Entwicklung des lithosphärischen Mantles unterhalb des Kaapvaal-Kratons vom Archaikum bis heute : in subkalzischen Granaten, Peridotiten und polymikten Brekzien aufgezeichnete Prozesse

The geodynamic processes and the chemical and thermal evolution of the mantle beneath the Kaapvaal craton (South Africa) was investigated with further regard to diamond formation. For this, 31 coarse-grained peridotites and 21 individual subcalcic garnets from heavy mineral concentrates (HMC) from the Finsch mine were studied for their major and trace element compositions, Lu-Hf and Sm-Nd isotope composition. Furthermore, processes in the Earth’s mantle that follow kimberlite sampling and propagation were studied in polymict peridotite breccia from Kimberley mines. Inter mineral equilibrium of the peridotites was tested by comparing the results from different, independent thermometers. These, well equilibrated peridotites stem from a restricted pressure of 5 to 6.5 GPa (depth ~160-200 km) and a temperature range of 1050-1250°C, following the 40 mW/m2 conductive geotherm. The majority of the samples display a well developed anti-correlation of oxygen fugacity with pressure, which is in contrast to the sheared and oxidised, younger kimberlite erupted peridotites from Kimberley. All analysed samples have homogeneous trace element mineral chemistry. Variations in trace elements among Finsch peridotites reflect their complex nature and the intricate development of the subcratonic mantle. The 3.6 Ga is the oldest crustal age recorded in the Kaapvaal craton, and is confirmed by the Lu-Hf model age of a highly radiogenic subcalcic garnet in this study. Therefore, this age probably represents the oldest depletion (partial melting event) of the subcratonic mantle beneath the Kaapvaal craton. Both, subcalcic garnets and Finsch peridotites yield Lu-Hf isochron ages of around 2.5 Ga, which probably represent the last depletion event of the Kaapvaal craton. Several older (than 2.5 Ga) depletions were also necessary to explain higher isochron initials of the both isochrones. The Cr# and HREE concentrations and ratios of the Finsch subcalcic garnets and peridotites indicate that partial melting of the Kaapvaal craton happened at different depths. One group of subcalcic garnets (group-1) experienced depletion at high pressure in the garnet stability field and another one (group-2) at low pressures in the spinel or plagioclase stability field. Major and trace elements indicate that up to 50%, of the melt was remover from the primitive (primer) mantle in at least two melting events. Thus, first continental crust was created early (> 2.5 Ga) from high degrees of partial melting of the lithospheric mantle. According to the Sm-Nd isotope signatures at least two metasomatic events took place significantly after 2.5 Ga. As monitored by group-1 subcalcic garnets, the first enrichment was produced by a fluid and occurred at around 1.3 Ga. The second metasomatic event was much later at 500-300 Ma ago and has changed both Nd and Hf isotopic compositions of group-2 subcalcic garnet as well as some Finsch peridotites. During partial melting any carbon species will be dissolved in the melt and removed from the residue. Therefore, any diamond growth before the last depletion (~2.5 Ga) would have been probably completely removed from the lithospheric mantle. Consequently, carbon was apparently reintroduced into the system, i.e. during Metasomatism, and triggered the growth of diamonds. The Sm-Nd isotope systematics of the subcalcic garnets of this study indicates that enrichment occurred at ~1.3 Ga or later, which implies non-Archean, late diamond growth in Finsch. Fertilisation of the subcontinental craton associated with the percolation of group-2 (~120 Ma) or even younger (~90 Ma) group-1 kimberlites and their precursors are not observed in Finsch peridotites, but are well presented in mantle xenoliths from Kimberley. Therefore, these younger events were studied on specific mantle xenoliths, polymict breccia from Kimberley. A polymict peridotite found at the Boshof road dump, Kimberley, represents a mechanical mixture of upper mantle clasts and minerals (opx, cpx, garnet and olivine) of different lithologies, cemented by fine-grained olivine and minute amounts of interstitial ilmenite, phlogopite and sulphide. According to Ni in garnet thermometry, single porphyroclastic garnets were sampled and mixed during ascent in a 100 km stratigraphic column, starting from ~250 km until ~120 km. During this ascent, melt has reacted with the porphyroclasts and at theirrims neoblastic minerals were formed, i.e. neoblastic opx around opx porphyroclast, neoblastic garnet around garnet porphyroclast, and neoblastic opx around cpx porphyroclast. Analyses of those neoblastic minerals indicate that volatile-rich, kimberlite-like melt was the agent that collected the mantle minerals and amalgamated this xenolith. Several complex processes were responsible for the formation of the polymict breccia. They comprise melt degassing at high pressures that probably created “explosive” Brecciation of the cratonic roots (~250 km), propagation of the melt that collected different porphyroclasts on a way and amalgamation at around 120 km. The whole process of “explosive” brecciation, turbulent transport and mixing of mantle porphyroclasts and melt, porphyroclast dissolution and neoblast precipitation happened very fast and was part of the kimberlite formation. Therefore, the here studied sample probably represents one frozen part (with variable mantle clasts) of the kimberlitic magma precursor, with kimberlite eruption at ~90 Ma years ago in Kimberley.In dieser Arbeit wurde die geodynamische, chemische und thermische Entwicklung des Kaapvaal Kratons (Südafrika) rekonstruiert. Dazu wurden 31 grobkörnige Peridotite und 21 einzelne subkalzische Granate aus Schweremineralkonzentraten der Finsch-Mine, bezüglich ihrer Haupt- und Spurenelementzusammensetzung und ihrer Lu-Hf und Sm-Nd Isotopensystematik, untersucht. Die Prozesse, die in Zusammenhang mit der Förderung von Peridotit-Xenolithen durch Kimberlitischschmelzen stehen, sind an polymikten Brekzien untersucht geworden. Ziel dieser Untersuchungen war es die Entwicklung des Mantels unterhalb des Kaapvaal Kratons zu rekonstruieren, mit Hinblick auf den Zeitpunkt und die Mechanismen der Diamantbildung. Die Anwendung unterschiedlicher unabhängiger Thermometer ergab, dass sich die Proben, bezüglich ihrer Hauptelemente, im Gleichgewicht befinden. Es stellte sich heraus, dass alle Peridotite aus einem beschränkten Druck- und Temperaturbereich (von 5-6.5 GPa und 1050-1250°C) kommen, und auf dem geothermischen Gradient von 40 mW/m2 liegen. Bei allen Peridotiten ist eine negative Korrelation von Sauerstofffugazität und Druck zu beobachten. Dies steht im Gegensatz zu den meisten Peridotiten jüngerer Kimberlitlokalitäten, welche deutlich geschert und oxidiert sind. Alle analysierten Minerale aus den Finsch-Peridotiten sind, bezüglich ihrer Spurenelemente, homogen. Die starken Spurenelementvariationen zwischen den Proben reflektieren die komplexe Geschichte der Peridotite und des subkratonischen Mantels. Die älteste Krustenbildung im Kaapvaal Kraton (Südafrika) ist vor ca. 3.6 Ga datiert und dieses Alter wurde auch in dieser Studie in Form des Lu-Hf Modellalters eines hoch-radiogenen subkalzischen Granats ermittelt. Dieses Alter vermutlich repräsentiert das älteste Schmelzbildungsereignis im Mantel des Kaapvaal Kratons. Die in dieser Studie untersuchten Finsch-Peridotite und subkalzische Granate bilden je eine Lu-Hf Isochrone, welche ein Minimalalter von 2.5 Ga für ein 2. (letztes) Schmelzereignis ergibt. Die hohen Initiale der Isochronen deuten auch auf deutlich ältere Schmelzbildungsereignisse hin. Die Schmelzbildung im heutigen Mantel des Kaapvaal Kratons hat in unterschiedlichen Tiefen, entweder bei hohen Drücken mit Granat im Residuum, oder bei niedrigen Drücken im Spinel- oder Plagioklasstabilitätsfeld, stattgefunden. Darauf deuten geochemische Parameter, wie z.B die Cr#, oder HREE-Gehalte und Verhältnisse hin. Insgesamt können Schmelzgrade von bis zu 50% erreicht worden sein, was ein Modell bestätigt, nachdem die frühe kontinentale Kruste durch extrem hohe Schmelzgrade im lithospherischen Mantel gebildet wurde. Wenigstens zwei metasomatische Ereignisse haben deutlich nach der partiellen Schmelzbildung vor 2.5 Ga stattgefunden und vor allem die Sm-Nd Isotopensystematik beeinflusst. Die subkalzischen Granaten der Gruppe-1 deuten darauf hin, dass vor ca. 1.3 Ga eine Metasomatose durch ein wässriges Fluid stattfand. Deutlich später, vor ca. 300-500 Ma, fand eine Metasomatose statt, welche sowohl die Nd, als auch die Hf-Isotopie beeinflusste und sich in den subkalzischen Granaten der Gruppe-2 und einigen Peridotiten widerspiegelt. Während der partiellen Aufschmelzung werden Kohlenstoffphasen von der Schmelze gelöst und somit dem Mantel entzogen. Von daher sollten sich Diamanten, die bereits vor der letzten Schmelzbildung vor 2.5 Ga gebildet wurden, bei diesem Ereignis wieder aufgelöst haben. Kohlenstoff für das Diamantwachstum muss dann anschließend wieder durch Metasomatose, in Zusammenhang mit subduziertem Material, eingebracht worden sein. Die Sm-Nd Isotopensystematik von subkalzischen Granaten impliziert, dass diese Metasomatose nicht vor ca. 1.3 Ga stattgefunden hat. Dies impliziert spätes (postarchaisches) Diamantwachstum in Finsch. „Rezente“ (120-90 Ma) Anreicherungsereignisse, welche mit dem Kimberlit oder mit dem Kimberlitvorläufer in Zusammenhang stehen, sind nicht in Finsch-, jedoch in Kimberley Peridotiten zu sehen. Diese rezenten Ereignisse wurden in der hier untersuchten polymikten Brekzie aufgezeichnet. Eine polymikter Peridotit, welcher in der Kimberley-Mine gefunden wurde, stellt eine mechanische Mischung von Klasten und Mineralen (Opx, Kpx, Granat und Olivin) verschiedener Lithologien des oberen Mantels dar, welche durch feinkörnigen Olivin und geringe Mengen an Ilmenit, Phlogopit und Sulfid zementiert wurde. Durch Temperaturbestimmungen mit dem „Ni-in-Granat“ Thermometer konnte gezeigt werden, dass die einzelnen porphyroklastischen Granate beim Aufstieg der Kimberlitschmelze entlang einer stratigraphischen Mantelsäule von ca. 100 km (von ca. 250 bis 120 km Tiefe) aufgesammelt wurden. Dabei hat die Schmelze mit den porphyroklastischen Mineralen reagiert und an den Rändern neoblastische Minerale gebildet. Analysen dieser neoblastischen Minerale deuten darauf hin, dass die Mantelminerale durch eine volatil-reiche, kimberlitische Schmelze gesammelt und der Xenolith amalgamisiert wurde. Insgesamt haben also eine Reihe komplexer Prozesse zur Bildung der polymikten Brekzie geführt. Diese beinhalten Entgasung, gekoppelt mit einer explosiven Brekzienbildung am Grund des kratonischen Kiels (~250 km), Schmelzaufstieg and Amalgamisierung bei ca. 120 km. Die erhaltenen chemischen Ungleichgewichte implizieren, dass der gesamte Prozess des turbulenten Transports, der Mischung der Porphyroklasten, ihr teilweises Anlösen und die Neoblastenbildung sehr schnell von Statten gegangen sein muss. Der hier untersuchte polymikte Peridotit repräsentiert vermutlich ein eingefrorenes Beispiel all dieser Prozesse, die zur Kimberlitbildung insgesamt führten und schließlich in einer Kimberliteruption vor ca. 90 Ma in Kimberley endeten

The Role of Water in the Stability of Cratonic Keels

Cratons are typically underlain by large, deep, and old lithospheric keels (to greater than 200 km depth, greater than 2.5 Ga old) projecting into the asthenosphere (e.g., Jordan, 1978; Richardson et al., 1984). This has mystified Earth scientists as the dynamic and relatively hot asthenosphere should have eroded away these keels over time (e.g., Sleep, 2003; O'Neill et al., 2008; Karato, 2010). Three key factors have been invoked to explain cratonic root survival: 1) Low density makes the cratonic mantle buoyant (e.g., Poudjom Djomani et al., 2001). 2) Low temperatures (e.g., Pollack, 1986; Boyd, 1987), and 3) low water contents (e.g., Pollack, 1986), would make cratonic roots mechanically strong. Here we address the mechanism of the longevity of continental mantle lithosphere by focusing on the water parameter. Although nominally anhydrous , olivine, pyroxene and garnet can accommodate trace amounts of water in the form of H bonded to structural O in mineral defects (e.g., Bell and Rossman, 1992). Olivine softens by orders of magnitude if water (1-1000 ppm H2O) is added to its structure (e.g., Mackwell et al., 1985). Our recent work has placed constraints on the distribution of water measured in peridotite minerals in the cratonic root beneath the Kaapvaal in southern Africa (Peslier et al., 2010). At P greater than 5 GPa, the water contents of pyroxene remain relatively constant while those of olivine systematically decrease from 50 to less than 10 ppm H2O at 6.4 GPa. We hypothesized that at P greater than 6.4 GPa, i.e. at the bottom of the cratonic lithosphere, olivines are essentially dry (greater than 10 ppm H2O). As olivine likely controls the rheology of the mantle, we calculated that the dry olivines could be responsible for a contrast in viscosity between cratonic lithosphere and surrounding asthenosphere large enough to explain the resistance of cratonic root to asthenospheric delamination

Isotope diffusion and re-equilibration of copper and evaporation of mercury during weathering of tetrahedrite in an oxidation zone

To understand the mobility of heavy metals during oxidative weathering of sulfides, we investigated weathering processes of tetrahedrite [(Cu,Fe,Zn,Hg)12(Sb,As)4S13] in an oxidation zone with abundant siderite (FeCO3) and baryte (BaSO4) at Rudňany (Slovakia). The focus of this work lied in the isotopic (δ65Cu, δ202Hg, δ34S) variations of the minerals during weathering and the interpretation of such changes. In the studied oxidation zone, Hg-rich tetrahedrite converts in situ to pockets of powdery cinnabar (HgS) and an X-ray amorphous mixture rich in Sb, Fe, and Cu that slowly re-crystallizes to Cu-rich tripuhyite (FeSbO4). Copper is mobile and precipitates as malachite [Cu2(OH)2(CO3)], azurite [Cu3(OH)2(CO3)2], or less abundant clinoclase [Cu3(AsO4)(OH)3]. The isotopic composition (δ65Cu) of tetrahedrite correlates well with the degree of weathering and varies between 0.0 ‰ and −4.0 ‰. This correlation is caused by isotopic changes during dissolution and subsequent rapid equilibration of δ65Cu values in the tetrahedrite relics. Simple diffusion models showed that equilibration of Cu isotopic values in the tetrahedrite relics proceeds rapidly, on the order of hundreds or thousands of years. Abundant secondary iron oxides draw light Cu isotopes from the aqueous solutions and shift the isotopic composition of malachite and azurite to higher δ65Cu values as the distance to the primary tetrahedrite increases. Clinoclase and tripuhyite have lower δ65Cu values and are spatially restricted near to the weathering tetrahedrite. The Hg and S isotopic composition of tetrahedrite is δ202Hg = −1.27 ‰, δ34S = −1.89 ‰, that of the powdery secondary cinnabar is δ202Hg = +0.07 ‰, δ34S = −5.50 ‰. The Hg isotopic difference can be explained by partial reduction of Hg(II) to Hg(0) by siderite and the following evaporation of Hg(0). The S isotopic changes indicate no involvement of biotic reactions in the oxidation zone, probably because of its hostility owing to high concentrations of toxic elements. This work shows that the Cu isotopic composition of the primary sulfides minerals changes during weathering through self-diffusion of Cu in those minerals. This finding is important for the use of Cu isotopes as tracers of geochemical cycling of metals in the environment. Another important finding is the Hg in the oxidation zones evaporates and contributes to the global cycling of this element through atmospheric emission

Changes in antimony isotopic composition as a tracer of hydrothermal fluid evolution at the Sb deposits in Pezinok (Slovakia)

In this work, we investigated in situ isotopic compositions of antimony (Sb) minerals from two substages of the ore deposits near Pezinok (Slovakia). The δ123Sb values of the primary Sb minerals range from −0.4 and +0.8‰ and increase progressively along the precipitation sequence. In the substage II, the early-formed gudmundite (FeSbS) shows in all sections the lowest δ123Sb values, followed by berthierite (FeSb2S4), stibnite (Sb2S3), and valentinite (Sb2O3) with the heaviest δ123Sb values. A similar trend was observed for the substage III, from the initially-formed stibnite, followed by kermesite (Sb2S2O), valentinite, senarmontite (both Sb2O3), and schafarzikite (FeSb2O4). The evolution can be rationalized by a Rayleigh fractionation model with a starting δ123Sb value in the fluid of +0.3‰, applying the same mineral-fluid fractionation factor to all minerals. Thus, the texturally observed order of mineralization is confirmed by diminishing trace element contents and heavier δ123Sb values in successively crystallized Sb minerals. Antimony in substage III was likely supplied from the oxidative dissolution of stibnite that formed earlier during substage II. The data interpretation, although limited by the lack of reliable mineral-fluid fractionation factors, implies that Sb precipitation within each substage occurred from an episodic metal precipitation, likely associated with a similar Sb isotope fractionation between fluid and all investigated Sb minerals. Large isotopic variations, induced by precipitation from a fluid as a response to temperature decrease, may be an obstacle in deciphering the metal source in hydrothermal ore deposits. However, Sb isotopes appear to be an excellent instrument to enhance our understanding on how hydrothermal systems operate

Water Content of Earth's Continental Mantle Is Controlled by the Circulation of Fluids or Melts

A key mission of the ARES Directorate at JSC is to constrain models of the formation and geological history of terrestrial planets. Water is a crucial parameter to be measured with the aim to determine its amount and distribution in the interior of Earth, Mars, and the Moon. Most of that "water" is not liquid water per se, but rather hydrogen dissolved as a trace element in the minerals of the rocks at depth. Even so, the middle layer of differentiated planets, the mantle, occupies such a large volume and mass of each planet that when it is added at the planetary scale, oceans worth of water could be stored in its interior. The mantle is where magmas originate. Moreover, on Earth, the mantle is where the boundary between tectonic plates and the underlying asthenosphere is located. Even if mantle rocks in Earth typically contain less than 200 ppm H2O, such small quantities have tremendous influence on how easily they melt (i.e., the more water there is, the more magma is produced) and deform (the more water there is, the less viscous they are). These two properties alone emphasize that to understand the distribution of volcanism and the mechanism of plate tectonics, the water content of the mantle must be determined - Earth being a template to which all other terrestrial planets can be compared

Metasomatic Control of Water in Garnet and Pyroxene from Kaapvaal Craton Mantle Xenoliths

Fourier transform infrared spectrometry (FTIR) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) were used to determine water, rare earth (REE), lithophile (LILE), and high field strength (HFSE) element contents in garnet and pyroxene from mantle xenoliths, Kaapvaal craton, southern Africa. Water enters these nominally anhydrous minerals as protons bonded to structural oxygen in lattice defects. Pyroxene water contents (150-400 ppm in clinopyroxene; 40-250 ppm in orthopyroxene) correlate with their Al, Fe, Ca and Na and are homogeneous within a mineral grains and a xenolith. Garnets from Jagersfontein are chemically zoned for Cr, Ca, Ti and water contents. Garnets contain 0 to 20 ppm H2 Despite the fast diffusion rate of H in mantle m inerals, the observations above indicate that the water contents of mantle xenolith minerals were not disturbed during kimberlite entrainment and that the measured water data represent mantle values. Trace elements in all minerals show various degrees of light REE and LILE enrichments indicative of minimal to strong metasomatism. Water contents of peridotite minerals from the Kaapvaal lithosphere are not related to the degree of depletion of the peridotites. Instead, metasomatism exerts a clear control on the amount of water of mantle minerals. Xenoliths from each location record specific types of metasomatism with different outcomes for the water contents of mantle minerals. At pressures . 5.5 GPa, highly alkaline melts metasomatized Liqhobong and Kimberley peridotites, and increased the water contents of their olivine, pyroxenes and garnet. At higher pressures, the circulation of ultramafic melts reacting with peridotite resulted in co-variation of Ca, Ti and water at the edge of garnets at Jagersfontein, overall decreasing their water content, and lowered the water content of olivines at Finsch Mine. The calculated water content of these melts varies depending on whether the water content of the peridotite (2 wt% HO. 2O) or individual m inerals (<0.5-13 wt% H2O) are used, and also depend on the mineral-melt water partition coefficients. These metasomatic events are thought to have occurred during the Archean and Proterozoic, meaning that the water contents measured here have been preserved since that time and can be used to investigate viscocity and longevity of cratonic mantle roots

Sulfide enrichment at an oceanic crust-mantle transition zone : Kane Megamullion (23°N, MAR)

Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 230 (2018): 155-189, doi:10.1016/j.gca.2018.03.027.The Kane Megamullion oceanic core complex located along the Mid-Atlantic Ridge (23°30′N, 45°20′W) exposes lower crust and upper mantle directly on the ocean floor. We studied chalcophile elements and sulfides in the ultramafic and mafic rocks of the crust-mantle transition and the mantle underneath. We determined mineralogical and elemental composition and the Cu isotope composition of the respective sulfides along with the mineralogical and elemental composition of the respective serpentines. The rocks of the crust-mantle transition zone (i.e., plagioclase harzburgite, peridotite-gabbro contacts, and dunite) overlaid by troctolites are by one order of magnitude enriched in several chalcophile elements with respect to the spinel harzburgites of the mantle beneath. Whereas the range of Cu concentrations in spinel harzburgites is 7–69 ppm, the Cu concentrations are highly elevated in plagioclase harzburgites with a range of 90–209 ppm. The zones of the peridotite-gabbro contacts are even more enriched, exhibiting up to 305 ppm Cu and highly elevated concentrations of As, Zn, Ga, Sb and Tl. High Cu concentrations show pronounced correlation with bulk S concentrations at the crust-mantle transition zone implying an enrichment process in this horizon of the oceanic lithosphere. We interpret this enrichment as related to melt-mantle reaction, which is extensive in crust-mantle transition zones. In spite of the ubiquitous serpentinization of primary rocks, we found magmatic chalcopyrites [CuFeS2] as inclusions in plagioclase as well as associated with pentlandite [(Fe,Ni)9S8] and pyrrhotite [Fe1−xS] in polysulfide grains. These chalcopyrites show a primary magmatic δ65Cu signature ranging from −0.04 to +0.29 ‰. Other chalcopyrites have been dissolved during serpentinization. Due to the low temperature (<300 °C) of circulating fluids chalcophile metals from primary sulfides have not been mobilized and transported away but have been trapped in smaller secondary sulfides and hydroxides. Combined with the Cu deposits documented in the crust-mantle transition zones of various ophiolite complexes, our results indicate that the metal enrichment, increased sulfide modes, and potentially formation of small sulfide deposits could be expected globally along the petrological Moho.This research was funded by a Diamond Grant of the Polish Ministry of Science and Higher Education (DI2012 2057 42 to J. Ciazela), and partly supported by grants of the U.S. National Science Foundation (OCE1434452 and OCE1637130 to H.J.B. Dick), and the German Science Foundation (Bo2941/4-1 to R. Botcharnikov)

Iron isotope and trace metal variations during mantle metasomatism: In situ study on sulfide minerals from peridotite xenoliths from Nógrád-Gömör Volcanic Field (Northern Pannonian Basin)

Sulfides from lherzolite and wehrlite xenoliths from the Nógrád-Gömör Volcanic Field (NGVF), located in the Northern Pannonian Basin, were studied to understand the behavior of chalcophile and siderophile elements during mafic melt – peridotite interaction. We applied in situ methods to analyze the major and trace elements, as well as Fe isotope compositions of sulfide minerals. Sulfides are more abundant in wehrlites (~0.03 vol%) and are often enclosed in silicates, whereas in lherzolites, they are scarcer (~0.01 vol%) and predominantly interstitial. Monosulfide solid solution and pentlandite are the most common sulfide phases in the lherzolite xenoliths, whereas in wehrlite xenoliths it is pyrrhotite and chalcopyrite. Consequently, wehrlitic sulfides show higher bulk Fe and Cu but lower bulk Ni and Co contents compared to the lherzolitic sulfides. Trace elements with both chalcophile and siderophile character (Ge, Se, Te, and Re) show lower, whereas highly chalcophile elements (Zn, Cd, Sb, and Tl) show higher concentrations in wehrlitic sulfides compared to lherzolitic ones. Highly siderophile elements show no systematic difference between the sulfides of the two xenolith series, which suggests moderate enrichment in these elements in wehrlite bulk rocks due to their higher sulfide content. Sulfide δ56Fe signature indicates variable isotopic composition both in lherzolites (δ56Fe: −0.13 to +0.56‰) and wehrlites (δ56Fe: −0.20 to +0.84‰) relative to the terrestrial mantle (δ56Fe: +0.025 ± 0.025‰; Craddock et al., 2013). However, irrespectively of the xenolith lithology, there is a significant difference between the δ56Fe of sulfides from the two sampling localities: NTB /North/: vary from −0.20 to +0.04‰ and NME /South/: vary from +0.56 to +0.84‰. This suggests that the Fe isotopic ratios of sulfides are not modified by the wehrlitization process. Difference in sulfide δ56Fe between the two xenolith localities is probably because of the higher, isotopically heavier (δ56Fe: from +1.28 to +1.60‰; Ciążela et al., 2019) chalcopyrite content in sulfides from the NME xenoliths compared to those from the NTB xenoliths irrespectively to their lithology. Our results also indicate sulfide and chalcophile element enrichment resulting from metasomatism in the subcontinental lithospheric mantle. We suggest that this process affected the regional metal distribution and has implications for global metal mass balance within the subcontinental lithosphere

Response of copper concentrations and stable isotope ratios to artificial drainage in a French Retisol

Copper is a redox-sensitive trace element, which can be both, an essential micronutrient and a pollutant. We therefore analyzed Cu concentrations and stable isotope ratios (δ65Cu values) in a drained Retisol to trace the response of Cu to a changing hydrological regime and enhanced clay eluviation. The study soil was artificially drained 16 years before sampling resulting in macroscopically visible pedogenetic changes and is thus a suitable site to investigate the influence of pedogenetic processes on the fate of Cu. Samples were collected from all horizons along a trench at four distances from the drain: 0.6 m, 1.1 m, 2.1 m and 4.0 m. In the E&Bt horizon, four different soil volumes (ochre, pale brown, white-grey and black) were sampled at all four distances from the drain. Furthermore, we analyzed soil solutions sampled with piezometer, porous cups, and at the drain outlet. The Cu concentrations were lowest in the surface (Ap) horizons (6.5–8.5 μg g− 1) and increased with depth to the clay-rich Bt horizons (10.5–12 μg g− 1), because of clay eluviation and associated Cu transport. The δ65Cu values significantly decreased from the surface (Ap = − 0.25 ± 0.07‰) to the deeper horizons, but showed no significant variation among the deeper horizons (− 0.41 ± 0.28‰) and no correlation with the clay content, indicating that clay eluviation did not significantly affect δ65Cu values. The isotopically heavier δ65Cu values in the Ap horizons can probably be explained by agricultural management practices like sludge application and fertilization. Close to the drain (position 0.6 m), Cu concentrations were depleted and the lighter Cu isotope was enriched (− 0.91 ± 0.15‰) in the uppermost part of the E&Bt horizon. We attribute this to the changing redox conditions, caused by the lowering of the water level close to the drain. Copper concentrations in black and ochre volumes were significantly higher than in pale-brown and white-grey volumes. The black volume had significantly higher δ65Cu values than the ochre volume indicating preferential sorption/occlusion of the heavy Cu isotope by Fe oxides. Enhanced clay eluviation in bulk soil close to the drain and in specific soil volumes did not affect δ65Cu values. Cu concentrations (2.1–14 μg L− 1) and δ65Cu (0.04–0.42‰) values in water samples showed no clear relation with redox changes along the trench perpendicular to the drain. The enrichment of the heavy Cu isotope in the solution samples (Δ65Cu(soil-solution) = − 0.61 ± 0.41) indicates that reductive Cu mobilization is not the main driver of Cu leaching, because this would preferentially mobilize isotopically light Cu. We conclude that the eluviation of the < 2 μm fraction, strongly controlled Cu concentrations, but had no discernible effect on δ65Cu values. The changing redox conditions did not seem to control Cu concentrations and the stable isotope distribution in most of the bulk soil, soil volumes and soil water. Instead, weathering, complexation of leached Cu, Cu application with fertilizers and sorption processes within the soil controlled its δ65Cu values

Sulfide enrichment along igneous layer boundaries in the lower oceanic crust: IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge

Reactive porous or focused melt flows are common in crystal mushes of mid-ocean ridge magma reservoirs. Although they exert significant control on mid-ocean ridge magmatic differentiation, their role in metal transport between the mantle and the ocean floor remains poorly constrained. Here we aim to improve such knowledge for oceanic crust formed at slow-spreading centers (approximately half of present-day oceanic crust), by focusing on specific igneous features where sulfides are concentrated. International Ocean Discovery Program (IODP) Expedition 360 drilled Hole U1473A 789 m into the lower crust of the Atlantis Bank oceanic core complex, located at the Southwest Indian Ridge. Coarse-grained (5–30 mm) olivine gabbro prevailed throughout the hole, ranging locally from fine- (&lt;1 mm), to very coarse-grained (&gt;30 mm). We studied three distinct intervals of igneous grain size layering at 109.5–110.8, 158.0–158.3, and 593.0–594.4 meters below seafloor to understand the distribution of sulfides. We found that the layer boundaries between the fine- and coarse-grained gabbro were enriched in sulfides and chalcophile elements. On average, sulfide grains throughout the layering were composed of pyrrhotite (81 vol.%; Fe1-xS), chalcopyrite (16 vol.%; CuFeS2), and pentlandite (3 vol.%; [Ni,Fe,Co]9S8), which reflect paragenesis of magmatic origin. The sulfides were most commonly associated with Fe-Ti oxides (titanomagnetites and ilmenites), amphiboles, and apatites located at the interstitial positions between clinopyroxene, plagioclase, and olivine. Pentlandite exsolution textures in pyrrhotite indicate that the sulfides formed from high-temperature sulfide liquid separated from mafic magma that exsolved upon cooling. The relatively homogenous phase proportion within sulfides along with their chemical and isotopic compositions throughout the studied intervals further support the magmatic origin of sulfide enrichment at the layer boundaries. The studied magmatic layers were likely formed as a result of intrusion of more primitive magma (fine-grained gabbro) into the former crystal mush (coarse-grained gabbro). Sulfides from the coarse-grained gabbros are Ir-Platinum Group Element-rich (PGE; i.e., Ir, Os, Ru) but those from the fine-grained gabbros are Pd-PGE-rich (i.e., Pd, Pt, Rh). Notably, the sulfides from the layer boundaries are also enriched in Pd-PGEs, and therefore elevated sulfide contents at the boundaries were likely related to the new intruding melt. Because S concentration at sulfide saturation level is dependent on the Fe content of the melt, sulfide crystallization may have been caused by FeO loss, both via crystallization of late-precipitating oxides at the boundaries, and by exchange of Fe and Mg between melt and Fe-bearing silicates (olivine and clinopyroxene). The increased precipitation of sulfide grains at the layer boundaries might be widespread in the lower oceanic crust, as also observed in the Semail ophiolite and along the Mid-Atlantic Ridge. Therefore, this process might affect the metal budget of the global lower oceanic crust. We estimate that up to ∼20% of the Cu, ∼8% of the S, and ∼84% of the Pb of the oceanic crust inventory is accumulated at the layer boundaries only from the interaction between crystal mush and new magma. © 2022 The Author