REE-rich Epidote Research Articles

Dating prograde metamorphism: U–Pb geochronology of allanite and REE-rich epidote in the Eastern Alps

We use U–Pb dating of allanite and REE-rich epidote in three polymetamorphosed units from the Eastern Alps to constrain the timing of prograde metamorphism. All three units (Ennstal, Wölz and Rappold Complex) record several metamorphic cycles (Variscan, Permian and Eoalpine) and presently define an Eoalpine (Cretaceous) metamorphic field gradient from lower greenschist to amphibolite facies. For U–Pb data, a method is introduced to test the magnitude of 230Th disequilibrium and potentially approximate the Th/U ratio of the reservoir out of which allanite and REE-rich epidote grew. We also show that the modelled stability of epidote-group minerals in the REE-free MnNCKFMASH and MnNCKFMASHTO systems and REE-bearing systems is nearly identical. By combining the stability fields of (clino-)zoisite and epidote modelled in REE-free systems with known geothermal gradients for the region, REE-rich epidote growth is constrained to 200–450 °C and 0.2–0.8 GPa during prograde metamorphism. In the Rappold Complex, allanite cores yield a Variscan age of ca. 327 Ma. In the Ennstal and Wölz Complex, allanite growth during the Permian event occurred at ca. 279–286 Ma. Importantly, recrystallized allanite laths and REE-rich epidote overgrowths in samples from all three units yield prograde Eoalpine ages of ca. 100 Ma, even though these units subsequently reached different peak conditions, most likely at different times. This suggests that all units were buried roughly at the same time during the onset of Eoalpine continental subduction. This interpretation leaves room for the model proposing that diachronous peak metamorphic conditions reported for the field gradient may be related to the inertia of thermal equilibration rather than tectonic processes.

Mineralogy, geochemistry, and 40Ar/39Ar dating of hornblendite/amphibolite pods in Penjween Ophiolite of northeastern Iraq: Implications for petrogenesis and tectonics

Penjween ophiolite is one of the ophiolitic complexes of northeastern Iraq. The serpentinites within the Penjween Ophiolite hosts many pods of hornblendite and amphibolite, and dikes of diorite among many other igneous bodies. These pods have very sharp contacts with the surrounding mantle serpentinized harzburgites. The hornblendites and amphibolites are usually intimately intergrown together as extremely hard, dark green to black colored, fine-to medium-grained pods with ∼2 x ∼2 x ∼(0.5–1) m dimensions. This study presents petrography, mineral chemistry, whole-rock major and trace element geochemistry, and electron probe microanalyses (EPMA) for the major minerals in the studied rocks. The hornblendite is composed entirely of amphiboles (>99% vol.), meanwhile the amphibolite consists of comparable amounts of amphibole and plagioclase which occasionally occurs as layered rocks with banded texture. The diorite dikes are white in color and consist dominantly of coarse-grained plagioclase and less amphiboles. The amphiboles of these rocks belong to pargasite (Mg# 0.69–0.77) ─ edenite (Mg# 0.74–0.79) endmembers where pargasite is by far the predominant mineral; meanwhile the plagioclase is albite (Ab93.4An6.4Or0.2). The amphiboles are replacement products of pyroxenes indicated from the relict pyroxene within their crystals. The amphiboles are abnormally rich in various dust-like inclusions of transparent minerals like REE-rich epidote, rutile, zircon, apatite, titanite, and ore minerals like ilmenite, and pyrrhotite, oriented along the crystallographic axes and form distinct zones in the core of amphibole crystals. The geochemical characteristics of the studied hornblendite (MgO = 13.07%, Ni = 260 ppm, Mg# = 66.57) as well as the high Sc (33 ppm) and V (254 ppm) concentrations are collectively consistent with a mantle-derived, igneous origin. The primitive-mantle normalized trace elements spidergram showed enrichment (hump) in Ba, Th, U, La, Ce, Pb, and Sr, and depletion (trough) in Nb, Ta, K, and Ti. The chondrite-normalized REE diagram showed enrichment of LREE relative to HREE, indicated from the smooth and steady decrease in the negative slope from LREE towards HREE with a negligible Eu-anomaly. Various tectonic discriminating diagrams showed that the studied hornblendite, amphibolite pods and diorite dikes are of igneous fore-arc origin, formed from calc-alkaline and/or tholeiitic magma within an active continental margin setting. The 40Ar/39Ar laser age of hornblendite is late Paleocene (Thanetian) (57.8 ± 5.1 Ma) which might represent an event during the obduction between the oceanic fore-arc Island and the Arabian Plate during the Late Cretaceous/Paleocene period.

A multi-mineral U-(Th)-Pb dating study of the Stetind pegmatite of the Tysfjord region, Norway, and implications for production of NYF-rare element pegmatites during orogenic collapse

Field relationships, textures, compositions and U-(Th)-Pb dates for minerals from the niobium‑yttrium‑fluorine (NYF-) Stetind pegmatite in the 1.798 Ga Tysfjord granitic gneiss in Norway are described. The pegmatite has a sub-horizontal orientation and anisotropic mineral layering oriented sub-parallel to contacts with the host-rock and oblique to its foliation. Epidote-group minerals have allanite cores and interstitial REE-rich epidote. Zircon, thorite, uraninite, fergusonite and xenotime, as well as apatite, fluorite, plagioclase and orthoclase are compositionally homogenous and display relict albite twinning and ternary junctions. Feldspars lack grain boundary recrystallisation textures, and no undulatory extinction is observed in quartz. Isotope UPb TIMS analysis of zircon, allanite, fergusonite and uraninite give dates between 400 and 410 Ma that have a high degree of concordance and no evidence of inheritance. A F-rich fluid dissolved fergusonite (YNbO4) and reacted with apatite to produce xenotime (YPO4) mantles around apatite. The reaction was dated by electron-probe micro-analysis total U-Th-Pb methods to 393 ± 12 Ma. The field and petrographic observations, coupled with mineral dates are interpreted to reflect primary emplacement and crystallisation of the Stetind NYF pegmatite after the peak of Scandian orogenesis and metamorphism during exhumation of the Tysfjord granitic gneiss. The xenotime forming event reflects a late magmato-hydrothermal reaction during the final stages of crystallisation. The compositional affinity of the Stetind pegmatite to its host rock suggests that the pegmatite-magma was produced by partial melting of the Tysfjord granitic gneiss or a rock with similar characteristics. The pegmatite is one of many rare-element pegmatites in the Tysfjord-Hamarøy pegmatite field, which may indicate a Scandian phase of pegmatite emplacement into the uppermost allochthon of the Norwegian Caledonides. The results validate models that propose the production of NYF-type pegmatites by anatexis during orogenic collapse.

Axinit a doprovodné minerály z lokality Jezuitský rybník východně od Golčova Jeníkova (moldanubikum, Česká republika)

A new occurrence of axinite at the locality Jezuitský rybník near Sirákovice (ENE from Golčův Jeníkov), situated in rocks of the Variegated (Drosendorf) Series (Moldanubian Zone of the Bohemian Massif), is a nice example of contaminated pegmatite in a Ca-skarn with intense superimposed hydrothermal overprint. Axinite [axinite-(Fe) to axinite-(Mg)] forms young hydrothermal infill of pocket/fissure in pegmatite cutting a brecciated Ca-skarn. The hydrothermal assemblage includes amphibole II (actinolite to ferro-actinolite), albite, K-feldspar II, chlorite, epidote (locally containing 0.20 - 0.30 apfu REE), muscovite and Al,F-enriched titanite (with up to 2 % SnO2) passing exceptionally to unnamed CaAlFSiO4. Quartz, plagioclase (andesine), K-feldspar I and amphibole I (mostly K-rich or even potassian ferro-pargasite to ferro-tschermakite) originated in magmatic stage associated with intrusion of externally derived pegmatite melt. Sporadic garnet (grossular-rich almandine) represents relics of mineral assemblage of the host skarn. Dominance of Nd among REE in the REE-rich epidote is explained in terms of chemical fractionation of REE, probably caused by the presence of strong REE-complexing ligands (F-, OH- and/or CO32-) in aqueous fluids enriched in MREE/HREE due to alteration of garnet. With regard to the presence of B, Cr and elevated XMg in some hydrothermal phases compared to the older Fe-Mg minerals, we suggest circulation of fluids affecting host rocks as well as additional rock types.

The trace element and U-Pb systematics of metamorphic apatite

Abstract Apatite is a common accessory mineral in igneous, sedimentary and metamorphic rocks. It has potential as a provenance indicator in sedimentary systems, as it can host a wide variety of trace elements in its crystal structure and can yield thermochronological age information. However, the processes controlling the trace element and U-Pb systematics of metamorphic apatite remain poorly understood, and metamorphic apatite remains significantly under-represented in compositional provenance databases linking apatite trace-element chemistry to its corresponding parent rock type. We investigate the trace-element and U-Pb systematics of metamorphic apatite from a suite of 22 bedrock samples of diverse metamorphic grade and protolith type, sampled from a variety of metamorphic terranes. Metamorphic apatite from low- to medium-grade metapelites and metabasites can be easily distinguished from granitic apatite as it is significantly depleted in Th, REE, and Y. Depletion in Th and REE + Y is attributed to growth of co-genetic epidote, which is the dominant carrier phase of the REE + Y, Th, and U in all the low- to medium-grade samples mapped by laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) and energy dispersive X-ray spectroscopy (EDS). Apatite U contents in low- to medium-grade metapelites and metabasites are more variable than the Th and REE + Y contents, but are typically low. Consequently, grains from these rock types are often undateable by the U-Pb LA-ICP-MS method and thus are underrepresented in apatite U-Pb detrital datasets, but can still be identified as low- to medium-grade metamorphic apatite by their trace-element characteristics. Low-grade metapelite apatite is difficult to distinguish from low-grade metabasite apatite, which is likely due to the growth of U-, Th-, and REE-rich epidote in both lithologies. Detrital (granitic) apatite is stable in very low-grade (e.g. pumpellyite-actinolite facies) metasedimentary samples, while neo- or re-crystallized metamorphic apatite is widespread by the upper-greenschist facies. LA-Q-ICP-MS imaging demonstrates that low REE + Y, Th, and U metamorphic apatite rims can nucleate on detrital igneous apatite precursors with high REE + Y, Th, and U. With increasing metamorphic grade, 1) relict detrital apatite is consumed, 2) the coherence of the U-Pb concordia systematics dating metamorphism improves, and 3) the degree of dispersion on metamorphic apatite multi-element plots decreases. The highest-grade metamorphic apatite samples investigated are paragneisses, some of which have locally undergone anatexis. Apatites from these samples yield well constrained TW concordia intercept ages and minor dispersion on multi-element plots (which is attributed to the absence of epidote) and closely resemble apatite from S-type granites in their trace element characteristics.

Rare earth elements and Sm-Nd isotope redistribution in apatite and accessory minerals in retrogressed lower crust material (Bergen Arcs, Norway)

In the Bergen Arcs (Norway), Grenvillian granulite retrogressed under eclogite- and amphibolite-facies conditions during Caledonian subduction/collision offer a unique opportunity to investigate rare earth elements (REE) and Sm-Nd isotope redistribution in accessory minerals during fluid-assisted metamorphism. Our sampling targeted apatite-bearing REE-rich protoliths (mangerite and jotunite) that preserve distinct mineral assemblages, depending on the external fluid availability and metamorphic conditions. REE concentrations in apatite are the highest in the granulite. Two populations are present: magmatic apatite (Ap1) relics that occur as inclusions in ilmenite-hematite, and intergranular apatite (Ap2) formed under granulite-facies conditions. The presence of abundant needle-like monazite and sulphide inclusions in Ap2 indicate that granulite reactions were fluid assisted. A thin (typically <10μm) rim of REE-rich epidote (Ep1) commonly surrounds Ap2. In these accessory minerals, U and Th contents are too low, or grains are too small, for in situ U-Th-Pb dating. Sm-Nd isotope data of Ap2, monazite and Ep1 give an isochron age of 601±69Ma, which is interpreted to represent a partially reset Grenvillian age, affected by Caledonian fluid-assisted mineral growth. In amphibolitized samples, granulite Ap2 is replaced by apatite (Ap3) with lower REE contents and no monazite inclusions. The REE released by this replacement are redistributed in a corona of epidote group minerals (Ep2) surrounding Ap3. The in situ Sm-Nd isotope data for Ep2 and titanite, found in replacement of ilmenite-hematite, return an isochron age of 395±65Ma, recording the timing of amphibolite-facies mineral growth when fluids were introduced into the rock. In eclogitized samples, eclogitic apatite (Ap4) occurs as polycrystalline aggregates, suggesting for a complex replacement process during deformation. REE contents of Ap4 are low, as REE originally contained in the precursor apatite were redistributed mainly into zoisite. Apatite shielded as inclusions in ilmenite and garnet preserve the REE-rich signature of the initial magmatic (Ap1) and granulite (Ap2) apatite, indicating these grains did not undergo further re-equilibration during Caledonian metamorphism. The resistance of apatite to compositional re-equilibration in this case confirms the petrological potential of apatite inclusions shielded in chemically inert minerals to track early magmatic, or metamorphic, crystallisation stages.

Zonal REE + Y profiles in garnet and their genetic implications: A case study of metapelites from the northern Ladoga region

The objective of this study is to provide insights into the REE and Y behavior during garnet porphyroblast formation in staurolite-bearing schists as a constituent of Late Paleoproterozoic metapelites of the Ladoga Complex. The MnNCKFMASH P–T pseudosection for a single sample and Grt–Bt thermometry indicate that the garnet core grew at 520°C and under 7.0–7.2 kbar in the Grt–Bt–Pl–Chl–Ms–Zo field, whereas the garnet rim was equilibrated at 590–600°C and under 3.5–4.0 kbar. The measured zoning profiles are strongly depleted in REE + Y in the garnet core containing high Mn and Ca concentrations. The intermediate zone of garnet is enriched in La, Ce, Pr, and Nd (inner LREE + Nd annulus), as well as in Dy, Er, Yb, Lu, and Y (outer HREE + Y + Dy annulus). According to pseudosection analysis, these peaks were probably produced owing to breakdown of epidote-group minerals (allanite, REE-rich epidote) at T 6.5 kbar. Towards the rim, the HREE + Y contents gradually decrease, whereas MREE (Sm, Eu, Gd) display an inverse trend. The rim also exhibits a negative Eu anomaly. The former tendency reflects an increase in temperature during garnet crystallization and partitioning of elements between garnet and monazite. It is thought that the latter is linked to oppositely directed change in garnet-monazite partition coefficients for HREE and MREE with increasing temperature.

Experimental constraints on the relative stabilities of the two systems monazite-(Ce) \u2013 allanite-(Ce) \u2013 fluorapatite and xenotime-(Y) \u2013 (Y,HREE)-rich epidote \u2013 (Y,HREE)-rich fluorapatite, in high Ca and Na-Ca environments under P-T conditions of 200\u20131000 MPa and 450\u2013750 \xb0C

The relative stabilities of phases within the two systems monazite-(Ce) – fluorapatite – allanite-(Ce) and xenotime-(Y) – (Y,HREE)-rich fluorapatite – (Y,HREE)-rich epidote have been tested experimentally as a function of pressure and temperature in systems roughly replicating granitic to pelitic composition with high and moderate bulk CaO/Na2O ratios over a wide range of P-T conditions from 200 to 1000 MPa and 450 to 750 °C via four sets of experiments. These included (1) monazite-(Ce), labradorite, sanidine, biotite, muscovite, SiO2, CaF2, and 2 M Ca(OH)2; (2) monazite-(Ce), albite, sanidine, biotite, muscovite, SiO2, CaF2, Na2Si2O5, and H2O; (3) xenotime-(Y), labradorite, sanidine, biotite, muscovite, garnet, SiO2, CaF2, and 2 M Ca(OH)2; and (4) xenotime-(Y), albite, sanidine, biotite, muscovite, garnet, SiO2, CaF2, Na2Si2O5, and H2O. Monazite-(Ce) breakdown was documented in experimental sets (1) and (2). In experimental set (1), the Ca high activity (estimated bulk CaO/Na2O ratio of 13.3) promoted the formation of REE-rich epidote, allanite-(Ce), REE-rich fluorapatite, and fluorcalciobritholite at the expense of monazite-(Ce). In contrast, a bulk CaO/Na2O ratio of ~1.0 in runs in set (2) prevented the formation of REE-rich epidote and allanite-(Ce). The reacted monazite-(Ce) was partially replaced by REE-rich fluorapatite-fluorcalciobritholite in all runs, REE-rich steacyite in experiments at 450 °C, 200–1000 MPa, and 550 °C, 200–600 MPa, and minor cheralite in runs at 650–750 °C, 200–1000 MPa. The experimental results support previous natural observations and thermodynamic modeling of phase equilibria, which demonstrate that an increased CaO bulk content expands the stability field of allanite-(Ce) relative to monazite-(Ce) at higher temperatures indicating that the relative stabilities of monazite-(Ce) and allanite-(Ce) depend on the bulk CaO/Na2O ratio. The experiments also provide new insights into the re-equilibration of monazite-(Ce) via fluid-aided coupled dissolution-reprecipitation, which affects the Th-U-Pb system in runs at 450 °C, 200–1000 MPa, and 550 °C, 200–600 MPa. A lack of compositional alteration in the Th, U, and Pb in monazite-(Ce) at 550 °C, 800–1000 MPa, and in experiments at 650–750 °C, 200–1000 MPa indicates the limited influence of fluid-mediated alteration on volume diffusion under high P-T conditions. Experimental sets (3) and (4) resulted in xenotime-(Y) breakdown and partial replacement by (Y,REE)-rich fluorapatite to Y-rich fluorcalciobritholite. Additionally, (Y,HREE)-rich epidote formed at the expense of xenotime-(Y) in three runs with 2 M Ca(OH)2 fluid, at 550 °C, 800 MPa; 650 °C, 800 MPa; and 650 °C, 1000 MPa similar to the experiments involving monazite-(Ce). These results confirm that replacement of xenotime-(Y) by (Y,HREE)-rich epidote is induced by a high Ca bulk content with a high CaO/Na2O ratio. These experiments demonstrate also that the relative stabilities of xenotime-(Y) and (Y,HREE)-rich epidote are strongly controlled by pressure.

Possible new Ca-REE-Bi phosphate minerals from a tungsten-rich calcsilicate skarn, Sierra Nevada Mountains, California

Scanning electron microscope and electron microprobe analyses of 3 to 15 μm diameter grains present within a garnet-quartz granofels from a tungsten skarn reveal the possible existence of at least two new rare earth element (REE)-bearing phosphate phases: Ca(Ce,La,Bi,Nd) 2 [(P,As)O 4 ] 2 (OH) 2 and Ca(La,Ce,Nd,Pr,Bi) 2 [(P,As)O 4 ] 2 (OH) 2 . The analyzed REEs constitute up to 50 wt% of the phases; bismuth oxide contents range from 4.1 to 16.1 wt%. Structural data has proved impossible to obtain from these tiny grains, presumably due to radiation damage by thorium decay. These potentially new phosphate minerals are present within alteration assemblages of REE-rich epidote crystals, as well as along grain boundaries and cracks cross-cutting the quartz-garnet host rock. Association with the zeolite brewsterite-Ba suggests that these hydroxyl phosphates formed during water-rich, low-temperature, retrograde mineralization in the skarn environment.

NOVÝ VÝSKYT SKARNU V GFÖHLSKÉ JEDNOTCE U VEVČIC U JEVIŠOVIC: MINERÁLNÍ ASOCIACE SKARNU A KONTAMINOVANÝCH AMFIBOLICKÝCH PEGMATITŮ

This work provides detailed information about a new occurrence of skarn near Vevčice (near Jevišovice, western Moravia), which is one of the most important outcrops of these rocks in the southeastern part of the Moldanubian Zone (Gföhl Unit). The skarn is characterized by a strong migmatization, which resulted in contaminated amphibole pegmatite veins penetrating the skarn. The mineral assemblage of the skarn is simple, with dominant garnets, both grossular-almandine (Alm55–57 Grs31–37 Adr3–8) and grossular-andradite-almandine (Grs45–53 Adr20–34 Alm20–24). Garnets clearly predominate over clinopyroxene, hedenbergite with a slightly increased “fassaite component”. The very low amount of quartz contrasts with that in the skarns in the surrounding Gföhl unit. Amphibole (potassium-rich hastingsite) is younger mineral in the skarn, and especially in contaminated pegmatites. Epidote after garnets, and eventually prehnite, belong to the youngest minerals in some types of rocks. Accessory titanite, sometimes rich in Sn, was found frequently, as well as metamict REE-rich epidote. Magnetite is rather exceptional, mainly in assemblages replacing garnet; fluorapatite, ilmenite and zircon are rare. The studied skarn is part of a lithologically varied sequence of the Gföhl Unit, with intercalations of Ca-metasediments, appearing in several non-contiguous horizons around the boundary of so-called Běhařovice-Vémyslice synform, with a granulite–serpentinite complex in its center. This sequence of strongly migmatized biotite paragneiss to leucocratic migmatites also contains diopside and scapolite-diopside gneiss, garnet-pyroxene and phlogopite-diopside skarns containing magnetite and exceptionally Au-Co-Bi and REE mineralization as well as rare occurrences of spinel-forsterite dolomitic marbles.

Predicting radioactive accessory mineral dissolution during chemical weathering: The radiation dose at the solubility threshold for epidote-group detrital grains from the Yangtze River delta, China

The influence of heavy-ion irradiation on the solubility of natural epidote-group minerals has been investigated. The epidote-group are calc-silicate minerals that, if soluble during terrestrial chemical weathering, are capable of consuming atmospheric CO2 over geologic time. The experimental design was to calculate the α-particle dose for actinide-rich epidote-group grains from the deltas of rivers draining large watersheds. Large watersheds should yield a suite of epidote-group grains spanning the range of compositions and radiation damage commonly observed in nature. These grains would be resistant to dissolution during chemical weathering and survive into fluvial sediments. Therefore, the highest α-particle dose calculated for the suite of grains represents a minimum solubility threshold for the epidote-group minerals during chemical weathering. The α-particle dose is calculated from the 232Th, 238U, and 235U content of a grain, and the grain’s date. Actinide-rich epidote-group grains were isolated from Yangtze River and River Nile delta sediments, although only the grains from the Yangtze delta had 232Th/208Pb ratios sufficiently high to yield meaningful dates.A total of 28 Th-rich epidote-group grains were isolated from the Yangtze delta sediment, with 24 being classified as allanite and four being classified as REE-rich epidote. Isotopic ratio data were measured by laser ablation microprobe-inductively coupled plasma-mass spectrometry. Dates and REE patterns revealed at least 17 different sources of grains, with ThO2 contents ranging from 0.057 to 4.37 wt. %, and dates ranging from 32±2 to 3788±263Ma. The calculated maximum α-particle dose of the suite of Yangtze delta epidote-group mineral grains, and hence the minimum solubility threshold, is 7.1 × 1015 α-decay mg−1. Therefore, to predict the likelihood of an epidote-group mineral dissolving during chemical weathering at a specific study site the α-particle dose may be calculated from the date of the rock, even if estimated, and the radioactinide concentrations of the mineral. A calculated α-particle dose below ~7.1 × 1015 α-decay mg−1 likely reflects epidote-group mineral stability during chemical weathering.Calculation of the α-particle dose may be superior to microscopic identification of accessory (<2% by volume) mineral dissolution during chemical weathering. This is because accessory minerals occur in relatively low abundances, may occur as relatively small grains, be highly soluble, and/or be heterogeneously distributed in the bedrock and regolith. Furthermore, the solubility of radioactively damaged accessory minerals cannot be readily predicted or quantified by geochemical thermodynamic and/or kinetic principles.The α-particle dose at the solubility threshold of ~7.1 × 1015 α-decay mg−1 value is approximately double that reported for zircon. Therefore, relative to zircon, epidote-group minerals can withstand more radiation prior to becoming soluble. This finding is surprising considering the chemical durability of zircon. This substantial variance in solubility threshold radiation doses between the two minerals cannot be explained by the bond strengths of non-tetrahedral cations and structural oxygen which are lower in allanite than zircon.

A-type magmatism in the sierras of Maz and Espinal: A new record of Rodinia break-up in the Western Sierras Pampeanas of Argentina

Two orthogneisses have been recognized in the sierras of Espinal and Maz (Western Sierras Pampeanas, NW Argentina) that were emplaced within a Grenvillian metasedimentary sequence. Microcline, plagioclase and quartz are the main rock-forming minerals, with accessory zircon, apatite-(CaF), magnetite, biotite (Fe/(Fe + Mg) = 0.88–0.91), ferropargasite (Fe total/(Fe total + Mg) = 0.88–0.89), titanite (with up to 1.61 wt% Y 2O 3) and an REE-rich epidote. REE-poor epidote and zoned garnet (Ca and Fe 3+-rich) are metamorphic minerals, while muscovite, carbonates and chlorite are secondary phases. Texture is mylonitic. Two representative samples are classified as granite (from Sierra de Espinal) and granodiorite/tonalite (from Sierra de Maz) on the grounds of immobile trace elements. Some trace element contents are rather high (Zr: 603 and 891 ppm, Y: 44 and 76 ppm, 10,000 × Ga/Al: 2.39–3.89) and indicate an affiliation with A-type granites (more specifically, the A 2 group). Both samples plot in the field of within-plate granites according to their Y and Nb contents. Concordant crystallization ages (zircon U–Pb SHRIMP) are 842 ± 5 and 846 ± 6 Ma, respectively. 87Sr/ 86Sr i (845) ratios are 0.70681 and 0.70666; ɛNd i (845) values are −1.5 and +0.3 and depleted-mantle Nd model ages (2T DM*) are 1.59 and 1.45 Ga, respectively. These values indicate the involvement of an isotopically evolved source. 2T DM* values are compatible with the presence of inherited zircon crystals of up to 1480 Ma in one of the rocks, thus implying that magmas incorporated material from Mesoproterozoic continental source. This is also indicated by the relatively high contents of Y, Ga, Nb and Ce compared to magmas derived from sources similar to those of oceanic-island basalts. These orthogneisses represent a period of extension at ca. 845 Ma affecting the Western Sierras Pampeanas continental crust that was already consolidated after the Grenvillian orogeny (1.2–1.0 Ga). They are thus a record of the early stages of Rodinia break-up. Metamorphic conditions during the subsequent Famatinian orogenic cycle (ca. 420 Ma, SHRIMP U-Pb on zircon) attained 7.7 ± 1.2 kbar and 664 ± 70 °C.

Long-term average mineral weathering rates from watershed geochemical mass balance methods: Using mineral modal abundances to solve more equations in more unknowns

The number of phases for which weathering rates can be determined by watershed geochemical mass balance is limited by the number of equations that can be constructed from elemental flux losses from the watershed and mineral stoichiometries. Mass balance studies of watershed weathering rates routinely use the flux losses of the six major cations SiO 2, Al, Na, K, Mg, and Ca. Analyses of these species in water are common, but following matrix algebraic methods limits the number of weathering rates that can be calculated to six. For the Brubaker Run watershed located in the northern Piedmont Physiographic Province of Pennsylvania (USA), long-term (10 3–10 6 year) watershed chemical flux losses have been determined using 10Be-derived total denudation rates and zirconium-normalized total chemical concentrations from bedrock and soils. Chemical flux losses calculated from solid-phase data have three advantages: They (1) Permit generation of a relatively large number of equations because both major and trace analyses are included; (2) eliminate the need for many years of regular (e.g., weekly) sampling and chemical analyses of stream water and atmospheric precipitation, and measurement of hydrologic parameters (i.e., precipitation, stream discharge, etc.); and (3) long-term weathering rate calculations need not address biomass. For Brubaker Run, eight minerals are involved in weathering; the five primary minerals are REE-rich epidote, ankerite, almandine–spessartine garnet, muscovite, chlorite, and the three secondary products are weathered muscovite, kaolinite, and gibbsite. The long-term average weathering rates of these minerals were calculated using the six major cations, and two trace elements selected from Rb, Sr, Ba, La, Pr, Nd, Sm, Gd, and Dy. Despite having the eight equations needed, geochemically reasonable weathering rates (e.g., positive primary mineral rates that reflect destruction) could not be achieved regardless of the two trace elements used in the mass balance calculations. For Brubaker Run, this is primarily attributable to the natural heterogeneity of the trace element concentrations within the host mineral grains, with trace element stoichiometries in some minerals varying by as much as an order of magnitude. Because the trace elements are hosted by a relatively small number of minerals, the computed weathering rates of other minerals become very sensitive to small variations in trace cation stoichiometry. REE-rich epidote, garnet, and ankerite within the Brubaker Run watershed together host nearly all of the Ca in the bedrock, and completely dissolve at or near the weathering front. Consequently, approximately all of the Ca in bedrock is lost from the regolith. In bedrock the mole-percentages of Ca hosted by REE-rich epidote, garnet, and ankerite are 49 mol%, 4 mol%, and 43 mol%, respectively, and are determined by the modal abundance of the mineral in the bedrock and its Ca stoichiometry. The weathering rates of REE-rich epidote, garnet, and ankerite can be determined by distributing to each mineral that fraction of the total watershed Ca flux loss for which it is responsible based on its mole-percent Ca in bedrock. By using a base cation that is completely lost from the regolith, and knowing the mole-percentage of that element in the mineral(s) undergoing weathering, additional equations may be added to the mass balance matrix. We term this technique the “flux distribution method.” The flux distribution method eliminates the need for additional equations established using trace elements. Based on the mineral weathering rates for the Brubaker Run watershed determined using the flux distribution method, the rates at which the weathering front penetrated the bedrock (the “saprolitization” rate) are 4.5 m Myr − 1 and 6.5 m Myr − 1 for chlorite and muscovite, respectively. These measured long-term average saprolitization rates compare very favorably with published theoretical values for the nearby northern Maryland Piedmont which range from 2.2 to 5.3 m Myr − 1 .

褐簾石の結晶化学と地球化学の最近の動向：島弧鉱物学の萌芽

Allanites from the Japanese island arc are not necessarily the same as those from other countries of the world. The study of minerals occurring in the Japanese islands context is integrated into “island arc mineralogy”, which concerns the following space-time continuum: (1) the present island arc, (2) the post-Paleozoic rocks, (3) subduction-related magmatism, (4) volcanic front, (5) tectonic discontinuities, and (6) an abundant variety of minerals, their paucity and characteristic accessory minerals. “Island arc mineralogy” of allanites from Japan is functionally related to (1), (2), (3) and (6). This paper focuses on tectonic setting (intracontinental settings,island arcs, and continental margin), formation age and a variety of host rock types containing allanite, crystal chemisry of REE-rich epidote group minerals, particularly allanite, and geochemical role of their chemical composition, crystal structure and mineral inclusion.

Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, Western China

Amphibolite-facies para-and orthogneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism associated with the Early Paleozoic continental collision of the Qilian and Qaidam microplates. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb and REE analyses of zircons from four eclogites yield weighted mean ages of 449 to 422 Ma, and REE patterns (flat HREE, no Eu anomaly) and inclusions of garnet, omphacite, and rutile indicate these ages record eclogite-facies metamorphism. The coherent field relations of these samples, and the similar range of individual ages in each sample suggests that the ∼25 m.y. age range reflects the duration of eclogite-facies conditions in the studied samples. Analyses from zircon cores in one sample yield scattered 433 to 474 Ma ages, reflecting partial overlap on rims, and constrain the minimum age of eclogite protolith crystallization. Inclusions of Th + REE-rich epidote, and zircon REE patterns are consistent with prograde metamorphic growth. In the Luliang Shan, approximately 350 km northwest in the North Qaidam terrane, ages interpreted to record eclogite-facies metamorphism of eclogite and garnet peridotite are as old as 495 Ma and as young as 414 Ma, which suggests that processes responsible for extended high-pressure residence are not restricted to the Dulan region. Evidence of prolonged eclogite-facies metamorphism in HP/UHP localities in the Northeast Greenland eclogite province, the Western Gneiss Region of Norway, and the western Alps suggests that long eclogite-facies residence may be globally significant in continental subduction/collision zones.

Chromian epidote in omphacite rocks from the Sambagawa metamorphic belt, central Shikoku, Japan

Chromian epidote was found in pebbles of omphacite rock derived from the Sambagawa metamorphic rocks in central Shikoku, Japan. The pebbles consist of chromian epidote, omphacite, amphibole, muscovite, phlogopite, chromite, albite, and zircon. The chromian epidote crystals are dark yellow to brown. Microscopically, they are subhedral and are pleochroic from yellowish orange to pale yellow-colorless. Chromian epidote has a zonal structure. Typically, cores of Sr-rich epidote are overgrown with Ca-epidote; alternatively Ca-epidotes are rimmed by and/or intergrown with REE-rich epidote. However, Cr distribution is not related to the zonal structure caused by Ca ↔ Sr and Ca + M3+ ↔ REE3+ + M2+ substitution since regions of higher Cr-concentration generally occur around chromite grains. The chromium content of epidote reaches 5.7 wt% Cr2O3 (0.36 Cr apfu/12.5 oxygens). In contrast, the Fe3+ content of the chromian epidote varies in a narrow range (5.1 to 8.9 wt% Fe2O3) irrespective of the chromium content. Associated minerals surrounding chromite (omphacite, amphibole, muscovite, and phlogopite) also tend to have a high and variable chromium content caused by Al ↔ Cr substitution, but with a nearly constant Fe content in individual minerals. Chromite is considered to be the source material of the chromian epidote and the associated minerals. The heterogeneous distribution of chromium may be attributed to the immobility of chromium under metamorphic conditions. The maximum Cr content of the chromian epidote in the present study is less than that of Fe3+-poor chromian epidote from other localities, whereas the Fe3+ content is greater. The substitution of Cr3+ for Al in the M3 site of the studied chromian epidote may be limited by the ferric iron occupying the M3 site to the extent of 0.52 apfu.

Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, Western China

Amphibolite-facies para-and orthogneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism associated with the Early Paleozoic continental collision of the Qilian and Qaidam microplates. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb and REE analyses of zircons from four eclogites yield weighted mean ages of 449 to 422 Ma, and REE patterns (flat HREE, no Eu anomaly) and inclusions of garnet, omphacite, and rutile indicate these ages record eclogite-facies metamorphism. The coherent field relations of these samples, and the similar range of individual ages in each sample suggests that the ∼25 m.y. age range reflects the duration of eclogite-facies conditions in the studied samples. Analyses from zircon cores in one sample yield scattered 433 to 474 Ma ages, reflecting partial overlap on rims, and constrain the minimum age of eclogite protolith crystallization. Inclusions of Th + REE-rich epidote, and zircon REE patterns are consistent with prograde metamorphic growth. In the Luliang Shan, approximately 350 km northwest in the North Qaidam terrane, ages interpreted to record eclogite-facies metamorphism of eclogite and garnet peridotite are as old as 495 Ma and as young as 414 Ma, which suggests that processes responsible for extended high-pressure residence are not restricted to the Dulan region. Evidence of prolonged eclogite-facies metamorphism in HP/UHP localities in the Northeast Greenland eclogite province, the Western Gneiss Region of Norway, and the western Alps suggests that long eclogite-facies residence may be globally significant in continental subduction/collision zones.

Chromian epidote in omphacite rocks from the Sambagawa metamorphic belt, central Shikoku, Japan

Chromian epidote was found in pebbles of omphacite rock derived from the Sambagawa metamorphic rocks in central Shikoku, Japan. The pebbles consist of chromian epidote, omphacite, amphibole, muscovite, phlogopite, chromite, albite, and zircon. The chromian epidote crystals are dark yellow to brown. Microscopically, they are subhedral and are pleochroic from yellowish orange to pale yellow-colorless. Chromian epidote has a zonal structure. Typically, cores of Sr-rich epidote are overgrown with Ca-epidote; alternatively Ca-epidotes are rimmed by and/or intergrown with REE-rich epidote. However, Cr distribution is not related to the zonal structure caused by Ca ↔ Sr and Ca + M3+ ↔ REE3+ + M2+ substitution since regions of higher Cr-concentration generally occur around chromite grains. The chromium content of epidote reaches 5.7 wt% Cr2O3 (0.36 Cr apfu/12.5 oxygens). In contrast, the Fe3+ content of the chromian epidote varies in a narrow range (5.1 to 8.9 wt% Fe2O3) irrespective of the chromium content. Associated minerals surrounding chromite (omphacite, amphibole, muscovite, and phlogopite) also tend to have a high and variable chromium content caused by Al ↔ Cr substitution, but with a nearly constant Fe content in individual minerals. Chromite is considered to be the source material of the chromian epidote and the associated minerals. The heterogeneous distribution of chromium may be attributed to the immobility of chromium under metamorphic conditions. The maximum Cr content of the chromian epidote in the present study is less than that of Fe3+-poor chromian epidote from other localities, whereas the Fe3+ content is greater. The substitution of Cr3+ for Al in the M3 site of the studied chromian epidote may be limited by the ferric iron occupying the M3 site to the extent of 0.52 apfu.

Accessory minerals and subduction zone metasomatism: a geochemical comparison of two mélanges (Washington and California, U.S.A.)

The ability of a subducted slab or subducted sediment to contribute many incompatible trace elements to arc source regions may depend on the stabilities of accessory minerals within these rocks, which can only be studied indirectly. In contrast, the role of accessory minerals in lower- T and - P metasomatic processes within paleo-subduction zones can be studied directly in subduction-zone metamorphic terranes. The Gee Point-Iron Mountain locality of the Shuksan Metamorphic Suite, North Cascades, Washington State, is a high- T mélange of metamafic blocks in a matrix of meta-ultramafic rocks. This mélange is similar in geologic setting and petrology to the upper part of an unnamed amphibolite unit of the Catalina Schist, Santa Catalina Island, southern California. Both are interpreted as shear zones between mantle and slab rocks that formed during the early stages of subduction. Some garnet amphibolite blocks from the Gee Point-Iron Mountain locality display trace-element enrichments similar to those in counterparts from the Catalina Schist. Some Catalina blocks are highly enriched in Th, rare-earth elements (REE), the high-field-strength elements Ti, Nb, Ta, Zr and Hf (HFSE), U and Sr compared to mid-ocean ridge basalt (MORB), and to other garnet amphibolite blocks in the same unit. Textural and geochemical data indicate that accessory minerals of metamorphic origin control the enrichment of Th, REE and HFSE in blocks from both areas. The Mg-rich rinds around blocks and the meta-ultramafic matrix from both mélanges are highly enriched in a large number of trace elements compared to harzburgites, dunites and serpentinites. Evidence for recrystallization or formation of accessory minerals in the former rocks suggests that these minerals control some of the trace-element enrichments. Data from the Gee Point and Catalina mélanges suggest that the accessory minerals titanite, rutile, apatite, zircon and REE-rich epidote play a significant role in the enrichment of trace elements in both mafic and ultramafic rocks during subduction-related fluid-rock interaction. Mobilization of incompatible elements, and deposition of such elements in the accessory minerals of mafic and ultramafic rocks may be fairly common in fluid-rich metamorphic environments in subduction zones.