Epidote Grains Research Articles

Tracing Detrital Epidote Derived from Alteration Halos to Porphyry Cu Deposits in Glaciated Terrains: The Search for Covered Mineralization

Abstract Distal alteration related to porphyry Cu mineralization is typically characterized by an abundance of green minerals, such as epidote, tremolite, and chlorite, within the propylitic and sodic-calcic alteration zones and extends far outside (>1 km) the mineralized zone(s). Glacial erosion and dispersal derived from rocks affected by propylitic and sodic-calcic alteration have resulted in the development of extensive dispersal trains of epidote in till (glacial sediment) that can reach 8 to 330 km2 as observed at four porphyry Cu study sites in the Quesnel terrane of south-central British Columbia: Highland Valley Copper, Gibraltar, Mount Polley, and Woodjam deposits. At each of these sites, epidote is more abundant in heavy mineral concentrates of till collected directly over and down-ice from mineralization and associated alteration. Epidote grains in till with >0.6 ppm Sb and >8 ppm As (as determined by laser ablation-inductively coupled plasma-mass spectrometry) are attributed to a porphyry alteration provenance. There is a greater abundance of epidote grains with high concentrations of trace elements (>12 ppm Cu, >2,700 ppm Mn, >7 ppm Zn, and >37 ppm Pb) in each porphyry district compared to background regions. This trace element signature recorded in till epidote grains is heterogeneously distributed in these districts and is interpreted to reflect varying degrees of metal enrichment from a porphyry fluid source. Tracing the source of the epidote in the till (i.e., geochemically tying it to porphyry-related propylitic and/or sodic-calcic alteration), coupled with porphyry vectoring tools in bedrock, will aid in the detection of concealed porphyry Cu mineralization in glaciated terrains.

Fluvial‐Aeolian Interaction and Compositional Variability in the River‐Fed Yarlung Tsangpo Dune System (Southern Tibet)

AbstractRiver‐fed dune fields characterized by relatively low quartz content are much more common on Earth than generally assumed, and the aeolian dune field system in the Yarlung Tsangpo (Upper Brahmaputra) river catchment is considered a typical example. As one of the highest‐altitude fluvial‐aeolian systems on Earth, the dune fields in different wide tracts of the Yarlung Tsangpo valley, made of sand entirely blown from the adjacent river bed, provides an excellent natural laboratory in which to investigate the diverse factors controlling textural and compositional changes induced by aeolian processes. To this aim, the textural (grain‐size, roundness) and compositional (petrography, heavy minerals) properties of multiple size classes of fluvial and aeolian sands were quantified. The results show that sediment composition is markedly controlled by grain size, with greater intrasample than intersample variability. Grain‐size controlled compositional differences cannot be exclusively ascribed to hydraulic‐sorting effects. Independent of their density or shape, quartz, sedimentary rock fragments, zircon, apatite, and epidote grains are observed to be on average smaller than feldspars, volcanic rock fragments, and pyroxenes. This indicates that the original size of minerals in source rocks also played a significant role, limiting their availability in finer (e.g., garnet) or coarser (e.g., apatite) classes. In the river‐fed dune system, the lower content of phyllosilicates and higher grain roundness could represent useful criteria to distinguish aeolian from fluvial sand. The efficient aeolian abrasion of river‐fed sediments on the Tibetan Plateau may be an effective mechanism for the production of dust deposited as vast loess covers during the Quaternary.

Optimal Raman-scattering signal for estimating the Fe3+ content on the clinozoisite–epidote join

Abstract. To provide a tool for fast estimation of the Fe3+ content in Ca2(Al, Fe3+)3Si3O12(OH) epidote grains, including in thin sections and crude-rock samples, we applied Raman spectroscopy to 33 areas from 15 natural samples with Fe3+ ranging from 0.22 to 1.13 atoms per formula unit (apfu), the chemistry of which was independently determined by wavelength-dispersive electron microprobe analysis (WD-EPMA). The Raman spectra were collected from the very areas subjected to WD-EPMA. We have analysed both the OH-stretching region (3215–3615 cm−1) and the spectral range generated by the framework vibrations (15–1215 cm−1). Similarly to the IR spectra, the Raman peaks in the OH-stretching region shift toward higher wavenumbers with increasing Fe. However, the quantification of Fe3+ based on OH-stretching Raman peaks can be hindered by the multicomponent overlapping and significant intensity variations with the crystal orientation. Among the Raman signals generated by framework vibrations, the position of four peaks (near 250, 570, 600, and 1090 cm−1) exhibit a steady linear regression with the increase in Fe content (in apfu). However, the peak near 250 cm−1 attributed to MO6 vibrations also depends on the crystal orientation and therefore is not always well resolved, which worsens the accuracy in Fe-content determination based on its position. The peaks near 570, 600, and 1090 cm−1 arise from Si2O7 vibrational modes, and although their intensities also vary with the crystal orientation, all three signals are well resolved in a random orientation. However, among the three Si2O7-related signals, the 570 cm−1 peak is the sharpest (peak width <10 cm−1) and is easily recognized as a separate peak. Hence, we propose to use the position of this peak as a highly reliable parameter to estimate the Fe content, via the linear trend given as ω570=577.1(3)-12.7(4)x, where ω is the wavenumber (cm−1) and x is Fe content (apfu), with accuracy ± 0.04 Fe3+ apfu. The peaks near 600 and 1090 cm−1 may be complementarily used for the Fe estimate, based on the following relations: ω600=611.6(2)-13.8(4)x and ω1090=1098.8(3)-13.5(5)x. Analyses of the effect of Sr as a substitution for Ca and Cr at the octahedral sites indicate that contents of Sr <0.12 apfu do not interfere with the quantification of Fe via the ω570 (x) relation, whereas Cr >0.16 apfu leads to overestimation of Fe; Cr presence can be recognized however by the broadening of the peaks near 95 and 250 cm−1.

Crystallographic preferred orientations and microtexture of the Himalayan eclogites revealing records of syn-deformation peak metamorphic stage and subsequent exhumation

Electron backscattered diffraction (EBSD) and crystallographic preferred orientations (CPO) studies on Himalayan high- and ultrahigh-pressure eclogites are reported. Subduction-related peak eclogite facies (omphacite, garnet, quartz, rutile) and exhumation-related post-eclogite facies (hornblende, epidote, titanite) minerals were investigated to understand subduction and exhumation-related deformation regimes. Petrographic features from the studied eclogite samples show preferably shape-oriented omphacites coexisting with random to weakly shape-oriented garnets, and overgrown by hornblende and symplectites. Quartz grains are also randomly shape-oriented in the eclogite matrix whereas epidote grains display shape-preferred orientations. Based on EBSD data, an L-type CPO was dominant in omphacites in the studied eclogites, exhibited by the maximum density of [001] axes parallel to the lineation and the high density of poles to the (010) and (110) planes forming girdles perpendicular to the foliation. This CPO likely developed in a constrictional strain regime during the peak eclogite-facies conditions due to dislocation creep and is associated with an intra-crystalline (110) [001] and (110) <110> slip systems. EBSD data of garnets exhibit complex fabric, suggesting they most likely behaved like rigid objects in the presence of omphacite, indicating insignificant plastic deformation. EBSD data of quartz in high-pressure eclogites show irregular CPO but in ultrahigh-pressure eclogites the fabric is relatively strong. EBSD data of rutile show CPO of [001] axes strongly aligned along the lineation, suggesting its deformation was coeval to the omphacite. Titanite, mainly overgrown around rutile, indicates a random CPO different from rutile, suggesting its growth during late-stages. Hornblende in both types of eclogites shows identical fabric to that of omphacite, displaying CPO of [001] axes parallel to the lineation and poles of (010) and (110) planes form girdles perpendicular to the foliation, suggesting topotactic replacement over omphacite during exhumation. The EBSD data, combined with textural features, indicate syn-deformation peak metamorphism as well as preserve records of subsequent exhumation.

Detecting Buried Porphyry Cu Mineralization in a Glaciated Landscape: A Case Study from the Gibraltar Cu-Mo Deposit, British Columbia, Canada

Abstract At the Gibraltar porphyry Cu-Mo deposit in south-central British Columbia, Canada, geochemical and mineralogical anomalies in till around a cluster of mineral occurrences form amoeboid-shaped dispersal patterns controlled by three phases of ice movement. The glacial dispersal patterns defined by elevated concentrations of ore (Cu, Mo) and pathfinder (Ag, Zn) elements and alteration oxides (Al2O3, K2O) in the till matrix (clay and silt plus clay fractions) extend over areas of 6–36 km2 and are centered on the main economic mineralization. Mineralogical dispersal trains were identified by the high abundances of chalcopyrite, pyrite, jarosite, titanite, and epidote, which were derived from the mineralization and its associated oxidation and alteration zones. These mineralogical anomalies in till extend approximately 30 km2 for chalcopyrite, 62 km2 for jarosite, and up to 136 km2 for epidote. Epidote grains in till with a trace element composition of &amp;gt;10 ppm As, &amp;gt;4 ppm Sb, &amp;lt;200 ppm total rare earth elements (ΣREEs), &amp;lt;45 ppm Y, &amp;lt;100 ppm Sc + Cr + Y, and/or &amp;lt;2 ppm Th + Hf are interpreted as being either principally derived from the intrusive rocks or from hydrothermal alteration of the porphyry mineralization. Rare epidote grains with &amp;gt;30 ppm Cu, interpreted to be related to the porphyry Cu mineralization, were detected in till up to 7 km down-ice from the economic mineralization. This study demonstrates that till geochemistry and mineralogy, combined with geochemical analyses of specific minerals such as epidote, are efficient methods for identifying porphyry Cu mineralization in terrains covered by glacial sediments.

Identification of Fe3+ content in Epidote from Varan, Urumieh-Dokhtar Magmatic Arc, Iran: using FTIR and Raman spectroscopy

This study is aimed to determine the Fe content in natural epidote from Varan area (Urumieh-Dokhtar Magmatic Arc, Iran) by using vibrational FTIR and Raman spectroscopy and EPMA analyses. Fe3+ concentration calculated from FTIR spectroscopic data is in the range of 0.96 to 1 apfu. The results are in complete agreement with EPMA data. The comparison between obtained Raman spectra of studied epidote grains and those from the RRUFF database suggest that epidote from Varan area is rich in Fe3+. High Fe3+ content might reflect moderate to high oxygen fugacity during the crystallization of epidote. This short paper demonstrates that the quantification of the Fe content in epidote via FTIR method is as good as EPMA, whereas the utilization of a low-cost Raman spectrometer helps in quickly distinguishing between Fe-rich and Fe-poor epidote, which even could be useful in the case of field studies.spectrometer helps in quickly distinguishing between Fe-rich and Fe-poor epidote, which even could be useful in the case of field studies.Key-words: Epidote, FTIR, Raman spectroscopy, Iron content, Urumieh-Dokhtar Magmatic Arc, Iran.

Corona formation around monazite and xenotime during greenschist-facies metamorphism and deformation

Abstract. Epidote/allanite–fluorapatite coronae around monazite and xenotime are investigated in Permian pegmatites deformed under greenschist-facies conditions during Alpine tectonometamorphism in the Austroalpine basement, Eastern Alps. The aim was to evaluate the replacement reactions involved in the formation of a corona microstructure, its age and relation to deformation. In the corona core, monazite and xenotime single crystals show domains with different composition and age. Monazite (Mnz1) and xenotime (Xen1) dating by electron microprobe (EPM) reveals an age of 250–287 Ma, consistent with the Permian magmatic age of the pegmatites. These are partly replaced by secondary monazite (Mnz2) and xenotime (Xen2) compositions yielding younger Mesozoic (170–210 Ma) and Alpine (30–120 Ma) ages. The same crystallographic orientation of the primary and secondary monazite and xenotime indicates interface-coupled dissolution–precipitation reactions. Allanite U–Th–Pb dating by laser ablation inductively coupled mass spectrometry in the corona revealed an age of 60±6 Ma, interpreted as the age of corona formation. The coronae around monazite consist of an inner zone of equant fluorapatite grains surrounded by prismatic allanite, which are surrounded by epidote enriched in heavy rare earth elements (HREEs) and REE-poor epidote grains. Compared to coronae around monazite, fluorapatite has higher REE contents and no allanite occurs in the coronae surrounding the xenotime. General reactions for monazite and xenotime breakdown can be written as follows: Mnz1+(Si,Ca,Al,Fe,F)fluid→Mnz2+LREE-Ap+Aln+HREE-Ep+Ep+(Th,U)O2+(Th,U)SiO4,Xen1+(Si,Ca,Al,Fe,F)fluid→Xen2+HREE-Ap+HREE-Ep+Ep+(Th,U)O2. The amount of replacement (judged by the relative proportions of monazite and fluorapatite) is low for monazite included in tourmaline but high within the mylonitic foliation. This dependence on the degree of replacement on the local surrounding microfabric indicates that fluid availability along grain boundaries in the matrix and cracks controlled reaction advancement, allowing the elementary mass transfer required for corona formation (e.g. input of Ca, Al, Si, Fe, F). The oblate shape of the coronae aligned within the foliation of the pegmatites and the deflected foliation around the coronae, without an outer rim of prismatic epidote showing signs of deformation, indicate that the main stage of corona formation took place during deformation and reactions were still ongoing after the main stage of deformation. The corona microstructure documents replacement reactions of a single reactant into multiple distinct mineral growth zones by dissolution and precipitation processes at non-isostatic, greenschist-facies conditions, which prevailed in the area to the north of the Defereggen–Antholz–Vals shear zone between the middle Cretaceous and the Oligocene. These reactions ceased before being completed, and REE gradients within single grains within the corona and on the thin-section scale are preserved, which suggests restricted and/or episodic transport of REE in the fluid phase and/or availability of fluid.

Using Mineral Chemistry to Aid Exploration: A Case Study from the Resolution Porphyry Cu-Mo Deposit, Arizona

AbstractThe giant, high-grade Resolution porphyry Cu-Mo deposit in the Superior district of Arizona is hosted in Proterozoic and Paleozoic basement and in an overlying Cretaceous volcaniclastic breccia and sandstone package. Resolution has a central domain of potassic alteration that extends more than 1 km outboard of the ore zone, overlapping with a propylitic halo characterized by epidote, chlorite, and pyrite that is particularly well developed in the Laramide volcaniclastic rocks and Proterozoic dolerite sills. The potassic and propylitic assemblages were overprinted in the upper parts of the deposit by intense phyllic and advanced argillic alteration. The district was disrupted by Tertiary Basin and Range extension, and the fault block containing Resolution and its Cretaceous host succession was buried under thick mid-Miocene dacitic volcanic cover, obscuring the geologic, geophysical, and geochemical footprint of the deposit.To test the potential of propylitic mineral chemistry analyses to aid in the detection of concealed porphyry deposits, a blind test was conducted using a suite of epidote-chlorite ± pyrite-altered Laramide volcaniclastic rocks and Proterozoic dolerites collected from the propylitic halo, with samples taken from two domains located to the north and south and above the Resolution ore zone. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data of epidote provided indications of deposit fertility and proximity. Competition for chalcophile elements (As, Sb, Pb) between coexisting pyrite and epidote grains led to a subdued As-Sb fertility response in epidote, consistent with epidote collected between 0.7 and 1.5 km from the center of a large porphyry deposit. Temperature-sensitive trace elements in chlorite provided coherent spatial zonation patterns, implying a heat source centered at depth between the two sample clusters, and application of chlorite proximitor calculations based on LA-ICP-MS analyses provided a precisely defined drill target in this location in three dimensions. Drilling of this target would have resulted in the discovery of Resolution, confirming that epidote and chlorite mineral chemistry can potentially add value to porphyry exploration under cover.

Epidote spherulites and radial euhedral epidote aggregates in a greenschist facies metavolcanic breccia hosting an UHP eclogite in Dabieshan (China): Implication for dynamic metamorphism

AbstractEpidote spherulites are identified in a greenschist facies metavolcanic breccia enclosing a body of coesite-bearing eclogite at Ganghe in the Dabie ultrahigh-pressure metamorphic belt, east-central China. The epidote spherulites are formed by fibrous, radially arranged, and rare earth element (REE)-rich epidote crystals (ΣREE = 0.13–0.36 (or slightly higher) cations per formula unit, cpfu) and interfibrillar REE-poor epidote (ΣREE ≤ 0.10 cpfu). Some of the epidote spherulites are overgrown by radially arranged euhedral epidote crystals, which also form aggregates around preexisting quartz, plagioclase, and/or epidote. The epidote grains in such aggregates display oscillatory zoning, with REE content varying from a negligible amount to about 0.44 cpfu. Epidote also occurs as REE-poor individual euhedral crystals about the radial epidote aggregates or form loose clusters of randomly oriented crystals. Thermodynamic modeling of the mineral assemblages in the plagioclase pseudomorphs and in the matrix shows that they formed at greenschist facies metamorphic conditions (435–515 °C and 5–7 kbar). The epidote spherulites and radial euhedral epidote aggregates, however, do not belong to these assemblages and are non-equilibrium textures. They imply crystal growth under large degrees of supersaturation, with relatively low ratios of the diffusion rate (D) to the crystal growth rate (G). At low D/G ratios, spiky interfaces are favorable for diffusion-controlled growth and the resultant texture is a collection of spikes around a growth center, forming a spherulite. The change of epidote texture from spherulite to radial euhedral crystal aggregate implies a decrease of supersaturation and an increase of D/G, such that the crystal morphology was controlled by its crystallographic structure. The crystallization of the individual epidote grains corresponds to a further drop of supersaturation and a further increase of the D/G ratio, approaching to the equilibrium conditions. Transiently higher P-T conditions are inferred from the spherulite-forming reactions, relative to the P-T estimates for the equilibrium assemblages. The fibrous crystals in the spherulites having relatively large interfacial energies would inevitably adjust their shapes to equilibrium ones with low interfacial energies if the P-T-H2O conditions were maintained for a sufficiently long period of time. The non-equilibrium epidote aggregates likely formed in response to P-T and fluid pulses, possibly related to seismicity.

Local tectonics and sedimentation in grabens of the Central Russian aulacogen (East European Platform)

The influence of local tectonic processes that determined individual development of each particular graben in the Central Russian aulacogen is best reflected in the asynchronous appearance of compositionally different clastic materials in their sedimentary sections, which accumulated with variable intensity. Against the background of the stable composition of the clastogenic framework of the terrigenous Molokovo Group, there are intervals of the sedimentary section from tens to a few thousands of meters thick, where the heavy fraction of sandstones is highly enriched (35–95%) with acute-angled epidote grains. Several features (relative instability of epidote in the hypergenesis zone, angular and fresh appearance of clasts, absence of correlation between the epidote influx and content of the main rock-forming components) imply the formation of epidote anomalies owing to local sources. Analysis of probable geodynamic mechanisms, which could determine development of the Central Russian aulacogen and its grabens, structure and composition of the upper part of the consolidated crust, made it possible to assume that the specific clastic material originates from epidote-rich blastomylonites occurring among metamorphic rocks of the basement in form of strata with anomalous petrogeophysical properties. Comparison between crystals and grains of epidote from blastomylonites and sediments shows that they are characterized by similar habit, sizes, and optical parameters and contain 25–30% of the pistacite component, which is typical of secondary epidote that forms pseudomorphs after biotite and amphibole under conditions of partial melting. Regularities in the distribution of epidote-rich intervals in the sedimentary section are explained by relations between attitude elements of planes of Neoproterozoic normal faults and Paleoproterozoic blastomylonite strata, which served as local sources of clastic material. The same factor also determined the facies properties of sedimentary complexes and structural evolution of sedimentation basins. Crosscutting normal faults stimulated the formation of grabens with the rheologically determined subsidence limit (Molokovo type), particularly, in situations characterized by the gentle attitude of blastomylonite bodies. In such a situation, the subsidence of granitoid rocks into the denser amphibolite substrate was limited by isostatic leveling forces. Under the stationary regional strain field after reaching the subsidence limit, grabens of this type experienced lateral extension, which resulted in the accumulation of regressive sedimentary successions with the irreversible transition from the lacustrine to the fluvial–proluvial facies. Such grabens are characterized by the one-act manifestation of the local source of clastics regardless of the structure development stage. The development of more expedient (in terms of energy) normal faults along blastomylonite strata (Roslyatino type) did not disturb the isostatic equilibrium and resulted in the formation of narrow deep grabens, where depositional environments remained virtually unchanged. The epidote influx was in progress during the entire life of the accommodation space, since progressive deepening of the graben stimulated constant activity of its local source.

Strontium and oxygen isotopic profiles through 3 km of hydrothermally altered oceanic crust in the Reykjanes Geothermal System, Iceland

The Iceland Deep Drilling Program well RN-17 was drilled 3km into a section of hydrothermally altered basaltic crust in the Reykjanes geothermal system in Iceland. The system is located on the landward extension of the Mid-Atlantic Ridge, and the circulating hydrothermal fluid is modified seawater, making Reykjanes a useful analog for mid-oceanic ridge hydrothermal systems. We have determined whole-rock Sr and O isotope compositions, and Sr isotope compositions of epidote grains from the RN-17 cuttings and RN-17B core. Whole rock oxygen isotope ratios range from −0.13 to 3.61‰ V-SMOW, and are isotopically lighter than fresh MORB (5.8±0.2‰). The concentrations of Sr in the altered basalt range from well below to well above concentrations in fresh rock, and appear to be strongly correlated with the dominant alteration mineralogy. Whole rock Sr isotope ratios ranged from 0.70329 in the least altered crystalline basalt, to 0.70609 in the most altered hyaloclastite samples; there is no correlation with depth. Sr isotope ratios in epidote grains measured by laser ablation MC-ICP-MS ranged from 0.70360 to 0.70731. Three depth intervals, at 1000m, 1350m, and 1650m depth, have distinctive isotopic signatures, where 87Sr/86Sr ratios are elevated (mean value>0.7050) relative to background levels (mean altered basalt value ~0.7042). These areas are proximal to geothermal feed zones, and the 1350m interval directly overlies the transition from dominantly extrusive to dominantly intrusive lithologies. Oxygen isotope measurements yield integrated water/rock ratios of 0.4 to 4.3, and suggest that hydrothermal fluids must have formerly had a component of meteoric water. Strontium isotopic measurements provide a more sensitive indication of seawater interaction and require significant exchange with seawater strontium. Both isotopic systems indicate that the greenschist-altered basalts were in equilibrium with hydrothermal fluids at a relatively high mean water/rock (Wt.) ratio ranging from about 0.5 to 4. These ratios are higher than estimates from ODP Hole 504B and IODP Hole 1256D, but are consistent with values inferred from vent fluids from 21° and 13°N on the East Pacific Rise (Albarède et al., 1981; Michard et al., 1984; Alt et al., 1996; Harris et al., 2015).

Magmatic zoisite and epidote in tonalite of the Ryoke belt, central Japan

Magmatic epidote and zoisite commonly occur in Cretaceous tonalite of the Hazu area in the Ryoke belt, central Japan. The tonalite is mainly composed of amphibole, biotite, plagioclase, quartz, and epidote/zoisite with minor ilmenite, magnetite, pyrite, zircon, and apatite. Small amounts of K-feldspar occur as an interstitial phase between other felsic phases or perthitic patches in plagioclase. Epidote occurs as inclusions in plagioclase, as interstitial phase in the matrix, and as secondary phase in chlorite pseudomorphs after biotite, and in saussuritized plagioclase. The XFe [= Fe3+/(Al + Fe3+)] value of the secondary epidote ranges from 0.27 to 0.39. Epidote inclusions in plagioclase and interstitial grains contain less Fe3+ (XFe = 0.08–0.29), Fe3+-poor epidote with XFe 0.20 and their host plagioclase. Epidote grains with XFe > 0.20 in plagioclase and the matrix are a magmatic phase that crystallized directly from the tonalite magma. The Fe3+-poor epidote (XFe < 0.20) and zoisite were probably formed by a local reaction between the trapped melt and its host plagioclase, and these are considered not to have been in equilibrium with the tonalite magma. Compositions of amphibole-plagioclase assemblages allowed for temperature estimates in the range of 730–770 °C and minimum pressures of 0.47–0.57 GPa for the epidote/zoisite-bearing tonalites of the Hazu area. Epidote/zoisite-free tonalites occur in other areas of the Ryoke belt. There may be several tonalite bodies that record different intrusion processes and solidification depths in the Ryoke belt.

Parageneses and Th-U distributions among allanite, monazite, and xenotime in Barrovian-type metapelites, Imjingang belt, central Korea

The paragenetic relationships and Th-U distributions among allanite, monazite, and xenotime were investigated in a progressive sequence of garnet- to kyanite-zone metapelites of the Imjingang belt, Korea. Allanite is predominant in the garnet and staurolite zones, whereas monazite and xenotime predominate in the kyanite zone. Epidote grains in the lower garnet zone are commonly zoned, from allanite (core) to relatively Y-rich, rare-earth-element (REE)-epidote to clinozoisite (rim), although both REE-epidote and clinozoisite disappear in higher-grade metapelites. Moreover, allanite and REE-epidote often contain minute inclusions of thorium silicate. The isogradic distributions and similarity of REE patterns between allanite and monazite suggest that the latter has grown at the expense of the former. In addition, the discontinuous Th zoning in monazite is apparently inherited from heterogeneous Th distribution and thorium silicate inclusions in allanite. Thus, thorium silicate possibly provided the additional Th and U necessary for the monazite formation. Paragenetic relationships of allanite and monazite inclusions within various index minerals suggest that at amphibolite-facies conditions allanite is stable at higher pressures than monazite. Xenotime grains in the staurolite zone are rarely produced by the breakdown of clinozoisite and REE-epidote, whereas those in the kyanite zone are grown primarily at the expense of garnet. Incorporation of Th and U into monazite and xenotime is governed mainly by the brabantite and thorite substitutions, respectively. Taken together, our results suggest that the allanite-to-monazite transformation is primarily responsible for the distributions of REEs, Th, and U among metapelitic phases, and that the xenotime formation was facilitated by the contribution from major silicates, particularly garnet.

Tracing the evolution of calc-alkaline magmas: In-situ Sm–Nd isotope studies of accessory minerals in the Bergell Igneous Complex, Italy

The common occurrence of Ca- and Nd-rich accessory minerals titanite, epidote, allanite and apatite in calc-alkaline plutonic suites makes them ideal targets for in-situ tracing of the temporal, chemical and isotopic evolution of tonalitic and granodioritic melts. The Sm–Nd isotope composition of accessory phases from the calc-alkaline Bergell Pluton and the peraluminous Novate leucogranite (central Alps) were investigated using laser ablation MC-ICPMS techniques. Grouping of individual LA-MC-ICPMS analyses produced average ε Nd values with typical uncertainties of < 0.3 (2σ) epsilon units. SHRIMP dating of magmatic allanite cores of epidote grains from a Bergell gabbro yielded a Th–Pb age of 32.4 ± 0.4 Ma, which provides a new timing constraint on the emplacement of juvenile members within the Bergell intrusive sequence. The Bergell bulk-rock mantle-crust isotopic mixing curve was reproduced, demonstrating that integration of U–Th-rich accessory mineral Nd isotope compositions with crystallisation age is capable of tracing the geochemical evolution of magmatic systems over time. Crucially, the isotopic composition of the mantle end-member was successfully constrained by measurement of magmatic REE-epidote, highlighting the compositional versatility of accessory phases. The removal or addition of feldspar controls the Eu signature of both the bulk-rock and single minerals and therefore presents a unique trace element indicator of magmatic differentiation and assimilation processes in accessory minerals. Therefore the in-situ determination of age, Sm–Nd isotopes and trace elements in accessory minerals permits efficient and accurate reconstruction of complex magmatic processes in calc-alkaline plutonic suites. Sub-grain isotopic heterogeneity in magmatic monazite from the Novate leucogranite was identifiable by LA-MC-ICPMS analysis and emphasizes the additional value of a micro-analytical approach to understanding geological processes.

Comparison of population and external detector method dating in epidote

Epidote fission-track dating (FTD) was performed through both population and external detector method (EDM) in five samples from the Mantiqueira Mountain range, southwestern Brazil. The ages obtained via population conducted to a weighted mean age of ( 114 ± 19 ) Ma . In order to perform EDM dating, it was necessary to measure the efficiency factor ( g) between muscovite mica and epidote. Three measurements were performed and the weighted mean g value obtained was ( 0.57 ± 0.03 ) . The weighted EDM age was ( 98 ± 7 ) Ma . The population error magnitude occurs due to the existence of few useful epidote grains. EDM provides dating with fewer grains, however there are difficulties concerning etching anisotropy that should be regarded. Both dating methods agreed.

Distribution of chromium among the octahedral sites in chromian epidote from Iratsu, central Shikoku, Japan

The crystal structures of chromian epidotes from the Sambagawa metamorphic rocks, central Shikoku, Japan, were refined using single-crystal data to investigate the distribution of chromium among the three independent octahedral sites. On the basis of the electron probe microanalysis of the chromian epidotes on the polished surface of thin sections, three chromian epidote grains in a thin section were selected for X-ray intensity data collection, and their Cr2O3 values were as much as 1.79, 4.99, and 4.74 wt%, respectively, which led to values of 0.113, 0.315, and 0.299 Cr apfu, respectively. Nevertheless, their Fe2O3 contents were nearly constant (8.1-8.7 wt%, 0.49-0.52 Fe apfu) and their SrO contents were not significant (1.8-3.2 wt%, 0.08-0.15 Sr apfu). The selected chromian epidote grains were picked out using a binocular optical microscope from the thin sections, and were labeled as CrEp1, CrEp2, and CrEp3, respectively. The oxidation state of chromium was found to be trivalent by the optical spectrum. 57Fe Mossbauer doublets with isomer shift of 0.28-0.53 mm/sec and quadrupole splitting of 1.77-2.24 mm/sec indicate an exclusive distribution of Fe3+ at the M3 site. The crystal structure of CrEp1 [a = 8.8815(6), b = 5.5956(5), c = 10.1532(5) A, β = 115.159(2)], CrEp2 [a = 8.9165(4), b = 5.6226(2), c = 10.1728(5) A, β = 115.2365(9)], and CrEp3 [a = 8.895(2), b = 5.610(1), c = 10.146(2) A, β = 115.177(2)] were refined using 1145, 1853 and 1438 unique reflections, respectively, and the calculations converged at R factors of 5.8%, 6.6%, and 3.2%, respectively. The resulting structural formula were: [Ca]A1[Ca0.86Sr0.14]A2[Al]M1[Al]M2[Al0.43Cr3+0.09Fe3+0.48]M3Si3O12(OH) for CrEp1, [Ca]A1[Ca0.90Sr0.10]A2[Al0.86Cr3+0.14]M1[Al]M2[Al0.34Cr3+0.17Fe3+0.49]M3Si3O12(OH) for CrEp2, and [Ca]A1[Ca0.88Sr0.12]A2[Al0.90Cr3+0.10]M1[Al]M2[Al0.37Cr3+0.12Fe3+0.51]M3Si3O12(OH) for CrEp3. The selective distribution of Cr3+ at the M3 site in CrEp1, and the KD values (KD = (Cr3+/Al)M1/(Cr3+/Al)M3) of 0.35(3) for CrEp2 and 0.41(3) for CrEp3 show that the preference of Cr3+ in the octahedral sites is M3 > M1 » M2. The variations in the M1-Oi and M3-Oi distances indicate an equidimensional expansion of the M1O6 and M3O6 octahedra by the substitution of Cr3+(+ Fe3+) for Al3+.

Crystal size of epidotes: A potentially exploitable geothermometer in geothermal fields?

Crystal size of epidotes crystallized in quartz + epidote veins is used as the basis for a new geothermometer from the fossil geothermal field of Saint Martin (Lesser Antilles). The epidote-bearing alteration paragenesis is developed as far as 3 km from a quartz diorite pluton at temperatures of 220-350 °C. The length/width ratio of the epidote grains is constant for all the analyzed samples and suggests isotropic growth environments. However, the length and width of the grains vary exponentially with temperature. The obtained results offer new perspectives for simple grain-size geothermomeery but must be extended to other geologic environments to clarify the influence of different rock types.

Leaching of Heavy Minerals Above and Below the Mid-Cretaceous Unconformity in the Ohuriawa Gorge Area of the Waipara River, North Canterbury, New Zealand

ABSTRACT Contrasting heavy mineral populations rich in 1) ilmenite and zircon, and 2) ilmenite, epidote and semi-opaque debris, exist in both indurated Mesozoic (Torlesse) basement sandstones and in overlying transgressive Cretaceous-Tertiary sandstones around Ohuriawa Gorge. The ilmenite-zircon population forms a zone over 100 m thick below the unconformity at the top of the Torlesse on its eastern boundary northeast of the gorge. The great structural complexity of the Torlesse contrasts with the parallelism between the ilmenite-zircon zone and the unconformity and, together with the lack of any change in overall detrital composition paralleling the change in heavy minerals, shows that the population contrast in the Torlesse rocks must result from alteration that post-dates deformation. The i menite-zircon zone in the overlying Cretaceous-Tertiary sediments extends up to near the Paleocene-Eocene boundary. Apatite in coal measures within this zone has been leached early during diagenesis and may be being affected by acidic pore fluids now associated with weathering of pyrite. The nature and distribution of solution effects on epidote grains in Tertiary samples and the absence of the same effects in Torlesse samples show that the change in the Tertiary sediments has not been inherited from the Torlesse but formed soon after deposition by leaching in a dominantly alkaline pore-fluid regime.