Thermochemistry of monazite-(La) and dissakisite-(La): implications for monazite and allanite stability in metapelites

Thermochemical properties have been either measured or estimated for synthetic monazite, LaPO4, and dissakisite, CaLaMgAl2(SiO4)3OH, the Mg-equivalent of allanite. A dissakisite formation enthalpy of −6,976.5±10.0kJmol−1 was derived from high-temperature drop-solution measurements in lead borate at 975K. A third-law entropy value of 104.9±1.6Jmol−1K−1 was retrieved from low-temperature heat capacity (C p) measured on synthetic LaPO4 with an adiabatic calorimeter in the 30-300K range. The C p values of lanthanum phases were measured in the 143-723K range by differential scanning calorimetry. In this study, La(OH)3 appeared as suitable for drop solution in lead borate and represents an attractive alternative to La2O3. Pseudo-sections were calculated with the THERIAK-DOMINO software using the thermochemical data retrieved here for a simplified metapelitic composition (La=∑REE+Y) and considering monazite and Fe-free epidotes along the dissakisite-clinozoïsite join, as the only REE-bearing minerals. Calculation shows a stability window for dissakisite-clinozoïsite epidotes (T between 250 and 550°C and P between 1 and 16kbar), included in a wide monazite field. The P-T extension of this stability window depends on the bulk-rock Ca-content. Assuming that synthetic LaPO4 and dissakisite-(La) are good analogues of natural monazite and allanite, these results are consistent with the REE-mineralogy sequence observed in metapelites, where (1) monazite is found to be stable below 250°C, (2) around 250-450°C, depending on the pressure, allanite forms at the expense of monazite and (3) towards amphibolite conditions, monazite reappears at the expense of allanit

Deformation of feldspar at greenschist facies conditions – the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps

Deformation microstructures of albitic plagioclase and K-feldspar were investigated in mylonitic pegmatites from the Austroalpine basement south of the western Tauern Window by polarized light microscopy, electron microscopy and electron backscatter diffraction to evaluate feldspar deformation mechanisms at greenschist facies conditions. The main mylonitic characteristics are alternating almost monophase quartz and albite layers, surrounding porphyroclasts of deformed feldspar and tourmaline. The dominant deformation microstructures of K-feldspar porphyroclasts are intragranular fractures at a high angle to the stretching lineation. The fractures are healed or sealed by polyphase aggregates of albite, K-feldspar, quartz and mica, which also occur along intragranular fractures of tourmaline and strain shadows around other porphyroclasts. These polyphase aggregates indicate dissolution–precipitation creep. K-feldspar porphyroclasts are partly replaced by albite characterized by a cuspate interface. This replacement is interpreted to take place by interface-coupled dissolution–precipitation driven by a solubility difference between K-feldspar and albite. Albite porphyroclasts are replaced at boundaries parallel to the foliation by fine-grained monophase albite aggregates of small strain-free new grains mixed with deformed fragments. Dislocation glide is indicated by bent and twinned albite porphyroclasts with internal misorientation. An indication of effective dislocation climb with dynamic recovery, for example, by the presence of subgrains, is systematically missing. We interpret the grain size reduction of albite to be the result of coupled dislocation glide and fracturing (low-temperature plasticity). Subsequent growth is by a combination of strain-induced grain boundary migration and formation of growth rims, resulting in an aspect ratio of albite with the long axis within the foliation. This strain-induced replacement by nucleation (associated dislocation glide and microfracturing) and subsequent growth is suggested to result in the observed monophase albite layers, probably together with granular flow. The associated quartz layers show characteristics of dislocation creep by the presence of subgrains, undulatory extinction and sutured grain boundaries. We identified two endmember matrix microstructures: (i) alternating layers of a few hundred micrometres' width, with isometric, fine-grained feldspar (on average 15&thinsp;µm in diameter) and coarse-grained quartz (a few hundred micrometres in diameter), representing lower strain compared to (ii) alternating thin layers of some tens of micrometres' width composed of fine-grained quartz (&lt;20&thinsp;µm in diameter) and coarse elongated albite grains (long axis of a few tens of micrometres) defining the foliation, respectively. Our observations indicate that grain size reduction by strain-induced replacement of albite (associated dislocation glide and microfracturing) followed by growth and granular flow simultaneous with dislocation creep of quartz are playing the dominating role in formation of the mylonitic microstructure.</p

Geochronological and thermometric evidence of unusually hot fluids in an Alpine fissure of Lauzière granite (Belledonne, Western Alps)

A multi-method investigation into Lauzière granite, located in the external Belledonne massif of the French Alps, reveals unusually hot hydrothermal conditions in vertical open fractures (Alpine-type clefts). The host-rock granite shows sub-vertical mylonitic microstructures and partial retrogression at temperatures of &lt;&thinsp;400&thinsp;∘C during Alpine tectonometamorphism. Novel zircon fission-track (ZFT) data in the granite give ages at 16.3&thinsp;±&thinsp;1.9 and 14.3&thinsp;±&thinsp;1.6&thinsp;Ma, confirming that Alpine metamorphism was high enough to reset the pre-Alpine cooling ages and that the Lauzière granite had already cooled below 240–280&thinsp;∘C and was exhumed to &lt;&thinsp;10&thinsp;km at that time. Novel microthermometric data and chemical compositions of fluid inclusions obtained on millimetric monazite and on quartz crystals from the same cleft indicate early precipitation of monazite from a hot fluid at T&thinsp;&gt;&thinsp;410&thinsp;∘C, followed by a main stage of quartz growth at 300–320&thinsp;∘C and 1.5–2.2&thinsp;kbar. Previous Th-Pb dating of cleft monazite at 12.4&thinsp;±&thinsp;0.1&thinsp;Ma clearly indicates that this hot fluid infiltration took place significantly later than the peak of the Alpine metamorphism. Advective heating due to the hot fluid flow caused resetting of fission tracks in zircon in the cleft hanging wall, with a ZFT age at 10.3&thinsp;±&thinsp;1.0&thinsp;Ma. The results attest to the highly dynamic fluid pathways, allowing the circulation of deep mid-crustal fluids, 150–250&thinsp;∘C hotter than the host rock, which affect the thermal regime only at the wall rock of the Alpine-type cleft. Such advective heating may impact the ZFT data and represent a pitfall for exhumation rate reconstructions in areas affected by hydrothermal fluid flow.</p

The origin and composition of carbonatite-derived carbonate-bearing fluorapatite deposits

Carbonate-bearing fluorapatite rocks occur at over 30 globally distributed carbonatite complexes and represent a substantial potential supply of phosphorus for the fertiliser industry. However, the process(es) involved in forming carbonate-bearing fluorapatite at some carbonatites remain equivocal, with both hydrothermal and weathering mechanisms inferred. In this contribution, we compare the paragenesis and trace element contents of carbonate-bearing fluorapatite rocks from the Kovdor, Sokli, Bukusu, Catalão I and Glenover carbonatites in order to further understand their origin, as well as to comment upon the concentration of elements that may be deleterious to fertiliser production. The paragenesis of apatite from each deposit is broadly equivalent, comprising residual magmatic grains overgrown by several different stages of carbonate-bearing fluorapatite. The first forms epitactic overgrowths on residual magmatic grains, followed by the formation of massive apatite which, in turn, is cross-cut by late euhedral and colloform apatite generations. Compositionally, the paragenetic sequence corresponds to a substantial decrease in the concentration of rare earth elements (REE), Sr, Na and Th, with an increase in U and Cd. The carbonate-bearing fluorapatite exhibits a negative Ce anomaly, attributed to oxic conditions in a surficial environment and, in combination with the textural and compositional commonality, supports a weathering origin for these rocks. Carbonate-bearing fluorapatite has Th contents which are several orders of magnitude lower than magmatic apatite grains, potentially making such apatite a more environmentally attractive feedstock for the fertiliser industry. Uranium and cadmium contents are higher in carbonate-bearing fluorapatite than magmatic carbonatite apatite, but are much lower than most marine phosphorites

Physico-chemical control on the REE-mineralogy in chloritoid-grade metasediments from a single outcrop (Central Alps, Switzerland)

International audienceThis study assesses the effects of fluid, whole-rock composition and oxygen fugacity, on the texture and composition of monazite, allanite, and xenotime. For this purpose, these were investigated in 13 mono-metamorphic metasediments from a single locality of the Central Alps (Switzerland), which record greenschist facies conditions (T ~ 400-450 °C). Two of the samples contain hydrothermal veins dominated by quartz and calcite + quartz, respectively. In metasediments devoid of veins, the light rare earth elements (LREE) are concentrated in allanite in all samples except for one metamarl (Ga06). Allanite formation is texturally coeval with apatite, chloritoid and xenotime, during the main tectono-metamorphic stage. Allanite formation implies significant mass transfer of Ca and P via a fluid phase, which is not clearly related to advective transport. In Ga06, elongate monazite grains have a detrital core rimmed by newly formed monazite. Significant arsenic contents are found in newly formed monazite, xenotime and apatite. Monazite texture and composition suggest (re)crystallization by pressure solution, at an oxygen fugacity sufficient to partly oxidize As, S, U, and Fe. Whether or not monazite is preserved appears to be but weakly dependent on the Ca/Al ratio and thus whole-rock composition at greenschist facies conditions. Samples with veins show peculiar features. Along carbonate-bearing veins allanite occurs as porphyroblasts overgrowing the main foliation of the host rock. As similar allanite porphyroblasts occur in calcite-bearing metamarl, their formation is attributed to environments rich in CO2-bearing fluid. A new generation of monazite is found in and along retrograde quartz veins. This study demonstrates that, at least in metasediments up to chloritoid-grade, REE minerals record fluid/rock interaction that occurred at different deformation stages. Arsenic concentrations in REE phosphates appear to reflect conditions of elevated oxygen fugacity. In cases where such conditions are not inherited from the sedimentary protolith, the oxidation reflects a hydrothermal event, the age of which may be directly datable by U/Th-Pb of As-rich monazite

Jiddat al Harasis 556: A howardite impact melt breccia with an H chondrite component

A petrographic and geochemical study was undertaken to characterize Jiddat al Harasis (JaH) 556, a howardite find from the Sultanate of Oman. JaH 556 is a polymict impact melt breccia containing highly shocked clasts, including mosaicized olivine and recrystallized plagioclase, set in a finely recrystallized vesicular matrix (grain diameter 76-92) and clinopyroxene (En48-62Wo7-15) are associated with orthopyroxene and olivine clasts like in a howardite. JaH 556 oxygen isotope data indicate that it has an anomalous bulk-rock composition as howardite, resulting from a mixture between HED material and at least one second reservoir characterized by a higher Δ17O. The bulk meteorite has a composition consistent with howardites, but it is enriched in siderophile elements (Ni = 3940 and Co = 159 ppm) arguing for a chondritic material as second reservoir. This is independently confirmed by the occurrence of chondrule relics composed of olivine (Fo56-80), orthopyroxene (En79Wo2), and plagioclase (An61-66). Based on oxygen isotopic signature, siderophile composition, and chondrule core Mg number (Fo80 and En79Wo2), it is proposed that JaH 556 is a howardite containing approximately 20% H chondrite material. This percentage is high compared with that observed petrographically, likely because chondritic material dissolved in the impact melt. This conclusion is supported by the observed reaction of orthopyroxene to olivine, which is consistent with a re-equilibration in a Si-undersaturated melt. JaH 556's unique composition enlarges the spectrum of howardite-analogs to be expected on the surface of 4 Vesta. Our data demonstrate that oxygen isotopic anomalies can be produced by a mixture of indigenous and impactor materials and must be interpreted with extreme caution within the HED group

Partial resetting of the U-Th-Pb systems in experimentally altered monazite: Nanoscale evidence of incomplete replacement

International audienc

Prograde metamorphic sequence of REE minerals in pelitic rocks of the Central Alps: implications for allanite–monazite–xenotime phase relations from 250 to 610 C

The distribution of REE minerals in metasedimentary rocks was investigated to gain insight into the stability of allanite, monazite and xenotime in metapelites. Samples were collected in the central Swiss Alps, along a well-established metamorphic field gradient that record conditions from very low grade metamorphism (250 C) to the lower amphibolite facies (600 C). In the Alpine metapelites\ud investigated, mass balance calculations show that LREE are mainly transferred between monazite and allanite during the course of prograde metamorphism. At very low grade metamorphism, detrital monazite grains (mostly Variscan in age) have two distinct populations in terms of LREE and MREE\ud compositions. Newly formed monazite crystallized during low-grade metamorphism (<440 C); these are enriched in La, but depleted in Th and Y, compared with inherited grains. Upon the appearance of chloritoid (440–450 C, thermometry based on chlorite–choritoid and carbonaceous material),\ud monazite is consumed, and MREE and LREE are taken up preferentially in two distinct zones of allanite distinguishable by EMPA and X-ray mapping. Prior to garnet growth, allanite acquires two growth zones of clinozoisite: a first one rich in HREE + Y and a second one containing low REE contents. Following garnet growth, close to the chloritoid–out zone boundary (556–580 C, based on phase equilibrium calculations), allanite and its rims are partially to totally replaced by monazite and xenotime, both associated with plagioclase (± biotite ± staurolite ± kyanite ± quartz). In these samples, epidote relics are located in the matrix or as inclusions in garnet, and these preserve their characteristic chemical and textural growth zoning, indicating that they did not experience reequilibration following their prograde formation. Hence, the partial breakdown of allanite to monazite offers the attractive possibility to obtain in situ ages, representing two distinct crystallization stages. In addition, the complex REE + Y and Th zoning pattern of allanite and monazite are essential monitors of crystallization conditions at relatively low metamorphic grade

Metamorphic rates in collisional orogeny from in situ allanite and monazite dating

The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), allanite is partly replaced by monazite at 560–580 °C. Relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430–450 °C). Coexisting monazite and allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb dating of allanite (31.5 ± 1.3 and 29.2 ± 1.0 Ma) and monazite (18.0 ± 0.3 and 19.1 ± 0.3 Ma) constrains the time elapsed between 430–450 °C and 560–580 °C, which implies an average heating rate of 8–15 °C/m.y. Combined with new fi ssion track ages (zircon, 10–9 Ma; apatite, 7.5–6.5 Ma), metamorphic rates of theentire orogenic cycle, from prograde to final cooling, can be reconstructed