Low δ11B Values Research Articles

A Sequential Leaching Protocol for δ11B and Trace Element Analyses of Multi‐Phase Carbonate Rocks

AbstractBoron geochemistry from biogenic carbonates offer valuable information about ocean pH and CO2 chemistry. However, application to geological carbonate deposits suffers from analytical difficulties in obtaining geochemical signals exclusively from the carbonate phase. Sequential leaching with reagents and acids has the potential to overcome such an issue. There is, however, little systematic investigation about the efficiency of sequential leaching in isolating carbonate‐associated boron from siliciclastic matrix. Here, we developed a sequential leaching protocol and applied it to methane‐derived‐authigenic‐carbonate samples. Using the leachate δ11B signatures, elemental composition, and mineral composition of residues, we show that the sequential leaching is able to improve the separation of boron from different phases. Buffered hydrogen peroxide removes organic matter and also some silicate phases resulting low δ11B values. Leaching with NH4Ac removes adsorbed boron though may also partially leach some carbonate phases. The first few leaching steps with diluted acetic acid dissolve carbonate phases. Depending on the sample type, these may also capture some remaining adsorbed boron from the preceding NH4Ac leaching. Once the adsorbed boron is completely removed, as indicated by the progressively higher δ11B values during the following acid leaching steps, representative carbonate composition can be derived. The accuracy of this protocol is demonstrated with leaching experiments using artificial deep sea coral carbonate and clay mixtures that give the representative carbonate‐associated δ11B within error of the pure coral value. Our results provide insights into characteristic signatures derived from silicates and organic matter that need to be considered in boron isotope analyses of impure marine carbonates.

Tourmaline composition probes serpentinite-derived fluid mobility in subduction zones

Serpentinite dehydration in subduction zones plays a pivotal role in geochemical cycling on Earth. A number of geochemical studies on arc magmas have elucidated the contributions of serpentinite-derived fluids to mantle sources. However, due to complex geological overprints during subduction zone processes, discerning serpentinite signatures in exposed metamorphic rocks within fossil subduction zones remains challenging. In this study we address these difficulties through in-situ investigations of tourmaline, the geochemistry of which reflects the host environment as well as potential fluid-induced processes. The presence of zonations in tourmaline makes it an excellent recorder of consecutive geological events. Integrated major and trace elements along with in-situ boron isotopes of tourmaline from the high-pressure Sopron area (Hungary) in the Eastern Alps were used to unravel fluid action sourced from serpentinite. Despite the presence of color zoning, tourmaline in the orthogneiss (Tur-G) has low XMg [Mg/(Mg + Fe)] of ca. 0.3–0.6 and δ11B values of around −11 ‰, along with variable trace element compositions. Petrological observations and geochemical analyses suggest that the inner domains of Tur-G are of igneous origin, while the outer rims are likely affected by subsequent metamorphic events. Tourmaline in metasomatized kyanite-quartzite (Tur-K) veins exhibits distinct geochemical zoning, and preserves metamorphic cores and fluid-induced rims. The inner domains of Tur-K display low XMg (<0.6), relatively high trace element concentrations and δ11B values of less than −10 ‰, whereas the overgrowths exhibit extremely high XMg values (>0.99), low trace element concentrations and high δ11B values reaching up to +21 ‰, clearly indicating the incorporation of serpentinite-derived Mg-11B-rich fluids. Through comparison with other metamorphic and metasomatic tourmalines in (ultra)high-pressure rocks globally, we establish that tourmaline with high XMg > 0.85 and δ11B values >0 ‰ may serve as an effective proxy for detecting serpentinite-derived fluids in subduction zones.

Deep recycling of volatile elements in the mantle: Evidence from the heterogeneous B isotope in intra-plate basalts

Volatiles in the mantle are crucial for Earth’s geodynamic and geochemical evolution. Understanding the deep recycling of volatiles is key for grasping mantle chemical heterogeneity, plate tectonics, and long-term planetary evolution. While subduction transfers abundant volatile elements from the Earth’s surface into the mantle, the fate of hydrous portions within subducted slabs during intensive dehydration processes remains uncertain. Boron isotopes, only efficiently fractionating near the Earth’s surface, are valuable for tracing volatile recycling signals. In this study, we document a notably large variation in δ11B values (−14.3‰ to +8.2‰) in Cenozoic basalts from the South China Block. These basalts, associated with a high-velocity zone beneath East China, are suggested to originate from the mantle transition zone. While the majority exhibit δ11B values (−10‰ to −5‰) resembling the normal mantle, their enriched Sr-Nd-Pb isotope compositions and fluid-mobile elements imply hydrous components in their source, including altered oceanic crust and sediments. The normal δ11B values are attributed to the dehydration processes. Remarkably high δ11B values in the basalts indicate the presence of subducted serpentinites in their mantle source. A small subset of samples with low δ11B values and radiogenic isotope enrichments suggests a contribution from recycled detrital sediments, though retaining minimal volatile elements after extensive dehydration. These findings provide compelling evidence that serpentinites within subducted slabs predominantly maintain their hydrous nature during dehydration processes in subduction zones. They may transport a considerable amount of water into deep mantle reservoirs, such as the mantle transition zone.

Magmatic evolution and sources of metals and fluids in the large granite-related Xiasai vein-type Ag–Pb–Zn(–Sn) deposit, northern Yidun terrane, SW China

The Xiasai Ag–Pb–Zn–(Sn) deposit in the northern Yidun terrane (eastern Tethys), hosted in Late Triassic metasandstones, is genetically associated with Late Cretaceous granites. We present chemical and Sr–Nd–Pb–Li–B isotope data of Late Cretaceous granites and their mafic microgranular enclaves (MME), late aplites, and Late Triassic metasedimentary rocks. The narrow range of 87Sr/86Sr99 (0.70627–0.70953) and εNd99 (–6.1 to –5.2) values of most Late Cretaceous granites reflects a mixed source with contributions from the mantle and Paleoproterozoic felsic crust. These granites have high SiO2 contents (>69 wt%) and strong depletions in Sr, Ba, and Eu. Major and trace element contents correlate with the fractionation index 1/TiO2, indicating that they experienced extensive fractionation. Local late aplite dikes were derived from similar source rocks, but seem to be younger.The Pb isotopic compositions reflect that the metal Pb was derived from the granite with variable contributions from the Late Triassic sedimentary rocks. Weakly and moderately altered metasandstones have similar δ7Li and δ11B values as most granites (δ7Li: 0.00 to 2.68 ‰; δ11B values: –14.93 to –12.36 ‰), indicating magmatic fluid sources. The magmatic fluids resulted in the initial enrichment of Sn, Cu, Pb, and Zn. Strong alteration of feldspar and biotite resulted in the metasandstones in negligible change of Li concentrations, significant loss of Sr, Na2O, CaO, MgO, and Fe2O3, but significant addition of B. This alteration lowered δ7Li from 1.01 to –3.65 ‰ and lowered δ11B from –9.43 to –19.18 ‰. The low δ7Li values and extremely low δ11B values reflect external fluids from the Late Triassic sedimentary wall rocks, which interacted with granites and mobilized metals (e.g., Sn, Ag, Pb, and Zn) from the granite and/or the sedimentary wall rocks. The location of the Xiasai deposit is controlled by regional NNW-trending faults induced by Late Cretaceous extension.

Fractionation of stable boron isotopes in alfisols: Application of a novel tool for tracing boron behavior in two soil profiles

Available boron (AB) and total boron (TB) in soil have distinct isotopic values, which are mainly influenced by the pH, organic matter and clay minerals, and can be used to explore the chemical behaviors of boron (B) in adsorption, weathering and eluviation processes. The B isotopic compositions of AB and TB in brown soil (slightly acidic) and cinnamon soil (neutral to slightly alkaline) profiles were investigated by multicollector inductively coupled plasma mass spectrometry. Variations in pH value and organic matter in the two soils directly affected the adsorption of B isotopes (dissolved 11B(OH)3 or 10B(OH)4-) in the aqueous solution, as shown by AB having an average negative δ11B value of −1‰ and a great range of −10.83‰ to +8.32‰ in the cinnamon soil. Furthermore, the long-term effects of adsorption and transportation processes were evidenced by the large range of δ11BTB values (-24.22‰ to +21.07‰) in the two soils. The topsoil layers with more organic matter and higher δ11B values indicated that B isotope fractionation occurred as a result of the input of rainfall, recycling of plant litter and the adsorption of organic matter and clay minerals. The lower δ11B values, the higher B contents and the element ratios in the underlying layers resulted from the weathering of different minerals and the adsorption of AB in the leaching process. This finding is supported by the relationship between AB and the clay/silt ratio, with more negative δ11B values being associated with finer clay particles via the adsorption of 10B(OH)4- in a weakly alkaline soil solution. The δ11B values in soils may be a useful tool for distinguishing geochemical cycling and processes and characterizing long-term processes that are difficult to assess.

Accretionary and collisional processes of the Brasiliano Orogeny recorded by the Tonian La Tuna ophiolite, Dom Feliciano Belt, Uruguay

Accretionary to collisional orogenies such as the Brasiliano Orogeny of eastern South America present the opportunity to distinguish between oceanic-continental (accretionary) and continental (collisional) processes. However, the multitude of geotectonic processes involved and the reduced degree of preservation of the accretionary stage restrain the interpretation of evidence. This study focuses on a metasomatite associated with the La Tuna Ophiolite in the southern Dom Feliciano Belt of the Uruguayan Shield, as these can provide crucial insights into the accretionary stage of the Brasiliano Orogeny. The metasomatite occurs as a meter-sized massive tourmalinite body surrounded by a meter-wide chloritite lens, both enclosed by the La Tuna serpentinite. Our SEM-EDS and EPMA data revealed that the serpentines from the La Tuna ophiolite are antigorite and lizardite, while chlorite was identified as clinochlore. Abundant inclusions of sheridanite chlorite in dravite tourmaline suggests that tourmaline overgrew chlorite. Dravite crystals revealed the presence of three diffuse zones with minor compositional variations, mainly enrichments in Ca and Fe towards the rims. The U-Pb LA-ICP-MS dating of metasomatic zircon yielded ages of 743.2 ± 3.4 Ma (n = 22) for the chloritite and 743.1 ± 2.4 Ma (n = 31) for the tourmalinite. High δ11B values (+12.1‰ to + 18.1‰) of tourmaline (LA-ICP-MS) suggest that the metasomatic event was caused by seawater-derived fluids in a fluid-dominated environment. The single metasomatic U-Pb age combined with the high δ11B values suggest that the metasomatites underwent more intense metasomatic events than those associated with the Bossoroca (−8.5‰ to +1.8‰) and the Ibaré ophiolites (+3.2‰ to +5.2‰) further north in the São Gabriel Terrane of the Dom Feliciano Belt. Nonetheless, a few spots in tourmaline located within fracture zones and in contact with chlorite displayed negative (c. −5‰) and low positive δ11B values (c. 2‰). These are interpreted as overprinting by crustal fluids, either during the tectonic emplacement of the La Tuna Ophiolite or during exhumation. The calculated TDM Hf model ages for zircon in tourmalinite and chloritite range from 0.74 to 1.09 Ga and their ɛHf(T) values range from +4.9 to +11.2, indicating derivation from depleted mantle sources. The data corroborate earlier results indicating that the associated La Tuna amphibolites have composition similar to N-MORB. Furthermore, the trace element composition of zircon in the metasomatites is consistent with an oceanic environment (U/Yb < 0.1). Geotectonically, the La Tuna metasomatites formed at 743 ± 2 Ma in an oceanic setting, most likely in a juvenile Adamastor Ocean basin. During the collisional stage of the Brasiliano Orogeny, the La Tuna Ophiolite was tectonically emplaced within the <666 Ma La Micaela Schist of the Paso del Drágon Complex. As such, this study on the La Tuna Ophiolite contributes direct evidence on the oceanic evolution leading up to the accretionary stage of the Brasiliano Orogeny and provides new constraints on the transition towards the collisional stage.

Magma and fluid sources in an intracontinental porphyry system: A case study of the Relin Mo–W(–Cu) deposit, southern Yidun terrane, SW China

The Relin Mo–W (–Cu) deposit in the northern Sanjiang area is bound to a Late Cretaceous intracontinental porphyry showing variable alteration. Here we present whole-rock chemistry and Sr, Nd, Pb, Li, and B isotope data to constrain the sources of ore-magmas and to understand how magmatic-hydrothermal processes mobilize the ore elements and alter the magmatic rocks. Chemical variations indicate the ore-bearing porphyries reflect two processes: fractional crystallization and late-magmatic alteration. Fresh and weakly altered porphyries are metaluminous to weakly peraluminous, showing I-type affinity. Chemical variation among these rocks can be explained by fractional crystallization. Most of these rocks show narrow ranges of isotopic compositions with –8.6 to –6.6 for εNd80, 0.70660 to 0.71028 for 87Sr/86Sr80, and high 207Pb/204Pb80 (15.57–15.66) and 208Pb/204Pb80 (39.21–39.51) values at 206Pb/204Pb80 values of 17.37 to 18.96. The chemical and isotopic compositions of these rocks indicate that the porphyries represent mantle melts that mixed with partial melts from the Paleoproterozoic crust. Fresh and weakly altered porphyries have uniform δ7Li (–2.3 to 1.5 ‰) and δ11B (–8.0 to –12.0 ‰). The strong sericite alteration of the porphyries resulted in the loss of Na2O and Sr (breakdown of feldspar) and the strong enrichment of the ore elements Cu, Mo, W, and Sn. Porphyries with varying degrees of alteration show large ranges of δ7Li (–6.0 ‰ to 11.4 ‰) and δ11B (–8.0 to –29.2 ‰). The anomalously high δ7Li and low δ11B values of the altered rocks indicate that the intrusions drove the flow of external fluids that altered the magmatic rocks and leached the ore elements W, Mo, Cu, and Sn from the porphyries and possibly the local wall rocks.

Boron isotope behavior during metamorphic dehydration and partial melting of continental crust: Constraints from tourmaline in metapelites and leucocratic dikes, Himalayan orogen

Crustal rocks in collisional orogens often underwent the metamorphic transition from dehydration to anatexis in response to an increase of geothermal gradients, but it remains obscure how the two types of metamorphism influenced each other. This issue can potentially be addressed by an integrated study of mineralogy and geochemistry for tourmaline in protoliths and their anatectic products. In this paper, we present new results from comprehensive analyses of major-trace elements and B isotopes in tourmaline together with whole-rock Sm–Nd isotopes in well-characterized metapelites and leucocratic dikes from the Himalayan orogen. The leucocratic dikes show whole-rock εNd(t) values of −15.7 to −15.4, falling within an εNd(t) range of −17.7 to −13.8 for the metapelites. In the metapelites, metamorphic tourmaline (Tur-M) is dravitic and has relatively low δ11B values of −12.0 to −9.0‰. As the metamorphic grade increases, the δ11B values of Tur-M decrease, which is linked to progressive metamorphic dehydration. In the leucocratic dikes, three types of tourmaline are identified by different petrological and geochemical criteria. The tourmaline cores (Tur-LC) are schorlitic and have relatively high δ11B values of −8.7 to −7.7‰, and this type is ascribed to an anatectic origin. The tourmaline rims (Tur-LR) corroding the Tur-LC are dravitic and have relatively high δ11B values of −9.6 to −7.6‰, and this type is attributed to a fluid origin. Another type of anatectic tourmaline (Tur-L) occurs as grains with homogeneous oscillatory zoning that are dravitic and have slightly higher δ11B values of −10.4 to −5.9‰. The anatectic Tur-LC in the leucocratic dikes has higher δ11B values than the metamorphic Tur-M in the metapelites. We attribute this difference to the addition of external fluids with high δ11B values to the metapelite source at the overlying crustal levels. The influxed fluids most likely originated from metamorphic dehydration of lithologically similar metapelites at deeper crustal levels, as evidenced by the Mg-rich characteristics of Tur-L and Tur-LR. The metamorphic fluids would ascend into shallow crustal levels through faults and shear zones, which could not only trigger fluid-fluxed (hydration) melting of the metapelites but also result in fluid activity in later stages. This is further verified by phase equilibrium modelling and model calculation of B geochemical behavior during both metamorphic dehydration and partial melting. On this basis, a spatiotemporally coupled dehydration-hydration melting mechanism is proposed to link the early dehydration metamorphism to the later anatectic metamorphism in the collisional orogen.

Geothermal input significantly influences riverine and oceanic boron budgets

The evolution of boron isotope compositions of seawater (δ11Bsw) over the Cenozoic has important implications for reconstructions of atmospheric CO2 and is tightly linked to boron input from the Himalaya-Tibetan Plateau. However, controls on evolution inδ11Bsw remain elusive. We report geochemical measurements of the Yarlung Tsangpo River draining the Tibetan Plateau and observe exceptionally high riverine boron concentrations and extremely low δ11B values. Calculation indicates that >50% of riverine boron is sourced from geothermal waters. Combined with global datasets, we show that global riverine boron input to the ocean is largely influenced by geothermal input. Mass-balance calculations indicate that an averaged 1.5-fold decrease in global geothermal inputs is sufficient to introduce 3‰ increase in Cenozoic δ11Bsw. Therefore, geothermal waters significantly affect global riverine and thus oceanic boron budgets. The increased δ11Bsw since the Cenozoic may be resulted partly from declining global geothermal activity.

The mobilization of boron and lithium in the hydrothermal system of the ∼3.48 Ga Dresser caldera: A stable isotope perspective

Voluminous hydrothermal circulation in the ∼3.48 Ga Dresser caldera produced zoned alteration haloes around major fluid pathways. Specifically, in the North Star Basalt (the footwall) hydrothermal alteration decreases with increasing distance from the margins of hydrothermal silica±barite veins, changing from argillic (i.e., kaolinite-quartz) to phyllic (i.e., illite-quartz), and then to either propylitic (i.e., chlorite-albite-epidote) or actinolitic (i.e., actinolite-albite-chlorite-epidote) assemblages at distal positions. This alteration series developed through hydrolysis reactions at decreasing acidic conditions, which promoted variable degrees of mobilization of major and trace elements in the hydrothermal fluids.In this study, we characterize the B and Li concentrations and the relative stable isotope ratios of the altered North Star Basalt to provide new insights into the hydrothermal processes that generated these complex alteration patterns. In particular, we focus on the magnitude and timing of the inputs from different fluid reservoirs, the mobilization of B and Li in the hydrothermal system of the Dresser caldera and their influence on the near-surface environment where ancient stromatolites were forming.Actinolitic and propylitic samples have homogeneous δ11B and δ7Li values (−17 to −15‰ and +1 to +3‰, respectively) that are associated with lower B and Li concentrations in actinolitic (4 to 11 ppm and 46 to 128 ppm, respectively) relative to propylitic samples (9 to 25 ppm and 176 to 305 ppm for B and Li, respectively). This pattern suggests that at least the propylitic assemblage interacted with alteration fluids of largely magmatic origin, thus excluding the possibility of an early seawater contributions to the hydrothermal system. Conversely, phyllic samples have distinctly higher B contents (57 to 99 ppm) and δ7Li values (+7 to +8‰), but have much lower δ11B values (−19 to −28‰) and Li contents (5 to 6 ppm) relative to actinolitic and propylitic samples. We argue that these striking differences are attributable to mineralogical and temperature controls that promoted the preferential removal of 6Li during chlorite dissolution and incorporation of 10B into illite. Furthermore, the highly negative δ11B values in phyllic samples suggest that the additional B incorporated into the altered North Star Basalt originated from the magma chamber underlying the Dresser caldera rather than from seawater or other crustal sources. The most intensely altered (argillic) samples have moderate B and Li contents (2 to 43 ppm and 41 to 71 ppm, respectively) and highly variable δ11B and δ7Li values (−24 to −0.4‰ and −10 to +11‰, respectively). These signatures are interpreted to represent the input of ‘external’ fluids into the upper portions of the hydrothermal system during periodic crack-seal events that allowed the influx of restricted amounts of seawater and, possibly, meteoric fluids.Overall, this study provides a novel non-traditional stable isotope perspective on the evolution of the hydrothermal system of the Dresser caldera that adds richness to our understanding of this complex environment. Both B and Li stable isotopes support a prominent magmatic contribution to the hydrothermal fluid budget, reinforcing the interpretation of a hot-spring origin of the tourmaline-bearing layers associated with the stromatolites of the North Pole Chert.

Stylasterid corals build aragonite skeletons in undersaturated water despite low pH at the site of calcification

Anthropogenic carbon emissions are causing seawater pH to decline, yet the impact on marine calcifiers is uncertain. Scleractinian corals and coralline algae strongly elevate the pH of their calcifying fluid (CF) to promote calcification. Other organisms adopt less energetically demanding calcification approaches but restrict their habitat. Stylasterid corals occur widely (extending well below the carbonate saturation horizon) and precipitate both aragonite and high-Mg calcite, however, their mode of biocalcification and resilience to ocean acidification are unknown. Here we measure skeletal boron isotopes (δ11B), B/Ca, and U/Ca to provide the first assessment of pH and rate of seawater flushing of stylasterid CF. Remarkably, both aragonitic and high-Mg calcitic stylasterids have low δ11B values implying little modification of internal pH. Collectively, our results suggest stylasterids have low seawater exchange rates into the calcifying space or rely on organic molecule templating to facilitate calcification. Thus, despite occupying similar niches to Scleractinia, Stylasteridae exhibit highly contrasting biocalcification, calling into question their resilience to ocean acidification.

Fluid sources in basement-hosted unconformity–uranium ore systems: tourmaline chemistry and boron isotopes from the Patterson Lake corridor deposits, Canada

The Patterson Lake corridor is a new uranium district located on the southwestern margin of the Athabasca Basin. Known resources extend almost 1 km below the unconformity in graphite- and sulfide-bearing shear zones within highly altered metamorphic rocks. Despite different host rocks and greater depths below the unconformity, alteration assemblages (chlorite, illite, kaolinite, tourmaline and hematite), ore grades and textures are typical of unconformity-related deposits. This alteration includes at least three generations of Mg-rich tourmaline (magnesio-foitite). The boron isotopic composition of magnesio-foitite varies with generation: the earliest generation, which is only observed in shallow samples from the Triple R deposit (Tur 1), contain the heaviest isotopic signature ( δ 11 B ≈ 19–26‰), whereas subsequent generations (Tur 2 and Tur 3) yield lighter and more homogeneous isotopic signatures ( δ 11 B ≈ 17.5–19.9‰). These results are consistent with precipitation from low-temperature, NaCl- and CaCl 2 -rich brine(s) derived from an isotopically heavy boron source (e.g. evaporated seawater) that interacted with tourmaline and silicates in the basement rocks and/or fluids derived from depth (with low δ 11 B values). The lower δ 11 B values in paragenetically later magnesio-foitite reflect greater contributions of basement-derived boron over time, whereas minor compositional variations reflect local metal sources (e.g. Cr, V, Ti) and evolving fluid chemistry (decreasing Na and Ca, increasing Mg) over time. The δ 11 B and chemical variation in magnesio-foitite over time reinforce the strong interactions with basement rocks in these systems while supporting incursion of basinal brines well below the unconformity contact. Supplementary material: Complete analytical dataset including reference materials are available at https://doi.org/10.6084/m9.figshare.c.5727555 Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Tourmaline boron isotopes trace metasomatism by serpentinite-derived fluid in continental subduction zone

Subduction of low-density continental crust to subarc depths is generally associated with preceding subduction of the oceanic slab. However, the original oceanic signature is often overprinted by subsequent fluid metasomatism from the subducting continental crust. As a result, it is generally difficult to trace the geochemical processes of previous oceanic subduction in collisional orogens. This issue can be potentially resolved by applying B isotopes to metamorphic rocks from continental subduction zones. Here a combined study of whole-rock geochemistry and in situ tourmaline B isotopes was carried out for coesite-bearing whiteschist and phengite schist, as well as country rock metagranitoids from the Dora-Maira Massif in the Western Alps. While all these metamorphic rocks have a similar protolith of Permian granites, whiteschist and phengite schist experienced Mg-rich fluid metasomatism during the continental subduction, evidenced by their much higher MgO contents than the metagranitoids. Tourmaline in the metagranitoid (Tur-G) is schorlitic (XMg = 15–55) with low δ11B values from −13 to −6‰, and is consistent with a magmatic origin. In contrast, tourmaline in metasomatic rocks (Tur-S) is mainly dravitic with the highest XMg worldwide (XMg = 90–98) and high δ11B values of −5 to +1‰. Integrated with whole-rock geochemistry and previous studies, Tur-S is interpreted to grow during the infiltration of external fluids that were highly enriched in MgO and relatively enriched in 11B. According to the B isotope compositions of precursor tourmalines, it is estimated that the external fluids had significantly higher δ11B values than +2.4‰. Based on B isotope compositions of both Tur-S and metasomatic fluids, our quantitative modeling suggests that the metasomatic fluids most likely originated from the mantle wedge serpentinite that was formed during the preceding oceanic subduction stage. Therefore, tourmaline B isotopes in the ultrahigh pressure metamorphic continental crust can be used to trace preceding fluid metasomatism at the interface between oceanic slab and mantle wedge. The mantle wedge serpentinite plays an important role in modifying the geochemical composition of deeply subducted supracrustal rocks and probably also the mantle sources of arc magmas.

Boron release and transfer induced by phengite breakdown in subducted impure metacarbonates

White mica is an important reservoir of boron (B) in subducted metasedimentary rocks. To quantify the effect of phengite breakdown on B release and transfer, we investigated the replacement processes of phengitic mica and associated element redistribution in exhumed ultra-high pressure (UHP) impure marbles from the Dabie terrane, China. Three types of microstructures, which are related to the liberation, transport, and precipitation of B, are recognized: (1) Type-I is the pseudomorphic replacement of phengite by the assemblage of biotite + plagioclase ± quartz ± epidote. The assemblage maintains a flaky shape of pre-existing phengite, which only rarely occurs as a relict phase in the product assemblages. Phase equilibria modelling indicates that the conditions of replacement reactions are 1.2–1.3 GPa at ~600 °C, with a relatively low XCO2 of 0.04–0.16. (2) Type-II is characterized by the local occurrence of fine-grained tourmaline (Tur-F) along the outer margin of Type-I texture domains, which represents the in situ precipitation of B released by the replacement reactions. (3) Type-III texture involves the irregularly shaped intergrowth of tourmaline (Tur-In) + quartz in the calcite matrix, isolated from the Type-I and Type-II texture domains. The Type-III minerals are in equilibrium with diopside and plagioclase and most likely represent the precipitation from a “transported”, B-bearing fluid.Mineral trace element analyses indicate that relict phengite in Type-I texture has significantly higher B contents (284–464 μg/g) than all of the product minerals (2–26 μg/g). A mass balance estimate indicates that the majority (>90%) of B in the reactants is lost during the process of phengite replacement. The calculations in terms of mineral-fluid partition indicate that the fluid involved in the replacement reactions has B contents of 1893–6858 μg/g. In situ B isotopic analyses indicate that Tur-F and Tur-In have a similar variation range of δ11B (+4 ~ +7‰). Both types of low-pressure tourmaline have lower δ11B values than the previously reported eclogite-facies tourmaline (reaching +15‰) from phengite-bearing marbles in the same study area. This result indicates two episodes of tourmaline growth and B transfer. Thus, a two-stage model of B loss from phengite during slab exhumation is proposed. Our results reveal that the breakdown of white mica in exhumed metasedimentary rocks can cause a strong release of B, which would largely influence the B budget at convergent plate margins.

Magmatic-hydrothermal fluids leaching older seafloor exhalative rocks to form the IOCG deposits of the Carajás Province, Brazil: Evidence from boron isotopes

The notable group of iron oxide-copper–gold (IOCG) deposits at the Carajás Province (Amazonian Craton, Brazil) contain remarkable copper reserves due to a complex Neoarchean metallogenetic evolution. The overlay of diachronic hydrothermal-mineralizing events is revealed by two clusters of boron isotope compositions of hydrothermal tourmaline obtained in IOCG deposits (Salobo, Igarapé Bahia, GT-46, Grota Funda and Furnas) with (i) high δ11B (+8.2 to +17.2‰) and (ii) low δ11B (+3.4 to +9.8‰) values. The isotopically heavy boron signatures, recorded in tourmaline cores, are inherited from the volcano-sedimentary sequences of the ca. 2.76 Ga Itacaiúnas Supergroup, which includes exhalative tourmaline and spilites formed in extensive exhalative systems prior to the IOCG formation. During the main IOCG event at ca. 2.55 Ga, the regional circulation of magmatic-hydrothermal boron-rich fluids originated tourmaline rims and new crystals with lower δ11B values. We describe the critical role of an older volcano-sedimentary as a source for boron and likely metals leached by magmatic-hydrothermal fluids to form the IOCG deposits of the Carajás Province.

Chemical and boron isotope composition of tourmaline from pegmatites and their host rocks, Sierra de San Luis, Argentina

ABSTRACT We report chemical and B-isotope analyses of tourmaline from Ordovician S-type granites, an aplite, LCT-type (lithium-cesium-tantalum) pegmatites, and metamorphic rocks of the Conlara Metamorphic Complex (CMC) in Sierra de San Luis, Argentina. For comparison, tourmaline from three LCT pegmatites in the adjacent Pringles Metamorphic Complex was also studied. Metamorphic tourmaline from the CMC has intermediate schorl–dravite compositions, with variable Fe# [100 * Fe/(Fe + Mg)] from 32 to 79. The δ11B values range from –14.8 to –8.9‰, which are typical values for continental metasediments and granites, ruling out a marine origin for the tourmalinite protoliths. Tourmaline from the S-type granites and aplite is more homogeneous, with Fe# from 48 to 60. The δ11B range (–14 to –9.8‰) of granitic tourmaline is within that of the metamorphic tourmaline, supporting the idea of boron recycling in the CMC during partial melting to form the granites. Tourmaline from CMC-hosted pegmatites is compositionally diverse and we distinguished three groups based on Fe#: Group 1: 42 to 50, Group 2: 50 to 62, and Group 3: 62 to 93. Regardless of strong variations in Fe#, tourmaline from all pegmatites in the CMC has δ11B values from –10.3 to –7.8‰. These values overlap with the range of related granites but are about 2 permil higher, which we attribute to crystallization of 10B-enriched minerals (mica and tourmaline) in the evolved magma from which the pegmatites formed. Pegmatites from the Pringles Metamorphic Complex contain tourmaline with a similar overall range of Fe# (45 to 84) as in the CMC but lower δ11B values (–13.2 to –11.2‰).

External fluids cause alteration and metal redistribution in the granite-hosted Tangziwa Sn[sbnd]Cu deposit, Gejiu district, China

The granite-hosted Tangziwa SnCu deposit in the Gejiu district shows variable alteration of the granite with associated mineralization. In this paper, we used whole rock chemistry mass balance calculations and Sr, Nd, Pb, Li, and B isotope data to constrain the source of metals and fluids in different alteration stages. The δ7Li and δ11B values of unaltered and weakly altered (Type I) granite samples fall in narrow ranges of −0.70 to 3.61‰ and −14.87 to −12.82 ‰, respectively, which is typical for granites. The Li and B isotopic composition of moderately (Type II) to strongly altered (Type III-1 and Type III-2) granite samples are shifted toward higher δ7Li (up to 13.61‰) and lower δ11B values (as low as −31.32‰). These extreme values reflect the addition of Li and B via an external fluid. Petrographic observations and isocon mass balance calculations demonstrate that early stage, magmatic fluids altered biotite and plagioclase to muscovite and clay minerals, respectively, and led to loss of Ca, Na, Fe, and Cs. The later stage, external fluids caused the destruction of feldspar and the precipitation of calcite, anhydrite/gypsum, fluorite, and sulfide minerals (pyrite, chalcopyrite, arsenopyrite, and stannite). This alteration is associated with an increase in the contents of Ca, S, Fe, Co, Cu, Sb, Cd, and Sn and a reduction of the contents of Pb, Sr, Cs, and LREEs. The high contents of Cu and S, which are not typically abundant in granite, could be added by the external fluids characterized by high δ7Li and low δ11B. The weakly and moderately altered granites have lower Sn contents than the unaltered granite. This indicates that late magmatic or external fluids may have leached Sn from weakly and moderately altered granite and that sulfide minerals in the strongly altered granites eventually scavenged this mobilized Sn.

Molybdenum and Boron Isotopic Compositions of Porphyry Cu Mineralization‐Related Adakitic Rocks in Central‐Eastern China: New Insights Into their Petrogenesis and Crust‐Mantle Interaction

AbstractThe voluminous Late Mesozoic porphyry ore‐related adakitic magmatism in central‐eastern China is crucial to understand the deep geodynamic process of the region and controls on porphyry deposit formation globally. Here, we present a novel study of whole‐rock Mo‐B isotopes on the mineralization‐related adakitic rocks from the Dexing (located in the interior of South China and associated with giant porphyry Cu deposits), and Middle‐Lower Yangtze River (MLYR) (host to one of the largest porphyry Cu‐Au deposit belts) areas in central‐eastern China. The Dexing adakitic rocks have δ98/95Mo ranging from −0.31‰ to −0.01‰ and δ11B from −17.65‰ to −11.49‰. The adakitic rocks from the MLYR area have δ98/95Mo values of −0.5‰ to 0.2‰ and δ11B values of −14.48‰ to −9.53‰. The δ98/95Mo values of both the Dexing and MLYR samples negatively correlate with the La/Yb and La/Sm ratios. Together with the high Mg#, Cr, and Ni contents, these results indicate a process of melt‐mantle interaction resulting in hybridization of initial adakitic melts with low δ98/95Mo and mantle‐derived melts/components with high δ98/95Mo. These adakitic rocks have distinct Mo isotope systematics compared with those of oceanic slab‐derived adakites and MORB‐type eclogites. Their relatively low δ11B values and the lack of correlation between δ11B and B contents support a mafic lower crust source for the initial adakitic melts. We thus suggest that these adakitic rocks were generated by partial melting of delaminated lower crust followed by interaction with the deep mantle, and that melt‐mantle interaction may facilitate Cu‐Mo‐Au mineralization.

Slab-controlled progressive evolution of the Kudi back-arc ophiolite in response to the rollback of the Proto-Tethys oceanic slab, in Western Kunlun, NW Tibetan Plateau

Slab-derived components can significantly diversify subduction-related magmatism, for example, fore-arc ophiolites evolving from normal mid-ocean ridge basalts to fore-arc basalts and then to boninites. Here we show a progressive evolution of back-arc ophiolites controlled by slab-derived components but different from fore-arc ophiolites through studying the Kudi back-arc ophiolite in Western Kunlun, NW Tibetan Plateau. Our results reveal that the Kudi ophiolite was formed at 516–512 Ma in a back-arc setting in response to the rollback of the Proto-Tethys oceanic slab. The back-arc setting is supported by geochemistry of basalts (e.g., low Th/Nb and V/Ti ratios), low melting pressure (<10 kbar) revealed by amphibole geobarometer and low δ11B values (−19.1 to −8.1‰) of the mantle units. In such a scenario, the early-stage depleted Kudi basalts are characterized by depletions in light rare earth elements and enrichments in fluid-mobile elements, probably resulting from decompression melting of the primary back-arc mantle slightly modified by slab-derived fluids. Subsequently, partial melting of the back-arc mantle enriched by progressive addition of slab-derived sediments resulted in the formation of residual harzburgites and corresponding primary magmas that evolved into dunites, gabbros, quartz diorites and enriched pillow lavas. Additionally, both the residual harzburgites and cumulated dunites experienced minor impregnation of melts, without significant interstitial melt-crystal reactions. Compared with the early-stage depleted Kudi basalts, the late-stage enriched pillow lavas are characterized by enrichments in light rare earth elements and depletions in high field strength elements, typical arc affinities, confirming the addition of slab-derived sediments. Compared with typical fore-arc ophiolitic series, it is possible that there is a progressive evolution in back-arc basins in which a “new” enriched mantle source (corresponding arc basalts) modified by slab-derived sediments displaces an “old” depleted mantle source (corresponding basalts characterized by enrichments in fluid-mobile elements) modified by slab-derived fluids.

Crustal maturation through chemical weathering and crustal recycling revealed by Hf–O–B isotopes

Juvenile continental crust is dominantly formed at intra-oceanic arcs via subduction zone magmatism. However, it remains unclear how basaltic oceanic arcs convert to granodioritic continental compositions. Here we present zircon U–Pb and Hf–O isotope data, whole-rock B isotope compositions, combined with a synthesis of over 1100 geochemical analyses from magmatic rocks that span a wide range of emplacement ages (∼490–270 Ma), from the Junggar intra-oceanic arc of the southern Central Asian Orogenic Belt (CAOB). Geochemical data show that the Junggar evolved from a juvenile oceanic arc composition (low SiO2, K2O and Rb, and high MgO contents) to one closer to continental crust (high SiO2, K2O and Rb, and low MgO contents) at ca. 300 Ma. All samples show very high and uniform Hf isotope ratios with εHf(t) values from +10.6 to +14.4. Zircon δ18O values and whole-rock δ11B values, however, are highly variable. The Silurian – Carboniferous (pre-300 Ma) rocks have distinctly lower zircon δ18O values ranging from 5.16 to 6.72‰ with an average of 5.78‰, and high δ11B values (−7.5–+12.2‰), suggesting that they were derived from the asthenospheric mantle wedge, which supports the genesis of primitive intra-oceanic crust during that time interval. In contrast, the Early Permian (post-300 Ma) rocks display much higher zircon δ18O values (8.24 to 10.29‰) and lower δ11B values (−9.0 to −12.2‰), combined with the presence of Carboniferous inherited zircons, which requires a source component comprising young, weathered, volcanogenic sediments from the Carboniferous Junggar intra-oceanic arc. The Early Permian rocks were produced by low degrees of partial melting of all sources (ca. 10%) and element and Hf–O–B isotope mixing calculations indicate a contribution of more than ∼50% from weathered sediments in those sources. Thus, the evolution of the Junggar segment from a primitive basaltic intra-oceanic arc toward a young continent was related to the recycling of crustal residues of chemical weathering and the younger magmatism was generated by crustal melting of these weathering products after they were buried to lower crustal depths. Our study highlights that an intra-oceanic arc's own chemical weathering history promotes its transformation into continental crust during collisional events and clarifies the relationship between continental crust formation and intra-oceanic arcs through time.