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1.
The thermal and redox state of the upper mantle beneath the Baikal-Mongolia region was estimated on the basis of the investigation
of the chemical composition (including iron oxidation state) of major minerals (olivine, orthopyroxene, clinopyroxene, and
spinel) in spinel and garnet-spinel peridotite xenoliths from the Cenozoic alkali basalts of the volcanic fields of the Dariganga
Plateau, Tariat Depression, and Vitim Plateau. At temperatures of 1030–1500°C and pressures of 29–47 kbar, the Δlog$
f_{O_2 }
$
f_{O_2 }
values relative to the FMQ buffer (calculated using the olivine-spinel oxygen barometer) range from −0.9 to −1.7 for the
xenoliths of the Dariganga Plateau, from −0.9 to −1.8 for the Tariat Depression, and from −0.8 to −0.1 for the Vitim Plateau.
The oxygen fugacity of peridotites from all of the areas is, in general, lower than that of the WM buffer. Oxygen fugacity
is usually below the CCO and EMOD/G buffers in the peridotites of the Dariganga Plateau and the Tariat Depression and higher
than these buffers in the peridotites of the Vitim Plateau. The T-PΔlog$
f_{O_2 }
$
f_{O_2 }
relationships in the xenoliths suggest the existence of spatial heterogeneity in the thermal and redox state of the upper
mantle of the Baikal-Mongolia region. This heterogeneity is probably related to the influence of the plume that was responsible
for the Late Mesozoic-Cenozoic intraplate magmatism of this region and reflects the different distance of the respective mantle
domains from the plume head. The C-O-H fluids in equilibrium with the upper mantle peridotites are composed mainly of water
and carbon dioxide. The mantle of the Dariganga Plateau and the Tariat Depression (Δlog$
f_{O_2 }
$
f_{O_2 }
< −0.9) is characterized by the dominance of H 2O, whereas CO 2-rich fluids are characteristic of the more oxidized mantle of the Vitim Plateau (Δlog$
f_{O_2 }
$
f_{O_2 }
is mostly higher than −0.8). 相似文献
2.
The Bereznyakovskoe ore field is situated in the Birgil’da-Tomino ore district of the East Ural volcanic zone. The ore field
comprises several centers of hydrothermal mineralization, including the Central Bereznyakovskoe and Southeastern Bereznyakovskoe
deposits, which are characterized in this paper. The disseminated and stringer-disseminated orebodies at these deposits are
hosted in Upper Devonian-Lower Carboniferous dacitic-andesitic tuff and are accompanied by quartz-sericite hydrothermal alteration.
Three ore stages are recognized: early ore (pyrite); main ore (telluride-base-metal, with enargite, fahlore-telluride, and
gold telluride substages); and late ore (galena-sphalerite). The early and the main ore stages covered temperature intervals
of 320–380 to 180°C and 280–300 to 170°C, respectively; the ore precipitated from fluids with a predominance of NaCl. The
mineral zoning of the ore field is expressed in the following change of prevalent mineral assemblages from the Central Bereznyakovskoe
deposit toward the Southeastern Bereznyakovskoe deposit: enargite, tennantite, native tellurium, tellurides, and selenides
→ tennantite-tetrahedrite, tellurides, and sulfoselenides (galenoclausthalite) → tetrahedrite, tellurides, native gold, galena,
and sphalerite. The established trend of mineral assemblages was controlled by a decrease in $
f_{S_2 }
$
f_{S_2 }
, $
f_{Te_2 }
$
f_{Te_2 }
and $
f_{O_2 }
$
f_{O_2 }
and an increase in pH of mineral-forming fluids from early to late assemblages and from the Central Bereznyakovskoe deposit
toward the Southeastern Bereznyakovskoe deposit. Thus, the Central Bereznyakovskoe deposit was located in the center of an
epithermal high-sulfidation ore-forming system. As follows from widespread enargite and digenite, a high Au/Ag ratio, and
Au-Cu specialization of this deposit, it is rather deeply eroded. The ore mineralization at the Southeastern Bereznyakovskoe
deposit fits the intermediate- or low-sulfidation type and is distinguished by development of tennantite, a low Au/Ag ratio,
and enrichment in base metals against a lowered copper content. In general, the Bereznyakovskoe ore field is a hydrothermal
system with a wide spectrum of epithermal mineralization styles. 相似文献
3.
Compositions of the major phenocryst minerals (olivine, phlogopite) and groundmass minerals (olivine, phlogopite, kalsilite), and a glass phase have been determined from a biotite mafurite occurring as an ejected block in the highly K-rich ultramafic rocks of south west Uganda. Comparison of the phenocryst mineral compositions with those determined from recent high pressure experiments on biotite mafurite composition suggests this rock may have formed by partial melting of a K-enriched mantle source containing both H 2O and CO 2 at approximately 1,250 ° C and 30 kb. The absence of crystalline leucite but its presence as a major component of the glass phase and textural relations in the groundmass indicate that the final consolidation of the biotite mafurite took place at pressures greater than atmospheric. The presence of phlogopite, olivine, kalsilite, and glass mainly of leucite composition may suggest that consolidation took place under the conditions where these phases were in equilibrium. Based on the experimentally determined conditions for the reaction of phlogopite break down to olivine+kalsilite +liquid+vapor, a crude estimation of the consolidation conditions for ejected blocks of biotite mafurite are 1,150 °–1,180 ° C at a
of 1–2 kb. 相似文献
4.
Fayalite is a common mineral of Fe-rich paralavas related to spontaneous combustion of coal seams. Fayalite has also been
found in parabasalts from burned coal waste piles of the Chelyabinsk coal basin. Among paralavas from different combustion
metamorphic (CM) complexes of the world, fayalite is the most widespread in the fused rocks of the Kuznetsk coal basin (Kuzbass)
and the Ravat area in Tajikistan. The optimal conditions for fayalite formation as products of coal fires in the Kuzbass and
Ravat resulted from a favorable combination of the composition of fused protolith (parental rocks) composed of pelitic and
Fe-rich sediments and the redox conditions of the deep subsurface ($
f_{O_2 }
$
f_{O_2 }
is lower than the QFM buffer). In the Kuzbass, fayalite is commonly hosted in high-silica aluminous Fe-rich paralavas composed
of Fe-cordierite (sekaninaite), tridymite, hercynite-magnetite, cristobalite, aluminous clinoferrosilite, and Al-K silicic
glass. The composition of all Kuzbass fayalites is close to the Fe 2SiO 4 end member. Kuzbass fayalites are characterized by a negligibly low CaO content and higher MnO and P 2O 5 contents like fayalites from burned rocks of other CM complexes. In Kuzbass paralavas, Fe-olivine is the late phase that
crystallized after sekaninaite and tridymite, immediately before melt quenching. 相似文献
5.
The experimental distribution coefficient for Ni/ Fe exchange between olivine and monosulfide (K D3) is 35.6±1.1 at 1385° C, \(f_{{\text{O}}_{\text{2}} } = 10^{ - 8.87} ,f_{{\text{S}}_{\text{2}} } = 10^{ - 1.02} \) , and olivine of composition Fo96 to Fo92. These are the physicochemical conditions appropriate to hypothesized sulfur-saturated komatiite magma. The present experiments equilibrated natural olivine grains with sulfide-oxide liquid in the presence of a (Mg, Fe)-alumino-silicate melt. By a variety of different experimental procedures, K D3 is shown to be essentially constant at about 30 to 35 in the temperature range 900 to 1400° C, for olivine of composition Fo97 to FoO, monosulfide composition with up to 70 mol. % NiS, and a wide range of \(f_{{\text{O}}_{\text{2}} } \) and \(f_{{\text{S}}_{\text{2}} } \) . 相似文献
6.
The results of thermodynamic modeling of equilibriums between Cu, Fe, and Zn sulfides and oxides pertaining to the Cu-Fe-Zn-S-O 2 system in water and aqueous chloride solution are presented. The system comprises solid phases of constant composition: pyrite,
pyrrhotite, hematite, magnetite, wüstite, γ-iron, chalcocite, covellite, cuprite, native copper, chalcopyrite, and bornite,
as well as more than 100 ions, complexes, and molecules in an aqueous solution.
The GIBBS program with the UNITHERM thermodynamic dataset used in calculations allows numerical analysis of phase assemblages
in a dry system and in equilibrium with an aqueous solution. How the temperature, pressure, and the composition of the solution
in the system opened for oxygen and sulfur affects the composition of phase assemblages was considered in temperature and
pressure ranges of 50–350 C and 100–1000 bar, respectively. Decrease in temperature leads to a shift in stability fields of
the studied phases toward the region of elevated oxygen and sulfur partial pressures. Variation of temperature is an important
factor affecting precipitation of ore minerals, primarily, Cu- and Zn-bearing. The calculation results are presented in tables
and diagrams. Each point in the $
(\log m_{S_{tot} } - \log f_{O_2 } )
$
(\log m_{S_{tot} } - \log f_{O_2 } )
diagram is characterized by a single possible assemblage of phases equilibrated with a solution of the given composition
within the considered temperature and pressure range. Since the composition of the mineral assemblage is controlled by physicochemical
conditions at the moment of mineral formation, comparison of the calculation results with mineral assemblages at ore deposits
makes it possible to estimate the parameters of ore deposition at the early stage of investigation, including oxygen and sulfur
activity and, occasionally, the composition and salinity of the solution. These parameters control the formation of such assemblages. 相似文献
7.
The oxygen fugacity condition of equilibration has been carefully determined from a spinel lherzolite from Mongolia, olivine xenocrysts from chrome pyrope-bearing peridotite nodules from kimberlites of Yakutia, and basaltic samples from ocean floor, iron arcs and the continental areas. These indicate that the spinel lherzolites occurring within alkali basalts from Mongolia, equilibrated under an \(f_{O_2 } \) condition similar to that of WM buffer. The diamond and chrome pyrope-bearing peridotites from the kimberlite pipes equilibrated between IW and WM buffers. Some of the ilmenite-bearing peridotite crystallized under \(f_{O_2 } \) conditions similar to that between WM and QFM buffers and chondrites equilibrated below the QFI buffer. It is concluded that during geochemical processes in the upper mantle the \(f_{O_2 } \) conditions vary broadly, and are similar to that between FMQ and IW buffers. There is a dramatic change in the composition of the kimberlitic fluid, which is CH 4-bearing at an early stage, but is in equilibrium with H 2O and CO 2 at a later stage. This is related to mass transfer of fluids from the lower part of the mantle with a low oxidation state to the upper part having a higher \(f_{O_2 } \) condition. 相似文献
8.
The paper reports data on the chemical composition of mantle peridotite xenoliths from kimberlites and alkaline basalts that
represent the continental lithospheric mantle (CLM) beneath Early Precambrian and Late Proterozoic-Cenozoic structures, respectively.
In order to identify compositional trends during the melting of primitive material and propose the most reliable criteria
for constraining the conditions of this process and its degree, we analyzed literature data on the melting of spinel and garnet
peridotites within broad temperature and pressure ranges. It was determined that the degree of melting (F%) of pristine peridotite
of composition close to that of the primitive mantle (PM) can be deduced from the Mg/Si and Al/Si ratios in the residue; an
equation was proposed for evaluating F from the Mg/Si ratio. The Ca/Al ratio of residues at low (1–1.5 GPa) pressures and
degrees of melting from 2–3 to 20–25% increases several times but decreases with increasing F at pressures higher than 3 GPa.
The Na partition coefficient between melt and residue decreases at increasing pressure and approaches one at a pressure close
to 20 GPa. Residues after low-degree melting are strongly depleted in Ti, Zr, Y, and Nb but are enriched in Cr. The application
of these criteria to the composition of xenoliths brought to the surface from the mantle occurring beneath tectonic structures
of various age led us to conclude that compositional heterogeneities of CLM (particularly the variations in the concentrations
of major and certain siderophile elements) are controlled, first of all, by the melting of the mantle source material. These
processes occurred under various thermodynamic conditions ( T, P, and $
f_{O_2 }
$
f_{O_2 }
) and differed in their intensity, and this predetermined the compositional diversity of the residual mantle material (its
concentrations of Mg, Al, Si, Ca, Na, K, Ni, Co, V, and Cr). Our results are principally consistent with the hypothesis of
the global magmatic ocean. It is thought that the early phases of its consolidation were variably controlled by the fractionation
of minerals, for example, majorite. Moreover, heterogeneities in the distribution of siderophile elements could be partly
predetermined by changes in the properties of these elements at ultrahigh temperatures and pressures. The processes of partial
melting were the most intense during the early evolution of the mantle (perhaps, in the Early Precambrian), and hence, the
mantle has different chemical composition beneath Archean cratons and Phanerozoic foldbelts. 相似文献
9.
Synthetic spinel harzburgite and lherzolite assemblages were equilibrated between 1040 and 1300° C and 0.3 to 2.7 GPa, under
controlled oxygen fugacity ( f
O
2). f
O
2 was buffered with conventional and open double-capsule techniques, using the Fe−FeO, WC-WO 2-C, Ni−NiO, and Fe 3O 4−Fe 2O 3 buffers, and graphite, olivine, and PdAg alloys as sample containers. Experiments were carried out in a piston-cylinder apparatus
under fluid-excess conditions. Within the P-T-X range of the experiments, the redox ratio Fe 3+/ΣFe in spinel is a linear function of f
O
2 (0.02 at IW, 0.1 at WCO, 0.25 at NNO, and 0.75 at MH). It is independent of temperature at given Δlog( f
O
2), but decreases slightly with increasing Cr content in spinel. The Fe 3+/ΣFe ratio falls with increasing pressure at given Δlog( f
O
2), consistent with a pressure correction based on partial molar volume data. At a specific temperature, degree of melting
and bulk composition, the Cr/(Cr+Al) ratio of a spinel rises with increasing f
O
2. A linear least-squares fit to the experimental data gives the semi-empirical oxygen barometer in terms of divergence from
the fayalite-magnetite-quartz (FMQ) buffer:
|
相似文献
10.
The solubility of pentatungstate of sodium ( PTS) Na 2W 5O 16 · H 2O and sodium tungsten bronzes ( STB) Na 0.16WO 3 in acid chloride solutions containing 0.026, 0.26, and 3.02 m NaCl have been studied at 500°C, 1000 bar, given fO 2 (Co-CoO, Ni-NiO, PTS-STB buffers), and constant NaCl/HCl ratio (Ta 2O 5-Na 2Ta 4O 11 buffer). Depending on experimental conditions, the tungsten content in the solutions after experiments varied from 10 −3 to 2 × 10 −2 mol/kg H 2O. Obtained data were used to calculate the formation constants of predominant tungsten complexes (VI, V): H 3W 3VIO 123−, W 3VO 93−, [W VW 4VIO 16] 3−, for reactions
|
$
\begin{gathered}
3H_2 WO_4^0 \leftrightarrow H_3 W_3 O_{12}^{3 - } + 3H^ + \log K_p = - 7.5 \pm 0.1, \hfill \\
3H_2 WO_4^0 \leftrightarrow W_3 O_9^{3 - } + 1.5H_2 O + 3H^ + + 0.75O_2 \log K_p = - 25.7 \pm 0.2, \hfill \\
5H_2 WO_4^0 \leftrightarrow \left[ {W^V W_4^{VI} O_{16} } \right]^{3 - } + 3H^ + + 3.5H_2 O + 0.25O_2 \log K_p = - 4.6 \pm 0.1 \hfill \\
\end{gathered}
$
\begin{gathered}
3H_2 WO_4^0 \leftrightarrow H_3 W_3 O_{12}^{3 - } + 3H^ + \log K_p = - 7.5 \pm 0.1, \hfill \\
3H_2 WO_4^0 \leftrightarrow W_3 O_9^{3 - } + 1.5H_2 O + 3H^ + + 0.75O_2 \log K_p = - 25.7 \pm 0.2, \hfill \\
5H_2 WO_4^0 \leftrightarrow \left[ {W^V W_4^{VI} O_{16} } \right]^{3 - } + 3H^ + + 3.5H_2 O + 0.25O_2 \log K_p = - 4.6 \pm 0.1 \hfill \\
\end{gathered}
相似文献
11.
The oxygen fugacity (
) of a C-O-H fluid in equilibrium with graphite has been determined in the range 10–30 kbar by equilibrating solid
-buffer assemblages in graphite capsules containing C-O-H fluid. By using different buffers (Fe xO-Fe 3O 4, Ni-NiO, Co-CoO, Mo-MoO 2), the
of the graphite-saturated fluid is bracketed within a narrow range. This technique produces a calibration for the
imposed on a sample contained within a graphite capsule. To achieve a thermodynamically-invariant system at fixed P and T,
the
was imposed on the system with an external buffer and the double-capsule technique. The experiments were performed in solid-media,
high pressure apparatus with 19 mm tale-pyrex assemblies. A series of experiments at 10, 15, 20, 25, and 30 kbar, 800–1600°
C, with
imposed by the Fe 2O 3-Fe 3O 4-H 2O equilibrium were conducted. The experimental results have been fitted to the following equation:
|
相似文献
12.
Larkman Nunatak (LAR) 06319 is an olivine-phyric shergottite whose olivine crystals contain abundant crystallized melt inclusions. In this study, three types of melt inclusion were distinguished, based on their occurrence and the composition of their olivine host: Type-I inclusions occur in phenocryst cores (Fo 77-73); Type-II inclusions occur in phenocryst mantles (Fo 71-66); Type-III inclusions occur in phenocryst rims (Fo 61-51) and within groundmass olivine. The sizes of the melt inclusions decrease significantly from Type-I (∼150-250 μm diameter) to Type-II (∼100 μm diameter) to Type-III (∼25-75 μm diameter). Present bulk compositions (PBC) of the crystallized melt inclusions were calculated for each of the three melt inclusion types based on average modal abundances and analyzed compositions of constituent phases. Primary trapped liquid compositions were then reconstructed by addition of olivine and adjustment of the Fe/Mg ratio to equilibrium with the host olivine (to account for crystallization of wall olivine and the effects of Fe/Mg re-equilibration). The present bulk composition of Type-I inclusions (PBC1) plots on a tie-line that passes through olivine and the LAR 06319 whole-rock composition. The parent magma composition can be reconstructed by addition of 29 mol% olivine to PBC1, and adjustment of Fe/Mg for equilibrium with olivine of Fo 77 composition. The resulting parent magma composition has a predicted crystallization sequence that is consistent with that determined from petrographic observations, and differs significantly from the whole-rock only in an accumulated olivine component (∼10 wt%). This is consistent with a calculation indicating that ∼10 wt% magnesian (Fo 77-73) olivine must be subtracted from the whole-rock to yield a melt in equilibrium with Fo 77. Thus, two independent estimates indicate that LAR 06319 contains ∼10 wt% cumulate olivine.The rare earth element (REE) patterns of Type-I melt inclusions are similar to that of the LAR 06319 whole-rock. The REE patterns of Type-II and Type-III melt inclusions are also broadly parallel to that of the whole-rock, but at higher absolute abundances. These results are consistent with an LAR 06319 parent magma that crystallized as a closed-system, with its incompatible-element enrichment being inherited from its mantle source region. However, fractional crystallization of the reconstructed LAR 06319 parent magma cannot reproduce the major and trace element characteristics of all enriched basaltic shergottites, indicating local-to-large scale major- and trace-element variations in the mantle source of enriched shergottites. Therefore, LAR 06319 cannot be parental to the enriched basaltic shergottites. 相似文献
13.
Solubility curves of water-hydrogen fluid were studied using a high-pressure gas apparatus at a pressure of 200 MPa under
variable fluid composition in haplogranite ( Ab
39
Or
32
Qtz
29, 950°C), Na-disilicate (Na 2Si 2O 5, 950°C), and albite melts (1200°C). The mole fraction of hydrogen in experiments was controlled directly by Ar-H 2 mixtures using a specially designed cell with a Shaw membrane. $
X_{H_2 }^{Ar - H_2 }
$
X_{H_2 }^{Ar - H_2 }
ranged from 0 to 1. In some experiments with haplogranite and Na-disilicate melts under oxidizing conditions, in order to
increase the accuracy of experimental parameters, the fugacities of oxygen and hydrogen were controlled using the double-capsule
technique and the solid-phase buffer mixtures Ni-NiO (NNO) and Co-CoO (CCO). The addition of H 2 to the H 2O-saturated systems ($
X_{H_2 }^{H_2 O - H_2 }
$
X_{H_2 }^{H_2 O - H_2 }
≥ 0.012) results in the appearance of a distinct maximum on the solubility curves at $
X_{H_2 }^{H_2 O - H_2 }
$
X_{H_2 }^{H_2 O - H_2 }
= 0.05–0.07 (H 2 mole fractions were calculated for real H 2O-H 2 mixtures of real gases), and the maximum content of H 2O-H 2 fluid increases relative to the H 2O-saturated melts by 1.51 wt % for haplogranite melt at $
X_{H_2 }
$
X_{H_2 }
= 0.063, 2.68 wt % for albite melt at $
X_{H_2 }
$
X_{H_2 }
= 0.066, and 3.54 wt % for Na-disilicate melt at $
X_{H_2 }
$
X_{H_2 }
= 0.067. A further increase in H 2 content in the gas mixture decreases the solubility of H 2O-H 2 fluid in the melts, and under pure H 2 pressure, the contents of fluid components are 0.08 wt % in haplogranite melt and 0.06 wt % in albite melt. The 1H NMR study of aluminosilicate and Na-silicate glasses obtained under the pressure of H 2O and H 2O-H 2 fluids suggests different mechanisms of the dissolution of H 2O and H 2O-H 2 fluids in magmatic melts. In addition to the spectra of dissolved water fluid, the spectra of quenched glasses synthesized
under H 2O-H 2 fluid pressure exhibited a narrow line of molecular hydrogen with a width at half height of 1.8–2.0 kHz at $
X_{H_2 }
$
X_{H_2 }
≥ 0.653 for albite and $
X_{H_2 }
$
X_{H_2 }
≥ 0.063 for Na-disilicate and two lines at $
X_{H_2 }
$
X_{H_2 }
≥ 0.063 for the haplogranite composition. 相似文献
14.
In this paper we describe the mineralogy and geochemistry of basanites and melt inclusions in minerals from the Tergesh pipe,
northern Minusinsk Depression. The rocks are composed of olivine and clinopyroxene phenocrysts and a groundmass of olivine,
clinopyroxene, titanomagnetite, plagioclase, apatite, ilmenite, and glass. Melt inclusions were found only in the olivine
and clinopyroxene phenocrysts. Primary melt inclusions in olivine contain glass, rh?nite, clinopyroxene, a sulfide globule,
and low-density fluid. The phase composition of melt inclusions in clinopyroxene is glass + low-density fluid ± xenogenous
magnetite. According to thermometric investigations, the olivine phenocrysts began crystallizing at T = 1280–1320°C and P > 3.5 kbar, whereas groundmass minerals were formed under near-surface conditions at T ≤ 1200°C. The oxygen fugacity gradually changed during basanite crystallization from oxidizing (NNO) to more reducing conditions
(QFM). The investigation of glass compositions (heated and unheated inclusions in phenocrysts and groundmass) showed that
the evolution of a basanite melt during its crystallization included mainly an increase in SiO 2, Al 2O 3, and alkalis, while a decrease in femic components, and the melt composition moved gradually toward tephriphonolite and trachyandesite.
Geochemical evidence suggests that the primary basanite melt was derived from a mantle source affected by differentiation.
Original Russian Text ? T.Yu. Timina, V.V. Sharygin, A.V. Golovin, 2006, published in Geokhimiya, 2006, No. 8, pp. 814–833. 相似文献
15.
The onset of hydrous partial melting in the mantle above the transition zone is dictated by the H 2O storage capacity of peridotite, which is defined as the maximum concentration that the solid assemblage can store at P and T without stabilizing a hydrous fluid or melt. H 2O storage capacities of minerals in simple systems do not adequately constrain the peridotite water storage capacity because
simpler systems do not account for enhanced hydrous melt stability and reduced H 2O activity facilitated by the additional components of multiply saturated peridotite. In this study, we determine peridotite-saturated
olivine and pyroxene water storage capacities at 10–13 GPa and 1,350–1,450°C by employing layered experiments, in which the
bottom ~2/3 of the capsule consists of hydrated KLB-1 oxide analog peridotite and the top ~1/3 of the capsule is a nearly
monomineralic layer of hydrated Mg# 89.6 olivine. This method facilitates the growth of ~200-μm olivine crystals, as well
as accessory low-Ca pyroxenes up to ~50 μm in diameter. The presence of small amounts of hydrous melt ensures that crystalline
phases have maximal H 2O contents possible, while in equilibrium with the full peridotite assemblage (melt + ol + pyx + gt). At 12 GPa, olivine and
pyroxene water storage capacities decrease from ~1,000 to 650 ppm, and ~1,400 to 1,100 ppm, respectively, as temperature increases
from 1,350 to 1,450°C. Combining our results with those from a companion study at 5–8 GPa (Ardia et al., in prep.) at 1,450°C,
the olivine water storage capacity increases linearly with increasing pressure and is defined by the relation
C\textH2 \textO\textolivine ( \text ppm ) = 57.6( ±16 ) × P( \text GPa ) - 169( ±18 ). C_{{{\text{H}}_{2} {\text{O}}}}^{\text{olivine}} \left( {\text{ppm}} \right) = 57.6\left( { \pm 16} \right) \times P\left( {\text{GPa}} \right) - 169\left( { \pm 18} \right). Adjustment of this trend for small increases in temperature along the mantle geotherm, combined with experimental determinations
of
D\textH2 \textO\textpyx/olivine D_{{{\text{H}}_{2} {\text{O}}}}^{\text{pyx/olivine}} from this study and estimates of
D\textH2 \textO\textgt/\textolivine D_{{{\text{H}}_{2} {\text{O}}}}^{{{\text{gt}}/{\text{olivine}}}} , allows for estimation of peridotite H 2O storage capacity, which is 440 ± 200 ppm at 400 km. This suggests that MORB source upper mantle, which contains 50–200 ppm
bulk H 2O, is not wet enough to incite a global melt layer above the 410-km discontinuity. However, OIB source mantle and residues
of subducted slabs, which contain 300–1,000 ppm bulk H 2O, can exceed the peridotite H 2O storage capacity and incite localized hydrous partial melting in the deep upper mantle. Experimentally determined values
of
D\textH2 \textO\textpyx/\textolivine D_{{{\text{H}}_{2} {\text{O}}}}^{{{\text{pyx}}/{\text{olivine}}}} at 10–13 GPa have a narrow range of 1.35 ± 0.13, meaning that olivine is probably the most important host of H 2O in the deep upper mantle. The increase in hydration of olivine with depth in the upper mantle may have significant influence
on viscosity and other transport properties. 相似文献
16.
The chemical compositions of melt inclusions in a primitive and an evolved basalt recovered from the mid-Atlantic ridge south
of the Kane Fracture Zone (23°–24°N) are determined. The melt inclusions are primitive in composition (0.633–0.747 molar Mg/(Mg+Fe 2+), 1.01–0.68 wt% TiO 2) and are comparable to other proposed parental magmas except in having higher Al 2O 3 and lower CaO. The primitive melt inclusion compositions indicate that the most primitive magmas erupted in this region are
not near primary magma compositions. Olivine and plagioclase microphenocrysts are close to exchange equilibrium with their
respective basalt glasses, whose compositions are displaced toward olivine from 1 atm three phase saturation. The most primitive
melt inclusion compositions are close to exchange equilibrium with the anorthitic cores of zoned plagioclases (An 78.3-An 83.1; the hosts for the melt inclusions in plagioclase) and with olivines more forsteritic (Fo 89-Fo 91) than the olivine microphenocrysts (the hosts for the melt inclusions in olivine). Xenocrystic olivine analyzed is Fo 89 but contains no melt inclusions. These observations indicate that olivines have exchanged components with the melt after
melt inclusion entrapment, whereas plagioclase compositions have remained the same since melt inclusion entrapment. Common
denominator element ratio diagrams and oxide versus oxide variation diagrams show that the melt inclusion compositions, which
represent liquids higher along the liquid line of descent, are related to the glass compositions by the fractionation of olivine,
plagioclase and clinopyroxene (absent from the mincral assemblage), probably occurring at elevated pressures. A model is proposed
whereby clinopyroxene segregates from the melt at elevated pressures (to account for its absence in the erupted lavas that
have the chemical imprint of clinopyroxene fractionation). Zoned plagioclases in the erupted lavas are thought to be survivors
of decompressional melting during magma ascent. Since similar primitive melt inclusions occur in olivine microphenocrysts
and in the cores of zoned plagioclases, any model must account for all phases present. 相似文献
17.
High pressure experimental studies of the melting of lherzolitic upper mantle in the absence of carbon and hydrogen have shown
that the lherzolite solidus has a positive d P/d T and that the percentage melting increases quite rapidly above the solidus. In contrast, the presence of carbon and hydrogen
in the mantle results in a region of ‘incipient’ melting at temperatures below the C,H-free solidus. In this region the presence
or absence of melt and the composition of the melt are dependent on the amount and nature of volatiles, particularly the CO 2, H 2O, and CH 4 contents of the potential C-H-O fluid. Under conditions of low
(IW to IW + 1 log unit at P ∼ 20–35kb), fluids such as CH 4+H 2O and CH 4+H 2 inhibit melting, having a low solubility in silicate melts. Under these conditions, carbon and hydrogen are mobile elements
in the upper mantle. At slightly higher oxygen fugacity (IW+2 log units, P∼20–35 kb) fluids in equilibrium with graphite or diamond in peridotite C-H-O are extremely water-rich. Carbon is thus not
mobile in the mantle in this
range and the melting and phase relations for the upper mantle lherzolite approximate closely to the peridotite-H 2O system. Pargasitic amphibole is stable to solidus temperatures in fertile lherzolite compositions and causes a distinctive
peridotite solidus, the ‘dehydration solidus’, with a marked change in slope (a ‘back bend’) at 29–30kb due to instability
of pargasite at high pressure. Intersections of geothermal gradients with the peridotite-H 2O solidi define the boundary between lithosphere (subsolidus) and asthenosphere (incipient melt region). This boundary is
thus sensitive to changes in
[affecting CH 4:H 2O:CO 2 ratios] and to the amount of H 2O and carbon (CO 2, CH 4) present. At higher
conditions (IW + 3 log units), CO 2-rich fluids occur at low pressures but there is a marked depression of the solidus at 20–21 kb due to intersection with the
carbonation reaction, producing the low temperature solidus for dolomite amphibole lherzolite ( T∼925°C, 21 to >31kb). Melting of dolomite (or magnesite) amphibole lherzolite yields primary sodic dolomitic carbonatite melt
with low H 2O content, in equilibrium with amphibole garnet lherzolite.
The complexity of melting in peridotite-C-H-O provides possible explanations for a wide range of observations on lithosphere/asthenosphere
relations, on mantle melt and fluid compositions, and on processes of mantle metasomatism and magma genesis in the upper mantle. 相似文献
18.
A technique is described for determining the cooling historyof olivine phenocrysts. The technique is based on the analysisof the diffusive re-equilibration of melt inclusions trappedby olivine phenocrysts during crystallization. The mechanismof re-equilibration involves diffusion of Fe from and Mg intothe initial volume of the inclusion. The technique applies toa single crystal, and thus the cooling history of differentphenocrysts in a single erupted magma can be established. Weshow that melt inclusions in high-Fo olivine phenocrysts frommantle-derived magmas are typically partially re-equilibratedwith their hosts at temperatures below trapping. Our analysisdemonstrates that at a reasonable combination of factors suchas (1) cooling interval before eruption (<350°C), (2)eruption temperatures (>1000°C), and (3) inclusion size(<70 µm in radius), partial re-equilibration of upto 85% occurs within 3–5 months, corresponding to coolingrates faster than 1–2°/day. Short residence timesof high-Fo phenocrysts suggest that if eruption does not happenwithin a few months after a primitive magma begins cooling andcrystallization, olivines that crystallize from it are unlikelyto be erupted as phenocrysts. This can be explained by efficientseparation of olivine crystals from the melt, and their rapidincorporation into the cumulate layer of the chamber. Theseresults also suggest that in most cases erupted high-Fo olivinephenocrysts retain their original composition, and thus compositionsof melt inclusions in erupted high-Fo olivine phenocrysts donot suffer changes that cannot be reversed. Short residencetimes also imply that large unzoned cores of high-Fo phenocrystscannot reflect diffusive re-equilibration of originally zonedphenocrysts. The unzoned cores are a result of fast efficientaccumulation of olivines from the crystallizing magma, i.e.olivines are separated from the magma faster than melt changesits composition. Thus, the main source of high-Fo crystals inthe erupted magmas is the cumulate layers of the magmatic system.In other words, olivine-phyric rocks represent mixtures of anevolved transporting magma (which forms the groundmass of therock) with crystals that were formed during crystallizationof more primitive melt(s). Unlike high-Fo olivine phenocrysts,the evolved magma may reside in the magmatic system for a longtime. This reconciles long magma residence times estimated fromthe compositions of rocks with short residence times of high-Foolivine phenocrysts. KEY WORDS: melt inclusions; olivine; picrites; residence time; diffusion 相似文献
19.
The paper summarizes experimental and calculation data on the effect of oxygen fugacity on the origin of mineral assemblages in Mn-bearing rocks and demonstrates the possibility of application of these data to the reconstruction of conditions under which metalliferous deposits were metamorphosed. A new variant of the T-log \(f_{O_2 } \) diagram is proposed for the Mn-Si-O system, which differs from previous ones by the location of the lines for the formation (decomposition) of braunite and tephroite. These two minerals are the most universal indicators of oxygen fugacity during the metamorphism of Mn-bearing deposits, because these minerals are widespread in nature and can be formed in diverse environments: braunite at high \(f_{O_2 } \) values in the pore solution, and tephroite at low \(f_{O_2 } \) values. The occurrence of Mn oxides and rhodonite (pyroxmangite) in a rock makes it possible to constrain the oxygen fugacity range. An original T-log \(f_{O_2 } \) diagram is constructed for the Ca-Mn-Si-O system. As follows from this diagram, a Ca admixture expands the stability field of rhodonite toward higher oxygen fugacity values. Johannsenite can be formed in these rocks at even higher \(f_{O_2 } \). The stability of both minerals is constrained in the region of low \(f_{CO_2 } \). The paper reports data on the Fe-Si-O and Mn-Fe-Si-O systems and discusses the possibility of applying the results of experiments in the Mn-Al-Si-O system to the estimation of conditions under which andalusite, spessartine, and galaxite can be formed in Mn-bearing rocks. Data on the mineralogy of numerous Mn deposits metamorphosed under various PTX parameters indicate that the origin of Mn-bearing mineral assemblages depends not so much on the temperature and pressure as on the oxygen fugacity, which is, in turn, controlled primarily by the composition of the pristine sediments (the presence or absence of organic matter in them) and host rocks and depends on the permeability of the rocks to oxygen, the P-T conditions, and the duration of the metamorphic processes. 相似文献
20.
A great wealth of analytical data for fluid inclusions in minerals indicate that the major species of gases in fluid inclusions are H 2O, CO 2, CO, CH 4, H 2 and O 2. Three basic chemical reactions are supposed to prevail in rock-forming and ore-forming fluids: $$\begin{gathered} H_2 + 1/2{\text{ O}}_{\text{2}} = H_2 O, \hfill \\ CO + 1/2{\text{ O}}_{\text{2}} = CO_2 , \hfill \\ CH_4 + 2{\text{O}}_{\text{2}} = CO_2 + 2H_2 O, \hfill \\ \end{gathered} $$ and equilibria are reached among them. \(\lg f_{O_2 } - T,{\text{ }}\lg f_{CO_2 } - T\) and Eh- T charts for petrogenesis and minerogenesis in the supercritical state have been plotted under different pressures. On the basis of these charts \(f_{O^2 } ,{\text{ }}f_{CO_2 } \) , Eh, equilibrium temperature and equilibrium pressure can be readily calculated. In this paper some examples are presented to show their successful application in the study of the ore-forming environments of ore deposits. 相似文献
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