + Q,Qz 190 30 33 1250 3.. 0 - - _ _ _ _ _ + Q 163 30 34 1200 2,. 2 - ______+ Q 164 30 33 1150 2,. 0 - - _ _ _ _ + Q 159 30 33 1100 2.. 0 - + tr + _ tr _ + Q,Qz 240 30 33 1000 6,. 0 - + + + - + _ + O.Q.Qz 248 30 27 950 17.. 0 - + + - + - + 0,Q,Qz 157 30 24 900 16.. 0 - + + + - + - ? Q,Qz 277 30 23 850 50.. 0 - + + + - + _ ? Q.Qopx 153 30 20 800 30 .0 - + •> + - + _ ? + 198 30 18 700 17 .0 + 4- - + - - Q

Runs in Pd capsules ?0 A930 260R 10 28 850 19 .0 ( + _ _ _ + + + )1 10 28 600 96 .0 + - _ _ _ + _ PI? 180 20 0 1300 1 .0 - - _ _ _ _ _ • Q 173 20 0 1250 2 .0 + + _ + _ _ _ + Q 185 20 0 1150 20 .0 + + - _ _ _ + Q 188R 20 0 1300 0 .5 ( - ______+> 20 0 1150 1 .0 + + _ _ _ + _ Q 255 20 32 1150 3 .0 - - _ _ _ _ _ + O, Q 282 20 32 1100 2 .0 - - - _ _ _ _ + Q 284 20 33 1075 4 .0 + + - + _ + _ + O.Q.Qz 247 20 24 1050 7 .3 7 + - + _ + _ + 0,Q 249 20 27 1000 6 .5 + + _ + _ + _ + 0 258R 20 32 1225 3 .0 ( - ______+) + 20 32 1000 21 .0 + _ + _ + _ + 0,Q 253 20 26 900 7 .0 + + - + _ + . _ + 0 256 20 23 850 22 .0 + + _ + _ + _ + 0 254 20 24 800 21 .0 + + _ + „ + _ + 0,Q 257 20 16 700 27 .0 + + + _ + _ + 0 176 30 0 1150 9 .0 - + - + - + - + Q,Qz

"R" denotes two-stage run; parentheses ( ) enclose assemblage presumed to have been stable at run conditions during the first stage; "tr", trace; "?", possible; "+", present; "-", absent.

"Hb", amphibole; "Cpx", clinopyroxene; "Opx", orthopyroxene; "Ga", garnet; "01", olivine; "Ru", rutile; "Sph", sphene; "Gl", glass quenched from liquid; "0", opaques; "Q", phases formed during quench, not stable at run conditions: amphibole, clinopyroxenes, rutile, mica; "Qz", quartz crystallized from vapor phase; "Qopx", quench orthopyroxene; "PI?", possible plagioclase.

"''Assemblage presumed to have been approximately that of Run No. 155 on completion of the first stage.

water-undersaturated surface of an olivine-enriched natural basa- it reacts out with decreasing temperature. Thus the volume for or- nite composition (Table 6), based on liquidus contours for 1, 2, 5, thopyroxene in P-T-Xh2o space for Green's composition is a shal- and 10 percent HaO, which were well located between 20 and 30 low wedge right at the liquidus surface. Contrast the wide tempera- kb but were only approximate for other pressures. The shape of ture interval for orthopyroxene below the water-saturated liquidus Green's liquidus surface differs considerably from that in Figure 6. surface for the olivine basanite composition in Figure 3. At pressures below about 20 kb, liquidus temperatures are higher. Millhollen and Wyllie (1970, 1974) did not observe orthopyrox- The surface is higher by about 100CC at 10 kb, for all water con- ene within the melting interval of brown hornblende mylonite from tents, and the field for primary olivine extends to pressures greater St. Paul's Rocks (Table 6) to 30 kb, either with excess HaO or with than 30 kb on the water-saturated liquidus boundary. There is no only 2 percent H20. amphibole recorded, perhaps because of the higher temperature of the liquidus surface, and the field for primary orthopyroxene is AMPHIBOLE STABILITY IN BASALT-WATER SYSTEMS limited to a small, narrow area between 24 to 29 kb and 1200° to 1325°C, with H20 contents between about 1.5 percent and 10 per- Figure 7 compares published curves for the maximum stability of cent. Green reported that where orthopyroxene is a liquidus phase, amphiboles in water-saturated basic liquids. Amphiboles appear to

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have greater P-T stability ranges in nephelinites and basanites than in basalts. Curves 1 and 2 are taken from Figures 1 and 3, respectively, and curve 3 is for brown hornblende mylonite from St. Paul's Rocks (Millhollen and Wyllie, 1970, 1974). Curves 1 and 3 were determined by reaction of low-pressure crystalline starting materials, 30 with consistent brackets obtained using two-stage reversal runs. Curve 2 was determined by reaction of the kaersutite eclogite starting assemblage, which contains "seed" crystals for both the high- pressure and low-pressure mineral phases. Results are consonant with curves 1 and 3. Curve 4 was reported by Allen and Boettcher o (1973) for an olivine nephelinite compositionally similar to the -Q kaersutite megacryst (Table 6). The pressure maximum at 1035°C, 31.5 kb, is based on reversal experiments, but the reversal tech- * 20 nique employed has not been published. Allen and Boettcher con- O) cluded that amphiboles may be stable to higher pressures in basic =3 rocks with lower silica activities. Haygood and others (1971) and cn Allen and others (1972) presented curve 5 for amphibole in an alk- CO ali olivine basalt, with shape similar to that of curve 4 but with CD pressure maximum at 1040°C, 24.5 kb. Curve 6 was presented by ¿ Essene and others (1970) for a synthetic alkali olivine basalt com- 10 position (the same as that used by Green and Ringwood, 1967b; see Table 6). They used starting materials of glass and mixtures of previously crystallized eclogite and amphibolite and found a wide interval for the reversal limits; curve 6 is the upper boundary of the band of uncertainty. Lambert and Wyllie (1968, 1972) located curve 7 near the solidus of a high-alumina olivine tholeiite, using crystalline starting material. Hill and Boettcher (1970) reported 0 curve 8 for a crystalline olivine tholeiite, but Allen and Boettcher (1971) and Allen and others, (1972) later revised this to curve 9 at 800 1000 1200 significantly higher temperatures. The most restricted amphibole stability is in the most silica-rich rock, curve 10 for a quartz tholeiite (Allen and others, 1972). Temperature °C We do not know why the experimental and analytical techniques Figure 7. Experimentally determined stability limits ("hb-out" field employed by Boettcher and associates produce amphibole stability boundaries) for calcic amphiboles in equilibrium with water-saturated curves with a pressure maximum, in contrast with curves deter- basaltic liquids. (1) Kaersutite megacryst, this paper; (2) kaersutite eclogite nodule, this paper; (3) brown hornblende mylonite (Millhollen and Wyllie, mined in this laboratory. Green (1973a), in his studies of am- 1970, 1974); (4) olivine nephelinite (Allen and Boettcher, 1973); (5) alkali phibole peridotite, noted that some amphibole prisms with optical olivine basalt (Haygood and others, 1971; and Allen and others, 1972); (6) characteristics of stable amphibole have compositions indicating alkali olivine basalt (Essene and others, 1970); (7) high-alumina olivine that they grew from liquid during the quench. tholeiite (Lambert and Wyllie, 1968, 1972); (8) kilauea 1921 olivine Host liquid compositions affect maximum amphibole stability tholeiite (Hill and Boettcher, 1970); (9) kilauea 1921 olivine tholeiite (Allen temperatures. Review of available data indicates that at 20 kb, and Boettcher, 1971; and Allen and others, 1972); (10) quartz tholeiite and to a lesser extent at 10 kb, amphiboles persist to higher tem- (Allen and others, 1972). peratures in systems with greater Ti02 contents but dissolve at lower temperatures with increasing Si02 contents and with increas- Kushiro (1970, 1972) inferred from experiments in the ing Na20/(Na20 + K20) ratios. No correlations could be found Mg0-Ca0-Na20-Al203-Si02-H20 system that partial melting of with variations in A1203 contents or other more complex chemical mantle peridotite with small amounts of H20 at depths >50 km parameters. ( 15 kb) produces nepheline-normative liquids, although higher H20 contents or increased degrees of melting lead to silica-satu- ORIGIN OF NEPHELINITIC AND rated liquids. Kushiro and others (1971) determined by micro- BASANITIC MAGMAS probe analysis that glass, produced by melting between 10 and 15 percent of a natural garnet-peridotite nodule containing 2 wt per- From results of reconnaissance experiments with hydrous olivine cent fluorine-rich phlogopite, at 30 kb (100 km), corresponds to nephelinite and picritic nephelinite compositions (Bultitude and olivine-rich basanite in composition. Green, 1968) and of experiments with hydrous olivine-rich basa- Modreski and Boettcher (1973) concluded that partial fusion of nite, Green (1969, 1970a, 1970b, 1971, 1972, 1973b) concluded phlogopite-peridotite produces quartz-normative liquids at depths that basanitic and nephelinitic liquids containing between 2 and 5 wt of 30 to 50 km but yields increasingly silica-undersaturated, percent H20 can be generated within the upper mantle by either (1) alkali-rich liquids at great depths. Mysen and Boettcher (1973) re- melting as much as about 6 percent of hydrous mantle peridotite ported that partial melting of four peridotites in the presence of (the pyrolite composition of Ringwood [1966] with 0.2 percent H20-C02 vapor with XH20 >0.5 produces quartz-normative H20) at depths between 80 and 100 km, leaving more refractory liquids up to 250°C above the solidus and to pressures >25 kb (at residual peridotite consisting of olivine, orthopyroxene, least 85 km). clinopyroxene, and garnet (Green, 1973a, 1973b) — with increas- Thus, there is considerable disagreement concerning the range of ing degree of partial melting, the liquid changes from nephelinite to mantle conditions necessary for the formation of nepheline- basanite; or (2) partial melting of mantle peridotite to yield a hy- normative or quartz-normative liquids by partial fusion of perido- drous picritic liquid, followed by fractional crystallization of or- tites in the presence of water. The arguments were reviewed in a thopyroxene, minor olivine, and possibly some garnet or clino- single volume by Boettcher (1973), Green (1973c), and Kushiro pyroxene, at depths greater than about 80 km. (1973). Green (1973c) concluded that analyses of glasses quenched

TABLE 4. MICR0PR08E ANALYSES OF GARNETS AMONG PRODUCTS OP EXPERIMENTS WITH THE KAERSUTITE ECLOGITE NODULE

T, C 1250 1100 1100

p, kbar 25 25 25

HjO pres 0.4 8.2 8.2

Run duration, hrs. 0.25 0.25 2

Run No. 289 291 292 start material * * * * Coexisting condensed phases cpx , L cpx, ru , opx, L cpx , ru, opx, L cpx , hb

Weight Percent

39.3 39.6 39.9 (39.4)

0.4 0.4 0.2 0.5

23.0 22.8 22.6 20.2

FeO 20.8 20.7 20.8 22.8

MgO 11.0 10.8 10.2 10.3

cao 6.3 6.1 6.0 6.8

0.0(4) 0.0(7) 0.1(1)

n.d. n.d. 0.0(8)

100.8 100.4 99.8 (100.0)

100 Mg/Mg + Fe

Cations proportional to 12 Oxygens

Si 2.942 2.968 2.997 IV Al 0.058 0.032 0.003 VI Al 1.973 1.98 5 2.002 1.808

Ti 0.020 0.02 0 0.014 0.029

Fe+3 0.028 0.120

Mg 1.222 1.202 1.148 1.168

Ca 0.506 0.491 0.484 0.554

Fe+2 1.272 1.300 1.311 1.330

Fe reported as FeO. n.d. = sought, but not detected. 2. Central portion of grain,

t = total includes 0.030 Na. * = coexisting phase analyzed. 3. Margin of grain.

1. Central portion of grain. 4. partial analysis of garnet in starting material. Mason, 1968b, Table 4.

from experiments are not reliable and emphasized that liquidus simple partial melting of hydrous mantle peridotite or by or- phases precipitating from liquid produced by partial melting of a thopyroxene + olivine fractionation of a parent magma. For a simi- rock must be identical in species and composition to minerals in the lar composition enriched in olivine, the primary olivine field in Fig- source rock at the same P-T conditions (Green, 1973a, 1973b). ure 6 could extend to higher pressures, possibly enlarging the range Liquids derived from mantle peridotite by small degrees of partial of P-T conditions under which olivine and orthopyroxene coexist melting should have olivine and orthopyroxene as liquidus phases on the liquidus. and possibly clinopyroxene and garnet (or spinel) as well. We conclude that liquids with the specific nephelinite and basa- Orthopyroxene does not occur on or near the liquidus of the nite compostions of Figures 5 and 6 cannot be produced directly by olivine nephelinite composition corresponding to the kaersutite partial melting of hydrous mantle peridotite, and that similar megacryst (Fig. 5), and olivine is stable only at pressures below magmas probably have evolved through differentiation of parental about 22 kb (80 km). According to this evidence, neither partial magmas by crystal fractionation and other processes. We have no melting of mantle peridotite nor orthopyroxene fractionation of a evidence that supports proposals that deep-seated orthopyroxene parent picritic magma is important during evolution of magma of fractionation plays an especially important role in such processes. this composition. For both compositions, the occurrence of amphiboles as pheno- For the olivine basanite composition corresponding to the kaer- crysts or as cumulate crystals, implying crystallization on or near sutite eclogite nodule, olivine and orthopyroxene coexist on or near the liquidus, is restricted to liquids containing at least 2 to 3 percent the liquidus only near 19 kb (Figs. 4 and 6), within a small, dissolved H20. Late crystallization of (postcumulus) amphibole in- water-rich field for hb + ol + cpx + opx. It is doubtful, therefore, terstitially between olivine and pyroxene occurs in liquids initially that significant amounts of this liquid can be produced either by containing less H20 (Figs. 2 and 4).

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TABLE 5. MICROPROBE ANALYSES OF CLINOPYROXENES AMONG PRODUCTS OF EXPERIMENTS WITH THE KAERSUTITE ECLOGITE NODULE

T, °C 1250 1100

P, kbar 25 25

HjO present, weight percent 0 4 8.2

Run duration, hrs. 0 25 2

Run no. 289 292 start material

* * * * Coexisting condensed phases ga L ga , ru, opx L ga , hb

Weight Percent 1 2 3 4 5

sio2 50.0 47.8 51.3 50.9 (49.4)

T i02 1.7 2.4 1.7 1.0 1.5

AI2O3 8.4 8.7 8.3 7.0 8.4

FeO 9.9 10.1 9.7 6.6 10.8

Mgo 9.9 12.8 10.2 14.8 10.5

Cao 16.8 16.1 15.8 16.6 16.4

Na20 2.8 2.8 2.7 2.0 3.0

n.d. n.d. K2° n.d. n.d.

Total 99.5 100.7 99.7 98.9 (100.0)

100 Mg/Mg + Fe 63.9 69.4 65.3 80.0 63.4

Cations Proportional to 6 Oxygens'^

Si 1.862 ) 1.767 , 1.891 1.874 1.862 ) IV 2.00 2.00 | 2.00 2.00 > 2.00 A1 0.138 ) 0.233 | 0.109 0.126 0.172 ) VI A1 0.230 0.146 0.252 0.175 0.194

Ti 0.047 0.067 0.046 0.028 0.042

+3 Fe 0.077 0.078 0.074 ) 0.051 ) 0.081 1 +2 Fe 0.232 1.99 0.234 2.07 0.224 ! 1.95 0.153 ¡2.02 0.253 1 2.01

Mg 0.547 0.708 0.562 ) 0.814 ) 0.579 )

ca 0.669 0.637 0.634 0.654 0.650

Na 0.205 0.199 0.164 0.144 0.215

Fe reported as FeO. n.d. = sought, but not detected. 2. Margin of grain (1).

2 2 3 t = Fe /{Fe + Fe ) assumed to be 0.758. 3. Central portion of grain remnant from start material.

* = coexisting phase analyzed. 4. Central portion of grain grown during run.

1. Central portion of grain. 5. Partial analysis of clinopyroxene in starting material. Mason, 1968b, Table 4.

Similarly, we note that orthopyroxene apparently was not in- percent dissolved H20. Green concluded that a primary olivine volved in evolution of the basanite composition represented by the basanite magma, formed under these specific conditions of depth, brown hornblende mylonite from St. Paul's Rocks (see Millhollen temperature, and water content, "could have dropped olivine prior and Wyllie, 1974, on Table 6), whether this resulted from direct fu- to picking up mantle xenoliths at 10-20 kb," and thus could have sion of mantle rocks or from deep-seated differentiation of a parent attained the composition of basanite erupted at the surface. magma. Chemical variations among basanites and nephelinites are irregu- In his search for a basanite possessing liquidus minerals consist- lar (shown by Table 6 and the thirteen analyses compared by ent with an origin by partial fusion of the mantle, Green (1973b) Bultitude and Green, 1971). We have reviewed evidence that three postulated that such magmas, formed at 25 to 30 kb, should con- of these compositions (Figs. 5 and 6; Millhollen and Wyllie, 1974) tain about 25 percent normative olivine. Therefore, in constructing were produced neither by partial fusion of mantle peridotite nor by an experimental starting material, he added 10 percent olivine to a deep-seated orthopyroxene fractionation of a picritic parent natural basanite, which increased its normative olivine content to magma. Green (1973b) concluded that a natural basanite had 28.4 percent (Table 6). He observed a limited area on the liquidus evolved through deep-seated olivine fractionation of an olivine-rich surface (corresponding to Fig. 6) of this mixture where the mantle basanite liquid (Table 6), which resulted from partial fusion of minerals olivine, orthopyroxene, clinopyroxene, and garnet do mantle peridotite at deeper, more specific conditions. coexist, between 25 and 30 kb, 1200° to 1300°C, and 2 to 7 wt These results suggest to us that many natural nephelinite and

basanite magmas are produced by differentiation of hydrous pic- 1200°C. This pressure interval is consistent with minimum esti- ritic magma though deep-seated fractionation of olivine, possibly mated pressures for crystallization and subsolidus re-equilibration with clinopyroxene and garnet but excluding orthopyroxene. of the minerals in the eclogite nodule. Green's (1973a, 1973b) evidence suggests that some basanitic Some of the smaller kaersutite megacrysts may have crystallized magmas can be generated by partial fusion of deep, hydrous mantle from the host magma during ascent at pressures between 24 and 13 peridotite; but the small volume available below the liquidus for kb, but it is unlikely that sufficient time was available during the orthopyroxene in P-T-Xmo space implies a limited role for or- rapid ascent for growth of the large kaersutite megacrysts. The thopyroxene fractionation in any subsequent evolution of small phenocrysts of olivine and titaniferous augite in the breccia nephelinitic or basanitic liquids. matrix (Dickey, 1965) probably crystallized from the rising nephelinite magma at pressures <20 kb (Fig. 5). ORIGIN OF THE MINERAL BRECCIA AT KAKANUI Petrogenetic History

Correlation of these experimental results with deductions from The following Sequence of events is consistent with the petrology the mineralogy arid petrology of the Mineral Breccia permits con- and with the experimental results: struction of a model describing origin of the breccia. An alkali basaltic magma (possibly basanite or nephelinite) with dissolved water rose through the mantle, fractionating by precipita- Petrological Conclusions tion of pyrope-rich garnet and omphacitic pyroxene. Cumulate crystals of garnet and clinopyroxene, which precipitated at temper- The liquid, which erupted to form the matrix of the Mineral atures between 1300° and 1200°C in the depth interval 75 to 85 km Breccia, was described by Dickey (1968; 1972, written commun.) (22 to 24 kb), became trapped in pockets with intercumulus liquid. as a melanephelinite, probably similar in composition to the kaer- These isolated pockets subsequently cooled toward the normal sutite megacrysts (Tables 1 and 6). The explosive eruption and the geotherm, while the intercumiilus liquid approached basanite or prominence of hydrous minerals indicate that the magma was vol- nephelinite in composition, reacting with the clinopyroxene and atile rich. forming, postcumulus kaersutite when the temperature reached Mineralogy arid petrology of the kaersutite eclogite nodules and about 1150°C (Fig. 6). Before crystallization was complete at about the megacrysts in the breccia were reviewed by Merrill (1973). I 750°C (Fig. 3), the eclogitic crystal mush was locally intruded into concluded that the nodules represent a sequence of high-pressure the solid mantle host, lherzolite. The kaersutite eclogite assemblage cumulates from afractionating magma different from that involved re-equilibrated subsolidus. in the eruption. Pockets of cumulus garnets and pyroxenes with Later, another rapidly ascending magma, nephelinite with about volatile-rich alkalic intercumulus liquid became isolated from the 5 wt percent dissolved H20, caught up fragments of the mantle magma. Upon cooling, intercumulus liquid converted some from the depth interval 75 to 85 km and carried them to the sur- clinopyroxene to aniphibole and then precipitated as postcumulus face. The inclusions that reached the surface are derived from kaersutite around the eclogite minerals. Before complete depths no greater than the depth of final equilibration between the solidification, some of the eclogitic cumulates intruded crystalline transporting liquid and its phenocrysts. The nephelinite host lherzolite, assumed to be host mantle rock. Exsolution of or- magma was in equilibrium with the megacrysts of garnet and thopyroxene and garnet from the clinopyroxene indicates that, clinopyroxene and probably also with the large kaersutite mega- after complete crystallization, the ecologite experienced low- crysts at a minimum depth of 75 km and temperature between temperature, high-pressure re-equilibration. Minor partial melting 1100° and 1200°C. Kaersutite eclogite fragments carried to the sur- of the nodules is attributed to decompression and heating as the ec- face in this hot liquid experienced partial melting (Figs. 3, 4, and 6). logite subsequently was carried to the surface by the melanepheli- Small kaersutite crystals were precipitated at depths between 75 nite magma. and 50 km, and precipitation of olivine and titaniferous augite The megacrysts of garnet, clinopyroxene, and kaersutite must probably continued as the magma rose from 65 km to near the sur- have experienced histories different from the nodules, because they face, where explosive eruption occurred. display no evidence of low-temperature re-equilibration. Merrill (1973) concluded that these probably are phenocrysts, cognate to OTHER GEOLOGIC APPLICATIONS the host magma that carried the nodules to the surface. Calcic amphiboles similar to those in the Kakanui Mineral Brec- Conclusions from Experimental Results cia occur commonly as components of inclusions within alkali basalts throughout the world and are conspicuous among inclu- Garnets and clinopyroxenes similar to the cumulate crystals in sions in alkali basalts of island-arc volcanic provinces. Results of the kaersutite eclogite nodule could precipitate from nephelinitic or experiments with the kaersutite megacryst and with the kaersutite basanitic magma containing a few weight percent dissolved HzO at eclogite assemblage suggest that calcic amphibole phenocrysts pressures above 20 or 25 kb, and at temperatures of 1200°C or found in nephelinitic or in basanitic basalts probably have crystal- higher (Figs. 5 and 6), or from an anhydrous alkali olivine basalt at lized from water-rich magmas shortly before their eruptions, at similar pressures (Green and Ringwood, 1967b). depths between about 40 and 80 km within the upper mantle (Figs. The mineral assemblage of the eclogite nodule re-equilibrated 5 and 6). The consequent conclusion, that typical water contents of subsolidus at depth, probably in the presence of water evolved dur- alkali basalt magmas within the upper mantle are high, is further ing the closing stages of crystallization. The mineralogy is most supported by observations of the high volatile contents and of the closely duplicated in the water-excess experiments (Fig. 3) at tem- rapid ascents, terminated by energetic and often explosive erup- peratures between 700° and 800°C in the pressure interval 20 to 30 tions, which seem to be characteristic of inclusion-bearing alkali kb. basalt magmas. Figure 5 shows that the large garnet, clinopyroxene, and am- Results of the experiments (Table 3) with the kaersutite eclogite phibole megacrysts could have crystallized simultaneously from the nodule from Kakanui provide information that may be useful in host nephelinite magma only under specific conditions: from interpreting relationships among similar eclogite lithologies, as- magma with more than 5 wt percent dissolved H20, at pressures sociated mafic and ultramafic rocks, and host magmas elsewhere. between 22 and 24 kb and temperatures between about 1100° and Possible applications include eclogites from nodules in alkali

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TABLE 6. SILICA-UNDERSATURATED BASALTIC COMPOSITIONS STUDIED IN HIGH-PRESSURE ROCK MELTING EXPERIMENTS

OLIVINE NEPHELINITES BASANITES

Weight per Cent Kakanui Bultitude & Allen & Kakanui Green & Ringwood, Ito & Kennedy, Green, 1973b Millhollen & recalculated Kaersutite Green, 1971 Boettcher, Kaersutite Eclogite 1967b 1968 Ol. Basanite Wyllie, 1974 volatile-free K—1 Glass 1973 K-14 Alkali 01. Basalt DH4 Glass MyIonite SE-13

Si02 41.5 44.3 38.9 41.4 45.4 45.0 44.7 38.0

Ti02 4.5 1.5 2.8 3.2 2.5 3.4 2.9 4.1

14.4 14.2 11.8 AX2°3 16.4 14.7 14.7 11.7 17.8

Fe203 ~ 0.5 5.3 2.9 1.9 5.1 1.7 2.9 * FeO 10.4 9.7 7.8 11.5 12.4 7.4 10.5 9.2

MhO 0.1 0.2 0.1 0.2 0.2 0.2 0.2 0.1

MgO 12.8 13.3 13.2 11.0 10.4 7.5 13.9 6.7

CaO 10.4 11.2 13.0 9.7 9.1 9.1 7.7 13.8

Ua20 2.8 3.6 4.3 2.3 2.6 4.2 3.7 4.0

K2O 2.2 1.0 1.2 0.9 0.8 2.3 2.0 0.8

— 0.5 1.1 0.1 P2°5 0.02 1.1 1.0 2.7

Total 99.1 100.0 99.5 99.6 100.0 100.0 100.0 100.1

100 Mq 69 71 75 63 Mg + Fe^ 60 64 70 56

C.I.P.W. NORMATIVE MINERALOGIES

or 5.2 6.1 0.6 5.1 4.5 13.3 11.8 4.9

ab — 2.0 ~ 7.9 18.0 15.7 11.5 2.2

an 20.0 19.4 9.2 31.5 26.2 14.5 9.6 28.2

lc 6.2 — 5.1 - ~ — — ~

ne 12.8 15.3 19.1 6.4 2.2 10.8 10.5 17.1

di 24.7 26.4 36.4 12.3 15.7 18.3 17.7 17.5

ol 17.3 25.9 14.3 25.2 25.8 11.6 28.4 11.7

ap -- 1.3 2.6 0.3 — 2.7 2.4 6.5

mt 4.5 0.7 7.5 4.2 2.9 7.4 2.6 4.1

il 8.5 2.9 5.2 6.1 4.8 6.5 5.5 7.8

2 2 * Total Fe reported as FeO. Assumed Fe /(Fe + Fe = 0.784 in calculating normative mineralogy. basalts, from nodules in kimberlite breccias, from members of fellowship held by R. B. Merrill. Brian Mason provided the some alpine-type ultramafic complexes, and from basal members of analyzed samples. R. J. Williams, J. R. Holloway, and D. H. Eggler some ophiolite complexes. contributed constructive criticisms which were received with ap- Experimental data rarely are sufficient within themselves to re- preciation. solve all questions concerning possible origins of natural assemblages. More commonly, such data supply constraints which, in REFERENCES CITED concert with information gathered from other sources (for example, results of stratigraphie, chemical, pétrographie, and structural Allen, J. C., and Boettcher, A. L., 1971, The stability of amphiboles in investigations), assist in establishing relative plausibilities of alter- basalts and andesites at high pressures: Geol. Soc. America, Abs. with native possibilities. We emphasize the uncertainties inherent in ex- Programs (Ann. Mtg.), v. 3, no. 7, p. 490. perimental data obtained on natural rocks with volatiles in piston- 1973, Phase relations and the stability of amphiboles in an olivine cylinder apparatus and note that specific applications should be nephelinite at high pressures [abs.]: EOS (Am. Geophys. Union made with caution. Trans.), v. 54, p. 481. Allen J. C., Modreski, P. J., Haygood, C., and Boettcher, A. L., 1972, The ACKNOWLEDGMENTS role of water in the mantle of the Earth: The stability of amphiboles and micas: Internat. Geol. Cong., 24th, Montreal 1972, Proc., sec. II, p. 231-240. Research was supported by the Earth Sciences Section, National Bell, B. P., and Williams, D. W., 1971, Pressure calibration in piston- Science Foundation Grant GA—32266X. The National Science cylinder apparatus at high temperature, in Ulmer, G. C., ed., Research Foundation provided general support of the Materials Research techniques for high pressure and high temperature: New York, Laboratory, and the Fannie and John Hertz Foundation provided a Springer-Verlag New York, p. 195-215.

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