The partitioning behavior of Cl and F between apatite and sediment melt has been investigated by performing piston-cylinder experiments at 2.5 GPa, 800 degrees C using a hydrous experimental pelite staring material (EPSM) with similar to 7 wt% H2O and variable Cl (similar to 0, 500, 1000, 2000, or 3000 ppm) and F (similar to 0, 700, or 1500 ppm) contents, relevant for subduction zone conditions. Cl and F partitioning between apatite and melt is non-Nernstian, with DClAp-melt varying from 1.9-10.6 and DFAp-melt varying from 16-72. In contrast, Cl and F partition coefficients between phengite/biotite and melt (DClPhen-melt, DClBi-melt, DFPhen-melt, and DFBi-melt) were determined to be 0.24 +/- 0.01, 0.86 +/- 0.05, 1.4 +/- 0.1, and 3.7 +/- 0.4, respectively. The Nernstian partitioning of Cl and F between phengite/biotite and melt suggests ideal mixing of F, Cl, and OH in phengite, biotite, and melt. Exchange coefficients for F, Cl, and OH partitioning between apatite and melt were determined, with K-d(ClOHK)Ap-meltd =3D 19-49, K-dFOHK(Ap-melt) =3D 164-512, and Kd(F-Cl)(Ap-meltd) =3D 7-21. The evident variation of K-d values was attributed to non-ideal mixing of F, Cl, and OH in apatite. A regular ternary solution model for apatite was developed by modeling the variation of K-d values for experiments from this study and those from Webster et al. (2009) and Doherty et al. (2014). Positive values (similar to 15 to similar to 15 kJ/mol) obtained for Margules parameters W-Cl-OH(Ap), and W-F-OH(Ap) at low-pressure conditions (0.2 GPa, 0.05 GPa, and 900 degrees C) are in contrast to zero or negative values at 2.5 GPa, 800 degrees C. Based on a thermodynamic framework for F, Cl, and OH exchange between apatite and melt, using values for -Delta(r)G(Cl-OH)(o) (P,T), -Delta(r)G(F-OH)(o)(P, T), -Delta(r)G(F-Cl)(o)(P, T), W-Cl-OH(Ap), W-F-Cl(Ap), and W-F-OH(Ap) obtained through regression, F and Cl contents in melt can be derived from apatite compositions.
Viete, Daniel R.
Hermann, Joerg
Lister, Gordon S.
Stenhouse, Iona R.
In this paper we examine the length scales of major element diffusion in garnet during the Barrovian metamorphism. The role of diffusion in the flattening of Mn zoning profiles in garnet with increasing metamorphic grade across the Barrovian metamorphic series is ambiguous. However, the loss of distinct Mn-defined secondary compositional zoning in Barrovian garnets with increasing metamorphic grade and preservation of geochemical textures around Mn-free inclusions within high-grade Barrovian garnets provide robust evidence for c. 1000 mu m Mn diffusion in sillimanite-zone garnets during the Barrovian regional metamorphism. Sillimanite-grade garnets from the Barrovian metamorphic series also preserve c. 100 mu m Mn diffusion textures between sillimanite-grade rim domains and lower-grade cores. Bimodality in diffusion length scales requires bimodality in thermal time scales. Length scales of chemical diffusion are considered within the context of recent duration estimates for the Barrovian metamorphism of a few million years. We conclude that heat associated with the Barrovian regional metamorphism accumulated following numerous, short time-scale heating events responsible for the smaller-scale diffusion textures. This self-similar thermal regime was likely accommodated by the operation of shear zones in the highest-grade regions of the Barrovian metamorphic series, which concentrated small-scale episodic heat sources such as sheeted magmas, fluids and/or mechanical work.
Acosta-Vigil, Antonio
Buick, Ian
Hermann, Joerg
Cesare, Bernardo
Rubatto, Daniela
London, David
Morgan, George B.
To shed light on the mechanisms of crustal anatexis, a detailed geochemical study has been conducted on minerals and glasses of quenched anatectic metapelitic enclaves and their host peraluminous dacites at El Hoyazo, SE Spain. Anatectic enclaves, composed of plagioclase + biotite + sillimanite + garnet + glass +/- K-feldspar +/- cordierite + graphite, formed during the rapid heating and overstepped melting of a greenschist-facies metapelite, and finally equilibrated at 850 +/- 50 degrees C and 5-7 kbar. Glass appears as melt inclusions within all mineral phases and in the matrix of the enclaves, and has a major element composition similar to that of peraluminous leucogranites. Melt inclusions and matrix glasses have normative quartz-orthoclase-albite compositions that plot in the vicinity of H(2)O-undersaturated haplogranite eutectics. Melt inclusions show some compositional variability, with high Li, Cs and B, low Y, first row transition elements (FRTE) and rare earth elements (REE), and zircon and monazite saturation temperatures of similar to 665-750 degrees C. They are interpreted as melts produced by muscovite-breakdown melting reactions at the onset of the process of rapid melting and mostly under H(2)O-undersaturated conditions. Compared with melt inclusions, matrix glasses show less compositional variability, lower large ion lithophile element contents, higher Y, FRTE and REE, and higher zircon and monazite saturation temperatures (similar to 695-815 degrees C). They are interpreted as former melts recording the onset of biotite dehydration-melting. Matrix glasses in the dacite are compositionally different from glasses in the enclaves, hence the genetic connection between metasedimentary enclaves and dacite is not as straightforward as previous petrographic and bulk major element data suggest; this opens the possibility for some alternative interpretation. This study shows the following: (1) melt inclusions provide a window of information into the prograde evolution of anatexis in the enclaves; (2) melting occurred for the most part under H(2)O-undersaturated conditions even if, because of the rapid heating, the protolith preserved most of the structurally bound H(2)O contained at greenschist facies up to the beginning of anatexis, such that the excess H(2)O maximized the amount of H(2)O-undersaturated melt generated during anatexis; (3) although a large proportion of accessory minerals are currently shielded within major mineral phases, they have progressively dissolved to a considerable extent into the melt phase along the prograde anatectic path, as indicated by the relative clustering of accessory mineral saturation temperatures and closeness of these temperatures to those of potential melting reactions; (4) the dacite magma was probably produced by coalescence of melts derived from several compositionally distinct metasedimentary protoliths.
Gregory, Courtney J.
Buick, Ian S.
Hermann, Joerg
Rubatto, Daniela
High-pressure amphibolite-facies migmatitic orthogneisses from the Cockburn Shear Zone (CSZ), northern Musgrave Block in central Australia, were formed during the 580520 Ma intraplate Petermann Orogeny. The shear-zone hosted orthogneisses are of an intermediate bulk composition that promoted the growth of rare earth element (REE)-bearing major phases (garne and hornblende), as well as numerous accessory phases (zircon, titanite, apatite, epidote and allanite), all of which are potential UThPb geochronometers and are involved in the distribution of REEs. We have integrated petrology and detailed in situ trace element analysis of major and accessory phases in samples collected outside and inside the CSZ to establish the relative timing of metamorphic mineral growth. This paper presents one of the first applications of newly developed in situ dating protocols on metamorphic allanite. Sensitive high-resolution ion microprobe geochronology on metamorphic zircon and allanite indicate that metamorphism and partial melting occurred between 559 6 and 551 6 Ma. Peak temperatures of 720750C, determined from rutile included in garnet, necessitate the presence of fluids to flux partial melting in the CSZ quartzofeldspathic rocks. Metamorphic zircon formed during cooling in the presence of melt near the granitic wet solidus at T 700C. In contrast, allanite formed at different stages of the CSZ PT path: (1) as a prograde sub-solidus phase (T 650C) formed in the presence of fluids, and (2) as melt-precipitated Th- and REE-rich overgrowths on pre-existing allanite. The ages of the two growth episodes are not isotopically resolvable by allanite dating. Trace element compositions indicate that in both melted and unmelted rocks, garnet and hornblende growth was primarily controlled by prograde sub-solidus hydration reactions that consumed feldspar below the metamorphic peak. REE compositions of the metamorphic zircon and allanite overgrowths that formed in the presence of melt also suggest disequilibrium with garnet. Thus, the major period of garnet and hornblende growth was not coeval with partial melting.
Sambridge, Malcolm
Gerald, John Fitz
Kovacs, Istvan
O'Neill, Hugh St. C.
Hermann, Joerg
A new approach to the use of spectroscopic absorbance measurements for anisotropic crystals allows results to be extracted using unpolarized light incident on random crystal orientations. The theory of light propagation in anisotropic absorbing crystals is developed from Maxwell's equations to devise an expression for the transmittance of linearly polarized light traveling in an arbitrary direction in weakly absorbing media. This theory predicts the distribution of transmittance and absorbance as a function of direction and polarization angle of incident light. It is shown how a previously deduced empirical expression, commonly used in infrared spectroscopy, is a good approximation to the full theory under a wide range of conditions. The new theory shows that principal polarized absorbances correspond to the eigenvalues of an absorbance ellipsoid. An expression is derived for the unpolarized absorbance as a function of the angles describing incident light direction, A(unpol)(phi,psi), and the principal polarized absorbances, A(a), A(b), A(c) in an anisotropic crystal A(unpol)(phi,psi) = 1/2[A(a)(cos(2)phi cos(2)psi + sin(2)psi) + A(b)(cos(2)phi sin(2)psi + cos(2)psi) + A(c) sin(2)phi]. Integration of this expression over all incident angles leads to a simple relationship between total measured unpolarized absorbance and the three principal polarized absorbances. Using this theory, a procedure is proposed for estimating both total (A(a) + A(b) + A(c)) and principal absorbances from spectroscopic measurements of absorbance using unpolarized light on a set of randomly oriented crystals.
Marocchi, Marta
Hermann, Joerg
Tropper, Peter
Bargossi, Giuseppe Maria
Mair, Volkmar
Ultramafic rocks in contact to gneisses in the Mt Hochwart HP melange (Eastern Italian Alps) preserve a series of metasomatic mineral zones A phlogopitite with minor tremolite and accessory zircon and apatite forms close to the gneiss (Zone 1). Zone 2 consists of tremolite, phlogopite and anthophyllite followed by Zone 3 with anthophyllite plus minor chlorite and talc. Zone 3 grades into an amphibole-garnet peridotite lens This reaction zone has been generated by infiltration of hydrous fluids at T of 660-700 degrees C and P<1 2 GPa, which occurred during exhumation of coupled continental crust and mantle peridotites. The reaction zone between a trace element-rich (gneiss) and a trace element depleted reservoir (peridotite) allows assessment of local trace element mobility in aqueous fluids. We present the results am situ LA-ICP-MS trace element analysis of minerals from the reaction zone. Phlogopite is the main host for Large Ion Lithophile Elements (LILE) and contributes significantly to the Li, Ti. Nb, Ta, Pb and Sc budget. Anthophyllite is the main host for Li whereas all other trace elements including Rare Earth Elements (REE) are preferentially incorporated into tremolite. Combined with the abundance of these minerals over the contact zone, the mineral trace element data suggests that the LILE and REE were mobile on a small scale of a few centimetres only Limited mobility of Ta, which is generally regarded as barely mobile in fluids, is documented in elevated contents of Ta in anthophyllite coupled with low Nb/Ta. The high Li content in minerals throughout the reaction zone suggests that Li was the most mobile element. The studied metasomatic zones mirror geochemical processes occurring in subduction zones at the slab-mantle interface. Phlogopite crystallization at the slab-mantle interface is an efficient mechanism to filter LILE from the aqueous fluid Thus, such reaction zones, forming at temperatures <660-700 degrees C, likely prevents that the typical slab signature with enriched LILE is transported by aqueous fluids over long distances in the mantle wedge However, if coupled to the downgoing slab, phlogopite- and tremolite-rich rocks from such reaction zones might be able to act as carriers of trace elements and water into deeper parts of the subduction zone (C) 2010 Elsevier B V All rights reserved
Buick, Ian S.
Hermann, Joerg
Maas, Roland
Gibson, Roger L.
In the Central Zone of the Limpopo Belt (South Africa), Palaeoproterozoic granulite-facies metamorphism was superimposed on an earlier Archaean orogenic history. Previously determined ages of similar to 2030-2020 Ma obtained from high-temperature chronometers (zircon, garnet, monazite) are generally thought to provide the best estimate of the peak of Palaeoproterozoic granulite-facies metamorphism in the Central Zone, whereas ages as young as similar to 2006 Ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period. We present a new MC-ICP-MS Pb-207-Pb-206 age of 2030.9 +/- 1.5 Ma for titanite found in amphibolite- to greenschist-facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed Central Zone metabasite. This age could potentially date cooling of rocks at this locality to temperatures below the wet solidus. Alternatively, the titanite could be inherited from the metabasite host, and the age determined from it date the peak of metamorphism. Integration of the geochronology with LA-ICP-MS trace element data for minerals from the metabasite, the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite-facies hydrothermal alteration, not at the metamorphic peak or during the greenschist-facies phase of veining. This suggests that high-grade rocks in the Central Zone have cooled differentially through the wet solidus, and provides timing constraints on when Palaeoproterozoic reworking in the Central Zone began. This study illustrates the potential of combined geochronological and highresolution geochemical studies to accurately match mineral ages to distinct crustal processes. (c) 2007 Elsevier B.V All rights reserved.
Zircon is of fundamental importance in the investigation of deeply subducted crustal rocks in which it is a trace constituent. Tiny mineral inclusions within zircon may be the only indicators that rocks were subducted to a depth of up to 150 km. Because zircon is resistant to physical and chemical changes, it preserves stages of the subduction and exhumation history within submillimetre-size grains. Advanced in situ techniques allow to date zircon domains and to determine their trace element composition. e can thus acquire a detailed knowledge of the temperature-pressure-time paths that these extraordinary rocks have experienced. Zircon studies provide evidence that subduction and exhumation act at plate tectonic speeds of 1-3 cm/year.
The concentration profiles of 13 trace elements (Li, Na, Al, P, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, and Y) were determined in a natural mantle olivine xenocryst included in a hybridized hydrous dioritic magma during cooling from similar to 1000 degrees C, which allow trace-element chemical diffusion coefficients to be evaluated relative to Mg-Fe interdiffusion under conditions that are difficult to access in the laboratory. The effective diffusion coefficients of many elements (Li, Ca, Sc, Mn, Co, Ni, and Y) fell within a factor of three of the corresponding Mg-Fe interdiffusion coefficient, in agreement with results from laboratory experiments at higher temperatures. By contrast, the concentration profiles for Na, Ti, and V implied much faster diffusion rates, while P showed no discernible diffusion. The Al and Cr profiles, which were well correlated with each other, were complex and variable on a small length scale due to local precipitation of spinel. These data show that the diffusion coefficients of cations in olivine are not simple functions of ionic charge or ionic radius, but they are likely controlled by the availability of suitable diffusion pathways.
Alberto Padron-Navarta, Jose
Hermann, Joerg
Garrido, Carlos J.
Lopez Sanchez-Vizcaino, Vicente
Teresa Gomez-Pugnaire, Maria
Piston cylinder experiments were performed to constrain the pressure and temperature conditions for two high-pressure antigorite dehydration reactions found in silica-enriched serpentinites from Cerro del Almirez (Nevado-Filabride Complex, Betic Cordillera, southern Spain). At 630-660A degrees C and pressures greater than 1.6 GPa, antigorite first reacts with talc to form orthopyroxene +/- A chlorite + fluid. We show that orthopyroxene + antigorite is restricted to high-pressure metamorphism of silica-enriched serpentinite. This uncommon assemblage is helpful in constraining metamorphic conditions in cold subduction environments, where antigorite serpentinites have no diagnostic assemblages over a large pressure and temperature range. The second dehydration reaction leads to the breakdown of antigorite to olivine + orthopyroxene + chlorite + fluid. The maximum stability of antigorite is found at 680A degrees C at 1.9 GPa, which also corresponds to the maximum pressure limit for tremolite coexisting with olivine + orthopyroxene. The high aluminium (3.70 wt% Al(2)O(3)) and chromium contents (0.59 wt% Cr(2)O(3)) of antigorite in the investigated starting material is responsible for the expansion of the serpentinite stability to 60-70A degrees C higher temperatures at 1.8 GPa than the antigorite stability calculated in the Al-free system. The antigorite from our study has the highest Al-Cr contents among all experimental studies and therefore likely constraints the maximum stability of antigorite in natural systems. Comparison of experimental results with olivine-orthopyroxene-chlorite-tremolite assemblages outcropping in Cerro del Almirez indicates that peak metamorphic conditions were 680-710A degrees C and 1.6-1.9 GPa.