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1.
Near-liquidus phase relationships of a spinel lherzolite-bearing olivine melilitite from Tasmania were investigated over a P, T range with varying
,
, and
. At 30 kb under MH-buffered conditions, systematic changes of liquidus phases occur with increasing
(
= CO2/CO2 +H2O+olivine melilitite). Olivine is the liquidus phase in the presence of H2O alone and is joined by clinopyroxene at low
. Increasing
eliminates olivine and clinopyroxene becomes the only liquidus phase. Further addition of CO2 brings garnet+orthopyroxene onto the liquidus together with clinopyroxene, which disappears with even higher CO2. The same systematic changes appear to hold at higher and lower pressures also, only that the phase boundaries are shifted to different
. The field with olivine- +clinopyroxene becomes stable to higher
with lower pressure and approaches most closely the field with garnet+orthopyroxene+clinopyroxene at about 27 kb, 1160 °C,
0.08 and
0.2 (i.e., 6–7% CO2+ 7–8% H2O). Olivine does not coexist with garnet+orthopyroxene+clinopyroxene under these MH-buffered conditions. Lower oxygen fugacities do not increase the stability of olivine to higher
and do not change the phase relationships and liquidus temperatures drastically. Thus, it is inferred that olivine melilitite 2927 originates as a 5% melt (inferred from K2
O and P2O5 content) from a pyrolite source at about 27kb, 1160 dg with about 6–7% CO2 and 7–8% H2O dissolved in the melt. The highly undersaturated character of the melt and the inability to find olivine together with garnet and orthopyroxene on the liquidus (in spite of the close approach of the respective liquidus fields) can be explained by reaction relationships of olivine and clinopyroxene with orthopyroxene, garnet and melt in the presence of CO2. 相似文献
2.
Trace element analyses of 1-atm and high-pressure experiments show that in komatiite and peridotite, the olivine (OL)/liquid (L) distribution coefficient for Al2O3 (
) increases with pressure and temperature. Olivine in equilibrium with liquid accepts as much as 0.2 wt% Al2O3 in solution at 6 GPa. Convergence to equilibrium compositions at this high level is shown by cation diffusion of Al into synthetic forsterite crystals of low-Al contents in the presence of melt. Convergence to low-Al equilibrium compositions at lower P and T is shown by diffusion of Al out of synthetic forsterite with high initial Al content. Isobaric and isothermal experimental data subsets reveal that temperature and pressure variations both have real effects on
. Variation in silicate melt composition has no detectable effect on
within the limited range of experimentally investigated mixtures. Least-squares regression for 24 experiments, using komatiite and peridotite, performed at 1 atm to 6 GPa and 1300 to 1960°C, gives the best fit equation:
Increase in
with increasingly higher-pressure melting is consistent with incorporation of a spinel-like component of low molar volume into olivine, although other substitutions possibly involving more complex coupling cannot be ruled out. High P-T ultrabasic melting residues, if pristine, may be recognized by the high
calculated from microprobe analyses of Al2O3 concentrations in residual olivines and estimated Al2O3 concentration in the last liquid removed. In general the low levels of Al in natural olivine from mantle xenoliths suggest that pristine residues are rarely recovered. 相似文献
3.
J. W. Jones 《Contributions to Mineralogy and Petrology》1972,37(4):291-306
In the Rogers Pass area of British Columbia the almandine garnet isograd results from a reaction of the form: 5.31 ferroan-dolomite+8.75 paragonite+4.80 pyrrhotite+3.57 albite+16.83 quartz+1.97 O2=1.00 garnet+16.44 andesine+1.53 chlorite+2.40 S2+1.90 H2O+10.62 CO2. The coefficients of this reaction are quite sensitive to the Mn content of ferroan-dolomite.Experimental data applied to mineral compositions present at the isograd, permits calculation of two intersecting P, T equilibrium curves. P=29088–39.583 T is obtained for the sub-system paragonite-margarite (solid-solution), plagioclase, quartz, ferroan-dolomite, and P=28.247 T–14126 is obtained for the sub-system epidote, quartz, garnet, plagioclase. These equations yield P=3898 bars and T=638° K (365° C). These values are consistent with the FeS content of sphalerite in the assemblage pyrite, pyrrhotite, sphalerite and with other estimates for the area.At these values of P and T the composition of the fluid phase in equilibrium with graphite in the system C-O-H-S during the formation of garnet is estimated as:
bars,
bars,
bars,
bars,
bars,
bars,
bars,
bars,
, bars,
bars. 相似文献
4.
Ignimbrites from the central North Island consist mainly of glass or its devitrified product (70–95%); their phenocryst mineralogy is varied and includes plag., hyp., ti-mag., ilm., aug., hblende, biot., san., qtz, ol., with accessory apatite, zircon and pyrrhotite. The Fe-Mg minerals can be used to divide the ignimbrites into four groups with hyp.+aug. reflecting high quench temperatures and biot.+hblende +hyp.+aug., low quench temperatures. Oxygen fugacities lie above the QMF buffer curve and even in ignimbrites with low crystal contents the solid phases apparently buffered fO2. Some ignimbrites contain the assemblage actinolite, gedrite, magnetite and hematite, reflecting post-eruption oxidation. The mineralogy also allows estimation of
using pyrrhotite and thence
,
. The assemblage biotite-sanidine can be used to estimate
and thence
. Water fugacity is calculated in a variety of ways using both biotite and hornblende as well as the combining reaction
. It is high and approaches P
total in most ignimbrites (~4kb) but is lower in unwelded pumice breccias. Comparison of temperature estimates using mineral geothermometers for the various phenocryst phases suggests that the ignimbrite magmas showed temperature differences of 60–100 °C and pressure differences of several kilobars. Individual magma chambers therefore, would have extended over several kilometres vertically. The chemical potential of water may have been constant through the magma. 相似文献
5.
The occurrence of critical assemblages among antigorite, diopside, tremolite, forsterite, talc, calcite, dolomite and magnesite in progressively metamorphosed ophicarbonate rocks, together with experimental data, permits the construction of phase diagrams in terms of the variables P, T, and composition of a binary CO2-H2O fluid. Equilibrium constants are given for the 30 equilibria that describe all relations among the above phases. Ophicalcite, ophidolomite, and ophimagnesite assemblages occupy partially overlapping fields in the
diagram. The upper temperature limit of ophicalcite rocks lies below that of ophidolomite and ophimagnesite. The fluid phase in ophicarbonate rocks has
0.8$$
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, and there are indications that
during their progressive metamorphism is approximately equal to P
total. 相似文献
6.
Near-liquidus melting experiments were performed on a high-K latite at fO2's ranging from iron-wustite-graphite (IWG) to nickel-nickel oxide (NNO) in the presence of a C-O-H fluid phase. Clinopyroxene is a liquidus phase under all conditions. At IWG
, the liquidus at 10 kb is about 1,150° C but is depressed to 1,025° C at NNO and
. Phlogopite and apatite are near-liquidus phases, with apatite crystallizing first at pressures below 10 kb. Phlogopite is a liquidus phase only at NNO and high
. Under all conditions the high-K latites show a large crystallization interval with phlogopite becoming the dominant crystalline phase with decreasing temperature. Increasing fO2 affects phlogopite crystallization but the liquidus temperature is essentially a function of
. The chemical compositions of the near-liquidus phases support formation of the high-K latites under oxidizing conditions (NNO or higher) and high
. It is concluded from the temperature of the H2O-saturated liquidus at 10 kb, the groundmass: crystal ratio and presence of chilled latite margins around some xenoliths that the Camp Creek high-K latite magma passed thru the lower crust at temperatures of 1,000° C or more. 相似文献
7.
Fe2+-Mg2+ partition between coexisting cordierite and garnet — a discussion of the experimental data
Bernard J. Wood 《Contributions to Mineralogy and Petrology》1973,40(3):253-258
The partition of iron and magnesium between cordierite and garnet depends on
as well as temperature. The apparently conflicting experimental data on the values of K
D
may be reconciled by considering the
pertaining during the different experiments. 相似文献
8.
Jonathan H. Berg 《Contributions to Mineralogy and Petrology》1977,64(1):33-52
Thermal aureoles surrounding intrusions of the Nain complex, Labrador, contain many unique or unusual mineral assemblages in aluminous gneisses and granulites, ironstones, and ultramafic rocks. Some of the limiting assemblages are (in addition to feldspars±pyrrhotite±ilmenite±graphite ±biotite±magnetite): Ga-Ol-Hy-Sp, Ga-Cd-Hy-Sp, Ga-Hy-Ol-Qz, Cd-Hy-Os-Qz, Ga-Cd-Sp-Si, Cd-Sp-Co-Si, Ga-Ol-Hy-Sp, Ga-Cd-Hy-Sp, Hy-Ol-Qz-Aug, and Ol-Hy-Sp(±Chl±Ca-amph±Aug). On the basis of some of these assemblages are (in addition to feldspars±pyrrhotite±ilmenite±graphite from 3.7–6.6 kbar and temperatures from 645–915 ° C. The paucity of hydrous phases, the preponderance of Ksp-Plag-Qz or even Cd-Ksp-Qz without evidence of a melt at these temperatures, the stable occurrence of osumilite, and the common presence of graphite suggest that
was extremely low. The presence of graphite-pyrrhotite, the compositions of ilmenites, the compositions of coexisting Fe-Ti oxides, and other mineralogical data indicate the
was relatively low and, despite the wide range in bulk compositions and rock types, may have followed an approximately buffered trend. It is possible that the magmas of the Nain complex may have acted as an external
buffer for the aureole rocks. The unique nature of these mineral assemblages appears to be a function of the extremely low
. 相似文献
9.
Tj. Peters V. Trommsdorff J. Sommerauer 《Contributions to Mineralogy and Petrology》1978,66(4):383-388
The compositions of coexisting pyroxmangites, rhodonites, rhodochrosites and manganese calcites in regional metamorphosed manganese cale-silicate marbles from Val Scerscen and Alagna were analysed by microprobe and permit definition of critical tie lines at metamorphic grades appropriate to temperatures between 400 and 450 °C.Variations in composition of coexisting mineral pairs in one and the same locality are attributed to variations in
and not to metamorphic temperatures. From the analysed assemblages isothermal
plots (with SiO2 as excess component) were constructed for the system CaO-MnO-SiO2-CO2. 相似文献
10.
Genesis of the calc-alkaline igneous rock suite 总被引:11,自引:1,他引:11
A high pressure experimental study of the partial melting fields of synthetic high-alumina olivine tholeiite, high-alumina quartz tholeiite, basaltic andesite, andesite, dacite and rhyodacite under dry and wet
conditions has been conducted in order to investigate possible origins of the calc-alkaline series from the upper mantle. Detailed analyses of crystallizing phases using the electron microprobe has enabled calculation of the liquid line of descent in these compositions at various pressures.At 27–36 kb garnet and clinopyroxene are the liquidus or near-liquidus phases in dry tholeiite, basaltic andesite and andesite, while quartz is the liquidus phases in dry dacite and rhyodacite. Under wet conditions at 27 kb garnet, not quartz, is the liquidus phase in the dacite. Qualitatively these results show that the low melting fraction of a quartz eclogite at 27–36 kb under dry conditions is of andesitic composition whereas under wet conditions it is rhyodacitic or granodioritic. At these pressures under dry conditions the andesite liquidus lies in a marked low temperature trough between the more basic and more acid compositions. Quantitatively, the calculated compositions of liquid fractionates for varying degrees of melting of the quartz eclogite bulk composition broadly follow the calc-alkaline trend.At 9–10 kb under wet conditions
sub-silicic amphibole and pyroxenes are the near-liquidus phases in tholeiite and basaltic andesite compositions. Calcic plagioclase and garnet occur nearer the solidus. The calculated liquid fractionates follow the calc-alkaline trend and demonstrate that the calc-alkaline series may be derived by the partial melting of amphibolite at lower crustal depths under wet conditions
, Or by the fractional crystallization of a hydrous basalt magma at similar depths.These experimental results support two complementary hypotheses for the derivation of the calc-alkaline igneous rock suite from the mantle by a two stage igneous process. In the first stage of both hypotheses large piles of basalt are extruded on the earth's surface. Subsequently this pile of basalt may, under dry conditions, transform to quartz eclogite, sink into the mantle and finally undergo partial melting at 100–150 kms depth. This partial melting gives rise to the calc-alkaline magma series leaving a residuum of clinopyroxene and garnet. Alternatively, if wet conditions prevail in the basalt pile and the geotherms remain high, partial melting of the basalt may take place near the base of the pile, at about 10 kb pressure
. The liquids so formed constitute the calc-alkaline suite and the residuum consists of amphibole, pyroxenes and possibly minor garnet and calcic plagioclase. Both models may be directly linked to the hypothesis of sea-floor spreading. 相似文献
11.
B. J. Watts 《Contributions to Mineralogy and Petrology》1973,40(3):225-238
In the 6 component system CaO-MgO-Al2O3-SiO2-CO2-H2 with 9 solid phases (quartz, plagioclase, epidote, tremolite, talc, chlorite, magnesite, calcite, dolomite) and a fluid phase, all 17 possible fluid-absent reactions have been set up and balanced. Using molar entropy and volume data for the solid phases, these reactions are arranged in P-T space about the 8 possible fluid-absent invariant points after the method of Schreinemakers. Field observations in Ordovician greenschist facies basic volcanics at Sofala N.S.W., indicate that neither talc+epidote nor magnesite+calcite are stable under the conditions of metamorphism. Assuming these conditions to apply to the theoretical study here, the fluid-absent invariant points are arranged in a relative fashion with fluid-absent reactions subdividing P-T space into smaller areas.A scheme which permits a fluid of composition
(i.e. a fluid containing CO2 and H2O together with other components), is modeled by treating H2O as a mobile component independent of CO2, and by allowing
values that lie off the
locus of binary H2O-CO2. Taking into account that neither talc+epidote nor magnesite +calcite is to be permitted, the fluid scheme is used to set up and balance all 39 possible fluid-bearing reactions. These are then arranged about 20 valid fluid-bearing invariant points in
space after the method of Korzhinskii and Sehreinemakers.A characteristic solid phase assemblage is defined for each P-T area using chemographic relations inherent from the fluid-absent boundary reactions. The fluid-bearing invariant points that have a solid assemblage compatible with the characteristic assemblage in a particular P-T area are stable within the P-T regime of that area. When these stable fluidbearing invariant points are arranged in a relative fashion in
space, they outline a fluid grid which can be used to study the possible effects of local variation in X
fluid over the particular P-T regime.Symbols Used
U
chemical potential
-
S
entropy
-
V
molar volume
-
n
coefficient of a phase in a reaction
-
X
mole fraction
-
T
temperature
-
P
pressure
-
F
number of degrees of freedom
-
C
number of components
-
p
number of phases
-
s
solid
-
slope of reaction
- 1
quartz
- 2
plagioclase
- 3
epidote
- 4
tremolite
- 5
talc
- 6
chlorite
- 7
dolomite
- 8
magnesite
- 9
calcite 相似文献
12.
Water in microcrystalline quartz of volcanic origin: Agates 总被引:2,自引:0,他引:2
O. W. Flörke B. Köhler-Herbertz K. Langer I. Tönges 《Contributions to Mineralogy and Petrology》1982,80(4):324-333
Agates of volcanic origin, containing the different quartz species, fibrous, length-fast chalcedony (CH), granular fine quartz (FQ), and fibrous, length-slow, to lepidospheric quartzine (QN), have been investigated to evaluate possible relations between microstructure, i.e. crystallite size and texture, refractive indices, densities, contents of trace elements and of water, as well as dehydration behaviour. By means of near infrared spectroscopy, total water contents
, could be differentiated quantitatively into contents of molecular water,
, and silanole-group water,
. Despite the low total water contents of the agates studied (
between 1 and 2 wt.%), near infrared spectroscopy results in reliable data on
and
.Wall-layering CH consists of fibrous quartz crystals and exhibits higher C-ratios,
, than horizontally layered FQ which consists predominantly of granular quartz crystals (C
CH=0.45±0.11 (N=6), C
FQ=0.36±0.10 (N=4). This result is interpreted to be due to analogy with the behaviour of C-ratios in fluid phase-deposited opals-AN (hyalithe) and liquid phase-deposited opals-AG (non-crystalline opal) or -CT (common opal) (Langer and Flörke 1974).Translucent layers of CH show mostly lower refractive indices, when measured parallel than when measured perpendicular to the axes of the quartz fibers. The same is true for milky layers of CH. Crystallite sizes are smaller in the latter than in the former.For all samples studied, exists a positive correlation between at% (1/2Ca+1/2Mg+Na+K+Li) and at% (Al3++Fe3+). This indicates that at least parts of (A13++ Fe3+) substitute for Si in the quartz structure. The charge is balanced by incorporation of di- and mono-valent cations in structural interstices. When the quantity at % H+, as obtained from
, is included into the sum at% (1/2 Me2++Me+), the above correlation is destroyed. This result could be indicative for a strong concentration of the Si-OH groups in the surface of the quartz microcrystallites. 相似文献
13.
Microphenocrystic pyrrhotites were observed in the glassy groundmass of two dacite rocks from Satsuma-Iwojima, southwest Kyushu, Japan. It suggests that the dacite magma was saturated with respect to pyrrhotite at the time of eruption, and thus the sulfur contents in the groundmass can be taken as the solubility of sulfur in the dacite magma. The solubility of sulfur in the dacite rocks thus calculated is 65 to 72 ppm sulfur at the estimated conditions of T=900±50°C,
and
atm. 相似文献
14.
The stability and phase relations of iron chlorite below 8.5 kb P_{{\text{H}}_{\text{2}} {\text{O}}}
Iron chlorites with compositions intermediate between the two end-members daphnite (Fe5Al2Si3O10(OH)8) and pseudothuringite (Fe4Al4Si2O10(OH)8) were synthesized from mixtures of reagent chemicals. The polymorph with a 7 Å basal spacing initially crystallized from these mixtures at 300 °C and 2 kb
after two weeks. Conversion to a 14 Å chlorite required a further 6 weeks at 550 °C. Shorter conversion times were required at higher water pressures. The products contained up to 20% impurities.The maximum equilibrium decomposition temperature for iron chlorite, approximately 550 °C at 2kb, is at an
between assemblages (1) and (2) listed below. Synthetic iron chlorite will break down by various reactions with variable P, T, and fugacity of oxygen. For the composition FeAlSi = 523, the sequence of high temperature breakdown products with increasing
traversing the magnetite field for P
total
=
=2kb is: (1) corierite+ fayalite+hercynite; (2) cordierite+fay alite+magnetite; (3) cordierite+magnetite+quartz; (4) magnetite+mullite+quartz. Almandine should replace cordierite in assemblages (1) and (2) but it did not nucleate. The significance of the relationship between iron cordierite and almandine in this system is discussed.At water pressures from 4 to 8.5 kb and
at the nickel-bunsite buffer, iron chlorite+quartz break down to iron gedrite+magnetite with temperature 550 to 640 °C along the curve. At temperatures 50 °C greater and along a parallel curve, almandine replaces iron gedrite. For
on this buffer curve, almandine is unstable below approximately 4 kb for temperatures to approximately 750 °C. 相似文献
15.
Nigel Harris 《Contributions to Mineralogy and Petrology》1981,76(2):229-233
Metasedimentary migmatites from the Archean charnockitic terrain of South India contain the five phase equilibrium assemblage spinel-cordierite-garnet-corundum-sillimanite. The assemblages is a result of anatexis which has generated a silica-deficient anhydrous restite. Peak metamorphic conditions are defined by the intersection of two divariant reactions in the A12O3-SiO2-FeO-MgO system at which the five phases coexist. These reactions are univariant and their intersection invariant if the Fe/Mg ratio of at least one femic phase is fixed.The location of the invariant point in P/T space is derived from extracting standard stage thermodynamic data from published equilibria experiments in the system Al2O3-SiO2-FeO. Microprobe analyses of coexisting spinel, almandine and cordierite specify the Fe/Mg distributions between phases and allow the computation of the five phase invariant point for
=P
total (770° C, 5.9 kb) and
=O (740° C, 4.8 kb). A low
, implied by evidence of extreme anatexis, indicates a P/T field of T=740±20° C and Ptotal=4.8±0.5 kb which is consistent with the field of equilibration of interlayered charnockites computed from garnet-hypersthene and garnet-plagioclase pairs. 相似文献
16.
Richard J. Arculus 《Contributions to Mineralogy and Petrology》1978,65(4):413-424
The mineralogy and petrology of volcanic and plutonic rocks from the island of Grenada are described. The volcanic rocks include basanitoids, alkalic and subalkalic basalts, andesites and dacites. Phenocryst phases in the basanitoids and basalts are olivine (Fo90–71), zoned calcic augite, spinel ranging from ferrian pleonaste through chromite to titaniferous magnetite, and plagioclase. Some of the basalts contain pargasitic amphibole. Andesites and dacites generally contain hypersthene and augite, and one pigeonite-hypersthene-augite-bearing andesite was found. Apatite commonly occurs as a phenocryst in the andesites and dacites and quartz is present in some dacites as well as being a possible xenocryst in both alkalic and subalkalic basalts. Plutonic cumulates found as ejected fragments in tuffs and ashes are composed of variable proportions of olivine, magnetite, calcic augite, amphibole and plagioclase. One peridotitic (ol-cpx-opx) fragment was found but spinel or garnet peridotitis are absent. Despite the alkalic nature of the association, calcalkalic characteristics such as calcic plagioclase, restricted Feenrichment in coexisting pyroxenes and generally low TiO2 content relative to oceanic suites are present in Grenada. Estimates of conditions of equilibration of the basanitoids with potential upper mantle materials using the results of high-pressure experiments are compared with estimates from thermodynamic data. Equating
and
basanitoid with hypothetical garnet peridotite assemblages gives a pressure and temperature of equilibration in the region of 35–38 kbar and 1550–1625 ° K. Experimental results are not supportive of these estimates. 相似文献
17.
Amphibole composition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer 总被引:66,自引:3,他引:63
Max W. Schmidt 《Contributions to Mineralogy and Petrology》1992,110(2-3):304-310
The Al-in-hornblende barometer, which correlates Altot content of magmatic hornblende linearly with crystallization pressure of intrusion (Hammarstrom and Zen 1986), has been calibrated experimentally under water-saturated conditions at pressures of 2.5–13 kbar and temperatures of 700–655°C. Equilibration of the assemblage hornlende-biotite-plagioclase-orthoclasequartz-sphene-Fe-Ti-oxide-melt-vapor from a natural tonalite 15–20° above its wet solidus results in hornblende compositions which can be fit by the equation: P(±0.6 kbar) = –3.01 + 4.76 Al
hbl
tot
r
2=0.99, where Altot is the total Al content of hornblende in atoms per formula unit (apfu). Altot increase with pressure can be ascribed mainly to a tschermak-exchange (
) accompanied by minor plagioclase-substitution (
). This experimental calibration agrees well with empirical field calibrations, wherein pressures are estimated by contact-aureole barometry, confirming that contact-aureole pressures and pressures calculated by the Al-in-hornblende barometer are essentially identical. This calibration is also consistent with the previous experimental calibration by Johnson and Rutherford (1989b) which was accomplished at higher temperatures, stabilizing the required buffer assemblage by use of mixed H2O-CO2 fluids. The latter calibration yields higher Altot content in hornblendes at corresponding pressures, this can be ascribed to increased edenite-exchange (
) at elevated temperatures. The comparison of both experimental calibrations shows the important influence of the fluid composition, which affects the solidus temperature, on equilibration of hornblende in the buffering phase assemblage. 相似文献
18.
The positions of the liquidi and the near-liquidus phases of olivine-melilitite+CO2 have been determined under MH-buffered and furnace-buffered conditions up to 40 kb. It is found that CO2 alone lowers the liquidus compared to dry conditions, yet its influence is minor compared to H2O. The major role of CO2 is to favour the growth of orthopyroxene and garnet over that of olivine at least at high pressures. CO2-contents of glasses from experiments just above the liquidus (MH-buffered) were determined as 5.1 % at 10kb; 7.5 % at 20kb, 9.3 % at 30kb and 10–11 % (estimated) at 40 kb. Experiments on (pyrolite –40 % olivine)+H2O+CO2 show that CO2 occurs under mantle conditions as carbonate under subsolidus conditions and dissolved in a melt above the solidus. At 30kb, the solidus lies between 1,000 ° C and 1,050 ° C for vapour-saturated conditions, at
and at
. 相似文献
19.
Donald J. DePaolo 《Contributions to Mineralogy and Petrology》1979,69(3):265-278
Thermodynamic calculations, modified after Nicholls et al. (1971), which relate the activity of silica in a lava to the temperature and pressure conditions at which the lava could be in equilibrium with a mantle mineral assemblage, have been extended to H2O-bearing magmas by using published experimental data to derive the dependence of
on the weight fraction of H2O dissolved in a magma. A petrogenetic grid has been calculated which gives the P-T conditions under which a magma with a given
at its liquidus at 1 atm could equilibrate with a mantle mineral assemblage containing olivine (ol) and orthopyroxene (opx) for different amounts of H2O in the magma at its source. This grid is in good agreement with the results of experimental studies as summarized by Green (1971) and Brey and Green (1975). The results show that the pressure at which a given magma composition can equilibrate with ol + opx increases for increasing amounts of H2O dissolved in the magma at depth.In addition, experimental data have been used to calculate the effect of olivine crystallization and removal on the
in the residual liquid to assess the effect of low-pressure differentiation on
. The results show that if 20 % olivine is added to a basalt magma, its calculated pressure of equilibration with ol+opx increases by 4–5 kbar for a given temperature. The calculated effects of olivine removal and H2O addition on
are reasonably consistent with the silicate mixing model of Burnham (1975).Thermodynamic calculations of this type may be useful for assessing the internal consistency of certain experimental data, and in extrapolating the results to other magma compositions. The application of these calculations to determining the possible depth of origin of natural lavas appears to be limited primarily by the difficulty in determining
in a lava at its liquidus temperature. 相似文献
20.
To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn2SiO4, electrical conductivity () of single crystals was measured along either the [100] or the [010] direction. The experiments were carried out at temperatures T=850–1200 °C and oxygen fugacities
atm under both Mn oxide (MO) buffered and MnSiO3 (MS) buffered conditions. Under the same thermodynamic conditions, charge transport along [100] is 2.5–3.0 times faster than along [010]. At high oxygen fugacities, the electrical conductivity of samples buffered against MS is 1.6 times larger than that of samples buffered against MO; while at low oxygen fugacities, the electrical conductivity is nearly identical for the two buffer cases. The dependencies of electrical conductivity on oxygen fugacity and temperature are essentially the same for conduction along the [100] and [010] directions, as well as for samples coexisting with a solid-state buffer of either MO or MS. Hence, it is proposed that the same conduction mechanisms operate for samples of either orientation in contact with either solid-state buffer.The electrical conductivity data lie on concave upward curves on a log-log plot of vs
, giving rise to two
regimes with different oxygen fugacity exponents. In the low-
regime
, the
exponent, m, is 0, the MnSiO3-activity exponent, q, is 0, and the activation energy, Q, is 45 kJ/mol. In the high
regime
10^{ - 7} {\text{atm}}} \right)$$
" align="middle" border="0">
, m=1/6, q=1/4–1/3, and Q=45 and 200 kJ/mol for T<1100 °c=" and=">1100>T>1100 °C, respectively. 相似文献