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• REE compositions distinguish between unaltered and altered BIF in Sirsi Shelf. • ε Nd (2.5Ga) of unaltered BIF overlap with contemporaneous global BIFs. • Negative Ce anomaly in the altered BIF represents post-depositional fluid overprint. • Alteration dated at ∼ 0.5 Ga, which coincides with the Pan-African orogeny. • Shales, affected by fluid alteration, derived from Goa Dharwar Sector granitoids. We report geochemical and Nd isotopic data for ca. 2.5 Ga old Banded Iron Formation (BIF) and associated clastic rocks which include red- and purple-coloured shales, mudstone, and sandstone from the Sirsi shelf of the Dharwar Craton. Based on lanthanide and non-lanthanide (yttrium) rare earth element (REE) concentration patterns, the Sirsi BIF samples are sub-divided into two groups. The group-1 BIF samples show low shale-normalized (SN) ratios of (La/Yb) SN (0.37–0.53) and (Pr/Yb) SN (0.29–0.34), and superchondritic Y/Ho (41–52); these samples also show insignificant negative Ce anomaly ((Ce/Ce*) SN = 0.83–1.22) compared to modern seawater and seawater-derived precipitates. In contrast, the group-2 BIF samples show strong light REE (LREE) enrichment with high (La/Yb) SN (1.73–18.48) and (Pr/Yb) SN (1.50–7.58), negative Ce anomaly ((Ce/Ce*) SN = 0.02–0.16), and mostly subchondritic Y/Ho (19–39). The group-1 BIF displays modern-day seawater-like REE patterns and their initial ε Nd at 2.5 Ga overlaps with contemporaneous global BIF, suggesting their precipitation from Archean seawater. The REE compositions of the group-2 Sirsi BIF cannot be explained by contribution of detritus and are best explained by alteration by a LREE-enriched and Ce-depleted fluid. The Sm–Nd isotopic compositions suggest that this alteration event took place approximately 0.5 Ga ago. The timing of the alteration event coincides with the Pan-African orogeny, which had regionally affected the Greater Dharwar Craton. Among the associated clastic rocks, red and purple shales show group-2-BIF-like REE signatures with negative Ce anomaly, and display high Chemical Index of Alteration (CIA) values (78–98), which suggest high degree of chemical alteration. The enrichment factors (calculated as [(X element /X Ti) sample /(X element /X Ti) reference -1] relative to the Archean Upper Continental Crust) of La, Pr, and Ce, in the Sirsi red shales further confirm post-depositional modifications by a LREE-enriched and Ce-depleted fluid, and indicate that negative Ce anomaly in ancient sedimentary rocks could be generated by post-depositional fluid overprint. Geochemical compositions of the Sirsi clastic rocks suggest a felsic provenance, while Nd isotopic compositions of selected shale samples indicate that these sediments were most likely derived from granitoids in the Goa-Dharwar Sector. [ABSTRACT FROM AUTHOR]

In this paper we present new data for the metamorphic Garm Block (150 × 40 km) that was traditionally shown on regional maps as fragment of a Precambrian basement. The Garm Block comprises migmatized gneisses and schists for some of which peak metamorphic conditions (600–660 °C at 7–9 kbar) characteristic for medium P/T series metamorphic belts were determined. U–Pb ages of overgrown rims of zircon grains and newly grown magmatic zircon grains from four migmatitic samples indicate that melting occurred during peak metamorphic conditions between 299 and 290 Ma. Thus the Garm Block is the first Early Permian regional metamorphic terrane established in the southern part of the Central Asian Orogenic Belt. A crosscutting granite intrusion yields a concordia age of 290 Ma identical to the age of metamorphism and similar to ages of post-collisional intrusions elsewhere in the Tien Shan. Detrital zircon grains from the Garm samples yield an age spectrum with major peaks at 700–550 Ma and 1150–700 Ma, and smaller ones at ca. 2.7–2.4 Ga and 3.5–3.3 Ga. The youngest grains define an Ediacarian maximum depositional age of the sedimentary protoliths. The presence of Grenvillian ages of 1150–800 Ma, which are characteristic for the Tarim Craton, and the lack of ca. 1.6–1.2 Ga ages, characteristic for the Northern Tien Shan, suggest an origin of the Precambrian zircon grains from southern continental blocks similar to the Tarim Craton. Six granitoid samples yield negative εNd values of −10.6 to −3.7 consistent with the reworking of crustal material with 1.8–1.1 Ga average crustal residence times and the formation of the South Tien Shan belt on a continental basement with Paleoproterozoic or older crust. The Garm Block formed as a consequence of lithospheric thickening during the Late Carboniferous collision of two continental blocks that created the South Tien Shan fold belt. Lateral crustal melt flow coupled with extension may have resulted in exhumation of the Permian metamorphic rocks. [ABSTRACT FROM AUTHOR]

The Mediterranean Outflow (MOW) is generated by deep and intermediate waters from different basins in the Mediterranean Sea. Despite the number of studies on Mediterranean water masses, little work has been done on the source and properties of intermediate waters in the westernmost Mediterranean Sea and their links with MOW. Here we examine three marine sediment records spanning the last 20 kyr, located at key depths to trace intermediate waters along the Alboran Sea. We use a combination of redox-sensitive elements, which can serve as proxies to reconstruct variations in the water column oxygenation and the Nd isotopic composition of foraminiferal ferromanganese coatings, in order to reconstruct water mass provenance of Eastern/Western Mediterranean waters. As measured, ε Nd < − 9.2 and a low U/Th ratio during glacial periods can be attributed to the presence of Western Mediterranean Deep Water (WMDW) at the study sites. During deglaciation, higher Nd isotopic compositions and U/Th ratios point to an enhanced contribution of the modified Levantine Intermediate Water (LIW). The comparison between our data and other LIW and MOW records suggests that i) the lower branch of MOW is linked to WMDW during the glacial period, ii) the middle MOW branch follows LIW activity during deglaciation, while iii) the upper branch is more active during late Holocene, coinciding with LIW formation increase after sapropel deposits. This reconstruction has significant implications for an understanding of the MOW evolution. [ABSTRACT FROM AUTHOR]

Pure marine chemical sediments, such as (Banded) Iron Formations, (B)IFs, are archives of geochemical proxies for the composition of Precambrian seawater and may provide information about the ancient hydrosphere-atmosphere system. We here present rare earths and yttrium (REY) and high precision Sm–Nd isotope data of ∼2.90 Ga old Superior-type BIFs from the Witwatersrand Supergroup, South Africa, and compare those with data for near-contemporaneous BIFs from the correlative Pongola Supergroup (Superior-type BIF) and from the Pietersburg Greenstone Belt (Algoma-type IF), respectively. All Witwatersrand samples studied display the typical general REY distribution of Archean seawater, but their REY anomalies are less pronounced and their immobile element concentrations are higher than those of other pure (B)IFs. These observations indicate the presence of significant amounts of detrital aluminosilicates in the Witwatersrand BIFs and question the reliability of the Contorted Bed and Water Tower BIFs (Parktown Formation, West Rand Group) as archives of Mesoarchean seawater. Significant post-depositional alteration of the REY budget and the Sm–Nd isotope system is not observed. The Nd isotopic compositions of the purest BIF samples, i.e. the most reliable archives for Witwatersrand seawater, show initial εNd values between −3.95 and −2.25. This range is more negative than what is observed in ambient shales, indicating a decoupling of suspended and dissolved loads in the “near-shore” Witwatersrand Basin seawater. However, εNd range overlaps with that of the correlative Pongola BIF (Alexander et al., 2008). The deeper-water Algoma-type Pietersburg BIF shows more positive (i.e. more mantle-like) εNd 2.9Ga values, supporting the hypothesis that a significant amount of its REY inventory was derived from black smoker-style, high-temperature hydrothermal fluids that had altered seafloor basalts. In marked contrast, the dissolved REY budgets (including the Nd isotopic compositions) of the Witwatersrand and Pongola seawater, however, were dominated by similar terrigenous REY sources from the Kaapvaal Craton. [ABSTRACT FROM AUTHOR]

Previous studies have suggested that northwest Cathaysia was a passive continental margin during Neoproterozoic time, given the absence of Neoproterozoic magmatism. However, this view has been challenged by recent discoveries of mafic and felsic magmatic rocks with geochemical affinities to continental arc or back-arc volcanic rocks from the 1.0–0.9 Ga Wuyi–Yunkai belt, indicating the development of an early Neoproterozoic continental margin arc and back-arc system in the Cathaysia block. In this paper we present new age data for the Miaohou and Shanhou arc complexes, demonstrating that the active continental margin of the Cathaysia block lasted until ca. 830 Ma. The Miaohou complex is composed of gabbro, diorite, and granite, the Shanhou complex of diorite and granite, and both complexes are bound by the Jiangshao Fault. Twelve samples from the Miaohou and Shanhou complexes give zircon U–Pb ages of ca. 830 Ma, indicating that the magmatism of the Miaohou and Shanhou complexes were coeval. The Miaohou gabbro has high zircon ɛ Hf ( t ) (+2.8 to +9.6) and whole-rock ɛ Nd ( t ) (+3.9) values together with the geochemical features of volcanic arc basalts, while the Miaohou and Shanhou diorites and granites show large variations and high values of zircon ɛ Hf ( t ) and ɛ Nd ( t ) [ ɛ Nd ( t ) (+1.1 to +5.5) and ɛ Hf ( t ) (−6.1 to +12.3) for the diorites, and ɛ Nd ( t ) (+0.3 to +3.1) and ɛ Hf ( t ) (−3.8 to +11.3) for the granites] together with a similar geochemistry to continental arc felsic rocks (e.g., Cordilleran batholiths). The geochemical and isotopic data suggest that the southeastwards subduction of an oceanic plate under the Cathaysia block caused the dehydration of sediments in the subducted slab, leading to the metasomatism of the mantle wedge by subduction-related fluids, partial melting of the mantle wedge, followed by varying degrees of assimilation and fractional crystallization (AFC) as the magmas ascended. These processes provide a reasonable explanation for the Neoproterozoic arc magmatism on the northwestern margin of the Cathaysia block, represented by the Miaohou and Shanhou complexes. [ABSTRACT FROM AUTHOR]

Dust deposition provides rock-derived nutrients such as phosphorus (P) to terrestrial ecosystems. Over pedogenic timescales, as bedrock sources of P are depleted, dust sources of P may support productivity in certain ecosystems, but controls on the spatial variability of dust in montane forested systems are largely unknown. Here, we use neodymium (Nd) isotope ratios in 31 ridgetop surface soils to evaluate the spatial variability of dust contributions to soil across ~ 100 km 2 in the Luquillo Mountains, Puerto Rico. Dust from the Sahara–Sahel region of Africa carries a distinct isotopic signature of − 12 ε Nd . Local bedrock, in contrast, has a ε Nd value of ~+ 7. End-member mixing calculations based on ε Nd reveal a wide range in dust influence on surface soils, with between 0% and 92% of the top 20 cm of ridgetop soil Nd derived from African dust. Using ε Nd paired with both dust and soil Nd content, the current soil dust content was calculated, ranging from 0 to 8%. There were no correlations between current dust content of soil and 10 Be-based denudation rate, elevation, rainfall, longitude, or forest type. Current soil dust content in the Luquillo Mountains is significantly higher in soils developed on volcaniclastic sandstone, breccia and mudstone than in soils developed on quartz diorite bedrock, which we attribute to greater retention capacity in the volcaniclastic soils. Current soil dust content also increases with increasing ridge-width, implying that small-scale topographic effects and other factors such as wind speed and turbulence influence local dust deposition rates. Higher current dust content of soil is also positively correlated with biologically cycled fractions of soil P on quartz diorite bedrock (r 2 = 0.24 and p = 0.002 for sum of extractable NaHCO 3 -P + NaOH-P), suggesting that atmospheric dust inputs contribute to the fertility of Luquillo Mountain ecosystems on the relatively P-poor quartz diorite bedrock. [ABSTRACT FROM AUTHOR]

High-spatial-resolution secondary-ion mass spectrometry (SIMS) U–Pb zircon ages, whole-rock Nd isotopic and geochemical data are reported for granites and granitic gneisses from a traverse across the Abas terrane, Yemen, a part of the Precambrian basement of southern Arabian Peninsula. SIMS U–Pb dating identifies two magmatic episodes, the first at c. 790–725 Ma represented by granitic gneisses, the second clearly post-tectonic at c. 625–590 Ma. The oldest sample in the post-tectonic group is slightly deformed while younger samples are undeformed indicating that penetrative deformation ceased at c. 625 Ma in the Abas region. Whole-rock Nd ( t ) values between −11 and +0.8, Nd model ages of 1.70–1.13 Ga indicate a significant contribution of evolved continental material in the genesis of the Abas granitoids, unlike most of the juvenile Arabian-Nubian Shield (ANS), although there are few inherited zircons. Secular variation in ɛ Nd ( t ) reflects a change in magma source with increasing juvenile magma and diminishing crustal input during post-tectonic (625–590 Ma) magmatism. The combination of subduction zone chemistry, absence of older rocks, paucity of inherited zircons, evolved Nd isotopic signatures and the I-type characteristics of the samples suggest that assimilation occurred at depth. [ABSTRACT FROM AUTHOR]

The origin and tectonic setting of the Mesoproterozoic (1.50–1.34 Ga) granite–rhyolite Provinces of the midcontinent region of the United States, and the coeval A-type granite plutons of the southwestern United States, have been an enigma for decades. In this paper we review the history of this problem, examine the significance of a wealth of previously published U–Pb zircon and whole-rock Sm–Nd data, and present new U–Pb zircon age and Lu–Hf isotopic data from selected zircon samples. Our data show that the initiation of this magmatism spans the interval 1.50–1.40 Ga across the continent, but there is a general younging westward. Younger, 1.39–1.34 Ga magmatism also occurred across the continent, but forms a major magmatic event only in the south-central mid-continent. There is generally a good correlation between Sm–Nd and Lu–Hf isotopic compositions and inferred mantle derivation model ages, except possibly in southeastern Oklahoma. The absence of older cores in zircons is compatible with calculated zircon saturation temperatures of ca. 850 °C, and supports mantle-driven basaltic underplating and resulting crustal partial melting as a petrogenetic model for the A-type magmatism. Our new data compilation refines the existence of the “Nd-line” that extends from southwestern Texas into southeastern Michigan and the western Grenville Province in Canada; it marks an isotopic boundary between granites and rhyolites of cratonal origin to the northwest (1.8–1.6 Ga model ages) and those of more juvenile origin to the southeast (1.5–1.4 Ga model ages). Our suggested tectonic model for the Mesoproterozoic A-type magmatism across eastern and southern Laurentia involves the development of an active Laurentian margin extending from eastern Canada, across what is now the midcontinent region of the United States, and at least into the current southwestern United States. Magmatism along this margin was likely related to asthenosphere–lithosphere interactions associated with the accretion of outboard arcs and possible exotic terranes that caused complex deformational and magmatic responses in the continental interior. This activity evidently spread from northeast to southwest over a period of ca. 70 million years. The occurrence of the younger, ca. 1.39–1.34 Ga event indicates that similar processes occurred some 130 million years later, suggesting a long-lived convergent plate margin and recurring basaltic underplating in some locations, perhaps due to inboard lithospheric delamination and/or mantle upwelling. [ABSTRACT FROM AUTHOR]

Mafic metavolcanic rocks and grey gneisses are the dominant components of many Archean terranes and preserve significant imprints and information of the crustal evolution. Here we present in situ zircon U–Pb geochronology, whole-rock geochemistry and Nd isotopes of the amphibolites and associated grey gneisses in the drill holes from the Jiaobei terrane, North China Craton. The LA-ICP-MS zircon U–Pb data show that the magmatic protoliths of two amphibolites and two grey gneisses crystallized at 2457–2503 Ma followed by the metamorphism at ∼1.8–1.9 Ga. These rocks exhibit heterogeneous whole-rock Nd isotopic compositions, with the most radiogenic analyses (ɛ Nd ( t ) = +3.8 to +6.9) plotting on or close to the depleted mantle evolution array and the most unradiogenic ɛ Nd ( t ) extending down to −2.4. The data suggest that both ∼2.5 Ga depleted mantle and ancient continental crust at least as old as 3.05 Ga contributed to the magma source. In combination with previous zircon Hf isotopic data, it is suggested that the ∼2.5–2.6 Ga magmatic events in the entire Jiaobei terrane were mostly derived from the reworking of the pre-existing ∼2.7–2.8 Ga crustal material. However, large variations in Hf–Nd isotopic compositions of abundant rocks at ∼2.5 Ga and ∼2.7–2.8 Ga in the Jiaobei terrane were likely to have resulted from interaction of mantle-derived juvenile magma with ancient crustal materials in the source region. Thus, the peaks of zircon Hf crustal model ages from these rocks do not represent the actual time of crustal growth, but could be artifacts of interaction of magmas from different sources. Geochemical features show that the protoliths of the amphibolites are tholeiitic basalts and can be subdivided into two groups: high-Mg tholeiite basalts (Group I) and high-Fe tholeiite basalts (Group II). Trace element systematics indicate that the Group I amphibolites show good affinity to subduction-related tholeiitic arc basalts, whereas the Group II amphibolites represent back-arc lavas modified by subduction components. The two groups of amphibolites and associated grey gneisses collectively define an arc-back-arc tectonic setting in the study area at ∼2.5 Ga. Integrated with the previous data from other areas in Jiaobei, it can be inferred that Archean-Proterozoic transition in the whole Jiaobei terrane witnessed subduction-related magmatism in a convergent margin. [ABSTRACT FROM AUTHOR]

High-precision high field strength element (HFSE: Zr, Hf, Nb, Ta, Th, U, W, Mo), Y/Ho, and Nd isotope chemostratigraphy of two contrasting Deccan Traps weathering profiles – an ancient, deeply weathered laterite, and a younger (Quaternary), more moderately weathered saprolite – are used to reconstruct different aspects of basalt weathering. Precision of the HFSE analyses is demonstrated through a report of the long-term concentrations and ratios determined in United States Geological Survey (USGS) and Geological Survey of Japan (GSJ) basalt rock standards (BHVO-1, BHVO-2, BIR-1, BCR-2, JB-2). The oxyanion-forming members (U, Mo, W) are the most mobile of the considered HFSE group. Extreme loss of W, far exceeding those of U and Mo during certain stages of basalt alteration, is first reported here. The ability to strongly fractionate Mo and W during weathering may contribute to solving the unresolved mass imbalance between the crustal and marine inventories of W. By contrast, Zr, Hf, Nb, and Ta preserve the ratios of the parent basalt in the profiles due to their limited mobility; these are therefore of great potential value in reconstructing basalt flow stratigraphy and correlating lava flows in weathered flood basalt provinces. Of the HFSE, Th is not a good choice as a conservative element because it is strongly susceptible to addition of aeolian dust; this is evidenced by strong excursions in Th/Nb that are correlatable with alkali element enrichment and less radiogenic 143 Nd/ 144 Nd ratios. The chemical fingerprints of dust were identified in a paleo-flow top of the saprolite profile, suggesting that dust accumulation occurred during periods of quiescence between basaltic eruptions. During protracted exposure and laterite development, the magnitude to which dust overprints the basalt chemistry increases substantially as evident from much less radiogenic Nd isotope ratios and higher Th/Nb ratios in the Bidar profile relative to the protolith basalt. Attempts at quantifying the magnitude of dust accumulation in the laterite based on Th enrichment indicate a mass fraction of greater than 0.5 when the dust is assumed to have the chemistry of average upper continental crust. Although mixing models between the basalt and assumed dust composition cannot unambiguously constrain the dust source, the Nd isotope mixture preserved in the laterite points to a relatively young crustal dust source (e.g., similar to loess in composition) rather than the Precambrian shield rocks in the vicinity of the Deccan Traps. The contrasting topologies of dust-derived Nd and dust-derived Th in the laterite appears to record both physical transport of dust (Th) through permeable laterite horizons as well as transport by chemical dissolution and precipitation (Nd) at an inferred paleo-water table and in deep saprolite zones. Yttrium and Ho fractionate substantially during all observed stages of weathering, with Y/Ho ratios ranging from 26.5 to 21.9 in the moderately weathered saprolite profile and from 30.2 to 14.7 in the laterite profile. The single strongly superchondritic Y/Ho ratio of 30.2 in the laterite is restricted to a sample at depth, and appears to fingerprint the deposition of REE derived from dissolution higher in the profile. Decrease in the Y/Ho ratio relative to the protolith basalt (24.4–24.7) in both profiles inversely correlates with chemical weathering indices, and suggests that Y/Ho ratios have significant potential as a silicate weathering proxy. Consequently, suspended vs. dissolved river loads may record the differing behaviour of these elements during weathering. [ABSTRACT FROM AUTHOR]

In the Forquilha area (NE Brazil), in NW Borborema Province, high to ultra-high pressure rocks are an important geological key to understanding West Gondwana amalgamation. U–Pb geochronological data for a retrograded eclogite sample yielded an upper intercept age of ca. 1520 Ma and a lower intercept age of ca. 620 Ma. These ages most likely represent the crystallization age of the basaltic protolith and the regional metamorphism, respectively. The retrograded eclogites are enclosed in migmatized quartz-feldspathic gneiss and sillimanite (after kyanite)-garnet-biotite gneiss. Detrital U–Pb zircon data for these paragneisses show only Paleoproterozoic zircon grains with ages clustering from ca. 1800 Ma (the maximum depositional age) to ca. 2480 Ma, and frequency peaks at 2.2–2.0 Ga. Combined with Nd isotopic data from the Forquilha paragneisses, one can assume a single Paleoproterozoic source. Basement rocks of the Ceará Central and the Rio Grande do Norte domains are the most likely candidates. The absence of Meso- and Neoproterozoic zircon grains suggest that the retrograded eclogite bodies possibly do not represent slivers of oceanic rocks captured in active margin sequences during subduction. It was identified that the high-pressure rocks of the Forquilha area are in tectonic contact with high-pressure granulite facies rocks of the Ceará Complex (Independência unit) that present detrital zircon records of an active margin setting, with ages ranging from ca. 660 Ma to 2200 Ma. Metamorphism of this sequence occurred at ca. 650 Ma. Considering previous studies, field relationships, and metamorphic paragenesis, a tectonic scenario is inferred, in which the Forquilha retrograded eclogites represent Mesoproterozoic basaltic rocks of an extensional event that were metamorphosed under eclogite facies conditions during Late Neoproterozoic continental subduction/collision, and juxtaposed to an active margin sequence during the exhumation process. [ABSTRACT FROM AUTHOR]

Several Brasiliano-Pan-African belts consist of large areas of reworked Paleoproterozoic rocks. Characterization of these rocks is needed to place better controls on Precambrian paleogeographic reconstructions. The Borborema Province, northeastern Brazil, occupies a central position in West Gondwana configuration, and knowledge of its geological evolution is crucial to infer relationships between Paleoproterozoic units in South America and Africa. Here, we report U–Pb ages, major- and trace-elements analyses and Sm–Nd isotopic data for orthogneisses in the eastern portion of Central Domain. The dominant basement units in the study area are banded gneisses of intermediate composition and relatively juvenile character, and migmatitic gneisses of granitic composition with Archean Nd T DM model ages. One sample of the banded gneiss yielded a weighted 207 Pb/ 206 Pb age of 2096 ± 23 Ma and an upper intercept age of 2044 ± 27 Ma, which we interpret, respectively, as ages of crystallization and metamorphism. Two large units of migmatitic gneiss in the southern and central parts of the area gave ages of, respectively, 2057 ± 20 Ma and 2055 ± 23 Ma; an orthoamphibolite associated with the latter yielded crystallization age of 2042 ± 11 Ma and metamorphic age of 1996 ± 13 Ma. All these rocks have geochemical signatures typical of subduction zone-related magmas. Combined with evidence provided by previous studies, we suggest that the evolution of the study area starts with island arc construction around 2.2 Ga, leading to an expressive volcanic arc edifice by 2.13–2.10 Ga. By 2.06 Ga, the crust had evolved enough to become intruded by magmas formed at the mantle wedge of the now largely continental magmatic arc, which continued to be intruded by mantle melts until at least 2.04 Ga. An augen gneiss in the northern part of the area, with an age of 2109 ± 15 Ma, and a migmatitic gneiss with a much older age (2183 ± 9 Ma), both of which have geochemical characteristics akin to intraplate magmas, suggest that crustal reworking also took place. The 1981 ± 23 Ma age of a small body of peraluminous felsic gneiss may be related to regional metamorphism and syncollisional magmatism at c. 2.0–1.98 Ga. [ABSTRACT FROM AUTHOR]

The radiogenic neodymium (Nd) isotope ratio 143 Nd/ 144 Nd (expressed in ε Nd ) is being used as a tracer in paleo and modern ocean circulation. However, the mechanisms controlling input, distribution, and internal cycling are far from understood. For example, globally, Nd concentration ([Nd]) commonly follows patterns of nutrient tracers, generally increasing with depth below the thermocline, while ε Nd , tends to reflect the water masses, which has often been referred to as the ‘Nd-paradox’. Here we present dissolved Nd isotopes and concentrations at unprecedented vertical and spatial resolution from the eastern part of the US GEOTRACES North Atlantic Zonal Transect (Gulf of Cadiz – Mauritanian Shelf – Cape Verde Islands). The [Nd] of all samples ranges from 12.3 to 36.7 pmol/kg, with lowest [Nd] usually found within the layer of highest chlorophyll-a levels (chl-max), suggesting removal through scavenging. The Nd isotope compositions range between ε Nd = − 13.4 and −9.9, with lower values at the surface within the extension of the Saharan dust plume and a benthic nepheloid layer (BNL). Less negative values are found in oligotrophic surface waters, Mediterranean Outflow Water (MOW), and near the Cape Verde Islands. Overall, water mass mixing derived from ε Nd is best visible at the Strait of Gibraltar, where MOW enters the Atlantic Ocean. Most of the sub-thermocline ε Nd varies within a small range with poor water mass distinction due to the dominance of North Atlantic Deep Water. High surface [Nd] associated with more negative ε Nd is interpreted to be the result of dust deposition and dissolution. Local [Nd] maxima with no apparent change in ε Nd compared to ambient seawater, observed within a zone of minimum oxygen concentration (OMZ) at ∼500 m depth off Mauritania, suggest minor input of lithogenic Nd but a rather high contribution through desorption of previously scavenged Nd. That is, Saharan dust in this area has only a minor influence on the isotope composition of water below the uppermost surface layer. The low ε Nd values and elevated [Nd] within the BNL on the other hand indicate release from detrital material. Our new high-resolution dataset gives valuable insights into the Nd distribution near continental margins, influenced by high atmospheric inputs and changing oxygen conditions. We show how vertical and lateral processes overlap to produce the observed Nd distribution, bringing us forward in understanding the Nd-paradox. [ABSTRACT FROM AUTHOR]

SHRIMP U–Pb zircon data obtained from a magnetite gneiss deposit in the southeast Gawler Craton indicate that the protoliths were deposited between 1750 and 1735 Ma. Previously, these iron-rich gneisses were thought to have been a metamorphosed banded iron formation and part of the Neoarchean to early Paleoproterozoic Sleaford Complex. Detrital zircon data provide maximum depositional ages between 1765 Ma and 1740 Ma, with ϵNd(1750)values between –4.6 and –3.3. Metamorphic zircon ages are 1735–1725 Ma, indicating that the magnetite gneiss was formed during the 1730–1690 Ma Kimban Orogeny. The pelitic mineralogy of the magnetite gneiss and presence of detrital zircon demonstrate that the protoliths were not a pure chemical sediment. Correlation with the greenschist facies Price Metasediments in the southern Gawler Craton suggests a series of iron-bearing basins were developed prior to the onset of the Kimban Orogeny and suggest that metamorphism played a role in ore formation. [ABSTRACT FROM AUTHOR]

Altogether 1130 U–Pb analyses on zircons from 25 samples across the Pajala shear zone in northern Fennoscandia were performed by LA-ICPMS methodology to study the origin of the zone. The samples were also analyzed for their Sm–Nd isotope systematics and geochemically. The new data is tested with existing geological information to elaborate the provenance and maximum depositional ages, the stratigraphic position of studied samples, and finally to assess the tectonic evolution in the Pajala shear zone area. Chromium-enriched metasedimentary rocks are found within and to the east and west of the N-S trending Pajala shear zone. No material originating from Proterozoic felsic igneous (zircons) or mafic (Sm–Nd) rocks was found in these rocks and thus, only Neoarchean or very early Paleoproterozoic ages of deposition are possible. Our new results indicate that the bimodal 1.99–1.97 Ga supracrustal rocks of the Rovaniemi Supersuite cover a large area east of the Pajala shear zone. Based on the absence of Archean zircons typical of the metasedimentary rocks covering the Archean basement, it seems that the basement and its sedimentary cover were not exposed at the time of deposition of the Rovaniemi Supersuite. The metasedimentary rocks of the Uusivirka Supersuite, located within the Pajala shear zone, vary from hornblende-bearing metasedimentary rocks, metapelite, and meta-arkose to orthoquartzite. Their characteristic features include a dominant Proterozoic zircon population (65–87%), with a high proportion of analyzed grains yielding ages between 1.96 Ga and 1.91 Ga, and maximum depositional ages of 1.92–1.91 Ga. We record at least three metamorphic events; at ca. 1.92–1.90 Ga, ca. 1.85 Ga and at 1.82–1.78 Ga, of which the youngest can possibly be separated into two events at 1.83–1.82 Ga and 1.79–1.77 Ga, respectively. We propose a tectonic model in which continental breakup occurred at 2.1–2.05 Ga, followed by the development of a magmatic arc (Kittilä arc) at ca. 2.0 Ga above a subduction zone. Two Archean continental units (Norrbotten and Karelia) collided at ca. 1.92–1.91 Ga, the Kittilä arc was obducted upon the Karelia continent as a foreland fold and thrust belt, and the metasedimentary rocks of the Uusivirka Supersuite were emplaced as foreland deposits. The proposed tectonic model is based on the absence of 2.44 Ga layered intrusions, and especially 2.06 Ga rocks and 1.99–1.97 Ga rocks of the Rovaniemi Supersuite that are found only east of the Pajala shear zone. In our model the Pajala shear zone originated as a divergent plate boundary, and was multiply reactivated after continental collision with both lateral and vertical movements. [ABSTRACT FROM AUTHOR]

Quantitative calculation of the supply ratio of nutrients by different water masses suggests that the intrusion of Kuroshio subsurface water (KSSW) may indeed be an important factor in the formation of harmful algal blooms (HABs) in the Min-Zhe coastal area. This region in China has a complex hydrographic structure as well as a high incidence of HABs. The results of the calculations of the dissolved neodymium and its isotopes show that KSSW is an important provider of dissolved inorganic nitrogen (30–90 %) and phosphates (80–100 %) compared to the cases of the Taiwan Warm Current and the Changjiang Diluted Water. Sensitivity analysis results indicate that the percent contribution of KSSW in volume increased to 50–60 % in June, supplying favorable conditions for the growth of harmful algae. Our study implies that the invasion of the Kuroshio is likely to be a driving factor of HABs and, accordingly, a key factor in predicting HABs. [Display omitted] • An isotope model was applied to evaluate the supply ratio of nutrients by water masses. • KSSW is an important provider of dissolved inorganic nitrogen and phosphates. • Contribution of nutrients from the TWC can be neglected although it is a key water mass in spring. • Kuroshio is likely to be a driving factor for HABs in the East China Sea. [ABSTRACT FROM AUTHOR]

Oceanic Anoxic Event (OAE) 2 coincided with the Cenomanian–Turonian boundary ~ 93.9 Ma, and was one of the two most prominent and globally significant of the major mid-Cretaceous OAEs. A confluence of global warming, major large igneous province volcanism, and intensified hydrologic cycling preconditioned the earth system for widespread preservation of organic matter during OAE2; however the ultimate necessary ingredient was enhanced nutrient availability in oceanic surface waters. Here we present new Cenomanian–Turonian interval seawater Neodymium isotope data from the proto-Indian Ocean that demonstrates increased water column stratification during the pre- and post-OAE2 interval, punctuated by a transient decrease in this stratification immediately prior to the onset of OAE2 recorded at Ocean Drilling Program Site 763. The direct oceanographic responses to climate change on both longer- and short-term time scales observed in this region are distinct from those recorded in the northern, tropical and southern Atlantic basins. The transient de-stratification of the water column in the eastern proto-Indian Ocean likely did not contribute to the accumulation of organic-rich sediments in the region, indicating that weathering and/or hydrothermal inputs promoted enhanced surface water nutrient levels and hence primary productivity. [ABSTRACT FROM AUTHOR]

The Outer Bytownite Gabbros of the Cuillin Centre on the Isle of Skye include a 175 m rhythmically layered sequence consisting of alternating layers of coarse-grained massive gabbro and finer-grained laminated gabbro. We infer that the massive layers formed from repeated influxes of plagioclase-phyric magmas, which flowed into an existing magma chamber and deposited their entrained phenocrysts on the chamber floor. The phenocrysts accumulated rapidly, trapping large amounts of pore liquid in the cumulus pile. During the intervals between influxes, laminated gabbro layers formed by crystallization of the resident magma; these layers accumulated more slowly, by crystal settling or in situ growth, allowing sufficient time for their pore liquids to be expelled by compaction or adcumulus crystallization. The resulting fractions of trapped pore liquid were <10% in laminated gabbros and 20–40% in massive gabbros. High porosity in the massive layers is attributable to rapid accumulation of a low-density cumulus assemblage, with trapping of pore liquids facilitated by the confining effects of impermeable laminated cumulates above and below. Among layers, whole-rock Sr and Nd isotope ratios reveal variable degrees of contamination by the Archean crust beneath Skye. In the lower part of the section, influxes of plagioclase-phyric magma are associated with a shift to more primitive mineral compositions, lower 87Sr/86Sr, and higher 143Nd/144Nd, indicating that the replenishing magmas were initially more primitive and less contaminated than those already residing in the chamber. In accordance with this interpretation, massive layers have lower 87Sr/86Sr than laminated layers, consistent with derivation of the plagioclase phenocrysts from a less contaminated source. In the upper part of the section, the shift toward more primitive and less contaminated compositions is reversed, suggesting that over time the replenishing magmas underwent increasing degrees of fractionation and contamination within the magmatic plumbing system that fed the intrusion. These observations and inferences, coupled with published evidence for other isotopic reversals, support a model for growth of the Cuillin Centre by frequent replenishment of a small-volume magma chamber with pulses of magma that underwent complex histories of fractionation and contamination during their transit through the Archean crust; periods dominated by assimilation and fractional crystallization within the feeder system alternated with periods of vigorous recharge by relatively primitive and less contaminated magmas. These cycles were superimposed on an overall trend toward less contamination through time, consistent with increasing dominance of the system by uncontaminated tholeiitic magmas as the Hebridean magmatic province matured. [ABSTRACT FROM AUTHOR]

In this study, the use of trace elements as a provenance indicator for Roman natron glass is evaluated. Suitable glass‐making sand raw materials are analysed for their trace elemental composition and compared to glass from known production centres. It is shown that the combined use of Nd isotopic and trace element analysis can be efficient for the provenancing of Roman glass. Trace elements associated with (de)colourants of glass are only present in small concentrations in glass‐making sands. Background levels introduced to the glass by the sand raw material are defined and a lower threshold for the concentrations influenced by recycling is determined. [ABSTRACT FROM AUTHOR]

Highlights: [•] Allochthonous klippen in the SW Grenville Province are mapped using Nd model ages. [•] These klippen are remnants of a sub-horizontal thrust sheet exhumed at a crustal ramp. [•] New cross sections of the SW Grenville show widespread ‘ramp-flat’ geometry. [Copyright &y& Elsevier]

Precise and accurate measurements of Nd concentrations and isotopic compositions of small sample masses (≤30ng Nd) are increasingly necessary in the geosciences. Here, we present a combined chemical separation and TIMS (thermal ionisation mass spectrometry) NdO+ method adapted for fossil deep-sea corals, which represents a significant improvement in precision compared to existing methods, with additional assessment of potential contamination of coral aragonite by external encrustations. After digesting and spiking samples, we purified sample Nd using RE and Ln-spec separation chemistries, and tested the efficiency of these on aliquots of USGS BCR-2. In addition, we prepared a matrix-matched in-house coral reference material to monitor the analytical precision on coral carbonate. Using a micro-loading technique and TaF5 activator on single W filaments, ion beam sizes of up to 0.60V per ng on mass 160 are achievable, but decline with increasing age of the activator. The long-term average for NdO+ measurements of 5ng and 15ng loads of the pure metal oxide JNdi-1 reference material (Geological Survey of Japan) was 0.512106±6 for 143Nd/144Nd (12ppm 2RSD, n=44), with an overall deviation from the reference value of 17ppm (0.512115±7; Tanaka et al., 2000). BCR-2 aliquots of 10ng and 30ng generated an average 143Nd/144Nd of 0.512643±8 (16ppm 2RSD, n=14), which is identical within uncertainty to the reference value (0.512637±12; Weis et al., 2006). Very similar external reproducibility was obtained on 10ng and 30ng aliquots of the in-house coral standard, with an average 143Nd/144Nd of 0.512338±8 (16ppm 2RSD, n=13). Precision and accuracy of Nd oxide measurements depend on well-characterised oxygen isotopic compositions over the range of measurement temperatures, and very small amounts of Ce and Pr in final chemistry cuts. Whilst the former is laboratory-specific, and can be determined precisely using a column-purified 150Nd spike (e.g. 17O/16O=0.000390±3 and 18O/16O=0.002073±8 for this study; n=12), our method shows that interference corrections for Ce and Pr are tolerable up to higher values than previously reported (e.g. 156/160 ratios≤0.148 and 157/160 ratios≤0.68). In reality, however, our separation was always better than this. An important issue with fossil corals concerns their apparently higher Nd concentrations compared to modern counterparts (~7–310ppb vs. ~3–51ppb respectively). Variable to high Nd concentrations could arise for a number of reasons, but here we focus solely on evaluating the effectiveness of coral cleaning to remove external, Nd-rich encrustations. To test this, we measured major and trace metal concentrations (Ca, Fe, Mn, Ti, Al, Ce, Nd, Th, U) in paired aragonite and FeMn coatings from the same samples. Mass balance calculations reveal negligible influence of FeMn coatings on the final εNd, although four samples showed some degree of contamination as demonstrated by elevated Fe, Mn, Ti and Th concentrations. [ABSTRACT FROM AUTHOR]

Abstract: Antarctic Intermediate Water (AAIW) is a key player in the global ocean circulation, contributing to the upper limb of the Atlantic Meridional Overturning Circulation (AMOC), and influencing interhemispheric heat exchange and the distribution of salinity, nutrients and carbon. However, the deglacial history of AAIW flow into the North Atlantic is controversial. Here we present a multicore-top neodymium isotope calibration, which confirms the ability of unclean foraminifera to faithfully record bottom water neodymium isotopic composition ( ) values in their authigenic coatings. We then present the first foraminifera-based reconstruction of from three sediment cores retrieved from within modern AAIW, in the western tropical North Atlantic. Our records reveal similar glacial and interglacial contributions of AAIW, and a pronounced decrease in the AAIW fraction during North Atlantic deglacial cold episodes, Heinrich Stadial 1 (HS1) and Younger Dryas (YD). Our results suggest two separate phases of reduced fraction of AAIW in the tropical Atlantic during HS1, with a greater reduction during early HS1. If a reduction in AAIW fraction also reflects reduced AMOC strength, this finding may explain why, in many regions, there are two phases of hydrologic change within HS1, and why atmospheric CO2 rose more rapidly during early than late HS1. Our result suggesting less flow of AAIW into the Atlantic during North Atlantic cold events contrasts with evidence from the Pacific, where intermediate-depth records may indicate increased flow of AAIW into the Pacific during the these same events. Antiphased behavior between intermediate depths of the North Atlantic and Pacific implies that the flow of AAIW into Atlantic and Pacific seesawed during the last deglaciation. [Copyright &y& Elsevier]

Kaersutite to pargasite phenocrysts from Tertiary alkali basalts (Rhön, Central European Province, Germany) yield new high-precision Ar/Ar ages of 24.0-24.1 Ma. Major and trace element compositions demonstrate that these high-TiO (4-7 wt%) amphiboles are in equilibrium with their host rock. Chemically, these amphibole phenocrysts resemble amphibole from magmatic veins in upper mantle rocks but differ from disseminated amphibole from peridotite. Most amphiboles have similar isotope characteristics to their alkaline basaltic host rocks (Sr/Sr = 0.7035-0.736, εNd = +3.8-+4.0, Pb/Pb = 19.21-19.37, Pb/Pb = 15.58-15.62, Pb/Pb = 38.95-39.16), but two samples show contrasting isotopic compositions (εNd = −4.0 and −2.9; Pb/Pb = 17.08 and 18.11; Pb/Pb = 15.51 and 15.58; Pb/Pb = 37.41 and 37.99), indicating involvement of an ancient crust-derived component during melting. The O isotopic composition of the amphibole phenocrysts ranges from 5.4 to 7.5 ‰, reflecting O isotope heterogeneity of the upper mantle sources. The contrasting isotopic composition of amphibole and host rock pairs furthermore indicates that phenocrysts record the early stages of the volcanic history of the Rhön volcanic field on a regional scale and at a different depth within the lithospheric mantle. Temperature and pressure estimates range from 1,010 and 1,080 °C and 0.7 and 1.0 GPa and are compatible with the experimental results on the stability of amphibole in alkaline rocks derived from the upper mantle. Geochemistry of major and trace elements and isotopic compositions shows that igneous amphibole from alkali basalts may preserve isotope heterogeneities suggesting that they sample heterogeneous upper mantle lithologies on a small scale. [ABSTRACT FROM AUTHOR]

There is a broad correlation between the ε values for rivers (including both the water and the particulate material it carries) and the age of the source terrain. This paper presents Nd isotope distribution data for soil, soil water, groundwater, and stream water samples gathered in a small catchment in northern Sweden. The results show that the release of Nd and Sm from boreal forests into streams and, eventually, into the oceans is more complicated than previously realized. The weathering of till causes changes in both the Nd isotopic composition and Sm/Nd ratios. Both the Sm/Nd ratio and ε were higher in strongly weathered soils horizons than in less weathered till, since minerals with high Sm/Nd ratios were, on average, more resistant to weathering than those with low Sm/Nd ratios. In contrast to the situation for the main minerals and the major elements, the weathering of rare earth elements (REE) was not restricted to the E-horizon: the measured REE concentrations continued to increase with depth in the C-horizon. In addition, REE released by weathering in the upper parts of the soil profile were partly secondarily retained at deeper levels. Therefore, the dissolved Nd released by weathering in the upper soil horizons was trapped and did not enter the groundwater directly. Rather, the Nd in the groundwater largely originated from weathering within the groundwater zone. However, this was not the only source of Nd in the stream water. The Nd isotope composition and Sm/Nd ratio were determined by the mixing between of Nd and Sm in the groundwater and REE-carrying organic material washed out of the soil profile. The groundwater close to the stream reaches the upper soil horizons during high discharge events such as snowmelts, and organic matter carrying Nd and Sm is washed out of the soils and thus released into the stream. Therefore, the Nd exported from catchment is derived from both the weathering within the groundwater zone, and the organic matter washed out from the soil. If longer timescales with more advanced weathering stages in the groundwater zone are considered, it cannot be ruled out that there will be a shift towards more radiogenic values in the exported Nd. Recorded shifts in the Nd isotopic composition in the ocean may thus not only reflect changed source regions, but also the weathering history of the same source region. [ABSTRACT FROM AUTHOR]

Abstract: Neodymium (Nd) isotopes, tracers of deep water mass source and mixing, were measured on sedimentary planktic foraminifera with authigenic coatings from a depth-transect of cores (1400–4800 m) from Chatham Rise in the southwest Pacific, over the past 30 ka. We observe deglacial variations in the Nd isotopic composition, which showed an average glacial composition of (1σ; ) for cores sites below 3200 mbsl. No significant deglacial variation was observed in the Nd isotopic composition of intermediate depth waters (1400 mbsl), in contrast with benthic foraminifera data. The deglacial shift of CDW in the southwest Pacific is consistent with changes observed in the deep South Atlantic and Equatorial Indian Ocean, but values are offset by -unit to more radiogenic values throughout the deglacial records, likely due to admixture of a Nd isotope signal which was modified in the Southern Ocean or Pacific, perhaps by boundary exchange. However, this modification did not overprint the deglacial Nd isotope change. The consistent deglacial evolution of in the South Atlantic, Equatorial Indian and southwest Pacific CDW, is evidence for the connection of CDW during the glacial, and propagation of diminished North Atlantic Deep Water export to the glacial Southern Ocean. In contrast, spatial heterogeneities in the benthic foraminifera of CDW have been observed in the Atlantic, Indian and Pacific basins of the deep glacial Southern Ocean. The Nd isotope data implies a well-connected deep Southern Ocean, which transported waters from the Atlantic to the Indian and Pacific oceans, during the glacial. This suggests that basin-scale variability in the glacial composition of CDW was unrelated to circulation changes. [Copyright &y& Elsevier]

Abstract: The combined use of Hf, Nd and Sr isotopes is more useful in understanding the supercontinent cycle than the use of only Hf isotopic data from detrital zircons. Sr and Nd seawater isotopes, although not as precise as ɛ Nd and ɛ Hf distributions, also record input from ocean ridge systems. Unlike detrital zircons where sources cannot be precisely located because of crustal recycling, both the location and tectonic setting often can be constrained for whole-rock Nd isotopic data. Furthermore, primary zircon sources may not reside on the same continent as derivative detrital zircons due to supercontinent breakup and assembly. Common to all of the isotopic studies are geographic sampling biases reflecting outcrop distributions, river system sampling, or geologists, and these may be responsible for most of the decorrelation observed between isotopic systems. Distributions between 3.5 and 2 Ga based on ɛ Hf median values of four detrital zircon databases as well as our compiled ɛ Nd database are noisy but uniformly distributed in time, whereas data between 2 and 1 Ga data are more tightly clustered with smaller variations. Grouped age peaks suggest that both isotopic systems are sampling similar types of orogens. Only after 1 Ga and before 3.5 Ga do we see wide variations and significant disagreement between databases, which may partially reflect variations in both the number of sample locations and the number of samples per location. External and internal orogens show similar patterns in ɛ Nd and ɛ Hf with age suggesting that both juvenile and reworked crustal components are produced in both types of orogens with similar proportions. However, both types of orogens clearly produce more juvenile isotopic signatures in retreating mode than in advancing mode. Many secular changes in ɛ Hf and ɛ Nd distributions correlate with the supercontinent cycle. Although supercontinent breakup is correlated with short-lived decreasing ɛ Hf and ɛ Nd (≤100 Myr) for most supercontinents, there is no isotopic evidence for the breakup of the Paleoproterozoic supercontinent Nuna. Assembly of supercontinents by extroversion is recorded by decreasing ɛ Nd in granitoids and metasediments and decreasing ɛ Hf in zircons, attesting to the role of crustal reworking in external orogens in advancing mode. As expected, seawater Sr isotopes increase and seawater Nd isotopes decrease during supercontinent assembly by extroversion. Pangea is the only supercontinent that has a clear isotopic record of introversion assembly, during which median ɛ Nd and ɛ Hf rise rapidly for ≤100 Myr. Although expected to increase, radiogenic seawater Sr decreases (and seawater Nd increases) during assembly of Pangea, a feature that may be caused by juvenile input into the oceans from new ocean ridges and external orogens in retreating mode. The fact that a probable onset of plate tectonics around 3 Ga is not recorded in isotopic distributions may be due the existence of widespread felsic crust formed prior to the onset of plate tectonics in a stagnant lid tectonic regime, as supported by Nd and Hf model ages. [Copyright &y& Elsevier]

Over 100 new Nd isotope analyses for the central Grenville Province in the Parent-Clova region of Quebec help fill a major gap in understanding the crustal accretion history of the province. Nd model ages show that the Parent-Clova region consists of three crustal blocks: the Archean parautochthon in the north; a central block with mixed ages interpreted as an ensialic arc; and a southerly block forming an extension of the Mesoproterozoic Quebecia arc terrane. The Allochthon Boundary Thrust is believed to define the edge of the Archean parautochthon, which is bordered for a distance of 300 km by the ensialic arc block, within which model ages decrease consistently away from the craton. A similar negative correlation between Nd model age and distance from the craton is seen in published data for the Algonquin terrane in Ontario, but with a lower range of model ages. These comparisons show that in the Parent-Clova region, a Mesoproterozoic ensialic arc was established directly on the Archean margin, but further west, the Mesoproterozoic arc was built on a younger margin consisting of accreted Palaeoproterozoic arc crust. The use of large Nd data sets allows these distinct regional growth patterns to become clear and, hence, allows an understanding of Mesoproterozoic crustal evolution in the province as a whole. [ABSTRACT FROM AUTHOR]

Abstract: Nd isotopes of fish debris collected from the English Chalk at Eastbourne (Sussex, UK) are used to reconstruct the history of ocean circulation in the NW European shelf sea during Oceanic Anoxic Event 2 (OAE 2, Cenomanian–Turonian). The Eastbourne ε Nd record exhibits a 1-unit negative excursion (decreasing from ∼−9 to ∼−10), immediately followed by a 3-unit positive excursion reaching ∼−7. The onset of the negative ε Nd excursion lags the global δ13C rise characteristic of OAE 2, suggesting stable patterns of ocean circulation in the NW European shelf sea at this time. Both negative and positive Nd-isotope excursions took place during a transient cooling episode within OAE 2. The negative ε Nd excursion is interpreted as due to a change in ocean circulation with northerly sourced water masses becoming the dominant bottom waters at Eastbourne. The positive excursion is best explained by the transport of radiogenic Nd derived from a volcanic source, possibly the High Arctic or Caribbean large igneous province (LIP). An input of volcanic Nd may reconcile the Eastbourne record with coeval ε Nd records on Demerara Rise in the western tropical Atlantic. The broad synchroneity of high ε Nd values (∼−7) registered at both sites suggests a possible period with efficient oceanic mixing between the tropical Atlantic and the NW European shelf sea during the cooling episode. The Eastbourne ε Nd record of OAE 2, together with coeval temperature reconstructions, provides evidence for the coincidence of changes in ocean circulation and transient climatic cooling, implying a tight coupling between the two phenomena during this interval. [Copyright &y& Elsevier]

The NNW- to NNE-trending Kaminak diabase dykes, exposed in the Hearne craton of the western Churchill Province, Canada, are components of a Neoarchean–Paleoproterozoic LIP that forms an important pin in plate tectonic reconstructions impacting models for the assembly of the Nuna supercontinent. This study provides the first detailed information on the age, geochemistry and isotopic composition of these dykes. A precise U–Pb baddeleyite date of 2497.6±1.1Ma, obtained for one dyke sample, provides the best estimate for the timing of Kaminak mafic magmatism. The dykes are tholeiitic basalts to basaltic andesites with consistent compositions over an areal extent of ∼10,000km2. Major and compatible trace element variations indicate that the dykes were not in equilibrium with anhydrous peridotitic mantle and evolved through fractionation of Pl+Cpx±Ol from parental melts in the deep crust. All dykes record remarkably similar, incompatible element enriched, closely parallel multi-element profiles with variable HFSE troughs, comparable to Phanerozoic low-TiO2 continental tholeiites. Time-corrected Nd isotopic compositions overlap, or are slightly below bulk Earth ( to −0.8), and yield T DM values (2.75–2.87Ga) comparable to those of igneous crust in the Hearne craton. Implied short residence times, along with the geological, geochemical and isotopic data, are interpreted to imply that parental magmas of the dykes were low-TiO2 tholeiites generated via contamination of primary, depleted N-MORB-like magmas by low-degree, calc-alkaline lamprophyre melts in the continental lithospheric mantle. The melts homogenized in chambers near the Moho and underwent extensive fractional crystallization (Pl+Cpx±Ol), and possibly minor assimilation of lower crust, prior to injection into the upper crust. Paucity of evidence for contemporaneous lithospheric rifting, homogeneous lithogeochemical and Nd isotopic compositions, and petrochemical variations distinct from the Matachewan and Mistassini swarms of the Superior Province suggest the Kaminak dykes are the magmatic products of a Late Archean–Early Paleoproterozoic mantle thermal anomaly that extended, but did not break apart the Hearne craton. The results of this study do not support direct correlation of the Kaminak and Matachewan or Mistassini dyke swarms. Further evaluation of the proposed linkage of the Hearne and Superior cratons prior to Nuna supercontinent assembly requires additional geochronological, lithogeochemical and radiogenic isotopic constraints. [ABSTRACT FROM AUTHOR]

Abstract: The Magallanes foreland basin, located along the northern periphery of the Scotia Arc and extending up to 51°S to its northern terminus, contains a near-complete sedimentary record of Fuegian Andes tectonics since the Early Cretaceous. Herein we report trace- and rare-earth-element (REE) geochemistry and Nd isotope ratios from Upper Cretaceous to lower Oligocene mudstones of the eastern Magallanes basin, Argentina. The REE patterns of all of these samples are typical of average post-Archean upper continental crust. The older Upper Cretaceous to middle Eocene samples contain an overall lower concentration of light REE (smaller LaN:SmN ratio) than do the younger, middle-upper Eocene and lower Oligocene samples, although a strict temporal change is not observed. These REE patterns suggest a Patagonian Batholith and mafic volcanic provenance for the older successions and an Eastern Andean metamorphic complex provenance for the younger sediments. The lower Th:Sc ratio of the older samples suggests a mafic origin, whereas the higher Th:Sc ratio of younger samples indicates a felsic origin. Nd isotope data show a broadly coeval shift in εNd values from less negative to more negative. These data, combined with previously reported geochronological and mineral composition provenance analysis, suggest an increased Eocene–Oligocene tectonic exhumation of the southern Andes, concurrent with independent evidence of the opening of the adjacent Drake Passage. [Copyright &y& Elsevier]

Abstract: Major, trace element concentrations and Nd, Sr isotopic data are reported for Neoproterozoic high-pressure (HP) mafic granulites from the Agou Igneous complex (AIC), which marks the suture zone of the Pan-African Dahomeyide orogen in southwestern Togo. The main rock types consist of garnet-free and garnet-bearing metagabbronorites (GFM and GBM) that are cross cut by subordinate leucocratic to melanocratic veins and dykes of garnet-bearing microgabbroic rocks. Geochemical data suggest that the HP granulites may have preserved the geochemical imprints of their igneous protoliths. They display either cumulitic features or liquid compositions varying from primitive to evolved basalts and andesites and then to more differenciated dacitic magmas as supported by major and trace element contents. Primary basalts display LREE and LILE enriched patterns and no Eu anomaly, but are depleted in Y and Yb suggesting a slightly enriched mantle source or a weak crustal contamination. Mafic veins and dykes display REE flat patterns (LaN/YbN =0.58–1.7) and a slightly negative Eu anomaly and ∑REE content ranging between 10 and 20. These features are consistent with protoliths of N- to E-MORB affinities. Mafic cumulates and andesitic basalts are characterized by various REE patterns which could be compared to those of the primary and/or the evolved basalts, probably identifying various igneous suites. The fractionated REE patterns (Ce/YbN ≈11.0–17.9) with slight LREE enrichment (Ce/SmN ≈3.1–3.8) and HREE depletion (Sm/YbN ≈3.6–4.8) could be induced by a garnet residue in the melt source. Neodymium isotopic data show that most mafic cumulative rocks have similar 143Nd/144Nd ratios between 0.5127 and 0.51289, corresponding to initial ε Nd(T) values between +2.26 and +6.33 that yielded T DM ages from 1.2 to 0.94Ga. The mafic mesocratic vein displays 143Nd/144Nd values of 0.51227, corresponding to initial ε Nd(T) values of +6.87 that yielded T DM =0.94Ga, and constrains the minimum age. However primitive basalts samples A17 (GFM) and other mafic cumulate Ag224 (GBM) give an older depleted mantle source derivation age of 2.31 and 1.68Ga respectively, suggesting crustal contamination by the surrounding gneissic Paleoproterozoic basement. The Nd isotopic compositions suggest that the parental magmas of the Agou Igneous Complex in southern Togo were derived from time-integrated depleted mantle source. Such geochemical features are consistent with an active continental margin environment and suggest that the AIC represents a magmatic body emplaced and cooled at the base of a continental crust above a subduction zone. [Copyright &y& Elsevier]

Abstract: We present SIMS U–Pb zircon ages and Nd isotope data for the felsic volcanic rocks from seven Neoarchean greenstone belts of the Eastern Dharwar craton (EDC) and from the Chitradurga greenstone belt in the Western Dharwar craton (WDC). Zircon ages show bimodal age distribution of felsic volcanism. The ca. 2.70–2.65Ga felsic volcanic event is contemporaneous with 2.7Ga mafic greenstone volcanism and emplacement of juvenile tonalitic to granodioritic crust, while 2.58–2.54Ga felsic volcanics are coeval and spatially (and probably genetically in some cases) linked to the major episode calc-alkaline magmatic accretion in the EDC. The Chitradurga and Veligallu greenstone belts host felsic volcanics of the first generation, the latter showing inheritance at ca. 2.95Ga. Four of the five greenstone belts hosting the second generation of felsic volcanics (Chitradurga, Kolar, Kadiri, Hutti) show crustal inheritance at ca. 2.6, 2.7, 2.9, 3.0, 3.1 and 3.3Ga. ɛNd t indicate derivation of the felsic volcanics from juvenile sources or short-lived crustal sources with minor influence of older crust. The new and existing data are consistent with two-stage growth of the Dharwar craton in the Neoarchean. First-stage accretion led to the growth of a 2.7–2.6Ga juvenile crustal province of mafic volcanics and felsic plutons along the eastern margin of the WDC. Second-stage accretion (2.58–2.52Ga) led to the emplacement of TTG and calc-alkaline plutons and felsic volcanics throughout the Eastern Dharwar craton. An active margin context could apply for the two-stage accretion scenario considering a west-dipping subducting slab beneath the craton in the framework of long-lived ultra-hot accretionary orogen. But the great width of influence of magmatic accretion and/or reworking, particularly during the second stage, would suggest large-scale mantle flow reorganization that would have generated large plume(s). [Copyright &y& Elsevier]

Abstract: Geological mapping, geochemical, and geochronological studies undertaken in the Archean granitoids of the Canaã area in the Carajás province, Amazonian craton, Brazil, led to the identification of new granitoid units. Two compositional groups of granitoids were recognized: (1) minor tonalitic-trondhjemitic units which are both geochemically different (Campina Verde and Pedra Branca) or similar to Archean TTGs (Rio Verde) and (2) major granitic units encompassing calc-alkaline (Canaã dos Carajás, Bom Jesus, and Serra Dourada), transitional (Cruzadão) and alkaline (Planalto) granites. These granitoids form four age groups from Mesoarchean to Neoarchean. (1) The protolith of the Pium complex, the Bacaba tonalite and other coeval rocks, as indicated by the presence of inherited zircons, were formed at 3.05–3.0Ga; (2) at 2.96–2.93Ga, the Canaã dos Carajás granite and the older rocks of the Rio Verde trondhjemite were crystallized; (3) at 2.87–2.83Ga, the Campina Verde tonalitic complex, the Rio Verde trondhjemite, and the Cruzadão, Bom Jesus and Serra Dourada granites were formed; (4) in the Neoarchean, at 2.75–2.73Ga, the Planalto and Pedra Branca suites and charnockite rocks were crystallized. The granitic units cover more than 60% of the Canaã surface. The Canaã area differs from the Rio Maria and other granite-greenstone terranes by the scarcity of TTGs, dominance of granites, and absence of sanukitoids. The evolution of the Canaã area started at least at 3.2Ga and was different from that of the juvenile Rio Maria terrane (2.98–2.86Ga), because of indicating a contribution from pre-existing crust. The Mesoarchean Canaã crust was strongly reworked during the Neoarchean (2.75–2.70Ga) and was probably the substratum of the Neoarchean Carajás basin. The evolution of the Carajás province differs from those of Dharwar and Karelian cratons. It approaches the evolution described in the Limpopo belt, in the sense that the Neoarchean granitoid magmatism was generated in hot zones of the deep crust, possibly due to collisional tectonic processes. [Copyright &y& Elsevier]

Abstract: Metamorphosed during the Variscan orogeny, sediments of the ca. 560Ma Młynowiec Formation and ca. 530Ma Stronie Formation in the Bystrzyckie and Orlickie Mountains (Central Sudetes, Poland) contain metabasites with a range of basaltic compositions. Immobile trace element and Nd isotope features allow distinction of dominant, either E-MORB-like (Group 1: Zr/Nb 9–20, εNd530 +2.6 to +6.7) or mildly enriched N-MORB-like tholeiites (Group 2: Zr/Nb 21–27, εNd530 +0.2 to +6.7), and scarce but genetically important OIB-like alkaline (Group 3: Zr/Nb 5, εNd530 +2.2) or depleted tholeiitic rocks (Group 4: Zr/Nb 67, εNd530 +7.9). Neither the radiogenic age nor age relationships between these four groups are known. However, field evidence suggests that the metabasites are younger than the Młynowiec Formation and that their emplacement must have been coeval with the accumulation of the Stronie Formation sediments. The OIB affinity of Group 3 is interpreted to reflect an enriched mantle (EM)-type asthenopheric source whilst the groups of tholeiitic rocks indicate involvement of depleted (locally slightly residual) MORB-type mantle (DMM). Several geochemical signatures, the decoupling between Nd isotope and trace element characteristics, and melting models indicate variable enrichment of the DMM-like source, here ascribed to asthenosphere-derived OIB-like melts (Group 1 and 2) and a contribution from a supra-subduction zone (Group 2 and 4). Based on contrasting back-arc basin (BAB)- and within-plate-like affinities of the metabasites, and on petrogenetic constraints from the spatially related infill of the Stronie Formation rift basin, the studied magmatic episode is suggested be related to cessation of the supra-subduction zone activity, presumably induced by ridge-trench collision. This event might have led to slab break-off, the development of a transform plate boundary, opening of a slab window and upward migration of sub-slab enriched asthenosphere. Decompression melting of the upwelling asthenosphere could then have produced OIB-like melts which segregated and infiltrated into the mantle of the former subduction zone, with randomly distributed slab-derived components. In an extensional regime, magmas generated at shallow levels from heterogeneous mantle regions were emplaced within sedimentary rocks of the overlying rift basin. The vestiges of subduction-related processes and within-plate style of mantle enrichment suggest that the metabasites could be related to final stages of the Cadomian orogeny and incipient Early Palaeozoic rifting of Gondwana that heralded the opening of the Rheic Ocean. [Copyright &y& Elsevier]

Abstract: We integrate micropaleontological and geochemical records (benthic stable isotopes, neodymium isotopes, benthic foraminiferal abundances and XRF-scanner derived elemental data) from well-dated Pacific Ocean successions (15–12.7Ma) to monitor circulation changes during the middle Miocene transition into a colder climate mode with permanent Antarctic ice cover. Together with previously published records, our results show improvement in deep water ventilation and strengthening of the meridional overturning circulation following major ice expansion at ∼13.9Ma. Neodymium isotope data reveal, however, that the provenance of intermediate and deep water masses did not change markedly between 15 and 12.7Ma. We attribute the increased δ 13C gradient between Pacific deep and intermediate water masses between ∼13.6 and 12.7Ma to more vigorous entrainment of Pacific Central Water into the wind-driven ocean circulation due to enhanced production of intermediate and deep waters in the Southern Ocean. Prominent 100kyr ventilation cycles after 13.9Ma reveal that the deep Pacific remained poorly ventilated during warmer intervals at high eccentricity, whereas colder periods (low eccentricity) were characterized by a more vigorous meridional overturning circulation with enhanced carbonate preservation. The long-term δ 13C decline in Pacific intermediate and deep water sites between 13.5 and 12.7Ma reflects a global trend, probably related to a re-adjustment response of the global carbon cycle following the last 400kyr carbon maximum (CM6) of the “Monterey Excursion”. [Copyright &y& Elsevier]

Abstract: Hainan soils contain a mixture of material derived from in situ weathering of parent material plus atmospheric inputs dominated by continental dust. We use Nd isotope geochemistry to evaluate the impact of continental dust on a chronosequence of Hainan soils. The results demonstrate that dust has a profound effect on soils in 180ka and older in tropical Hainan Island. Dust is the dominant source of Nd in the oldest, most intensely weathered soils, with radiogenic dust-derived isotopic signatures (εNd(0)=−6.46) and significant high mass fraction of dust-derived Nd (75%). The fdust Nd values for soils show an increasing trend with time, implying that dust accumulates with the ages of soils. We also calculate a long term average dust accretion rates of 14.90 and 12.39mgcm−2 ka−1 at the 180 and 300ka sites; whereas the rates of the old sites are low, indicating underestimates of time-averaged dust deposition rates over million year time scales. On the basis of the geomorphic stability sites, we think that the decrease of dust inputs due to climate changes and the loss of dust due to weathering are the dominant loss mechanisms for dust and lead to underestimates of time-averaged dust deposition rates in the old sites. The results also underscore the potential for neodymium isotopes to constrain the origin of soils and paleosols. [Copyright &y& Elsevier]

Abstract: Abundant marine sedimentary deposits accumulated in the epeiric seas of North America during the Carboniferous Period (359–299Ma) when the Earth''s climate shifted from a global “greenhouse” to “icehouse”. Geochemical records extracted from these deposits are often assumed to reflect global ocean processes and are increasingly being used to infer relationships between tectonism, carbon cycling and climate change. We analyzed radiogenic isotopes (Sr, Nd) in biogenic apatite along a North American transect to constrain the degree of coupling between the epicontinental seas and the open ocean. In many samples the primary Sr signal is altered, however the original Nd isotopic composition of the biogenic apatite seems to be preserved. The εNd(t) values range from −8.6 to −4.5 in the US midcontinent basins and from −11.4 to −5.7 at Arrow Canyon (Nevada, USA), indicating the development of an east–west geochemical gradient throughout the Pennsylvanian. This apparent decoupling of the epicontinental seas from the open ocean was established by the latest Mississippian. Thus, circulation of waters between epicontinental seaways of western US and the basins to the east and south was periodically restricted. Shifts toward unradiogenic Nd at Arrow Canyon seem to correspond with glacio-eustatic changes across the mid-Carboniferous boundary. These εNd(t) variations may have been caused by changes in weathering rates/sources, marine circulation or a combination of both. [Copyright &y& Elsevier]

The intermediate and deep waters of the Pacific Ocean play a crucial role for regulating global climate changes on the millennium timescale. However, due to poor preservation of carbonate sediments in the deep Pacific, little attention has been received to better understand deep ocean circulation and its relationship with abrupt climate events. Here we present the first authigenic Nd isotope (ε Nd) record extracted from Fe–Mn oxyhydroxides of the South China Sea (SCS) sediments (SO17940-2, 1727m water depth) in order to investigate changes of intermediate and deep-water circulations in the Pacific Ocean since the Last Glacial Maximum (LGM). In general, the seawater ε Nd record extracted from the SCS sediments paralleled the existing Nd isotopes of fish teeth/debris from the eastern North Pacific core during the last deglaciation, demonstrating that our authigenic ε Nd can faithfully record the seawater ε Nd evolution in the deep Pacific Ocean. At the onset of the deglacial period, our data exhibit a clear negative shift in ε Nd towards a gradually increased component of Antarctica Intermediate Water (AAIW) and reached a maximal influx during the mid Heinrich Stadial 1 (HS1, ∼16kyr BP). Between the HS1 and Younger Dryas, deep-water circulation gradually shifted back to the modern condition during the Bølling-Allerød warm interval. A pronounced negative ε Nd excursion had occurred during the Pre-Boreal (PB), suggesting a rapid reorganization of deep-water ventilation in the Pacific Ocean and was predominated by AAIW. This is most likely due to poor ventilation in the western North Pacific associated with an intensified summer monsoon during the PB-early Holocene warm period. After the PB period, the seawater ε Nd values were once again dominated by Pacific Deep Water at the study site. [ABSTRACT FROM AUTHOR]

The Kuruman Kimberlite Province contains the oldest known kimberlites in the Kaapvaal Craton (∼1·6–1·8 Ga), but only limited geochronological and isotopic data exist for this province. To provide better constraints on the sources of the kimberlite magmas and the timing of magmatism, we have carried out in situ U–Pb dating and Sr–Nd isotopic analysis of groundmass perovskite from four of the Kuruman kimberlites (Bathlaros, Elston, Helpmekaar and Zero). We also have measured the Sr-isotope composition of groundmass apatite and carbonate in several of the kimberlites, for comparison with published whole-rock isotopic data. Attempts to date perovskite using in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were hindered by extensive Pb loss, even in apparently unaltered grains, yielding anomalously young emplacement ages. The assumption of a Mesoproterozoic common-Pb component allows the extraction of intercept ages from the least altered grains (Bathlaros, 1640 ± 13 Ma; Elston, 1663 ± 49 Ma; Helpmekaar, 1638 Ma; Zero, 1845 ± 45 Ma). The smaller sampling volume of secondary ion mass spectrometry and the ability to measure 204Pb, thus allowing more precise common-Pb corrections, produces concordant ages. These reveal that kimberlite intrusion occurred over a time span of ∼200 Myr (Bathlaros, 1647 ± 27 Ma; Zero, 1835 ± 25 Ma), with emplacement ages younging to the east. The results of this study suggest that alteration may disrupt the U–Pb system in perovskite. In situ Sr and Nd isotope analyses [LA-multi-collector (MC)-ICP-MS] of perovskite show a moderate range in values (87Sr/86Sr(i) = 0·70383–0·70507; &varepsilon;Nd(i) = –2·7 to 1·5), but similar mean values among the four kimberlites (87Sr/86Sr(i) = 0·70396–0·70425; &varepsilon;Nd(i) = –0·8 to 0·8). The Nd-isotope compositions of perovskite are similar to those of whole-rock samples of the host kimberlite (&varepsilon;Nd(i) = 1·0 to −3·0), whereas Sr in perovskite and apatite (87Sr/86Sr(i) = 0·70424–0·70427) is more radiogenic than the whole-rock values (87Sr/86Sr(i) = 0·70293–0·70382). However, groundmass carbonates generally yield lower values (87Sr/86Sr(i) = 0·70348–0·70405), closer to those of the kimberlite samples, indicating that the whole-rock Sr isotope compositions are largely controlled by carbonates, probably assimilated from the immediate country rocks. The isotopic data suggest that the Kuruman kimberlite magmas were initially derived from a deeper sub-lithospheric source region, but their distinctive compositional characteristics were imposed through assimilation, first of metasomatized sub-continental lithospheric mantle and later of crustal material, during magma ascent. [ABSTRACT FROM PUBLISHER]