Wilhelm Heinrich

Experimental resetting of the U–Th–Pb systems in monazite

Abraded fragments (200–400 µm) of a large, chemically homogeneous, and non-metamict Brazilian monazite crystal, characterised by a concordant U–Pb ages of 474 +/- 1 Ma (208Pb/206Pb = 19.5), were hydrothermally treated at varying temperatures with solutions of different compositions. Product monazites were analysed with Scanning Electron Microscope (SEM), Electron Microprobe (EMP), Secondary Ion Mass Spectrometer (SIMS) and Isotope Dilution–Thermal Ionisation Mass Spectrometer (ID-TIMS). Experiments with pure water over a temperature range of 800–1200°C, at 700 MPa and durations ranging from 5 to 60 days showed that even at 1200°C any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain. Neither Pb diffusion at the EMP scale, nor significant discordancy were observed. We performed experiments at 800 and 1000°C for different durations using different fluid compositions at quartz saturation: a 10 wt.% CaCl2 fluid, a 10 wt.% SrCl2 fluid, a 10 wt.% NaCl fluid and a fluid containing NBS 982 Pb standard (208Pb/206Pb = 1). For all runs, EMP traverses revealed no Pb-diffusion profiles. Significant overgrowths of newly formed monazite are documented by SEM analyses. They occurred only in the 1000°C experiments when either CaCl2 or Pb-bearing fluids were present. In the CaCl2 experiment, two zones could be distinguished within the crystal: a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution/precipitation, which was enriched in Ca and Pb-free. ID-TIMS dating of single grains treated with SrCl2 and CaCl2 solutions at 1000°C are significantly discordant. Experiments employing the NBS Pb-standard produced sub-concordant monazite, for which the 207Pb/206Pb apparent age has become older than prior to the experiment (544 Ma at 800°C and 495 Ma at 1000°C). The newly grown monazite rim had obviously incorporated Pb from the fluid. None of our reaction products contained a detectable diffusion profile. The only resetting mechanism we detected involved dissolution/precipitation. The extent of the dissolution/precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks.

Monazite-xenotime thermobarometry; experimental calibration of the miscibility gap in the binary system CePO 4 -YPO 4

Abstract Solid solutions of (Ce5Y)PO4 were synthesized hydrothermally between 300 ℃ and 1000 ℃ at pressures of 2, 5, 10, and 15 kbar. Experiment products were analyzed by electron microprobe, and their lattice parameters were refined by Rietveld analysis of XRD powder patterns. Over a wide range of bulk compositions, two immiscible phases formed: monazite with a P21/n structure (REEO9 polyhedron) and xenotime with an I41/amd structure (REEO8 polyhedron).I Variations of the unit-cell dimensions are directly correlated with variations in composition. The boundaries of the asymmetric miscibility gap are strongly dependent on temperature. At 2 kbar and from 300-1000 ℃, maximum Y concentrations in monazite coexisting with xenotime increase from 3-16 mol%. With increasing pressure, this limb is shifted to higher Y contents so that at 15 kbar it ranges from 6 to about 25 mol% for the same temperature range. In contrast, Ce concentrations in xenotime do not exceed 3 mol% at 1000 ℃ over the entire pressure range. Our experimentally determined boundaries fit surprisingly well with empirical calibrations of the (LREE-HREE,Y)PO4-solvus boundaries derived from a suite of low pressure metapelites (3 to 5 kbar) that equilibrated at peak temperatures ranging from 400-700 °C. The behavior of the natural (REE5Y)PO4 system is probably well described by the simple (Ce5Y)PO4 binary, at least for metapelitic compositions. Many natural samples of monazite coexisting with xenotime show prograde growth zonation with respect to the incorporation of HREE + Y The combination of our thermometer along with U-Pb and Sm-Nd age determinations of high spatial resolution may thus provide information about prograde branches of metamorphic PTt paths.

Experimental determination of Thorium partitioning between monazite and xenotime using analytical electron microscopy and X-ray diffraction Rietveld analysis

The Thorium distribution between monazite and xenotime has been determined experimentally using the coupled substitution Th + Si=REE + P. Experiments have been conducted in standard cold seal hydrothermal and internally heated pressure vessels at 200MPa in the range of 600-1100°C. Starting mixtures were prepared from gels composed of equal amounts of CePO4 and YPO4 with addition of 10, 20 and 50 mol%ThSiO4. The grain sizes of the run products were in the range of a few microns. Analytical electron microscopy (AEM) methods were applied to obtain reliable chemical compositions of the reaction products. Lattice parameters of run products were determined using Rietveld analysis. For runs with 10 and 20 mol% ThSiO4 component in the bulk the ThSiO4 component distributes almost exclusively into monazite at all temperatures. The amount of the YPO4 component in monazite increases relative to the Th-free system if significant amounts of ThSiO4 are present within the structure. ThSiO4 favours incorporation of YPO4 resulting in a shift of the monazite limb and the shrinkage of the monazite-xenotime miscibility gap in the CePO4-YPO4-ThSiO4 ternary diagram. Thermometric calculations based on monazite-xenotime equilibria must be corrected for this effect. For runs with 50 mol% ThSiO4 in the bulk, thorite formed as an additional phase at 600 to 900°C but was absent at higher temperatures. At high Xbulk ThSiO4 and low T, the system is three-phase. The three-phase stability field strongly shrinks with increasing temperature. A tentative phase diagram of the ternary system CePO4-YPO4-ThSiO4 is proposed and discussed in the light of monazite-xenotime-thorite-bearing assemblages in natural rocks.

Composition and SmNd isotopic data of the lower crust beneath San Luis Potosí, central Mexico: Evidence from a granulite-facies xenolith suite

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.

High-pressure ammonium-bearing silicates: Implications for nitrogen and hydrogen storage in the Earth’s mantle

Abstract 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 (NH4)2O(-MgO)-Al2O3-SiO2- H2O [N(M)ASH] using multi-anvil and piston-cylinder equipment. Syntheses included NH4-hollandite (NH4AlSi3O8) at 12.3 GPa, 700 °C; NH4-Si-wadeite [(NH4)2Si4O9] at 10 GPa, 700 °C; NH4-cymrite (NH4AlSi3O8⋅H2O) at 7.8 GPa, 800 °C; and NH4-phengite [NH4(Mg0.5Al1.5)(Al0.5Si3.5)O10(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 NH4 end-members are: a = 9.4234(9) Å, c = 2.7244(3) Å, V = 241.93(5) Å3 (NH4-hollandite); a = 6.726(1) Å, c = 9.502(3) Å, V = 372.3(1) Å3 (NH4-Si-wadeite); a = 5.3595(3) Å, c = 7.835(1) Å, V = 194.93(5) Å3 (NH4-cymrite). NH4-phengite consisted of a mixture of 1M, 2M1, 2M2, 3T, and 2Or polytypes. The most abundant polytype, 2M1, has cell dimensions a = 5.2195(9) Å, b = 9.049(3) Å, c = 20.414(8) Å, β = 95.65(3)°, V = 959.5(5) Å3. All unit-cell volumes are enlarged in comparison to the potassium analogues. Substitution of NH4 for K does not cause changes in space group. NH4 incorporation was confirmed by the appearance of NH4-vibration modes ν4 and ν3 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.

Thermal history of the upper mantle beneath a young back-arc extensional zone; ultramafic xenoliths from San Luis Potosi, central Mexico

At the San Luis Potosí (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.

Contrasting fluid flow patterns at the Bufa del Diente contact metamorphic aureole, north-east Mexico: evidence from stable isotopes

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.

The alkaline Meidob volcanic field (Late Cenozoic, northwest Sudan)

The Meidob volcanic field (MVF) forms part of the Darfur Volcanic Province and developed from 7 Ma to 5 ka as indicated by K/Ar, thermoluminescence and 14C ages. It is situated in an uplifted high of the Pan-African basement, which consists of greenstones, high-grade gneisses and granites, and which is covered by Cretaceous sandstone. The MVF basaltic lavas, which originated from more than 300 scoria cones, formed a lava plateau of 50×100 km and up to 400 m thickness in the time between 7 and < 0.3 Ma. Young phonolitic mesa flows, together with rare trachyticbenmoreitic lava flows, trachytic pumice fallout deposits, ignimbrites and maars, form the central part of the field. The total amount of volcanic rocks is between 1400 and 1800 km3, with 98 vol.% being basaltic rocks, which results in an integrated magma output rate of ∼ 0.0002 km3 a−1. A combination of age data of the lavas with erosional features yields uplift rates for the Darfur Dome of ∼30 m Ma-1 in the MVF area. Magma was generated by 3–5% melting of predominantly asthenospheric mantle with a HIMU contribution. Fractionation of olivine, pyroxene, An-poor plagioclaseanorthoclase, magnetite and apatite leads to a differentiation from basanite to phonolite. Assimilation of crustal rocks near the top of the phonolitic upper crustal magma chambers - facilitated by volatile enrichment - produced magmas which gave way to benmoreitic and trachytic lavas, as well as to trachytic ignimbrites and pumice fallout deposits. Ultramafic cumulate xenoliths indicate the existence of major magma reservoirs at the crust-mantle boundary during MVF activity. Magma ascent occurred in a tensional regime, which changed its orientation at around 1 Ma. Early during MVF development, west-east and subordinately northeastsouthwest trending lineaments were active whereas volcanic activity younger than 1 Ma took place along northwest-southeast and northeast-southwest trending systems. The Central African Fault Zone, a transcontinental, lithospheric shear zone, played an important role for the rise of magmas in the Darfur Dome.

Ca-Sr distribution among amphibole, clinopyroxene, and chloride-bearing solutions

Abstract The distribution of Sr between a 1 M (Ca,Sr)Cl2 solution, (Ca,Sr)-tremolite and (Ca,Sr)-diopside was determined at 750 °C and 200 MPa. The synthesized crystals of (Ca,Sr)-tremolite (2000 × 30 μm) and (Ca,Sr)-diopside (1500 × 20 μm) were large enough for accurate electron microprobe analysis. The experimental results indicate that Ca2+ can be replaced completely by Sr2+ on the M4-site in tremolite and on the M2-site in diopside. The compositions of the product fluid were analyzed by atomic absorption spectroscopy. In both the (Ca,Sr)-tremolite-fluid and (Ca,Sr)-diopside-fluid systems, Sr strongly fractionated into the fluid. For bulk compositions having low Sr concentrations, mineral/fluid partition coefficients, DSrmineral/fluid, of 0.045 for (Ca,Sr)-tremolite/fluid and 0.082 for (Ca,Sr)- diopside/fluid were derived. The experimental results were evaluated thermodynamically assuming Henry’s law and simple mixing properties for SrCl2 and CaCl2 in the fluid. The mixing energies of the solids were calculated using a regular solution model. In the (Ca,Sr)-tremolite-(Ca,Sr)Cl2 system, ∆μ° is 59.0 kJ and WCaSramph = 9.8 kJ. In the system (Ca,Sr)-diopside-(Ca,Sr)Cl2 ∆μ° is 30.8 kJ and WCaSrpx is 11.7 kJ. The high ∆μ° values and, to a much lesser extent, the WCaSr values cause the strong fractionation of Sr into the fluid. The moderate values for WCaSramph and WCaSrpx strongly suggest that complete solid solution exists for (Ca,Sr)-tremolite and (Ca,Sr)-diopside at experimental run conditions. However, for the (Ca,Sr)-tremolite and (Ca,Sr)-diopside joins, limited miscibilities were calculated below 316 and 430 °C, respectively. The experimentally derived thermodynamic properties were used to determine Ca/Sr ratios of Srrich metasomatic fluids that penetrated a metaeclogite in Bjørkedalen, southwest Norway. The derived Ca/Sr ratios from amphibole-fluid equilibria are in good agreement with those calculated from plagioclase-fluid equilibria.

Trace-element analysis of individual synthetic and natural fluid inclusions with synchrotron radiation XRF using Monte Carlo simulations for quantification

The trace element composition of individual fluid inclusions was investigated by using synchrotron radiation X-ray fluorescence for analysis and Monte Carlo simulations for quantification. To validate the input parameters of the Monte Carlo simulation code for the applied spectrometer, area scan measurements on synthetic standard reference glasses (NIST 612 and NIST 610) were performed. Results yielded an accuracy of generally better than 20 % for all elements with Z between 37 and 92 at a standard deviation of less than 7 %. The analytical procedure was then applied to synthetic inclusions in quartz: single point analysis yielded an accuracy between 10 and 30 % with a reproducibility of ± 20 % and a repeatability of ± 7 % for the elements Cu, Rb, Sn and Cs. Ten trace elements (Mn, Fe, Cu, Zn, As, Rb, Nb, Sn, Sb, Cs) with concentrations between 2 and 10500 ppm were determined in fluid inclusions of hydrothermal quartz hosts from the Ehrenfriedersdorf Complex, Germany. Inclusions of three evolutionary stages of the Ehrenfriedersdorf Complex were studied. Vapour-rich inclusions of the pegmatite stage have low concentrations of all elements with Z > 20. Inclusions of the hydrothermal stage record progressive enrichment and differentia- tion in trace elements. Fluids of the early hydrothermal stage are enriched in Sn, Cu and As. Subsequently, they evolve to Fe-, Mn-, Zn- and Cs-rich fluids.