We performed decompression experiments to simulate the ascent of a phenocryst-bearing rhyolitic magma in a volcanic conduit. The starting materials were bubble-free rhyolites water-saturated at 200 MPa–800°C under oxidizing conditions: they contained 6.0 wt% dissolved H₂O and a dense population of hematite crystals (8.7 ± 2 × 10⁵ mm⁻³). Pressure was decreased from the saturation value to a final value ranging from 99 to 20 MPa, at constant temperature (800°C); the rate of decompression was either 1,000 or 27.8 kPa/s. In all experiments, we observed a single event of heterogeneous bubble nucleation beginning at a pressure P_N equal to 63 ± 3 MPa in the 1,000 kPa/s series, and to 69 ± 1 MPa in the 27.8 kPa/s series. Below P_N, the degree of water supersaturation in the liquid rapidly decreased to a few 0.1 wt%, the nucleation rate dropped, and the bubble number density (BND) stabilized to a value strongly sensitive to decompression rate: 80 mm⁻³ at 27.8 kPa/s vs. 5,900 mm⁻³ at 1,000 kPa/s. This behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite-H₂O system and in numerical simulations of vesiculation in ascending magmas. Similar degrees of water supersaturation were measured at 27.8 and 1,000 kPa/s, implying that a faster decompression rate does not result in a larger departure from equilibrium. Our experimental results imply that BNDs in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate $$ \left| {\left. {{\text{d}}P/{\text{d}}t} \right|} \right. $$ and the number density of phenocrysts, especially the number density of magnetite microphenocrysts (1–100 mm⁻³), which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation. Very low BNDs (≈1 mm⁻³) are predicted in the case of effusive eruptions ( $$ \left| {\left. {{\text{d}}P/{\text{d}}t} \right|} \right. $$  ≈ 0.1 kPa/s). High BNDs (up to 10⁷ mm⁻³) and bimodal bubble size distributions are expected in the case of explosive eruptions: (1) a relatively small number density of bubbles (1–100 mm⁻³) will first nucleate in the lower part of the conduit ( $$ \left| {\left. {{\text{d}}P/{\text{d}}t} \right|} \right. $$  ≈ 10 kPa/s), either at high pressure on magnetite or at lower pressure on quartz and feldspar (or by homogeneous nucleation in the liquid) and (2) then, extreme decompression rates near the fragmentation level ( $$ \left| {\left. {{\text{d}}P/{\text{d}}t} \right|} \right. $$  ≈ 10³ kPa/s) will trigger a major nucleation event leading to the multitude of small bubbles, typically a few micrometers to a few tens of micrometers in diameter, which characterizes most silicic pumices.

The new phosphate bearthite, Ca₂Al(PO₄)₂HO, found in high-pressure metamorphic rocks, has been synthesized from a stoichiometric mixture of γ-Al₂O₃ and CaHPO₄·2H₂O between 4 kbar water pressure, 485°C and 24.5 kbar, 800°C. Its upper temperature stability limit, bearthite ⇆ hydroxyapatite + corundum + berlinite + H₂O, has been tightly reversed between 485°C, 1 kbar, and 850°C, 10 kbar. We extracted the following thermodynamic parameters for bearthite:H_f^298/o =−4327.25 kJ/mol andS₂₉₈^o =214.5 J/mol.K, using a new Δ_rCp approximation for dehydration reactions. Additional experiments on the reaction trolleite ⇆ berlinite + corundum + H₂O yield new thermodynamic data for trolleite, Al₄(PO₄)₃(OH)₃ (H_f^298/o =−6530 kJ/mol,S₂₉₈^o =252.7 J/mol.K). We derived the phase relations in the CaO−P₂O₅−Al₂O₃−H₂O system considering the phosphates bearthite, trolleite, berlinite, augelite, and hydroxyapatite. Extension to the SiO₂-bearing system barely affects the stability field of bearthite but reveals a chemical control on bearthite occurrence. The stability of the assemblages hydroxyapatite + aluminium silicate + (quartz or corundum), with respect to bearthite + Ca−Al-silicate, restricts bearthite to very Capoor (Dora-Maira massif) or P-rich rocks (Monte-Rosa massif; Hohe Tauern; Västan», Sweden). The coexistence of bearthite with a Ca−Al-silicate (lawsonite) may be possible only for pressures exceeding ca. 15 kbar under unusually low gradients of 5° C/km. Bearthite is otherwise not a high-pressure index mineral but provides thermal constraints in relatively high-grade phosphate-bearing deposits or metasediments. Its stability under mantleP-T conditions makes bearthite a sink for Sr and REE in subducted Ca-poor rocks.

Three possibilities exist for the geometry of the upper mantle geotherm determined from study of garnet lherzolite xenoliths from Cretaceous kimberlites of northern Lesotho, southern Africa: (1) the geotherm is inflected to a lower thermal gradient at greater depth (“low-T inflection”); (2) the geotherm is uninflected; and (3) the geotherm is inflected to a higher thermal gradient at greater depth (“high-T inflection”). In the past two years all three possibilities have been advocated. Finnerty and Boyd (1984, 1987) found that many independent thermometers yield similar P-T arrays, so that features of xenolith geotherms cannot be artifacts of the method of temperature estimation. Hence the current controversy centers on the barometers used for pressure estimation. Bertrand et al. (1986) calibrated a new aluminous-enstatite barometer using 50–100 kbar data of Yamada and Takahashi (1983), and presented a southern Africa geotherm displaying a low-T inflection. The high-P alumina solubility data are incompatible with lower-P data, however, with the result that the new barometer underestimates pressure: a diamond-bearing xenolith falls at least 5.7 kbar into the stability field of graphite. Thus, the Bertrand et al. (1986) barometer does not adequately test the reality of inflected geotherms. Carswell and Gibb (1987 a, b) modified the aluminous enstatite barometer of Nickel and Green (1985) to account for Jadeite molecule in orthopyroxenes containing relatively high concentrations of Na. When applied to xenoliths of northern Lesotho the apparent inflection is minimized but still evident. In this suite Na content of orthopyroxenes increases systematically with greater T or greater depth. Sodium correlates poorly with T (and depth) in a suite of xenoliths from Farm Louwrencia, Namibia, and application of the Nickel and Green (1985) barometer (with or without modification) destroys the correlation of T with P expected for a geotherm. The decorrelation of P from T in the Louwrencia suite is caused by errors in the Na correction. The minimization of the inflection in the northern Lesotho suite is caused by the correlation of Na with T (and depth) in that suite and does not result from an improved correction scheme for the aluminous enstatite barometer. Hence, the Carswell and Gibb (1987a, b) formulation of the barometer does not support the absence of an inflection in the northern Lesotho geotherm. Adams and Bishop (1986) recalibrated the olivine barometer (Finnerty and Boyd 1978) and presented a southern African P-T array that appears uninflected. Their barometer, however, underestimates pressure by 10–20 kbar: all xenoliths from the southern African diamond-bearing kimberlites plot well within the graphite stability field. This barometer is also too imprecise to judge whether an inflection is present or absent. Finnerty (1989), employing an empirical fit to data for Ca solubility in olivine and several different independent thermometers, presented northern Lesotho P-T arrays that satisfy the diamond-graphite constraint with minimal scatter and display high-T inflections. Because the inflection is evident with independent thermometers and independent barometers, it cannot be an artifact of the method of P-T estimation. The arguments contesting such an interpretation are flawed and so it is concluded that a high-T inflection is a fundamental property of the Cretaceous upper mantle geotherm beneath southern Africa.

Lead isotopic analyses of a suite of basaltic rocks from the Juan de Fuca-Gorda Ridge and nearby seamounts confirm an isotopically heterogeneous mantle known since 1966. The process of mixing during partial melting of a heterogeneous mantle necessarily produces linear data arrays that can be interpreted as secondary isochrons. Moreover, the position of the entire lead isotope array, with respect to the geochron, requires that U/Pb and Th/Pb values are progressively increased over the age of the earth. Partial melting theory also dictates analogous behavior for the other incompatible trace elements. This process explains not only the LIL element character of MOR basalts, but also duplicates the spread of radiogenic lead data collected from alkali-rich oceanic basalts. This dynamic, open-system model of lead isotopic and chemical evolution of the mantle is believed to be the direct result of tectonic flow and convective overturn within the mantle and is compatible with geophysical models of a dynamic earth.

Laser-ablation microanalysis of a large suite of silicate and sulfide melt inclusions from the deeply eroded, Cu-Au-mineralizing Farallón Negro Volcanic Complex (NW Argentina) shows that most phenocrysts in a given rock sample were not formed in equilibrium with each other. Phenocrysts in the andesitic volcano were brought together in dominantly andesitic—dacitic extrusive and intrusive rocks by intense magma mixing. This hybridization process is not apparent from macroscopic mingling textures, but is clearly recorded by systematically contrasting melt inclusions in different minerals from a given sample. Amphibole (and rare pyroxene) phenocrysts consistently contain inclusions of a mafic melt from which they crystallized before and during magma mixing. Most plagioclase and quartz phenocrysts contain melt inclusions of more felsic composition than the host rock. The endmember components of this mixing process are a rhyodacite magma with a likely crustal component, and a very mafic mantle-derived magma similar in composition to lamprophyre dykes emplaced early in the evolution of the complex. The resulting magmas are dominantly andesitic, in sharp contrast to the prominently bimodal distribution of mafic and felsic melts recorded by the inclusions. These results severely limit the use of mineral assemblages to derive information on the conditions of magma formation. Observed mineral associations are primarily the result of the mixing of partially crystallized magmas. The most mafic melt is trapped only in amphibole, suggesting pressures exceeding 350 MPa, temperatures of around 1,000 °C and water contents in excess on 6 wt%. Upon mixing, amphibole crystallized with plagioclase from andesitic magma in the source region of porphyry intrusions at 250 MPa, 950 °C and water contents of 5.5 wt%. During ascent of the extrusive magmas, pyroxene and plagioclase crystallized together, as a result of magma degassing at low pressures (150 MPa). Protracted extrusive activity built a large stratovolcano over the total lifetime of the magmatic complex (>3 m.y.). The mixing process probably triggered eruptions as a result of volatile exsolution.

Phengites from eclogites and pegmatites (3T, 2M₁, coarse-grained and recrystallized) of the Münchberg Massif (Weissenstein and Oberkotzau) have been dated by the ⁴⁰Ar/³⁹Ar method. 3T-micas from the eclogites yielded plateau and isochron ages of 365±7 Ma. 2M₁-micas show disturbed degassing spectra. Micas from pegmatites show a slight excess Ar component, with an isochron age of 353 to 351±3 Ma. An age component of approximately 300 Ma was also detected. In combination with age values from the literature, the cooling history of the Münchberg Massif from eclogite-facies conditions (390 Ma) to cooling below 350°C (350 Ma) is documented. The age component of 300 Ma is attributed to a low-grade stage of mineral growth accompanied by a transitional ductile-brittle deformation. The petrological effects include formation of pumpellyite-prehnite-facies minerals, frequently precipitated in microcraks and cleavage planes of earlier formed minerals. This stage has to be seen in conjunction with the intrusions of the Fichtelgebirge granite.

Coronite-bearing anorthositic granulites consisting of olivine, orthopyroxene, clinopyroxene, garnet and plagioclase assemblages are particularly well preserved at Gaupås and Holsnöy in the Bergen Arc of West Norway. The coronites are considered to have formed near T= 900° C and P=10 Kb by two stages of subsolidus reaction from an anorthositic gabbro parent. The first reaction involved ol+plag→cpxI+opxI+sp and the second cpxI+opxI+sp+pl→cpxII+opxII+gn. The incomplete reaction products are preserved to varying extents in different corona structures. Sm-Nd isotopic data for each of four coronas yield precise isochrons, and demonstrate isochronism both between the constituent phases of the corona assemblages and dispersed ground mass phases. Three individual coronas not associated with shear zones yield ages of 907±9 my, 912±18 my and 905±37 my. Eclogite facies mineralogy is developed locally in shear zones, which are shown by Sm-Nd and Rb-Sr analyses to be Caledonian in age. Where relict corona structures survive unsheared within these zones, Nd exchange between the constituent phases cannot be resolved. This observation together with sympathetic Ca/Mg and Sm/Nd zoning preserved at T∼900° C in the garnet mantles of coronites places a limit on the diffusivity of Nd in pyropic garnets which is no higher than published experimental values for Mg in pyrope-almandine garnets. Consequently even in slowly-cooled granulite terrains, garnet grains are expected to yield Sm-Nd chronologies very close to the time of mineral growth.

The solubility of alumina in complex orthopyroxenes crystallised in equilibrium with garnet has been determined over the pressure-temperature range 8–30 Kb and 800–1250° C. The results are in good agreement with predictions made using the simple thermodynamic model of Wood and Banno (1973). The model has been refined using a combination of the new data on orthopyroxenes of about En₆₀Fs₄₀ (with variable calcium content) and previously published data on more magnesian systems. Given analyses of coexisting Orthopyroxene and garnet in natural assemblages it is possible to calculate a P-T curve for the rock which should, in most cases, be accurate to within 2 or 3 kbar.