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1.
Clinopyroxene geobarometry of magmatic rocks Part 1: An expanded structural geobarometer for anhydrous and hydrous, basic and ultrabasic systems 总被引:15,自引:0,他引:15
Crystal-structure modeling of experimental Ca-rich clinopyroxenes [Ca + Na > 0.5 apfu; Mg/(Mg + Fe2+) > 0.7] coexisting with basic and ultrabasic melts was utilized for calibration of geobarometers based on unit-cell volume (Vcell) vs M1-site volume (VM1). The clinopyroxene database includes over one hundred experiments from literature and sixteen previously unpublished experiments on basanite and picrobasalt starting materials. The coexisting melts span a wide range of petrologically relevant anhydrous and hydrous compositions (from quartz-normative basalt to nephelinite, excluding high-Al basalts and melts coexisting with garnet or melilite) at pressure conditions pertinent to the earth's crust and uppermost mantle (P= 0–24 kbar) in a variety of fO 2 conditions (from CCO-buffered to air-buffered) and mineral assemblages (Cpx ± Opx ± Pig ± Ol ± Plag ± Lc ± Ne ± Spl ± Amp ± Ilm). As previously found for near-liquidus products of basaltic melts, the experimental clinopyroxenes follow two distinct trends: (i) at a given P, Vcell is linearly and negatively correlated with VM1. This corresponds with the extent of Tschermak-type substitutions, which depends strongly on aSiO2 and a CaO; (ii) for a fixed melt composition, Vcell and VM1 decrease linearly as P increases, due to a combination of M1, M2 and T site exchanges. Despite the chemical complexity of these relationships, P could be modeled as a linear function of Vcell and VM1. A simplified solution for anhydrous magmas reproduced the experimental pressures with an uncertainty of 1.75 kbar (=1 ; max. dev. = 5.5 kbar; N = 135). An expanded T-dependent solution capable of recovering the measured pressures of both anhydrous and hydrous experiments with an uncertainty of 1.70 kbar (=1 ; max. dev. = 5.4 kbar; N = 157) was obtained by correcting unit-cell and M1-site volumes for thermal expansivity and compressibility. The corrected formulation is more resistant to the effects of temperature variations and is therefore recommended. Nevertheless, it requires an independent, accurate estimate of crystallization T. Underestimating T by 20 °C propagates into a 1-kbar increase of calculated P. The applicability of the T-dependent formulation was tested on hydrous ultramafic to gabbroic rocks of the southern Adamello batholith for which P-T evolution could independently be constrained by field observation, petrography and experimentally determined phase relations. The pressure estimates obtained by clinopyroxene structural geobarometry closely matched those predicted by phase equilibria of a picrobasaltic melt parental to the investigated magmatic rocks. To facilitate application of the present geobarometers, both anhydrous and corrected solutions were implemented as MS-DOS® and UNIX® software programs (CpxBar) designed to permit retrieval of the pressure of crystallization directly from a chemical analysis or from uncorrected unit-cell and M1-site volume X-ray data. 相似文献
2.
Experimental clinopyroxenes synthesized at 850–1500 °C and 0–60 kbar in the CMS and CMAS-Cr systems and in more complex lherzolitic
systems have been used to calibrate a Cr-in-Cpx barometer and an enstatite-in-Cpx thermometer for Cr-diopsides derived from
garnet peridotites. The experiments cover a wide range of possible natural peridotitic compositions, from fertile pyrolite to refractory, high-Cr lherzolite. The barometer is based on the Cr exchange between clinopyroxene and garnet. Pressure is formulated as
a function of temperature and clinopyroxene composition:
where a
CaCrTs
Cpx=Cr−0.81·Cr#·(Na+K) and Cr#= , with elements in atoms per 6 oxygens. This formulation reproduces the experimental pressures to ±2.3 kbar (1σ) and has a
temperature dependence (1.2–2.4 kbar/50 °C, varying with composition) that is weaker than that of the widely used Al-in-Opx
barometer (2–3 kbar/50 °C). The enstatite-in-Cpx thermometer includes corrections for the effect of minor components and is
formulated as
where K)). The thermometer reproduces the experimental temperatures to ±30 °C (1σ).
The uncertainties of the present formulations are comparable to, or better than, those of the most widely used thermobarometers
for garnet peridotites. P-T estimates obtained for diamond-bearing and graphite-bearing lherzolite xenoliths and peridotitic clinopyroxene inclusions
in kimberlitic and lamproitic diamonds confirm the reliability of the thermobarometer. Cr-diopside thermobarometry appears
to be a potential tool for obtaining information on the thermal state of the upper mantle and the extent of mantle sampling
by deep-seated magmas. We consider the Cr-in-Cpx barometer to be the best alternative to the Al-in-Opx barometer for the evaluation
of pressure conditions of equilibration of natural garnet lherzolites. P-T conditions of equilibration can be directly retrieved from the composition of Cr-diopside alone, thus allowing application
to partially altered xenoliths, inclusions in diamonds, and loose grains from sediments. We foresee application of the present
thermobarometer to evaluation of the diamond potential of kimberlite and lamproite provinces and in diamond exploration where
Cr-diopside from deep mantle sources is preserved in the surficial weathering environment.
Received: 16 August 1999 / Accepted: 17 March 2000 相似文献
3.
A clinopyroxene geobarometer for basaltic systems based on crystal-structure modeling 总被引:9,自引:0,他引:9
Paolo Nimis 《Contributions to Mineralogy and Petrology》1995,121(2):115-125
The crystal chemical response of basalt clinopyroxene to increasing pressure was investigated by means of crystal-structure
simulation (a procedure that enables modeling of the structural parameters of a clinopyroxene of known chemistry without requiring
direct X-ray diffraction analysis) using available experimental chemical data. Pressure proved the main physical variable
governing clinopyroxene behavior in a magmatic environment. The general internal consistency of the simulation data permitted
construction of an empirical geobarometer based on the relationship of cell volume (Vc) vs M1-site volume (VM1). The straightforward
geobarometric formulation in the absence of direct X-ray analysis is:
P(kbar)=698.443+4.985⋅AlT−26.826⋅Fe2+
M1−3.764⋅Fe3+
+53.989⋅AlM1+3.948⋅Ti+14.651⋅Cr
−700.431⋅Ca−666.629⋅Na−682.848⋅MgM2−691.138⋅Fe2+
M2−688.384⋅Mn−6.267⋅(MgM2)2−4.144⋅(Fe2+
M2
where:
(Fe2+
M1⋅MgM2)/(Fe2+
M2⋅MgM1)=e**(0.238⋅R3++0.289⋅CNM−2.315), CNM=Ca+Na+Mn, and R3+=AlM1+Fe3++Ti+Cr,
with cations in atoms per formula unit. The geobarometer reproduces experimental pressures within ±2 kbar (=1σ; max. dev.
≤5 kbar; N=29) in the range 0–24 kbar and is applicable to near-liquidus C2/c clinopyroxenes crystallized from basaltic melts in the absence of garnet (excepting high-Al2O3 basalts). It is therefore suitable for many natural clinopyroxenes occurring as mega- or phenocrysts or forming well-preserved
cumulate pyroxenites. If the above restrictions are not wholly satisfied, the Vc vs VM1 plot can also be used qualitatively
to deduce the relative pressure conditions of clinopyroxenes forming from similar batches of magma. The structural simulation of experimental data
also provided insight into the influence of minor chemical changes of the parental magma on the crystal chemistry of clinopyroxene
at high pressure. Within the considered compositional space at given P-T, a
CaO and a
SiO2 in the melt have opposite effects on M2- and T-site cation populations. As a result, under similar physical conditions, clinopyroxenes
from higher-CaO or more undersaturated basalts have higher VM2, VT and Vc and lower VM1. For basalts with normal contents of Al2O3 (<18 wt %), variations of major elements in the melt do not reduce the accuracy of the geobarometer.
Received: 3 April 1994 / Accepted: 23 December 1995 相似文献
4.
An alkali basalt near Glen Innes, northeastern New South Wales, contains a suite of Cr-diopside group ultramafic xenoliths
which includes some spinel peridotites but which is dominated by a diverse spinel pyroxenite assemblage. Pyroxenite xenoliths
range from subcalcic clinopyroxenites (composed largely of unmixed prismatic subcalcic clinopyroxene megacrystals and lesser
orthopyroxene megacrystals) to equant mosaic textured websterites (orthopyroxene and Ca-rich clinopyroxene ± spinel). Rare
orthopyroxenite xenoliths also occur. The pyroxenite xenoliths are characterised by high 100Mg/(Mg + Fe2+) ratios (M˜ 90) and low concentrations of Ti, K, P, La, Ce and Zr. The websterites are mineralogically and chemically similar to many
spinel pyroxenites occurring as layers or dykes in peridotite massifs such as those at Ronda in southern Spain and at Ariège
(French Pyrénées). T / P estimates indicate crystallization temperatures of 1250–1350 °C for subcalcic clinopyroxene-orthopyroxene megacrystal pairs
and 900–1000 °C for the equilibrated mosaic textured websterites and associated peridotites at pressures of 9–13 kbar. Subcalcic
clinopyroxene megacrystals, websterites and orthopyroxenites have LREE-depleted chondrite-normalised REE abundances with (La/Yb)CN < 1 and their convex-upwards REE patterns are typical of subcalcic clinopyroxene-dominated cumulates. The pyroxenites are
not residua from partially melted pyroxenite layers or dykes in mantle peridotites nor are they completely crystallized protobasaltic
or protopicritic magmas. They are interpreted as high-pressure crystal segregations from basaltic magmas (probably mildly
alkaline or transitional) flowing within narrow mantle conduits (the flow crystallization model of Irving, 1980). The parental
magma(s) was Ti-poor (0.6–0.7% TiO2) and relatively Mg-rich (M˜ 74 − 70). Pyroxenite genesis was a two-stage process involving crystallization of tschermakitic subcalcic clinopyroxenes
and orthopyroxenes ±spinel as liquidus or near-liquidus phases at 1250–1350 °C and 9–13 kbar to yield “primary” subcalcic
clinopyroxenites which then re-equilibrated at 900–1000 °C and similar pressures to produce the mosaic textured “secondary”
websterites. The pyroxenites show a wide range of 143Nd/144Nd and 87Sr/86Sr values (0.513298–0.512473 and 0.702689–0.704659, respectively). Their isotopic ratios appear to have been variably modified
by exchange with adjacent mantle peridotites or migrating basaltic melts.
Received: 11 December 1995 / Accepted: 3 December 1996 相似文献
5.
J. L. Rosenberg P. G. Spry C. E. Jacobson N. J. Cook F. M. Vokes 《Mineralium Deposita》1998,34(1):19-34
The Bleikvassli massive sulfide ore deposit is hosted by Proterozoic pelitic, quartzofeldspathic, and amphibolitic rocks
of the Uppermost Allochthon of the Scandinavian Caledonides. Staurolite-garnet-biotite and kyanite-staurolite-biotite assemblages
indicate that metamorphism reached the kyanite zone of the amphibolite facies. Geothermobarometry was conducted on rocks in
and around the deposit using a variety of silicate and sulfide calibrations. Temperature determinations are most reliant on
the garnet-biotite exchange reaction, with analyses obtained from 259 garnet rims and adjacent biotite. Results from nine
calibrations of the garnet-biotite geothermometer are considered, but compositional limitations of many calibrations involving
high Ca and Mn contents in garnet and AlVI and Ti in biotite make many of the coexisting mineral pairs unsuitable. Average temperatures calculated from the two calibrations
that most closely address the garnet-biotite compositions observed at Bleikvassli are 584 °C ± 49 °C and 570 °C ± 40 °C. The
application of two calibrations of the garnet-staurolite geothermometer on a limited number of samples yields 581 °C ± 2 °C
and 589 °C ± 12 °C, assuming a
H2O=0.84, based upon calculations of the modal proportions of gaseous species. Pressure determinations are less constrained.
Phengite and plagioclase-biotite-garnet-muscovite geobarometers give average pressures of approximately 5.0 kbar and 8.5 ± 1.2 kbar,
respectively. Pressures obtained from the sphalerite-hexagonal pyrrhotite-pyrite barometer average 7.7 ± 1.0 kbar. In consideration
of these results, the peak metamorphic conditions at the Bleikvassli deposit are estimated to be 580 °C and 8 kbar.
Received: 18 June 1997 / Accepted: 14 May 1998 相似文献
6.
K. Putirka Marie Johnson Rosamond Kinzler John Longhi David Walker 《Contributions to Mineralogy and Petrology》1996,123(1):92-108
Models for estimating the pressure and temperature of igneous rocks from co-existing clino-pyroxene and liquid compositions
are calibrated from existing data and from new data obtained from experiments performed on several mafic bulk compositions
(from 8–30 kbar and 1100–1475° C). The resulting geothermobarometers involve thermodynamic expressions that relate temperature
and pressure to equilibrium constants. Specifically, the jadeite (Jd; NaAlSi2O6)–diopside/hedenbergite (DiHd; Ca(Mg, Fe) Si2O6) exchange equilibrium between clinopyroxene and liquid is temperature sensitive. When compositional corrections are made
to the calibrated equilibrium constant the resulting geothermometer is
(i) 104
T=6.73−0.26* ln [Jdpx*Caliq*FmliqDiHdpx*Naliq*Alliq] −0.86* ln [MgliqMgliq+Feliq]+0.52*ln [Caliq]
an expression which estimates temperature to ±27 K. Compared to (i), the equilibrium constant for jadeite formation is more
sensitive to pressure resulting in a thermobarometer
(ii) P=−54.3+299* T104+36.4* T104 ln [Jdpx[Siliq]2*Naliq*Alliq] +367*[Naliq*Alliq]
which estimates pressure to ± 1.4 kbar. Pressure is in kbar, T is in Kelvin. Quantities such as Naliq represent the cation fraction of the given oxide (NaO0.5) in the liquid and Fm=MgO+FeO. The mole fractions of Jd and diopside+hedenbergite (DiHd) components are calculated from a
normative scheme which assigns the lesser of Na or octahedral Al to form Jd; any excess AlVI forms Calcium Tschermak’s component (CaTs; CaAlAlSiO6); Ca remaining after forming CaTs and CaTiAl2O6 is taken as DiHd. Experimental data not included in the regressions were used to test models (i) and (ii). Error on predictions
of T using model (i) is ±40 K. A pressure-dependent form of (i) reduces this error to ±30 K. Using model (ii) to predict pressures,
the error on mean values of 10 isobaric data sets (0–25 kbar, 118 data) is ±0.3 kbar. Calculating thermodynamic properties
from regression coefficients in (ii) gives VJd
f of 23.4 ±1.3 cm3/mol, close to the value anticipated from bar molar volume data (23.5 cm3/mol). Applied to clinopyroxene phenocrysts from Mauna Kea, Hawaii lavas, the expressions estimate equilibration depths as
great as 40 km. This result indicates that transport was sufficiently rapid that at least some phenocrysts had insufficient
time to re-equilibrate at lower pressures.
Received: 16 May 1994/Accepted: 15 June 1995 相似文献
7.
Olivine and augite minette powders have been equilibrated from one bar to nearly 2.0 kbar (water-saturated), and from 900
to 1300° C, and then quenched rapidly, at oxygen fugacities controlled between the nickel-nickel oxide (NNO) and hematite-magnetite
(HM) oxygen buffers. The liquidus of both samples is suppressed ∼100° C at water-saturated conditions and 1500 bar. Both lavas
contained between 3 and 4 wt% water at the stage of phenocryst precipitation. The partitioning of ferric and ferrous iron
between phlogopite and liquid has been determined on eight samples across 3 log f O2 units; when these determinations are combined with previous studies, Fe2O3/(Σ FeO total) of Mg-rich biotite can be calculated knowing log f
O2, T, and X
Fe. Thermodynamic modelling of biotite-liquid equilibria results in two expressions for calculating activity coefficients (γ)
for annite and phlogopite in natural biotites. Based on the partitioning of BaO and TiO2 between biotite and liquid, we have formulated a thermometer and barometer. Over the range of 400° C, TiO2 partitioning between phlogopite and liquid is a function of temperature (±50° C), and is insensitive to pressure and H2O and O2 activities. BaO partitioning between phlogopite and liquid is a function of both temperature and pressure (±4 kbar), the
latter being most important. Applying the TiO2 and BaO partitioning expressions to lamprophyre and lamproite suites shows that Mexican minettes equilibrated at low pressures
(5 to 15 kbar;±4 kbar) and temperatures (1090 to 1160° C; ±50° C), while Australian lamproites equilibrated at higher P (up to 30 kbar; ±4 kbar) and T (1125 to 1400° C; ±50° C). Experimental glass compositions and phenocryst fractionation calculations, together with the BaO-
and TiO2- based pressure calculations indicate that felsic minettes from the Mexican suite of lavas can be generated by simple fractionation
of a more mafic parent minette at mid to lower crustal pressures.
Received: 1 August 1994/Accepted: 30 June 1995 相似文献
8.
The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario,
and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite.
These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite
± plagioclase show that the vein minerals were stable at T < 600 °C, XCO2 < 0.4 at P ≈ 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T < 600 °C and XCO2 < 0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T < 500 °C with XCO2 > 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from
H2O-rich to CO2-rich. The calcite in the retrograde veins has δ18O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ13C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated
underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived
either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince.
Vein quartz contains CO2-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO2 ≈ 1.13 to 1.05 g/cm3). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of
vein formation. It has been argued that some high density CO2-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO2-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO2-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions
after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements
with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment.
All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path.
Received: 2 April 1996 / Accepted: 15 November 1996 相似文献
9.
The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric
mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been
attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions.
Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic
K2O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase).
The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few
centimetres by vein–wallrock exchange reactions which continued down to subsolidus temperatures for Mg–Fe. Thermobarometric
data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P ≥ 13 kbar and recrystallized at P < 12 kbar. The high AlVI/AlIV ratio (>1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for
parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar
to transitional alkali basalt with about 1–3 wt% H2O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus
phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg=0.6; 4–6 wt% H2O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process.
Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P ≤ 10 kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle
is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms
of extensive dissolution of olivine and orthopyroxene from wallrock peridotite by alkaline melts produced at the mechanical
boundary layer/thermal boundary layer transition (about 45–50 km deep). Continuous fluid ingress allowed remelting of the
deeper veined mantle to produce the basanitic, strongly volatiles enriched, melts that precipitated hornblendites. A similar
model could be valid for the few orthopyroxene-rich hydrous pyroxenites described in basalt-hosted mantle xenoliths.
Received: 15 September 1999 / Accepted: 31 January 2000 相似文献
10.
Stability and chemical composition of pargasitic amphibole in MORB pyrolite under upper mantle conditions 总被引:18,自引:1,他引:17
The stability field of pargasitic amphibole in a model mantle composition (MORB pyrolite) has been experimentally determined
for a fixed water content. A solidus for a pargasite-bearing lherzolite has been defined at pressures below the limit of amphibole
stability of 30 kbar at T = 925 °C. The maximum temperature for pargasitic amphibole in MORB pyrolite occurs at 1075 °C between P = 18 and 25 kbar. This maximum lies between that determined for a fertile peridotite composition (Hawaiian pyrolite) and
a depleted peridotite composition (Tinaquillo lherzolite). A comparison of the new results with those from earlier studies
suggests that the stability for a particular bulk H2O content is mostly controlled by alkali content of the lherzolite composition. The systematic compositional variation of
pargasitic amphibole as a function of pressure and temperature can be represented as an increase of the richterite component
with increase in both pressure and temperature. For a given pressure the tschermakite component increases with increasing
temperature. The compositions of coexisting clinopyroxenes also show a systematic variation with pressure and temperature.
The phase relationships in MORB pyrolite combined with the modal abundance of coexisting phases show that the breakdown reactions
of pargasitic amphibole occur continuously throughout the subsolidus region studied. The temperature stability limit of pargasitic
amphibole coincides with the water-undersaturated solidus (amphibole-dehydration solidus) at pressures below 30 kbar. The
experimental results are applicable to pargasitic amphibole-bearing natural peridotites. Cooling and decompression paths and
heating events observed in natural peridotites can be interpreted from changes in the composition of pargasitic amphibole.
The data are also applicable to a model for peridotite melting and hydration process in the subduction environment.
Received: 27 October 1997 / Accepted: 6 November 1998 相似文献
11.
The near-solidus transition from garnet lherzolite to spinel lherzolite 总被引:20,自引:1,他引:19
The position of the transition from spinel lherzolite to garnet lherzolite in the system CaO-MgO-Al2O3-SiO2 (CMAS) has been determined experimentally at near-solidus temperatures. In reversed experiments, the transition occurs between
18 and 20 kbar at 1200 °C and between 26 and 27 kbar at 1500 °C, corresponding to higher pressures than previously envisaged.
A position for the transition deeper within the Earth further complicates the explanation of the so-called garnet signatures
in the trace element and isotope patterns of mid-ocean ridge basalts. If melting during adiabatic upwelling beneath a mid-ocean
ridge begins at the depth required for the stability of garnet in peridotitic compositions, simple melting models predict
that the amount of melt produced should be much greater than the observed thickness of the oceanic crust. A partial solution
to the apparent conflict might be that (1) the rather simplistic melting models are in error, (2) that melting begins in garnet
pyroxenite veins that are believed to be stable at lower pressures than garnet lherzolite or (3) that melting does not involve
garnet at all, but it is clinopyroxene causing the trace element patterns observed in basalts erupted at mid-ocean ridges.
A second set of reversal experiments were conducted to investigate the solubility of alumina in both orthopyroxenes and clinopyroxenes
at the high temperatures near the solidus in the system CMAS. The results are compatible with most previous studies, and may
be used as a starting point to calibrate thermodynamic models for pyroxenes in chemical systems, approximating upper mantle
chemistry.
Received: 9 August 1999 / Accepted: 29 October 1999 相似文献
12.
The beginning of dehydration melting in the tonalite system (biotite-plagioclase-quartz) is investigated in the pressure
range of 2–12 kbar. A special method consisting of surrounding a crystal of natural plagioclase (An45) with a biotite-quartz mixture, and observing reactions at the plagioclase margin was employed for precise determination
of the solidus for dehydration melting. The beginning of dehydration melting was worked out at 5 kbar for a range of compositions
of biotite varying from iron-free phlogopite to iron-rich Ann70, with and without titanium, fluorine and extra aluminium in the biotite. The dehydration melting of phlogopite + plagioclase
(An45) + quartz begins between 750 and 770°C at pressures of 2 and 5 kbar, at approximately 740°C at 8 kbar and between 700 and
730°C at 10 kbar. At 12 kbar, the first melts are observed at temperatures as low as 700°C. The data indicate an almost vertical
dehydration melting solidus curve at low pressures which bends backward to lower temperatures at higher pressures (> 5 kbar).
The new phases observed at pressures ≤ 10 kbar are melt + enstatite + clinopyroxene + potassium feldspar ± amphibole. In addition
to these, zoisite was also observed at 12 kbar. With increasing temperature, phlogopite becomes enriched in aluminium and
deficient in potassium. Substitution of octahedral magnesium by aluminium and titanium in the phlogopite, as well as substitution
of hydroxyl by fluorine, have little effect on the beginning of dehydration melting temperatures in this system. The dehydration
melting of biotite (Ann50) + plagioclase (An45) + quartz begins 50°C below that of phlogopite bearing starting composition. Solid reaction products are orthopyroxene +
clinopyroxene + potassium feldspar ± amphibole. Epidote was also observed above 8 kbar, and garnet at 12 kbar (750°C). The
experiments on the iron-bearing system performed at ≤ 5 kbar were buffered with NiNiO. The f
O
2 in high pressure runs lies close to CoCoO. With the substitution of octahedral magnesium and iron by aluminium and titanium,
and replacement of hydroxyl by fluorine in biotite, the beginning of dehydration melting temperatures in this system increase
up to 780°C at 5 kbar, which is 70°C above the beginning of dehydration melting of the assemblage containing biotite (Ann50) of ideal composition. The dehydration melting at 5 kbar in the more iron-rich Ann70-bearing starting composition begins at 730°C, and in the Ann25-bearing assemblage at 710°C. This indicates that quartz-biotite-plagioclase assemblages with intermediate compositions of
biotite (Ann25 and Ann50) melt at lower temperatures as compared to those containing Fe-richer or Mg-richer biotites. This study shows that the dehydration
melting of tonalites may begin at considerably lower temperatures than previously thought, especially at high pressures (>5
kbar).
Received: 27 December 1995 / Accepted: 7 May 1996 相似文献
13.
Faruk Aydin Richard M. Thompson Orhan Karsli Hinako Uchida Jason B. Burt Robert T. Downs 《Contributions to Mineralogy and Petrology》2009,158(1):131-147
Chemical and structural data are reported for C2/c pyroxene phenocrysts collected from three potassic series (Group A: basanite-tephrite, Group B: tephrite-phonolitic tephrite,
Group C: alkaline basalt-trachybasalt) of the Neogene alkaline volcanics (NAVs) in northeastern Turkey, in order to investigate
the evolution of the magmatic plumbing system and the location of magma chamber(s) with crystallization conditions. The rock
series hosting the clinopyroxene phenocrysts show generally porphyritic texture and have a variable phenocryst-rich nature
(20–58%), with phenocryst assemblages characterized by cpx ± ol ± plag ± foid ± amp ± bio. The clinopyroxene phenocrysts can
be chemically classified as Ti- and Fe3+-rich Al-diopsides for Groups A and B (AB-cpxs) and Ti- and Fe3+-poor Al-diopsides for Group C (C-cpxs). They have poorly variable composition, clustering in the diopside field. Structurally,
the diopside groups have nearly similar a (ranging from 9.73 to 9.75 ?), V
cell (437.2–440.9 ?3), and 〈beta〉 angle values (106.01°–106.23°), but some differences in polyhedral parameters and geometries of the AB-cpxs and C-cpxs have
been observed. For example, the AB-cpxs are characterized by larger c (5.27–5.30 vs. 5.25–5.28 ?), V
T (2.27–2.30 vs. 2.23–2.28 ?3), and V
M2 (25.53–25.72 vs. 25.41–25.59 ?3) values and smaller b (8.87–8.88 vs. 8.88–8.91 ?) and V
M1 (11.49–11.63 vs. 11.64–11.83 ?3) values with respect to the C-cpxs. In addition, the AB-cpxs show higher values of V
M2/V
M1 (2.20–2.23) due to large V
M2 and small V
M1 compared to the V
M2/V
M1 ratios of the C-cpxs (<2.19). Such differences in the crystal structure of the AB-cpxs and C-cpxs from the NAVs are partly
related to different crystallization pressures, but mostly related to variation in melt composition and, possibly, the influence
of other crystallizing mineral phases. In particular, R(M2-O1) and R(M1-O2) (i.e. bond lengths) differences in the clinopyroxenes
of different groups support the presence of evolved host rocks with different alkaline character (i.e. silica-undersaturated
Groups A–B and silica-saturated Group C). Based on the cpx-geothermobarometry, the crystallization pressures for the C-cpxs
are lower than 4.5 kbars, but the AB-cpxs have relatively high-pressure values (5.6–10.6 kbars), suggesting that the AB-cpxs
crystallized in higher pressure environments. The relatively higher crystallization temperatures of the AB-cpxs also indicate
higher cooling rates. The P–T estimates suggest that the source regions of the clinopyroxene phenocrysts from the NAVs were crustal magma chambers in a
closed plumbing system at a moderate- to low-pressure regime. 相似文献
14.
This study of La Gloria pluton in the Chilean Andes evaluates what information about magmatic conditions can be extracted
from minerals in a granitic pluton, despite lower-temperature re-equilibration. The pluton is zoned vertically from granodiorite/quartz
monzodiorite to quartz monzonite at the roof, with the uppermost 1500 m showing the strongest modal and compositional trends.
This mimics the pattern frequently inferred from zoning in voluminous ignimbrites: a strongly zoned cap overlying a more homogeneous
main␣body. The presence of large, euhedral amphibole ± biotite at the chamber margins and roof indicate that water was concentrated
there. Biotite and amphibole compositions indicate a roofward increase in magmatic f
HF, f
HCl and F/Cl ratio, analogous to pre-eruptive volatile gradients recorded in zoned ignimbrites. Hornblende that crystallized
directly from the melt in the volatile-rich wall and roof zones yields total-Al solidification pressures of ˜1 kbar, consistent
with the estimated 4000 m of cover at the time of emplacement. In the core of the pluton, actinolitic amphibole formed by
reaction of melt with early-crystallized clinopyroxene. Plag-cpx cumulate clots in the lower level are interpreted as early
crystallizing phases entrained in rising granitic magma. Cores of amphibole phenocrysts in mafic enclaves suggest initial
crystallization at pressures of 2–3 kbar. Lower Ti and Al contents of rims and acicular groundmass amphibole, overlapping
the composition of amphibole in the host granitoid, indicate that the enclaves equilibrated with the host at the present exposure
level in the presence of interstitial melt. A roofward relative increase in fO2 of the magma is recorded by an increasing proportion of Fe-Ti oxides as a fraction of the mafic phases, greater Mn content
of ilmenite, and constant or higher Mg/(Mg+Fe) in hornblende and biotite despite declining whole-rock MgO contents. Association␣of
subhedral biotite and magnetite with actinolitic amphibole in clots implies a reaction: K-Ti-hb + O2(gas) = bi + mt + actinolitic amph + titanite. Magnetite coexisting with biotite with Fe/(Fe+Mg) = 0.34– 0.40 implies temperatures
of equilibration no lower than about 720–750 °C, i.e., late-magmatic rather than subsolidus. Saturation with respect to a
water-rich vapor and subsequent diffusive loss of hydrogen may have caused this oxidation trend, which resulted in the most
magnesian mafic phases occurring in the most compositionally evolved rocks, opposite to trends in most zoned ignimbrites,
which presumably record conditions nearer the liquidus and prior to exsolution of a water-rich vapor.
Two-feldspar and Fe-Ti-oxide geothermometers record subsolidus conditions in the pluton and yield higher temperatures for
samples from the roof zone, suggesting that slower cooling at deeper levels allowed these minerals to continue to equilibrate
to lower temperatures. Individual minerals span wide ranges in composition at any given level of the pluton, from those appropriate
for phenocrysts, to those that record conditions well below the solidus. We suggest that the shallow level and isolated position
of the pluton led to rapid escape of magmatic volatiles and rapid cooling, thereby preventing development of a long-lived
hydrothermal system. Resulting small water/rock ratios may account for why late-magmatic and subsolidus re-equilibration were
not pervasive.
Received: 23 August 1996 / Accepted: 18 October 1996 相似文献
15.
H2O activities in supercritical fluids in the system KCl-H2O-(MgO) were measured at pressures of 1, 2, 4, 7, 10 and 15 kbar by numerous reversals of vapor compositions in equilibrium
with brucite and periclase. Measurements spanned the range 550–900 °C. A change of state of solute KCl occurs as pressures
increase above 2 kbar, by which H2O activity becomes very low and, at pressures of 4 kbar and above, nearly coincident with the square of the mole fraction
(x
H2O). The effect undoubtedly results primarily from ionic dissociation as H2O density (ρH2O) approaches 1 gm/cm3, and is more pronounced than in the NaCl-H2O system at the same P-T-X conditions. Six values of solute KCl activity were yielded by terminal points of the isobaric brucite-periclase T-x
H2O curves where sylvite saturation occurs. The H2O mole fraction of the isobaric invariant assemblage brucite-periclase-sylvite-fluid is near 0.52 at all pressures, and the
corresponding temperatures span only 100 °C between 1 and 15 kbar. This remarkable convergence of the isobaric equilibrium
curves reflects the great influence of pressure on lowering of both KCl and H2O activities. The H2O and KCl activities can be expressed by the formulas: a
H2O = γH2O[x
H2O+(1 + (1 + α)x
KCl)], and a
KCL = γKCl[(1 + α)x
KCl/(x
H2O +(1 + α)x
KCl)](1 + α), where α is a degree of dissociation parameter which increases from zero at the lowest pressures to near one at high pressures
and the γ's are activity coefficients based on an empirical regular solution parameter W: ln γi = (1 − xi)2W. Least squares fitting of our H2O and KCl activity data evaluates the parameters: α = exp(4.166 −2.709/ρH2O) − 212.1P/T, and W = (−589.6 − 23.10P) /T, with ρH2O in gm/cm3, P in kbar and T in K. The standard deviation from the measured activities is only ± 0.014. The equations define isobaric liquidus curves,
which are in perfect agreement with previous DTA liquidus measurements at 0.5–2 kbar, but which depart progressively from
their extrapolation to higher pressures because of the pressure-induced dissociation effect. The great similarity of the NaCl-H2O and KCl-H2O systems suggests that H2O activities in the ternary NaCl-KCl-H2O system can be described with reasonable accuracy by assuming proportionality between the binary systems. This assumption
was verified by a few reconnaissance measurements at 10 kbar of the brucite-periclase equilibrium with a Na/(Na + K) ratio
of 0.5 and of the saturation temperature for Na/(Na + K) of 0.35 and 0.50. At that pressure the brucite-periclase curves reach
a lowest x
H2O of 0.45 and a temperature of 587 °C before salt saturation occurs, values considerably lower than in either binary. This
double-salt eutectic effect may have a significant application to natural polyionic hypersaline solutions in the deep crust
and upper mantle in that higher solute concentrations and very low H2O activities may be realized in complex solutions before salt saturation occurs. Concentrated salt solutions seem, from this
standpoint, and also because of high mechanical mobility and alkali-exchanging potential, feasible as metasomatic fluids for
a variety of deep-crust and upper mantle processes.
Received: 9 August 1996 / Accepted: 15 November 1996 相似文献
16.
Margaret Hanrahan Gerhard Brey Alan Woodland Rainer Altherr Hans-Micheal Seitz 《Contributions to Mineralogy and Petrology》2009,158(2):169-183
An eclogite barometer has profound importance in the study of upper mantle processes and potential application to diamond
prospecting. Studies on the partitioning of Li between clinopyroxene (cpx) and garnet (grt) in natural samples have shown
that this particular element is very sensitive to changes in pressure and could be calibrated as the barometer demanded for
bimineralic eclogites. Experiments were performed from 4 to 13 GPa and 1,100–1,400°C in the CMAS (CaO, MgO, Al2O3, SiO2) system with Li added as Li3PO4 to quantify this pressure dependence into a barometer expressed in the following equation: P = (0.00255T – ln K
d)/0.2351 where P is in GPa, T is in °C and K
d is defined as the partition coefficient of Li (in ppm) between clinopyroxene and garnet. The experimental pressures are reproduced
to ±0.38 GPa (1σ) by this equation. This barometer is strictly applicable only to CMAS. Experiments at 1,300°C, 8–12 GPa showed
that Henry’s Law is fulfilled for Li partitioning between cpx and grt in the concentration range of approximately 0.01–1 wt%
Li. Direct application of the equation to experiments in natural systems performed at 1,300°C from 4 to 13 GPa consistently
overestimates pressures by approximately 2 GPa.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
17.
The diffusivity of water has been investigated for a haplogranitic melt of anhydrous composition Qz28Ab38Or34 (in wt %) at temperatures of 800–1200°C and at pressures of 0.5–5.0 kbar using the diffusion couple technique. Water contents
of the starting glass pairs varied between 0 and 9 wt %. Concentration-distance profiles for the different water species (molecular
water and hydroxyl groups) were determined by near-infrared microspectroscopy. Because the water speciation of the melt is
not quenchable (Nowak 1995; Nowak and Behrens 1995; Shen and Keppler 1995), the diffusivities of the individual species can
not be evaluated directly from these profiles. Therefore, apparent chemical diffusion coefficients of water (D
water) were determined from the total water profiles using a modified Boltzmann-Matano analysis. The diffusivity of water increases
linearly with water content <3 wt % but exponentially at higher water contents. The activation energy decreases from 64 ± 10 kJ/mole
for 0.5 wt % water to 46 ± 5 kJ/mole for 4 wt % water but remains constant at higher water contents. A small but systematic
decrease of D
water with pressure indicates an average activation volume of about 9 cm3/mole. The diffusivity (in cm2/s) can be calculated for given water content (in wt %), T (in K) and P (in kbar) by
in the ranges 1073 K ≤ T ≤ 1473 K; 0.5 kbar ≤ P≤ 5␣kbar; 0.5 wt % ≤ C
water ≤ 6 wt %. The absence of alkali concentration gradients in the glasses after the experiments shows that interdiffusion of
alkali and H+ or H3O+ gives no contribution to the transport of water in aluminosilicate melts. The H/D interdiffusion coefficients obtained at
800°C and 5 kbar using glass pieces with almost the same molar content of either water or deuterium oxide are almost identical
to the chemical diffusivities of water. This indicates that protons are transported by the neutral component H2O under these conditions.
Received: 26 March 1996 / Accepted: 23 August 1996 相似文献
18.
E. Schmädicke M. Okrusch W. Schubert B. Elwart U. Görke 《Mineralogy and Petrology》2001,72(1-3):77-111
Summary In the Odenwald Crystalline Complex, calc-silicate rocks are concentrated at the margins of the marble layer of Auerbach.
They were presumably formed by metasomatic exchange between the calcite marble and the neighbouring granodioritic and quartz-dioritic
intrusives. The investigated samples contain the characteristic mineral assemblages: garnet + clinopyroxene + epidote/clinozoisite + calcite + quartz ± titanite
(1) and wollastonite + clinopyroxene + garnet + calcite ± quartz ± epidote/clinozoisite ± titanite (2). Microprobe analyses
revealed the following compositional ranges: garnet grs40–98adr2–55alm<5.5sps<5.5pyp<1; clinopyroxene di46–88hed9–47joh0–5cats0–6; epidote/clinozoisite ps20–80. Different phase diagrams were calculated in the system CaO-MgO-Al2O3-TiO2-SiO2-CO2-H2O (CMATSCH) to decipher the P-T-XCO2 parameters of metamorphism: isobaric T-XCO2 sections and a P-T projection with mixed volatiles. The phase diagrams illustrate that the observed assemblages can only
form in the presence of an H2O-rich volatile phase. The assemblages are stable over a large range of temperatures, from 580 °C to < 400 °C (at 4 kbar)
and at XCO2 values of less than 0.055 (at 4 kbar). Higher temperatures can be inferred from reaction textures which indicate that garnet + plagioclase
(T > 580 °C, at 4 kbar) and wollastonite + plagioclase (T > 660 °C, at 4 kbar) coexisted during an early metamorphic stage. A minimum pressure of 3.5 kbar can be inferred for the
early high-temperature stage. Furthermore, on the basis of the calculated phase diagrams, combined with modal abundances in
thin sections, it is possible to evaluate fluid behaviour; in the investigated specimens, infiltration of fluids from an external
reservoir occurred. A minimum fluid:rock ratio of 3.6:1 can be estimated.
Received July 29, 1999; accepted March 28, 2000 相似文献
Zusammenfassung Phasenbeziehungen in Kalksilikat-Paragenesen des Marmorzuges von Auerbach, Odenwald-Kristallin-Komplex, Deutschland Im kristallinen Odenwald konzentrieren sich kalksilikatische Gesteine in den Randbereichen des Marmorzuges von Auerbach. Die kalksilikatischen Partien wurden vermutlich duch metasomatischen Austausch zwischen dem Calcit-Marmor und benachbarten Granodioriten und Quarzdioriten gebildet. Die untersuchten Proben enthalten die charakteristischen Mineralparagenesen: Granat + Klinopyroxen + Epidot/Klinozoisit + Calcit + Quarz + Titanit (1) und Wollastonit + Klinopyroxen + Granat + Calcit ± Quarz ± Epidot/Klinozoisit ± Titanit (2). Mikrosondenanalysen ergaben folgendes Zusammensetzungsspektrum: Granat grs40–98adr2–55alm<5.5sps<5.5pyp<1; Klinopyroxen di46–88hed9–47joh0–5cats0–6s; Epidot/Klinozoisit ps20–80. Verschiedene Phasendiagramme wurden für das Modellsystem CaO-MgO-Al2O3-TiO2-SiO2-CO2-H2O (CMATSCH) berechnet, um die P-T-XCO2-Parameter der Metamorphose einzugrenzen: Isobare T-XCO2-Schnitte und eine P-T-Projektion mit einer Fluid-Mischphase. Die Phasendiagramme verdeutlichen, da? die beobachteten Paragenesen nur in Anwesenheit eines H2O-reichen Fluids gebildet werden k?nnen. Die Paragenesen sind über einen gro?en Temperaturbereich hinweg stabil, von 580 °C bis < 400 °C (bei 4 kbar) und bei XCO2-Gehalten von < 0.055 (bei 4 kbar). Ursprünglich h?here Temperaturen k?nnen anhand von Reaktions-Texturen rekonstruiert werden, die zeigen, da? Granat + Plagioklas (T > 580 °C, bei 4 kbar) und Wollastonit + Plagioklas (T > 660 °C, bei 4 kbar) w?hrend eines früheren Metamorphosestadiums koexistierten. Ein Minimaldruck von 3.5 kbar kann für dieses frühe Hochtemperatur-Stadium abgeleitet werden. Mit Hilfe der berechneten Phasendiagramme, in Kombination mit beobachteten Modalgehalten, ist es m?glich, das Verhalten der fluiden Phase abzusch?tzen. Die untersuchten Gesteine implizieren Fluidinfiltration, wobei ein minimales Verh?ltnis Fluid:Gestein von 3.6:1 abgesch?tzt werden kann.
Received July 29, 1999; accepted March 28, 2000 相似文献
19.
Eclogitic metasediments from the Tso Morari area (Ladakh, Himalaya): evidence for continental subduction during India-Asia convergence 总被引:9,自引:0,他引:9
S. Guillot J. de Sigoyer J. M. Lardeaux G. Mascle 《Contributions to Mineralogy and Petrology》1997,128(2-3):197-212
Metasediments in the Tso Morari area (Ladakh, Himalaya) provide new insights into the Higher Himalayan metamorphism in the
northwestern part of the Himalayan belt. Whole-rock analyses and petrologic observations show that the metasediments correspond
to Fe-rich metapelites, Mg-rich metapelites, intermediate metapelites and metagreywackes of the Indian continental margin.
Jadeite + chloritoid + paragonite + garnet in the Fe-rich metapelites indicate pressures of 20 ±2 kbar at temperatures of
550 ±50 °C according to major element partitioning thermobarometry, stability fields of minerals and Thermocalc P-T estimates. These results are consistent with P-T estimates on other metasediments and with the occurrence of eclogites. Subsequent retrogression at the eclogite-blueschist
facies transition (from 18 to 13 kbar and 540 ±50 °C) was followed by an increase in temperature to 630 ±30 °C at amphibolite
facies conditions. The metamorphic evolution is related to subduction of the Indian continental margin beneath the southern
Asian margin at the onset of the Indian-Eurasian collision.
Received: 17 April 1996 / Accepted: 19 February 1997 相似文献
20.
In order to elucidate high-pressure transformations of high-P clinopyroxene (C2/c) at kinetically low temperature where atoms are not thermally activated, the transformation processes of FeGeO3 clinopyroxene (C2/c) have been investigated at pressures up to 20 GPa and 365 °C by powder X-ray diffraction using a synchrotron radiation source
and TEM observation. With increasing pressure up to 20 GPa at room temperature, FeGeO3 high-P clinopyroxene (C2/c) reversibly transforms into a new high-pressure phase, FeGeO3(II). On increasing the temperature up to 365 °C, this phase rapidly transforms into FeGeO3 ilmenite within about 2 h. Intensity analysis of the X-ray diffraction pattern reveals that the high-pressure phase of FeGeO3(II) has an intermediate structure between clinopyroxene and ilmenite: the cation arrangement is similar to that of clinopyroxene
and the oxygen arrangement is similar to that of ilmenite. The comparison of the crystal structures of these polymorphs suggests
that clinopyroxene to FeGeO3(II) and FeGeO3(II) to ilmenite transformations are performed by the slight deformation of the oxygen packing and the short-range movement
of cations, respectively. It is shown that this high-P clinopyroxene transforms into ilmenite through a low-activation energy
path under the low-temperature condition.
Received: 30 August 2000 / Accepted: 10 February 2001 相似文献