The intensity of amphibole OH bands in the infrared absorption spectrum     

Henrik Skogby  George R. Rossman 《Physics and Chemistry of Minerals》1991,18(1):64-68

Polarized IR spectra of single-crystals of amphiboles show that the molar absorptivity of the fundamental vibrational OH band is strongly wavenumber dependent. The intensity increases by a factor of 1.6 from 3674 cm^-1 (tremolite endmember; MgMgMg) to 3618 cm^-1 (grunerite endmember; FeFeFe). Spectra obtained from Ca and Fe-Mg amphiboles consist of sharp, well-resolved bands on a well-defined background. The high intensity of the OH bands in Ca and Fe-Mg amphiboles makes it sometimes necessary to thin the samples to under 2 μm thickness, whereas alkali amphiboles can be measured at 25–50 μm thickness.  相似文献   

Voluminous arc dacites as amphibole reaction-boundary liquids     

Dawnika?L.?BlatterEmail author  Thomas?W.?Sisson  W.?Ben?Hankins 《Contributions to Mineralogy and Petrology》2017,172(5):27

Dacites dominate the large-volume, explosive eruptions in magmatic arcs, and compositionally similar granodiorites and tonalites constitute the bulk of convergent margin batholiths. Shallow, pre-eruptive storage conditions are well known for many dacitic arc magmas through melt inclusions, Fe–Ti oxides, and experiments, but their potential origins deeper in the crust are not well determined. Accordingly, we report experimental results identifying the P–T–H₂O conditions under which hydrous dacitic liquid may segregate from hornblende (hbl)-gabbroic sources either during crystallization–differentiation or partial melting. Two compositions were investigated: (1) MSH–Yn?1 dacite (SiO₂: 65 wt%) from Mount St. Helens’ voluminous Yn tephra and (2) MSH–Yn?1?+?10% cpx to force saturation with cpx and map a portion of the cpx?+?melt?=?hbl peritectic reaction boundary. H₂O-undersaturated (3, 6, and 9 wt% H₂O) piston cylinder experiments were conducted at pressures, temperatures, and fO₂ appropriate for the middle to lower arc crust (400, 700, and 900 MPa, 825–1100?°C, and the Re–ReO₂ buffer?≈?Ni–NiO?+?2). Results for MSH–Yn?1 indicate near-liquidus equilibrium with a cpx-free hbl-gabbro residue (hbl, plg, magnetite, ± opx, and ilmeno-hematite) with 6–7 wt% dissolved H₂O, 925?°C, and 700–900 MPa. Opx disappears down-temperature consistent with the reaction opx?+?melt?=?hbl. Cpx-added phase relations are similar in that once ~10% cpx crystallizes, multiple saturation is attained with cpx, hbl, and plg, +/? opx, at 6–7 wt% dissolved H₂O, 940?°C, and 700–900 MPa. Plg–hbl–cpx saturated liquids diverge from plg–hbl–opx saturated liquids, consistent with the MSH–Yn?1 dacite marking a liquid composition along a peritectic distributary reaction boundary where hbl appears down-temperature as opx?+?cpx are consumed. The abundance of saturating phases along this distributary peritectic (liquid?+?hbl?+?opx?+?cpx?+?plg?+?oxides) reduces the variance, so liquids are restricted to dacite–granodiorite–tonalite compositions. Higher-K dacites than the Yn would also saturate with biotite, further limiting their compositional diversity. Theoretical evaluation of the energetics of peritectic melting of pargasitic amphiboles indicates that melting and crystallization of amphibole occur abruptly, proximal to amphibole’s high-temperature stability limit, which causes the system to dwell thermally under the conditions that produce dacitic compositions. This process may account for the compositional homogeneity of dacites, granodiorites, and tonalites in arc settings, but their relative mobility compared to rhyolitic/granitic liquids likely accounts for their greater abundance.  相似文献   

Some chemical controls on igneous amphibole compositions     

R.Grant Cawthorn 《Geochimica et cosmochimica acta》1976,40(11):1319-1328

The composition of igneous amphiboles is extremely sensitive to magma composition and the nature of the co-existing phases. The extent of solid solution in amphiboles may be depicted in the basalt tetrahedron. They are more silica-undersaturated than the liquid from which they have crystallized for liquids containing about 11% normative nepheline. For liquids containing more than 11% normative nepheline the reverse is true. The existence of this compositional divide may be demonstrated for both intrusive and extrusive suites. Amphiboles are generally nepheline-normative, except when crystallizing from granodioritic to granitic magmas.Fractionation factors between amphibole and magma composition for MgO and FeO appear to increase with decreasing temperature; those for TiO₂ and Na₂O decrease with increasing pressure. The CaO content of amphiboles appears to be insensitive to the CaO content of the magma.  相似文献   

Crystal clots,amphibole fractionation and the evolution of calc-alkaline magmas   总被引：1，自引：0，他引：1  

Michael O. Garcia  Sara S. Jacobson 《Contributions to Mineralogy and Petrology》1979,69(4):319-327

Petrographic and microprobe investigations of calc-alkaline (CA) rocks from the High Cascade Range (i.e., Mt. St. Helens, Mt. Jefferson, Crater Lake and Mt. Shasta) of western North America show that crystal clots represent primary igneous phase assemblages and are not products of amphibole reactions with melt. For each eruptive complex, crystal clots display diverse modal proportions even within a single eruptive unit. Nevertheless, in all cases the crystal-clot minerals are also represented in the rock as phenocrysts or microphenocrysts. Basalts contain clots of ol+plag+mgt, ol+mgt, cpx+ plag+mgt, cpx+mgt and plag+mgt; andesites, clots of cpx+mgt, opx+mgt, cpx+opx+plag+mgt, cpx+plag+mgt, opx+plag+mgt and plag±mgt; and dacites, clots of opx+mgt, cpx+opx+plag+ mgt, opx+plag+mgt, amph+plag+mgt±ilm, amph+mgt±ilm and plag±mgt. The bulk compositions of most of these clot assemblages could not have been derived from amphibole percursors. Although some amphiboles in dacitic rocks display a breakdown reaction of amph=plag+cpx+opx +mag, these mineral clusters, unlike those of clots, typically have a relict amphibole crystal outline and a fine-grained metamorphic texture. Plagioclase grains in the mineral clusters lack oscillatory zoning which is typical of crystal clot plagioclase grains. The euhedral to subhedral shapes of most clot minerals and the oscillatory zoning present in most clot plagioclase grains are not likely to have formed from the breakdown of amphibole. Crystal clots are also observed in Hawaiian and ocean floor basalts, although amphibole fractionation has not been proposed for those lavas. Magnetite fractionation may be the controlling process limiting iron enrichment in CA magmas rather than amphibole fractionation. Textural evidence indicates that magnetite is an early-forming phase in CA magmas. V, which is concentrated in magnetite, shows a strong decrease with increasing silica in many CA rocks, supporting a magnetite fractionation model.Hawaii Institute of Geophysics Contrib. No. 969  相似文献   

Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer   总被引：13，自引：4，他引：9  

Jonathan D. Blundy  Timothy J. B. Holland 《Contributions to Mineralogy and Petrology》1990,104(2):208-224

There is currently a dearth of reliable thermobarometers for many hornblende and plagioclase-bearing rocks such as granitoids and amphibolites. A semi-empirical thermodynamic evaluation of the available experimental data on amphibole+plagioclase assemblages leads to a new thermometer based on the Al^iv content of amphibole coexisting with plagioclase in silica saturated rocks. The principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is \(\left( {Na\square _{ - 1} } \right)^A \left( {AlSi_{ - 1} } \right)^{T1}\) . We have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1. $$1. Edenite + 4 Quartz = Tremolite + Albite$$ 2. $$2. Pargasite + 4 Quartz = Hornblende + Albite.$$ The equilibrium relation for both (1) and (2) leads to the proposed new thermometer $$T = \frac{{0.677P - 48.98 + Y}}{{ - 0.0429 - 0.008314 ln K}} and K = \left( {\frac{{Si - 4}}{{8 - Si}}} \right)X_{Ab}^{Plag} ,$$ where Si is the number of atoms per formula unit in amphiboles, with P in kbar and T in K; the term Y represents plagioclase non-ideality, RTlnγ_ab, from Darken's Quadratic formalism (DQF) with Y=0 for X _ab>0.5 and Y=-8.06+25.5(1-X _ab)² for X _ab<0.5. The best fits to the data were obtained by assuming complete coupling between Al on the T1 site and Na in the A site of amphibole, and the standard deviation of residuals in the fit is ±38°C. The thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende-plagioclase assemblages with uncertainties of around ±75° C for rocks equilibrated at temperatures in the range 500°–1100° C. The thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than An₉₂ and with amphiboles containing less than 7.8 Si atoms pfu. Good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks. Our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry.  相似文献   

An electron microscopic study of amphibole lamellae in augite     

P. P. K. Smith 《Contributions to Mineralogy and Petrology》1977,59(3):317-322

Ion-thinned samples of augite from four plutonic igneous rocks have been examined in the electron microscope at 100 kV. Lamellae less than 0.08 m thick were observed parallel to (010) in all four samples. Electron diffraction shows that the lamellae consist of clino-amphibole, space group I2/m, with crystallographic axes parallel to those of the augite. Analytical electron microscopy of the lamellae in one specimen shows that they are a hornblende. The amphibole lamellae have nucleated at the interface between the augite host and exsolution lamellae of orthopyroxene parallel to (100) of the augite. The interface between the amphibole lamellae and the augite is coherent.It is thought that the amphibole lamellae have formed by exsolution from the augite, implying the existence of finite solid solution between members of the pyroxene and amphibole groups of minerals.  相似文献   

Continuous reactions with amphibole, garnet and plagioclase     

A. J. ZINGG 《Journal of Metamorphic Geology》1995,13(3):431-443

Abstract End-member, continuous and degenerate reactions are derived for the multisystem with the six components Na₂O, CaO, (Mg/Fe)O, Al₂O₃, SiO₂, H₂O among the phases plagioclase_ss, garnet_ss, amphibole_ss, cpx, opx, olivine, spinel, quartz and an aqueous fluid. The chemography of this system is degenerate due to the co-linearity 2Opx = Ol + Qtz. This co-linearity has its implications both on reaction space and phase equilibria. From a total of 28 reaction systems, reaction space is derived for nine subsystems (phases in parentheses are absent): Case A₁: (Cpx,Ol) (Cpx,Opx) and (Cpx,Qtz), Case A₂: (Spl,Ol) (Spl,Opx) and (Spl,Qtz), Case B: (Ol,Opx) (Ol,Qtz) and (Opx,Qtz). In the absence of either cpx or spl (case A), three reactions form an invariant point, either [Cpx] or [Spl], where the co-linear phases olivine, opx and quartz coexist on the transformation line 2Opx = Ol + Qtz. Changing mineral compositions force invariant points to move along the line with the different reaction curves changing their relative position according to Schreinemakers’rules. Zero contours, i.e. the location where (a) phase(s) disappear(s) in reaction space correspond to singular points in phase diagrams. Two types are distinguished; singular points of indispensable and of substitutable phases. In the first case the phase disappears from the entire bundle while in the second it disappears from a single reaction. In the specific case where the substitutable phases are also the co-linear ones, two of the three co-linear phases disappear simultaneously. Two of the three reaction curves coincide. In the system including Cpx and Spl (Case B) three reactions, (Ol,Opx) (Ol,Qtz) and (Opx,Qtz), oppose three invariant points, [Ol], [Opx] and [Qtz]. Invariant points no longer move along the line 2Opx = Ol + Qtz. The coincidence of the zero contours of all three co-linear phases in reaction space-the result of the chemographic degeneracy-causes the respective singular points to coincide in the phase diagrams. This is the location where curves must be rearranged in a bundle to conform Schreinemakers’rules. The reaction Grs₁Prp₂= 2 Ol + An is fourth order degenerate and part of all nine subsystems (cases A and B). It can be used to relate the different phase diagrams to one another.  相似文献   

NaSi⇌CaAl exchange equilibrium between plagioclase and amphibole     

Frank S. Spear 《Contributions to Mineralogy and Petrology》1980,72(1):33-41

The exchange equilibrium between plagioclase and amphibole, 2 albite+tschermakite=2 anorthite+glaucophane, has been calibrated empirically using data from natural amphibolites. The partition coefficient, K _D, for the exchange reaction is (X _an/X _ab)_plag ·(Na, M4/Ca, M4)_amph.. Partitioning is systematic between plagioclase and amphibole in suites collected from single exposures, but the solid solutions are highly non-ideal: values of In K _D range from –3.0 at X _an=0.30 to –1.0 at X _an=0.90 in samples from a single roadcut. Changes in both K _D and the topology of the ternary reciprocal exchange diagram occur with increasing metamorphic grade. Temperature dependence of In K _D is moderate with ¯H35 to 47 kcal at X _an=0.25; pressure dependence is small with ¯V –0.24 cal/bar. Usefulness of this exchange equilibrium as a geothermometer is restricted by uncertainties in the calculation of the amphibole formula from a microprobe analysis, especially with regard to Na, M4 in amphibole, to approximately ±50 ° C.  相似文献   

Novel amphibole geobarometer for high-magnesium andesite and basalt magmas     

A. G. Simakin  O. Yu. Shaposhnikova 《Petrology》2017,25(2):226-240

Experimental data on amphibole crystallization in water-saturated high-Mg andesite from Shiveluch volcano, Kamchatka, at 2, 3, and 5 kbar were used to calibrate a new geobarometer. The main parameters chosen to indicate the crystallization pressure of equilibrium amphibole is the ratio of tetrahedrally coordinated Al to the sum (Ti⁴⁺ + Fe³⁺), which is calculated by the 13eCNK technique with regard for crystal chemical considerations. The newly derived geobarometer is designed to evaluate pressure in the range of 2–12 kbar using amphibole from high-Mg andesite and basalt. The accuracy and reliability of the pressure estimates by the new geobarometer were tested by applying this geobarometer in studying amphibolized ultramafic xenoliths from the Dish Hill dike, California; magmatic amphibole hosted in cortlandite from the Shanuch intrusion, Kamchatka; and magmatic amphibole in the Pekul’ney Bay area, Chukotka. All estimates made with the newly developed geobarometer were reasonably close to values obtained by independent techniques.  相似文献