Isotopic Compositions Of Minerals Research Articles

In-situ trace element and Sr isotope in mantle xenoliths from the West Qinling orogen: Evidence for carbonate metasomatism

Subduction process plays an important role in triggering chemical metasomatism of the lithospheric mantle beneath orogenic belts. The lithospheric mantle beneath the West Qinling orogen is extensively heterogeneous, but the lithospheric modification process and related metasomatic agents remain poorly constrained. In this paper, we report in-situ elemental and isotopic compositions of minerals in mantle xenoliths from the West Qinling orogen. The mantle xenoliths are lherzolites and can be categorized into two types. Lherzolite with refractory compositions represents the residue of lithospheric mantle that have undergone high degrees of melt extraction. In contrast, lherzolites with fertile compositions contain clinopyroxenes variably enriched in incompatible elements and radiogenic Sr isotopes. These fertile lherzolites represent lithospheric mantle modified by metasomatic process in different degrees. High (La/Yb)N, Ca/Al, Nb/Ta, Nd/Yb, 87Sr/86Sr ratios and low Ti/Eu ratios of the clinopyroxenes suggest the metasomatic agent was dominated by carbonate-rich melts. This inference is supported by the identification of carbonate inclusion, reaction structures, and clinopyroxene chemical zoning in the lherzolites. Considering the tectonic background of the West Qinling orogen, the orogenic lithospheric mantle would have been metasomatized by carbonate-rich melts derived from partial melting of subducted carbonate-bearing Paleo-Tethyan oceanic slab. In this regard, petrological observations, along with in-situ elemental and Sr isotopic compositions of clinopyroxenes in mantle xenoliths can be powerful means to identify carbonate metasomatism and constrain the evolution of the lithospheric mantle beneath orogenic belts.

The magmatic and post-magmatic processes of granitic magma: Constraints from in situ element and isotope compositions of minerals

The magmatic and post-magmatic processes of granitic magma: Constraints from in situ element and isotope compositions of minerals

Limitations in using the Cu isotopic composition of minerals from ancient copper mines for archaeometric purposes – a case study

High-precision Cu isotopic analysis in archaeometry suffers from large δ65Cu variability between ore samples from the same location.

Recommendations for offline combustion-based nitrogen isotopic analysis of silicate minerals and rocks.

Due to isotope fractionations during partial nitrogen release from minerals and rocks, the complete extraction of nitrogen for analysis is crucial to ensure high-quality nitrogen isotopic data. However, the appropriate nitrogen extraction conditions (e.g. temperature, duration) have not been established for most silicate minerals and rocks. Nitrogen in a number of common minerals and rocks was extracted using the most robust sealed-tube offline combustion techniques, purified and quantified in a custom-made metal manifold, and carried by helium gas to an isotope ratio mass spectrometer for isotopic measurement at nanomolar nitrogen level. Each mineral or rock was combusted in a variety of temperature and duration conditions to compare the nitrogen yields and isotopic compositions. The nitrogen yields and isotopic compositions of minerals and rocks are strongly affected by combustion temperature and duration. The optimal combustion temperature is lowest for cyclosilicate minerals, followed by phyllosilicate, tectosilicate and inosilicate minerals. Preheating of samples can induce significant nitrogen loss and δ15 N shift. Heating of samples above their optimal temperatures may cause nitrogen re-assimilation by the residual mineral or rock. Each mineral or rock has a characteristic optimal temperature and duration for complete nitrogen release. Preheating, under-heating or over-heating can cause nitrogen loss and isotopic shift. Therefore, we recommend using the offline combustion techniques and the optimal combustion conditions obtained in this study for nitrogen quantification and isotopic analysis of silicate minerals and rocks.

Badzhal Tin Magmatic-Fluid System, Far East, Russia: Transition from Granite Crystallization to Hydrothermal Ore Deposition

The Badzhal tin magmatic-fluid system of the eponymous volcanoplutonic zone in the Middle Amur Region has been studied to reveal special characteristics of the transition from granite crystallization to rare-metal deposition. Therefore, the authors have conducted an in-depth study of melt, fluid-melt, and fluid inclusions and oxygen isotope composition of minerals from granitic rocks of the Verkhneurmiisky pluton within the Badzhal volcanoplutonic zone and minerals of the Pravourmiisky and Blizhnee Sn-W deposits. Greisen alteration and hydrothermal veins at the Pravourmiisky and Blizhnee deposits resulted from a single magmatic-fluid system related to the Verkhneurmiisky granite pluton, which is one of domes of the Badzhal batholith. The transition has been traced from magmatic crystallization of granite to hydrothermal ore formation and evolution of magmatic fluid from its separation to Sn and W deposition. Mixed fluid-melt inclusions directly support tin-bearing fluid separation during melt crystallization. The glass compositions indicate that granite and porphyry granite crystallized from felsic metaluminous to peraluminous melts with an ASI index and alkali content ranging from 0.95 to 1.33 and from 6.02 to 9.02 wt %, respectively. The Cl and F concentrations in glasses are 0.03–0.14 and 0.14–0.44 wt %, respectively, and are higher than those in the bulk rock compositions, 0.02 and 0.05–0.13 wt %, respectively. These differences indicate that Cl and F could have been removed from a granitic melt during its crystallization and degassing. The H2O concentration estimated based on the electron deficiency of summed microprobe analyses is 8–11 wt %. This was estimated taking into account the possible effect of “sodium loss” (Nielsen and Sigurdson, 1981) when analyzing hydrated glasses. Taking into account the high uncertainty of such estimation (Devine et al., 1995), this value is extremely uncertain and the studied melts should be considered as containing 9.5–10.0 wt % H2O. The melt inclusion study shows that some of the magmatic rocks of the Badzhal ore-magmatic system formed at approximately 650°C. The melt from which these rocks crystallized was felsic, moderate in fluorine, and meta- and peraluminous. Low-temperature crystallization is probably caused by high water pressure and elevated fluorine content. These inclusions most likely characterize the final stage of the pluton, at which crystals, residual melt, and magmatic fluid phase coexist. Fluid responsible for the formation of greisens at the Pravourmiisky deposit has features very close to those of supercritical fluid trapped by magmatic minerals. Its salinity, ranging from ∼9 to 12 wt % NaCl equiv., and maximal temperature of 550°C (taking into account pressure correction for ∼1 kbar) are close to those of magmatic fluid. This makes it possible to relate the origin of the fluid to cooling of the pluton. Greisen and quartz-cassiterite-topaz veins of the Pravourmiisky deposit formed from magmatic fluid at a decreased temperature from 550–450 to 480–380°C. The fluid responsible for the formation of quartz-cassiterite veins at the Blizhnee deposit also separated from magma as indicated by the oxygen isotope composition ( $${\delta^{18}}{{\rm{O}}_{{{\rm{H}}_2}{\rm{O}}}}\sim8.5{\%\circ}$$ ), which evolved in a shallower environment under much lower pressure. This resulted in fluid with a salinity of ∼13 wt % NaCl equiv. at 420–340°C being separated into a low-density low-saline vapor and brine with a salinity of 33.5–37.4 wt % NaCl equiv. The oxygen isotope composition of the mineralizing fluid was governed by equilibrium with granite within a wide temperature range (from ∼700°C to the onset of greisen crystallization). The agreement between the measured data and those calculated based on the suggested model indicates that a significant volume of external fluid having different isotopic features and not in equilibrium with the Verkhneurmiisky granite did not enter the magmatic-fluid system. The revealed differences in the physicochemical formation conditions of the two studied deposits are not critical and support their formation within a single magmatic-fluid system.

High-temperature inter-mineral Cr isotope fractionation: A comparison of ionic model predictions and experimental investigations of mantle xenoliths from the North China Craton

Recent works have proposed that chromium (Cr) isotopes could be fractionated during peridotite partial melting and basaltic magma crystallization. However, until now, inter-mineral Cr isotope fractionation behavior for major mantle minerals has been poorly constrained. To investigate the mechanism and magnitude of equilibrium inter-mineral Cr isotope fractionation and to explore its implications for planetary mantle and crust lithology evolutions, we performed a systematic study of equilibrium Cr isotope fractionation between major mantle minerals by coupling theoretical ionic modeling with laboratory isotope analyses of natural samples and Cr valence determinations by X-ray absorption near edge spectroscopy (XANES). The ionic model considers both charges and coordination environments of Cr species in mantle minerals, which have proven to be critical factors affecting inter-mineral isotope fractionation. The ionic modeling results predict a general order of spinel (Spl) > pyroxene (Py) ≥ olivine (Ol) in 53Cr/52Cr. Systematic Cr isotope analyses of coexisting mantle minerals of seventeen peridotite xenoliths from Beiyan in the North China craton were performed. Chromium isotope results for different mineral pairs of lherzolites (e.g., Δ53CrSpl–Ol of 0.11‰ to 0.16‰, Δ53CrSpl–Py of 0.04‰ to 0.11‰ and Δ53CrPy–Ol of 0.05‰ to 0.10‰ at 870 °C to 970 °C), document measurable and systematic inter-mineral Cr isotope fractionation, in excellent agreement with the modeling results predicted with XANES-determined Cr2+/ΣCr values of the mineral separates. This fractionation order could essentially account for Cr isotope behaviors during partial melting and magmatic crystallization observed in terrestrial peridotites and lunar basalts.In contrast, we found that metasomatisms could influence Cr isotope compositions of minerals from Beiyan metasomatized clinopyroxene- (Cpx-) rich lherzolites and wehrlites by means of mineral–melt interaction and/or kinetic diffusion, leading to disequilibrium inter-mineral Cr isotope fractionation.

Small-scale Sr and O isotope variations through the UG2 in the eastern Bushveld Complex: The role of crustal fluids

A detailed study of the Sr- and O- isotopic composition of minerals in the Upper Group 2 (UG2) unit of the eastern Bushveld Complex revealed significant variations in the pegmatoidal footwall pyroxenite and the UG2 chromitite layer itself. Initial 87Sr/86Sr ratios (Sri) of interstitial plagioclase in the footwall rocks are significantly higher than in adjacent rocks of the Critical Zone. The δ18O values of plagioclase-pyroxene and plagioclase-chromite mineral pairs in footwall pyroxenite and chromitite are consistent with oxygen isotope exchange from crystallization temperatures down to 700 °C and 800 °C, respectively, whereas in other parts of the UG2, plagioclase and pyroxene δ18O values are consistent with O isotope equilibration at liquidus crystallization temperatures of approximately 1150 °C.We propose that the higher initial 87Sr/86Sr of plagioclase in the UG2 chromitite and the footwall pyroxenite reflects infiltration of fluid derived from dehydrating country rocks beneath the Bushveld Complex. Fluids could have been introduced either late or during chromitite crystallization. The chromitite layer and its pegmatoidal footwall could have acted as barriers to entrap late stage crustal fluids. Alternatively, reaction of fluids with the chromite-bearing crystallizing mush layer would induce hydration melting of pyroxene and plagioclase supplying Cr and Al to the melt, thus invoking further chromite formation. This dissolution-reaction releases Cr from pyroxene, which along with pre-existing disseminated chromite could refine the chromitite layers. The pegmatoidal UG2 footwall constitutes the reaction front-line of this process, which recrystallized to a coarse-grained rock upon cooling.This study emphasizes the role of crustal fluids and late- and post-magmatic processes in layered intrusions taking place at subsolidus temperatures, whereby isotopic and chemical compositions may be significantly changed.

Evaluation of Mantle Processes in an Extensional Regime: Insight from In Situ O and Sr Isotope Systematics of Mantle Xenoliths from Ethiopia

AbstractNew O and Sr isotope compositions of minerals from well-characterized and variably metasomatized mantle xenoliths in Cenozoic basalts from the northwestern plateau (Gundeweyn area) and southern rift zone (Dillo and Megado areas) of Ethiopia were acquired to further evaluate the nature and effects of metasomatic processes affecting the lithospheric mantle beneath the East African Rift System (EARS), where a transition from plume impingement to rifting is recorded. The studied samples are dominantly fresh spinel lherzolites with subordinate spinel harzburgites and clinopyroxenites. The in situ δ18O composition of olivine is largely homogeneous, with average values of 5.31‰ for Gundeweyn, 5.22‰ for Dillo, and 5.25‰ for Megado xenoliths. The values are within the range of normal mantle olivine (δ18O = 5.1‰–5.4‰), and there is no systematic variation of δ18O values with petrographic characteristics, elemental compositions, Sr isotopic ratios, or tectonic setting (i.e., plateau [Gundeweyn] vs. rift [Dil...

Stable isotope composition of minerals in the Belaya Zima plutonic complex, Russia: Implications for the sources of the parental magma and metasomatizing fluids

The Belaya Zima alkaline–carbonatite complex in the East Sayan Mountains, Russia, is a multiphase concentric intrusion, which comprises diverse alkaline silicate rocks and carbonatites. Melteigites are the earliest products of crystallization and they are followed by ijolites–urtites, nepheline syenites, and finally, calcite, calcite–dolomite and ankerite carbonatites. In this paper, we present new geochronological data (Ar–Ar method), chemical and stable isotope (H, C and O) compositions of the main rock-forming minerals in carbonatites and the silicate rocks of the complex. The Ar–Ar dating of phlogopite from ankerite carbonatites is consistent with the age of syenite emplacement and implies that the silicate rocks and carbonatites are likely to be comagmatic. The evolutionary trends of pyroxene, fluorapatite, amphibole and phlogopite are consistent with fractional crystallization from a single parental magma. Primary Nb mineralization is represented by pyrochlore, whereas columbite-(Fe) is a replacement product of pyrochlore at the post magmatic, hydrothermal stage. At that stage, interaction of calcite carbonatites with hydrothermal fluids resulted in the formation of hydrothermal paragenesis comprising dolomite–ankerite, monazite-(Ce), ancylite-(Ce), minerals of burbankite group and Ca–REE fluorcarbonates. The hydrothermal processes were also responsible for cation leaching from pyrochlore, the replacements of phlogopite by tetraferriphlogopite and early Ca-rich pyroxene by aegirine. The stable isotope data suggest that the rocks crystallized from a 18O-depleted magma. Shifts toward higher δ18O values relative to the primary compositions are commonly observed in minerals from carbonatites, which is attributed to hydrothermal processes.

Isotopic disequilibrium and lower crustal contamination in slowly ascending magmas: Insights from Proterozoic anorthosites

Many Proterozoic anorthosite massifs show crustal isotopic signatures that have, for decades, fuelled debate regarding the source of these temporally-restricted magmas. Are these signatures indicative of lower crustal melting or of significant assimilation of crustal material into mantle-derived magmas? Traditional whole rock isotopic tracers (Sr, Nd, Pb and Os), like other geochemical, petrological and experimental tools, have failed to identify unambiguously the origins of the crust-like signature and resolve the source controversies for these feldspathic, cumulate intrusives. We make use of high precision Sr, Nd and Pb isotopic compositions of mineral phases (plag, opx, mag) and comagmatic, high-pressure orthopyroxene megacrysts as well as whole rock anorthosites/leuconorites from the Mealy Mountains Intrusive Suite (MMIS) and the Nain Plutonic Suite (NPS) to probe the origin of the crustal isotopic signatures and assess the importance of differentiation at lower crustal depths. This selection of samples represents fragments from various stages of the polybaric ascent of the magmas, while the study of the Mealy Mountains Intrusive Suite and the Nain Plutonic Suite is instructive as each is intruded into crust of significantly different age and isotopic composition. We observe marked differences in the whole-rock isotopic composition of Proterozoic anorthosites and high-pressure megacrysts (e.g. εNd;T=+2 to −10) intruded into crustal terranes of different ages and isotopic compositions. Evidence for varying degrees of internal isotopic disequilibrium (ΔNd, ΔSr, ΔPb) in anorthosites from these different terranes reinforces the notion that crustal contamination, and more importantly, the nature of the crustal assimilant, has a profound influence on the chemical signature of Proterozoic anorthosites. While most samples from the MMIS and NPS show significant and measurable ΔNd and ΔPb disequilibrium, ΔSr compositions cluster around zero. This decoupling in disequilibrium geometries cannot be explained by melting of the lower crust. Assimilation of crust with distinctive Sr, Nd and Pb isotopic compositions does, however, explain the origin of decoupling in internal mineral isotopic compositions. We also find unexpected patterns of internal isotopic disequilibrium, such as isotopically radiogenic orthopyroxene relative to plagioclase and differences in plagioclase isotopic disequilibrium between orthopyroxene- and olivine-bearing samples. These various lines of evidence provide strong support for the generation of crustal isotopic signatures through assimilation, and not anatexis, of the lower crust. These isotopic data show that anorthosite petrogenesis likely involves significant differentiation and solidification at lower crustal depths, followed by ascent of high-crystallinity bodies (⩾50% crystallinity) to mid- or upper crustal levels. We show that protracted lower crustal differentiation imparts a clear chemical and isotopic signature on mantle-derived magmas of Proterozoic anorthosites and that this process is central in the development of such slowly ascending, plagioclase-rich magmas.

Structures, mineralogy, and fluid regime of ore formation in the polygenetic Malo-Taryn gold field, Northeast Russia

Detailed structural study of ore zones and deformations in host rocks reveals the multistage tectonic evolution of the Malo-Taryn ore field. Gold–quartz mineralization is confined to the interstratal and longitudinal thrust faults, where vein–stringer bodies are systematically oriented relative to the structures of the thrust stage. Antimony mineralization is superposed on gold–quartz mineralization in the same structures, being associated with veins formed during the shear stage of deformations. It is determined that the types of mineralization were formed in the following sequence: gold–bismuth, low-sulfide gold–quartz, berthierite–antimonite, and silver–antimony. Data on mineral assemblages and typomorphic features of minerals are reported. The isotope composition of minerals, temperature conditions, and composition of orebearing fluids are studied. It is established that the same minerals ascribed to different types of mineralization sharply differ in typomorphic features, which is caused by the different composition and genesis of the hydrothermal fluids. The superposition of different types of mineralization on one another within repeatedly activated tectonic structures that were fluid-permeable for a long time is an important indication of large-scale deposits.

Recycled oceanic crust-derived fluids in the lithospheric mantle of eastern China: Constraints from oxygen isotope compositions of peridotite xenoliths

The oxygen isotope compositions of minerals of peridotite xenoliths from the Subei basin, Eastern China were investigated to detect a possible crustal signature in mantle rocks. The δ18O values of olivine (ol), orthopyroxene (opx) and clinopyroxene (cpx) vary from 5.10 to 5.98‰, 5.46 to 6.78‰, and 5.06 to 6.31‰, respectively. Olivine and opx have δ18O values similar to-, or above the “expected” mantle range, whereas the δ18O values of cpx range from higher to lower than average mantle. These characteristics are inherited from the peridotite mantle source, and likely result from mantle domains that experienced fluid-assisted metasomatism. Coexisting high δ18O values of ol and opx and low δ18O values of cpx are unlikely to result from selective interactions with the fluids with different δ18O values during a single metasomatic event. The measured O-isotope values can be reconciled with a simple two-stage metasomatic model characterized by an early high-δ18O fluid/peridotite interaction followed by a late low-δ18O fluid infiltration. Model calculations of mineral diffusivities predict that the negative Δ18Ocpx-ol values are unlikely to be preserved at mantle conditions for time interval>80Ma. Provided the investigated mantle xenoliths were entrained and brought to surface by ~9Ma basaltic eruptions, this also limits the occurrence of the late metasomatic event, and the subducting Pacific plate is the best candidate to provide such fluids.

Combined thermodynamic–geochemical modeling in metamorphic geology: Boron as tracer of fluid–rock interaction

Abstract Quantitative geochemical modeling is today applied in a variety of geological environments from the petrogenesis of igneous rocks to radioactive waste disposal. In addition, the development of thermodynamic databases and computer programs to calculate equilibrium phase diagrams has greatly advanced our ability to model geodynamic processes. Combined with experimental data on elemental partitioning and isotopic fractionation, thermodynamic forward modeling unfolds enormous capacities that are far from exhausted. In metamorphic petrology the combination of thermodynamic and trace element forward modeling can be used to study and to quantify processes at spatial scales from μm to km. The thermodynamic forward models utilize Gibbs energy minimization to quantify mineralogical changes along a reaction path of a chemically open fluid/rock system. These results are combined with mass balanced trace element calculations to determine the trace element distribution between rock and melt/fluid during the metamorphic evolution. Thus, effects of mineral reactions, fluid–rock interaction and element transport in metamorphic rocks on the trace element and isotopic composition of minerals, rocks and percolating fluids or melts can be predicted. Here we illustrate the capacities of combined thermodynamic–geochemical modeling based on two examples relevant to mass transfer during metamorphism. The first example focuses on fluid–rock interaction in and around a blueschist-facies shear zone in felsic gneisses, where fluid-induced mineral reactions and their effects on boron (B) concentrations and isotopic compositions in white mica are modeled. In the second example, fluid release from a subducted slab, the associated transport of B as well as variations in B concentrations and isotopic compositions in liberated fluids and residual rocks are modeled. We compare the modeled results of both examples to geochemical data of natural minerals and rocks and demonstrate that the combination of thermodynamic and geochemical models enables quantification of metamorphic processes and insights into element cycling that would have been unattainable if only one model approach was chosen.

Geochemistry of stable oxygen and hydrogen isotopes in minerals and corundum-bearing rocks in northern Karelia as an indicator of their unusual genesis

The paper presents newly obtained data on the oxygen and hydrogen isotopic composition of minerals in corundum occurrences and their host rocks in northern Karelia. Minerals in the Khitoostrov and Varaka corundum-bearing zones have extremely low δ18O (lower than −26‰) and δ D (lower than −215‰), which suggest that the mineral-forming process involved glacial waters and that the minerals preserve the isotopic ratios of their protolith. Aluminous corundum plagioclasites were produced by high-pressure Svecofennian (1.9-1.8 Ga) metamorphism of Paleoproterozoic rocks that had been metasomatized with the involvement of meteoric waters during the Guronian glaciation epoch.

Water contents and hydrogen isotopes in nominally anhydrous minerals from UHP metamorphic rocks in the Dabie-Sulu orogenic belt

A continuous flow method, by a combination of thermal conversion elemental analyzer (TC/EA) with isotope ratio mass spectrometry (MS), was developed to determine both H isotope composition and H2O concentration of ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu orogenic belt. By using the developed step-heating technique, we have studied H2O concentration and H isotope composition of the different forms of water (structural OH and molecular H2O) in garnet. The quantitative measurements of H2O concentration and H isotope composition of minerals in UHP metamorphic rocks from several typical outcrops indicate that the gneisses can release more amounts of water than the eclogites during exhumation of the deeply subducted continental crust. Therefore, by decompression dehydration at the contact between eclogite and gneiss, the released water could flow from the gneiss to the eclogite and result in significant hydration of the eclogite adjacent to the gneiss. The measured maximum water contents of minerals in eclogites indicate that garnet and omphacite have the maximum water solubilities of 2500 and 3500 ppm, respectively, under the peak UHP metamorphic conditions. eclogite, gneiss, mineral, structural hydroxyl, molecular water, metamorphic fluid

Oxygen isotopes effects due to dehydration of the blueschist: The experimental data under P-T conditions of subduction zone

The oxygen isotope composition of minerals from metamorphic rocks carries information on the nature of the protolith, the temperature of the metamorphism peak, and the isotope composition and content of the fluid phase participating in the metamorphic process. Among the main sources of fluid in highgrade meta� morphism are hydrous minerals, for example, those delivered to the subduction zone with sedimentary and metamorphic rocks of the oceanic slab. The process of dehydration should result in the oxygen isotope shift in the dehydrated rock (restite), the surrounding crustal rocks, and the overlying rocks of the mantle wedge. Currently it is generally accepted that the oxygen isotope effect under metamorphic dehydration is very low (<1 ‰) (1). Moreover, small isotope shifts become poorly reflected against the background of the variable lithology of rocks in metamorphic complexes. More and more rocks with anomalously light or widely ranging oxygen isotope composition are found in granulitic and eclogitic metamorphic complexes. Signif� icant oxygen isotope lightening ( δ 18 O up to -11‰) was observed in eclogite and gneiss of the Dabie-Sulu belt in China; moderate oxygen isotope lightening (up to -4‰), in eclogite of the Kokchetav massif; and extreme oxygen isotope lightening (δ 18 O from -16 to

Eclogite–gneiss complex of the Muya block (East Siberia): age, mineralogy, geochemistry, and petrology

Results of study of eclogite–gneiss complex of the Muya Block (East Siberia) are presented. Several structural types of the studied eclogites have been recognized. Kyanitic eclogite has been found for the first time. The host granite-gneisses are two-mica and biotite varieties, mainly garnet-bearing. The exposure of eclogites from different depths of the subducted plate at the present-day denudation level might be the reason for the wide range of the equilibrium temperatures of the Muya block eclogites (590–740 °C). The Sm–Nd dating of the eclogites and host gneisses showed the Neoproterozoic age of high-pressure metamorphism (∼630 Ma). The model age (TDM) of the eclogites (720 Ma) differs considerably from the model age of the host gneisses (>1.3 Ga). The geochemical features of the eclogites point to the mobility of LILE (Rb, Cs, Ba, K) and LREE during their interaction with fluids, whereas the gneisses in the same process showed the mobility of LILE only. The oxygen isotope composition of minerals in the eclogites varies over a narrow range (δ18O = 5.5–3.9) and is close to the average mantle value, which evidences a negligible interaction between the eclogite protoliths and meteoric or sea water. The study of fluid inclusions in quartz from the eclogites and host gneisses showed a predominance of liquid-nitrogen inclusions in the former and carbon dioxide inclusions in the latter.

Preserving the Burgess Shale

Paleontology![Figure][1] CREDIT: GAINES ET AL., PROC. NATL. ACAD. SCI. U.S.A. 109 , 10.1073/PNAS.1111784109 (2012) The Burgess Shale in British Columbia, Canada, is one of the most famous fossil fields in the world, containing remarkably preserved soft-bodied marine life from just after the Cambrian explosion. In most other fossil locations, these soft, fragile tissues usually degrade in sediments soon after they are deposited. By examining the composition of the overlying sediments and measuring the sulfur isotope composition of minerals surrounding the fossils from the Burgess Shale, the Chengjiang in China, and five other Burgess Shale–type deposits, Gaines et al. propose that the unique depositional environment and seawater chemistry composition were the primary factors that facilitated preservation. The seafloor at this time in Earth's history was probably anoxic and also deprived of other oxidants such as sulfate, which inhibited initial bacterial degradation of recently deposited organic remains on the seafloor. Soon after deposition, the fossils were entombed in fine-grained clay sediments and then capped by a thick carbonate cement—a result of increasingly alkaline seawater. The cement further prevented the exchange of oxidants into sediment pore water and allowed the organic matter to remain, eventually transforming into the carbonaceous compressions seen today. Proc. Natl. Acad. Sci. U.S.A. 109 , 10.1073/pnas.1111784109 (2012). [1]: pending:yes

Modelling changes in stable isotope compositions of minerals during net transfer reactions in a contact aureole: Wollastonite growth at the northern Hunter Mountain Batholith (Death Valley National Park, USA)

One of the world's largest wollastonite deposits was formed at the contact of the northern Hunter Mountain Batholith (California, USA) in Paleozoic sediments. Wollastonite occurs as zones of variable thickness surrounding layers or nodules of quartzite in limestones. A minimum formation temperature of 650 °C is estimated from isolated periclase-bearing lenses in that area. Contact metamorphism of siliceous carbonates has produced mineral assemblages that are consistent with heterogeneous, and partly limited infiltration of water-rich fluids, compatible with 18O/ 16O and 13C/ 12C isotopic patterns recorded in carbonates. Oxygen isotope compositions of wollastonites in the study area may also not require infiltration of large quantities of externally-derived fluids that were out of equilibrium with the rocks. δ 18O values of wollastonite are high (14.8‰ to 25.0‰; median: 19.7‰) and close to those of the host limestone (19.7‰ to 28‰; median: 24.9‰) and quartz (18.0‰ to 29.1‰; median: 22.6‰). Isotopic disequilibrium exists at quartz/wollastonite and wollastonite/calcite boundaries. Therefore, classical batch/Rayleigh fractionation models based on reactant and product equilibrium are not applicable to the wollastonite rims. An approach that relies on local instantaneous mass balance for the reactants, based on the wollastonite-forming reaction is suggested as an alternative way to model wollastonite reaction rims. This model reproduces many of the measured δ 18O values of wollastonite reaction rims of the current study to within ± 1‰, even though the wollastonite compositions vary by almost 10‰.

Patterns in the Copper Isotope Composition of Minerals in Porphyry Copper Deposits in Southwestern United States

Cu isotope measurements of minerals from American southwest porphyry Cu deposits reveal a positive relationship with the grade of Cu and display spatial patterns. We measured the Cu isotope composition of 87 minerals contained within veins from drill core and hand specimen samples from three deposits (the Silver Bell, Ray, and Morenci mines). The overall range of Cu isotope ratios varies from −13.5 to +8.3 per mil. The deposits possess three different Cu reservoirs in which each has unique Cu isotope signatures, where δ65Cu for leach cap minerals < hypogene minerals < supergene enrichment minerals. The pattern exists in all three deposits and resulted from oxidative weathering of Cu at surface and reprecipitation of Cu at depth. A Rayleigh distillation model mimics the Cu isotope patterns found in the leach cap and quantifies the degree of leaching. A comparison of the isotopic composition of the three reservoirs among the three deposits reveals distinct differences—the Silver Bell mine possesses leach cap minerals that are more depleted in 65Cu and enrichment minerals that are more enriched in 65Cu than both the Morenci and Ray mines. A possible explanation for the difference among the deposits could indicate that the Silver Bell deposit experienced a greater degree of leaching than the Ray and Morenci deposits. The comparative analysis of the Cu isotope data reveals the exploration potential of using Cu isotope fractionation in the supergene environments.