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1.
Summary ?A single-crystal X-ray investigation was performed on crystals of P21/c natural pigeonite with varying Ca and Fe* ( = Fe2+ + Mn2+) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal
chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal
free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains
and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae.
The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than
that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated.
The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples.
In particular, it is shown that Ca and Fe* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and
decrease the difference between the M2–O3 distances.
Received October 18, 2001; revised version accepted February 15, 2002 相似文献
2.
L. Melluso S. F. Sethna M. D’Antonio P. Javeri L. Bennio 《Mineralogy and Petrology》2002,74(2-4):323-342
Summary Major element, trace element, Sr- and Nd-isotopes and mineral chemical data are reported for alkaline rocks (lamprophyres,
tephrites, melanephelinites, nephelinites and nepheline syenites) cross-cutting the Deccan Trap lava flows south (Murud-Janjira
area) and north of Mumbai (Bassein). These rocks range from sodic to potassic and have a large span in MgO (12–2 wt%). The
lamprophyres have high content of incompatible elements (e.g., TiO2 > 3.8 wt%, Nb > 130 ppm, Zr > 380 ppm, Ba > 1200 ppm), and relatively high initial (at 65 Ma) 143Nd/144Nd (0.5128) and low 87Sr/86Sr (0.7038–0.7042). They are likely to be small-degree melts (2–3%) of volatile- and incompatible element-enriched mantle
sources, similar to other alkaline rocks in the northern Deccan, though slightly more potassium-rich. The nepheline-rich rocks
have highly porphyritic textures (up to 57% phenocrysts of diopside ± olivine), and anomalously low contents of incompatible
elements (e.g., TiO2 < 1.3 wt%, Nb < 24 ppm, Zr < 100 ppm) indicating that they could not represent liquid compositions. Moreover, their very
low initial 143Nd/144Nd ratios (0.5116–0.5120), at 87Sr/86Sr = 0.7045–0.7049, are unusual in the rocks related to the Deccan Traps and identify a new end-member in this province, that
could be identified as “Lewisian-type” lower crust and/or enriched mantle. The melting episode that generated these alkaline
rocks likely occurred close to the base of the ca. 100 km-thick Indian lithosphere, very shortly after the main eruption of
the Deccan tholeiites.
Received January 14, 2000; revised version accepted September 28, 2001 相似文献
3.
A precise U–Pb age on cassiterite from the Xianghualing tin-polymetallic deposit (Hunan,South China)
Shunda Yuan Jiantang Peng Ruizhong Hu Huimin Li Nengping Shen Dongliang Zhang 《Mineralium Deposita》2008,43(4):375-382
We report the first precise U–Pb isotope data on cassiterite from the large Xianghualing tin-polymetallic deposit in the
central Nanling district, South China. The results show that four separates from sample XF-51 have a relatively narrow range
of 206Pb/238U apparent ages, varying from 152 to 157 Ma, and the three 206Pb/238U apparent ages yield a weighted average value of 156 ± 4 Ma (MSWD = 0.32). Separates from two other cassiterite samples do
not have sufficient radiogenic Pb to generate a reliable 206Pb/238U age. Seven separates from the above three cassiterite samples define a well-constrained 238U–206Pb isochron corresponding to an age of 157 ± 6 Ma (MSWD = 34). A comparison of the U–Pb cassiterite ages with published Ar–Ar
dates on muscovite from this deposit and K–Ar age data on biotite from the pluton genetically related to the tin mineralization
in this area demonstrates that the U–Pb isotope system of cassiterite is a potential geochronometer. Combined with the Ar–Ar
dates of muscovite from this deposit, we can constrain the absolute age of tin-polymetallic mineralization in Xianghualing
at 154–157 Ma. The dates obtained in this study, consistent with the published geochronological results from other important
deposits in this region, reveal that the large-scale tungsten–tin mineralization in the central Nanling region was predominantly
emplaced during 150–161 Ma. 相似文献
4.
Factors controlling copper solubility and chalcopyrite deposition in the Sungun porphyry copper deposit, Iran 总被引:14,自引:0,他引:14
The Sungun porphyry copper deposit is hosted in a Diorite/granodioritic to quartz-monzonitic stock that intruded Eocene volcanosedimentary
and Cretaceous carbonate rocks. Copper mineralization is associated mainly with potassic alteration and to a lesser extent
with sericitic alteration. Based on previously published fluid inclusion and isotopic data by Hezarkhani and Williams-Jones
most of the copper is interpreted to have deposited during the waning stages of orthomagmatic hydrothermal activity at temperatures
of 400 to 300 °C. These data also indicate that the hydrothermal system involved meteoric waters, and boiled extensively.
In this work, thermodynamic data are used to delineate the stability fields of alteration and ore assemblages as a function
of fS2, fO2 and pH. The solubility of chalcopyrite was evaluated in this range of conditions using recently published experimental data.
During early potassic alteration (>450 °C), Copper solubility is calculated to have been >50 000 ppm, whereas the copper content
of the initial fluid responsible for ore deposition is estimated, from fluid inclusion data, to have been 1200–3800 ppm. This
indicates that initially the fluid was highly undersaturated with respect to chalcopyrite, which agrees with the observation
that veins formed at T > 400 °C contain molybdenite but rarely chalcopyrite. Copper solubility drops rapidly with decreasing temperature, and at
400 °C is approximately 1000 ppm, within the range estimated from fluid inclusion data, whereas at 350 °C it is only 25 ppm.
These calculations are consistent with observations that the bulk of the chalcopyrite deposited at Sungun is hosted by veins
formed at temperatures of 360 ± 60 °C. Other factors that, in principle, may reduce chalcopyrite solubility are increases
in pH, and decreases in fO2 and aCl−. Our analysis shows, however, that most of the change in pH occurred at high temperature when chalcopyrite was grossly undersaturated
in the fluid, and that the direction of change in fO2 increased chalcopyrite solubility. We propose that the Sungun deposit formed mainly in response to the sharp temperature
decrease that accompanied boiling, and partly as a result of the additional heat loss and decrease in aCl−, which occurred as a result of mixing of acidic Cu-bearing magmatic waters with cooler meteoric waters of lower salinity.
Received: 8 July 1998 / Accepted: 8 April 1999 相似文献
5.
Summary The kinetics of phytoplankton frustule dissolution has generally been studied as the appearance of silicic acid in a batch
reactor. Unfortunately, this approach, though often illuminating, has not so far been successful because of the difficulty
of parameterising the full reaction curve. This current study shows how the initial rate approach to chemical kinetics offers
a way around this bottleneck, thereby allowing much chemical kinetics information about frustule dissolution to be collected.
The technique is shown to be flexible and suited to short reaction times which facilitate detailed quantitative kinetics investigation,
indeed, as would be expected in a solution phase, kinetics study. The technique is exemplified by a dissolution study of uncleaned
frustules of Cyclotella crypticaat 40 °C and above. The frustules were found to yield the same dissolution rate after 5 weeks dark storage, at 4 °C. Meanwhile,
log dissolution rate was found to vary linearly with pH, with gradient 0.38 ± 0.01 (r
2=0.990). Linearity was upheld even at pHs as high as 14. Finally, a robust Arrhenius plot was established between 40 and 90 °C
yielding an activation energy for dissolution of 84 ± 3 kJ mol
−1. Follow through with the Eyring equation yielded an activation enthalpy, ΔH
‡, and an activation entropy, ΔS
‡, of 81 and 85 J mol
−1 K
−1, respectively. The discussion brings salient aspects of existing knowledge about diatom frustule dissolution kinetics into
the wider context of silicate mineral dissolution. 相似文献
6.
J. P. Richards C. J. Bray D. M. DeR. Channer E. T. C. Spooner 《Mineralium Deposita》1997,32(2):119-132
The Porgera gold deposit in Papua New Guinea is a world-class example of an alkalic-type epithermal gold system (stage II),
which overprints a precursor stage of magmatic-hydrothermal gold mineralization (stage I). Gas and ion chromatographic analyses
of fluid inclusions contained in vein minerals from both mineralization stages have been carried out in order to constrain
the compositions of the fluids involved in, and the processes attending, ore deposition. These data indicate the presence
of three end-member liquids, the most dilute of which was present throughout the mineralization history and is interpreted
to represent evolved groundwater of meteoric origin. Its composition is estimated to have been approximately 500 mM Na+, 10 mM K+, 5 mM Li+, 250 mM Cl−, 0.15 mM Br−, and 0.01 mM I−, plus significant concentrations of dissolved gases. More saline liquids were also present during the two main stages of
ore formation, and although their compositions differ, both are interpreted to have been derived at least in part from magmatic
fluids, and to have been the media by which gold was introduced into the system. Stage I minerals contain fluid inclusions
which decrease in salinity towards this dilute end-member composition through the vein paragenesis, reflecting progressive
dilution at depth of the magmatic fluid source by groundwaters. Ore deposition is thought to have been caused largely by simple
cooling and/or wallrock reactions, although limited in situ fluid mixing may also have occurred. The most saline fluids, present
in early quartz and pyrite, contain at least 810 mM Na+, 530 mM Ca2+, 130 mM K+, 12 mM Li+, 87 mM SO4
2−, 960 mM Cl−, 1.1 mM Br−, and 0.05 mM I−, plus significant but variable concentrations of dissolved gases. Fluid inclusions from stage II hydraulic breccia veins
reveal the presence of two distinct liquids with contrasting salinities, which were present at different times during vein
formation. A higher salinity liquid appears to have predominated during mineralization, whereas lower salinity groundwaters
filled the structures during intervening periods. The ore-forming fluid may have been forcibly injected into the veins from
depth during fracturing and depressurization events, displacing the resident groundwaters in the process. The original composition
of this fluid is estimated to have been at least 1770 mM Na+, 59 mM K+, 180 mM Li+, 210 mM SO4
2−, 680 mM Cl−, 1.4 mM Br−, and 0.09 mM I−, plus 1.5 mol% CO2, 0.19 mol% CH4, and 0.04 mol% N2. Gas chromatographic analyses of fluid inclusions from stage II samples show a decrease in total gas content between early
unmineralized veins and post-mineralization vuggy quartz (suitable samples could not be obtained from the ore stage itself).
Post-mineralization samples plot along an experimental gas-saturation curve in the CO2-CH4-H2O-NaCl system, obtained at conditions similar to those attending stage II ore deposition at Porgera (200–300 bar, ˜165 °C).
These results are interpreted to indicate a period of depressurization-induced phase separation during hydraulic fracturing,
which resulted in rich ore deposition. Volatile gases such as CH4 and N2, in addition to CO2 in solution, are shown to have a significant negative effect on total gas solubility. This effect may be of critical importance
in lowering the temperature and increasing the depth (pressure) at which phase separation can occur in epithermal systems.
Received: 28 November 1995 / Accepted: 17 July 1996 相似文献
7.
Iron tracer diffusion experiments in diopside have been performed using natural and synthetic single crystals of diopside,
and stable iron tracers enriched in 54Fe, at temperatures in the range 950–1100 °C, total pressure 1 atm, for times up to 29 days. Iron isotope diffusion profiles
were determined with an ion microprobe. For experiments performed at log pO2 = −13, in directions parallel to the c axis and the b axis of two natural, low iron (Fe ∼ 1.8 at %) diopsides, the data obey
a single Arrhenius relationship of the form D = 6.22−5.9
+49.6×10−15 exp(−161.5 ± 35.0 kJ mol−1/RT) m2 s−1. A single datum for iron diffusion in iron-free, single-crystal diopside at 1050 °C, is approximately 1 order of magnitude
slower than in the natural crystals. The pO2 dependence of iron diffusion in natural crystals at 1050 °C (power exponent = 0.229 ± 0.036) indicates a vacancy mechanism;
this is consistent with the results of unpublished atomistic simulation studies. There is no evidence of anisotropy for iron
diffusion in diopside.
Received: 16 March 1999 / Accepted: 10 April 2000 相似文献
8.
We present Raman and infrared spectra of gypsum to 21 GPa at 300 K. Our measurements encompass the internal modes of the
(SO4)−4 group that lie between 400 and 1150 cm−1, hydroxyl-stretching vibrations between 3200 and 3600 cm−1, and a libration and bending vibrations of the molecular H2O group. All vibrations of the sulfate group have positive pressure shifts, while the hydroxyl-stretching and -bending vibrations
have a mixture of positive and negative pressure shifts: the effect of pressure on the hydrogen bonding of the water molecule
thus appears to be complex. Near 5 GPa, the two infrared-active bending vibrations of the water molecule coalesce, and the
morphology of the hydroxyl-stretching region of the spectrum shifts dramatically. This behavior is consistent with a pressure-induced
phase transition in gypsum in the vicinity of 5–6 GPa, which is observed to be reversible on decompression to zero pressure.
The spectral observations are consistent with the onset of increased disorder in the position of the water molecule in gypsum:
the sulfate vibrations are largely unaffected by this transition. The Raman-active symmetric stretch of the sulfate group
undergoes an apparent splitting near 4 GPa, which is interpreted to be produced by Fermi resonance with an overtone of the
symmetric bending vibration. The average mode Grüneisen parameter of the 20 vibrational modes we sample is less than 0.05,
in contrast to the bulk thermal Grüneisen parameter of 1.20. Accordingly, the vibrations of both water and sulfate units within
gypsum are highly insensitive to volumetric compaction. Therefore, in spite of the changes in the bonding of the water unit
near 5 GPa, metastably compressed gypsum maintains strongly bound molecular-like units to over 20 GPa at 300 K.
Received: 31 July 2000 / Accepted: 5 April 2001 相似文献
9.
Andrea Marzoli Paul R. Renne Enzo M. Piccirillo Castorina Francesca Giuliano Bellieni Adolpho J. Melfi Jean B. Nyobe Jean N'ni 《Contributions to Mineralogy and Petrology》1999,135(2-3):133-150
The intraplate Cameroon Volcanic Line (CVL) straddles the African-South Atlantic continent-ocean boundary and is composed
mainly of alkaline basic volcanic rocks. Voluminous silicic volcanics characterize the continental sector of the CVL. We present
here new geochemical, isotopic (Sr-Nd-O) and 40Ar/39Ar geochronological data on the main silicic volcanic centres of the Western (Mt. Oku, Sabga and Mt. Bambouto) and Eastern
(Ngaoundere plateau) Cameroon Highlands. The silicic volcanism of Mt. Oku, Sabga and Mt. Bambouto occurred between 25 and
15 Ma and is represented by voluminous quartz-normative trachytes and minor rhyolitic ignimbrites. At Mt. Bambouto central
volcano about 700 m of silicic volcanics erupted in less than 2.7 million years. These silicic volcanics are associated with
slightly to moderately alkaline basalts and minor basanites. In general, onset of the silicic volcanism migrated from NE (Oku:
25 Ma) to SW (Sabga: 23 Ma; Bambouto: 18 Ma; and Mt. Manengouba: 12 Ma). The silicic volcanism of the Ngaoundere plateau (eastern
branch of the CVL) is instead dominated by nepheline-normative trachytes which are associated with strongly alkaline basalts
and basanitic rocks. These Ne-trachytes are younger (11-9 Ma) than the Q-trachytes of the Western Highlands. The least differentiated
silicic volcanics are isotopically similar (87Sr/86Sr < 0.70380; 143Nd/144Nd > 0.51278) to the associated alkaline basalts suggesting differentiation processes without appreciable interaction with
crustal materials. Such interactions may, however, have played some role in the genesis of the most evolved silicic volcanics
which have 87Sr/86Sr as high as 0.705–0.714. Fractional crystallization is the preferred mechanism for genesis of the silicic melts of both
Western and Eastern Highlands, as shown by modeling major and trace element variations. The genesis of the least evolved Q-trachytes
from the Western Highlands, starting from slightly to moderately alkaline basalts, is compatible with fractionation of dominantly
plagioclase, clinopyroxene and magnetite. Crystal fractionation may have occurred at low pressure and at QFM buffer f
O2conditions. Parental magmas of the Ngaoundere Ne-trachytes are likely instead to have been strongly alkaline basalts which
evolved through crystal fractionation at higher P (6-2 kbar) and f
O2 (QFM + 2). The migration (25 to 12 Ma) of the silicic volcanism from NE to SW in the continental sector of the CVL is reminiscent
of that (31-5 Ma) of the onset of the basic volcanism in the oceanic sector (Principe to Pagalu islands) of the CVL. These
ages, and that (11-9 Ma) of the silicic volcanism of the Ngaoundere plateau, indicate that the Cameroon Volcanic Line as a
whole may not be easily interpreted as the surface expression of hot-spot magmatism.
Received: 24 February 1998 / Accepted: 22 September 1998 相似文献
10.
Unit-cell dimensions of a natural phlogopite from Pargas, Finland, have been determined in the temperature interval of 27–1050 °C
by X-ray powder diffraction technique. Expansion rates vary discontinuously with temperature with a break at 412 °C. Below
this temperature, the linear expansions (α) for a, b and c axis lengths are 3.74 × 10−5 K−1, 1.09 × 10−5 K−1, and 1.19 × 10−5 K−1, respectively, and above that they are 0.86 × 10−5 K−1, 0.80 × 10−5 K−1, and 1.93 × 10−5 K−1. The volume thermal expansion coefficients are 6.26 × 10−5 K−1 and 3.71 × 10−5 K−1 for low-temperature and high-temperature intervals, respectively. The observed kink in the rate of thermal expansions with
temperature could be due to the different mode of structural changes. Thermogravimetric analysis of the sample indicates the
oxidation of iron in the temperature range of 500–600 °C and dehydroxylation as well as decomposition of phlogopite in the
temperature range of 900–1200 °C.
Received: 8 September 1998 / Accepted: 28 February 2000 相似文献
11.
Oxygen isotope constraints on the petrogenesis of the Sybille intrusion of the Proterozoic Laramie Anorthosite Complex 总被引:2,自引:0,他引:2
The origin of monzonitic intrusions that are associated with Proterozoic massif-type anorthosite complexes is controversial.
A detailed oxygen isotope study of the Sybille intrusion, a monzonitic intrusion of the Laramie Anorthosite Complex (Wyoming),
indicates that either derivation from a basaltic magma of mantle origin with a metasedimentary component (∼20%) incorporated
early in its magmatic history, or a partial melt of lower crustal rocks is consistent with the data. The oxygen isotope compositions
of plagioclase, pyroxene and zircon from the Sybille monzosyenite, the dominant rock type in the Sybille intrusion, were analyzed
in order to establish the isotopic composition of the source of the magma. Plagioclase δ18O values range from 6.77 to 9.17‰. We interpret the higher plagioclase δ18O values (average 8.69 ± 0.30‰, n = 19) to be magmatic in origin, lower plagioclase δ18O values (average 7.51 ± 0.44‰, n = 22) to be the result of variable subsolidus alteration, and pyroxene δ18O values (average 6.34 ± 0.38‰, n = 19) to be the result of closed-system diffusional exchange during cooling. Low magnetic zircons, which have been shown
to retain magmatic oxygen isotope values despite high grade metamorphism and extensive subsolidus hydrothermal alteration,
have δ18O values (7.40 ± 0.24‰, n = 11) which are consistent with our interpretation of the plagioclase and pyroxene results. Oxygen isotope data from all
three minerals indicate that the magmatic oxygen isotope composition of the Sybille intrusion is enriched in 18O relative to the composition of average or “normal” mantle-derived magmas. This enrichment is approximately twice the oxygen
isotope enrichment that could result from closed-system fractionation, rendering a closed-system, comag- matic petrogenetic
model between the Sybille intrusion and the mantle-derived anorthositic lithologies of the Laramie Anorthosite Complex improbable.
Received: 7 April 1998 / Accepted: 19 January 1999 相似文献
12.
Friedrich Lucassen Sven Lewerenz Gerhard Franz José Viramonte Klaus Mezger 《Contributions to Mineralogy and Petrology》1999,134(4):325-341
Crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the Cretaceous Salta Rift at
Quebrada de Las Conchas, Provincia Salta, Argentina were investigated to get information about the age and the chemical composition
of the lower crust. Most of the xenoliths have a granitoid composition with quartz-plagioclase-garnet-rutile ± K-feldspar
as major minerals. The exceedingly rare mafic xenoliths consist of plagioclase-clinopyroxene-garnet ± hornblende. All xenoliths
show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned. Thermobarometric calculations indicate
equilibration of the mafic xenoliths in the granulite facies at temperatures of ca. 900 °C and pressures of ca. 10 kbar. The
Sm-Nd mineral isochron ages are 95.1 ± 10.4 Ma, 91.5 ± 13.0 Ma, 89.0 ± 4.2 Ma (granitoid xenoliths), and 110.7 ± 23.6 Ma (mafic
xenolith). These ages are in agreement with the age of basanitic volcanism (ca. 130–100 and 80–75 Ma) and are interpreted
as minimum ages of metamorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature
of the Sm-Nd system (>600–700 °C). The Sm-Nd and Rb-Sr isotopic signatures (147Sm/144Nd = 0.1225–0.1608; 143Nd/144Ndt
0 = 0.512000–0.512324; 87Rb/86Sr = 0.099–0.172; 87Sr/86Srt
0 = 0.708188–0.7143161) and common lead isotopic signatures (206Pb/204Pb = 18.43–18.48; 207Pb/204Pb = 15.62–15.70; 208Pb/204Pb = 38.22 –38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic
basement from NW Argentina, apart from the lower 208Pb/204Pb ratio of the basement. The Sm-Nd depleted mantle model ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic
basement point to a similar Proterozoic protolith. Time constraints, the well equilibrated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca.
90 Ma and to a prominent thermal anomaly in the lithosphere. The composition of the xenoliths is similar to the composition
of the Early Paleozoic basement in the Andes of NW Argentina and northern Chile. A thick mafic lower crust seems unlikely
considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths.
Received: 21 July 1998 / Accepted: 27 October 1998 相似文献
13.
Dependence on pressure of conduction by hopping of small polarons in minerals of the Earth's lower mantle 总被引:1,自引:0,他引:1
Electrical conductivity of the lower mantle-like assemblage (Mg,Fe)SiO3 perovskite-(Mg,Fe)O magnesiowüstite is usually analyzed using the quasi-chemical Arrhenian approach of diffusion. The conductivity
of this assemblage has often been attributed to hopping of small polarons, because of the low value of the activation energy
and the small negative activation volume. However, the solid-state physics approach can provide more arguments, for or against
conduction by polarons. We have tried to bridge the gap between the two approaches and identify the physical quantities entering
the phenomenological activation parameters. In particular, we have investigated the pressure dependence of the activation
energy, and the physical meaning of the activation volume. Hopping is controlled by the binding energy of the polaron and
by the value of the exchange integral, which increases with pressure causing the observed decrease of the activation energy.
From the physical theory and the results of experiments at pressures up to 40 GPa and temperatures up to 400 ∘C, we have estimated the values of parameters characteristic of polarons: radius, mobility, time between jumps and adiabaticity.
These values are compatible with conduction by small adiabatic polarons. The consequences for extrapolations to lower mantle
conditions of the presence of a temperature dependent preexponential term in the expression for conductivity have been examined.
It was found that the extrapolations are not significantly different from those using the Arrhenius equation.
Received: 5 November 1998 / Revised, accepted: 4 May 1999 相似文献
14.
S. A. T. Redfern G. Artioli R. Rinaldi C. M. B. Henderson K. S. Knight B. J. Wood 《Physics and Chemistry of Minerals》2000,27(9):630-637
The partitioning of Fe and Mg between the M1 and M2 octahedral sites of olivine has been investigated by in situ time-of-flight
neutron powder diffraction. The degree of M-cation order was determined from direct measurements of site occupancies in a
synthetic sample of Fo50Fa50 heated to 1250 °C at the Fe-FeO oxygen buffer. Fe shows slight preference for M1 at temperatures
below about 600 °C, progressively disordering on heating to this temperature. Above 630 °C, the temperature at which site
preferences cross over (T
cr), Fe preferentially occupies M2, becoming progressively more ordered into M2 on increasing temperature. The cation-ordering
behaviour is discussed in relation to the temperature dependence of the M1 and M2 site geometries, and it is suggested that
vibrational entropy, crystal field effects and changes in bond characteristics play a part in the cross-over of partitioning
behaviour. The temperature dependence of site ordering is modelled using a Landau expansion of the free energy of ordering
of the type ΔG = −hQ + gTQ + (T − T
c)Q
2 + Q
4, with a/h = 0.00406 K−1, b/h = 2.3, T
c = 572 K and g/h = 0.00106 K−1. These results suggest that the high-temperature ordering behaviour across the forsterite-fayalite join will have a bearing
on the activity-composition relations of this important rock-forming mineral, and indicate that Fe-Mg olivine solid solutions
become less ideal as temperature increases.
Received: 12 August 1999 / Accepted: 25 April 2000 相似文献
15.
“Extreme boiling” model for variable salinity of the Hokko low-sulfidation epithermal Au prospect, southwestern Hokkaido, Japan 总被引:1,自引:0,他引:1
The Hokko prospect is located in the Minamikayabe area southwestern Hokkaido, Japan, where gold-bearing quartz veins of Pliocene
age are exposed at the surface. The alteration mineral assemblage is typical of low-sulfidation epithermal systems, with the
quartz veins associated with adularia alteration overprinted on Late Miocene propylitic alteration. Fluid inclusion studies
of the vein quartz reveal mean homogenization temperatures of approximately 220 °C, and the co-existence of low-salinity (<2
wt.% NaCl equivalent) and moderate salinity (2 to 12 wt.% NaCl equivalent) fluid inclusions within the same veins. The moderate
salinity fluid inclusions (2–12 wt.% NaCl equivalent) typically have relatively low homogenization temperatures between 150°
to 200 °C. The results obtained from stable isotope analysis of δ18O in quartz vein material showed a gradual decrease in δ18O signatures with increasing depth. The majority of the samples have calculated fluid source signatures (δ18OH2O) between −8.0 and −10.0‰, but there is a significant change in the composition above 185 m drill depth. The shallower samples
in particular show a wide range of oxygen isotope signatures that are associated with the moderate salinity fluid inclusions.
We interpret that low-salinity inclusions within the Hokko system represent the composition of the liquid phase of the fluid,
before boiling, and that the moderate-salinity inclusions are representative of the residual liquid phase, after extensive
non-adiabatic boiling and vapor loss in an open system. This mechanism resulted in the entrapment of fluids with variable
salinities at the same time, and in close proximity to each other. This is also reflected in the δ18OH2O values which become more variable and heavier where the moderate-salinity inclusions occur. Deposition of ore minerals within
the Hokko vein system also occurred at this time as a result of boiling and gas loss.
Received: 30 May 1997 / Accepted: 6 January 1998 相似文献
16.
Metamorphic fluid origins in the Osborne Fe oxide–Cu–Au deposit,Australia: evidence from noble gases and halogens 总被引:1,自引:1,他引:0
The Osborne iron oxide–copper–gold (IOCG) deposit is hosted by amphibolite facies metasedimentary rocks and associated with
pegmatite sheets formed by anatexis during peak metamorphism. Eleven samples of ore-related hydrothermal quartz and two pegmatitic
quartz–feldspar samples contain similarly complex fluid inclusion assemblages that include variably saline (<12–65 wt% salts)
aqueous and liquid carbon dioxide varieties that are typical of IOCG mineralisation. The diverse fluid inclusion types present
in each of these different samples have been investigated by neutron-activated noble gas analysis using a combination of semi-selective
thermal and mechanical decrepitation techniques. Ore-related quartz contains aqueous and carbonic fluid inclusions that have
similar 40Ar/36Ar values of between 300 and 2,200. The highest-salinity fluid inclusions (47–65 wt% salts) have calculated 36Ar concentrations of approximately 1–5 ppb, which are more variable than air-saturated water (ASW = 1.3–2.7 ppb). These fluid
inclusions have extremely variable Br/Cl values of between 3.8 × 10−3 and 0.3 × 10−3, and I/Cl values of between 27 × 10−6 and 2.4 × 10−6 (all ratios are molar). Fluid inclusions in the two pegmatite samples have similar 40Ar/36Ar values of ≤1,700 and an overlapping range of Br/Cl and I/Cl values. High-salinity fluid inclusions in the pegmatite samples
have 2.5–21 ppb 36Ar, that overlap the range determined for ore-related samples in only one case. The fluid inclusions in both sample groups
have 84Kr/36Ar and 129Xe/36Ar ratios that are mainly in the range of air and air-saturated water and are similar to mid-crustal rocks and fluids from
other settings. The uniformly low 40Ar/36Ar values (<2,200) and extremely variable Br/Cl and I/Cl values do not favour a singular or dominant fluid origin from basement-
or mantle-derived magmatic fluids related to A-type magmatism. Instead, the data are compatible with the involvement of metamorphic
fluids that have interacted with anatectic melts to variable extents. The ‘metamorphic’ fluids probably represent a mixture
of (1) inherited sedimentary pore fluids and (2) locally derived metamorphic volatilisation products. The lowest Br/Cl and
I/Cl values and the ultra-high salinities are most easily explained by the dissolution of evaporites. The data demonstrate
that externally derived magmatic fluids are not a ubiquitous component of IOCG ore-forming systems, but are compatible with
models in which IOCG mineralisation is localised at sites of mixing between fluids of different origin.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorised users. 相似文献
17.
A. K. Kleppe A. P. Jephcoat H. Olijnyk A. E. Slesinger S. C. Kohn B. J. Wood 《Physics and Chemistry of Minerals》2001,28(4):232-241
Raman spectra of hydrous β-Mg2SiO4 (1.65 wt% H2O) have been measured in a diamond-anvil cell with helium as a pressure-transmitting medium at room temperature to 50 GPa.
We observe three OH-stretching modes, a doublet with components at 3329 and 3373 cm−1, which decrease linearly with pressure, and a single mode at 3586 cm−1, which remains nearly constant up to 24 GPa before decreasing at higher pressures. Assessment of the mode frequencies and
their pressure dependence, together with previous results from X-ray and IR data, are consistent with protonation of the O1
site in agreement with previous studies. Strict assignment of Raman activity awaits detailed structural models. The nature
of the protonation in wadsleyite may require more specific experimental probes for full solution of the hydrogen-site problem.
Received: 18 July 2000 / Accepted: 22 November 2000 相似文献
18.
Chris Harris H. Stuart Smith Anton P. le Roex 《Contributions to Mineralogy and Petrology》2000,138(2):164-175
We have measured the δ18O values of the major phenocrysts (olivine, clinopyroxene and plagioclase) present in lavas from Tristan da Cunha and Gough
Island. These islands, which result from the same mantle plume, have enriched radiogenic isotope ratios and are, therefore,
prime candidates for an oxygen isotope signature that is distinct from that of MORB. Consistent differences between the δ18O values of olivine, pyroxene and feldspar in the Gough lavas show that the phenocrysts in the mafic Gough Island lavas are
in oxygen isotope equilibrium. The olivines in lavas with SiO2 <50 wt% have a mean δ18O value of 5.19‰, consistent with crystallization from a magma having the same oxygen isotope composition as MORB. Phenocrysts
in all the Gough lavas show a systematic increase in δ18O value as silica content increases, which is consistent with closed-system fractional crystallization. The lack of enrichment
in δ18O of the Gough magmas suggests that the mantle source contained <2% recycled sediment. In contrast, the Tristan lavas with
SiO2 >48 wt% contain phenocrysts which have δ18O values that are systematically ∼0.3‰ lower than their counterparts from Gough. We suggest that the parental mafic Tristan
magmas were contaminated by material from the volcanic edifice that acquired low δ18O values by interaction with water at high temperatures. The highly porphyritic SiO2-poor lavas show a negative correlation between olivine δ18O value and whole-rock silica content rather than the expected positive correlation. The minimum δ18O value occurs at an SiO2 content of about 45 wt%. Below 45 wt% SiO2, magmas evolved via a combination of assimilation, fractionational crystallization and crystal accumulation; above 45 wt%
SiO2, magmas appeared to have evolved via closed-system fractional crystallization.
Received: 23 November 1998 / Accepted: 27 September 1999 相似文献
19.
Hannes K. Brueckner Jane A. Gilotti Allen P. Nutman 《Contributions to Mineralogy and Petrology》1998,130(2):103-120
High-pressure metamorphic assemblages occur in mafic, ultramafic and a few intermediate rocks in a gneiss complex that covers
an area of approximately 400 × 100 km in the North-East Greenland Caledonides. Detailed petrologic and geochronologic studies
were carried out on three samples in order to clarify the P-T-t evolution of this eclogite province. Geothermobarometry yields temperature estimates of 700–800 °C and pressure estimates
of at least 1.5 GPa from an eclogite sensu stricto and as high as 2.35 GPa for a garnet websterite. The eclogite defines a garnet-clinopyroxene-amphibole-whole rock Sm-Nd isochron
age of 405 ± 24 Ma (MSWD 0.9). Isofacial garnet websterites define garnet-clinopyroxene-orthopyroxene-amphibole-whole rock-(biotite)
ages of 439 ± 8 Ma (MSWD =2.1) for a coarse-grained sample and 370 ± 12 Ma (MSWD=0.6) for a finer-grained variety. Overgrowths
on zircons from the fine-grained pyroxenite and the eclogite give a pooled 206Pb/238U SHRIMP age of 377 ± 7 Ma (n=4). Significantly younger Rb-Sr biotite ages of 357 ± 8, 330 ± 6 and 326 ± 6 agree with young Rb-Sr, K-Ar and 40Ar/39Ar mineral ages from the gneiss complex and indicate slow cooling of the eclogitic rocks. High-pressure metamorphism may have
been at least 439 Ma old (Siluro-Ordovician) with cooling through amphibolite-facies conditions in the Devonian and continued
crustal thinning and exhumation well into the Carboniferous. Sm-Nd whole rock model ages indicate the eclogite protoliths
are Early Proterozoic in age, while 207Pb/206Pb SHRIMP ages of 1889 ± 18 and 1981 ± 8 from anhedral zircon cores probably reflect Proterozoic metasomatism. The samples
have negative ɛNd values (−5 to −16) and elevated 87Sr/86Sr ratios (0.708–0.715), consistent with field evidence that the eclogite protoliths were an integral part of the continental
crust long before Caledonian metamorphism. The presence of a large Caledonian eclogite terrane in Greenland requires modification
of current tectonic models that postulate subduction of Baltica beneath Laurentia during the Caledonian orogeny.
Received: 9 October 1996 / Accepted: 7 July 1997 相似文献
20.
Taras Nahnybida Sarah A. Gleeson Brian G. Rusk Len I. Wassenaar 《Mineralium Deposita》2009,44(8):837-848
A bulk geochemical study has been carried out on fluid inclusion leachates extracted from quartz veins from porphyry Cu deposits
in Butte, Montana, USA and Bingham Canyon, Utah, USA. The leachates mostly represent low-salinity magmatic–hydrothermal fluid
inclusions. Their halogen ratios (Br/Cl) of fluid inclusion leachates were determined by ion chromatography, and δ37Cl values of the leachates were measured by continuous-flow isotope ratio mass spectrometry. Br/Cl ratios from early pre-Main
stage and later Main stage veins at Butte range from 0.60 to 1.88 × 10−3 M. Ratios are similar in pre-Main stage veins with sericite bearing selvages and Main stage samples ranging from 0.81 to
1.08 × 10−3 and from 0.92 to 1.88 × 10−3 M, respectively, clustering below seawater (1.54 × 10−3 M) and overlapping mantle values (~1–2 × 10−3 M). Two samples associated with early pre-Main stage potassic alteration yield distinctly lower Br/Cl ratios of 0.60 and
0.64 × 10−3 M. Butte δ37Cl values range from −0.8‰ to −2.3‰ with no significant difference between pre-Main stage and Main stage samples. Br/Cl ratios
for quartz veins from Bingham Canyon range from 0.18 to 3.68 × 10−3 M. Br/Cl ratios from Bingham range above and below previously reported for porphyry copper deposits. In contrast to Butte,
δ37Cl values for Bingham are lower, ranging from −0.9‰ to −4.1‰. In the absence of any processes which can significantly fractionate
chlorine isotopes at high temperatures, we suggest that the porphyry system at Bingham, and to a lesser extent at Butte, have
inherited negative chlorine isotopic signatures from the subducting slab generated at low temperatures. 相似文献