Experimental study of barium partitioning between phlogopite and silicate liquid at upper-mantle pressure and temperature (original) (raw)
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Evidence for the role of phlogopite in the genesis of alkali basalts
Contributions to Mineralogy and Petrology, 1971
Evaluation of available experimental and petrochemical evidence suggests that variations of potassium, titanium and aluminium in basalts can be explained by the melting of titaniferous phlogopite at depths exceeding about 60 km in the upper mantle. For tholeiitie basalts, an inverse covariance is reeognised between titanium and aluminium which is probably controlled by the depth of magma segregation. The relative effects of pressure and phlogopite breakdown have been estimated for selected suites of alkali basalt.
Phase Relations of Phlogopite with Magnesite from 4 to 8 GPa
To evaluate the stability of phlogopite in the presence of carbonate in the Earth’s mantle, we conducted a series of experiments in the KMAS-H2O-CO2 system. A mixture consisting of synthetic phlogopite (phl) and natural magnesite (mag) was prepared (phl90-mag10; wt%) and run at pressures from 4 to 8 GPa at temperatures ranging from 1150 to 1550°C. We bracketed the solidus between 1200 and 1250°C at pressures of 4, 5 and 6 GPa and between 1150 and 1200°C at a pressure of 7 GPa. Below the solidus, phlogopite coexists with magnesite, pyrope and a fluid. At the solidus magnesite is the first phase to react out, and enstatite and olivine appear. Phlogopite melts over a temperature range of ~150°C. The amount of garnet increases above solidus from ~10 to ~30 modal% to higher pressures and temperatures. A dramatic change in the composition of quench phlogopite is observed with increasing pressure from similar to primary phlogopite at 4 GPa to hypersilicic at pressures ≥5 GPa. Relative to CO2-free systems, the solidus is lowered such, that, if carbonation reactions and phlogopite metasomatism take place above a subducting slab in a very hot (Cascadia-type) subduction environment, phlogopite will melt at a pressure of ~7.5 GPa. In a cold (40 mWm-2) sub-continental lithospheric mantle, phlogopite is stable to a depth of 200 km in the presence of carbonate, and can coexist with a fluid that becomes Si-rich with increasing pressure. Ascending kimberlitic melts that are produced at greater depths could react with peridotite at the base of the sub-continental lithospheric mantle, crystallizing phlogopite and carbonate at a depth of 180 to 200 km.
Phase relations of phlogopite with and without enstatite up to 8 GPa
Stability relations of phlogopite and phlogopite+enstatite systems without additional water have been experimentally determined at pressures of 4-8 GPa and at temperatures of 1200-1500˚C by using a uniaxial split-sphere apparatus. The phlogopite gradually dissociates into pyrope and Al 2 O 3-deficient phlogopite at pressures above 5 GPa. The Al 2 O 3 content in the phlogopite decreases from 14.4 wt% at 6 GPa to 12.9 wt% at 8 GPa and concurrently the mode of pyrope increases from <5 modal% at 5.GPa to 20-30 % at 8 GPa, in the phlogopite system. Thus increase in pressure enhances two cation substitutions, Al+Al=Mg+Si and Mg+2Al= []+2Si, in the phlogopite structure. The solidi of the phlogopite and phlogopite+enstatite systems reach maximum temperatures of 1350˚C and 1300˚C, respectively, at about 5 GPa. Pyrope is a residual phase under hypersolidus conditions in both the systems above 5 GPa. The stabilities of phlogopite determined in the present study indicate that phlogopite can be stable in garnet harzburgite in the subcratonic lithosphere down to 210 km depth. The observed decomposition of the phlogopite into an assemblage of garnet and fluid or hydrous silicate melt suggests that phlogopite can be secondarily formed by a reaction of garnet and upwelling metasomatic agents near the base of continental lithosphere.
JOURNAL OF MINERALOGY, PETROLOGY AND ECONOMIC GEOLOGY, 1988
Barium-rich phlogopite occurs in a mantle derived xenolith incorporated into the Upper Canada Mine kimberlite, Ontario, Canada. The phlogopite is characterized by high BaO (up to 4.3 wt.%) and Na2O (up to 2.2 wt.%) and extremely low TiO2 (0.02 wt.%) contents with a relatively high ratio (0.95) of Mg/(Mg+Fe). The chemical characteristics distinguish the phlogopite from Ba-rich micas reported from various alkalic mafic-ultramafic igneous rocks and suggest that the phlogopite has been formed by mantle metasomatism involving Ba-and alkali rich, Ti-poor hydrous fluids. The Ba content of phlogopite in this study is about five times higher than that of phlogopite previously reported from mantle derived xenoliths. A compilation of data suggests a distinct correlation between Ba and K in mantle derived xenoliths and various continen tal volcanic rocks (alkali basalts, ultrapotassic basic rocks, lamprophyres, lamproites, and kimber lites). A range of Ba/K in the xenoliths and the volcanic rocks examined is comparable with that of mantle phlogopite, suggesting that phlogopite may be a principal reservoir for Ba as well as K in the upper mantle source regions.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2003
The origin of subduction-related magmas is still a matter of debate in the Earth Sciences. These magmas are characterised by their distinctive trace element compositions compared to magmas from other tectonic settings, e.g. mid-ocean ridges or rifts. The distinct trace element composition of these magmas is generally attributed to alteration of the source region by a contaminating agent: either a silicate melt or a hydrous fluid, possibly chlorine-enriched. In this study, we have used lPIXE (proton induced X-ray emission) to analyse synthetic samples obtained from a micro-experimental petrology study that aims to determine the partitioning behaviour of two key elements, Ba and Pb, between silicate melt and both pure water and saline fluids. Our experiments were performed at high-pressure (>0.34-1.53 GPa) and high-temperature (697-1082°C) in a hydrothermal diamond anvil cell, that was used as a transparent rapid quench autoclave. We observed that at high pressure and temperature, in the presence of pure water, Ba and Pb are not strongly fractionated into one phase or the other. The partition coefficient of Pb is ranging from 0.46 to 1.28. Results from one experiment performed at 0.83 GPa and 847°C, in the presence of a saline fluid indicate that the presence of Cl induces strong fractionation of Pb and moderate fractionation of Ba both into the silicate melt. In addition, our data indicate that Cl is strongly partitioned into the fluid phase.
The formation of mantle phlogopite in subduction zone hybridization
Contributions to Mineralogy and Petrology, 1982
Extrapolation and extension of phase equilibria in the model system KA1SiO4-Mg2SiO4-SiOz-H20 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, orthopyroxerie, 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) HzO-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.
Contributions to Mineralogy and Petrology, 2010
We performed a series of piston-cylinder experiments on a synthetic pelite starting material over a pressure and temperature range of 3.0-5.0 GPa and 1,100-1,600°C, respectively, to examine the melting behaviour and phase relations of sedimentary rocks at upper mantle conditions. The anhydrous pelite solidus is between 1,150 and 1,200°C at 3.0 GPa and close to 1,250°C at 5.0 GPa, whereas the liquidus is likely to be at 1,600°C or higher at all investigated pressures, giving a large melting interval of over 400°C. The subsolidus paragenesis consists of quartz/ coesite, feldspar, garnet, kyanite, rutile, ±clinopyroxene ±apatite. Feldspar, rutile and apatite are rapidly melted out above the solidus, whereas garnet and kyanite are stable to high melt fractions ([70%). Clinopyroxene stability increases with increasing pressure, and quartz/coesite is the sole liquidus phase at all pressures. Feldspars are relatively Na-rich [K/(K ? Na) = 0.4-0.5] at 3.0 GPa, but are nearly pure K-feldspar at 5.0 GPa. Clinopyroxenes are jadeite and Ca-eskolaite rich, with jadeite contents increasing with pressure. All supersolidus experiments produced alkaline dacitic melts with relatively constant SiO 2 and Al 2 O 3 contents. At 3.0 GPa, initial melting is controlled almost exclusively by feldspar and quartz, giving melts with K 2 O/Na 2 O *1. At 4.0 and 5.0 GPa, lowfraction melting is controlled by jadeite-rich clinopyroxene and K-rich feldspar, which leads to compatible behaviour of Na and melts with K 2 O/Na 2 O ) 1. Our results indicate that sedimentary protoliths entrained in upwelling heterogeneous mantle domains may undergo melting at greater depths than mafic lithologies to produce ultrapotassic dacitic melts. Such melts are expected to react with and metasomatise the surrounding peridotite, which may subsequently undergo melting at shallower levels to produce compositionally distinct magma types. This scenario may account for many of the distinctive geochemical characteristics of EM-type ocean island magma suites. Moreover, unmelted or partially melted sedimentary rocks in the mantle may contribute to some seismic discontinuities that have been observed beneath intraplate and island-arc volcanic regions.
European Journal of Mineralogy, 2009
The separate effects of pressure (10 À4 and 1.0 GPa), water, CO 2 , oxygen fugacity and calcium doping on the liquid line of descent of a primitive leucite-basanite magma (SiO 2 ¼ 47.06 wt%, MgO ¼ 12.76 wt% and Mg# ¼ 75.1) from the Montefiascone Volcanic Complex (Vulsini volcanoes, central Italy) were experimentally investigated in the 1350-1160 C temperature range. Results indicate that low-pressure liquidus temperatures are 1280 C and that the high-pressure T liquidus is 1350 C under anhydrous conditions; the latter is lowered to $ 1275 C by the addition of 3 wt% water. Cr-spinel is always the liquidus phase. At comparable fO 2 values, high and low pressure runs produced the same phase assemblage (spinel þ olivine þ clinopyroxene) up to 50 % crystallization, although olivine was partially or totally replaced by phlogopite in hydrous experiments. An increase in oxygen fugacity and the addition of CaO determine an increase in both the degree of melt crystallization and the stability field of clinopyroxene. These determine contrasting effects on the composition of residual liquids: the former increases SiO 2 content, whereas the latter induces the desilication of melts. The replacement of olivine by phlogopite, induced by increasing amounts of water, leads to the production of glass with lower potassium contents.
Lithos, 2004
A model metasomatized lherzolite composition contains phlogopite and pargasite, together with olivine, orthopyroxene, clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at z 1.5 GPa, phlogopite is stable above the dehydration solidus, determined by the melting behaviour of pargasite and coexisting phases. At 2.8 GPa, melts with residual phlogopite + garnet lherzolite mineralogy at 1195 jC and with garnet lherzolite mineralogy at 1250 jC are both olivine nephelinite with K/Na (atomic) = 0.51 and K/Na = 0.65, respectively. Recent work shows that melting along the dehydration (fluid-absent) solidus of the phlogopite + pargasite lherzolite at pressures < 1.5 GPa is very different with the presence of phlogopite, decreasing the solidus below that of pargasite lherzolite. At 1.0 GPa, both phlogopite and pargasite disappear at temperatures at or slightly above the solidus. The compositions of two melts at 1.0 GPa, 1075 jC (with different water contents), in equilibrium with residual spinel lherzolite mineralogy are silica-saturated trachyandesite ( f 5% melt fraction, f 3% H 2 O) to silica-oversaturated basaltic andesite ( f 8% melt fraction, 4.5% H 2 O). Both compositions may be classified as 'shoshonites' on the basis of normative compositions, silica-saturation, and K/Na ratio. Decompression melting of metasomatized lithospheric lherzolite with minor phlogopite and pargasite may produce primary 'shoshonitic' magmas by dehydration melting at f 1 GPa, 1050 -1150 jC. Such magmas may be parental to Proterozoic batholithic syenites occurring in Brazil. D