摘要:

AbstractIn a thermodynamic description of a mineral solid solution, it is customary to select a minimal group of end-members that constitutes an independent set, and yet in practical calculations it commonly occurs that a different independent set or subset is required. Given the simplification of the symmetric formalism, it is straightforward to derive the enthalpies of formation of dependent end-members as well as the interaction energies between the end-members in the new independent set, in terms of those in the original set. For example, a simplified ternary solid solution of Fe-free Ca-amphiboles might be described with independent end-members tremolite, tschermakite, and pargasite, and yet some calculations may require the use of end-members such as edenite or hornblende. Not only are their end-member properties dependent on those of the first three, but the mixing properties of any of the binary joins involving edenite or hornblende are dependent on those in the original independent set. Examples drawn from pyroxenes, amphiboles, and biotite micas show that such dependencies may prove invaluable in using experimental information or heuristics to help constrain the mixing properties of complex solid solutions. In particular, it is found that in non-ideal solid solutions that involve Fe and Mg distributed over two or more non-equivalent sites, equipartition of Fe and Mg implies extreme restrictions on the ratios of the interaction parameters and the magnitude of the Fe-Mg exchange energy. The thermodynamics can only be formulated generally and consistently when Fe- Mg ordering is explicitly included, and this is done most simply via ordered Fe-Mg endmembers.

ISSN:0003-004X    

DOI:10.2138/am-1999-1-201

出版商:Mineralogical Society of America    

年代:1999

数据来源: Degruyter

摘要:

AbstractBiotite solid-solutions are a significant H2O reservoir in the lithosphere, and the assemblage Bt + Opx + Kfs + Qtz is commonly used to estimate aH₂Oin high-grade metamorphic and magmatic rocks. Here we report experimental constraints on subsolidus mineral equilibria involving biotite and orthopyroxene in the system KAlO2-MgO-FeO-SiO2-H2O-CO2. Our experiments address the question of stability of biotite of a given XFe in the assemblage Bt + Qtz ± Sa, or the stability of the assemblage Opx + Sa ± Qtz. Clemens (1993) and Clemens et al. (1997) concluded that CO2has no effect other than to lower aH₂Oand thereby raise the solidus T. Our data at XFlH₂O<1 extend these conclusions to encompass Fe-bearing systems more similar to natural rocks. From a comparison of experimental data and calculated isopleths of biotite composition in the divariant assemblage Bt + Opx + Kfs + Qtz + fluid, it appears that phlogopite-annite solid-solutions must be significantly non-ideal (at T , 800 °C) or that enstatite-ferrosilite solid-solutions must have negative values for their Margules-type parameters. Ignoring these factors would result in any calculated aH₂Ovalues being too low. Although various models allow us to estimate XH₂Oin H2O-CO2fluids, we are still unable to use biotite equilibria to estimate aH₂Oaccurately during high-grade metamorphism and magma crystallization. We also consider qualitatively the effects Fe-Mg biotite solid-solution on partial melting equilibria in fluid-poor (rock-dominated) systems in which hydration-dehydration reactions control the fluid composition.

ISSN:0003-004X    

DOI:10.2138/am-1999-1-202

出版商:Mineralogical Society of America    

年代:1999

数据来源: Degruyter

摘要:

AbstractThe viscosities of hydrous melts (0.65 to 2.8 wt% H2O) with quartzofeldspathic compositions corresponding to Ab, Ab74Qz26, and Ab48Qz52(mole proportions calculated on the basis of eight oxygen atoms; Ab = NaAlSi3O8, Qz = Si4O8) have been determined between 980 and 1375 °C at pressures between 190 and 360 MPa using the falling sphere technique. The use of large bubble-free hydrous glass cylinders (placed in internally heated pressure vessels) previously prepared and already containing markers and platinum spheres allows falling distances up to several centimeters to be measured with a precision of ± 50 to 200 μm. This results in a precision of ± 15% relative or less for most viscosity data (± 10% relative or less if the temperature is known within ± 5 °C).For a water content of 2.8 wt% H2O, viscosity increases with increasing Qz content. In the investigated viscosity range, no significant deviation from Arrhenian behavior is observed and the activation energy of viscous flow increases slightly with decreasing water content of the melt (for Ab). Combining the experimental data obtained in this study with data for a haplogranitic composition investigated previously by Schulze et al. (1996) shows that the viscosities, and hence, the activation energies of viscous flow are similar for compositions with the same atom ratio (Si + Al)/(H + Na + K) (SA/HNK). Thus, melt viscosity is constant if Al, charge balanced by Na or K, is exchanged with Si + H (H incorporated as OH or H2O). The viscosities (in dPa·s) of all investigated hydrous haplogranite compositions with water contents ranging between 0.7 and 8.2 wt% H2O can be calculated to better than ±0.15 log units using the expression:logη = - 1.8 + [940 + 5598·(SA/HNK)0.3774]·1/Twhere T is expressed in Kelvin and varies from 1073 to 1650 K.

ISSN:0003-004X    

DOI:10.2138/am-1999-1-203

出版商:Mineralogical Society of America    

年代:1999

数据来源: Degruyter

摘要:

AbstractTitanite of variable Al and F content was found in granulite- to amphibolite-facies calcsilicates in Central Dronning Maud Land, Antarctica. The highest observed Al content corresponds to an XAl[= Al/(Al + Ti)] of 0.53. Previously, such high values of XAlwere reported from high-pressure rocks, but the titanite of this study is from a lowpressure terrain. The compositional variations in titanite can be described for all samples by a set of three linearly independent exchange vectors added to the CaTiSiO5endmember titanite. In most rocks, these vectors are Al1F1Ti-1O-1, Ti-0.25M0.25O-1OH1, and OH1F-1; in one sample, the Ti-0.25⃞0.25O-1OH1vector is replaced by a Si-0.25⃞0.25O-1OH1vector. The actual amount of exchange along these vectors and, therefore, the amount of Al in titanite, depends on P and T, on the composition of the coexisting fluid phase in terms of its H2O/HF fugacity ratio, and on host rock composition in terms of Al2O3/TiO2activity ratio. It is inferred that, in suitable chemical environments, high-Al titanite is stable over a wide P-T range. Therefore, the Al content of titanite should not be used in geothermobarometry, even qualitatively. Additionally, because of the coupled substitutions Al1F1Ti-1O-1and Al1OH1Ti-1O-1, the concentration of F in titanite is strongly dependent on the host rock chemistry. This rules out the easy use of titanite as a monitor of fluid composition.

ISSN:0003-004X    

DOI:10.2138/am-1999-1-204

出版商:Mineralogical Society of America    

年代:1999

数据来源: Degruyter

摘要:

AbstractWe describe a new quench technique that allows for controlled and reproducible constant quench rates exceeding 100 °C/s in the temperature range of 1400-700 °C and ~ 10 °C/s in the range 1400-200 °C at 1 atm total pressure. Our technique uses a 1 atm infrared image furnace and a blower unit capable of discharging cooling air through the infrared furnace at speeds approaching 100 m/s. The control protocol consists of operating the furnace at a constant power setting, sufficient to reach the highest desired temperature, and modulating the blower output continuously; blower output is controlled via a siliconcontrolled rectifier (SCR) using a proportional-integral-derivative (PID) algorithm on a personal computer (PC). A special autotuning procedure was used that enables the finetuning of PID parameters necessary for precise temperature control. Temperatures are controlled to ± 1 °C over the entire temperature range under isothermal conditions and to within 10 °C of the setpoint during quench. The range of accessible quench rates using our technique opens up new temperature-time paths for quantitative study. Potential applications include detailed studies on chemical diffusion and the kinetics of bubble and crystal formation under conditions of rapid temperature changes. Such studies have direct relevance to the crystallization, degassing, and structural relaxation of silicate melts during rapid temperature changes such as those encountered during volcanic eruptions.

ISSN:0003-004X    

DOI:10.2138/am-1999-1-205

出版商:Mineralogical Society of America    

年代:1999

数据来源: Degruyter

摘要:

AbstractPervasive metasomatism that involved the formation of tourmaline-rich rocks and influx of Li. F. and Cs into Ordovician psammo-pelitic metasediments occurred in the Valdeflores area (Cáceres. Spain). Numerous Li- and Sn-bearing. mineralized, greisen-type veins also can be observed here, in the vicinity of geochemically specialized granites. Tourmaline- rich rocks appear as: (1) massive, fine-grained, dark green to black rocks: and (2) fine- scale tourmaline-rich laminae, which alternate with quartz-rich layers parallel to the bedding.Electron microprobe analyses indicate that the tourmaline lies mostly within the space defined by the exchange vectors from dravite: FeMg-1(schorl), ⃞AlNa-1Mg-1(foitite), AlOMg-1(OH)-1(olenite), and CaMgNa-1Al-1(uvite). The Fe/(Fe+Mg) ratio ranges main­ly from 0.87 to 0.54 and increases with Al in the Y-site. Analytical results and substitu­tional relations show an insignificant elbaite component. Mica in the tounnalinized rocks is very fine-grained (mostly<50 μm). White mica ranges from lithian muscovite-phengite to lepidolite/zinnwaldite, containing up to 8.40 wt% F. 6.0 wt% Li2O, and 10.73 wt% FeO. Dark mica shows a variable color and has compositions characterized by relatively high contents of Cs2O (1.14-2.78 wt%) and F (1.94-8.08 wt%), with a deficit in K2O (5.75- 9.04 wt%). Log (fH₂O/fHF) of fluids in equilibrium with biotite in the tourmaline-rich rocks was 4.02-4.17 at T ≈ 400 °C. Log (fH₂O/fHF) values of fluids in equilibrium with topaz (XF≈ 0.8) in country rock adjacent to contacts with veins, and in equilibrium with am- blygonite-montebrasite (Xamb= 0.2) in the veins were about 4.30-4.60 and 6.4-6.7, respectively. Tliese variations denote the existence of gradients in relative aHFmore than differences of temperature during metasomatism. The lack of tourmaline in the veins is interpreted to reflect the alkalinity and low Fe-Mg contents in the fluids, which precluded the formation of tourmaline. Consequently, most of the boron was expelled into metase­diments where tourmaline was produced as a result.

ISSN:0003-004X    

DOI:10.2138/am-1999-1-206

出版商:Mineralogical Society of America    

年代:1999

数据来源: Degruyter

摘要:

AbstractThis work presents a procedure developed for trace element analysis using the electron microprobe (EMP). The method is demonstrated by analysis of seven glasses prepared from reference rock powders, with compositions of granite, granodiorite, andesite, diabase, and basalt. The melting process was adapted to prevent the loss of volatile elements and to obtain homogeneous samples. A routine procedure for analysis of major and minor (above 1000 ppm) elements yielded results in fairly good agreement with published values. The methods presented in this study produced detection limits as low as: (1) 6-8 ppm when the Kα peaks of transition metals of the first row (Cr and Ni) were used; (2) 23 ppm with soft Lα peaks (Y, Zr, and Sr); (3) 15 ppm with high-energy Lα peaks (rare earth elements); and (4) 35 ppm with the Ma peaks of heavy elements (Pb and Th). These limits were achieved after total counting times (peak + two background measurements) of about 15 min with a beam of 35 kV and 500 nA. Precision (± 2σ) on the weight percent concentrations was below 50% for concentrations above 13 ppm for group 1 elements, 35 ppm for group 2, 25 ppm for group 3, and 55 ppm for group 4. In the present work, a commercial software package was adapted for quantitative trace element analysis. One modification addresses beam-sensitive materials for which the long counting times required for measurements need to be divided into subsets (10-20 s count), each acquired from different sites of the sample surface, to minimize damage. This analytical mode is referred to as ‘‘multi-site’’ mode.

ISSN:0003-004X    

DOI:10.2138/am-1999-1-207

出版商:Mineralogical Society of America    

年代:1999

数据来源: Degruyter

摘要:

AbstractFor estimations of P - T conditions of igneous and metamorphic rocks, Fe3+in coexisting minerals is either assumed to be zero or is calculated from electron microprobe analyses (EMPA) based upon stoichiometry and charge balance. Geothermobarometers that involve Fe2+- Mg2+exchange can be significantly affected by either neglecting Fe3+or using incorrect values. Ratios of Fe3+/ΣFe in garnet and clinopyroxene measured by a Mössbauer milliprobe were compared to those calculated from EMPA of garnet and clinopyroxene from eclogite xenoliths from the Udachnaya kimberlite in Yakutia. The effects of Fe3+contents in garnet and clinopyroxene on temperature estimations were evaluated.The following Fe3+/ΣFe (in at%) values were obtained (EMPA/Mössbauer): Gt = 9.4/6.0; 11.5/7.0; 19.4/16.0; and 24.7/15.0; Cpx = 22.0/22.9; 34.2/22.0. The effects of Fe3+in clinopyroxenes on calculated temperatures are illustrated by taking eclogitic clinopyroxene compositions and changing contents of certain elements within the range of standard deviations for EMPA of those particular elements. Increasing Na2O contents from 5.67 to 5.74 wt% (<2.0% relative error) would lead to increasing Fe3+/ΣFe from 31.6 to 47.1%, thereby decreasing the calculated temperature from 1026 to 941 °C. Various Fe3+/ΣFe values for garnet and clinopyroxene were also tested for their effects on calculated temperatures: for clinopyroxene, T decreases with increasing Fe3+/ΣFe whereas for garnet, T increases with increasing Fe3+/ΣFe. This compensation effect between garnet and clinopyroxene moderates the variation in temperature estimations of eclogites based on Fe3+corrected vs. uncorrected microprobe analyses.Little correlation exists between EMPA-calculated and Mössbauer-measured Fe3+/ΣFe values for these mantle-derived garnets and clinopyroxenes. Even a small relative error in Fe3+may significantly change calculated temperatures of equilibration, seriously affecting petrologic interpretations. In particular, uncertainty in Fe3+calculated from EMPA of silicate minerals leads to serious questions with regard to KDvalues obtained from natural assemblages.

ISSN:0003-004X    

DOI:10.2138/am-1999-1-208

出版商:Mineralogical Society of America    

年代:1999

数据来源: Degruyter

摘要:

AbstractAmphiboles were synthesized along the joins tremolite-fluorotremolite, richterite-fluororichterite and potassic-richterite-potassic-fluororichterite at 750 °C and 1 kbar P(H2O). Infrared spectra of the amphiboles were recorded in the principal OH-stretching region. Amphiboles of the tremolite-fluorotremolite series show one-mode behavior, a single band due to a local MgMgMg-OH-A⃞ (⃞ = vacancy) arrangement; this behavior is consistent with no coupling between NNN (next-nearest-neighbor) O3 anions either through the O3- O3 edge or across the vacant A-site cavity. The amphiboles of the richterite-fluororichterite and potassic-richterite-potassic-fluororichterite series show two-mode behavior, two bands due to the local arrangements MgMgMg-OH-ANa-OH and MgMgMg-OH-ANa-F (and their K equivalents); this behavior is consistent with coupling between NNN O3 anions across the filled A-site cavity through Na or K that occupies the A-site. A mathematical model is developed to describe local (OH,F) ordering in amphiboles as a function of F content. The variation in infrared band intensities is consistent with complete short-range disorder of OH and F in the synthetic amphiboles of the richterite-fluororichterite and potassicrichterite- potassic-fluororichterite series.

ISSN:0003-004X    

DOI:10.2138/am-1999-1-209

出版商:Mineralogical Society of America    

年代:1999

数据来源: Degruyter

摘要:

AbstractThe29Si MAS NMR spectrum of armenite from Wasenalp (Valais, Switzerland) indicates complete Si, Al ordering. The same chemical shifts (- 82.3, - 95.0, and - 101.8 ppm) were also measured for armenite from Rémigny (Quebec, Canada), however, the intensity ratios of the NMR bands for the latter sample differed. A full sphere of X-ray single-crystal data on an optically homogeneous domain of armenite from Rémigny was collected on a threecircle diffractometer equipped with a CCD area-detector. The crystal structure was refined in the acentric space group Pnc2 [a = 18.660(2), b = 10.697(1) c = 13.874(2) Å ] to R1 5 3.6% based on 4275 reflections. These results confirm complete Si, Al ordering. Mean Si-O distances range between 1.615 and 1.629 Å ; mean Al-O distances between 1.734 and 1.742 Å . Calcium is sevenfold coordinated by six framework O atoms and one H2O molecule. Barium is 12-fold coordinated by framework O atoms. Polarized IR spectra in the region of OH absorptions (between 5700 and 1300 cm-1) were recorded on polished slabs of Wasenalp armenite and structurally related milarite from Val Giuf. In general, milarite and armenite show similar anisotropy of H2O related absorptions.

ISSN:0003-004X    

DOI:10.2138/am-1999-1-210

出版商:Mineralogical Society of America    

年代:1999

数据来源: Degruyter