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1.
Yusuke Seto Daisuke Hamane Takaya Nagai Kiyoshi Fujino 《Physics and Chemistry of Minerals》2008,35(4):223-229
We report on high-pressure and high-temperature experiments involving carbonates and silicates at 30–80 GPa and 1,600–3,200 K,
corresponding to depths within the Earth of approximately 800–2,200 km. The experiments are intended to represent the decomposition
process of carbonates contained within oceanic plates subducted into the lower mantle. In basaltic composition, CaCO3 (calcite and aragonite), the major carbonate phase in marine sediments, is altered into MgCO3 (magnesite) via reactions with Mg-bearing silicates under conditions that are 200–300°C colder than the mantle geotherm.
With increasing temperature and pressure, the magnesite decomposes into an assemblage of CO2 + perovskite via reactions with SiO2. Magnesite is not the only host phase for subducted carbon—solid CO2 also carries carbon in the lower mantle. Furthermore, CO2 itself breaks down to diamond and oxygen under geotherm conditions over 70 GPa, which might imply a possible mechanism for
diamond formation in the lower mantle. 相似文献
2.
Lin-gun Liu 《Contributions to Mineralogy and Petrology》2002,144(1):16-21
Diamonds containing ferropericlase (Mg,Fe)O and other silicate (enstatite [(Mg,Fe)SiO3], in particular) assemblages are generally believed to be derived from the Earth's lower mantle. On the basis of the observed ratio between ferropericlase and enstatite inclusions and the FeO content of these ferropericlases, it is concluded that most of these minerals entrapped in diamonds may not represent the lithology of the lower mantle itself as has been suggested by many investigators. Instead, ferropericlases in these diamonds represent most likely the disproportionate product of ferromagnesite [(Mg,Fe)CO3], which underwent a decarbonation reaction to form both diamond and ferropericlase simultaneously in the lower mantle. The wide variation in the Mg# of ferropericlase inclusions in diamonds is attributed to the decarbonation "loop" of the MgCO3-FeCO3 solid solutions. Some of the enstatite inclusions coexisting with these ferropericlases in the same diamond may represent the most abundant mineral species of (Mg,Fe)SiO3-perovskite in the lower mantle. The latter mineral phase experienced a retrogressive transition into enstatite during the transport of diamonds to the Earth's surface. 相似文献
3.
Wenjun Yong E. Dachs A. C. Withers E. J. Essene 《Physics and Chemistry of Minerals》2006,33(3):167-177
The low-temperature heat capacity (C
p
) of KAlSi3O8 with a hollandite structure was measured over the range of 5–303 K with a physical properties measurement system. The standard entropy of KAlSi3O8 hollandite is 166.2±0.2 J mol−1 K−1, including an 18.7 J mol−1 K−1 contribution from the configurational entropy due to disorder of Al and Si in the octahedral sites. The entropy of K2Si4O9 with a wadeite structure (Si-wadeite) was also estimated to facilitate calculation of phase equilibria in the system K2O–Al2O3–SiO2. The calculated phase equilibria obtained using Perple_x are in general agreement with experimental studies. Calculated phase relations in the system K2O–Al2O3–SiO2 confirm a substantial stability field for kyanite–stishovite/coesite–Si-wadeite intervening between KAlSi3O8 hollandite and sanidine. The upper stability of kyanite is bounded by the reaction kyanite (Al2SiO5) = corundum (Al2O3) + stishovite (SiO2), which is located at 13–14 GPa for 1,100–1,400 K. The entropy and enthalpy of formation for K-cymrite (KAlSi3O8·H2O) were modified to better fit global best-fit compilations of thermodynamic data and experimental studies. Thermodynamic calculations were undertaken on the reaction of K-cymrite to KAlSi3O8 hollandite + H2O, which is located at 8.3–10.0 GPa for the temperature range 800–1,600 K, well inside the stability field of stishovite. The reaction of muscovite to KAlSi3O8 hollandite + corundum + H2O is placed at 10.0–10.6 GPa for the temperature range 900–1,500 K, in reasonable agreement with some but not all experiments on this reaction. 相似文献
4.
Stability of various hydrous phases in CMAS pyrolite-H<Subscript>2</Subscript>O system up to 25 GPa 总被引:1,自引:0,他引:1
We carried out a series of melting experiments with hydrous primitive mantle compositions to determine the stability of dense
hydrous phases under high pressures. Phase relations in the CaO–MgO–Al2O3–SiO2 pyrolite with ˜2 wt% of water have been determined in the pressure range of 10–25 GPa and in the temperature range between
800 and 1400 °C. We have found that phase E coexisting with olivine is stable at 10–12 GPa and below 1050 °C. Phase E coexisting
with wadsleyite is stable at 14–16 GPa and below 900 °C. A superhydrous phase B is stable in pyrolite below 1100 °C at 18.5
GPa and below 1300 °C at 25 GPa. No hydrous phases other than wadsleyite are stable in pyrolite at 14–17 GPa and 900–1100
°C, suggesting a gap in the stability of dense hydrous magnesium silicates (DHMS). We detected an expansion in the stability
field of wadsleyite to lower pressures (12 GPa and 1000 °C). The H2O content of wadsleyite was found to decrease not only with increasing temperature but also with increasing pressure. The
DHMS phases could exist in a pyrolitic composition only under the conditions present in the subducting slabs descending into
the lower mantle. Under the normal mantle and hot plume conditions, wadsleyite and ringwoodite are the major H2O-bearing phases. The top of the transition zone could be enriched in H2O in accordance with the observed increase in water solubility in wadsleyite with decreasing pressure. As a consequence of
the thermal equilibration between the subducting slabs and the ambient mantle, the uppermost lower mantle could be an important
zone of dehydration, providing fluid for the rising plumes.
Received: 9 September 2002 / Accepted: 11 January 2003
Acknowledgements The authors are thankful to Y. Ito for the assistance with the EPMA measurement, A. Suzuki, T. Kubo and T. Kondo for technical
help with the high-pressure experiments and Raman and X-ray diffraction measurements and C.R. Menako for technical support.
K. Litasov thanks H. Taniguchi for his continuous encouragement and the Center for Northeast Asian Studies of Tohoku University
and the Japanese Society for the Promotion of Science for the research fellowships. This work was partially supported by the
Grant-in-Aid of Scientific Research of the Priority Area (B) of the Ministry of Education, Science, Sport, and Culture of
the Japanese government (no. 12126201) to E. Ohtani. 相似文献
5.
Bowen's petrogenetic grid was based initially on a series of decarbonation reactions in the system CaO-MgO-SiO2-CO2 with starting assemblages including calcite, dolomite, magnesite and quartz, and products including enstatite, forsterite, diopside and wollastonite. We review the positions of 14 decarbonation reactions, experimentally determined or estimated, extending the grid to mantle pressures to evaluate the effect of CO2 on model mantle peridotite composed of forsterite(Fo)+orthopyroxene(Opx)+clinopyroxene(Cpx). Each reaction terminates at an invariant point involving a liquid, CO2, carbonates, and silicates. The fusion curves for the mantle mineral assemblages in the presence of excess CO2 also terminate at these invariant points. The points are connected by a series of reactions involving liquidus relationships among the carbonates and mantle silicates, at temperatures lower (1,100–1,300° C) than the silicate-CO2 melting reactions (1,400–1,600° C). Review of experimental data in the bounding ternary systems together with preliminary data for the system CaO-MgO-SiO2-CO2 permits construction of a partly schematic framework for decarbonation and melting reactions at upper mantle pressures. The key to several problems in the peridotite-CO2 subsystem is the intersection of a subsolidus carbonation reaction with a melting reaction at an invariant point near 24 kb and 1,200°C. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO2-saturated liquidus surfaces. Conclusions include the following: (1) Peridotite Fo+Opx+Cpx can be carbonated with increasing pressure, or decreasing temperature, to yield Fo+Opx+Cpx+Cd (Cd=calcic dolomite), Fo+Opx+Cd, Fo+Opx+Cm (Cm=calcic magnesite), and finally Qz+Cm. (2) Free CO2 cannot exist in subsolidus mantle peridotite with normal temperature distributions; it is stored as carbonate, Cd. (3) The CO2 bubbles in peridotite nodules do not represent free CO2 in mantle peridotite along normal geotherms. (4) CO2 is as effective as H2O in causing incipient melting, our preferred explanation for the low-velocity zone. (5) Fusion of peridotite with CO2 at depths shallower than 80 km produces basic magmas, becoming more SiO2-undersaturated with depth. (6) The solubility of CO2 in mantle magmas is less than about 5 wt% at depths to 80 km, increasing abruptly to about 40 wt% at 80 km and deeper. (7) Deeper than 80 km, the first liquids produced are carbonatitic, changing towards kimberlitic and eventually, at considerably higher temperatures, to basic magmas. (8) Kimberlite and carbonatite magmas rising from the asthenosphere must evolve CO2 at depths 100-80 km, which contributes to their explosive emplacement. (9) Fractional crystallization of CO2-bearing SiO2-undersaturated basic magmas at most pressures can yield residual kimberlite and carbonatite magmas. 相似文献
6.
High-Mg# andesitic lavas of the Shisheisky Complex,Northern Kamchatka: implications for primitive calc-alkaline magmatism 总被引:1,自引:0,他引:1
J. A. Bryant G. M. Yogodzinski T. G. Churikova 《Contributions to Mineralogy and Petrology》2011,161(5):791-810
Primitive arc magmatism and mantle wedge processes are investigated through a petrologic and geochemical study of high-Mg#
(Mg/Mg + Fe > 0.65) basalts, basaltic andesites and andesites from the Kurile-Kamchatka subduction system. Primitive andesitic
samples are from the Shisheisky Complex, a field of Quaternary-age, monogenetic cones located in the Aleutian–Kamchatka junction,
north of Shiveluch Volcano, the northernmost active composite volcano in Kamchatka. The Shisheisky lavas have Mg# of 0.66–0.73
at intermediate SiO2 (54–58 wt%) with low CaO (<8.8%), CaO/Al2O3 (<0.54), and relatively high Na2O (>3.0 wt%) and K2O (>1.0 wt%). Olivine phenocryst core compositions of Fo90 appear to be in equilibrium with whole-rock ‘melts’, consistent
with the sparsely phyric nature of the lavas. Compared to the Shisheisky andesites, primitive basalts from the region (Kuriles,
Tolbachik, Kharchinsky) have higher CaO (>9.9 wt%) and CaO/Al2O3 (>0.60), and lower whole-rock Na2O (<2.7 wt%) and K2O (<1.1 wt%) at similar Mg# (0.66–0.70). Olivine phenocrysts in basalts have in general, higher CaO and Mn/Fe and lower Ni
and Ni/Mg at Fo88 compared to the andesites. The absence of plagioclase phenocrysts from the primitive andesitic lavas contrasts
the plagioclase-phyric basalts, indicating relatively high pre-eruptive water contents for the primitive andesitic magmas
compared to basalts. Estimated temperature and water contents for primitive basaltic andesites and andesites are 984–1,143°C
and 4–7 wt% H2O. For primitive basalts they are 1,149–1,227°C and 2 wt% H2O. Petrographic and mineral compositions suggest that the primitive andesitic lavas were liquids in equilibrium with mantle
peridotite and were not produced by mixing between basalts and felsic crustal melts, contamination by xenocrystic olivine,
or crystal fractionation of basalt. Key geochemical features of the Shisheisky primitive lavas (high Ni/MgO, Na2O, Ni/Yb and Mg# at intermediate SiO2) combined with the location of the volcanic field above the edge of the subducting Pacific Plate support a genetic model
that involves melting of eclogite or pyroxenite at or near the surface of the subducting plate, followed by interaction of
that melt with hotter peridotite in the over-lying mantle wedge. The strongly calc-alkaline igneous series at Shiveluch Volcano
is interpreted to result from the emplacement and evolution of primitive andesitic magmas similar to those that are present
in nearby monogenetic cones of the Shisheisky Complex. 相似文献
7.
Robert W. Luth 《Contributions to Mineralogy and Petrology》1995,122(1-2):152-158
The carbonation reaction CaMg(CO3)2 (dolomite)+2SiO2 (coesite)=CaMgSi2O6 (diopside)+2 CO2 (vapor) has been determined experimentally between 3.5 and 6 GPa in a multiple-anvil, solid-media apparatus. This reaction,
a candidate for carbonation of eclogites (garnet+clinopyroxene) in the Earth’s mantle, lies at higher pressure for a given
temperature than do the carbonation reactions for peridotites (olivine+orthopyroxene±clinopyroxene). A depth interval may
exist within the Earth’s mantle under either ‘normal’ or ‘subduction’ thermal regimes where carbonated peridotite could coexist
with carbonate-free, CO2-bearing eclogite.
Received: 25 May 1994/Accepted: 13 June 1995 相似文献
8.
Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive
dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and
have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that
crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three
primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical
calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt
of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest
overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted
mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of
Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element
patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or
fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite
unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction
component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel
lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully
modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less
likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux. 相似文献
9.
The phase relations in the Fe2SiO4–Fe3O4 binary system have been determined between 900 and 1200 °C and from 2.0 to 9.0 GPa. At low to moderate pressures magnetite
can accommodate significant Si, reaching XFe2SiO4=0.1 and 0.2 at 3.0 and 5.0 GPa respectively, with temperature having only a secondary influence. At pressures below 3.5 GPa
at 900 °C and 2.6 GPa at 1100 °C magnetite-rich spinel coexists with pure fayalite. This assemblage becomes unstable at higher
pressures with respect to three intermediate phases that are spinelloid polytypes isostructural to spinelloids II, III and
V in the Ni-aluminosilicate system. The phase relations between the spinelloid phases are complex. At pressures above ≈8.0 GPa
at 1100 °C, the spinelloid phases give way to a complete spinel solid solution between Fe3O4 and Fe2SiO4. The presence of small amounts of Fe3+ stabilises the spinel structure to lower pressures compared to the Fe2SiO4 end member. This means that the fayalite–γ-spinel transition is generally unsuitable as a pressure calibration point for
experimental apparatuses. The molar volumes of the spinel solid solutions vary nearly linearly with composition, having a
small negative deviation from ideal behaviour described by Wv=−0.15(6) cm3. Extrapolation yields V°(298) = 41.981(14) cm3 for the Fe2SiO4-spinel end member. The cell parameters and molar volumes of the three spinelloid polytypes vary systematically with composition.
Cation disorder is an important factor in stabilising the spinelloid polytypes. Each polytype exhibits a particular solid
solution range that is directly influenced by the interplay between its structure and the cation distributions that are energetically
favourable. In the FeO–FeO1.5–SiO2 ternary system Fe7SiO10 (“iscorite”) coexists with the spinelloid phases at intermediate pressures on the SiO2-poor, or Fe3+-poor side of the Fe2SiO4–Fe3O4 join. On the SiO2 and Fe3+-rich side of the join, orthopyroxene or high-P clinopyroxene coexists with the spinelloids and spinel solid solutions. The
assemblage pyroxene+spinel+SiO2 is stable over a wide range of bulk composition. The stability of spinelloid III is of particular petrologic interest since
this phase has the same structure as (Mg,Fe)2SiO4–wadsleyite, indicating that Fe3+ can be easily incorporated in this important phase in the Earth's transition zone, in addition to silicate spinel. This has
important implications for the redox state of the Earth's transition zone and for the depth at which the olivine to spinel
transition occurs in the mantle, potentially leading to a shift in the “410 km” seismic discontinuity to shallower depths
depending on the prevailing redox state. In addition, a coupled tetrahedral substitution of Fe3++OH for Si+O could provide a further mechanism for the incorporation of H2O in wadsleyite.
Received: 10 January 2000 / Accepted: 12 May 2000 相似文献
10.
Subduction carries atmospheric and crustal carbon hosted in the altered oceanic crystalline basement and in pelagic sediments
back into the mantle. Reactions involving complex carbonate solid solutions(s) lead to the transfer of carbon into the mantle,
where it may be stored as graphite/diamond, in fluids or melts, or in carbonates. To constrain the thermodynamics and thus
reactions of the ternary Ca–Mg–Fe carbonate solid solution, piston cylinder experiments have been performed in the system
CaCO3–MgCO3–FeCO3 at a pressure of 3.5 GPa and temperatures of 900–1,100°C. At 900°C, the system has two miscibility gaps: the solvus dolomite–calcite,
which closes at X
MgCO3 ~0.7, and the solvus dolomite–magnesite, which ranges from the Mg to the Fe side of the ternary. With increasing temperature,
the two miscibility gaps become narrower until complete solid solutions between CaCO3–Ca0.5Mg0.5CO3 is reached at 1,100°C and between CaCO3–FeCO3 at 1,000°C. The solvi are characterized by strong compositional asymmetry and by an order–disorder mechanism. To deal with
these features, a solid solution model based on the van Laar macroscopic formalism has been calculated for ternary carbonates.
This thermodynamic solid solution model is able to reproduce the experimentally constrained phase relations in the system
CaCO3–MgCO3–FeCO3 in a broad P–T range. To test our model, calculated phase equilibria were compared with experiments performed in carbonated
mafic protolithes, demonstrating the reliability of our solid solution model at pressures up to 6 GPa in complex systems. 相似文献
11.
Evidence for adiabatic decompression melting in the Southern Mariana Arc from high-Mg lavas and melt inclusions 总被引:1,自引:1,他引:1
Edward J. Kohut Robert J. Stern Adam J. R. Kent Roger L. Nielsen Sherman H. Bloomer Matthew Leybourne 《Contributions to Mineralogy and Petrology》2006,152(2):201-221
Unusually magnesian (Mg# ∼76) basalts have been sampled from a small submarine volcano situated on the Mariana arc magmatic front. Total alkalis range from 1.7 to 1.94%, Al2O3 from 9.09 to 10.3% and CaO from 13.9 to 14.09%. These lavas can be classified based on mineralogy as picrite and ankaramite. Olivine-hosted melt inclusions (MIs) have median MgO contents of 17.17–17.86 wt%, 0.35–0.5% TiO2, 42–50% SiO2 and 1.66–3.43% total alkalis, which suggest that the parental magmas were primitive mantle melts. Trace element concentrations for both MIs and lavas are arc-like, although more depleted than most arc lavas. Chlorine (182–334 ppm) and H2O contents (0.11–0.64 wt%) in the MIs are consistent with the estimated median oxygen fugacities (log ΔFMQ of + 1.53–1.66) which lie at the low end of the range estimates for arc basalts and picrites (ΔFMQ = + 1 to + 3). Isotopic compositions of Sr, Nd, Hf and Pb are similar to those of other Mariana arc lavas and indicate derivation from an Indian Ocean mantle domain. The averaged magmatic temperature estimate from several geothermometers was 1,367°C at 1–1.5 GPa. We propose that high-Mg magmagenesis in this region results from the adiabatic decompression melting of relatively anhydrous but metasomatized mantle wedge. This melting is attributed to enhanced upwelling related to unusual tectonics on the over-riding plate related to a tear or other discontinuity on the subducted slab.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
12.
One of the main uncertainties in mineralogical models of the Earth's lower mantle is the nature of the aluminous mineral:
it is not clear whether Al forms its own minerals or is mainly contained in (Mg,Fe)SiO3-perovskite. This question is very important, since it is known that if Al were mainly hosted by perovskite, it would radically
change Fe/Mg-partitioning and phase equilibria between mantle minerals, and also alter many physical and chemical properties
of perovskite, which is currently believed to comprise ca. 70% of the volume of the lower mantle. This, in turn, would require
us to reconsider many of our geochemical and geophysical models for the lower mantle. This work considers the possibility
of a V3O5-type structured modification of Al2SiO5 to be the main host of Al in the lower mantle, as proposed by previous workers. We report ab initio calculations, based on density functional theory within the generalised gradient approximation (GGA) with plane wave basis
set and nonlocal pseudopotentials. We consider polymorphs of Al2SiO5 (kyanite, andalusite, sillimanite, and hypothetical V3O5-like and pseudobrookite-like phases), SiO2 (stishovite, quartz) and Al2O3 (corundum). Computational conditions (e.g., plane-wave energy cutoff, Brillouin zone sampling) were carefully chosen in order
to reproduce small energy changes associated with phase transitions between the Al2SiO5 polymorphs. Good agreement of crystal structures, bulk moduli, atomisation energies and the phase diagram of Al2SiO5 with experimental data was found. Strong disagreement between the calculated lattice parameters and density of V3O5-like phase of Al2SiO5 and experimental values, assigned to it by previous workers, suggests that a V3O5-structured phase of Al2SiO5 was never observed experimentally. In addition, we found that the most stable high-pressure assembly in Al2SiO5 system is corundum+stishovite, and the value of the transition pressure at T = O K (113 kbar) is in excellent agreement with experimental estimates (95–150 kbar). We explain the instability of octahedrally
coordinated silicates of Al to decomposition on the basis of Pauling's second rule.
Received: 18 May 1999 / Accepted: 5 November 1999 相似文献
13.
Kankan diamonds (Guinea) II: lower mantle inclusion parageneses 总被引:3,自引:2,他引:1
Thomas Stachel Jeff W. Harris Gerhard P. Brey Werner Joswig 《Contributions to Mineralogy and Petrology》2000,140(1):16-27
Frequent inclusions of ferropericlase, some coexisting with phases of MgSiO3, CaSiO3 and SiO2 composition, suggest that a large proportion of diamonds from Guinea are derived from the lower mantle. Low aluminium contents
in MgSiO3 inclusions indicate derivation from the uppermost lower mantle, where Al solubility in perovskite is low. Trace element analyses
(SIMS) of CaSiO3 inclusions reveal extreme degrees of LREE (200–2000 times chondritic) and Sr enrichment (70–1000 times chondritic) together
with negative and positive Eu anomalies. This implies a highly enriched lower mantle source, possibly a product of a subducted
oceanic slab. A number of phases that are only stable in the upper mantle are found to coexist with lower mantle phases and
thereby indicate retrograde equilibration during slow exhumation within a rising plume or convection cell. In one case, however,
an inclusion paragenesis of ferropericlase and olivine can be shown to have formed within the upper mantle, indicating that
the occurrence of ferropericlase inclusions alone is an unreliable indicator of lower mantle origin.
Received: 26 January 2000 / Accepted: 18 May 2000 相似文献
14.
Calc-silicate granulites from Rayagada, north-central sector of Eastern Ghats granulite belt show a wide range of mineral
assemblages and chemical compositions, which can be grouped as Gr. I (grossular- rich garnet-wollastonite-scapolite-calcite-clinopyroxene),
Gr. II (andradite-rich garnet-scapolite-calcite-clinopyr- oxene), and Gr. III (scapolite-calcite-clinopyroxene-plagioclase)
assemblages. Petrographic features suggest the following several reactions in the CaO–Al2O3–SiO2-vapor system: Mei+4Wo+Cal=3Grs+Qtz +2CO2, Mei+3Wo+2Cal=3Grs+CO2, Mei= 3An+Cal, Wo+CO2=Cal+Qtz, Mei+5Wo =3Grs+2Qtz+CO2, An+Wo=Grs+Qtz, Mei+ 5Cal+3Qtz=3Grs+6CO2, and the following reactions in the CaO–FeO–MgO–Al2O3–SiO2-vapor system: Cpxss+Scp+Wo=Grtss+Qtz+CO2, 4Hd+ 2Cal+O2=2Adr+2Qtz+2CO2, Cpxss+Scp= Grtss+Cal+Qtz. These reactions have been used to estimate peak T-X
CO2 condition for these granulites. A maximum temperature of ∼920 °C has been calculated at an estimated pressure of 9 kbar.
A T-X
CO2 diagram shows an isobaric cooling from ∼920 °C to ∼815 °C. A range of X
CO2 (0.50 at 920 °C to 0.25 at 815 °C) has been observed for Gr. I calc-silicate granulites based on the reaction sequences including
coronal garnet-forming reactions. This sequence is suggestive of internal fluid buffering rather than external fluid influx
and the differences in X
CO2 conditions has been thought to be due to local buffering of fluid phases. Group II and Gr. III calc-silicate granulites,
on the other hand, exhibit relatively lower temperature conditions.
Received: 11 September 1995/Accepted: 20 June 1996 相似文献
15.
Kazufusa Ishibashi Kei Hirose Nagayoshi Sata Yasuo Ohishi 《Physics and Chemistry of Minerals》2008,35(4):197-200
The high-pressure stability limit of calcium aluminosilicate (CAS) phase has been examined in its end-member CaAl4Si2O11 composition at 18–39 GPa and 1,670–2,300 K in a laser-heated diamond-anvil cell (LHDAC). The in-situ synchrotron X-ray diffraction
measurements revealed that the CAS phase decomposes into three-phase assemblage of cubic Al-bearing CaSiO3 perovskite, Al2O3 corundum, and SiO2 stishovite above 30 GPa and 2,000 K with a positive pressure–temperature slope. Present results have important implications
for the subsolidus mineral assemblage of subducted sediment and the melting phase relation of basalt in the lower mantle. 相似文献
16.
The petrogenetic potential of in situ laser ablation Hf isotope data from melt precipitated zircons was explored through the
analyses of about 700 individual crystals derived from about 20 different granitic intrusions covering the Variscan basement
segment of eastern Bavaria, SE Germany. In combination with geochemical features, four major suites of granitic rocks can
be distinguished: (1) NE Bavarian redwitzites (52–57 wt% SiO2, intrusion ages around 323 Ma) have chondritic εHf(t) values (+0.8 to –0.4). The redwitzites are hybrid rocks and the Hf
data are permissive of mixing of a mantle progenitor and crustal melts. (2) Various intermediate rock types (dioritic dyke,
granodiorite, palite, 59–63 wt% SiO2, 334–320 Ma) from the Bavarian Forest yield negative εHf(t) values between –3.4 and –5.1. These values which apparently contradict
a mantle contribution fingerprint an enriched (metasomatized) mantle component that was mixed with crustal material. (3) Voluminous,
major crust forming granites sensu stricto (67–75 wt% SiO2, 328–298 Ma) are characterized by a range in εHf(t) values from –0.5 to –5.6. Different crustal sources and/or modification
of crustal melts by various input of juvenile material can explain this variation. (4) Post-plutonic (c. 299 Ma) porphyritic dykes of dacitic composition (64–67 wt% SiO2) from the southern Bavarian Forest have chondritic εHf(t) values (+0.6 to –1.1) and display large intergrain Hf isotope variation.
The dykes form a separate petrogenetic group and the Hf data suggest that the zircons crystallized when a pristine mantle-derived
parental melt was modified by infiltration of crustal material. The zircon Hf data form a largely coherent positive array
with the whole-rock Nd data and both systems yield similar two-stage depleted mantle model ages (1.1–1.7 Ga). 相似文献
17.
The Tuwu–Yandong porphyry copper belt lies in the eastern Tianshan mountains, eastern section of the Central Asian orogenic belt. The copper mineralization is mainly hosted in plagiogranite porphyries intruded into early Carboniferous volcanic rocks of the Paleozoic Dananhu island arc between the Tarim and Siberian plates. The plagiogranite porphyries have contents of 65–73 wt% SiO2, 14–17 wt% Al2O3, 0.9–2.2 wt% MgO, 3–16 ppm Y, 0.4–1.40 ppm Yb, 347–920 ppm Sr, and positive Eu anomalies. The rocks also exhibit positive ɛ
Nd(t) values (+5.0 to +9.4) and low initial 87Sr/86Sr values (0.70316–0.70378). Such features are similar to those of adakites derived from partial melting of a subduction-related oceanic slab. The mineralization age is early Carboniferous (350–320 Ma), which is close to that of the porphyries. The close relationship between the Cu mineralization and the porphyry is also indicated by their similar Sr-Nd-Pb isotopic compositions. We suggest that the copper porphyry (magma) system in the Dananhu island arc was formed by direct melting of an obliquely subducting early Carboniferous oceanic slab. 相似文献
18.
Quaternary minettes and associated volcanic rocks of Mascota,western Mexico: a consequence of plate extension above a subduction modified mantle wedge 总被引:12,自引:0,他引:12
I. S. E. Carmichael R. A. Lange James F. Luhr 《Contributions to Mineralogy and Petrology》1996,124(3-4):302-333
Pleistocene-Holocene volcanism in the Jalisco block of western Mexico is confined to two conspicuous grabens, where potassic
eruptives range from absarokites (48–52% SiO2) and minettes (49–54% SiO2) through basaltic andesites (53–57% SiO2), the most voluminous type, to andesites and their lamprophyric equivalent spessartite (58–62% SiO2); there are no contemporary rhyolitic rocks. This suite has high concentrations of Mg, Cr (<550 ppm) and Ni (<450 ppm) accompanied
by large concentrations of K, P, Ba (<4000 ppm) and Sr (<5000 ppm) and elements such as LREE and Zr (<600 ppm). No combination
of crystal fractionation and/or crustal contamination can reproduce the compositional range of these magmas, which nevertheless
are believed to be genetically related because of their proximity in time and space. Hydrous minerals in the lamprophyres
and the typical absence of plagioclase phenocrysts in both basaltic andesites and andesites reflect the relatively high concentrations
of water in the magmas, which suppressed the crystallisation of feldspar. Experimental verification of the minimal amounts
of water required to reproduce the phenocryst assemblages in selected rocks range from 3.5 to 6%. During ascent in a volcanic
conduit, andesitic magma may lose water and consequently precipitate plagioclase, or it may ascend more rapidly, retaining
more of its initial water, which stabilises phenocrysts of hornblende at the expense of plagioclase. Our estimates of water
concentrations, which are consistent with the various low pressure phenocryst assemblages, will be minimal for the magmas
in their source regions, and the process of magmatic dewatering on ascent may be typical in well established volcanic conduits.
In accord with the compositions of phenocrystic olivine in the basaltic andesites and the minettes, the values of FeO and
Fe2O3 of the bulk lavas and scoriae are demonstrably pristine. As a consequence, there are two characteristic features of the Mascota
suite: the high range of relative oxygen fugacities (ΔNNO=1–5) and the high Mg# (MgO/MgO+FeO) that ranges from 0.70 to 0.91
(with only one andesite as low as 0.66). From the evidence of phlogopite phenocrysts, a partial melt involving phlogopite
would have a higher Mg# than one from olivine (Fo90) and pyroxene alone. As the Mascota series shows a correlation between K2O and Mg#, we conclude that it was generated by partial fusion of the mantle wedge, with a variable contribution of phlogopite
and apatite from veins throughout the lherzolitic assemblage. In conformity with an origin by varying increments of partial
fusion of a phlogopite-bearing mantle, all incompatible elements vary linearly with Ti (or K) as if phlogopite (+apatite)
in the source dominated their contribution to the partial melts. Fluids from dehydration of the subducting slab presumably
deposit hydrous and other minerals in veins in the mantle wedge and also increase its redox state. As the Mascota volcanism
occurs in grabens closer to the trench than the main andesite arc, it is concluded that the eruption of these small volumes
of hydrous magmas require the tectonically favored ascent paths offered by the extensional grabens to reach the surface from
their mantle sources.
Received: 24 January 1995 / Accepted: 21 February 1996 相似文献
19.
A. A. Gurenko Thor H. Hansteen Hans-Ulrich Schmincke 《Contributions to Mineralogy and Petrology》1996,124(3-4):422-435
Picritic units of the Miocene shield volcanics on Gran Canaria, Canary Islands, contain olivine and clinopyroxene phenocrysts
with abundant primary melt, crystal and fluid inclusions. Composition and crystallization conditions of primary magmas in
equilibrium with olivine Fo90-92 were inferred from high-temperature microthermometric quench experiments, low-temperature microthermometry of fluid inclusions
and simulation of the reverse path of olivine fractional crystallization based on major element composition of melt inclusions.
Primary magmas parental for the Miocene shield basalts range from transitional to alkaline picrites (14.7–19.3 wt% MgO, 43.2–45.7
wt% SiO2). Crystallization of these primary magmas is believed to have occurred over the temperature range 1490–1150° C at pressures
≈5 kbar producing olivine of Fo80.6-90.2, high-Ti chrome spinel [Mg/ (Mg+Fe2+)=0.32–0.56, Cr/(Cr+Al)=0.50–0.78, 2.52–8.58 wt% TiO2], and clinopyroxene [Mg/(Mg+Fe)=0.79–0.88, Wo44.1-45.3, En43.9-48.0, Fs6.8-11.0] which appeared on the liquidus together with olivine≈Fo86. Redox conditions evolved from intermediate between the QFM and WM buffers to late-stage conditions of NNO+1 to NNO+2. The
primary magmas crystallized in the presence of an essentially pure CO2 fluid. The primary magmas originated at pressures >30 kbar and temperatures of 1500–1600° C, assuming equilibrium with mantle
peridotite. This implies melting of the mantle source at a depth of ≈100 km within the garnet stability field followed by
migration of melts into magma reservoirs located at the boundary between the upper mantle and lower crust. The temperatures
and pressures of primary magma generation suggest that the Canarian plume originated in the lower mantle at depth ≈900 km
that supports the plume concept of origin of the Canary Islands.
Received: 23 October 1995/Accepted: 21 February 1996 相似文献
20.
K-rich glass-bearing wehrlite xenoliths from Yitong,Northeastern China: petrological and chemical evidence for mantle metasomatism 总被引:9,自引:1,他引:9
Y. Xu J.-C. C. Mercier Chuanyong Lin Lanbin Shi M. A. Menzies J. V. Ross B. Harte 《Contributions to Mineralogy and Petrology》1996,125(4):406-420
Ultramafic xenoliths in Cenozoic alkali basalts from Yitong, northeast China comprise three types in terms of their modal
mineralogy: lherzolite, pyroxenite and wehrlite. The wehrlite suite always contains interstitial pale/brown glass which occupies
several per cent by volume of the whole rock. The texture of the wehrlites is porphyroclastic with some large strained grains
of olivine (0.5–1 mm) scattered in a very fine grained matrix (0.1 mm), implying a metamorphic origin for the protolith rather
than an igneous origin. The host minerals are compositionally zoned, showing evidence of reaction with a melt. Petrological
evidence for resorption of spinel (lherzolite) and orthopyroxene (wehrlite) by infiltrating melt further supports the hypothesis
that the wehrlites result from interaction between a partial melting residue and a melt, which preferentially replaced primary
spinel, Cr-diopside and enstatite to produce secondary clinopyroxene (cpx) + olivine (ol) ± chromite ± feldspar (fd). The
composition of the mineral phases supports this inference and, further indicates that, prior to melt impregnation, the protoliths
of these wehrlites must have been subjected to at least one earlier Fe-enrichment event. This explanation is consistent with
the restricted occurrence of glasses in the wehrlite suite. The glass is generally associated with fine-grained (0.1 mm) minerals
(cpx+ol+chromite ±fd). Electron microprobe analyses of these glasses show them to have high SiO2 content (54–60 wt%), a high content of alkalis (Na2O, 5.6–8.0%; K2O, 6.3–9.0%), high Al2O3 (20–24%), and a depletion in CaO (0.13–2.83%), FeO (0.89–4.42%) and MgO (0.29–1.18%). Ion probe analyses reveal a light rare
earth element-enrichment in these glasses with chondrite normalised (La)n = 268–480. The high K2O contents in these glasses and their mode of occurrence argue against an origin by in-situ melting of pre-existent phases. Petrographic characteristics and trace element data also exclude the possibility of percolation
of host-basalt related melts for the origin of these glasses. Thus the glasses must have resulted from local penetration of
mantle metasomatic melts which may have been produced by partial melting of peridotites with involvement of deep-seated fluids.
Such melts may have been significantly modified by subsequent fractional crystallization of ol, cpx and sp, extensive reaction
with the mantle conduit and the xenolith transport process.
Received: 1 August 1995 / Accepted: 19 June 1996 相似文献