Search

Many studies dealing with the synthesis of tourmaline report a sharp intragranular chemical gradient and extensive porosity in the core zones of the crystals, both of which still lack a reliable explanation. Using the example of olenitic tourmaline, we show that these features are likely explained by the occurrence of a precursor phase during tourmaline formation. Time-dependent piston–cylinder synthesis experiments were performed in the system SiO2–Al2O3–B2O3–NaCl–H2O at 700 °C/40 kbar with run durations of 0.5 h, 2.5 h and 216 h, starting from quartz–Al2O3–H3BO3 solid mixtures and NaCl solutions. Sharply zoned olenitic tourmaline ([4]B-rich cores, [4]B-poor rims) formed in all experiments, with its abundance increasing with increasing run duration. The amount of the additional solid product phases coesite and jeremejevite decreased with time. Extensive porosity is recognized in jeremejevite and in the cores of early grown acicular tourmaline. Textural relationships indicate that olenitic tourmaline grows at the expense of jeremejevite which acts as a crystalline precursor in this system. A possible reaction is: ![Formula][1] The transformation likely proceeds via a dissolution/re-precipitation mechanism, which triggers the sharp chemical zonation in olenite. Based on Rayleigh fractionation modelling, we estimate a minimum B isotope fractionation between jeremejevite and fluid with Δ11Bjer-fluid ≈ −2.8‰ at 700 °C/40 kbar. Due to the progressive dissolution of jeremejevite, the fluids δ11B values continuously decrease with increasing run duration. Hence, olenite growth concomitant with the dissolution of jeremejevite will produce scattered or inverse boron isotope patterns (heavy cores, light rims) in tourmaline, which cannot result from simple Rayleigh fractionation. Similar reactions involving jeremejevite or other precursor phases might explain chemical zonation and porous textures in tourmaline core zones reported in many experimental studies. The occurrence of natural tourmaline overgrowing jeremejevite in pegmatites of the Erongo Mountains, Namibia, gives rise to the assumption that jeremejevite might also act as a precursor for tourmaline formation in natural systems. [1]: /embed/mml-math-1.gif

Due to the similar ionic radius of K + and NH 4 + , K-silicates can incorporate a significant amount of NH 4 . As tourmaline is able to accommodate K in its crystal structure at high and ultrahigh pressure, we test if this also holds true for NH 4 . Piston-cylinder experiments in the system (NH 4 ) 2 O-MgO-SiO 2 -Al 2 O 3 -B 2 O 3 -H 2 O at 4.0 GPa, 700 °C, with B 2 O 3 and NH 4 OH in excess produce an assemblage of tourmaline, phengite, and coesite. The tourmaline crystals are up to 10 × 40 μm in size. EMP analyses indicate that the tourmalines contain 0.22 (±0.03) wt% (NH 4 ) 2 O and are solid solutions mainly along the magnesio-foitite and “NH 4 -dravite” join with the average structural formula X [(NH 4 ) 0.08(1) □ 0.92(1) ] Y [Mg 2.28(8) Al 0.72(8) ] Z [Al 5.93(6) Si 0.07(6) ] T [Si 6.00(5) O 18 ](BO 3 ) 3 (OH) 4 . NH 4 incorporation is confirmed by characteristic stretching and bending modes in the IR-spectra of single crystals on synthetic tourmaline. Further evidence is the increased unit-cell parameters of the tourmaline [ a = 15.9214(9) A, c = 7.1423(5) A, V = 1567.9(2) A 3 ] relative to pure magnesio-foitite. Incorporation of NH 4 in natural tourmaline was tested in a tourmaline-bearing mica schists from a high- P /low- T (>1.2 GPa/550 °C) metasedimentary unit of the Erzgebirge, Germany, rich in NH 4 . The NH 4 -concentrations in the three main NH 4 -bearing phases are: biotite (~1400 ppm) > phengite (~700 ppm) > tourmaline (~500 ppm). This indicates that tourmaline can act as important carrier of nitrogen between the crust and the deep Earth, which has important implications for a better understanding of the large-scale light element cycle.

The cation exchange equilibria Muscovite + NH 4 Cl ⇔ Tobelite + KCl KAl 2 AlSi 3 O 10 ( OH ) 2 + NH 4 Cl aq ⇔ NH 4 Al 2 AlSi 3 O 10 ( OH ) 2 + KCl aq K - feldspar + NH 4 Cl ⇔ Buddingtonite + KCl KAlSi 3 O 8 + NH 4 Cl aq ⇔ NH 4 AlSi 3 O 8 + KCl aq are determined by hydrothermal experiments at 400, 500, 600 °C at 400 MPa, and piston cylinder experiments at 500, 600 °C at 1500 MPa along the entire K–NH4 compositional range. The composition of solid phases are determined by XRD, EMP and FTIR-methods, NH4–K ratios in coexisting 2–3 molal chloride solutions by ion chromatography. Muscovite–tobelite and K-feldspar–buddingtonite form complete solid solutions series at all conditions. Consistent phase relations and mass balances indicate equilibrium between solid solutions and fluids. In both the (K,NH4)-muscovite and (K,NH4)-feldspar-fluid systems, NH4 strongly fractionates into the fluid. In the range from 400 to 600 °C, 400 to 1500 MPa, fractionations are slightly temperature and pressure-dependent. The distribution coefficient K D solid - fluid = X NH 4 solid X K solid X K fluid X NH 4 fluid varies within the experimental range from 0.10 at XKbulk=0.81 to 0.46 at XKbulk=0.20 for muscovite-fluid, and from 0.05 at XKbulk=0.91 to 0.70 at XKbulk=0.15 for feldspar-fluid. NH4 is preferentially incorporated into feldspar relative to mica with KDfsp-ms ranging between 1.36 and 2.0 at 400 MPa, and between 1.13 and 1.5 at 1500 MPa as XKbulk varies from 0.15 to 0.89. Results are evaluated thermodynamically assuming equal mixing of NH4Cl and KCl in the fluid. Mixing energies of solids were estimated using multidimensional regression and a regular solution model. For the muscovite–tobelite equilibrium, values of Δs°=−11.7 J K−1 mol−1, Δv°=−2.1 J MPa−1 mol−1, and W=4.6 kJ mol−1, and for the K-feldspar–buddingtonite equilibrium, Δs°=−8.8 J K−1 mol−1, Δv°=−1.1 J MPa−1 mol−1, and W=5.4 kJ mol−1 were derived. Results are used to calculate partition coefficients D among phases at very low NH4 bulk concentrations. Between 400 and 600 °C, DNH4fluid-ms ranges from 7 to 8 at 400 MPa and is 5 at 1500 MPa. DNH4fluid-ms is 6 to 7 at 400 MPa, and about 5 at 1500 MPa. DNH4fsp-ms is between 1 and 1.2 at all conditions. Partition coefficients are valid for NH4 contents of up to several hundred ppm in mica and feldspar, well within the concentration range observed in many rocks. Combining the data with that of K–NH4-partitioning between phlogopite and fluid results in DNH4phl-ms≈3.5 and DNH4phl-ms≈3 at 550 °C, 200 to 400 MPa. NH4 concentrations in coexisting muscovite, biotite, and K-feldspar from a variety of rocks show near-equilibrium distributions. NH4–K-partitioning between major K-bearing minerals and saline fluids allows for assessment of the dehydration history of metamorphic rocks. During prograde metamorphism, water is progressively produced by dehydration reactions and expelled along rock's P–T path. Nitrogen is subsequently removed due to preferred fractionation of NH4 into fluids. The remaining NH4 is continuously redistributed among muscovite and biotite, and at higher grades, K-feldspar with biotite as the main solid phase carrier of ammonium. The large fractionation effect of NH4 among most phases highlights its potential as tracer of devolatilization processes and fluid–rock interactions.

The composition of synthetic amphiboles was investigated experimentally along the tremolite-tschermakite join in the system CaO-MgO-Al 2 O 3 -SiO 2 -H 2 O-Br 2 . Compositions of these amphiboles were studied within the phase assemblages: amphibole-anorthite-quartz-diopside (I), amphiboleanorthite-quartz-talc (II), amphibole-anorthite-quartz-enstatite (III), amphibole-anorthite-talcclinochlore (IV), amphibole-zoisite-talc-quartz (V), and amphibole-zoisite-talc-clinochlore (VI). Assemblages were synthesized from oxide-hydroxide mixtures in the presence of a CaBr 2 -bearing solution between 600-800 °C and 200-2000 MPa. Solid phases were investigated using SEM, HRTEM, EMP, and XRD techniques. EMP data show that the amphiboles produced are solid solutions of the ternary system tremolite-tschermakite-cummingtonite. Enstatite, diopside, talc, and clinochlore showed small deviations from their respective end-member compositions due to incorporation of some Al. The thermodynamic properties of the tschermakite end-member and the mixing properties along the tremolite-tschermakite join were extracted from corresponding exchange reactions of the unreversed synthesized phase assemblages I-VI. Various ideal mixing models were tested for Al-Mg and Al-Si substitution at octahedral M2 and M3 sites and at tetrahedral T1-sites. Best fits were obtained for a two-site coupled model, resulting in Δ f H 0 t s =-12528.3 ′ 11.7 kJ/mol and S 0 t s = 556.5 ′ 12.0 J/(mol.K) for the tschermakite end-member. Similar calculations were carried out for magnesiohornblende, and values of Δ f H 0 M g h b =-12418.7 ′ 5.9 kJ/mol and S 0 M g h b = 562.8 ′ 6.1 1 J/(mol.K) were extracted. Calculated phase relations and amphibole compositions agree well with experimental data if the derived thermodynamic data of tschermakite and a two-site mixing model for Al incorporation in amphibole solid solutions are applied.

The distribution of Rb-Na and Rb-K between richterite and a 2-molal aqueous (Na, K, Rb)-chloride solution has been investigated with hydrothermal experiments at 800∘C and 200 MPa. Experiments were performed as syntheses in which amphiboles grew in the presence of an excess fluid containing the exchangeable cations Na+-Rb+ or Na+-K+-Rb+. The obtained amphiboles were large enough (up to 20 m in width) for reliable EMP analysis. They were chemically homogeneous and HRTEM investigations showed that they were structurally well ordered. The Rb, Na, K, Ca and Mg concentrations in coexisting fluids were measured by ICP-AES. According to the possible incorporation of Na, K and Rb on the A-site, solid solutions in the ternary Na(NaCa) Mg5[Si8O22/(OH)2] (richterite)-K(NaCa)Mg5[Si8O22/(OH)2] (K-richterite)-Rb(NaCa)Mg5[Si8O22/(OH)2] (Rb-richterite) were expected. However, Rb-rich richterites always had significant amounts of A-site vacancy concentrations (X □ amph=□ A /(RbA+KA +NaA+□A) of up to 0.42 in the K-free (Na,Rb)-richterites and of up to 0.67 in the (Na, K, Rb)-richterites which corresponds to the same content of tremolite+cummingtonite-component. Amphiboles containing practically only Rb besides vacancies and no Na and/or K on the A-site were also synthesized, however. The Rb-Na and Rb-K exchange coefficients between fluid and richterites are similar. Rubidium always fractionated strongly into the fluid phase. For low Rb-concentrations in richterite (X Rb amph

At the Bufa del Diente contact-metamorphic aureole, brine infiltration through metachert layers embedded in limestones produced thick wollastonite rims, according to Cc+Qz=Wo+CO2. Fluid inclusions trapped in recrystallized quartz hosts include: (1) high salinity four phase inclusions [Th(V-L)=460–573° C; Td(salts)=350–400° C; (Na+K)Cleq=64–73 wt%; X CO 2≤0.02]; (2) low density vapour-rich CO2-bearing inclusions [Th(L-V)≈500±100° C; X CO 2=0.22–0.44; X NaCl≤0.01], corresponding to densities of 0.27± 0.05 gcm−3. Petrographical observations, phase compositions and densities show that the two fluids were simultaneously trapped in the solvus of the H2O-CO2-salts system at 500–600° C and 700±200 bars. The low density fluid was generated during brine infiltration at the solvus via the wollastonite producing reaction. Identical fluid types were also trapped as inclusion populations in wollastonite hosts 3 cm adjacent to quartz crystals. At room temperature, both fluid types additionally contain one quartz and one calcite crystal, generated by the back-reaction Wo+CO2=Cc+Qz of the host with the CO2-proportion of the fluid during retrogression. All of the CO2 was removed from the fluid. On heating in the microstage, the reaction progress of the prograde reaction was estimated via volume loss of the calcites. In vapour-rich fluids, 50% progress is reached at 490–530° C; 80% at 520–560° C; and 100% at 540–590° C, the latter representing the trapping temperatures of the original fluid at the two fluid solvus. The progress is volume controlled. With knowledge of compositions and densities from unmodified inclusions in quartz and using the equation of state of Duan et al. (1995) for H2O-CO2-NaCl, along with f CO 2-values extracted from it, the reaction progress curve was recalculated in the P-T-X-space. The calculated progress curve passes through the two fluid solvus up to 380° C/210 bars, continues in the one fluid field and meets the solvus again at trapping conditions. The P-T slope is steep, most of the reaction occurs above 450° C and there is high correspondence between calculated and measured reaction progress. We emphasize that with the exception of quartz, back-reactions between inclusion fluids and mineral hosts is a common process. For almost any prograde metamorphic mineral that was formed by a devolatilization reaction and that trapped the equilibrium fluid or any peak metamorphic fluid as an inclusion, a fluid-host back-reaction exists which must occur somewhere along the retrograde path. Such retrograde reactions may cause drastic changes in density and composition of the fluid. In most cases, however, evidence of the evolving mineral assemblages is not given for they might form submicroscopical layers at the inclusion walls.

The growth rate and internal organization of bimineralic diopside (CaMgSi2O6)-merwinite (Ca3MgSi2O8) reaction rims produced by a solid-state reaction between monticellite (CaMgSiO4) and wollastonite (CaSiO3) single crystals was determined at 900 °C, 1.2 GPa, and run durations from 5 to 65 h using conventional piston-cylinder equipment. Overall reaction rim thickness ranges from 3.8 to 20.9 μm and increases with the square root of time, indicating that rim growth is diffusion controlled. Symmetrical makeup of the internal microstructure implies that rims grow from the original interface toward both reactants at identical rates, indicating that diffusion of MgO across the rim controls overall growth, with an effective bulk diffusion coefficient of D bulk, MgODi+Mw = 10−16.3 ± 0.2 m2/s. At the initial stages, a “lamellar type” microstructure of alternating palisade-shaped diopside and merwinite grains elongated normal to the reaction front is generated, which gradually transforms into a “segregated multilayer type” microstructure with almost perfectly monomineralic Mw|Di|Mw layers oriented parallel to the reaction fronts at long run durations. This is due to changes in relative component mobilities. Whereas the “lamellar” microstructure develops when MgO is substantially more mobile than the other components, formation of the “segregated multilayer” microstructure requires additional mobility of at least one of the other components, CaO or SiO2. We assume that a significant change in relative component mobilities is caused by continuous entrance of minute amounts of water from the piston-cylinder solid pressure medium through the capsule walls, as revealed by the presence of OH-defects in a reactant after the runs, and supported by water-containing powder experiments that only produce monomineralic Mw|Di|Mw layers. Traces of water have a major influence on relative component mobilities, internal rim organization, and the microstructural development of reaction zones.

In the inner aureole of the Bufa del Diente alkali syenite (north-east Mexico), thin calcareous argillite bands horizontally embedded in impure marbles acted as contact-metamorphic aquifers for hypersaline brines of magmatic origin. Thick-bedded marbles were largely impervious. From 180 m up to the intrusion contact, argillites were completely decarbonated, resulting in melilite + wollastonite + phlogopite + perovskite-bearing parageneses. In marbles, this assemblage is confined to a narrow 7-12-m-wide infiltration zone adjacent to the contact. Up to this distance, calcite + wollastonite + diopside + alkali feldspar + titanite was stable, indicating that the fluid evolution in these marbles was internally buffered. Brine infiltration from the metaargillite aquifer into the marbles occurred perpendicular to the marble-metaargillite boundaries and was confined to a zone 4-6 cm wide above the boundaries. This is documented by the three reactions Cc + Di = Mel + CO2, (1) Cc + Kfs + Di + H2O = Phl + Wo + CO2, (2) Cc + Ttn = Prv + Wo + CO2, (3) Melilites (Ak32-45Gh13-32Sm32-40 to Ak52-72Gh0-1Sm28-48) occur as rims around diopsides and become continuously thicker towards the metaargillite beds. Fluid inclusion observations suggest that the infiltrating brine was hypersaline (NaCl + KClcq∼ 65 wt%) and that the reactions took place at the water-rich side of the H2O-CO2-salts immiscibility field at about 600d C (2, 3) and 660 to 680d C (1) at P∼ 1200 bar and Xco2∼ 0.02. Mass balance calculations show that the amount of brine infiltrated from the aquifer into the marble was very low and decreased continuously with increasing distance from the boundary. The maximum width of brine infiltration was about 6 cm. This confirms that brine flow was largely parallel to the aquifer, not perpendicular to it. The CO2 produced by the decarbonation reactions probably escaped as an immiscible low-density H2O-CO2 fluid of Xco2≤ 0.5 into overlying marble via grain-edge flow. The metaargillite-marble boundary acted as a semipermeable membrane 6 cm in thickness keeping back the brine in the aquifer and losing the in-situ produced low-density CO2-rich fluid.

At the San Luis Potosi (SLP) volcanic field (Central Mexico), Quaternary basanites and tuff breccias have sampled a suite of ultramafic xenoliths, predominately spinel lherzolites, spinel-olivine websterites, spinel pyroxenites, and hornblende-rich pyroxenites. Spinel lherzolites from the La Ventura maars have protogranular to equigranular textures, those from the Santo Domingo maars are strongly sheared. Both spinel-lherzolite types show similar whole-rock major and trace-element abundances. They are fertile to slightly depleted with mineralogical and geochemical heterogeneities induced by partial melting processes. Pyroxenites with either magmatic or metamorphic textures are high-pressure cumulates. Hornblende-rich pyroxenites are genetically linked to the host basanites. Most of the protogranular spinel lherzolites contain veinlets of glass along grain boundaries. These glasses are chemically homogeneous and have trachybasaltic to trachyandesitic compositions. Mg- and Fe2+-partitioning between olivine and glass suggests chemical equilibrium between the melts represented by the glasses and the spinel-lherzolite mineral assemblage at about 1,000°C and 10 to 15 kbar. The melts are interpreted to be of upper mantle origin. They may have been formed by in-situ partial melting in the presence of volatiles or represent percolating melts chemically buffered by the spinel-lherzolite mineral assemblage at uppermost mantle conditions. Mineral chemistry in all rock types of the whole xenolith suite reveals distinct disequilibrium features reflecting partial re-equilibration stages towards lower temperatures estimated to be from 1,050°C to 850°C at 9 to 15 kbar. The presence of similar zoning and exsolution features mainly documented in pyroxenes along with similar maximum and minimum temperatures requires all sampled xenoliths to have undergone the same temperature regime within the upper mantle. The sheared spinel lherzolites from the Sto. Domingo field are interpreted as formerly protogranular material which was sheared during uplift and cooling. The estimated mantle temperatures are higher than those predicted by low heat-flow measurements at the SLP fild, indicating that surface heat flow has not equilibrated to elevated temperatures at depth. This strongly supports a young perturbation event beneath the SLP area and connects the onset of uplift and cooling of the SLP-mantle segment with the back-arc extensional regime of the Quaternary volcanic cycle of the Transmexican Volcanic Belt.

The ammonium analogues of the high-pressure potassium-bearing silicate phases K-hollandite, K-Si-wadeite, K-cymrite, and phengite were synthesized in the system (NH 4 ) 2 O(-MgO)-Al 2 O 3 -SiO 2 -H 2 O [N(M)ASH] using multi-anvil and piston-cylinder equipment. Syntheses included NH 4 -hollandite (NH 4 AlSi 3 O 8 ) at 12.3 GPa, 700 °C; NH 4 -Si-wadeite [(NH 4 ) 2 Si 4 O 9 ] at 10 GPa, 700 °C; NH 4 -cymrite (NH 4 AlSi 3 O 8 ·H 2 O) at 7.8 GPa, 800 °C; and NH 4 -phengite [NH 4 (Mg 0.5 Al 1.5 )(Al 0.5 Si 3.5 )O 10 (OH) 2 ] at 4 GPa, 700 °C. Run products were characterized by SEM, FTIR, and powder XRD with Rietveld refinements. Cell parameters of the new NH 4 end-members are: a = 9.4234(9) A, c = 2.7244(3) A, V = 241.93(5) A 3 (NH 4 -hollandite); a = 6.726(1) A, c = 9.502(3) A, V = 372.3(1) A 3 (NH 4 -Si-wadeite); a = 5.3595(3) A, c = 7.835(1) A, V = 194.93(5) A 3 (NH 4 -cymrite). NH 4 -phengite consisted of a mixture of 1 M , 2 M 1 , 2 M 2 , 3 T , and 2 Or polytypes. The most abundant polytype, 2 M 1 , has cell dimensions a = 5.2195(9) A, b = 9.049(3) A, c = 20.414(8) A, β = 95.65(3)°, V = 959.5(5) A 3 . All unit-cell volumes are enlarged in comparison to the potassium analogues. Substitution of NH 4 for K does not cause changes in space group. NH 4 incorporation was confirmed by the appearance of NH 4 -vibration modes ν 4 and ν 3 occurring in the ranges of 1397–1459 and 3223–3333 cm −1 , respectively. Ammonium in eclogite facies metasediments is mainly bound in micas and concentrations may reach up to a few thousand parts per million. It can be stored to greater depths in high-pressure potassium silicates during ongoing subduction. This possibly provides an important mechanism for nitrogen and hydrogen transport into the deeper mantle.

Experiments in the system K 2 O-(NH 4 ) 2 O-Al 2 O 3 -SiO 2 -H 2 O-HCl have been conducted between temperatures of 400 and 600 °C at pressures of 200, 400, 500, and 1500 MPa. The run products consisted of solid solutions of the K-feldspar–buddingtonite, muscovite–tobelite series, and quartz. The run products were characterized by electron microprobe, powder X-ray diffraction with Rietveld analysis, and infrared spectroscopy. It can be shown that complete solid solution exists for both series for temperatures at least above 400 °C. The grain size of the synthesized phases was usually small and rarely exceeded 10 μm. In addition the micas were quite thin and the thickness was lower than 100 nm. Due to the small grain size and the (NH 4 ) 2 O-content, reliable electron-microprobe results were difficult to obtain. Rietveld analyses did show that the synthesized micas consisted of a complex mixture of 1 M, 2 M 1 , 2 M 2 , 3 T, and 2 Or polytypes. The lattice parameters of the feldspar and mica solid solutions are linear combinations of the end-members and show practically no excess volume. Reasonable compositions of the solid solutions can be derived from the lattice parameters. The mica solid solutions are not binary mixtures but at elevated pressure increasing amounts of pyrophyllite component were observed. By using the NH 4 -deformation mode at around 1430 cm −1 and the OH-stretching vibrations ν OH at 3600–3700 cm −1 the NH 4 -contents of the micas could be determined also by IR spectroscopy.

Detailed melt and fluid inclusion studies in quartz hosts from the Variscan Ehrenfriedersdorf complex revealed that ongoing fractional crystallization of the highly evolved H2O-, B-, and F-rich granite magma produced a pegmatite melt, which started to separate into two immiscible phases at about 720°C, 100 MPa. With cooling and further chemical evolution, the immiscibilty field expanded. Two conjugate melts, a peraluminous one and a peralkaline one, coexisted down to temperatures of about 490°C. Additionally, high-salinity brine exsolved throughout the pegmatitic stage, along with low-density vapor. Towards lower temperatures, a hydrothermal system gradually developed. Boiling processes occurred between 450 and 400°C, increasing the salinities of hydrothermal fluids at this stage. Below, the late hydrothermal stage is dominated by low-salinity fluids. Using a combination of synchrotron radiation-induced X-ray fluorescence analysis and Raman spectroscopy, the concentration of trace elements (Mn, Fe, Zn, As, Sb, Rb, Cs, Sr, Zr, Nb, Ta, Ag, Sn, Ta, W, rare earth elements (REE), and Cu) was determined in 52 melt and 8 fluid inclusions that are representative of distinct stages from 720°C down to 380°C. Homogenization temperatures and water contents of both melt and fluid inclusions are used to estimate trapping temperatures, thus revealing the evolutionary stage during the process. Trace elements are partitioned in different proportions between the two pegmatite melts, high-salinity brines and exsolving vapors. Concentrations are strongly shifted by co ncomitant crystallization and precipitation of ore-forming minerals. For example, pegmatite melts at the initial stage (700°C) have about 1,600 ppm of Sn. Concentrations in both melts decrease towards lower temperatures due to the crystallization of cassiterite between 650 and 550°C. Tin is preferentially fractionated into the peralkaline melt by a factor of 2–3. While the last pegmatite melts are low in Sn (64 ppm at 500°C), early hydrothermal fluids become again enriched with about 800 ppm of Sn at the boiling stage. A sudden drop in late hydrothermal fluids (23 ppm of Sn at 370°C) results from precipitation of another cassiterite generation between 400 and 370°C. Zinc concentrations in peraluminous melts are low (some tens of parts per million) and are not correlated with temperature. In coexisting peralkaline melts and high-T brines, they are higher by a factor of 2–3. Zinc continuously increases in hydrothermal fluids (3,000 ppm at 400°C), where the precipitation of sphalerite starts. The main removal of Zn from the fluid system occurs at lower temperatures. Similarly, melt and fluid inclusion concentrations of many other trace elements directly reflect the crystallization and precipitation history of minerals at distinctive temperatures or temperature windows.

The 31.6 ± 0.3 Ma old Bufa del Diente alkali-syenite (NE Mexico) intruded a sequence of Cretaceous limestones with intercalated sub-horizontal chert layers. The cherts acted as aquifers that facilitated transport of brines and pegmatitic melts within the shallow-level ( Aqueous brines of magmatic origin produced thick wollastonite mantles around the chert layers. Wollastonite formation occurred at the expense of limestone and chert and generated CO2. This CO2 induced fluid unmixing into an aqueous brine and a low-density CO2-rich fluid, which was lost to the overlying marble where it oxidized organic matter and caused d13C and d18O shifts in a zone some 5 to 10 cm wide. After wollastonite formation, the chert aquifers were locally intruded by pegmatite veins carrying alkali feldspar, quartz, aegirine-augite, eudialyte, zircon, and apatite. Aqueous fluids that exsolved during crystallization of the pegmatite veins escaped along late cross-fractures and migrated along the inner and outer borders of the wollastonite margins. Chemical dispersion patterns of U, Al, Na+K, P, S, Fe, and REE across the chert-to-marble boundary and its metasomatic rims are shown by autoradiography and neutron-induced radiography. Scavenging of cations at mineralogical contacts and cation transport into the marbles occurred only on the mm to cm scale. Isotopic data for Pb and Sr across a simple metachert-marble boundary and for Pb, Sr, Nd, B, and Li across a metachert-pegmatite-marble sequence demonstrate the following: (1) The Pb and Sr isotopic signature of early fluids was buffered by the carbonate wallrock. Only late fluids, shielded from wallrock interaction by a wollastonite mantle, variably preserved a memory of their initial magmatic signature. (2) Since the Nd isotope signature of marble and chert is bound to calcite and clay minerals, systematic shifts to unradiogenic Nd in marble reflect loss of carbonate-bound Nd as the wollastonite margin is approached. Nd in the wollastonite margin is dominated by Nd originally bound to clay minerals. The later emplacement of the pegmatite, which carried the Nd isotope signature of its alkali-syenite source, had little effect on the Nd isotopic composition of the wollastonite rim. (3) Although the Li and B isotopic compositions reflect the alkali-syenite source, they are also affected by isotopic fractionation and partitioning between melt, fluid, and solids.

Contributions to Mineralogy and Petrology, 147 (6), ISSN:0010-7999, ISSN:1432-0967

Phase equilibria in the system CaO-MgO-SiO2-CO2-H2O-NaCl are calculated to illustrate phase relations in metacarbonates over a wide range of P-T-X[H2O-CO2-NaCl] conditions. Calculations are performed using the equation of state of Duan et al. (1995) for H2O-CO2-NaCl fluids and the internally consistent data set of Gottschalk (1997) for thermodynamic properties of solids. Results are presented in isothermal-isobarical plots showing stable mineral assemblages as a function of fluid composition. It is shown that in contact-metamorphic P-T regimes the presence of very small concentrations of NaCl in the fluid causes almost all decarbonation reactions to proceed within the two fluid solvus of the H2O-CO2-NaCl system. Substantial flow of magma-derived fluids into marbles has been documented for many contact aureoles by shifts in stable isotope geochemistry of the host rocks and by the progress of volatile-producing mineral reactions controlled by fluid compositions. Time-integrated fluid fluxes have been estimated by combining fluid advection/dispersion models with the spatial arrangement of mineral reactions and isotopic resetting. All existing models assume that minerals react in presence of a single phase H2O-CO2 fluid and do not allow for the effect that fluid immiscibility has on the flow patterns. It is shown that fluids emanating from calc-alkaline melts that crystallize at shallow depths are brines. Their salinity may vary depending mainly on pressure and fraction of crystallized melt. Infiltration-driven decarbonation reactions in the host rocks inevitably proceed at the boundaries of the two fluid solvus where the produced CO2 is immiscible and may separate from the brine as a low salinity, low density H2O-CO2 fluid. Most parameters of fluid-rock interaction in contact aureoles that are derived from progress of mineral reactions and stable isotope resetting are probably incorrect because fluid phase separation is disregarded.

The reaction kinetics and fluid expulsion during the decarbonation reaction of calcite + quartz = wollastonite + CO2 at water-absent conditions were experimentally investigated using a Paterson-type gas apparatus. Starting materials consisted of synthetic calcite/quartz rock powders with variable fractions of quartz (10, 20, and 30 wt%) and grain sizes of 10 µm (calcite) and 10 and 30 µm (quartz). Prior to reaction, samples were HIPed at 700°C and 300 MPa confining pressure and varying pore pressures. Initital porosity was low at 2.7 to 5.9 %, depending on pore pressure during HIP and the amount and grain size of quartz particles. Samples were annealed at reaction temperatures of 900°C and 950°C at 150 MPa and 300 MPa confining pressures, well within the wollastonite stability field. Run durations were between 10 min and 20 hours. SEM micrographs of quenched samples show growth of wollastonite rims on quartz grains and CO2-filled pores between rims and calcite grains and along calcite grain boundaries. Measured widths of wollastonite rims vs time indicate a parabolic growth law. The reaction is diffusion-controlled and reaction progress and CO2 production are continuous. Porosity increases rapidly at initial stages of the reaction and attains about 10 to 12% after a few hours. Permeability at high reaction temperatures is below the detection limit of 10-21 m2 and not affected by increased porosity. This makes persistent pore connectivity improbable, in agreement with observed fluid inclusion trails in form of unconnected pores in SEM micrographs. Release of CO2 from the sample was measured in a downstream reservoir. The most striking observation is that fluid release is not continuous but occurs episodic and in pulses. Ongoing continuous reaction produces increase in pore pressure, which is, once having attained a critical value (Pcrit), spontaneously released. Connectivity of the pore space is short-lived and transient. The resulting cycle includes pore pressure build-up, formation of a local crack network, pore pressure release and crack closure. Using existing models for plastic stretching and decrepitation of pores along with critical stress intensity factors for the calcite matrix and measured pore widths, it results that Pcrit is about 20 MPa. Patterns of fluid flow based on mineralogical and stable isotope evidence are commonly predicted using the simplifying assumption of a continuous and constant porosity and permeability during decarbonation of the rock. However, simple flow models, which assume constant pore pressure, constant fluid filled porosity, and constant permeability may not commonly apply. Properties are often transient and it is most likely that fluid flow in a specific reacting rock volume is a short-lived episodic process.

Diffusion-controlled growth rates of polycrystalline enstatite reaction rims between forsterite and quartz were determined at 1,000 °C and 1 GPa in presence of traces of water. Iron-free, pure synthetic forsterite with normal oxygen and silicon isotopic compositions and quartz extremely enriched in 18O and 29Si were used as reactants. The relative mobility of 18O and 29Si in reactants and rims were determined by SIMS step scanning. The morphology of the rim shows that enstatite grows by a direct replacement of forsterite. Rim growth is modelled within a mass-conserving reference frame that implies advancement of reaction fronts from the initial forsterite–quartz interface in both directions. The isotopic compositions at the two reaction interfaces are controlled by the partial reactions Mg2SiO4 = 0.5 Mg2Si2O6 + MgO at the forsterite–enstatite, and MgO + SiO2 = 0.5 Mg2Si2O6 at the enstatite–quartz interface, implying that grain boundary diffusion of MgO is rate-controlling. Isotopic profiles show no silicon exchange across the propagating reaction interfaces. This propagation, controlled by MgO diffusion, is faster than the homogenisation of Si by self-diffusion behind the advancing fronts. From this, and using D , at dry conditions from the literature, results a D′ Si,En δ value of 3×10–24 m3 s–1 at 1,000 °C. The isotopic profiles for oxygen are more complex. They are interpreted as an interplay between the propagation of the interfaces, the homogenisation of the isotope concentrations by grain boundary self-diffusion of O within the rim, and the isotope exchange across the enstatite–quartz interface, which was open to 18O influx from quartz. Because of overlapping diffusion processes, boundary conditions are unstable and D Ox,En δ cannot be quantified. Using measured rim growth rates, the grain boundary diffusivity D MgO δ of MgO in iron-free enstatite is 8×10–22 m3 s–1 at 1,000 °C and 1 GPa. Experiments with San Carlos olivine (fo92) as reactant reveal lower rates by a factor of about 4. Our results show that isotope tracers in rim growth experiments allow identification of the actual interface reactions, recognition of the rate-controlling component and further calculation of D´δ values for specific components.

The Sr and Pb isotopes from the 31.6 ± 0.3 Ma (2σ) old Diente del Bufa alkali syenite, northeastern Mexico, and marbles of its contact aureole were used to trace the sources and the mobility of these metals during hydrothermal activity. Chert layers form aquifers within the marbles. The marbles represent aquitards. During fluid-wallrock reaction, the chert layers developed wollastonite rims. Early wollastonite rims have Sr and Pb isotopic compositions similar to those of their immediate host marbles, which indicates that the isotopic composition of Sr and Pb is initially buffered by the marble. Later wollastonite and other replacement minerals rimming the aquifer have Sr and Pb isotopic compositions that carry with time increasingly larger contributions from the high-salinity magmatic brine. The Sr and Pb contributions from the alkali syenite can be traced isotopically for more than 90 m away from the contact of the intrusion. In contrast, Sr and Pb originating from the alkali syenite are traceable within the marbles only for 3 to 5 cm from the aquifer-marble boundary. This distance is comparable to the spatial distribution of isotopic alterations of C and O implying that Sr and Pb were transported into the marbles through a fluid phase. The isotopic variation of Sr, Pb, C, and O across the aquifer-marble profiles reflects infiltration as a transport mechanism rather than diffusion. Because Sr and Pb are minor components in both the infiltrating fluid and the rock and because their concentrations are strongly affected by the distribution coefficients among the solid phases present, there is little correlation between the isotopic compositions of the trace elements Sr and Pb and those of C and O, which are major components in fluid and rock. Very thin meta-argillite rinds at the outer margin of the aquifer represent residual material after the dissolution of calcite. They are distinctly enriched in Rb, Sr, and U. The Rb and Sr are to some extent residual from the original limestone mineralogy, whereas U is dominantly derived from the magmatic fluid and leaked from the aquifer with the escaping immiscible CO2-rich H2O-CO2 fluid that was produced by decarbonation. The 238U/204Pb values ranging from 100 to 250 and distinctly lowered Th/U in the meta-argillite rims (1) demonstrate that U was transported with the magmatic fluid along the aquifer and (2) imply that during unmixing of the highly saline magmatic fluid U fractionated into the CO2-rich H2O-CO2 fluid from which it precipitated selectively in the meta-argillite band across the aquifer. Radioautographs demonstrate that the upper meta-argillite rim has 20 to 40 times more U than the lower rim, which implies that 20 to 40 times more CO2-rich H2O-CO2 fluid has left through the upper aquifer contact.

The cation exchange equilibrium has been investigated by hydrothermal experiments at 700 and 800°C at 200 MPa. To avoid equilibration problems of conventional exchange experiments, we synthesized amphiboles with an excess fluid allowing exchange between solid and fluid during the experiment. The exchangeable cations Na and K were provided as excess 1 to 2n chloridic solution. These exchange syntheses can be described by the reaction equation with (aq) for hydroxides and chlorides in aqueous solutions and ( s ) and ( p ) = start and product fluid. The amphiboles grew in presence of the exchange fluid and adjusted their stoichiometry in equilibrium with the fluid phase. The solid products consist of more than 99% amphibole (Na,K-richteritess) with traces of diopside and quartz. The amphiboles are up to 1 mm long and often ≈ 40 μm thick. Detailed EMP- and HRTEM-observations show that they are chemically homogeneous and structurally wellordered. The experimental results give consistent phase relations in the reciprocal ternary system Na-richterite–K-richterite–NaCl–KCl. We analysed the product fluid with AAS- and ICP-methods. The Na-K distribution coefficients between fluid and amphiboles of the richterite–K-richterite join are close to unity at 700°C and 800°C at 200 MPa. Small systematic deviations are explained by a symmetric solution model for the A-position of the amphiboles. Using ideal mixing for H2O-NaCl-KCl fluids, a mixing model for the system richterite–K-richterite is presented. We suggest that the composition of richterite solid solutions can be used as a sensor for NaCl/KCl-ratios in metamorphic fluids.

Impure limestones with interstratified metachert layers were contact metamorphosed and metasomatized by the Bufa del Diente alkali syenite. Massive marbles exhibit mineralogical and stable isotope evidence for limited fluid infiltration, confined to a 17 m wide zone at the contact. Influx of magmatic brines along most metacherts produced up to 4 cm thick wollastonite rims, according to calcite (Cc)+quartz (Qz)= wollastonite (Wo)+CO2, and were observed at distances of up to 400 m from the contact. The produced CO2 exsolved as an immiscible low density CO2-rich fluid. Chert protolith isotope compositions were δ18O (Qz)=27–30%. and δ18O (Cc)=24–27%.. Many wollastonites in infiltrated metacherts have low δ18O ranging from 11–17‰ and confirm that decarbonation occurred in presence of a magmatic-signatured fluid. Large gradients in δ18O (Wo) across the rims may reach 6‰ The δ18O of remaining quartz is often lowered to 15–20‰ whereas caleites largely retained their original compositions. The isotopic reversals of up to 10‰ between quartz and calcite along with reaction textures demonstrate non-equilibrium between infiltrating fluid in the aquifer and the assemblage calcite+quartz+wollastonite. This is compatible with the assumption of a down-temperature flow of magmatic fluids that occurred exclusively in the remaining quarzite layer. The δ13C (Cc) and δ18O (Cc) of marble calcites measured perpendicular to two metachert bands reveal significant isotopic alterations along distances of 4.5 cm and 7.5 cm from the wollastonite-marble boundary only into the hanging wall marble, suggesting an advection process caused by a fluid phase which movel upwards. Covariation trends of δ13C (Cc) and δ18O (Cc) across the alteration front indicate that this fluid was CO2-rich. Mass balance calculations show that all CO2-rich fluid produced by the decarbonation reaction was lost into overlying marble. The metachert aquifers did not leak with respect to water-rich fluids.

Quaternary volcanics from San Luis Potosi (SLP) in central Mexico sampled a xenolith suite comprising uppermantle nodules (spinell lherzolites, hornblende pyroxenites, spinel pyroxenites) and lower-crustal xenoliths. The latter consist of granulite-facies, mafic to intermediate metaigneous rocks and high-grade metasediments. Precursors of the metaigneous suite range from gabbroic to tonalitic compositions. Whole-rock REE patterns permit classification of metaigneous rocks into cumulates and fractionated melts. Mafic xenoliths consist of plagioclase ± clinopyroxene ± orthopyroxene ± garnet ± amphibole ± apatite ± rutile ± scapolite. Garnet has reacted extensively with clinopyroxene, forming corona textures of secondary plagioclase, orthopyroxene and amphibole. Intermediate xenoliths are garnet-free and quartz-bearing. All samples display similar CaAl zoning patterns documented in orthopyroxene, clinopyroxene and plagioclase which are, along with the presence of coronas around garnet, interpreted as representing partial re-equilibration stages from 940 ± 60°C and 7–11 kbar down to lower temperatures and pressures. There is no indication of a secondary heating event in these xenoliths. SmNd isotopic compositions of the spinel lherzolites indicate a heterogeneous composition of the mantle segment beneath SLP. Similar Sr- and Nd-isotopic signatures reveal that the hornblende pyroxenites are high-pressure cumulates linked to the host basanites. Four mainly intermediate metaigneous granulites from a single vent, which are free of secondary amphibole, and two spinel pyroxenites, regarded as the most primitive member of the metaigneous suite, define a regression line in a SmNd isochron diagram, yielding an age value of 1248 ± 69 Ma and an initial ϵNd-value of +2.3 Metaigneous xenoliths containing secondary amphibole plot significantly above the regression line, which is probably due to the introduction of externally derived Nd during amphibole formation under retrograde p-T conditions. Discussing the geological significance of the calculated age value, there are some arguments for interpreting this age as representing the intrusion age of the magmatic precursors. This gives evidence for formation of lower crust of basic to intermediate compositions during the mid-Proterozoic, now situated beneath SLP.

The kinetics of the reaction of tremolite plus dolomite to give forsterite, calcite, CO2, and H2O were investigated by powder experiments at 500 MPa. In the first group of experiments, the mole fraction of CO2( $$X_{\operatorname{CO} _2 } $$ ) of the carbon dioxide-H2O mixture (present in all runs) lay in the range of 0.60–0.69. At this $$X_{\operatorname{CO} _2 } $$ -value, three series of experiments were performed at temperatures of 750°, 730°, and 720° C. The equilibrium temperature at 500 MPa and $$X_{\operatorname{CO} _2 } $$ is 690°±10° C. Throughout the second group of experiments, the $$X_{\operatorname{CO} _2 } $$ was 0.10–0.18 and the run temperatures of 3 series of experiments were 670°, 660°, and 650° C. The equilibrium temperature at 500 MPa and $$X_{\operatorname{CO} _2 } $$ is 620°±10° C. For each of the 6 run temperatures, conversion/time curves were determined. The group of runs performed in the presence of the CO2-rich fluid with a maximum conversion at about 55% consistently revealed smaller rates of reaction than the runs with the H2O-rich fluid. By contrast, total conversion was obtained in the H2O-rich experiments. The different maximum conversions observed in the two groups of experiments can be explained by the mechanisms of forsterite and calcite growth, which are a function of the CO2 concentration of the fluid. The time dependence of the experimentally determined reaction conversion U can be described by the simple equation dU/dt=k if one additionally takes into account the decrease of the dolomite surface and the approach of equilibrium conditions during the course of the experiments. Assuming that the reaction takes place far from equilibrium, the rate constants k of the overall reaction found for the conditions of the run experiments are given by the following two equations: $$\operatorname{l} \operatorname{n} k_1 = \frac{{610897(J)}}{{RT}} + 63.780{\text{ }}[X_{\operatorname{CO} _2 } = 0.60 - 0.69]$$ $$\operatorname{l} \operatorname{n} k_2 = \frac{{582824(J)}}{{RT}} + 67.687{\text{ }}[X_{\operatorname{CO} _2 } = 0.10 - 0.18]$$ The apparent activation energies are 611±150kJ ( $$X_{\operatorname{CO} _2 } $$ =0.6) and 583±170 kJ ( $$X_{\operatorname{CO} _2 } $$ =0.1). Allowing a maximum influence on kinetics due to approach to equilibrium, which is strongly dependent of the formulation of the mass-dependent ΔG of the reaction, the apparent activation energies are reduced to minimum values of 429 kJ ( $$X_{\operatorname{CO} _2 } $$ =0.6) and 409 kJ ( $$X_{\operatorname{CO} _2 } $$ =0.1). The rate data determined were applied to contact metamorphism by combining the experimental rate data with the heating rate of 1° C in 50 years. Assuming that at $$X_{\operatorname{CO} _2 } $$ =0.1 for nucleation of forsterite and calcite, the equilibrium temperature has to be overstepped by 10° C, the reaction is completed after 30 years under unfavourable conditions (dolomite grains = 10 mm and n=0.1), and the total overstepping of the equilibrium temperature during the conversion of 100% is 11° C.