Cogenetic Minerals Research Articles

Heterogeneous Archean oceanic protoliths in Neoproterozoic retrogressed eclogites from Northeast Brazil: Petrological, geochemical and Sr–Nd–Pb–Hf isotopic constraints

Eclogites are reliable indicators of high to ultrahigh-pressure (HP-UHP) conditions for oceanic and continental crust and are essential rocks to understand the lithospheric convergence from subduction to post-collisional setting. Furthermore, the origin and nature of eclogite precursors may reveal crucial constraints on the crustal history before HP-UHP metamorphism. Here, we provide petrological, geochemical and Pb-Sr-Nd-Hf isotopic constraints from the Campo Grande retrogressed eclogites, Northeast Brazil, to determine the source and evolution of their Archean protoliths. Eclogites were formed under HP conditions (18 ± 1.0 kbar and 680 ± 20 °C) followed by retrogression in the amphibolite facies, which is characterized by Mg-rich garnet porphyroblasts surrounded by clinopyroxene + plagioclase + amphibole + orthopyroxene symplectites (stable at <6.0 kbar). These retrogressed eclogites have chemical compositions corresponding to enriched-type tholeiites such as E-MORB protoliths (G1 eclogitic group). A second group of retrogressed eclogites with higher amphibole content (G2 eclogitic group) shows similar chemical patterns to G1, however with higher enrichment in LILE, REE and incompatible elements. The G1 eclogite is characterized by higher modal concentration of garnet and clinopyroxene, positive εNd(t) (+2.1 to +5.8) with low 87Sr/86Sr(i) ratios (0.7024 to 0.7131), whereas G2 displays dominant coarse-grained amphibole, positive and negative εNd(t) (−7.5 to +8.5) with high radiogenic 87Sr/86Sr(i) ratios (0.7109 to 0.7255). The chemical and isotopic G1 values show typical E-MORB signatures, whereas G2 indicate an E-MORB enriched by crustal components in a supra-subduction zone setting. A LuHf isochron provided the age of 2667.9 ± 30.7 Ma, which is close to the published ∼2.65 Ga crystallization age for the G1 and G2 oceanic protoliths. Therefore, Hf isotopes in whole rock and cogenetic minerals were useful for constraining the crystallization time of metamafic protoliths. In contrast, SmNd isochron age of 606.9 ± 24.8 Ma falls within the range of 623 to 590 Ma HP metamorphism recorded in the G1 and G2 rocks. Lastly, a third type of eclogite group (G3 eclogitic group) is represented by a banded Mn-rich-garnet (spessartite) and Al-rich clinopyroxene with detrital zircon cores from 3.42 to 2.15 Ga and recrystallized rims at 607.7 ± 5.4 Ma. Mineral texture and composition, geochemistry, strongly negative εNd(t) (−4.3 to −1.5) and higher radiogenic 87Sr/86Sr(i) ratios (0.7434–0.7501) suggest a different Mn-rich protolith for the G3 eclogites. Integrated data reveal that retrogressed eclogites retain the memory of the original chemical and isotopic patterns of Archean oceanic protoliths and its diachronous reworking processes at deep crustal levels.

Clumped isotope geothermometry in Archean mesothermal hydrothermal systems (Augmitto-Bouzan orogenic gold deposit, Abitibi, Québec, Canada): A note of caution and a look forward

This study evaluates the applicability of the clumped isotope thermometry to mesothermal hydrothermal systems (5–10 km depth; 250–450 °C). We measured Δ47, δ18O and δ13C of calcite as well as δ18O of cogenetic minerals from typical quartz-calcite ± tourmaline±chlorite orogenic veins from the Neoarchean Augmitto-Bouzan orogenic gold deposits (Abitibi, Canada). Our findings show that caution is required when utilizing the clumped isotope thermometry in the study of old mesothermal deposits. Temperatures calculated from Δ47 values are systematically and significantly shifted to low temperature, i.e., ∼150 °C rather than the ∼350 ± 50 °C expected for orogenic gold deposit formation and documented using oxygen isotope equilibrium between mineral pairs. We show that the low temperatures estimates resulted from solid-state reordering that occurred in calcite grains during the cooling history of the vein-hosting rocks. Because the fast cooling rates required to preserve the initial temperature of calcite formation in mesothermal hydrothermal systems are geologically unrealistic, we suggest that refractory minerals (i.e., minerals with higher blocking temperature such as magnesite, dolomite, ankerite) should be investigated to apply clumped isotope thermometry in such context.

Crustal Thickening of the Northern Central Andean Plateau Inferred From Trace Elements in Zircon

AbstractThe timing of crustal thickening in the northern Central Andean Plateau (CAP), at 13–20°S, and its relationship to surface uplift is debated. Zircon qualitatively records crustal thickness as its trace element chemistry is controlled by the growth of cogenetic minerals and relative uptake of light and heavy Rare Earth Elements. Jurassic to Neogene zircons from volcanic rocks, sandstones, and river sediments reveal shifts in trace element ratios suggesting major crustal thickening at 80–55 Ma and 35–0 Ma, coincident with high‐flux magmatism. An intervening magmatic lull due to shallow subduction obscures the magmatic record from 55 to 35 Ma during which thickening continued via crustal shortening. Protracted thickening since the Late Cretaceous correlates with early elevation gain of the CAP western margin, but contrasts with Miocene establishment of near modern elevation in the northern CAP and the onset of hyperaridity along the Pacific coast, highlighting their complex spatial and temporal relationship.

Inter-cratonic geochronological and geochemical correlations of the Derim Derim–Galiwinku/Yanliao reconstructed Large Igneous Province across the North Australian and North China cratons

Large igneous provinces (LIPs) are often used as a powerful tool for constraining continental reconstructions, due to their potential to correlate radiometric emplacement ages across scattered fragments of once continuous igneous bodies, and establish the presence of coeval magmatic suites. Mesoproterozoic magmatism in northern Australia and north China has been suggested to be the dispersed fragments of a once contiguous LIP. Here we present new geochronology and geochemical data from the under-resolved northern Australian fragments of this province (the Derim Derim and Galiwinku suites), and review existing data on related Mesoproterozoic mafic sills in the North Australian Craton and the North China Craton. We propose an updated framework for shared magmatic genesis and cratonic proximity during magmatic emplacement. Apatite geochronology has been applied to date dolerite intrusions, using cogenetic minerals to constrain the initial common-Pb constraints of the dolerite samples which provides a significant advantage for dating mafic rocks. The ages of samples collected throughout the extent of the mafic magmatic province in northern Australia and north China supports coincident ca. 1330–1295 Ma magmatism across both regions, while geochemical evidence indicates a mantle plume derived melt as the probable source for both suites. Coeval intrusion of geochemically analogous sills within temporally and stratigraphically comparable sedimentary basins is supportive of plume activity proximal to a once continuous sedimentary system that covered much of the North Australian Craton and North China Craton, which supports a close connection between the cratons within the supercontinent Nuna in the Mesoproterozoic.

U-Pb ID-TIMS reference ages and initial Pb isotope compositions for Durango and Wilberforce apatites

We present new Isotope Dilution Thermal Ionisation Mass Spectrometry (ID-TIMS) U-Pb reference ages for apatites from type locations Durango, Cerro de Mercado, Mexico and Wilberforce, Ontario, Canada. A weighted mean 206Pb/238U obtained from 14 Durango apatite fragments yields an age of 32.716 ± 0.061/0.063/0.072 Ma (2σ analytical uncertainty; analytical and tracer uncertainty combined; analytical, tracer, and decay constant uncertainties combined; MSWD = 0.81). Correction of excess 206Pb from Th-disequilibrium in Durango apatite, using a Th/Umelt of 3.5 ± 1, yields a weighted mean 206Pb/238U age of 32.29 ± 0.12/0.12/0.12 Ma. A concordia-constrained linear 3-D isochron yields an age of 32.683 ± 0.050 Ma for Durango apatites (ratio + error (%) + correlation coefficients; MSWD = 7.6; no propagation of decay constant and Pbc composition uncertainty, isoplot v. 4.15). A weighted mean 206Pb/238U obtained from 9 Wilberforce apatite fragments yield an age of 930.9 ± 8.0/8.0/8.1 Ma (MSWD = 0.65). Excess 206Pb correction for Wilberforce is negligible (<100kyr) relative to the uncertainty. A concordia-constrained linear 3-D isochron yields an age of 923 ± 14 Ma (ratio + error (%) + correlation coefficients; MSWD = 367; no propagation of decay constant and Pbc composition uncertainty, isoplot v. 4.15) for Wilberforce apatites. Initial Pb isotope compositions for these apatites were obtained by regression of the U-Pb data (3-D linear regression) and yielded a 206Pb/204Pb of 18.19 ± 0.12 and 207Pb/204Pb of 15.67 ± 0.10 for the Durango apatite reference material, and a 206Pb/204Pb of 17.06 ± 0.37 and 207Pb/204Pb of 15.33 ± 0.29 for the Wilberforce apatite reference material. Varying the assumed initial Pb composition can have significant effects on the calculated U-Pb age of apatites, which is highlighted with synthetic apatite data based on the composition of the Durango apatite reference material. For Durango apatites, the frequently used estimates of common Pb from two-stage Pb evolution models can artificially shift the U-Pb data and yield inaccurate dates, relative to the use of 3-D isochrons constructed using data obtained from Durango apatite. The use of 3-D isochrons to extract initial Pb isotope data should be restricted to samples/material with single U-Pb date population (closed system) and where estimates from Pb-bearing-U-free co-genetic mineral phases is unavailable and evaluated on a case-by-case basis.

Age and petrogenesis of the beryl-bearing granitic magmatism of the Velasco Pegmatite District, Pampeana Province, NW Argentina

Two porphyritic granitoids (Huaco and Sanagasta) in the Velasco district of the Pampeana Pegmatite Province in Northwestern Argentina are recognized. They are considered the fertile granites of the beryl-bearing pegmatites and can be described as post-orogenic and peraluminous A-type granites formed in an intraplate tectonic setting during an extensive regime, whose magma source is predominantly of cortical origin and to a lesser extent, mantle-derived. The pegmatites are classified as Rare Elements of the beryl type and beryl-columbite-phosphate subtype, while the entire district shows characteristics related to the NYF (Nb-Y-F) petrogenetic family. From rocks and cogenetic minerals of an individual ‘Ismiango’ pegmatite of the Velasco district, two Rb/Sr isochrons have been constructed. They define an age of 330.3 ± 8.3 and 331.7 ± 2.3 Ma and fall in the Lower Carboniferous period, consistent with the age of the parental and host-rock, the Huaco granite. As the Ismiango pegmatite has a similar composition and structure to the other beryl mineralized pegmatites of the Velasco district, the obtained age is attributed extensively for the entire district. According to the initial 87Sr/86Sr value obtained of 0.713, the pegmatite-magmatism of the Velasco District might be mainly derived from the crust with some minor participation of mantle materials.

An experimentally calibrated thermobarometric solubility model for titanium in coesite (TitaniC)

Experiments were conducted to quantify the temperature and pressure effects on the solubility of titanium in coesite. Powdered amorphous silica, titania (anatase), zirconia, and water were added to silver capsules and run in the coesite stability field (at 32, 35, and 40 kbar) from 700 to 1050 °C using a piston–cylinder apparatus. Crystallization of coesite, rutile, and zircon from silica-, titania-, and zircon-saturated aqueous fluids was confirmed by Raman spectroscopy. Cathodoluminescence images and electron microprobe measurements showed that coesite crystals are relatively homogenous. The Ti concentrations of coesite crystals are significantly higher than concentrations predicted using the Ti-in-quartz calibration (Wark and Watson in Contrib Mineral Petrol 152:743–754, 2006. https://doi.org/10.1007/s00410-006-0132-1 ; Thomas et al. in Contrib Mineral Petrol 160:743–759, 2010. https://doi.org/10.1007/s00410-010-0505-3 ). Titanium K-edge X-ray absorption near edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in coesite at all conditions studied. A model was calibrated to describe the effects of pressure and temperature on the solubility of titanium in coesite by using a least-squares method to fit Ti concentrations in coesite to the simple expression: $$RT\ln X_{{{\text{TiO}}_{2} }}^{\text{coesite}} = - 55.068 + 0.00195 \times T\;({\text{K}}) - 1.234 \times P\;({\text{kbar}}) + RT\ln a_{{{\text{TiO}}_{2} }}^{\text{rutile}} ,$$ where R is the gas constant 8.3145 × 10−3 kJ/K, P is pressure in kbar, T is temperature in kelvin, $$X_{{{\text{TiO}}_{2} }}^{\text{coesite}}$$ is the mole fraction of TiO2 in coesite, and $$a_{{{\text{TiO}}_{2} }}^{\text{rutile}}$$ is the activity of TiO2 in the system referenced to rutile. Ti-in-coesite solubility can be used as a thermobarometer for natural samples when used in combination with another indicator of temperature or pressure, such as another thermobarometer in a cogenetic mineral (e.g. rutile) or other phase equilibria (e.g. graphite = diamond). Applications of the Ti-in-coesite thermobarometer to samples from the western Alps and Papua New Guinea are presented.

Unprecedented 34 S-enrichment of pyrite formed following microbial sulfate reduction in fractured crystalline rocks.

In the deep biosphere, microbial sulfate reduction (MSR) is exploited for energy. Here, we show that, in fractured continental crystalline bedrock in three areas in Sweden, this process produced sulfide that reacted with iron to form pyrite extremely enriched in 34 S relative to 32 S. As documented by secondary ion mass spectrometry (SIMS) microanalyses, the δ34 Spyrite values are up to +132‰V-CDT and with a total range of 186‰. The lightest δ34 Spyrite values (-54‰) suggest very large fractionation during MSR from an initial sulfate with δ34 S values (δ34 Ssulfate,0 ) of +14 to +28‰. Fractionation of this magnitude requires a slow MSR rate, a feature we attribute to nutrient and electron donor shortage as well as initial sulfate abundance. The superheavy δ34 Spyrite values were produced by Rayleigh fractionation effects in a diminishing sulfate pool. Large volumes of pyrite with superheavy values (+120±15‰) within single fracture intercepts in the boreholes, associated heavy average values up to +75‰ and heavy minimum δ34 Spyrite values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture-specific δ34 Spyrite variability and overall average δ34 Spyrite values (+11 to +16‰) lower than the anticipated δ34 Ssulfate,0 support this hypothesis. The superheavy pyrite found locally in the borehole intercepts thus represents a late stage in a much larger fracture system undergoing Rayleigh fractionation. Microscale Rb-Sr dating and U/Th-He dating of cogenetic minerals reveal that most pyrite formed in the early Paleozoic era, but crystal overgrowths may be significantly younger. The δ13 C values in cogenetic calcite suggest that the superheavy δ34 Spyrite values are related to organotrophic MSR, in contrast to findings from marine sediments where superheavy pyrite has been proposed to be linked to anaerobic oxidation of methane. The findings provide new insights into MSR-related S-isotope systematics, particularly regarding formation of large fractions of 34 S-rich pyrite.

Sphene and Zircon Fission Track Analysis of Syenite Rocks of the Sushina Hills, Purulia-Bankura Shear Zone (TPSZ)

Sushina hills are situated in the Purulia-Bankura Shear Zone. This is tectonically disturbed narrow zone nearly 100 Km long having WNW-ESE trend. Two different rock assemblages namely Chhotanagpur Gneissic Complex and rocks of Singhbhum Group occur on the two sides of this shear zone. In this paper, I have used fission track ages of two cogenetic minerals and their corresponding closure temperatures to describe the denudation history. The difference of their fission track ages provides the rate of denudation 4 m/Ma during the period 970Ma-1458Ma due to tectonic activity. The youngest age of zircon of 970Ma represents the timing of the initiation of denudation.

Trace Element Geochemistry and Metasomatic Origin of Alluvial Sapphires From the Orosmayo Region, Jujuy Province, Northwest Argentina

Abstract This study primarily sought to investigate the trace element composition of alluvial sapphires from the Orosmayo region, Jujuy Province, northwest Argentina. In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis ( n = 69) showed that the sapphires exhibit a predominant metasomatic trace element signature, including low Fe/Mg (4–117) and Ga/Mg (0.3–10.4) ratios. However, the sapphires also display magmatic associations, including elevated Ga (up to 239 ppm) and Si (up to 3890 ppm), which indicates magmatic-metasomatic inputs. Enrichments in LREEs (Y, La, Ce, and Nd), Zn, and Pb, likely contained within remnant formational fluids, and elevated Be, Nb, Ta, Sn, Th, and U contents, likely contained within mineral inclusions, suggest that the sapphires were influenced by highly evolved felsic melts resembling those of granitic composition. The analyses were complemented by mineral inclusion identification using visual microscopy, and confirmed by energy dispersive spectroscopy (EDS). Identifiable co-genetic mineral inclusions include Al and Ti-rich magnetite, pyrochlore, albite, columbite-(Fe), rutile, phlogopite, trikalsilite, pyrite, baryte, monazite-(La), xenotime-(Y), and zircon. Secondary inclusions included hematite and Mg-hercynite which formed as reaction rims surrounding three of the sapphires. The inclusion mineralogy supports the notion of a mixed genetic scenario, with a distinct bimodal nature highlighting interactions between Si-poor and evolved Si-rich melts. Morphological and textural analysis using scanning electron microscopy (SEM) revealed dominant corrosive features on the sapphires. These corrosive features, coupled with the presence of spinel reaction rims, suggests that the sapphires were transported to the surface as xenocrystal fragments that were in chemical dis-equilibrium with their transport medium. Detailed consideration of their geochemical composition in relation to the known geology of the surrounding region was used to narrow down potential sources. Until other evidence surfaces, a working hypothesis for sapphire genesis involves a three-stage process, including: (1) their crystallization via metasomatic fluid exchange between an evolved crustal felsic system, such as crustal pegmatitic/granitic fluids, and intruding coeval mantle-derived carbonatized lamprophyre during the Lower Cretaceous (∼150–100 Ma) as part of alkaline magmatism within NW Argentina; (2) their incorporation as xenocrysts within the effusive volcanic products of the Neogene Granadas Volcanic Complex (∼9.8 Ma); and (3) subsequent host-rock erosion to form the present-day alluvial placer deposits of Orosmayo. The large trace element variation within individual sapphire grains identified in this work also highlights the importance of detailed in situ analysis across corundum grains in order to better understand the complex changing physiochemical conditions during their growth.

Uplift History of Syenite Rocks of the Sushina Hill, Tamar Porapahar Shear Zone (TPSZ), Purulia: Constraints from Fission-track Ages of Two Cogenetic Minerals

Fission-track ages of cogenetic minerals namely apatite, and zircon from syenite rocks of the Sushina hill, Tamar Porapahar Shear Zone (TPSZ), coupled with the corresponding closure temperatures of the minerals have been used to reveal the uplift history of syenite rocks. Offset of their FT ages indicates that the samples uplifted at the rate of 9.97 m/Ma during the period 535 Ma-970 Ma.

Interpreting U–Pb data from primary and secondary features in lunar zircon

In this paper, we describe primary and secondary microstructures and textural characteristics found in lunar zircon and discuss the relationships between these features and the zircon U–Pb isotopic systems and the significance of these features for understanding lunar processes. Lunar zircons can be classified according to: (i) textural relationships between zircon and surrounding minerals in the host breccias, (ii) the internal microstructures of the zircon grains as identified by optical microscopy, cathodoluminescence (CL) imaging and electron backscattered diffraction (EBSD) mapping and (iii) results of in situ ion microprobe analyses of the Th–U–Pb isotopic systems. Primary zircon can occur as part of a cogenetic mineral assemblage (lithic clast) or as an individual mineral clast and is unzoned, or has sector and/or oscillatory zoning. The age of primary zircon is obtained when multiple ion microprobe analyses across the polished surface of the grain give reproducible and essentially concordant data. A secondary set of microstructures, superimposed on primary zircon, include localised recrystallised domains, localised amorphous domains, crystal–plastic deformation, planar deformation features and fractures, and are associated with impact processes. The first two secondary microstructures often yield internally consistent and close to concordant U–Pb ages that we interpret as dating impact events. Others secondary microstructures such as planar deformation features, crystal–plastic deformation and micro-fractures can provide channels for Pb diffusion and result in partial resetting of the U–Pb isotopic systems.

Vein mineral assemblage in partially serpentinized peridotite xenolithsfrom Hyblean Plateau (south-eastern Sicily, Italy)

Periodico di Mineralogia (2011), 80, 2, 247-266 - DOI:10.2451/2011PM0019 Vein mineral assemblage in partially serpentinized peridotite xenoliths from Hyblean Plateau (south-eastern Sicily, Italy) Fabio C. Manuella Dipartimento di Scienze Geologiche, Universita di Catania, C.so Italia 57, I-95129 Catania - Italy fmanuella@alice.it Abstract A mineralogical study was performed on seven selected peridotite xenoliths, found in diatremic tuff-breccia deposits from Valle Guffari (Hyblean Plateau, Sicily, Italy), in order to investigate evidence for serpentinization. Petrographic examinations of spinel-harzburgites revealed the presence of the pervasive network of composite serpentine veins, suggesting a variable degree of alteration of 50-80%. Veins were characterized by X-ray Powder Diffraction (XRPD), which allowed the identification of chrysotile 2 M c1 and lizardite 1 T . No peaks referable to antigorite polymorphs were detected. Microprobe WDS (wavelength dispersion system) data evidenced a different iron content between chrysotile [average composition (Mg2.58Fe 2+ 0.27Al0.01)Σ2.86Si2.06O5(OH)4, Fe/(Fe+Mg) = 0.09] and lizardite [average composition (Mg2.49Fe 2+ 0.37Al0.01)Σ2.87 Si2.05O5(OH)4, Fe/(Fe+Mg) = 0.12]. Chlorine was detected in both polytypes, with an average concentration of 0.09 wt% in chrysotile and 0.05 wt% in lizardite. A series of cogenetic secondary minerals are enclosed in serpentine veins, described hereunder: a) Sulphides, predominantly S-poor phases (atomic sulphur/metal ratio 1), such as millerite [mean composition (Ni0.95Fe0.03)Σ0.98S] and polydymite-violarite solid solution [mean composition (Fe1.38Ni1.69)Σ3.07S4]. b) Interstitial dendritic aggregates of Na-rich sylvite, whose molar ratio of NaCl/(NaCl+KCl) ( X NaCl) varies from 0.03 to 0.21. c) Widespread veinlets of (Ca, Na, S)-rich phosphates (CaO 50.30-52.70 wt%, Na2O 1.50-3.10 wt%, P2O5 31.76-34.60 wt%, SO3 7.40-12.60 wt%), whose chemical composition is similar to (Na, S)-rich apatite-(CaOH)- M (CaO 48.70-54.30 wt%, Na2O 0.10-3.90 wt%, P2O5 32.01-40.82 wt%, SO3 0.40-11.40 wt%). d) Aragonite veins. e) Fe-rich saponit. f) Chalcedony. The studied secondary mineral assemblage hints that serpentinization...

THE REDISTRIBUTION OF RARE-EARTH ELEMENTS IN SECONDARY MINERALS OF HYDROTHERMAL VEINS, SCHWARZWALD, SOUTHWESTERN GERMANY

Minerals of the rare-earth elements (REE) occur as supergene phases in the Schwarzwald ore district, southwestern Germany. They form by alteration of hydrothermal fluorite – barite – quartz – carbonate veins with various associations, including Cu– Pb, Pb–Zn and Co–Bi–Ag–U assemblages in sandstones, gneisses and granites. The REE minerals, including mixite-group minerals ([REE,Bi,Ca,Pb]Cu 6 (AsO 4 ,AsO 3 OH) 3 OH 6 •3H 2 O), rhabdophane and churchite (REEPO 4 •H 2 O and REEPO 4 •2H 2 O), chukhrovite (Ca 3 REEAl 2 SO 4 F 13 •10H 2 O) and bastnasite (REECO 3 F), were analyzed by electron microprobe and LA–ICP–MS. In addition, REE concentrations in secondary fluorite, calcite and Mn oxides cogenetic with the REE minerals were determined by LA–ICP–MS. Results of analyses of 74 mixite-group samples from 20 localities in the Schwarzwald ore district show that continuous miscibility is possible between REE and Ca and between Bi and Ca end-members. In contrast, no miscibility seems to exist between the Bi and REE end-members, and only Ca-rich members can accommodate small amounts of both Bi and REE. The REE phosphates churchite and rhabdophane do not occur at the same locality in the Schwarzwald, which is probably dependent on which REE (light or heavy) predominate at a certain locality. Whereas churchite incorporates heavy REE (HREE), rhabdophane prefers light REE (LREE). Dependent on the source of the REE, either HREE or LREE dominate in an alteration fluid and, consequently, only one type of REE phosphate forms. The HREE dominate in such a fluid if REE originate from dissolved “pitchblende”, whereas the LREE dominate if the altered host-rock or dissolved fluorite are the source of the REE. The REE contents of Mn oxides and calcite cogenetic with REE-bearing mixite-group minerals show that the REE distribution of secondary minerals can be influenced by each other. Whereas the Mn oxides incorporate Ce 4+ resulting in a positive Ce anomaly, a cogenetic mixite-group mineral develops a negative anomaly, which shows that Ce anomalies are coupled. Calcite intergrown with a mixite-group mineral can only incorporate those REE that are not consumed by the latter. These processes involve the distribution of REE between minerals on the micrometer scale and do not take place until minerals precipitate. However, sorption and complexation processes also take place during transport on a larger scale and produce decoupled Ce anomalies. This interplay between large- and small-scale processes results in a complex redistribution of REE during remobilization.

Significance of trace elements in syntaxial quartz cement, Haushi Group sandstones, Sultanate of Oman

Trace element contents and distributions in authigenic quartz cement in deeply buried (2500–4000 m) Haushi Group sandstones from wells in Oman have been investigated in order to determine the factors that control trace element uptake during precipitation. Scanning electron microscope-cathodoluminescence images show well developed growth zones within the quartz cement, which correlate with chemical zonations observed in electron microprobe Al distribution maps. The most abundant trace elements are Al (50–3000 μg g −1), Li (1–100 μg g −1), Na (1–40 μg g −1), and Ge (0.3–5 μg g −1) with a strong linear correlation between Li and Al and a weaker one between Ge and Al. The molar concentration of Li (+ Na) accounts only for ~ 15% of the charge compensation for Al 3+ substitution of Si 4+. Though H was not measured in this study, these data indicate a major role of H in charge balancing Al 3+. The samples belong to the same stratigraphic unit and have similar petrography, but show considerable variability in absolute trace element concentrations between different wells. This variability does not correlate with either sample depth or temperature and shows no regional pattern, but seems to reflect petrophysical and tectonic differences within the sedimentary basin. Petrographic observations of the cogenetic mineral assemblages and hydrochemical modelling indicate that a change from the equilibrium assemblage quartz–kaolinite (–dolomite) to quartz–illite (–dolomite) reflects a decrease in the CO 2 concentration and concurrent variations of the Al concentration. It is concluded that changes in the CO 2 concentrations are responsible for fluctuations in fluid Al concentrations and thus likely also in the investigated quartz cements.

The comparative behavior of apatite‐zircon U‐Pb systems in Apollo 14 breccias: Implications for the thermal history of the Fra Mauro Formation

Abstract— We report secondary ion mass spectrometry (SIMS) U‐Pb analyses of zircon and apatite from four breccia samples from the Apollo 14 landing site. The zircon and apatite grains occur as cogenetic minerals in lithic clasts in two of the breccias and as unrelated mineral clasts in the matrices of the other two. SIMS U‐Pb analyses show that the ages of zircon grains range from 4023 ± 24 Ma to 4342 ± 5 Ma, whereas all apatite grains define an isochron corresponding to an age of 3926 ± 3 Ma. The disparity in the ages of cogenetic apatite and zircon demonstrates that the apatite U‐Pb systems have been completely reset at 3926 ± 3 Ma, whereas the U‐Pb system of zircon has not been noticeably disturbed at this time. The apatite U‐Pb age is slightly older than the ages determined by other methods on Apollo 14 materials highlighting need to reconcile decay constants used for the U‐Pb, Ar‐Ar and Rb‐Sr systems. We interpret the apatite age as a time of formation of the Fra Mauro Formation. If the interpretation of this Formation as an Imbrium ejecta is correct, apatite also determines the timing of Imbrium impact. The contrast in the Pb loss behavior of apatite and zircon places constraints on the temperature history of the Apollo 14 breccias and we estimate, from the experimentally determined Pb diffusion constants and an approximation of the original depth of the excavated samples in the Fra Mauro Formation, that the breccias experienced an initial temperature of about 1300–1100 °C, but cooled within the first five to ten years.

A Preliminary Study on Skarn‐Related Calc‐silicate Rocks Associated with the Batu Hijau Porphyry Copper‐Gold Deposit, Sumbawa Island, Indonesia

AbstractBatu Hijau is a world‐class gold‐rich porphyry copper deposit, situated in Sumbawa Island, Indonesia. Deep drilling indicates that several intervals of calc‐silicate rock were intersected, where they are apparently interbedded with volcaniclastic rocks. The calc‐silicate rocks occur at the contact with copper‐gold‐bearing tonalite porphyries. The rocks are fine‐grained and granular with green, reddish‐brown and white layers. The green layers consist mostly of fine‐grained clinopyroxene (diopside and hedenbergite) and the reddish‐brown layers consist mostly of garnet (andradite), whereas the white layers are commonly composed of calcite and zeolite (chabazite). The calc‐silicate rocks were formed by contact metasomatism of andesitic volcaniclastic rocks, as it is calcic in composition. Paragenesis study reveals at least two stages of calc‐silicate mineralization. Stage 1 (prograde) is characterized by the presence of garnet (andradite), clinopyroxene (diopside and hedenbergite), anorthite and quartz at 340–360 C (high salinity 35–45 NaCl wt percentage eqn.). Stage 2 (retrograde) is characterized by chlorite and rare epidote at 280–300 C (low salinity 1–10 NaCl wt% eqn.). Late calcite ± quartz veinlets and calcite + chabazite veins/veinlets may also be related to this stage and cross cut the oldest mineral assemblages. Mineralization (magnetite, chalcopyrite and pyrite) may occur during the retrograde stage. Clinopyroxene and garnet were modified by Fe‐rich hydrothermal fluid (oxidizing condition) indicated by increase of Fe from core to rim of both the cogenetic minerals. The presence of the calc‐silicate rocks associated with massive magnetite‐chalcopyrite‐pyrite assemblage indicates the occurrence of calcic‐exoskarn surrounding the Batu Hijau porphyry copper‐gold deposit.

The Physico-Chemical Properties of a Subducted Slab from Garnet Zonation Patterns (Sesia Zone, Western Alps)

Garnets in continentally derived high-pressure (HP) rocks of the Sesia Zone (Western Alps) exhibit three different chemical zonation patterns, depending on sample locality. Comparison of observed garnet zonation patterns with thermodynamically modelled patterns shows that the different patterns are caused by differences in the water content of the subducted protoliths during prograde metamorphism. Zonation patterns of garnets in water-saturated host rocks show typical prograde chemical zonations with steadily increasing pyrope content and increasing XMg, together with bell-shaped spessartine patterns. In contrast, garnets in waterundersaturated rocks have more complex zonation patterns with a characteristic decrease in pyrope and XMg between core and inner rim. In some cases, garnets show an abrupt compositional change in core-to-rim profiles, possibly due to water-undersaturation prior to HP metamorphism. Garnets from both water-saturated and water-undersaturated rocks show signs of intervening growth interruptions and core resorption. This growth interruption results from bulk-rock depletion caused by fractional garnet crystallization. The water content during burial influences significantly the physical properties of the subducted rocks. Due to enhanced garnet crystallization, water-undersaturated rocks, i.e. those lacking a free fluid phase, become denser than their water-saturated equivalents, facilitating the subduction of continental material. Although water-bearing phases such as phengite and epidote are stable up to eclogite-facies conditions in these rocks, dehydration reactions during subduction are lacking in water-undersaturated rocks up to the transition to the eclogite facies, due to the thermodynamic stability of such hydrous phases at high P–T conditions. Our calculations show that garnet zonation patterns strongly depend on the mineral parageneses stable during garnet growth and that certain co-genetic mineral assemblages cause distinct garnet zonation patterns. This observation enables interpretation of complex garnet growth zonation patterns in terms of garnet-forming reactions and water content during HP metamorphism, as well determination of detailed P–T paths.

Oxygen isotope equilibrium between eclogite minerals and its constraints on mineral Sm-Nd chronometer

Sm-Nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the Sulu terrane in eastern China. The results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and Sm-Nd isotope systems of eclogite minerals. The omphacite-garnet pairs of oxygen isotope equilibrium at eclogite-facies conditions yield meaningful Triassic Sm-Nd isochron ages, whereas those of oxygen isotope disequilibrium give non-Triassic ages of geological meaninglessness. This can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than, or close to, those of Nd diffusion, and thus attainment of isotopic equilibrium in the omphacite-garnet O system suggests achievement of Nd isotope equilibrium in the same mineral pairs. The presence or absence of fluid in the eclogite protoliths is a major rate-controlling factor for isotopic equilibration during high-grade metamorphism. It appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the Sm-Nd mineral chronometer. In addition, the exact timing of the ultrahigh pressure metamorphism in the Dabie-Sulu terranes is constrained at Early Triassic rather than Late Triassic.

The use of sector field ICP-mass spectrometry for Rb-Sr geochronological dating.

Rb-Sr dating, one of the most important tools in geochronology, requires determination of the Rb/Sr concentration ratios and the 87Sr/86Sr isotope ratios in co-genetic minerals or rocks and is traditionally performed by thermal ionization mass spectrometry (TIMS). In this work we investigated whether sector field inductively coupled plasma mass spectrometry (ICP-MS), which is characterized by a high sample throughput and straight-forward sample introduction, could be used as an alternative to TIMS. To avoid spectral overlap of the ion signals of the isobaric nuclides 87Sr and 87Rb, Sr was separated from Rb by cation-exchange chromatography. A mathematical correction was applied to take into account the small amount of Rb that can be present in the Sr fraction. The isotope ratio accuracy and precision attainable with ICP-MS were evaluated by analysis of several reference materials from the US Geological Survey. The results of this evaluation show that excellent accuracy could be achieved; the internal precision (repeatability) of the isotope ratio (expressed as the relative standard deviation for 10 successive 1-min measurements) was 0.04-0.12%. An attempt was made to calculate the total or combined uncertainty on the isotope ratio results, by also taking into account other possible error sources (corrections for mass discrimination, detector dead time, blank signal and Rb fraction). Finally, the same procedure was used for dating two rock formations that were 2,500 Ma and 350 Ma old, according to age determinations previously performed by the Rb-Sr laboratory of the University of München (Germany) using TIMS. The ICP-MS results (2,520 +/- 150 Ma and 379 +/- 48 Ma) obtained for these formations compare well with the corresponding TIMS data (2,509 +/- 120 Ma and 357 +/- 25 Ma).