Lower Temperature Terminations of the Three-phase Region Plagioclase-Alkali Feldspar-Liquid

Stewart, David B.; Roseboom, Eugene H.

doi:10.1093/petrology/3.3.280

Cited by 48 publications

(11 citation statements)

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“…[31] The evolution in feldspar compositions as a function of increasing total crystallinity and evolving melt is consistent with the low-pressure, H 2 O-saturated phase relations in the system Q-Ab-An-Or-H 2 O with a Q = 1 [Stewart and Roseboom, 1962;Nekvasil, 1992]. Plagioclase crystallizes alone until the melt composition intersects the two-feldspar curve.…”

Section: Feldspar Compositionssupporting

confidence: 57%

An experimental study of the kinetics of decompression‐induced crystallization in silicic melt

2002

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[1] Experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression. Pinatubo dacite was held at 780°C, 220 MPa, f O2 = NNO + 2, H 2 O-saturated conditions for an equilibration period, decompressed to final pressures, P f , ranging from 175 to 5 MPa, and then held for 0.3-931 hours. According to the plagioclase liquidus curve in P H2O -T space for the relevant melt composition, these decompressions impose effective undercoolings, ÁT eff , of 34-266°C. Growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 P f < 150 MPa (ÁT eff = 34-93°C), and equilibrium crystal modes are achieved in <168 hours. Microlite nucleation is the dominant transformation process for 10 < P f < 50 MPa (ÁT eff = 125-241°C), and chemical equilibrium is not attained by 168 hours under these conditions. Slow, steady decompressions typically produced normally zoned, euhedral, and planar-faceted feldspar crystals, although anhedral morphologies were produced at very low P f . Contrary to expectation, slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure. Although counterintuitive, these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth (controlled by ÁT eff and melt viscosity) experienced during each decompression path. Analysis of liquid to solid transformation kinetics using TTT-style diagrams shows that crystallization occurs most rapidly at $100 MPa by a crystal growth mechanism. The next most efficient crystallization conditions are at 25 MPa, in a crystal nucleationdominated regime.INDEX TERMS: 3630 Mineralogy and Petrology: Experimental mineralogy and petrology, 3640 Mineralogy and Petrology: Igneous petrology, 8434 Volcanology: Magma migration, 8439 Volcanology: Physics and chemistry of magma bodies;

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Section: Feldspar Compositionssupporting

confidence: 57%

An experimental study of the kinetics of decompression‐induced crystallization in silicic melt

2002

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“…A quartz-saturated ternary feldspar cotectic melt could form if some ternary composition had a solidus temperature lower than minimum/eutectic melting on the Ab + Or + Qtz sideline (see, for example, Stewart & Roseboom 1962;Carmichael 1963;Abbott 1978). The existence of such a quartz-saturated ternary feldspar cotectic melt would mean that a wide range of bulk compositions would produce the same initial melt composition.…”

Section: Ternary Minimum Meltingmentioning

confidence: 99%

“…This simplified behaviour allows a straightforward thermodynamic treatment of the migration of the minimum temperature and composition with both pressure and a H2O , within a relatively small triangular volume of Ab + Or + Qtz+ fl H2O Luth et al (1964) concluded that at H 2 O-saturation the fourcomponent system Ab + Or + Qtz + H 2 O changed from minimum to eutectic melting at pressures above about 3.6 kbar. By including information on the pressure dependence of the alkali feldspar critical temperature, the interpretation of Stewart & Roseboom (1962) would displace this intersection to lower than 3 kbar P HlO -The P-T of the critical curve, presented by Waldbaum and Thompson (1969), allowed revision of this intersection point to about 2.5 kbar. The projection of the P-T of the alkali feldspar critical curve (AFC) into Ab + Or + Qtz phase relations, under anhydrous and H 2 O saturation conditions, was shown by Huang and Wyllie (1975).…”

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confidence: 99%

Fertility of crustal rocks during anatexis

Thompson

1996

Earth and Environmental Science Transactions of the Royal Socie

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After many years of systematic experimental investigations, it is now possible to quantify the conditions for optimum fertility to melt production of most common crustal rock types as functions of temperature and a HlO , to about 30 kbar pressure. Quartzo-feldspathic melting produces steady increases in melt proportion with increasing temperature. The exact melt fraction depends on the mineral mode relative to quartz-feldspar eutectics and the temperatures of mica dehydration melting reactions. Mica melting consumes SiO 2 from residual quartz during the formation of refractory Al,SiO 5 , orthopyroxene, garnet or cordierite.A simple graphical interpretation of experimental results allows a deduction of the proportions of mica and feldspar leading to optimum fertility. In effect, the mica dehydration melting reactions, at specific pressure and a HiO , are superimposed on quartz-feldspar melting relations projected onto Ab-An-Or. Fertility to melt production varies with the mica to feldspar ratio and pressure. Pelites are more fertile than psammites at low pressures (e.g. 5 kbar), especially if they contain An 40 to An 50 plagioclase. At higher pressure (e.g. 10-20 kbar) and for rocks containing albitic plagioclase, psammites are more fertile than pelites. For a typical pelite (e.g. with An 25 at 20 kbar), the cotectic with muscovite lies at higher a H2O (*0-6) and X Ab (*0-42) than with biotite (a H , o :*0-35; X Ab *O-32), thus dehydration melting of muscovite requires 10% more plagioclase for fertility than does biotite.The first melts from dehydration melting of muscovite (with Plg + Qtz) are more sodic and form at lower temperatures than the first melts from Bio + Plg + Qtz. With increasing pressure, to at least 30 kbar, granite minimum and mica dehydration melts become more sodic. This indicates that fl H 2 o °f such melts is greater than 0-3.

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“…Zoning of feldspars is one of the most common manifestations of disequilibrium (Stewart and Roseboom, 1962). One may therefore expect that the homogeneous feldspar megacrysts crystallized under equilibrium conditions (Albarede and Bottinga, 1972).…”

Section: Equilibrium Crystallization From An Acid Liquid At Shallow Dmentioning

confidence: 99%

Anorthoclase megacrysts: physical conditions of formation

Bahat¹

1979

Mineral. mag.

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SUMMARY. Previous interpretations relating to precipitation of anorthoclase megacrysts from alkaline basaltic magmas under high-pressure conditions are contradicted, It is proposed that these feldspars were crystallized above the solvus from hydrous melts at shallow depths, under low-pressure equilibrium conditions. These melts had been considerably enriched with volatiles of sodium, strontium, and barium, and to some extent also of rubidium.

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Lower Temperature Terminations of the Three-phase Region Plagioclase-Alkali Feldspar-Liquid

Cited by 48 publications

References 0 publications

An experimental study of the kinetics of decompression‐induced crystallization in silicic melt

An experimental study of the kinetics of decompression‐induced crystallization in silicic melt

Fertility of crustal rocks during anatexis

Anorthoclase megacrysts: physical conditions of formation

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