Amphibole Stability Research Articles

Experimental studies to 10 kb of the bulk composition tremolite50-tschermakite50+excess H2O

Phase relations for the magnesio-hornblende bulk composition, 2 CaO·4 MgO·Al2O3·7 SiO2+ excess H2O, have been investigated to 10 kb employing hydrothermal and piston-cylinder techniques. The low-temperature limit of amphibole in this system lies at 519° C, 1,000 bars, 541° C, 2,000 bars, and 718° C, 10 kb. The low-T assemblage consists of an+chl+di+tc(+f), and is related to the adjacent high-T equilibrium assemblage, amph+an+chl+f, by the solid-solid reaction (A): 2 di+tc=tr. Small amounts of aluminum, hypothesized to be preferentially dissolved in the cpx (and in the tc) relative to amph, may account for the broad P-T stability range of the di+tc assemblage in the synthetic work relative to systems involving stoichiometric tr, ∘Ca2Mg5Si8O22(OH)2, such as are common in natural, Al-poor calc-silicate parageneses. Alternatively, the low-temperature assemblage produced in the experiments may be metastable. For the investigated bulk composition, synthetic tremolitic-cummingtonitic amphibole contains relatively modest amounts of ts, ∘Ca2Mg3Al2IVSi6-Al2IVO22(OH)2; at pressures of 1,000–3,000 bars, solid solution extends from near tremolite only to about cu11tr69ts20, analogous to most analyzed natural magnesio-hornblendic specimens. At 10 kb fluid pressure, the solid solution reaches approximately cu06tr53ts41 for the investigated bulk composition, and appears to be virtually independent of temperature. Amphibole and 14 A chl react within the amphibole stability field, along curve (B), at about 704° C and 2,000 bars, to produce an, en, fo and f (ΔH=40.9 kcal/ mole); at pressures greater than approximately 7kb, due to the incompatibility of an and fo, the higher temperature assemblage consists of amph, an, en, sp and f. Above Pfluid−T curve (B), the amphibole coexists with an+en+fo+f at low pressures; at higher pressures, the amphibole, which is in equilibrium with an+en+sp+f, is relatively more aluminous. The high-T stability limit of aluminous tr+fo lies approximately 20–25° C below the dehydration curve for stoichiometric tremolite on its own bulk composition. Reaction (C), tr+fo=2 di+5 en+f (ΔH = 39.4 kcal/mole), produces an+di+en+f, the highest temperature subsolidus assemblage investigated for the tr50ts50 bulk composition. Hydrous melt is encountered at temperatures at least as low as 900° C at 10 kb, and at that fluid pressure coexists with amphibole over an interval of more than 60° C. Limited solid solution observed between tr and ts in nature (tr100-70) is accounted for by the restricted range of amphibole compositions produced in the present study. Such amphiboles, moreover, appear to have both high- and low-temperature stability limits, as demonstrated by the experimental results.

Stability and composition relations of calcic amphiboles in ultramafic rocks

Calcic amphiboles are observed in ultramafic rocks that have equilibrated under a broad span of geological conditions and might prove to be good indicators of metamorphic grade if their stabilities could be determined as a function of their compositions. Experiments were performed on the stability of tremolite plus forsterite in the system H2O-CaO-MgO-SiO2 from 5 to 20 kbar. A univariant curve was fitted to the experimental brackets using volume, water fugacity, and heat capacity data. The results indicate that the maximum stability of tremolite in the presence of forsterite is about 825° C at 5 kbar. Addition of Al2O3 to this system increases the stability of tremolitic amphibole by only 20°–40° C and induces solubility of 5–7 wt.% Al2O3 in the amphibole, as determined from quantitative SEM analyses of individual amphibole crystals. Thus substitution of the tschermakite component (Ca2(Mg3Al2) (Si6Al2) O22(OH)2) alone cannot lead to the greatly enhanced Al2O3 contents or thermal stability of natural calcic amphiboles.

The role of apatite in mantle enrichment processes and in the petrogenesis of some alkali basalt suites

Analyses of Sr and REE in apatites from a variety of mantle-derived parageneses are used in conjunction with trace element data from the literature to investigate relationships between alkali basalts and apatite-rich materials in upper-mantle source regions. Despite difficulties in interpretation, positive P-anomalies in the hygromagmatophile element abundance patterns of some continental primary alkali basalts suggest either P-enrichment of their source or assimilation of P-rich material, or both. Amphibole- and apatite-rich xenoliths occur in several alkali-basalt provinces, and by virtue of the P and LREE enrichment represent a probable source of the P anomalies and part of the other trace element enrichments of these magmas. Incorporation of such apatite-rich materials by later primary magmas would be enhanced by the high P 2O 5 concentrations required to achieve apatite saturation in basaltic liquids. In the early stages of mantle diapirism an undersaturated magma, produced by slight partial melting of garnet peridotite, might fractionate as it rises to the range of amphibole stability. Hygromagmatophile element patterns of clinopyroxenite xenoliths indicate that clinopyroxene fractionation could produce P-enriched liquids which might subsequently crystallize amphibole- and apatite-rich materials now represented by xenoliths. During generation of later primary magma, apatite-rich materials might preferentially contaminate the liquids, to yield positive P-anomalies. This model requires that magmas undergo prolonged fractionation at considerable depth (~ 100 km), a process which is apparently most probable in subcontinental environments. An apatite- and zircon-bearing mica-clinopyroxenite xenolith from Matsoku provides a link between the S. African MARID suite and amphibole and apatite-rich xenoliths from various alkali basalt provinces. Unusual REE patterns ( La N < Ce N < Nd N , Ce N/Y N −10 ) of apatites in this xenolith suggest a link between the MARID suite xenoliths and postulated pre-Karroo mantle metasomatism.

The formation of mantle phlogopite in subduction zone hybridization

Extrapolation and extension of phase equilibria in the model system KAlSiO4-Mg2SiO4-SiO2-H2O suggests that at depths greater than 100 km (deeper than amphibole stability), hybridism between cool hydrous siliceous magma, rising from subducted oceanic crust, and the hotter overlying mantle peridotite produces a series of discrete masses composed largely of phlogopite, orthopyroxene, and clinopyroxene (enriched in Jadeite). Quartz (or coesite) may occur with phlogopite in the lowest part of the masses. The heterogeneous layer thus produced above the subducted oceanic crust provides: (1) aqueous fluids expelled during hybridization and solidification, which rise to generate in overlying mantle (given suitable thermal structure) H2O-undersaturated basic magma, which is the parent of the calc-alkalic rock series erupted at the volcanic front; (2) masses of phlogopite-pyroxenites which melt when they cross a deeper, high-temperature solidus, yielding the parents of alkalic magmas erupted behind the volcanic front; and (3) blocks of phlogopite-pyroxenites which may rise diapirically for long-term residence in continental lithosphere, and later contribute to the potassium (and geochemically-related elements) involved in some of the continental magmatism with geochemistry ascribed to mantle metasomatism.

Leucitites from Gaussberg, Antarctica

Leucitities of probable late Pleistocene age from the extinct volcano of Gaussberg on the coast of East Antarctica consist of microphenocrysts of olivine, diopside, and leucite in a largely glassy groundmass. Their extreme K contents and K/Na ratios, as well as high Rb, Ba, and possibly Ti, are compatible with an origin by small degrees of partial melting of phlogopite-rich mantle below the level of amphibole stability, whereas their enrichment in P, Sr, Zr, Nb, La, Ce, Pb, and Th indicates the presence of minor phases such as apatite in the source material. Although the Mg/(Mg+Fe) values (∼0.70) and relatively high Ni and Cr contents suggest that the Gaussberg lavas represent near-primary melts, some degree of fractionation involving an aluminous phase (probably garnet) may be necessary to produce liquids with atomic K>Al. Alternatively, an as yet poorly understood process such as wall-rock reaction, liquid immiscibility, or mantle metasomatism may have been a critical factor in the genesis of these unusual rocks. Gaussberg is situated on a passive continental margin and does not appear to be related to any other known area of Cainozoic volcanic activity.

Coexisting sodic and calcic amphiboles from high-pressure metamorphic belts and the stability of barroisitic amphibole

SummaryCompositions of glaucophanes and actinolite-hornblende solid solutions occurring in chemically similar metabasaltic rocks from blueschist terranes in east-central Shikoku, W. California, Valtournanche (W. Alps), and W. Liguria are compared. Chemical contrasts among coexisting Na and Ca amphibole pairs, which reflect disparate P-T histories under the presumed attendance of local equilibrium, include: Na contents are rather high among barroisitic hornblendes from the western and Ligurian Alps, as well as among high-grade tectonic blocks from California; in contrast, actinolitic amphiboles from both lower-grade Franciscan tectonic blocks and in situ schists and the blueschists of Shikoku are impoverished in Na relative to blue-green hornblendes. Sodic amphiboles contain less than 0. 5 Aliv per formula unit, whereas Alvi is very high; a situation reversed among calcic amphiboles. The Na + Ca contents ofglaucophanes are strongly clustered around the sum of 2.0 (i.e. A site vacant) whereas calcic amphiboles have a wider range with the A site variably occupied. No solvus has been detected within either sodic or calcic amphiboles under blueschist facies conditions. For low-grade metabasaltic parageneses, a miscibility gap separates these two amphibole groups; at relatively high grade such compositions have sodic calcic amphiboles of barroisitic type; this may mean that glaucophane + hornblende assemblages are metastable, accounting for textural relations indicating that the sodic amphibole typically did not grow at the same time as the barroisite. Ti, Mn, and K appear to be concentrated in calcic amphibole compared to coexisting glaucophane, probably in the M2, M4, and A sites, respectively.Contrasts in coexisting amphibole tie lines are thought to be a consequence of the fact that the parageneses of Shikoku and California reflect high P and very high P prograde P-T paths respectively, whereas those from Valtournache and W. Liguria show evidence of decompression recrystallization (or back-reaction) of high P (i.e. eclogitic) protoliths. Comparison of the inferred physical conditions operating during the production of these four contrasting paragenetic sequences allows the provisional assignment of a P-T stability region for barroisitic amphibole in metabasaltic rocks as: P 4–5 kb at c. 350°; P 5–7 kb at c. 450 °C.

The role of sphene as an accessory phase in the high-pressure partial melting of hydrous mafic compositions

In a high-pressure experimental study of reactions and possible melt products occurring in the deep continental crust or in subducted oceanic crust, sphene has been identified over a pressure range of 10–18 kbar and to temperatures of 1020°C. Sphene may be a refractory phase with up to 60% partial melting for hydrous mafic compositions. Sphene breaks down at lower pressure than the maximum pressure stability of amphibole in hydrous mafic compositions, and rutile rather than sphene is the important Ti-bearing accessory phase at pressures greater than 16–18 kbar. Sphene and rutile coexist to pressures as low as 14 kbar. This implies that amphibole eclogites containing primary sphene and no rutile have most likely formed at depths less than 45 km. The presence of minor sphene as a residual phase in equilibrium with low-Ti silicic liquids may have a marked effect on the REE distribution in derivative liquids. Thus melts in equilibrium with a garnet and sphene-bearing residuum may have less light-REE-enriched patterns than those predicted when garnet is a residual phase without coexisting sphene. This effect is modelled using REE patterns for sphenes from high-grade metamorphic terrains of western Norway. Both the new REE data and the experimental study have important implications for the genesis of low-Ti magmas formed in continental margins and island arcs.

Light element metasomatism of the continental mantle: The evidence and the consequences

The highly potassic volcanics of South West Uganda and the sodi-potassic volcanics of the West Eifel, Germany, represent two distinctive aspects of the highly alkaline volcanism characteristic of uplifted and rifted continental cratons. The sparse lavas are feldspathoidal clinopyroxenites and the associated explosive volcanism provides highly typical ultramafic nodules composed of dark mica + clinopyroxene ± amphibole with titano-magnetite, sphene and apatite. The Eifel suite also includes lherzolites, harzburgites, wehrlites and dunites. Metasomatic textures, plus a mineralogical and chemical transition between spinel lherzolite and alkali clinopyroxenite, suggest that mica clinopyroxenite nodules represent altered mantle. The existence of metasomatized mantle implied by these nodules is consistent with the nature of the magmatism and its geological context. Calculations show that alteration of anhydrous peridotite mantle to hydrous alkali clinopyroxenite would provide the density contrast required of the anomalous low-density mantle below these regions, producing the observed, geologically long-persistent uplifts. It is proposed that the metasomatism occurs during upward passage of mobile elements through the mantle, accompanying the volatile emission so evident in regions of alkaline volcanism. Close affinities between lavas and nodule suites suggest that the lavas are derived by partial melting of the metasomatized mantle. Highly potassic lavas are generated where the continental geothermal gradient is low, from phlogopite-rich mantle below the level of amphibole stability: sodi-potassic lavas are formed at higher levels, when amphibole-bearing mantle becomes involved in melting. In the oceans highly potassic lavas are not recorded. This accords with the present hypothesis, because under conditions of steep oceanic geotherms phlogopite will persitt to depths only slightly greater than that of amphibole: thus there will be very little opportunity for a layer of potassium-enriched mantle to develop, except possibly in the older and cooler parts of the oceanic lithosphere, near the margins.

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

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.

Melting of Gabbro (Quartz Eclogite) with Excess Water to 35 Kilobars, with Geological Applications

Crystalline, fine-grained, high-alumina olivine tholeiite with excess water was reacted in sealed platinum capsules in piston-cylinder apparatus between 10 and 35 kbar pressure. Runs were planned to determine the curve for the beginning of melting, but combining these with published results at lower pressures permitted delineation of the major features of the phase diagram through the melting interval. Amphibolite melts below 10 kbar; quartz eclogite melts above 25 kbar; between them is a melting interval dominated by the breakdown of amphibole and formation of garnet and jadeitic pyroxene. The results introduce two features for geological applications. The solidus changes slope at about 13.5 kbar; the amphibole maximum-stability curve changes slope in the interval 12.5-15 kbar to such an extent that the amphibole stability field is more restricted at high pressures than anticipated from previous studies. With free water, eclogite is Stable only at depths greater than about 70 km, and amphibolite is Stable in the deep crust. Amphibolite crust thickened in the depth range 40-60 km with aqueous pore fluid melts, forming a liquid enriched in silica and albitic plagioclase; this is a potential source of water-undersaturated liquids for batholiths. The upper boundary of the mantle low-velocity zone could be the boundary between rocks with interstitial amphibole and those with interstitial hydrous silicate liquid. In oceanic crust forming the upper part of a subducted lithosphere slab, it appears that most hydrous minerals dehydrate or melt before they reach 100 km depth. If so, dehydration of subducted oceanic crust does not supply water for andesitic magmatism beyond the arc-trench gap, nor contribute to the chemical variations recorded in andesites across arc complexes (K_2O, K/Rb).

Amphibole stability in H 2O-undersaturated calc-alkaline melts

The upper stability limit of amphibole in silicate melts has previously been shown to increase in temperature when the mole fraction of H 2O in the fluid phase in equilibrium with the melt was decreased from 1.0 to 0.5. New experiments on amphibole stability in melts of andesite composition show that the amphibole stability limit also decreases in temperature when mole fraction H 2O in fluid is less than 0.5; these experiments imply that there is an isobaric temperature maximum of stability. At 5.5 kb this maximum occurs at a melt composition of 4.5 percent H 2O for andesite composition. The maximum at 5.5 kb may be explained by a change in amphibole breakdown from an incongruent melting reaction to a dehydration reaction. Calculations in the pressure range 0–10 kb indicate that the isobaric temperature maximum of amphibole stability occurs at a mole fraction of H 2O in the fluid phase ranging from 1.0 to 0.4, depending on total pressure. The maximum may represent either a melting or a dehydration reaction. Calculations also suggest that above 8 kb amphibole is as stable in silicate melt when the mole fraction of H 2O in the fluid phase is 0.3 as when the mole fraction is 1.0. Such conditions affirm the possibility of hydrous partial melting of amphibole-bearing basalt in the earth's mantle to form the calc-alkaline series.