Diachronous Redistribution of Hf and Nd Isotopes at the Crystal Scale—Consequences for the Isotopic Evolution of a Poly-Metamorphic Crustal Terrane

In metamorphic rocks, mineral species react over a range of pressure–temperature conditions that do not necessarily overlap. Mineral equilibration can occur at varied points along the metamorphic pressure–temperature (PT) path, and thus at different times. The sole or dominant use of zircon isotopic compositions to constrain the evolution of metamorphic rocks might then inadvertently skew geological interpretations towards one aspect or one moment of a rock’s history. Here, we present in-situ U–Pb/Sm–Nd isotope analyses of the apatite crystals extracted from two meta-igneous rocks exposed in the Saglek Block (North Atlantic craton, Canada), an Archean metamorphic terrane, with the aim of examining the various signatures and events that they record. The data are combined with published U–Pb/Hf/O isotope compositions of zircon extracted from the same hand-specimens. We found an offset of nearly ca. 1.5 Gyr between U-Pb ages derived from the oldest zircon cores and apatite U–Pb/Sm–Nd isotopic ages, and an offset of ca. 200 Ma between the youngest zircon metamorphic overgrowths and apatite. These differences in metamorphic ages recorded by zircon and apatite mean that the redistribution of Hf isotopes (largely hosted in zircon) and Nd isotopes (largely hosted in apatite within these rocks), were not synchronous at the hand-specimen scale (≤~0.001 m3). We propose that the diachronous redistribution of Hf and Nd isotopes and their parent isotopes was caused by the different PT conditions of growth equilibration between zircon and apatite during metamorphism. These findings document the latest metamorphic evolution of the Saglek Block, highlighting the role played by intra-crustal reworking during the late-Archean regional metamorphic event.

Supplemental Material: Rifting of the Indian passive continental margin: Insights from the Langjiexue basalts in the central Tethyan Himalaya, southern Tibet

Text S1: Analytical methods. Figure S1: Zr versus selected element variation diagrams to highlight the effects of alteration and metamorphism for the basalts from Langjiexue area. Figure S2: (A) Ti/Y vs. TiO2, and (B) Ti/Y vs. MgO diagrams for the basalt samples from the Langjiexue in Tethyan Himalaya. Table S1: Representative Permian-Triassic magmatic events along the Tethyan Himalaya. Table S2: Zircon LA-ICP-MS U-Pb in-situ analyzing results for zircons from the Langjiexue basalts. Table S3: Whole-rock major, trace element and Sr-Nd isotope data of Langjiexue basalts.

Supplemental Material: Rifting of the Indian passive continental margin: Insights from the Langjiexue basalts in the central Tethyan Himalaya, southern Tibet

Text S1: Analytical methods. Figure S1: Zr versus selected element variation diagrams to highlight the effects of alteration and metamorphism for the basalts from Langjiexue area. Figure S2: (A) Ti/Y vs. TiO2, and (B) Ti/Y vs. MgO diagrams for the basalt samples from the Langjiexue in Tethyan Himalaya. Table S1: Representative Permian-Triassic magmatic events along the Tethyan Himalaya. Table S2: Zircon LA-ICP-MS U-Pb in-situ analyzing results for zircons from the Langjiexue basalts. Table S3: Whole-rock major, trace element and Sr-Nd isotope data of Langjiexue basalts.

A tandem-column extraction chromatography for Nd separation: minimizing mass-independent isotope fractionation for ultrahigh-precision Nd isotope-ratio analysis

The short-lived 146Sm-142Nd isotope system traces key early planetary differentiation processes that occurred during the first 500 million-years of solar system history. The variations of 142Nd/144Nd in terrestrial samples, typically...

Paleoproterozoic Pt-Pd Fedorovo-Pansky and Cu-Ni-Cr Monchegorsk Ore Complexes: Age, Metamorphism, and Crustal Contamination According to Sm-Nd Data

This paper continues the Sm-Nd isotope geochronological research carried out at the two largest Paleoproterozoic ore complexes of the northeastern Baltic Shield, i.e., the Cu-Ni-Cr Monchegorsk and the Pt-Pd Fedorovo-Pansky intrusions. These economically significant deposits are examples of layered complexes in the northeastern part of the Fennoscandian Shield. Understanding the stages of their formation and transformation helps in the reconstruction of the long-term evolution of ore-forming systems. This knowledge is necessary for subsequent critical metallogenic and geodynamic conclusions. We applied the Sm-Nd method of comprehensive age determination to define the main age ranges of intrusion. Syngenetic ore genesis occurred 2.53–2.85 Ga; hydrothermal metasomatic ore formation took place 2.70 Ga; and the injection of additional magma batches occurred 2.44–2.50 Ga. The rock transformation and redeposited ore formation at 2.0–1.9 Ga corresponded to the beginning of the Svecofennian events, widely presented on the Fennoscandian Shield. According to geochronological and Nd-Sr isotope data, rocks of the Monchegorsk and the Fedorovo-Pansky complexes seemed to have an anomalous mantle source in common with Paleoproterozoic layered intrusions of the Fennoscandian Shield (enriched with lithophile elements, εNd values vary from −3.0 to +2.5 and ISr 0.702–0.705). The data obtained comply with the known isotope-geochemical and geochronological characteristics of ore-bearing layered intrusions in the northeastern Baltic Shield. An interaction model of parental melts of the Fennoscandian layered intrusions and crustal matter shows a small level of contamination within the usual range of 5–10%. However, the margins of the Monchetundra massif indicate a much higher level of crustal contamination caused by active interaction of parental magmas and host rock.

Petrogenesis and tectonic setting of Late Devonian high-Mg andesites in the Beishan orogen, NW China

High-Mg andesites (HMAs) are crucial for the reconstruction of plate tectonics, continental margin formation and lithospheric evolution. In this study, we present new fossil age, whole-rock geochemical and Sr–Nd isotope data on the newly discovered Dundunshan Group HMAs in the Dundunshan area of the Beishan orogen (central-southern Central Asian Orogenic Belt). The Dundunshan HMA samples are characterized by high MgO (6.47–7.02 wt%) contents and high Mg# values (67.27–68.77), with SiO2 (58.57–62.13 wt%), Al2O3 (14.49–16.07 wt%) and CaO (5.05–6.24 wt%) resembling typical HMAs. The Dundunshan HMA samples are calc-alkaline and strongly enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), with slightly negative Eu anomaly and high-field-strength element (HFSE) depletions. Their (87Sr/86Sr)i ratios (0.7041–0.7057) and ϵNd(t) (3.73–5.59) indicate that the Dundunshan HMAs were mainly formed by the interactions between subducted oceanic sediment-derived melts and mantle peridotites. Fossil evidence and published radiometric age data constrain the formation of the Dundunshan HMAs to early Late Devonian time. Sedimentological features of the Middle Devonian Sangejing Formation and regional tectonic correlation suggest that the Hongliuhe–Niujuanzi–Xichangjing Ocean in the Dundunshan area was likely closed during late Middle Devonian time, and that the Dundunshan HMAs were formed in a post-collision extensional setting.

Neodymium Isotope Geochemistry of a Subterranean Estuary

Rare earth elements (REE) and Nd isotope compositions of surface and groundwaters from the Indian River Lagoon in Florida were measured to investigate the influence of submarine groundwater discharge (SGD) on these parameters in coastal waters. The Nd flux of the terrestrial component of SGD is around 0.7±0.03 μmol Nd/day per m of shoreline across the nearshore seepage face of the subterranean estuary. This translates to a terrestrial SGD Nd flux of 4±0.2 mmol/day for the entire 5,880 m long shoreline of the studied portion of the lagoon. The Nd flux from bioirrigation across the nearshore seepage face is 1±0.05 μmol Nd/day per m of shoreline, or 6±0.3 mmol/day for the entire shoreline. The combination of these two SGD fluxes is the same as the local, effective river water flux of Nd to the lagoon of 12.7±5.3 mmol/day. Using a similar approach, the marine-sourced SGD flux of Nd is 31.4±1.6 μmol Nd/day per m of shoreline, or 184±9.2 mmol/day for the investigated portion of the lagoon, which is 45 times higher than the terrestrial SGD Nd flux. Terrestrial-sourced SGD has an εNd(0) value of −5±0.42, which is similar to carbonate rocks (i.e., Ocala Limestone) from the Upper Floridan Aquifer (−5.6), but more radiogenic than the recirculated marine SGD, for which εNd(0) is −7±0.24. Marine SGD has a Nd isotope composition that is identical to the εNd(0) of Fe(III) oxide/oxyhydroxide coated sands of the surficial aquifer (−7.15±0.24 and −6.98±0.36). These secondary Fe(III) oxides/oxyhydroxides formed during subaerial weathering when sea level was substantially lower during the last glacial maximum. Subsequent flooding of these surficial sands by rising sea level followed by reductive dissolution of the Fe(III) oxide/oxyhydroxide coatings can explain the Nd isotope composition of the marine SGD component. Surficial waters of the Indian River Lagoon have an εNd(0) of −6.47±0.32, and are a mixture of terrestrial and marine SGD components, as well as the local rivers (−8.63 and −8.14). Nonetheless, the chief Nd source is marine SGD that has reacted with Fe(III) oxide/oxyhydroxide coatings on the surficial aquifer sands of the subterranean estuary.

Determination of Sm/Nd and Sr isotopic composition using an ICP-MS Neptune Plus equipped with an NWR 213 attachment for laser ablation

Research subject. The Sm/Nd isotope system was investigated using inter-laboratory natural samples of apatite, titanite, allanite, monazite, as well as intra-laboratory samples of apatite (from carbonatites, Ilmenogorsk massif, Ural), monazite (from pegmatites of the Aduy granite massif and its framing, Middle Urals) and titanite (from calcite veins, Saranov skoye chromite deposit, Middle Urals and from alkaline pegmatite, Shpat mine, Vishnevy mountains, South Urals). The Sr isotope system was investigated using inter-laboratory natural apatite samples and intra-laboratory apatite samples (from the apatite-carbonate vein, Slyudyanogorskoe deposit, Irkutsk region and from carbonatites, Ilmenogorsk massif, Ural).Methods. The research was carried using a Neptune Plus multicollector mass spectrometer with inductively coupled plasma (ThermoFisher) equipped with an NWR 213 (ESI) laser ablation attachment, located in a room of ISO class 7 at the “Geoanalyst” Center for Collective Use (IGG Ural Branch of the Russian Academy of Sciences, Ekaterinburg). Results. The article describes methodological approaches for studying Sm/Nd and Sr isotope systems in natural phosphate and silicate minerals by inductively coupled plasma mass spectrometry with laser ablation, implemented on the equipment of the Center for Collective Use “Geoanalyst” (IGG Ural Branch of the Russian Academy of Sciences, Ekaterinburg). A comparative analysis of the obtained results with those reported in literature showed their satisfactory agreement. The developed analytical approaches were used to study apatite samples (analysis of the Sr isotope system) and those of apatite, monazite, titanite (analysis of the Sr isotope system). Conclusions. The developed approaches to the analysis of Sm/Nd and Sr isotopic systems can be recommended for investigating such minerals, as apatite, titanite, allanite, monazite (analysis of the Sm/Nd isotope system); apatite (analysis of the Sr isotope system). The achieved analysis errors allow the results to be used for interpreting various geochemical processes.