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1.
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
2.
Summary Supra-solidus phase relations at temperatures and pressures ranging from 800 to 1700 °C and 2 to 6.4 GPa have been determined
experimentally for three silica-rich lamproites: hyalo-leucite phlogopite lamproite (Oscar, West Kimberley); sanidine richterite
lamproite (Cancarix, Murcia-Almeria); and phlogopite transitional madupitic lamproite (Middle Table Mountain, Wyoming). All
samples have extended melting intervals (500–600 °C). Bulk composition has a significant control on the nature of the initial
liquidus phases, with orthopyroxene occurring at low pressures (<4 GPa) in the relatively calcium-poor Oscar and Cancarix
lamproites. At higher pressure (>6 GPa) orthopyroxene is replaced by garnet plus clinopyroxene as near-liquidus phases in
the Oscar lamproite and by orthopyroxene plus clinopyroxene in the Cancarix sample. Clinopyroxene is a near-liquidus phase
at all pressures in the Middle Table Mountain lamproite. Near-solidus phase assemblages at high pressure (>5 GPa) are: clinopyroxene + phlogopite + coesite + rutile + garnet
(Oscar); clinopyroxene + garnet + coesite + K–Ti-silicate (Cancarix); clinopyroxene + phlogopite + apatite + K–Ti-silicate
(Middle Table Mountain). In all compositions olivine is never found as a liquidus phase at any of the temperatures or pressures
studied here. The phase relationships are interpreted to suggest that silica-rich lamproites cannot be derived by the partial
melting of lherzolitic sources. Their genesis is considered to involve high degrees of partial melting of ancient metasomatic
veins within a harzburgitic-lherzolitic lithospheric substrate mantle. The veins are considered in their mineralogy to be
similar to the experimentally-observed, high pressure, near-solidus phase assemblages. The composition of silica-rich primary
lamproite magmas differs between cratons as a consequence of differing mineralogical modes of the source veins and different
relative contributions from the veins and wall-rocks to the partial melts.
Received February 21, 2000; revised version accepted July 3, 2001 相似文献
3.
We have experimentally determined the solidus position of model lherzolite in the system CaO-MgO-Al2O3-SiO2-CO2 (CMAS.CO2) from 3 to 7 GPa by locating isobaric invariant points where liquid coexists with olivine, orthopyroxene, clinopyroxene,
garnet and carbonate. The intersection of two subsolidus reactions at the solidus involving carbonate generates two invariant
points, I1A and I2A, which mark the transition from CO2-bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing lherzolite respectively. In CMAS.CO2, we find I1A at 2.6 GPa/1230 °C and I2A at 4.8 GPa/1320 °C. The variation of all phase compositions along the solidus has also been determined. In the pressure range
investigated, solidus melts are carbonatitic with SiO2 contents of <6 wt%, CO2 contents of ˜45 wt%, and Ca/(Ca+Mg) ratios that range from 0.59 (3 GPa) to 0.45 (7 GPa); compositionally they resemble natural
magnesiocarbonatites. Volcanic magnesiocarbonatites may well be an example of the eruption of such melts directly from their
mantle source region as evidenced by their diatremic style of activity and lack of associated silicate magmas. Our data in
the CMAS.CO2 system show that in a carbonate-bearing mantle, solidus and near-solidus melts will be CO2-rich and silica poor. The widespread evidence for the presence of CO2 in both the oceanic and continental upper mantle implies that such low degree SiO2-poor carbonatitic melts are common in the mantle, despite the rarity of carbonatites themselves at the Earth's surface.
Received: 9 April 1997 / Accepted: 25 November 1997 相似文献
4.
The influence of water on melting of mantle peridotite 总被引:47,自引:8,他引:39
This experimental study examines the effects of variable concentrations of dissolved H2O on the compositions of silicate melts and their coexisting mineral assemblage of olivine + orthopyroxene ± clinopyroxene ± spinel ± garnet.
Experiments were performed at pressures of 1.2 to 2.0 GPa and temperatures of 1100 to 1345 °C, with up to ∼12 wt% H2O dissolved in the liquid. The effects of increasing the concentration of dissolved H2O on the major element compositions of melts in equilibrium with a spinel lherzolite mineral assemblage are to decrease the
concentrations of SiO2, FeO, MgO, and CaO. The concentration of Al2O3 is unaffected. The lower SiO2 contents of the hydrous melts result from an increase in the activity coefficient for SiO2 with increasing dissolved H2O. The lower concentrations of FeO and MgO result from the lower temperatures at which H2O-bearing melts coexist with mantle minerals as compared to anhydrous melts. These compositional changes produce an elevated
SiO2/(MgO + FeO) ratio in hydrous peridotite partial melts, making them relatively SiO2 rich when compared to anhydrous melts on a volatile-free basis. Hydrous peridotite melting reactions are affected primarily
by the lowered mantle solidus. Temperature-induced compositional variations in coexisting pyroxenes lower the proportion of
clinopyroxene entering the melt relative to orthopyroxene. Isobaric batch melting calculations indicate that fluid-undersaturated
peridotite melting is characterized by significantly lower melt productivity than anhydrous peridotite melting, and that the
peridotite melting process in subduction zones is strongly influenced by the composition of the H2O-rich component introduced into the mantle wedge from the subducted slab.
Received: 7 April 1997 / Accepted: 9 January 1998 相似文献
5.
Trace element distribution in Central Dabie eclogites 总被引:16,自引:0,他引:16
R. Sassi B. Harte D. A. Carswell H. Yujing 《Contributions to Mineralogy and Petrology》2000,139(3):298-315
Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace
element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite
facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina
titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable
that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite
facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They
vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show
consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite
all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals
are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending
on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content
of the host phases, with D
REE
Cpx/Grt
decreasing with decreasing D
Ca
. This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation
of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the
eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate
Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory
manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids
will be strongly depleted in LREE–MREE.
Received: 11 February 1999 / Accepted: 31 January 2000 相似文献
6.
Summary Mesozoic melilite-bearing ultramafic lamprophyres are developed as sill, dyke and plug-like intrusive bodies in the East
Antarctic Beaver Lake area. They consist of varying amounts of olivine, melilite, phlogopite, nepheline, titanomagnetite and
perovskite as major phases, accompanied by minor amounts of apatite, carbonate, spinel, glass and, rarely, monticellite. The
rocks are mineralogically and geochemically broadly similar to olivine melilitites, differing in higher CO2 and modal phlogopite and carbonate contents. The ultramafic lamprophyres are MgO-rich (13.4–20.5 wt%) and SiO2-poor (32.8–37.2 wt%), indicative of a near-primary nature. Major and trace element features are consistent with minor fractionation
of olivine and Cr-spinel from melts originating at depths of 130–140 km.
Primary melts originated by melting of upper mantle peridotite which had been veined by phlogopite + carbonate + clinopyroxene-bearing
assemblages less than 200 Ma before eruption. The presence of the veins and their time of formation is required to explain
high incompatible trace element contents and growth of 87Sr/86Sr, leaving 143Nd/144Nd unaffected. The major element, compatible trace element, and most radiogenic isotope characteristics are derived from melting
of the wall-rock peridotite. The depth of about 130 km is indicated by the presence of phlogopite rather than amphibole in
the veins, by control of the REE pattern by residual garnet, by the high MgO content of the rocks, and by the expected intersection
of the rift-flank geotherm with the solidus at this depth. The higher CO2 contents than are characteristic for olivine melilitites favoured the crystallization of melilite at crustal pressures, and
suppressed the crystallization of clinopyroxene. The Beaver Lake ultramafic lamprophyres are a distal effect of the breakup
of Gondwanaland, too distal to show a geochemical signature of the Kerguelen plume. Upward and outward movement of the asthenosphere-lithosphere
boundary beneath the Lambert-Amery rift led first to the production of phlogopite- and carbonate-rich veins, and later to
the generation of the ultramafic lamprophyres themselves.
Received March 31, 2000; revised version accepted September 3, 2001 相似文献
7.
Robert W. Luth 《Contributions to Mineralogy and Petrology》2006,151(2):141-157
The melting relationships in the system CaMgSi2O6 (Di)–CO2 have been studied in the 3–8 GPa pressure range to determine if there is an abrupt decrease in the temperature of the solidus
accompanying the stabilization of carbonate as a subsolidus phase. Such a decrease has been observed previously in peridotitic
and some eclogitic systems. In contrast, the solidus in the Di–CO2 system was found to decrease in a gradual fashion from 3 to 8 GPa. This decrease accompanies an evolution in the composition
of the melt at the solidus from silicate-rich with minor CO2 at 3 GPa to carbonatitic at 5.5 GPa, where the carbonation reaction Diopside + CO2 = Dolomite (Dol) + Coesite (Cst) intersects the solidus. The near-solidus melt remains carbonatitic at higher pressure, consistent
with carbonate being the dominant contributor to the melt. Based on previous studies in both eclogitic and peridotitic systems,
this conclusion can be extended to more complicated systems: once carbonate is a stable subsolidus phase, it plays a major
role in controlling both the temperature of melting and the composition of the melt produced. 相似文献
8.
The effect of CO2 on mantle peridotites is modeled by experimental data for the system CaO-MgO-SiO2-CO2 at 2.7 GPa. The experiments provide isotherms for the vapor-saturated liquidus surface, bracket piercing points for field
boundaries on the surface, and define the positions and compositions of isobaric invariant liquids on the boundaries (eutectics
and peritectics). CO2-saturated carbonatitic liquids (>80% carbonate) exist through approximately 200 °C above the solidus, with a transition to
silicate liquids (>80% silicate) within ∼75 °C across a plateau on the liquidus. Carbonate-rich magmas cannot cross the silicate-carbonate
liquidus field boundary, so the carbonate liquidus field is therefore a forbidden volume for liquid magmas. This confirms
the fact that rounded, pure carbonates in mantle xenoliths cannot represent original liquids. A P-T diagram is constructed
for the carbonation and melting reactions for mineral assemblages corresponding to lherzolite, harzburgite, websterite and
wehrlite, with carbonate, CO2 vapor (V), or both. The changing compositions of liquids in solidus reactions on the P-T diagram are illustrated by the changing
compositions of eutectic and peritectic liquids on the liquidus surface. At an invariant point Q (∼2.8 GPa/1230 °C), all peridotite
assemblages coexist with a calcite-dolomite solid solution (75 ± 5% CaCO3) and a dolomitic carbonatite melt [57% CaCO3 (CC), 33% MgCO3 (MC), 10% CaMgSi2O6 (Di)], with 63% CC in the carbonate component. At higher pressures, dolomite-lherzolite, dolomite-harzburgite-V, and dolomite-websterite-V
melt to yield similar liquids. Magnesian calcite-wehrlite is the only peridotite melting to carbonatitic liquids (more calcic)
at pressures below Q (∼70 km). Dolomitic carbonatite magma rising through mantle to the near-isobaric solidus ledge near Q
will begin to crystallize, releasing CO2 (enhancing crack propagation), and metasomatizing lherzolite toward wehrlite.
Received: 20 March 1998 / Accepted: 7 July 1999 相似文献
9.
Clinopyroxene is an essential mineral in eclogitic rocks. It commonly contains minor amounts of the defect-bearing Ca-Eskola (CaEs, Ca0.5□0.5AlSi2O6) component, with higher concentrations generally considered to indicate a high-pressure origin at least within the coesite stability field. Changes in pressure and temperature conditions can lead to exsolution of this component as a free SiO2 phase, which may have a number of petrological implications. This makes it important to understand the factors that maximize CaEs incorporation in clinopyroxene. We have undertaken a series of experiments at high pressures and temperatures (4–10 GPa and 1000–1350 °C) to further investigate the systematics of CaEs incorporation in eclogite-like clinopyroxene and the factors responsible for maximizing CaEs contents. Two simple chemical systems were chosen that allow unambiguous interpretation of the results: (1) CMAS + H2O and (2) two compositions in the NCMAS system. All experimental products contained clinopyroxene and garnet along with either a free SiO2 phase or a silicate melt. Coexisting garnet is grossular-rich, generally with X gr ≥ 0.67. Compositional variations are attributable to the presence or absence of melt and changes in modal amounts of garnet at different pressure–temperature conditions. Even small amounts of H2O lower the solidus temperature and the presence of a melt reduces the SiO2 activity, which destabilizes the CaEs component in clinopyroxene. The CaEs and the Ca-Tschermaks (CaTs, CaAl2SiO6) components in clinopyroxene decrease with increasing jadeite mole fraction, which is also a function of pressure and bulk Al content. Modeling X-ray powder diffraction data yields a molar volume for the CaEs endmember of V CaEs = 60.87(63) cm3, which reasonably agrees with a literature value that was estimated from natural samples. In the presence of coexisting coesite, the CaEs and CaTs do not vary independently of each other, being controlled by the internal equilibrium 2CaEs = CaTs + 3SiO2 (coesite). This relation, observed in simple systems (i.e., CMAS ± Na), is also obeyed by clinopyroxene in more complex, natural analog bulk compositions. An assessment of available experimental data reveals a maximum of 15–18 mol% CaEs in eclogitic clinopyroxene at conditions corresponding to 130–180 km depth. CaEs contents are maximized at high temperatures; i.e., at or near the solidus in the presence of coesite. Thus, this study supports the role of CaEs exsolution in contributing to melt generation during upwelling of eclogite bodies in the mantle, albeit with some caveats. Somewhat higher maximum CaEs contents (~20 mol%) are found in Ca and Al-rich bulk compositions, such as grospydite xenoliths. Such bulk compositions also seem to require the coexistence of kyanite. Other Ca and Al-rich rock types, like rodingites, should have the potential of containing CaEs-rich clinopyroxenes, except that they are SiO2-undersaturated. This emphasizes the further role of bulk composition, in addition to high temperatures, in achieving maximum CaEs contents in high-pressure clinopyroxene. 相似文献
10.
Y. D. Chen Suzanne Y. O'Reilly W. L. Griffin T. E. Krogh 《Contributions to Mineralogy and Petrology》1998,130(2):154-161
Mafic rocks dominate the lower crustal and upper mantle xenolith suites within the Jurassic Delegate basaltic diatremes in
the Paleozoic Lachlan Fold Belt, SE Australia. Two upper mantle mafic xenoliths from the Delegate pipes, a garnet pyroxenite
and a garnet granulite (equilibrated at 1060 and 1140 °C, and 40–50 km), yield garnet-clinopyroxene Sm-Nd ages of 160 ± 4
Ma and 153 ± 10 Ma, respectively. Both ages are indistinguishable from the time of eruption of the diatremes, and are interpreted
as showing continuous isotopic equilibrium within the mantle of Sm and Nd between garnet + clinopyroxene at temperatures ≥ 1050 °C.
A lower crustal, 2-pyroxene granulite xenolith (equilibrated at 810–850 °C and ca. 25 km) yields a clinopyroxene + plagioclase + whole
rock Sm-Nd isochron ages of 283 ± 26 Ma. This age probably reflects partial resetting of the isotopic systems of much older
granulite during slow cooling, or after a heating event in the lower crust associated with the Jurassic magmatic activity
represented by the basaltic host rock. Metamorphic zircons from the 2-pyroxene granulite xenolith were dated by the U-Pb method
at 398±2 and 391 ± 2 Ma. These ages are considered to date granulite facies metamorphic events in the lower crust of the region.
The age gap between the granulite facies metamorphism and granitoid plutonism in the region (420–410 Ma) indicates that the
dated granulite is unlikely to represent residue after partial melting and magma extraction that generated the regional granitoids.
It is suggested that these ages may record a relatively slow cooling following the cessation of mafic magmatic intrusion that
formed the xenolith protoliths and that was probably the heat source responsible for granite production. At about 25 km, this
thermal relaxation accounts for the change from an olivine + plagioclase + 2-pyroxene gabbroic assemblage into the granulite
facies 2-pyroxene + plagioclase + spinel field.
Received: 17 May 1995 / Accepted: 24 March 1997 相似文献
11.
Evolution of metamorphic volatiles during exhumation of microdiamond-bearing granulites in the Western Gneiss Region, Norway 总被引:1,自引:0,他引:1
Rune B. Larsen Elizabeth A. Eide Ernst A. J. Burke 《Contributions to Mineralogy and Petrology》1998,133(1-2):106-121
Fluid inclusions in garnet, kyanite and quartz from microdiamond-bearing granulites in the Western Gneiss Region, Norway,
document a conspicuous fluid evolution as the rocks were exhumed following Caledonian high- and ultrahigh-pressure (HP–UHP) metamorphism. The most important of the various fluid mixtures and daughter minerals in these rocks are: (N2 + CO2 + magnesian calcite), (N2 + CO2 + CH4 + graphite + magnesian calcite), (N2 + CH4), (N2 + CH4 + H2O), (CO2) and (H2O + NaCl + CaCl2 + nahcolite). Rutile also occurs in the N2 + CO2 inclusions as a product of titanium diffusion from the garnet host into the fluid inclusions. Volatiles composed of N2 + CO2 + magnesian calcite characterise the ambient metamorphic environment between HP–UHP (peak) and early retrograde metamorphism. During progressive decompression, the mole fraction of N2 increased in the fluid mixtures; as amphibolite-facies conditions were reached, CH4 and later, H2O, appeared in the fluids, concomitant with the disappearance of CO2 and magnesian calcite. Graphite is ubiquitous in the host lithologies and fluid inclusions. Thermodynamic modelling of the
metamorphic volatiles in a graphite-buffered C-O-H system demonstrates that the observed metamorphic volatile evolution was
attainable only if the f
O2 increased from c. −3.5 (±0.3) to −0.8 (±0.3) log units relative to the FMQ oxygen buffer. External introduction of oxidising
aqueous solutions along a system of interconnected ductile shear zones adequately explains the dramatic increase in the f
O2. The oxidising fluids introduced during exhumation were likely derived from dehydration of oceanic crust and continental
sediments previously subducted during an extended period of continental collision in conjunction with the Caledonian orogeny.
Received: 15 December 1997 / Accepted: 25 May 1998 相似文献
12.
Mineral inclusions in diamonds from Namibia document a range of mantle sources, including eclogitic, websteritic and peridotitic parageneses. Based on unusual textural features a group of inclusions showing websteritic, peridotitic and transitional chemical features is assigned to an 'undetermined suite' (12% of the studied diamonds). The mutual characteristic of this group is the occurrence of lamellar intergrowths of clinopyroxene and orthopyroxene. In addition, the 'undetermined suite' is associated with a number of uncommon phases: in one diamond MgCO3 is enclosed by clinopyroxene. Other minerals that form touching inclusions with the pyroxene lamellae are (1) a SiO2 phase observed in three diamonds, together with CaCO3 in one of them, (2) phlogopite and a Cr-rich 'titanate' (probably lindsleyite). The inclusions document a metamorphic path of decreasing pressures and temperatures after entrapment in diamond. First, homogeneous low-Ca clinopyroxenes were entrapped at high temperatures. They subsequently exsolved orthopyroxene and probably also SiO2 (coesite) on cooling along a P,T trajectory that did not allow garnet to be exsolved as well. Phlogopite, carbonates and LIMA phases are the result of overprint of a peridotitic source rock by a carbon-rich agent. The resulting unusual, olivine-free mineral association and the host diamonds are interpreted as products of extensive carbonation of the peridotite. 相似文献
13.
A detailed study based on textural observations combined with microanalysis [back scattered electron imaging (BSE) and electron
microprobe analysis (EMPA)] and microstructural data transmission electron microscopy (TEM) has been made of K-feldspar micro-veins
along quartz–plagioclase phase and plagioclase–plagioclase grain boundaries in granulite facies, orthopyroxene–garnet-bearing
gneiss's (700–825 °C, 6–8 kbar) from the Val Strona di Omegna, Ivrea–Verbano Zone, northern Italy. The K-feldspar micro-veins
are commonly associated with quartz and plagioclase and are not found in quartz absent regions of the thin section. This association
appears to represent a localised reaction texture resulting from a common high grade dehydration reaction, namely: amphibole + quartz
= orthopyroxene + clinopyroxene + plagioclase + K-feldspar + H2O, which occurred during the granulite facies metamorphism of these rocks. There are a number of lines of evidence for this.
These include abundant Ti-rich biotite, which was apparently stable during granulite facies metamorphism, and total lack of
amphibole, which apparently was not. Disorder between Al and Si in the K-feldspar indicates crystallisation at temperatures
>500 °C. Myrmekite and albitic rim intergrowths in the K-feldspar along the K-feldspar–plagioclase interface could only have
formed at temperatures >500–600 °C. Symplectic intergrowths of albite and Ca-rich plagioclase between these albitic rim intergrowths
and plagioclase suggest a high temperature grain boundary reaction, which most likely occurred at the start of decompression
in conjunction with a fluid phase. Relatively high dislocation densities (>2 × 109 to 3 × 109/cm2) in the K-feldspar suggest plastic deformation at temperatures >500 °C. We propose that this plastic deformation is linked
with the extensional tectonic environment present during the mafic underplating event responsible for the granulite facies
metamorphism in these rocks. Lastly, apparently active garnet grain rims associated with side inclusions of K-feldspar and
quartz and an exterior K-feldspar micro-vein indicate equilibrium temperatures within 20–30 °C of the peak metamorphic temperatures
estimated for the sample (770 °C). Contact between these K-feldspar micro-veins and Fe-Mg silicate minerals, such as garnet,
orthopyroxene, clinopyroxene or biotite along the interface, is observed to be very clean with no signs of melt textures or
alteration to sheet silicates. This lends support to the idea that these micro-veins did not originate from a melt and, if
fluid induced, that the water activity of these fluids must have been relatively low. All of these lines of evidence point
to a high grade origin for the K-feldspar micro-veins and support the hypothesis that they formed during the granulite facies
metamorphism of the metabasite layers in an extensional tectonic environment as the consequence of localised dehydration reactions
involving the breakdown of amphibole in the presence of quartz to orthopyroxene, clinopyroxene, plagioclase, K-feldspar and
H2O. It is proposed that the dehydration of the metabasite layers to an orthopyroxene–garnet-bearing gneiss over a 4-km traverse
in the upper Val Strona during granulite facies metamorphism was a metasomatic event initiated by the presence of a high-grade,
low H2O activity fluid (most likely a NaCl–KCl supercritical brine), related to the magmatic underplating event responsible for
the Mafic Formation; and that this dehydration event did not involve partial melting.
Received: 15 February 2000 / Accepted: 26 June 2000 相似文献
14.
Experimental comparison of trace element partitioning between clinopyroxene and melt in carbonate and silicate systems, and implications for mantle metasomatism 总被引:13,自引:2,他引:11
Experiments in the systems diopside-albite (Di-Ab) and diopside-albite-dolomite (Di-Ab-Dmt), doped with a wide range of trace
elements, have been used to characterise the difference between clinopyroxene-silicate melt and clinopyroxene-carbonate melt
partitioning. Experiments in Di-Ab-Dmt yielded clinopyroxene and olivine in equilibrium with CO2-saturated dolomitic carbonate melt at 3 GPa, 1375 °C. The experiments in Di-Ab were designed to bracket those conditions
(3 GPa, 1640 °C and 0.8 GPa, 1375 °C), and so minimise the contribution of differential temperature and pressure to partitioning.
Partition coefficients, determined by SIMS analysis of run products, differ markedly for some elements between Di-Ab and Di-Ab-Dmt
systems. Notably, in the carbonate system clinopyroxene-melt partition coefficients for Si, Al, Ga, heavy REE, Ti and Zr are
higher by factors of 5 to 200 than in the silicate system. Conversely, partition coefficients for Nb, light REE, alkali metals
and alkaline earths show much less fractionation (<3). The observed differences compare quantitatively with experimental data
on partitioning between immiscible carbonate and silicate melts, indicating that changes in melt chemistry provide the dominant
control on variation in partition coefficients in this case. The importance of melt chemistry in controlling several aspects
of element partitioning is discussed in light of the energetics of the partitioning process. The compositions of clinopyroxene
and carbonate melt in our experiments closely match those of near-solidus melts and crystals in CMAS-CO2 at 3 GPa, suggesting that our partition coefficients have direct relevance to melting of carbonated mantle lherzolite. Melts
so produced will be characterised by elevated incompatible trace element concentrations, due to the low degrees of melting
involved, but marked depletions of Ti and Zr, and fractionated REE patterns. These are common features of natural carbonatites.
The different behaviour of trace elements in carbonate and silicate systems will lead to contrasted styles of trace element
metasomatism in the mantle.
Received: 15 July 1999 / Accepted: 18 February 2000 相似文献
15.
Tsutomu Ota Katsura Kobayashi Tomoo Katsura Eizo Nakamura 《Contributions to Mineralogy and Petrology》2008,155(1):19-32
Pressure–temperature conditions of tourmaline breakdown in a metapelite were determined by high-pressure experiments at 700–900°C
and 4–6 GPa. These experiments produced an eclogite–facies assemblage of garnet, clinopyroxene, phengite, coesite, kyanite
and rare rutile. The modal proportions of tourmaline clearly decreased between 4.5 and 5 GPa at 700°C, between 4 and 4.5 GPa
at 800°C, and between 800 and 850°C at 4 GPa, with tourmaline that survived the higher temperature conditions appearing corroded
and thus metastable. Decreases in the modal abundance of tourmaline are accompanied by decreasing modal abundance of coesite,
and increasing that of clinopyroxene, garnet and kyanite; the boron content of phengite increases significantly. These changes
suggest that, with increasing pressure and temperature, tourmaline reacts with coesite to produce clinopyroxene, garnet, kyanite,
and boron-bearing phengite and fluid. Our results suggest that: (1) tourmaline breakdown occurs at lower pressures and temperatures
in SiO2-saturated systems than in SiO2-undersaturated systems. (2) In even cold subduction zones, subducting sediments should release boron-rich fluids by tourmaline
breakdown before reaching depths of 150 km, and (3) even after tourmaline breakdown, a significant amount of boron partitioned
into phengite could be stored in deeply subducted sediments. 相似文献
16.
P -T paths from anatectic pelites 总被引:2,自引:1,他引:1
Frank S. Spear Matthew J. Kohn John T. Cheney 《Contributions to Mineralogy and Petrology》1999,134(1):17-32
A relatively simple petrogenetic grid for partial melting of pelitic rocks in the NCKFMASH system is presented based on the
assumption that the only H2O available for melting is through dehydration reactions. The grid includes both discontinuous and continuous Fe-Mg reactions;
contours of Fe/(Fe+Mg) for continuous reactions define P-T vectors along which continuous melting will occur. For biotite-bearing assemblages (garnet+biotite + sillimanite + K-feldspar + liquid
and garnet + biotite + cordierite + K-feldspar + liquid), Fe/(Fe+Mg) contours have negative slopes and melting will occur
with increasing temperature or pressure. For biotite-absent assemblages (garnet + cordierite + sillimanite + K-feldspar + liquid
or garnet + cordierite + orthopyroxene + K-feldspar + liquid) Fe/(Fe + Mg) contours have flat slopes and melting will occur
only with increasing pressure. The grid predicts that abundant matrix K-feldspar should only be observed if rocks are heated
at P < 3.8 kbar, that abundant retrograde muscovite should only be observed if rocks are cooled at P > 3.8 kbar, and that generation of late biotite + sillimanite replacing garnet, cordierite, or as selvages around leu- cosomes
should be common in rocks in which melt is not removed. There is also a predicted field for dehydration melting of staurolite
between 5 and 12 kbar. Textures in migmatites from New Hampshire, USA, suggest that prograde dehydration melting reactions
are very nearly completely reversible during cooling and crystallization in rocks in which melt is not removed. Therefore,
many reaction textures in “low grade” migmatites may represent retrograde rather than prograde reactions.
Received: 5 March 1998 / Accepted: 7 August 1998 相似文献
17.
Cliff S. J. Shaw 《Contributions to Mineralogy and Petrology》1999,135(2-3):114-132
A large body of recent work has linked the origin of Si-Al-rich alkaline glass inclusions to metasomatic processes in the
upper mantle. This study examines one possible origin for these glass inclusions, i.e., the dissolution of orthopyroxene in
Si-poor alkaline (basanitic) melt. Equilibrium dissolution experiments between 0.4 and 2 GPa show that secondary glass compositions
are only slightly Si enriched and are alkali poor relative to natural glass inclusions. However, disequilibrium experiments
designed to examine dissolution of orthopyroxene by a basanitic melt under anhydrous, hydrous and CO2-bearing conditions show complex reaction zones consisting of olivine, ± clinopyroxene and Si-rich alkaline glass similar
in composition to that seen in mantle xenoliths. Dissolution rates are rapid and dependent on volatile content. Experiments
using an anhydrous solvent show time dependent dissolution rates that are related to variable diffusion rates caused by the
saturation of clinopyroxene in experiments longer than 10 minutes. The reaction zone glass shows a close compositional correspondence
with natural Si-rich alkaline glass in mantle-derived xenoliths. The most Si-and alkali-rich melts are restricted to pressures
of 1 GPa and below under anhydrous and CO2-bearing conditions. At 2 GPa glass in hydrous experiments is still Si-␣and alkali-rich whereas glass in the anhydrous and
CO2-bearing experiments is only slightly enriched in SiO2 and alkalis compared with the original solvent. In the low pressure region, anhydrous and hydrous solvent melts yield glass
of similar composition whereas the glass from CO2-bearing experiments is less SiO2 rich. The mechanism of dissolution of orthopyroxene is complex involving rapid incongruent breakdown of the orthopyroxene,
combined with olivine saturation in the reaction zone forming up to 60% olivine. Inward diffusion of CaO causes clinopyroxene
saturation and uphill diffusion of Na and K give the glasses their strongly alkaline characteristics. Addition of Na and K
also causes minor SiO2 enrichment of the reaction glass by increasing the phase volume of olivine. Olivine and clinopyroxene are transiently stable
phases within the reaction zone. Clinopyroxene is precipitated from the reaction zone melt near the orthopyroxene crystal
and redissolved in the outer part of the reaction zone. Olivine defines the thickness of the reaction zone and is progressively
dissolved in the solvent as the orthopyroxene continues to dissolve. Although there are compelling reasons for supporting
the hypothesis that Si-rich alkaline melts are produced in the mantle by orthopyroxene – melt reaction in the mantle, there
are several complications particularly regarding quenching in of disequilibrium reaction zone compositions and the mobility
of highly polymerized melts in the upper mantle. It is considered likely that formation of veins and pools of Si-rich alkaline
glass by orthopyroxene – melt reaction is a common process during the ascent of xenoliths. However, reaction in situ within
the mantle will lead to equilibration and therefore secondary melts will be only moderately siliceous and alkali poor.
Received: 24 August 1998 / Accepted: 2 December 1998 相似文献
18.
Carl Spandler Greg Yaxley David H. Green Dean Scott 《Contributions to Mineralogy and Petrology》2010,160(4):569-589
We performed a series of piston-cylinder experiments on a synthetic pelite starting material over a pressure and temperature
range of 3.0–5.0 GPa and 1,100–1,600°C, respectively, to examine the melting behaviour and phase relations of sedimentary
rocks at upper mantle conditions. The anhydrous pelite solidus is between 1,150 and 1,200°C at 3.0 GPa and close to 1,250°C
at 5.0 GPa, whereas the liquidus is likely to be at 1,600°C or higher at all investigated pressures, giving a large melting
interval of over 400°C. The subsolidus paragenesis consists of quartz/coesite, feldspar, garnet, kyanite, rutile, ±clinopyroxene
±apatite. Feldspar, rutile and apatite are rapidly melted out above the solidus, whereas garnet and kyanite are stable to
high melt fractions (>70%). Clinopyroxene stability increases with increasing pressure, and quartz/coesite is the sole liquidus
phase at all pressures. Feldspars are relatively Na-rich [K/(K + Na) = 0.4–0.5] at 3.0 GPa, but are nearly pure K-feldspar
at 5.0 GPa. Clinopyroxenes are jadeite and Ca-eskolaite rich, with jadeite contents increasing with pressure. All supersolidus
experiments produced alkaline dacitic melts with relatively constant SiO2 and Al2O3 contents. At 3.0 GPa, initial melting is controlled almost exclusively by feldspar and quartz, giving melts with K2O/Na2O ~1. At 4.0 and 5.0 GPa, low-fraction melting is controlled by jadeite-rich clinopyroxene and K-rich feldspar, which leads
to compatible behaviour of Na and melts with K2O/Na2O ≫ 1. Our results indicate that sedimentary protoliths entrained in upwelling heterogeneous mantle domains may undergo melting
at greater depths than mafic lithologies to produce ultrapotassic dacitic melts. Such melts are expected to react with and
metasomatise the surrounding peridotite, which may subsequently undergo melting at shallower levels to produce compositionally
distinct magma types. This scenario may account for many of the distinctive geochemical characteristics of EM-type ocean island
magma suites. Moreover, unmelted or partially melted sedimentary rocks in the mantle may contribute to some seismic discontinuities
that have been observed beneath intraplate and island-arc volcanic regions. 相似文献
19.
We have experimentally investigated melting phase relation of a nominally anhydrous, carbonated pelitic eclogite (HPLC1) at
2.5 and 3.0 GPa at 900–1,350°C in order to constrain the cycling of sedimentary carbon in subduction zones. The starting composition
HPLC1 (with 5 wt% bulk CO2) is a model composition, on a water-free basis, and is aimed to represent a mixture of 10 wt% pelagic carbonate unit and
90 wt% hemipelagic mud unit that enter the Central American trench. Sub-solidus assemblage comprises clinopyroxene + garnet + K-feldspar + quartz/coesite + rutile + calcio-ankerite/ankeritess. Solidus temperature is at 900–950°C at 2.5 GPa and at 900–1,000°C at 3.0 GPa, and the near-solidus melt is K-rich granitic.
Crystalline carbonates persist only 50–100°C above the solidus and at temperatures above carbonate breakdown, carbon exists
in the form of dissolved CO2 in silica-rich melts and as a vapor phase. The rhyodacitic to dacitic partial melt evolves from a K-rich composition at near-solidus
condition to K-poor, and Na- and Ca-rich composition with increasing temperature. The low breakdown temperatures of crystalline
carbonate in our study compared to those of recent studies on carbonated basaltic eclogite and peridotite owes to Fe-enrichment
of carbonates in pelitic lithologies. However, the conditions of carbonate release in our study still remain higher than the
modern depth-temperature trajectories of slab-mantle interface at sub-arc depths, suggesting that the release of sedimentary
carbonates is unlikely in modern subduction zones. One possible scenario of carbonate release in modern subduction zones is
the detachment and advection of sedimentary piles to hotter mantle wedge and consequent dissolution of carbonate in rhyodacitic
partial melt. In the Paleo-NeoProterozoic Earth, on the other hand, the hotter slab-surface temperatures at subduction zones
likely caused efficient liberation of carbon from subducting sedimentary carbonates. Deeply subducted carbonated sediments,
similar to HPLC1, upon encountering a hotter mantle geotherm in the oceanic province can release carbon-bearing melts with
high K2O, K2O/TiO2, and high silica, and can contribute to EM2-type ocean island basalts. Generation of EM2-type mantle end-member may also
occur through metasomatism of mantle wedge by carbonated metapelite plume-derived partial melts. 相似文献
20.
Melt inclusions in kimberlitic and metamorphic diamonds worldwide range in composition from potassic aluminosilicate to alkali-rich carbonatitic and their low-temperature derivative, a saline high-density fluid (HDF). The discovery of CO2 inclusions in diamonds containing eclogitic minerals are also essential. These melts and HDFs may be responsible for diamond formation and metasomatic alteration of mantle rocks since the late Archean to Phanerozoic. Although a genetic link between these melts and fluids was suggested, their origin is still highly uncertain. Here we present experimental results on melting phase relations in a carbonated pelite at 6 GPa and 900–1500 °C. We found that just below solidus K2O enters potassium feldspar or K2TiSi3O9 wadeite coexisting with clinopyroxene, garnet, kyanite, coesite, and dolomite. The potassium phases react with dolomite to produce garnet, kyanite, coesite, and potassic dolomitic melt, 40(K0.90Na0.10)2CO3·60Ca0.55Mg0.24Fe0.21CO3 + 1.9 mol% SiO2 + 0.7 mol% TiO2 + 1.4 mol% Al2O3 at the solidus established near 1000 °C. Molecular CO2 liberates at 1100 °C. Potassic aluminosilicate melt appears in addition to carbonatite melt at 1200 °C. This melt contains (mol/wt%): SiO2 = 57.0/52.4, TiO2 = 1.8/2.3, Al2O3 = 8.5/13.0, FeO = 1.4/1.6, MgO = 1.9/1.2, CaO = 3.8/3.2, Na2O = 3.2/3.0, K2O = 10.5/15.2, CO2 = 12.0/8.0, while carbonatite melt can be approximated as 24(K0.81Na0.19)2CO3·76Ca0.59Mg0.21Fe0.20CO3 + 3.0 mol% SiO2 + 1.6 mol% TiO2 + 1.4 mol% Al2O3. Both melts remain stable to at least 1500 °C coexisting with CO2 fluid and residual eclogite assemblage consisting of K-rich omphacite (0.4–1.5 wt% K2O), almandine-pyrope-grossular garnet, kyanite, and coesite. The obtained immiscible alkali‑carbonatitic and potassic aluminosilicate melts resemble compositions of melt inclusions in diamonds worldwide. Thus, these melts entrapped by diamonds could be derived by partial melting of the carbonated material of the continental crust subducted down to 180–200 km depths. Given the high solubility of chlorides and water in both carbonate and aluminosilicate melts inferred in previous experiments, the saline end-member, brine, could evolve from potassic carbonatitic and/or silicic melts by fractionation of Ca-Mg carbonates/eclogitic minerals and accumulation of alkalis, chlorine and water in the residual low-temperature supercritical fluid. Direct extraction from the hydrated marine sediments under conditions of cold subduction would be another possibility for the brine formation. 相似文献