Enstatite-diopside solvus and geothermometry

The enstatite-diopside solvus presents certain interesting thermodynamic and crystal-structural problems. The solvus may be considered as parts of two solvi one with the ortho-structure and the other with clino-structure. By assuming the standard free energy change for the two reactions (MgMgSi₂O₆)opx ? (MgMgSi₂O₆)cpx and (CaMgSi₂O₆) opx ? (CaMgSi₂O₆) cpx as 500 and 1 000 to 3 000 cal/mol respectively, it is possible to calculate the regular solution parameter W for orthopyroxene and clinopyroxene. These W's essentially refer to mixing on M2 sites. The expression for the equilibrium constant by assuming ideal mixing for Fe-Mg, Fe-Ca and non-ideal mixing for Ca-Mg on binary M1 and ternary M2 sites is given by 1 $$K_a = \frac{{X_{{\text{Mg - cpx}}}^{{\text{M1}}} X_{{\text{Mg - cpx}}}^{{\text{M2}}} \exp \left[ {\frac{{W_{{\text{cpx}}} }}{{RT}}\left\{ {X_{{\text{Ca - cpx}}}^{{\text{M2}}} \left( {X_{{\text{Ca - cpx}}}^{{\text{M2}}} + X_{{\text{Fe - cpx}}}^{{\text{M2}}} } \right)} \right\}} \right]}}{{X_{{\text{Mg - cpx}}}^{{\text{M1}}} X_{{\text{Mg - opx}}}^{{\text{M2}}} \exp \left[ {\frac{{W_{{\text{cpx}}} }}{{RT}}\left\{ {X_{{\text{Ca - opx}}}^{{\text{M2}}} \left( {X_{{\text{Ca - opx}}}^{{\text{M2}}} + X_{{\text{Fe - opx}}}^{{\text{M2}}} } \right)} \right\}} \right]}}$$ where X's are site occupancies, R is 1.987 and T is temperature in ^oK. Temperature of pyroxene crystallization may be estimated by substituting for T in the above equation until the equation ?RT In K _a=500 is satisfied. The shortcomings of this method are the incomplete standard free energy data on the end member components and the absence of site occupancy data in pyroxenes at high temperatures. The assumed free energy data do, however, show the possible extent of inaccuracy in temperature estimates resulting from the neglect of Mg-Ca non ideality.     

Na,K, P and Ti in garnet,pyroxene and olivine from peridotite and eclogite xenoliths from African kimberlites

Electron microprobe analyses sensitive to 20ppmw (2σ) were made for Na, P, K and Ti in garnet, pyroxenes and olivine from peridotite and eclogite xenoliths from African kimberlites and volcanic rocks in Tanzania. Average concentrations (ppmw) in peridotite (mostly garnet lherzolite) are: Na₂O gt 340 ol 90 opx 1070 cpx 2.1 (wt.%); P₂O₅ gt 460 ol 130 opx 50 cpx 350; K₂O gt <20 ol <20 opx 30 cpx 170; TiO₂ gt 1470 ol 130 opx 480 cpx 1630. For eclogites and a cpx megacryst with gt inclusions: Na₂O gt 610 cpx 4.3 (wt.%); P₂O₅ gt 530 cpx 300; K₂O gt <20 cpx 370; TiO₂ gt 1990 cpx 1980.In garnet, Na can be explained by coupled substitution with P and Ti, and there is no need to invoke six-coordinated silicon. The Na distribution between garnet and clinopyroxene correlates with the Fe/Mg distribution for both eclogites and peridotites, and for the peridotites correlates with estimates of pressure and temperature from pyroxene composition. When calibrated experimentally, the Na distribution may be a useful indicator of physical conditions at depths for which the Fe/Mg distribution is insensitive; furthermore the Na distribution may be less sensitive to oxidation state.     

Distribution of REE between clinopyroxene and basaltic melt along a mantle adiabat: effects of major element composition, water, and temperature

The distribution of rare earth elements (REE) between clinopyroxene (cpx) and basaltic melt is important in deciphering the processes of mantle melting. REE and Y partition coefficients from a given cpx-melt partitioning experiment can be quantitatively described by the lattice strain model. We analyzed published REE and Y partitioning data between cpx and basaltic melts using the nonlinear regression method and parameterized key partitioning parameters in the lattice strain model (D ₀, r ₀ and E) as functions of pressure, temperature, and compositions of cpx and melt. D ₀ is found to positively correlate with Al in tetrahedral site (Al ^T ) and Mg in the M2 site (Mg^M2) of cpx and negatively correlate with temperature and water content in the melt. r ₀ is negatively correlated with Al in M1 site (Al^M1) and Mg^M2 in cpx. And E is positively correlated with r ₀. During adiabatic melting of spinel lherzolite, temperature, Al ^T , and Mg^M2 in cpx all decrease systematically as a function of pressure or degree of melting. The competing effects between temperature and cpx composition result in very small variations in REE partition coefficients along a mantle adiabat. A higher potential temperature (1,400°C) gives rise to REE partition coefficients slightly lower than those at a lower potential temperature (1,300°C) because the temperature effect overwhelms the compositional effect. A set of constant REE partition coefficients therefore may be used to accurately model REE fractionation during partial melting of spinel lherzolite along a mantle adiabat. As cpx has low Al and Mg abundances at high temperature during melting in the garnet stability field, REE are more incompatible in cpx. Heavy REE depletion in the melt may imply deep melting of a hydrous garnet lherzolite. Water-dependent cpx partition coefficients need to be considered for modeling low-degree hydrous melting.     

Temperature-dependent distribution of Cr between olivine and pyroxenes in lherzolite xenoliths

The temperature effect on the exchange reaction Cr₂O₃(ol)=Cr₂O₃(px) was studied for coexisting olivine and both clino and ortho pyroxenes. The distribution of Cr between olivine and clinopyroxene in 31 coarse garnet lherzolites and 10 porphyroclastic garnet lherzolites from kinberlites, and in 17 coarse spinel lherzolites from basalts, obeys a van't Hoff relation (c.f. Stosch 1981) with the Wells two-pyroxene temperature: T(Kelvin)=8,787 (In D _Cr+ 2.87) where D _Cr(opx/ol)=wt.% Cr(clinopyroxene)/Cr(olivine). An analogous exchange for olivine and orthopyroxene with 0.7–1.6 wt.% Al₂O₃ in 41 garnet lherzolites from kimberlites shows considerable scatter about the following relation: T(Kelvin)=5,540/(ln D _cr+1.86) where D _cr(opx/ol)= wt.% Cr(orthopyroxene)/Cr(olivine). Spinel lherzolites and a garnet lherzolite from the Malaita alnöite do not obey the second relation. For orthopyroxene with 2.5–5.1 wt.% Al₂O₃, D _cr(opx/ol) is 1.7 to 3 times higher, and for 0.1 wt.% Al₂O₃ is 2 times lower than for the garnet lherzolites. Experimental calibration is needed, especially to check the possible effect of Al on D _cr(opx/ol).     

Partitioning of Ru, Rh, Pd, Re, Ir, and Au between Cr-bearing spinel, olivine, pyroxene and silicate melts

A series of high temperature experiments was undertaken to study partitioning of several highly siderophile elements (HSE; Ru, Rh, Pd, Re, Os, Ir, Pt and Au) between Cr-rich spinel, olivine, pyroxene and silicate melt. Runs were carried out on a Hawaiian ankaramite, a synthetic eucrite basalt, and a DiAn eutectic melt, at one bar, 19 kbar, and 20 kbar, respectively, in the temperature range of 1200 to 1300°C, at oxygen fugacities between the nickel-nickel oxide (NNO) and hematite-magnetite (HM) oxygen buffers. High oxygen fugacities were used to suppress the formation of HSE-rich “nuggets” in the silicate melts. The resulting oxide and silicate crystals (<100 μm) were analyzed using both SIMS and LA-ICP-MS, with a spatial resolution of 15 to 50 μm. Rhenium, Au and Pd were all found to be incompatible in Cr-rich spinel (D_Re^sp/melt = 0.0012-0.21, D_Au^sp/melt = 0.076, D_Pd^sp/melt = 0.14), whereas Rh, Ru and Ir were all found to be highly compatible (D_Rh^sp/melt = 41-530, D_Ru^sp/melt = 76-1143, D_Ir^sp/melt = 5-22000). Rhenium, Pd, Au and Ru were all found to be incompatible in olivine (D_Re^oliv/melt = 0.017-0.073, D_Pd^oliv/melt = 0.12, D_Au^oliv/melt = 0.12, D_Ru^oliv/melt = 0.23), Re is incompatible in orthopyroxene and clinopyroxene (D_Re^opx/melt = 0.013, D_Re^cpx/melt = 0.18-0.21), and Pt is compatible in clinopyroxene (D_Pt^cpx/melt = 1.5). The results are compared to and combined with previous work on HSE partitioning among spinel-structured oxides, and applied to some natural magmatic suites to demonstrate consistency.     

Voluminous arc dacites as amphibole reaction-boundary liquids

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.     

Water contents of the Cenozoic lithospheric mantle beneath the western part of the North China Craton: Peridotite xenolith constraints

Nominally anhydrous phases (clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol)) of peridotite xenoliths hosted by the Cenozoic basalts from Beishan (Hebei province), and Fansi (Shanxi province), Western part of the North China Craton (WNCC) have been investigated by Fourier transform infrared spectrometry (FTIR). The H₂O contents (wt.) of cpx, opx and ol are 30–255 ppm, 14–95 ppm and ~ 0 ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivine, pyroxenes (cpx and opx) largely preserve the H₂O content of their mantle source inferred from (1) the homogenous H₂O content within single pyroxene grains, and (2) equilibrium H₂O partitioning between cpx and opx. Based on mineral modes and assuming a partition coefficient of 10 for H₂O between cpx and ol, the recalculated whole-rock H₂O contents range from 6 to 42 ppm. In combination with previously reported data for other two localities (Hannuoba and Yangyuan from Hebei province), the H₂O contents of cpx, opx and whole-rock of peridotite xenoliths (43 samples) hosted by the WNCC Cenozoic basalts range from 30 to 654 ppm, 14 to 225 ppm, and 6 to 262 ppm respectively. The H₂O contents of the Cenozoic lithospheric mantle represented by peridotite xenoliths fall in a similar range for both WNCC and the eastern part of the NCC (Xia et al., 2010, Journal of Geophysical Research). Clearly, the Cenozoic lithospheric mantle of the NCC is dominated by much lower water content compared to the MORB source (50–250 ppm). The low H₂O content is not caused by oxidation of the mantle domain, and likely results from mantle reheating, possibly due to an upwelling asthenospheric flow during the late Mesozoic–early Cenozoic lithospheric thinning of the NCC. If so, the present NCC lithospheric mantle mostly represents relict ancient lithospheric mantle. Some newly accreted and cooled asthenospheric mantle may exist in localities close to deep fault.     

Trace element distribution between orthopyroxene and clinopyroxene in peridotites from the Gakkel Ridge: a SIMS and NanoSIMS study

Clinopyroxenes (cpx) in abyssal and ophiolitic peridotites are commonly analyzed for lithophile trace element abundances in order to estimate degrees of melting and porosity conditions during melt extraction, assuming that these data reflect near-solidus conditions. During cooling, however, cpxs always exsolve into parallel lamellae of low-Ca enstatite and high-Ca diopside. This may potentially lead to redistribution of the initial trace element budget. Since orthopyroxene (opx) cannot significantly host most incompatible trace elements, exsolution will lead to an enrichment in the cpx lamellae. In order to address a possibly exsolution-controlled partitioning between cpx and opx, we have obtained major and trace element mineral compositions on 14 plagioclase-free ocean floor mantle rocks. They cover the entire abyssal peridotite compositional spectrum from very fertile to highly depleted compositions. The mean volume proportion of opx lamellae in cpx porphyroclasts lies around 15% of the original cpx. For the light to middle rare earth elements, the enrichment in the measured cpx exsolution is exclusively controlled by these phase proportions. Relative to these highly incompatible trace elements, solely Ti and Yb partition significantly into opx. Lamellar interpyroxene partition coefficients, estimated from NanoSIMS analyses, are around three times as high as the ones for near-solidus bulk pyroxene. The equilibration temperatures for the exsolution lamella are slightly higher than 800°C. The bulk cpx can be reconstructed using the lamellar proportions and their relative partitioning. The implication of such a reconstruction is that the cpx rare earth element patterns shift almost in parallel to lower values. These shifts, however, do not affect mantle melting models proposed thus far for mid-ocean ridges. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

The solubility of BaCO3(cr) (witherite) in CO2-H2O solutions between 0 and 90°C,evaluation of the association constants of BaHCO3+(aq) and BaCO30(aq) between 5 and 80°C,and a preliminary evaluation of the thermodynamic properties of Ba2+(aq)

One hundred and fifty new measurements of the solubility of witherite were used to evaluate the equilibrium constant of the reaction BaCO₃(cr) = Ba²⁺(aq) + CO₃²⁻(aq) between 0 and 90°C and 1 atm total pressure. The temperature dependence of the equilibrium constant is given by logK = 607.642 + 0.121098T − 20011.25/T − 236.4948 logT where T is in degrees Kelvin. The logK of BaCO₃(cr), the Gibbs energy, the enthalpy and entropy of the reaction at 298.15 K are −8.562, 48.87 kJ · mol⁻¹, 2.94 kJ · mol⁻¹ and −154.0 J · mol⁻¹ · K⁻¹, respectively. The equilibrium constants are consistent with an aqueous model that includes the ion pairs BaHCO₃⁺(aq) and BaCO₃⁰(aq) Three different methods were used to evaluate the association constant of BaHCO₃⁺(aq), and all yielded similar results. The temperature dependence of the association constant for the reaction Ba²⁺(aq) + HCO₃⁻(aq) = BaHCO₃⁺(aq) is given by logK_BaHCO3⁺ = −3.0938 + 0.013669T.The log of the association constant, the Gibbs energy, the enthalpy and entropy of the reaction at 298.15°K are 0.982, −5.606 kJ · mol⁻¹, 23.26 kJ · mol⁻¹ and 96.8 J · mol⁻¹ · K⁻¹, respectively. The temperature dependence of the equilibrium constant for the reaction Ba²⁺(aq) + CO²⁻₃(aq) = BaCO₀³(aq) is given by logK_BaCO3⁰ = 0.113 + 0.008721T.The log of the association constant, the Gibbs energy, the enthalpy and entropy of the reaction at 298.15° K are 2.71, −15.49 kJ · mol⁻¹, 14.84 kJ · mol⁻¹ and 101.7 J· mol⁻¹ · K⁻¹.The above model leads to reliable calculations of the aqueous speciation and solubility of witherite in the system BaCO₃-CO₂-H₂O from 0 to more than 90°C. Literature data on witherite solubility were re-evaluated and compared with the results of this study.Problems in the thennodynamic selections of Ba compounds are considered. Newer data require the revision of Δ_fH° and Δ_fG° of Ba²⁺(aq) to −532.5 and −555.36 kJ · mol⁻¹, respectively, for agreement with solubility data.     

Experimental determination of the temperature and pressure dependence of the Fe-Mg partition coefficient for coexisting garnet and clinopyroxene

An experimental study initiated to calibrate the distribution coefficient \(K_D = \frac{{({\text{FeO}}/{\text{MgO}})_{{\text{ga}}} }}{{{\text{(FeO}}/{\text{MgO)}}_{{\text{cpx}}} }}\) in eclogites as a geothermometer has been done on (a) a mineral mis, (b) a glass of the typical tholeiite composition and (c) a series of glasses of tholeiite compositions with \(6.2 < \frac{{100{\text{Mg}}}}{{{\text{Mg}} + {\text{Fe}}^{ + + } }} < 93.\) The mineral mix was found to be unsuitable as reactant due to incomplete equilibration but the minimum K _D of the mineral mix and the K _D from glass of tholeiite composition are identical within experimental uncertainty. These data constitute a reversal of the garnet/clinopyroxene partition relationship and provide justification of the use of glass as a reactant for the project. To eliminate any uncertainty in interpretation of mineral analyses due to possible variation in Fe⁺⁺⁺/Fe⁺⁺ between runs, experiments were carried out in iron capsules on the nine tholeiite glasses, thus maintaining iron as Fe⁺⁺. Microprobe analytical techniques yielded mineral analyses of comparable accuracy to analyses of natural phases for experiments within the temperature range from 600° C to 1500° C and a pressure range from 20 kb to 40 kb. It has been shown that for \(6.2 < \frac{{100{\text{Mg}}}}{{{\text{Mg}} + {\text{Fe}}^{ + + } }} < 85\) , the bulk chemical composition does not perceptibly affect the K _D value. At 30 kb the K _D value ranges from 18.0 at 600° C to 1.45 at 1400° C, defining the linear relationship in a ln K _D vs 1/T(°K) plot. The pressure dependence of the K _D -value has been shown to be greater than previously predicted. There is a straight line relationship in ln K _D vs Pressure (Kb) between 20 and 40 kb at constant temperature (1100°C). This enables us to determine K _D =fn (T, P) and \(T(^\circ {\text{K}}) = \frac{{3686 + 28.35 \times P({\text{Kb}})}}{{\ln K_D + 2.33}}\) . This expression uniquely determines the temperature of equilibration of natural eclogites of basaltic bulk composition when the K _D ^ga,cpx is known and a pressure estimate can be given.     

Associated chemical and carbon isotopic composition variations in diamonds from Finsch and Premier kimberlite,South Africa

The carbon isotopic composition of 66 inclusion-containing diamonds from the Premier kimberlite, South Africa, 93 inclusion-containing diamonds and four diamonds of two diamond-bearing peridotite xenoliths from the Finsch kimberlite, South Africa was measured. The data suggest a relationship between the carbon isotopic composition of the diamonds and the chemical composition of the associated silicates. For both kimberlites similar trends are noted for diamonds containing peridotite-suite inclusions (P-type) and for diamonds containing eclogite-suite inclusions (E-type): Higher δ¹³C P-type diamonds tend to have inclusions lower in SiO₂ (ol), Al₂O₃ (opx, gt), Cr₂O₃, MgO, $\text{Mg}\text{(Mg + Fe)}$ (ol, opx, gt) and higher in FeO (ol, opx, gt) and CaO (gt). Higher δ¹³C E-type diamonds tend to have inclusions lower in SiO₂, Al₂O₃ (gt, cpx), MgO, $\text{Mg}\text{(Mg + Fe)}$ (gt), Na₂O, K₂O, TiO₂ (cpx) and higher in CaO, $\text{Ca}\text{(Ca + Mg)}$ (gt, cpx).Consideration of a number of different models that have been proposed for the genesis of kimberlites, their xenoliths and diamonds shows that they are all consistent with the conclusion that in the mantle, regions exist that are characterized by different mean carbon isotopic compositions.     

Temperature,composition, and f O2 effects on intersite distribution of Mg and Fe2+ in olivines

Single-crystal X-ray structure refinement of natural olivines equilibrated at high temperature under controlled oxygen fugacity (f _O2) conditions, coupled with a structure-energy model were used to establish the influence of T, f _O2 and bulk chemistry on intracrystalline disorder.The results are: 1) The k _D (k _D = [(Fe M1·Mg M2)/(Fe M2·Mg M1)]) factor describing site population on M1, M2 polyhedra increases from values lower than 1 at T below 400–600° C (depending on composition) to values higher than 1 at higher temperature. 2) The increase of k _D with T is quite regular. 3) At constant temperature and pressure, k _D increases with increasing fayalite content in the mixture; 4) Contrary to previous observations (Will and Nover 1979; Nover and Will 1981) varying f _O2, within the stability range of the substance, has a negligible influence on intracrystalline disorder. 5) As ancillary results, the model confirms the defect scheme of Nakamura and Schmalzried (1983) for the investigated solids. Moreover, it shows that cationic vacancies are always created on M 1 site at the expense of Mg ions, while trivalent iron is always stabilized on M2 site. This explains the marked anisotropies observed in Fe-Mg interdiffusion (Buening and Busek 1973; Misener 1974; Schock et al. 1989).     

Clinopyroxene-orthopyroxene major and rare earth elements partitioning in spinel peridotite xenoliths from Assab (Ethiopia)

Major element and rare earth element (REE) partitioning among coexisting clinopyroxene-orthopyroxene pairs from mantle xenoliths of the Assab Range (Ethiopia) are discussed in terms of crystal-chemistry.Major element partitioning indicates relatively uniform conditions of subsolidus equilibration over a narrow range of temperatures (mean value about 1100 C) in the spinel peridotite stability field. Major element distributions and correlations, moreover, seem to indicate that the mantle material studied underwent slightly different depletions prior to the metamorphic equilibration.In spite of the rather homogeneous major element compositions for both cpx and opx, clinopyroxenes show chondrite-normalized REE patterns which are widely variable both in shape and absolute values, whereas orthopyroxenes exhibit more restricted ranges and concordant profiles.REE activity ratios have been investigated by applying Iiyama's (Bull. Soc. fr. Minéral. Christallogr.97, 143–151) thermodynamic model: the estimated activity patterns exhibit a good coherence for the different pyroxene pairs, in spite of the contrasting features of their REE concentration ratios. The wide ranges in the measured partition values for the same rare earth element in different pyroxene pairs have been related to coupled substitutions involving A1 in the Z site and REE in the M₂ site of clinopyroxene.     

X-ray diffraction study of Fe2+-Mg order-disorder in orthopyroxene. Some kinetic results

Kinetic rates of Fe²⁺-Mg disordering in three orthopyroxenes (mean value of X_Fe = Fe²⁺/(Fe²⁺+Mg) = 0.175,0.482,0.770 respectively) have been determined employing heating experiments and single crystal X-ray structural refinements. Disordering rate constants \((\vec K)\) (550800° C) for two pyroxenes are given by: ln \((\vec K)\) = 27.107(±5.177)?32062(±783)T^?1(X_Fe = 0.175) ln \((\vec K)\) = 16.142(±0.057)?18227(±423)T^?1(X_Fe = 0.770) The distribution coefficients K_D (representing a steady state of disordering Fe_M2 + Mg_M1 ? Fe_M1 + Mg_M2) are given by: ln K_D = 5.016(±0.223)-7033(±1473) T^?1(X_Fe = 0.175) ln K_D = 1.988(±0.122)-3809(±913)T^?1(X_Fe = 0.770) These distribution coefficients provide the constraint of the disordering reaction on the value of the equilibrium constant for Fe²⁺-Mg order-disorder. Until the low temperature dependence of K_D is well constrained, the calculation of cooling rates of pyroxenes and host rocks cannot be done reliably.     

Long-term earthquake prediction in western Anatolia with the time- and magnitude-predictable model

Instrumental and historical data on mainshocks for 13 seismogenic sources in western Anatolia have been used to apply a regional time- and magnitude-predictable model. Considering the interevent time between successive mainshocks, the following two predictive relations were computed: log T _t = 0.13 M _min + 0.21 M _p ? 0.15 log M ₀ + 2.93 and M _f = 0.87 M _min ? 0.06 M _p + 0.33 log M ₀ ? 6.54. Multiple correlation coefficient and standard deviation have been computed as 0.50 and 0.29, respectively, for the first relation, and 0.65 and 0.47, respectively, for the second relation. The positive dependence of T _t on M _p and the negative dependence of M _f on M _p indicate the validity of time- and magnitude-predictable model on the area considered in this study. On the basis of these relations and using the occurrence time and magnitude of the last main shocks in each seismogenic source, the probabilities of occurrence P(Δt) of the next main shocks during the next 50 years with decade interval as well as the magnitude of the expected main shocks were determined. The highest probabilities P ₁₀ = 80 % (M _f = 6.8 and T _t = 13 years) and P ₁₀ = 32 % (M _f = 7.6 and T _t = 29 years) were estimated for the seismogenic source 11 (Golhisar-Dalaman-Rhodes) for the occurrence of a strong and a large earthquake during the future decade, respectively.     

Experimental partitioning of halogens and other trace elements between olivine, pyroxenes, amphibole and aqueous fluid at 2 GPa and 900–1,300 °C

We present new partition coefficients for various trace elements including Cl between olivine, pyroxenes, amphibole and coexisting chlorine-bearing aqueous fluid in a series of high-pressure experiments at 2 GPa between 900 and 1,300 °C in natural and synthetic systems. Diamond aggregates were added to the experimental capsule set-up in order to separate the fluid from the solid residue and enable in situ analysis of the quenched solute by LA–ICP–MS. The chlorine and fluorine contents in mantle minerals were measured by electron microprobe, and the nature of OH defects was investigated by infrared spectroscopy. Furthermore, a fluorine-rich olivine from one selected sample was investigated by TEM. Results reveal average Cl concentrations in olivine and pyroxenes around 20 ppm and up to 900 ppm F in olivine, making olivine an important repository of halogens in the mantle. Chlorine is always incompatible with Cl partition coefficients D _Cl ^{olivine/fluid} varying between 10^?5 and 10^?3, whereas D _Cl ^{orthopyroxene/fluid} and D _Cl ^{clinopyroxene/fluid} are ~10^?4 and D _Cl ^{amphibole/fluid} is ~5 × 10^?3. Furthermore, partitioning results for incompatible trace element show that compatibilities of trace elements are generally ordered as D ^amph/fluid ≈ D ^cpx/fluid > D ^opx/fluid > D ^ol/fluid but that D ^{mineral/fluid} for Li and P is very similar for all observed silicate phases. Infrared spectra of olivine synthesized in a F-free Ti-bearing system show absorption bands at 3,525 and ~3,570 cm^?1. In F ± TiO₂-bearing systems, additional absorption bands appear at ~3,535, ~3,595, 3,640 and 3,670 cm^?1. Absorption bands at ~3,530 and ~3,570 cm^?1, previously assigned to humite-like point defects, profit from low synthesis temperatures and the presence of F. The presence of planar defects could not be proved by TEM investigations, but dislocations in the olivine lattice were observed and are suggested to be an important site for halogen incorporation in olivine.     

Experimental studies of oxygen and hydrogen isotope fractionations between precipitated brucite and water at low temperatures

Oxygen and hydrogen isotope fractionation factors between brucite and water were experimentally determined by chemical synthesis techniques at low temperatures of 15° to 120°C. MgCl₂, Mg3N₂, and MgO were used as reactants, respectively, to produce brucite in aqueous solutions. All of the synthesis products were identified by x-ray diffraction (XRD) for crystal structure and by scanning electron microscope (SEM) for morphology. It is observed that oxygen isotope fractionations between brucite and water are temperature dependent regardless of variations in aging time, the chemical composition, and pH value of solutions. Brucites derived from three different starting materials yielded consistent fractionations with water at the same temperatures. These suggest that oxygen isotope equilibrium has been achieved between the synthesized brucite and water, resulting in the fractionation equation of 10³lnα=1.56×10⁶/T²−14.1. When the present results for the brucite–water system are compared with those for systems of gibbsite–water and goethite–water, it suggests the following sequence of ¹⁸O-enrichment in the M−OH bonds of hydroxides: Al³⁺ − OH > Fe³⁺ − OH > Mg²⁺ − OH.Hydrogen isotope fractionations between brucite and water obtained by the different synthesis methods have also achieved equilibrium, resulting in the fractionation equation of 10³lnα=−4.88×10⁶/T²−22.5. Because of the pressure effect on hydrogen isotope fractionations between minerals and water, the present calibrations at atmospheric pressure are systematically lower than fractionations extrapolated from hydrothermal exchange experiments at high temperatures of 510° to 100°C and high pressures of 1060 to 1000 bar. Comparison of the present results with existing calibrations involving other low-temperature minerals suggests the following sequence of D-enrichment in hydroxyl-bearing minerals: Al³⁺ − OH > Mg²⁺ − OH > Fe³⁺ − OH.     

Solubility of Ca6[Al(OH)6]2(CrO4)3·26H2O,the chromate analog of ettringite; 5–75°C

Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O⇌6Ca²⁺+2Al(OH)⁻₄+3CrO²⁻₄+4OH⁻+26H₂O varies with pH unless a CaCrO_4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca²⁺+CrO²⁻₄=CaCrO_4(aq) was obtained by minimizing the variance in the IAP for Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log K_SP) of Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O at 25°C is −41.46±0.30. The temperature dependence of the log K_SP is log K_SP=A−B/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG⁰_r,298 and ΔH⁰_r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol⁻¹. the values of ΔC⁰_P,r,298 and ΔS⁰_r,298 are −1506±140 and −534±83 J mol⁻¹ K⁻¹. Using these values and published standard state partial molal quantities for constituent ions, ΔG⁰_f,298=−15,131±19 kJ mol⁻¹, ΔH⁰_f,298=−17,330±8.6 kJ mol⁻¹, ΔS⁰₂₉₈=2.19±0.10 kJ mol⁻¹ K⁻¹, and ΔC⁰_Pf,298=2.12±0.53 kJ mol⁻¹ K⁻¹, were calculated.     

A thermodynamic theory of the Grüneisen ratio at extreme conditions: MgO as an example

The Grüneisen ratio, γ, is defined as γy=αK _TV/C_v. The volume dependence of γ(V) is solved for a wide range in temperature. The volume dependence of αK _T is solved from the identity (? ln(αK _T)/? ln V)T ≡ δ _T-K′. α is the thermal expansivity; K _T is the bulk modulus; C _V is specific heat; and δ _Tand K′ are dimensionless thermoelastic constants. The approach is to find values of δ _T and K′, each as functions of T and V. We also solve for q=(? ln γ/? ln V) where q=δ _T -K′+ 1-(? ln C _V/? ln V)T. Calculations are taken down to a compression of 0.6, thus covering all possible values pertaining to the earth's mantle, q=? ln γ/? ln V; δ _T=? ln α/? ln V; and K′= (?K _T/?P)T. New experimental information related to the volume dependence of δ _T, q, K′ and C _V was used. For MgO, as the compression, η=V/V ₀, drops from 1.0 to 0.7 at 2000 K, the results show that q drops from 1.2 to about 0.8; δ _T drops from 5.0 to 3.2; δ _T becomes slightly less than K′; ? ln C _V/? In V→0; and γ drops from 1.5 to about 1. These observations are all in accord with recent laboratory data, seismic observations, and theoretical results.     

Petrology and P–T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai Complex,China

The Wutai Complex represents the best preserved granite-greenstone terrane in the North China Craton. The complex comprises a sequence of metamorphosed ultramafic to felsic volcanic rocks, variably deformed granitoid rocks, along with lesser amounts of siliciclastic and carbonate rocks and banded iron formations. Petrological evidence from the Wutai amphibolites indicates four metamorphic evolutionary stages. The M₁ assemblage is composed of plagioclase+quartz+actinolite+chlorite+epidote+biotite+rutile, preserved as mineral inclusions in garnet porphyroblasts. The metamorphic conditions for this assemblage cannot be quantitatively estimated. The M₂ stage is represented by garnet porphyroblasts in a matrix of quartz, plagioclase, amphibole, biotite, rutile and ilmenite. P–T conditions for this assemblage have been estimated using the program Tweequ at 10–12 kbar and 600–650°C. The M₃ assemblage is shown by amphibole+plagioclase±ilmenite symplectic coronas around embayed garnets and yields P–T conditions of 6.0–7.0 kbar and 600–650°C. M₄ is represented by chlorite and epidote rimming garnet, chlorite rimming amphibole and epidote replacing plagioclase under greenschist-facies conditions of 400–500°C and relatively lower pressures. Taken together, the qualitative P–T estimates from M₁ and M₄ and the quantitative P–T estimates from M₂ and M₃ define a clockwise P–T path for the Wutai amphibolites.The estimated P–T path from the four stages suggests that the Wutai Complex underwent initial burial and crustal thickening (M₁+M₂), subsequent isothermal exhumation (M₃), and finally cooling and retrogression (M₄). This tectonothermal path, along with those of the Fuping and Hengshan complexes, which bound the southeast and northwest margins, respectively, of the Wutai Complex, is considered to record the early Paleoproterozoic collision between the eastern and western segments of the North China craton.