Rb-Sr Isotopic Systematics Research Articles

Rb-Sr isotope systematics of the oldest crustal rock in the solar system

The RbSr isotopic system is an important cosmochemical tool for constraining the evolution of moderately volatile (Rb) and refractory (Sr) elements in the earliest Solar System. Presented here are RbSr isotopic systematics of twenty-two fractions of the evolved achondrite Erg Chech 002, the oldest crustal rock yet identified in the Solar System. Although the data show a large amount of scatter, likely due to terrestrial alteration, an errorchron yields an age within uncertainties of published PbPb ages. Initial 87Sr/86Sr ratios of both the errorchron and of individual mineral fractions show that Erg Chech 002 formed from materials evolving with a typical CI-chondrite nebular Rb/Sr ratio and that any Rb depletion on its parent body must have occurred almost simultaneously with the crystallization of the meteorite. Additionally, LuHf systematics of bulk Erg Chech 002 overlap with the chondritic range. Other early-formed parent bodies similarly show initial 87Sr/86Sr ratios close to a CI-chondrite evolution line. Despite nucleosynthetic variations of 84Sr/86Sr being evident between inner and outer Solar System materials, no variations are detectable in 87Sr/86Sr for Erg Chech 002 or other early-formed parent bodies. Although the initial 87Sr/86Sr of Erg Chech 002 overlaps with that of all other early-accreted bodies, the initial 87Sr/86Sr of asteroid 4 Vesta (BABI) remains the most precise and relevant initial value for the bulk silicate Earth.

Mantle-plume model for the Zun-Kholba orogenic gold deposit (Eastern Sayan, Russia): results of mineralogical, Rb-Sr and <sup>40</sup>Ar-<sup>39</sup>Ar geochronological and Pb-Pb isotopic studies

The large Zun-Kholba deposit, belonging to a numerous group of orogenic-type gold deposits, is in the Eastern Sayan (Russia) – a segment of the Altai-Sayan fold system of the Central Asian Orogenic Belt. The paper discusses the results of mineralogical, geochemical, geochronological and Pb-isotopic studies of gold mineralization, the purpose of which was to verify the genetic model of the deposit. Mineralogical and geochemical data obtained for ore bodies located between hypsometric levels from 1290 to 2090 m indicate a complex distribution of mineral associations at the deposit. There is no vertical and horizontal zonation in the distribution of mineral associations, in the chemical composition of the main ore minerals, as well as in the content of impurity elements in them. In dating ore-forming processes at the Zun-Kholba deposit, a comprehensive approach based on the study of K-Ar and Rb-Sr isotope systems of metasomatites was applied. The combination of Rb-Sr and 40Ar-39Ar methods allowed us to determine that the age of gold mineralization is 411 ± 2 Ma, while the age of the superimposed event, which is associated with redistribution of ore, as well as disturbance of the Rb-Sr and K-Ar isotopic systems of metasomatites, is about 380 Ma. Pb-Pb isotopic study of ore mineralization at the deposit and host Precambrian rocks allowed to prove the leading contribution of the latter in ore lead supply to the mineral-forming system. The model of formation of the Zun-Kholba orogenic gold deposit assumes a genetic relationship between ore-forming processes and intraplate alkaline mafic magmatism due to the impact of the plume in the Early Devonian time on the lithosphere of the Tuva-Mongolian terrane.

Geochemical Study of the Osumi Granodiorite, Southwestern Japan

The Osumi Granodiorite, located on the Osumi Peninsula in southwest Japan, is an example of outer zone granites that were formed during a limited period (13–15 Ma) in response to the subduction of the Philippine Sea Plate. This event, which is linked to the separation of southwest Japan from continental Asia, resulted in unique igneous activity. The Osumi Granodiorite is the largest Miocene granite body in the region. It intrudes into the Mesozoic to Paleogene accretionary complex of the Shimanto Belt and affects contact metamorphism. Despite considerable research on the Osumi Granodiorite, limited geochemical studies, especially on trace and rare earth element (REE) analyses, have been conducted. Furthermore, there are insufficient data on the Rb–Sr isotopic system, leaving the formation process unclear. This study presents whole-rock geochemical and Rb-Sr isotopic data to investigate the petrogenesis of the Osumi Granodiorite. The results suggest a common magma origin for this pluton, as indicated by linear trends on the Harker diagrams and similar REE patterns. The presence of a relatively large Eu anomaly implies formation under a reducing environment. The AKF diagram indicates predominant contamination by pelitic rocks of the Shimanto Belt during magma formation. The Rb–Sr whole-rock isochron diagram and SrI–1000/Sr diagram suggest that the Osumi Granodiorite body was formed by heterogeneous assimilation of magma into the Shimanto Belt. Furthermore, the whole-rock isochron age is 64.3 Ma, which differs by approximately 50 My from the previously reported biotite K–Ar age (14–22 Ma). This age is considered to be a pseudo-isochron age, rather than the consolidation age. During the middle Miocene, the compressive stress field in the outer zone south of the Butsuzo Tectonic Line made it difficult for magma to rise. As a result, it reacted with the sedimentary rocks of the Shimanto Belt to various degrees. The Osumi Granodiorite underwent magma differentiation upon intrusion into the Shimanto Belt. It subsequently ascended, cooled, and interacted with pelitic rocks under stable geological conditions.

Elemental differentiation and isotopic fractionation during space weathering of Chang’E-5 lunar soil

To investigate the chemical variation during space weathering of young mare basalts, here we report elemental, radiogenic Sr-Nd and stable Fe-Mg-Ca isotopic data of Chang’E-5 sieved soils and breccias. From the coarse fraction to the fine one, the sieved soils display increasing Al2O3 (10.34 wt%–13.36 wt%) and Sr (248 ppm–307 ppm) but decreasing FeO (23.50 wt%–20.22 wt%), MgO (6.88 wt%–5.78 wt%), FeO/Al2O3 (2.27–1.51) and MgO/Al2O3 (0.67–0.43). The contents of rare earth elements (except Eu) and high field strength trace elements do not vary with particle size but correlate with P2O5 contents. Given the limited contribution from contamination by meteorites and exotic materials ejected far away from the landing site, these elemental variations can be explained by differential comminution and distribution behaviors of plagioclase and mesostasis phases. These sieved soils yield a Sm-Nd isochron age (1.84 ± 0.83 Ga) comparable to that of basaltic clasts obtained by U-Pb dating (∼2.0 Ga). However, their Rb-Sr isotopic system is disturbed as indicated by their relatively homogeneous 87Sr/86Sr (0.701425–0.701592) despite variable Rb/Sr (0.017–0.028). These results suggest the Sm-Nd isotopic system is more robust to impact disturbance during space weathering compared to the Rb-Sr isotopic system. Given that the bulk soil still plots on the 2.03 Ga Rb-Sr reference isochron from the pristine plagioclases in CE-5 basalts, this disturbance did not affect the Rb-Sr isotopic system on the bulk scale. The CE-5 bulk soil has higher Mg# (33.6), 87Rb/86Sr (0.06) and present-day 87Sr/86Sr (0.701542) than the mean composition of reported basaltic clasts (Mg#: ∼28; 87Rb/86Sr: ∼0.038; 87Sr/86Sr: ∼0.700941), possibly implying that the bedrocks in CE-5 landing site consist of multiple magma pulses. The δ56Fe (0.122 ± 0.002 ‰ to 0.199 ± 0.008 ‰) and δ26Mg (−0.204 ± 0.016 ‰ to −0.109 ± 0.006 ‰) of sieved CE-5 soils increase with decreasing particle sizes but their δ44/42Ca (0.38 ± 0.04 ‰ to 0.44 ± 0.02 ‰) are relatively homogeneous. Mass balance modelling indicates that differential comminution has limited influence on the Fe-Mg-Ca stable isotopic compositions. We further dismiss the role of solar-wind sputtering, as Ca and Mg are more susceptible to sputtering and thus would be expected to show larger isotope fractionations compared to Fe, which is inconsistent with the observations. Free evaporation may explain the elevated δ56Fe and δ26Mg in fine fractions at given very limited depletion in FeO and MgO. The observed positive correlation between δ56Fe and δ26Mg, however, is much steeper than the slope expected for free evaporation, indicating also other mechanisms (e.g., Fe-Mg inter-diffusion). Since the CE-5 soil has a unique composition compared with Apollo and Luna soils, the chemical differentiation identified in this study provides new insights for establishing a connection between the chemistry and reflectance spectral properties of lunar soil. Our combined Fe-Mg-Ca isotopic study also provides a paradigm to distinguish the role of solar-wind sputtering and impact evaporation, and shows that the inter-particle diffusion process may be an important mechanism for the isotope fractionation among lunar soil components.

Petrogenesis and Geodynamic Evolution of the Archean Shawmere Anorthosite Complex and Associated Gneisses, Kapuskasing Uplift, Superior Province, Canada

Abstract In this study, we integrated extensive field, petrographic, whole-rock major and trace element, and Nd–Pb–Sr–O isotope, and zircon U–Pb ages, trace element and Lu–Hf isotope data from the Neoarchean Shawmere Anorthosite Complex and surrounding gneisses to unravel their petrogenetic origin and tectonic history. The ~2765 Ma Shawmere Anorthosite Complex is interpreted to have been emplaced into a sequence of interlayered greywacke and basalt deposited in an intra-continental arc rift system above a north-dipping subduction zone. The complex consists mainly of anorthosite, leucogabbro, gabbro, and hornblendite that were emplaced as several batches of magmas and crystal mushes originating from sub-arc mantle sources. In contrast to the previous studies, our field and petrographic data suggest an igneous origin for the most hornblende in the complex, implying hydrous parental magmas. A hydrous magma origin is also consistent with the high-anorthite content (mostly 70–90%) of the plagioclase in the complex. Percolation of hydrous basaltic melts through gabbroic cumulates in crustal magma chambers led to extensive (>50%) replacement of igneous clinopyroxene by igneous hornblende. Continued subduction resulted in the closure of the intra-arc rift system and the intrusion of the complex by tonalite, granodiorite and diorite between 2765 and 2680 Ma in an Andean-type margin. The complex and surrounding gneisses underwent hornblende granulite-facies metamorphism mainly between 2680 and 2620 Ma, overlapping with mid-crustal east-west extension between 2660 and 2640 Ma. The granulite-facies metamorphism is recorded by the replacement of hornblende, plagioclase and clinopyroxene by garnet and the development of a garnet-orthopyroxene-plagioclase metamorphic assemblage with a granoblastic texture. Tectonic rebounding of mid-crustal rocks to upper crustal levels after 2620 Ma led to the formation of an extensive network of extensional fractures and retrograde metamorphism. Migration of CO2-rich hydrous fluids along the extensional fractures and grain boundaries resulted in the precipitation of many metasomatic minerals mainly at the expense of hornblende and plagioclase, including epidote, clinozoisite, tremolite, actinolite, paragonite, margarite, titanite, quartz, calcite, sillimanite, dolomite, and chlorite. Prevalent replacement of hornblende by garnet during prograde metamorphism and metasomatic replacement of hornblende and plagioclase by retrograde mineral assemblages disturbed the Sm–Nd, U–Th–Pb, and Rb–Sr isotope systems.

Sr–Nd Isotopic Evidence of a Heterogeneous Magmatic Source of Alkaline–Carbonatite Rocks of the Odikhincha Alkaline–Carbonatite Massif (Maimecha-Kotui Province, Siberia)

This study focuses on the igneous rocks composing the Odikhincha massif. The massif is typical ring alkaline–ultrabasic massif with carbonatites, second largest in the Maimecha-Kotui province. The Sr-Nd isotopic values of the traps of the Arydzhang Formation and the host dolomites were also determined for comparison. The Rb–Sr isotope system of phlogopite and calcite from the Od-16-19 carbonatite of the Odikhincha massif is disturbed; the obtained age on the mineral isochrone (245 ± 3 Ma) is close to the time of formation of the Siberian traps and rocks of the ultrabasic–alkaline Maimecha-Kotui complex, but the large scatter of analytical points (MSWD = 22) does not allow this date to be considered as reliable. The disturbance of the isotope system is probably related to the fact that the strontium isotope ratio in the fluid was not constant during autometasomatic phlogopitization of carbonatite. The U–Pb isotopic system of titanite and perovskite from the same carbonatite sample Od-16-19 also appeared to be disturbed, since data points formed discordia. The U–Pb age obtained for titanite and perovskite are 244 ± 5 Ma (MSWD = 1.8) and 247 ± 18 Ma (MSWD = 4), respectively. Apparently, the age values provided by the two isotopic systems (245 ± 3 Ma by Rb–Sr and 247 ± 18 and 244 ± 5 Ma by U–Pb) are consistent with each other and reflect the time of metasomatic processes, i.e., phlogopitization and iolitization. Rb–Sr and Sm–Nd isotope data for ultrabasic–alkaline intrusive rocks with carbonatites of the Odikhincha massif and volcanics of the Arydzhang Formation indicate an enriched, relative to the composition of the convecting mantle, isotopically heterogeneous source of their parent melts. This source could be a combination of ultrabasic mantle rocks and rocks of basic composition (basites). The latter played the role of an enriched component. No signs of contamination of the melts with the host sedimentary rocks in situ were found, however, variations of Sr and Nd isotopic ratios in the rocks of the Odikhincha massif may indicate that during the introduction of deep magmas their interaction and substance exchange with the surrounding rocks of the lithosphere continued up to complete solidification of the melts, as indicated by the nature of local isotopic heterogeneity within the Odikhincha intrusion.

Sr–Nd Isotopic Evidence of a Heterogeneous Magmatic Source of Alkaline–Carbonatite Rocks of the Odikhincha Alkaline–Carbonatite Massif (Maimecha-Kotui Province, Siberia)

This study focuses on the igneous rocks composing the Odikhincha massif. The massif is typical ring alkaline–ultrabasic massif with carbonatites, second largest in the Maimecha-Kotui province. The Sr-Nd isotopic values of the traps of the Arydzhang Formation and the host dolomites were also determined for comparison. The Rb–Sr isotope system of phlogopite and calcite from the Od-16-19 carbonatite of the Odikhincha massif is disturbed; the obtained age on the mineral isochrone (245 ± 3 Ma) is close to the time of formation of the Siberian traps and rocks of the ultrabasic–alkaline Maimecha-Kotui complex, but the large scatter of analytical points (MSWD = 22) does not allow this date to be considered as reliable. The disturbance of the isotope system is probably related to the fact that the strontium isotope ratio in the fluid was not constant during autometasomatic phlogopitization of carbonatite. The U–Pb isotopic system of titanite and perovskite from the same carbonatite sample Od-16-19 also appeared to be disturbed, since data points formed discordia. The U–Pb age obtained for titanite and perovskite are 244 ± 5 Ma (MSWD = 1.8) and 247 ± 18 Ma (MSWD = 4), respectively. Apparently, the age values provided by the two isotopic systems (245 ± 3 Ma by Rb–Sr and 247 ± 18 and 244 ± 5 Ma by U–Pb) are consistent with each other and reflect the time of metasomatic processes, i.e., phlogopitization and iolitization. Rb–Sr and Sm–Nd isotope data for ultrabasic–alkaline intrusive rocks with carbonatites of the Odikhincha massif and volcanics of the Arydzhang Formation indicate an enriched, relative to the composition of the convecting mantle, isotopically heterogeneous source of their parent melts. This source could be a combination of ultrabasic mantle rocks and rocks of basic composition (basites). The latter played the role of an enriched component. No signs of contamination of the melts with the host sedimentary rocks in situ were found, however, variations of Sr and Nd isotopic ratios in the rocks of the Odikhincha massif may indicate that during the introduction of deep magmas their interaction and substance exchange with the surrounding rocks of the lithosphere continued up to complete solidification of the melts, as indicated by the nature of local isotopic heterogeneity within the Odikhincha intrusion.

The Singhbhum Craton (India) records a billion year of continental crust formation and modification

The petrogenesis of continental crust from its ultimate mantle source can be reconstructed from the element abundances and radiogenic isotope compositions of ideally pristine igneous rocks. The initial isotope compositions of igneous rocks provide geochemical constraints on the age, composition and evolution of their source(s). Determining initial isotope ratios for rock samples can be challenging, especially in rocks with a long and protracted thermal history. The Rb-Sr system is highly sensitive to parent-daughter element fractionation during magma differentiation. This makes the Rb-Sr isotope systematics ideal to trace the precursor composition of Archean felsic crust and constrain the time of element fractionation during the formation and subsequent modification of continental crust. Initial isotope compositions can be obtained directly from minerals that strongly prefer the daughter element and effectively exclude the parent element of the radio-isotope system of interest. Apatite, having a near zero Rb/Sr ratio, is ideal for preserving its initial 87Sr/86Sr and zircon records initial 176Hf/177Hf compositions. Combined modelling of Sr and Hf isotope data from granitoids of the Archean Singhbhum Craton, indicates that the older Paleoarchean granitoids, emplaced between 3.53 Ga and 3.44 Ga, were derived from a mafic precursor (∼52–54 wt% SiO2) sourced from a depleted mantle at ∼3.71 Ga. Initial 87Sr/86Sr isotope signatures of matrix apatite and apatite inclusions in zircon from the younger Paleoarchean granitoids (3.4–3.2 Ga) of the Singhbhum Craton indicate these younger granitoids were produced by mixing of magma generated from an older mafic source and partial melts derived from the older granitoids. The combined Sr-Hf isotope modelling links the timing of mantle extraction of the precursor material for Paleoarchean Singhbhum granitoids with a known mafic crust extraction event at ∼3.71 Ga. In combination, the new Sr isotope data from apatite combined with whole rock and zircon Hf isotope data from the literature reveal a ∼1 Ga protracted crustal growth and differentiation history of the nucleus of the Singhbhum Craton. By combining radio-isotope systems like 87Rb-87Sr and 176Lu-176Hf, the petrogenesis of Archean felsic crust from the extraction of mafic material from the mantle to reworking in an orogenic cycle to emplacement can be reconstructed. This approach can be applied to other greenstone-gneiss terranes to quantify the spatio-temporal and compositional evolution of voluminous felsic crust and the formation of cratons in the Archean.

Age and Isotope-Geochemical Characteristics of Ta, Nb, W, Sn Mineralization Associated with Rare-Metal Granites (Khangilay Ore District, Eastern Transbaikalia)

The age relations between the formation of the parent massif and the crystallization time of the associated ore mineralization were established based on isotope-geochronological study of the massif-deposits of the Khangilay ore cluster with various metallogenic specialization in Eastern Transbaikalia. In the Orlovka Li–F granite massif, the crystallization time of columbite–tantalite (145 ± 1 Ma) and cassiterite (144.2 ± 0.3 Ma) (U-Pb, ID-TIMS) is almost identical to the crystallization time of zircon (140.6 ± 2.9 Ма (U-Pb, SHRIMP) and 145 ± 1 Ma (U-Pb, CA-ID-TIMS)), which is an age marker of the formation of massifs. This fact testifies to the magmatogenic nature of rare-metal mineralization. In the Spokojnoye massif – the “standard type” of rare-metal peraluminous granites—a 0.6–3.8 Ma time gap was revealed between the time of massif formation (141.3 ± 1.8 Ма, U-Pb, SHRIMP, 146.9 ± 0.7 Ма, Rb-Sr isotopic system) and crystallization of wolframite (141.8 ± 0.6, Rb-Sr isotopic system and 140.1 ± 1.4 Ма, Sm-Nd isotopic system). This interval likely corresponds to the life time of the hydrothermal system, which produced tungsten mineralization.

Metamorphic P–T conditions, in‐situ LA–ICP–MS biotite–muscovite Rb–Sr geochronology and tectonic implications of two‐mica–quartz schist from the Kuanping Group in the North Qinling Orogenic Belt

The Kuanping Group is an important lithotectonic unit in the North Qinling Tectonic Belt, which has experienced metamorphism from greenschist to high amphibolite facies and is of great significance in defining the orogenic process of the Qinling Orogenic Belt. This study presents the results of petrology, geothermobarometry, phase equilibrium modelling, in‐situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) biotite–muscovite Rb–Sr analysis, for the two‐mica–quartz schist of the Kuanping Group in the Hongtuling and Heihe areas. These results are employed to better understand their metamorphic P–T conditions and ages, as well as geological implications. Garnet‐bearing two‐mica–quartz schist in the Hongtuling area records a clockwise P–T trajectory on the basis of phase equilibrium modelling, with peak P–T conditions of ~600°C and ~0.85 GPa. In addition, the Ti‐in‐biotite thermometry yielded metamorphic temperature conditions of ~578°C–580°C and ~590°C–596°C for the garnet‐bearing two‐mica–quartz schist from the Hongtuling area and the two‐mica–quartz schist from the Heihe area, respectively. These results indicate that metamorphic temperature conditions in both areas are higher than the closure temperature of the Rb–Sr isotopic system of muscovite and biotite. In‐situ LA–ICP–MS biotite and muscovite Rb–Sr analyses yielded three groups of isochron age, i.e., ca. 410 Ma, ca. 360 Ma and ca. 320–300 Ma. On the basis of previously published data and the data in this study, the Kuanping Group underwent ca. 440 Ma regional metamorphism in the NQB, retrograde cooling at ca. 410–360 Ma due to exhumation and ca. 320–300 Ma tectonothermal overprinting.

Age and Isotope-Geochemical Characteristics of Ta, Nb, W, Sn Mineralization Associated with Rare-Metal Granites (Khangilay Ore District, Eastern Transbaikalia)

The age relations between the formation of the parent massif and the time of crystallization of the associated ore mineralization were established on the basis of isotope-geochronological study of the massifs-deposits of the Khangily ore district with various metallogenic specialization in Eastern Transbaikalia. In the Orlovka massif of Li-F granites, the crystallization time of columbite-tantalite (145 ± 1 Ma) and cassiterite (144.2 ± 0.3 Ma) (U-Pb, ID-TIMS) is almost identical to the crystallization time of zircon (140.6 ± 2.9 Ма (U-Pb, SHRIMP) and 145 ± 1 Ma (U-Pb, CA-ID-TIMS)) – an age marker of the formation of massifs. This fact testifies to the magmatogenic nature of rare-metal mineralization. In the Spokojnoye massif – the “standard type” of rare-metal peraluminous granites – a time gap with an interval of 0.6–3.8 Ma was revealed between the time of formation of the massif (141.3 ± 1.8 Ма, U-Pb, SHRIMP, 146.9 ± 0.7 Ма, Rb-Sr isotopic system) and crystallization of wolframite (141.8 ± 0.6, Rb-Sr isotopic system and 140.1 ± 1.4 Ма, Sm-Nd isotopic system). Perhaps this interval corresponds to the time of formation of the hydrothermal system, with which tungsten mineralization is genetically related.

The Accumulation of the REE by Bacterial Fe Oxyhydroxide

The current study was focused on Fe oxyhydroxides from the north-western part of the East-European platform. Modern-day Fe oxyhydroxides of bacterial origin demonstrate an enhanced concentration of rare earth elements (up to 1200 ppm), compared to samples without iron bacteria. The 143Nd/144Nd ratio in bacterial Fe oxyhydroxide has the value from 0.511532 to 0.512588 and corresponds to the geochemical signature of the waters, oxyhydroxides precipitated from. Samples of iron hydroxides from Quaternary and recent continental ore deposits with different Nd and Sr isotope composition were used for the laboratory reduction of Fe3+ up to emergence of magnetite (T ~ 1000°C). 143Nd/144Nd and 87Sr/86Sr ratios in the newly formed mineral phases show insignificant discrepancy with parent iron ore. The persistency of Sm–Nd and Rb–Sr isotope systems in the process of bog iron ore experimental melting permits it’s applying to paleoenvironment reconstructions and archaeometry.

Gallium isotopic constraints for the origin of the Earth-Moon system

Comparatively heavy isotopic compositions of moderately volatile elements (MVE) in lunar rocks have been advocated to reflect the loss of light isotopes during devolatilization processes from the Moon. In this study we present new gallium (Ga) isotope data for lunar highland rocks, with a focus on the Ferroan Anorthosite Suite (FAS). These are commonly thought to be direct crystallization products from the late lunar magma ocean (LMO) and should contain the majority of the Ga inventory of the Moon. As such, FAS rocks are crucial for identifying the processes that drove Ga isotope fractionation as well as for inferring the Ga isotopic composition of the bulk Moon.Our data reveal that FAS samples have a range in δ71Ga from −0.27 to 0.22‰ and are generally isotopically light in Ga compared to other lunar lithologies, but straddle values typical of terrestrial rocks. Although Ga is defined as an MVE, these Ga isotope variations do not correspond with concentrations of more volatile elements, indicating that Ga isotope variations in the FAS are not primarily controlled by devolatilization processes. Instead, the Ga isotopic compositions of bulk FAS rocks broadly correlate with the composition of plagioclase, with the calcium content of plagioclase decreasing as Ga becomes isotopically heavier. This suggests that fractionation of Ga isotopes in FAS rocks was caused by the preferential incorporation of isotopically light Ga into plagioclase during the later solidification stages of the LMO. The progressive crystallization and extraction of plagioclase forces the residual melt towards increasingly heavier Ga isotope ratios, corroborating similar conclusions derived from correlations between δ71Ga and Eu* in the mare basalt suite.Using Ga isotope partitioning calculations, we demonstrate that an isotope fractionation coefficient between plagioclase and coexisting melt of −0.3 to -0.4‰ could explain the observed range of δ71Ga values in FAS, mare basalt suite rocks, and KREEP. These calculations allow for a first order estimate of the Ga isotopic composition of the bulk silicate Moon prior to plagioclase fractionation and suggest it was close to the composition of the bulk silicate Earth. This would imply that the Moon did not lose a substantial fraction of its Ga inventory during accretion, consistent with new constraints from Rb-Sr isotope systematics that indicate the Moon's volatile deficit was primarily inherited from Theia. In conjunction with the overlap in non-mass-dependent isotope ratios, these collective observations could be reconciled if Theia and the proto-Earth formed in similar regions of the inner Solar System that were already volatile-depleted.

Zircon U–Pb geochronology and Sm–Nd and Rb–Sr isotope systematics of Neoproterozoic granitoïds from Bou Azzer (Anti-Atlas - Morocco): The obduction trigger of the central Anti-Atlas terrane

This study presents new U–Pb zircon ages for the Neoproterozoic granitoids of the Bou Azzer inlier at the central part of the Anti-Atlas chain (Morocco), in the northwestern edge of the West African Craton (WAC). Analyzes of zircon grains yield the following ages: 658 ± 3 Ma for Ait Ahmane pluton, 647 ± 5 Ma for Bou-Izbane intrusion, 656 ± 3 Ma for Bou Azzer quartz-diorites, 583 ± 12 Ma in the Oustrate intrusion and 644 ± 4 Ma for the Ait Ahmane leucogranite. These ages confirm a major magmatic event between 640 and 660 Ma contemporaneous with the obduction of oceanic crust marking the formation of the ophiolites of Bou Azzer and Sirwa. The emplacement of granitoids would have led to a decrease in the density of the oceanic crust and triggered the process of obduction. The granitoids display the geochemical features of volcanic arc magmas and have positive initial ϵNd (+4.67 to +6.57) and low initial 87Sr/86Sr (0.7002–0.7055). The quartz-diorite group displays young Depleted Mantle Model (TDM) ages that range from 904 to 1065 Ma. The leucogranites have younger TDM of 869–884 Ma. These new data attest to their juvenile nature and the absence of any crustal contribution. The NNW-SSE extended inlier band of Bou Azzer - Sirwa constitutes an exotic terrane of Neoproterozoic age and of a juvenile nature wedged between the SW and NE domains, respectively, formed by or surmounting a Meso-Paleoproterozoic or even Archean basement. The Bleida granodiorite, dated at 559 ± 4, displays older U–Pb ages from inherited zircon cores at 1234, 1748 and 2085 Ma. The Bleida granodiorite was emplaced during a later magmatic episode that belongs to the Ouarzazate Group, generated during the post-collisional metacratonic evolution of the northern edge of the West African craton.

Rare earth elements and neodymium and strontium isotopic constraints on provenance switch and post-depositional alteration of fossiliferous Ediacaran and lowermost Cambrian strata from Arctic Norway

The Digermulen Peninsula in northeastern Finnmark, Arctic Norway, comprises one of the most complete Ediacaran–Cambrian transitions worldwide with a nearly continuous record of micro- and macrofossils from the interval of the diversification of complex life. Here, we report on the provenance and post-depositional alteration of argillaceous mudstones from the Digermulen Peninsula using rare earth elements and Sm–Nd and Rb–Sr isotopic systematics to provide an environmental context and better understand this important transition in Earth’s history. The studied sections comprise a mid-Ediacaran glacial–interglacial cycle, including the Nyborg Formation (ca. 590 Ma) and Mortensnes Formation (related to the ca. 580 Ma-old Gaskiers glaciation), and the Stáhpogieddi Formation (ca. 560–537 Ma), which yields Ediacara-type fossils in the Indreelva Member and contains the Ediacaran–Cambrian boundary interval in the Manndrapselva Member and basal part of the informal Lower Breidvika member (ca. 537–530 Ma). Three sample groups, (1) Nyborg and Mortensnes formations, (2) the lowermost five samples from the Indreelva Member and (3) the remaining samples from the Indreelva as well as from the Manndrapselva and Lower Breidvika members, can be distinguished, belonging to distinct depositional units. All samples have negative εNd(T) values (−6.00 to − 21.04) indicating a dominant input of terrigenous detritus with an old continental crust affinity. Significant shifts in Sm–Nd isotope values are related to changes in the sediment source, i.e. Svecofennian province vs Karelian province vs Svecofennian province plus in addition likely some juvenile (late Neoproterozoic volcanic) material, and probably reflect palaeotectonic reorganisation along the Iapetus-facing margin of Baltica. The combined Rb–Sr isotopic data of all samples yield an errorchron age of about 430 Ma reflecting the resetting of the Rb–Sr whole-rock isotope systems of the mudstones during the Scandian tectono-metamorphic event in the Gaissa Nappe Complex of Finnmark. Preservation of palaeopascichnids coincides with the sedimentation regimes of sample groups 2 and 3 while other Ediacara-type fossils, e.g. Aspidella-type and frondose forms, are limited to the sample group 3. Our results are similar to those of earlier studies from the East European Platform in suggesting oxic seafloor conditions during the late Ediacaran.

Constraining the geothermal parameters of in situ Rb–Sr dating on Proterozoic shales and their subsequent applications

Abstract. Recent developments in tandem laser ablation mass spectrometer technology have demonstrated the capacity for separating parent and daughter isotopes of the same mass online. As a result, beta-decay chronometers can now be applied to the geological archive in situ as opposed to through traditional whole-rock digestions. One novel application of this technique is the in situ Rb–Sr dating of Proterozoic shales that are dominated by authigenic clays such as illite. This method can provide a depositional window for shales by differentiating signatures of early diagenetic processes versus late-stage secondary alteration. However, the hydrothermal sensitivity of the Rb–Sr isotopic system across geological timescales in shale-hosted clay minerals is not well understood. As such, we dated the Mesoproterozoic Velkerri Formation from the Altree 2 well in the Beetaloo Sub-basin (greater McArthur Basin), northern Australia, using this approach. We then constrained the thermal history of these units using common hydrocarbon maturity indicators and modelled effects of contact heating due to the intrusion of the Derim Derim Dolerite. In situ Rb–Sr dating of mature, oil-prone shales in the diagenetic zone from the Velkerri Formation yielded ages of 1448 ± 81, 1434 ± 19, and 1421 ± 139 Ma. These results agree with previous Re–Os dating of the unit and are interpreted as recording the timing of an early diagenetic event soon after deposition. Conversely, overmature, gas-prone shales in the anchizone sourced from deeper within the borehole were dated at 1322 ± 93 and 1336 ± 40 Ma. These ages are younger than the expected depositional window for the Velkerri Formation. Instead, they are consistent with the age of the Derim Derim Dolerite mafic intrusion intersected 800 m below the Velkerri Formation. Thermal modelling suggests that a single intrusion of 75 m thickness would have been capable of producing a significant hydrothermal perturbation radiating from the sill top. The intrusion width proposed by this model is consistent with similar Derim Derim Dolerite sill thicknesses found elsewhere in the McArthur Basin. The extent of the hydrothermal aureole induced by this intrusion coincides with the window in which kerogen from the Velkerri Formation becomes overmature. As a result, the mafic intrusion intersected here is interpreted to have caused kerogen in these shales to enter the gas window, induced fluids that mobilize trace elements, and reset the Rb–Sr chronometer. Consequently, we propose that the Rb–Sr chronometer in shales may be sensitive to temperatures of ca. 120 ∘C in hydrothermal reactions but can withstand temperatures of more than 190 ∘C in thermal systems not dominated by fluids. Importantly, this study demonstrates a framework for the combined use of in situ Rb–Sr dating and kerogen maturation indicators to help reveal the thermochronological history of Proterozoic sedimentary basins. As such, this approach can be a powerful tool for identifying the hydrocarbon potential of source rocks in similar geological settings.

The timing of blueschist-facies metamorphism in the Makrotantalon Unit on Andros Island, Greece: Cretaceous and Eocene high-pressure/low-temperature events?

AbstractThis study corroborates interpretations suggesting that the Makrotantalon Unit on Andros represents a tectonic slice with Pelagonian affinity in the nappe stack of the Cycladic Blueschist Unit. Previously reported Cretaceous 40Ar–39Ar dates of a garnet-glaucophane schist from the Makrotantalon Unit could not be reproduced by Rb–Sr geochronology, but this is not an indication of contamination with excess Ar. Instead, the newly dated samples record disturbance of the Rb–Sr isotope system by partial recrystallization. Subsets of these phengite populations, representing the smaller grain-size fractions, yielded low-precision dates ranging from c. 21 to c. 15 Ma that document deformation-related resetting and recrystallization of a presumably Cretaceous white mica population. Although these Miocene dates cannot be linked with blueschist-facies metamorphism, they provide time constraints on the formation of shear zones that overprinted the original thrust contact during exhumation. The geological relevance of a Cretaceous high-pressure event is confirmed by a Rb–Sr date of c. 121 Ma for an epidote-glaucophane schist collected further away from the tectonic contact. The occurrence of a second blueschist-facies event in the Eocene is verified by Rb–Sr dates of two epidote-glaucophane schists (c. 40 Ma and c. 44 Ma) that can unambiguously be assigned to the Makrotantalon Unit.

Sr-Nd-Hf Isotopic Disequilibrium During the Partial Melting of Metasediments: Insight From Himalayan Leucosome

Radiogenic isotopes of granitoids are widely applied to fingerprint the source of granitoids and study the magma mingling and assimilation processes, aiming to decipher the planetary differentiation. This weapon is based on the assumption that crustal melts inherit the radiogenic isotopes of protoliths. However, complicated melting processes in the crust would drive the radiogenic isotopes of melt away from the source, thus calling for a clear understanding of the behaviors of radiogenic isotopes during crustal melting. This study investigated the joint behavior of Rb-Sr, Sm-Nd, and Lu-Hf isotope systems during the melting of metasediments. Leucosome, schist, and leucogranite samples were collected from the Nyalam, South Tibet, where the leucosome was produced by muscovite dehydration melting of metapelites. Results show that the leucosome has a broad range of Sr-Nd-Hf isotopes (87Sr/86Sri: 0.763,48–0.875321, εNd(t): 14.6–11.6, εHf(t): 15.0–4.4) that deviate from the metasediments. We attributed it to the non-modal or disequilibrium melting of metapelites. The variation of Sr isotopes of leucosome is formed due to preferential entry into the melt of muscovite relative to plagioclase during melting. The changing 87Sr/86Sri and 87Rb/86Sr of leucosome define an errorchron at ∼ 500 Ma indicating an early Paleozoic tectono-thermal event supported by the U-Pb age of zircon cores (∼460 Ma) in the leucosome. The Nd isotopes of leucosome are mainly controlled by the preferential dissolution of apatite relative to monazite. The inadequate dissolution of zircon caused more depleted initial Hf isotopes compositions in the leucosome than the source. However, the leucosome with a higher crystallization temperature has radiogenic isotopic composition closer to the source. It indicates that the melting temperature greatly affects the isotope equilibrium between source and melt.

Decoupling of Sr-Nd Isotopic Composition Induced by Potassic Alteration in the Shapinggou Porphyry Mo Deposit of the Qinling–Dabie Orogenic Belt, China

In our previous study on petrogenesis of quartz syenite and granite porphyry, the host rocks of the Late Mesozoic Shapinggou Mo deposit in the Qinling–Dabie orogenic belt, we found that the initial Sr isotopic composition of the host rocks is strongly affected by the degree of K-alteration. Here, we provide further isotopic evidence of the host rocks and their minerals to investigate the geochemical behaviour of trace elements and isotopes during the alteration and to explain the phenomenon of decoupling of Sr–Nd isotopic composition. The quartz syenite and granite porphyry are altered by K-alteration in varying degrees and have high K2O and Rb contents and low Na2O, CaO, Sr, and Ba contents. Rock samples of both quartz syenite and granite porphyry have variable Rb/Sr ratios and initial 87Sr/86Sr values (even < 0.70) but contain quite homogeneous εNd(t) values (−12.8 to −14.8). Minerals from the rocks of moderate to intense K-alteration have very low initial 87Sr/86Sr values (even < −17), while those from the weakly altered rocks have 87Sr/86Sr(t) values of 0.7044 to 0.7084. The same phenomenon of the decoupling in Sr–Nd isotopic composition can be observed from several Mo deposits within the eastern Qinling–Dabie orogenic belt. This fact suggests similar hydrothermal features and a comparable origin for both the magmatic rocks and hydrothermal fluids in this belt. A comparison between porphyry Mo and porphyry Cu deposits shows that elements and the Rb–Sr isotope system have different behaviours during the K-alteration, implying distinct material sources and igneous rocks for porphyry Mo and porphyry Cu deposits, respectively.

Rb–Sr geochronology of metamorphic rocks from the Central Indonesian Accretionary Collision Complex: Additional age constraints for the Meratus and Luk Ulo complexes (South Kalimantan and Central Java)

Metamorphic rocks of the Central Indonesian Accretionary Collision Complex are exposed in widely scattered occurrences on the islands of Java, Sulawesi, and Kalimantan. Their relationship to one or more subduction zones is controversial which can partly be explained by insufficient geochronological investigation. Using Rb–Sr geochronology we have dated various schists from the Meratus Complex, South Kalimantan, and the Luk Ulo Complex, Central Java. The Meratus schists first experienced high-pressure/low-temperature (HP/LT) metamorphism and were later overprinted at medium pressure conditions. Due to the rarity and poor preservation of such rocks, isotopic ages for well-preserved HP/LT rocks could not be determined yet. Previously obtained K–Ar white mica dates (180–160 Ma) were attributed to the HP/LT event, however, this was speculative as the dated rocks only record greenschist- to amphibolite-facies P–T conditions. A newly dated partially overprinted but still glaucophane-rich sample yielded a well-defined Rb–Sr isochron age (phengite, epidote) of 112.9 ± 0.7 Ma (all uncertainties on Rb–Sr dates indicate 95% confidence level), providing a minimum age for the HP/LT stage. Internal mineral isochrons of more severely overprinted samples show poor linear fits between mica and associated phases indicating incomplete resetting of the Rb–Sr isotope system during the later lower pressure metamorphic event. Isochrons that are derived only from different grain-size fractions of white mica indicate dates between 115 and 110 Ma (± 3 to 6 Ma). In contrast to previous interpretations that linked high-grade rocks of the Meratus Complex to collision-induced crustal thickening it is here considered more likely that these rocks either represent exotic fragments from NW Australia or the metamorphic sole of the Meratus ophiolite. Differences in temperature estimates for HP/LT rocks from the Luk Ulo Complex probably indicate erroneous textural interpretation of equilibrium mineral assemblages and/or compositions. Neither petrological analysis nor geochronological dating has yet unambiguously confirmed different P–T–t paths for eclogites with and without tourmaline (Trm) from the Luk Ulo area. The newly dated Trm-bearing epidote blueschist from the serpentinitic Muncar River mélange yielded a well-defined Rb–Sr isochron of 119.0 ± 0.7 Ma (phengite, epidote, tourmaline) that perfectly matches with existing K–Ar and Rb–Sr data for Trm-free HP/LT rocks. Two samples of the more widespread garnet-phengite schists of the Luk Ulo area yielded Rb–Sr ages (phengite, feldspar) of 118.0 ± 0.8 Ma and 114.7 ± 1.3 Ma. A meter-sized epidote amphibolite block included in these clastic metasedimentary rocks provided a Rb–Sr isochron age of 116.8 ± 0.7 Ma (phengite, epidote, whole rock). These ages match existing K–Ar data for severely overprinted rocks from the Luk Ulo area and further corroborate the geological significance of Early Cretaceous metamorphism in Central Java. Petrological and geochronological data of the Meratus and Luk Ulo complexes are in accordance with a model suggesting that both occurrences belong to the same Cretaceous subduction system. The existence of an additional Cretaceous suture zone cutting through Sulawesi is considered likely.