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Experimental study on the pseudobinary H 2 O+NaAlSi 3 O 8 at 600-800°C and to 2.5 GPa
... Because of the proximity of the critical endpoint close to 1.6 GPa, the miscibility gap is vanishingly small not and thus is not included in Fig. 3d. Schmidt et al. (2014) found that paragonite is a stable phase with liquid at 700 °C and pressures above 1.0 GPa for H 2 O-rich compositions on the join NaAlSi 3 O 8 -H 2 O (see their Fig. 6). We found paragonite to be a stable phase on this join at 650 °C and 1.25 GPa ( Table 2, Run Ab74), where it appeared in abundant thick, felted masses completely replacing albite, similar to textures noted by Manning et al. (2010). ...
... Paragonite is not stable even on its own composition at 800 °C and 1.25 GPa (Chatterjee 1970;Holland and Powell 1998). Schmidt et al. (2014) reported metastable paragonite at temperatures up to 771 °C at 1.1 GPa; a true stability region of paragonite on the join NaAlSi 3 O 8 -H 2 O awaits experimental definition. Our critical curve for the system NaAlSi 3 O 8 -H 2 O is similar to that of Shen and Keppler (1997), but with substantially smaller uncertainty and with a systematic offset to slightly higher pressures (see Fig. 3.5 B in Manning 2018). ...
... Our critical curve for the system NaAlSi 3 O 8 -H 2 O is similar to that of Shen and Keppler (1997), but with substantially smaller uncertainty and with a systematic offset to slightly higher pressures (see Fig. 3.5 B in Manning 2018). The systematic offset of our critical curve to higher pressures closely matches the determination of Schmidt et al. (2014). We found only congruent melting of albite at all conditions, so our results do not support a singular point as proposed by Boettcher and Wyllie (1969), where hydrous albite melting becomes incongruent (i.e. ...
Supercritical fluids in rock–H2O systems have been proposed to be important agents of mass transfer in subduction zone environments. New experimental studies were conducted on the simple model granite system NaAlSi3O8 (Ab)–H2O in order to investigate phase relations and to develop the thermodynamic mixing properties between aqueous fluid (a.k.a. vapor, V) and silicate melt (a.k.a. liquid, L) at pressures (P) and temperatures (T) approaching those of critical mixing. We established liquidus and solvus phase relations by analyzing the quenched run products from piston–cylinder experiments over a range of \(P - T - X_{{{\text{H}}_{2} {\text{O}}}}\) conditions from 1.0 to 1.7 GPa, 630–1060 °C and 4–92 wt% H2O. Equations for the critical curve, solidus temperatures, albite solubility at the solidus, and vapor-saturated solidus H2O content were formulated as functions of \(P - T - X_{{{\text{H}}_{2} {\text{O}}}}\). We constructed a subregular solution model to describe the solvus curves using P and T dependent Margules coefficients (\(W_{{{\text{H}}_{2} {\text{O}}}}\) and \(W_{\text{Ab}}\)). Activities of H2O and Ab (\(a_{{{\text{H}}_{2} {\text{O}}}}\) and \(a_{\text{Ab}}\)) could be formulated using only the input at the solidus and the critical point at each pressure because of the nearly linear dependence of the parameters on T. The solvus curves were confirmed independently by means of criteria established for classification of quenched products as L, L + V, or V and are in excellent agreement with the compositions that can be calculated using the Margules coefficients. At 1.6 GPa, the H2O content at the vapor-saturated solidus is 44.5 ± 5.5 wt% and the solubility of albite at the solidus is 42.95 ± 0.99 wt%, indicating the imminent intersection of the two curves and thus a stable critical endpoint at some slightly higher pressure. We constrain the critical endpoint at 1.63 ± 0.02 GPa, 659 ± 5 °C, and a composition of ~ 44.7 wt% H2O based on the intersections of the pressure dependent solidus curves with the critical curve, the pressure dependent albite solubility curve with the vapor-saturated solidus curve. The 1.7 GPa experiments showed no evidence for liquid–vapor immiscibility across a wide range of compositions and temperatures (4–80 wt% H2O and 630–1050 °C, and furthermore, that low albite is stable in the presence of the supercritical fluid near the breakdown of albite to jadeite and quartz. These results provide a comprehensive account of the solution properties of subcritical and supercritical fluids in this model granite system at temperatures and pressures corresponding to the deep-crust regions of granite magma generation.
... A somewhat different expression was given for more NaCl-rich solutions. The solubility of Al 2 O 3 in fluids in the NaAlSi 3 O 8 -H 2 O system is another example of effects on solubility of added components at high temperature and pressure (Currie 1968;Anderson and Burnham 1983;Woodland and Walther 1987;Schmidt et al. 2014). The total aluminosilicate solubility in the NaAlSi 3 O 8 -H 2 O system is on the order of 1 wt%. ...
The Earth's fluid budget is dominated by species in the system C–O–H–N–S together with halogens such as F and Cl. H 2 O is by far the most abundant. Such fluids are one of the two main mass transport agents (fluid and magma) in the Earth. Among those, in particular aqueous fluids are efficient solvents of geochemically important components at high temperature and pressure. The solution capacity of aqueous fluids can be enhanced further by dissolved halogens and sulfur. CO 2 or nitrogen species has the opposite effect.
Fluid-mediated transport in the Earth is by fluids passing through cracks at shallow depth and via percolation channels along grain boundaries at greater depth. Percolation velocity is linked to permeability, which, in turn is governed by rock porosity. Porosity is controlled by wetting angles, θ , at the interface between fluid and mineral surfaces. When θ < 60°, fluid will wet all grain boundaries of an isotropic crystalline material, whereas when greater than 60°, grain boundary wetting does not occur as readily, and fluid-mediated transport efficiency can be greatly reduced. The size of the wetting angle is negatively correlated with the solubility of silicate components in the fluids, which means that fluid composition, temperature, and pressure affect the wetting angles and, therefore, fluid-mediated mass transport efficiency in the interior of the Earth.
Geophysical and geochemical anomalies in the Earth's interior have been linked to the presence of fluids. Fluid infiltration in crustal and mantle rocks will enhance electrical conductivity and seismic wave attenuation. For example, 5–10% H 2 O-rich fluids in the mantle wedge above subducting plates have been suggested from enhanced electrical conductivity. Similar fluid fractions have been suggested to be consistent with seismic velocities in these regions. The geochemistry of the crust and the mantle can be affected by fluid-mediated transport of major, minor, and trace elements. When such altered materials serve as source rocks of partial melts, those geochemical alterations also lead to changes in partial melt compositions. As an example, the presence of such aqueous fluid in the mantle wedge above subducting and dehydrating subducting slabs is consistent with partial melting of an H 2 O-bearing mantle wedge above subducted oceanic crust.
... This phase has been shown to be stable at conditions consistent with those in felsic magma chambers, [ 700°C and 1-10 kbar (Sowerby and Keppler 2002;Thomas et al. 2000), and that solubilities of otherwise incompatible elements can reach extremely high levels (Bureau et al. 2007). Such a phase has also been produced and observed in HDAC experiments (Kawamoto et al. 2014;Schmidt et al. 2014). We propose that this phase may prove to be the intermediary between felsic magmas and the orebodies and hydrothermal fluids that form them, and that the properties and behavior of these fluids are critical in the process of sequestering the otherwise rare elements from huge volumes of magma. ...
Abstract In this paper, we show that supercritical fluids
have a greater significance in the generation of pegmatites,
and for ore-forming processes related to granites than is
usually assumed. We show that the supercritical melt or
fluid is a silicate phase in which volatiles; principally H2O
are completely miscible in all proportions at magmatic
temperatures and pressures. This phase evolves from felsic
melts and changes into hydrothermal fluids, and its unique
properties are particularly important in sequestering and
concentrating low abundance elements, such as metals. In
our past research, we have focused on processes observed
at upper crustal levels, however extensive work by us and
other researchers have demonstrated that supercritical melt/
fluids should be abundant in melting zones at deep-crustal
levels too. We propose that these fluids may provide a
connecting link between lower and upper crustal magmas,
and a highly efficient transport mechanism for usually melt
incompatible elements. In this paper, we explore the unique
features of this fluid which allow the partitioning of various
elements and compounds, potentially up to extreme levels,
and may explain various features both of mineralization
and the magmas that produced them.
... This phase has been shown to be stable at conditions consistent with those in felsic magma chambers, [ 700°C and 1-10 kbar (Sowerby and Keppler 2002;Thomas et al. 2000), and that solubilities of otherwise incompatible elements can reach extremely high levels (Bureau et al. 2007). Such a phase has also been produced and observed in HDAC experiments (Kawamoto et al. 2014;Schmidt et al. 2014). We propose that this phase may prove to be the intermediary between felsic magmas and the orebodies and hydrothermal fluids that form them, and that the properties and behavior of these fluids are critical in the process of sequestering the otherwise rare elements from huge volumes of magma. ...
In this paper, we show that supercritical fluids have a greater significance in the generation of pegmatites, and for ore-forming processes related to granites than is usually assumed. We show that the supercritical melt or fluid is a silicate phase in which volatiles; principally H2O are completely miscible in all proportions at magmatic temperatures and pressures. This phase evolves from felsic melts and changes into hydrothermal fluids, and its unique properties are particularly important in sequestering and concentrating low abundance elements, such as metals. In our past research, we have focused on processes observed at upper crustal levels, however extensive work by us and other researchers have demonstrated that supercritical melt/fluids should be abundant in melting zones at deep-crustal levels too. We propose that these fluids may provide a connecting link between lower and upper crustal magmas, and a highly efficient transport mechanism for usually melt incompatible elements. In this paper, we explore the unique features of this fluid which allow the partitioning of various elements and compounds, potentially up to extreme levels, and may explain various features both of mineralization and the magmas that produced them.
... These experiments back up other experiments in HDAC cells that showed the phase changes that occur in volatile-rich melts at up to 10 kbar pressure (e.g. Schmidt et al., 2014;Thomas et al., 2011a, b,c;. ...
Using water and major and trace element data obtained from melt inclusions primarily in pegmatite quartz we have shown in this, and previous papers, that melt–melt–fluid immiscibility is widespread and an important process during the generation of granitic pegmatites. Furthermore we have shown that the formation of pegmatites actually begins in the supercritical melt/fluid stage. In this study we extend previous work, and in particular explore the behavior of 10 different elements (Be, F, P, S, Cl, As, Rb, Sn, Cs, and Ta) in the supercritical range. From preliminary studies we suggest that other elements such as B, Na, K, Zn, Nb, and Sb will behave similarly near the solvus crest of a generalized pseudo-binary melt–water system. This provides important evidence that pegmatite-forming processes have already begun at high temperatures, in the range of 850–750 °C, and that may require a re-think on the partitioning behavior of metals in late stage residual melts exsolved from granitic magmas. The existence of this parental supercritical silicate melt/fluid, although derived from granitic magmas, imposes significant constraints of its own, so drawing conclusions about pegmatite-forming processes from data gained from slightly modified granite compositions becomes highly problematic.
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