Low δ11B 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.

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.

Light Mo Isotopes of Post‐Collisional Ultrapotassic Rocks in Southern Tibet Derived From Subducted Indian Continental Crust

AbstractRecycling of molybdenum isotopes in continental subduction zones remains debated. In this contribution, we re‐visit the Mo isotope compositions of the Sailipu post‐collisional ultrapotassic rocks in the Himalaya‐southern Tibet orogen. These ultrapotassic rocks have very varying δ98/95Mo values of −0.66 to −0.07‰ and Mo/Ce ratios of 0.0008–0.005, which are lower than those of mid‐ocean ridge basalts (MORB; δ98/95Mo = −0.20 ± 0.06‰, and Mo/Ce = 0.03) and oceanic subduction‐related (i.e., mantle source involving fluids, residual slab, or oceanic sediments) magmatic rocks (e.g., modern arc lavas, Cenozoic OIB‐type basalts in eastern China and the central Mariana Trough basalts in the back‐arc basin, syn‐collisional andesitic rocks in southern Tibet). Combined with the light Mo isotopes of the Himalayan schists and gneisses, we suggest that the light Mo isotopic signature of the Sailipu ultrapotassic rocks is derived from subducted Indian continental crust. This is consistent with the extremely low δ11B (−17.4 to −9.7‰) and B/Nb (0.16–1) values and enriched Sr‐Nd‐Pb isotopes of the Sailipu ultrapotassic rocks. Thus, this study reveals the recycling of light Mo‐B isotopes during continental subduction and demonstrates that Mo‐B isotopes can effectively distinguish between continental and oceanic subduction.

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.

Different B[sbnd]Mo isotopic fractionation processes controlled by redox conditions in the subduction zone

The control of redox conditions on the geochemical recycling process in subduction zones is still poorly understood due to large uncertainties in oxygen fugacity (fO2) for subducted slabs. We present the first systematic geochemical and B-Mo-Sr-Nd-Hf-Pb isotopic data for continental arc basalts (CABs) and back-arc basalts (BABs) from Sumatra to investigate the relationships between oxygen fugacity and BMo isotopic fractionation processes during subduction. The Sumatran CABs and BABs yield high FeOT/MgO ratios and low K2O contents (0.48–1.42 wt%) and can be classified as tholeiitic. They have variable Sr-Nd-Hf-Pb isotopic compositions, consistent with the input of different amounts of melt from subducted terrigenous sediments into mantle sources. The BAB samples show much lower δ11B (−9.0‰ to −7.3‰) values than the CABs (δ11B = −7.0‰ to +0.17‰), reflecting significant B isotopic fractionation during multistage melting of subducted sediments. This is distinct from the similar δ98/95Mo values for the Sumatran CABs (−0.21‰ to −0.01‰) and BABs (−0.17‰ to −0.08‰), suggesting limited Mo isotopic fractionation during subduction. The Sumatran CABs show low V/Yb ratios, suggesting that melts from subducted sediments were possibly generated at fO2 values lower than FMQ + 1.5. The limited Mo isotopic fractionation could be induced by the relatively low fO2 during melting of subducted sediments, which would significantly decrease the mobility of Mo but have no influence on B. Alternatively, rocks from highly oxidized arcs, such as the Izu arc (e.g., >FMQ + 3), exhibit across-arc lightening trends for both B and Mo isotopes, which is consistent with enhanced Mo isotopic fractionation due to elevation of fO2 during subduction. Thus, the BMo isotopes of arc rocks could provide diagnostic indicators for distinct geochemical recycling processes controlled by changes in redox conditions in subduction zones.

Temperature-induced arsenic accumulation in groundwater from Pliocene aquifers of a semiarid continental basin

Contributions of geothermal reservoir and aquifer rock weathering to dissolved arsenic (As) concentrations remain poorly understood. This study investigated geochemical and isotopic characteristics of groundwater from the Quaternary aquifer (Quaternary-GW) and groundwater from the middle-Pliocene aquifer (Pliocene-GW) to reveal As enrichment mechanisms in geothermal groundwater. The results showed that As concentrations in Pliocene-GW were higher than in Quaternary-GW. δ7Li values varied in a wider range in Quaternary-GW than those in Pliocene-GW, and δ11B values were generally lower in Pliocene-GW than those in Quaternary-GW. Normally, δ7Li values in Pliocene-GW showed a negative correlation with the ratio of Li to Na+, and increased along the flow path, which indicated that the prolonged residence time was responsible for high δ7Li. Negative correlations between δ11B and HCO3–, CO32– and B in Pliocene-GW revealed the processes of B desorption and silicate weathering in the middle-Pliocene aquifer. The processes of B desorption and silicate weathering were enhanced by geothermal reservoir, which was evidenced from the negative correlation between δ11B and calculated reservoir temperature. Arsenic concentration increased with increasing δ7Li along the groundwater flow path in the middle-Pliocene aquifer, indicating that prolonged residence time of Pliocene-GW promoted As enrichment. Meanwhile, As concentrations were negatively correlated with δ11B in Pliocene-GW, illustrating that silicate weathering and desorption driven by geothermal conduction were important processes for As mobilization. In samples of Pliocene-GW with similar δ7Li and relatively low δ11B, As was mainly released from the original groundwater-rock interactions, especially desorption and silicate weathering facilitated by geothermal conduction. This study provides quantitative insights into As enrichment mechanisms in geothermal groundwater.

Tourmaline chemistry, boron, and strontium isotope systematics trace multiple melt–fluid–rock interaction stages in deeply subducted continental crust

The generation, transport, and recrystallization of slab-derived melts/fluids play a critical role in the deep recycling of elements in subduction zones. While boron (B) isotope systematics have been invoked as an important tracer of these processes, its behavior during metamorphic dehydration and partial melting of deeply subducted continental slabs, and the partitioning of B isotopes between minerals and melts/fluids is not fully understood. Here, we investigate these processes through an in situ study of the major, trace-element, and B-Sr isotope variations in different occurrences of tourmaline in migmatite from the Yuka terrane, North Qaidam orogen (China), which resulted from partial melting of a continental slab at different stages during subduction and exhumation. Based on textural and detailed high-resolution X-ray mapping studies, tourmaline was classified into four paragenetic generations (Tur-I, Tur-II, Tur-III, and Tur-IV). Dravitic Tur-I occurs in melanosomes and shows increasing Fe, Ca, and Ti contents from the core to the outer rim. In addition, it has relatively homogeneous Sr isotope values (0.7407–0.7416) and decreasing δ11B values (-3.8 to −8.6 ‰) and XMg (Mg/(Mg + Fe)) ratios, indicating formation in a rock buffered by an aqueous fluid during the prograde to peak metamorphism. Schorlitic Tur-II occurs within selvage zones between melanosomes and leucosomes, and yields high-Fe values and low δ11B (-13.5 to −10.9 ‰) and more variable 87Sr/86Sr (0.7343–0.7418) values, indicating crystallization in the presence of a hydrous melt external derived from breakdown of Fe-rich mineral(s) during partial melting of the subducted slab. Dravitic Tur-III formed in the matrix and also enveloped Tur-II. It shows homogeneous 87Sr/86Sr values (0.7411–0.7420) and decreasing δ11B values (-6.8 to −9.9 ‰) and XMg ratios as well as increasing Fe and Ti contents from core to outer rim. Formation of Tur-III reflects a transitional stage from hydrous melt to aqueous fluid during exhumation. Tur-IV in the leucosomes is essentially a schorl-dravite solid solution with small amounts of Ca. Its 87Sr/86Sr values (0.7402–0.7416) and δ11B values (-11.4 to −8.5 ‰) are intermediate between the respective values of Tur-II and Tur-III. The formation of Tur-IV likely results from interaction between melt and fluid and, based on its chronological sequence, and is interpreted to have formed during the exhumation stage of the Yuka terrane.Overall, the variable XMg ratios and δ11B values in the different generations of tourmalines are a consequence of the evolution of the melt/fluid at different depths within the deeply subducted slab. Decreasing δ11B values from Tur-I to Tur-II and Tur-III are controlled by the breakdown of different minerals during partial melting or metamorphic dehydration of the subducted slab, while the co-variations of the elemental geochemistry and B isotopic compositions of tourmaline reflect different depths of formation during subduction and exhumation of the lithosphere. These observations suggest tourmaline may serve as a useful tracer of multiple melt/fluid–rock interactions and of boron cycling in continental subduction zones. The heterogeneity of δ11B in melts/fluids at different depth levels of the continental subducted slab may also result in locally variable B isotope values in syn- and post-collisional magmas.

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.

Contribution of continental subduction to very light B isotope signatures in post-collisional magmas: Evidence from southern Tibetan ultrapotassic rocks

Understanding the subduction and recycling of continental crust is crucial for reconstructing the long-term evolutionary history of Earth's mantle and crust. The Himalaya-Tibet orogen is arguably the world's best natural laboratory for investigating these processes. Cenozoic post-collisional ultrapotassic volcanic rocks are common in the Lhasa block of southern Tibet and they can provide important clues to crust-mantle interaction in a well-characterized continental collision zone. Understanding the sources and processes that generated these lavas can contribute to our understanding of the thermal and compositional characteristics of the deep mantle and geodynamic processes in this region, including Indian continental subduction. In this contribution, we report Sr-Nd-Pb-O-B isotope and elemental chemistry data for post-collisional (13-11 Ma) ultrapotassic rocks from the TangraYumco-Xuruco rift (TYXR) in the Lhasa block. The arc-like trace-element signatures and markedly enriched Sr-Nd-Pb-O isotope compositions indicate that these mafic rocks originate from a mantle source containing recycled crustal components. Unlike pre-collisional (∼64 Ma) ultrapotassic rocks in the Lhasa block with arc-like B/Nb (0.85-1.89) and δ11B (−9.0 to −2.5‰) values, the TYXR post-collisional ultrapotassic rocks with much lower B/Nb (0.05-0.85) and δ11B (down to −20.5‰) values resemble Miocene K-rich volcanic rocks from western Anatolia. These western Anatolian rocks have been formed by either progressive dehydration of a stalled slab or deep-subducted continental crust. However, some TYXR samples have lower B/Nb ratios than MORB, consistent with a fluid-starved source. These markedly negative δ11B in conjunction with low B/Nb cannot be explained by the addition of melts from oceanic sediments, which generally yield lower B/Nb but higher δ11B values than MORB (e.g., the Armenia post-collisional mafic rocks). Given the low δ11B of Indian upper continental crust and its similar Sr-Nd-Pb isotopic signatures to the post-collisional lavas, it is clear that the post-collisional ultrapotassic rocks in the Lhasa block contain a significant component derived from subducted Indian continental crust. Combined with the temporal evolution of regional magmatism, tectonics and geophysical data, we propose that the break-off and tearing of subducted Indian continental slab induced post-collisional magmatism in the Lhasa block. Our case study provides evidence that continental subduction contributes to very light B isotope compositions of post-collisional magmas, which suggests that B isotopes have the potential to discriminate between oceanic subduction and continental subduction.

Boron isotopes of tourmalines from the central Himalaya: Implications for fluid activity and anatexis in the Himalayan orogen

The relationship between fluid activity and anatexis in the Himalaya is ambiguous in regard to whether it is dominated by fluid-saturated melting triggered by Lesser Himalayan Sequence (LHS) fluids or dehydration melting of Greater Himalayan Sequence (GHS) metasedimentary rocks. Boron isotopes are a sensitive probe for tracing the source of fluids and their role during partial melting. However, a lack of constraints on LHS fluid activity has limited our understanding of anatectic processes in the Himalayan orogen. Here we report the results of in situ tourmaline and muscovite B isotope analyses of a suite of samples from the LHS and GHS, including chlorite schist from the LHS, quartz–tourmaline rock from the Main Central Thrust Zone, and leucogranites, leucosomes, mica schist, and garnet amphibolite from the GHS. Tourmaline B isotopic compositions fall in the range of δ11B = −18.3‰ to −8.2‰, with tourmalines from leucogranites showing δ11B values of −17.2‰ to −8.2‰, from leucosomes of −11.8‰ to −10.7‰, from schists of −17.8‰ to −13.9‰, from quartz–tourmaline rock of −18.3‰ to −12.8‰, and from garnet amphibolite of −16.9‰ to −13.6‰. Muscovites from leucogranite and mica schist yield relatively low B isotopes of −23.3‰ to −17.2‰ and −22.4‰ to −15.4‰, respectively. Different source compositions, prograde devolatilization, tourmaline fractionation, fluid exsolution, and external fluid metasomatism are inferred to be the main mechanisms responsible for the varied B isotopes in this study. Our data suggest that the LHS is characterized by depleted 11B, with LHS-derived dehydration fluids calculated to have δ11B values of −15.2‰ to −12.0‰ at 600 °C, whereas metamorphic fluids in the GHS have δ11B values of −11.8‰ to −4.8‰. Thus, we propose that dehydration of LHS hydrous minerals produced relatively light-B fluids. Consequently, leucogranites derived from hydration melting triggered by LHS fluids exhibit light B isotopic compositions compared with those derived from dehydration melting in the GHS. To investigate the source of heavy B in GHS leucogranites, we modeled B isotopic fractionation during muscovite prograde devolatilization, with results showing that substantial fractionation (∆11B > 7‰) would occur during muscovite dehydration. Hence, B lost from micas could be a source of heavy B in the leucogranites. On the basis of these results, we propose a model whereby LHS hydrous metasediments underwent intense dehydration, with the resultant fluids migrating upward and triggering hydration melting on the edge of melt channel in the GHS. Hence, tourmalines that crystallized from the hydrated magmas inherited the low δ11B characteristic of the LHS fluids, whereas those derived from dehydration melting of micas in the interior of melt channel acquired higher δ11B values.

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.

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.

Interactions between deep formation fluid and gas hydrate dynamics inferred from pore fluid geochemistry at active pockmarks of the Vestnesa Ridge, west Svalbard margin

Seafloor seepage sites along the Vestnesa Ridge off west-Svalbard have been, for decades, a natural laboratory for the studies of fluid flow and gas hydrate dynamics along passive continental margins. The lack of ground truth evidence for fluid composition and gas hydrate abundance deep in the sediment sequence however prohibits us from further assessing the current model of pockmark evolution from the region. A MARUM-MeBo 70 drilling cruise in 2016 aims to advance our understanding of the system by recovering sediments tens of meters below seafloor from two active pockmarks along Vestnesa Ridge. We report pore fluid composition data focusing on dissolved chloride, stable isotopes of water (δ18O and δD), and the isotopic composition of dissolved boron (δ11B). From one of the seepage sites, we detect a saline formation water with two layers where gas hydrates were recovered. This saline formation pore fluid is characterized by elevated chloride concentrations (up to 616 mM), high B/Cl ratios (9 × 10−4 mol/mol), high δ18O and δD isotopic signatures (+0.6‰ and +3.8‰, respectively) and low δ11B signatures (+35.0‰), which collectively hint to a high temperature modification at great depths. Based on the dissolved chloride concentration profiles, we estimated up to 47% of pore space occupied by gas hydrate in the sediments shallower than 11.5 mbsf. The observation of bubble fabric in the recovered gas hydrates suggests formation during past periods of intensive gaseous methane seepage. The presence of these gas hydrates without associated positive anomalies in dissolved chloride concentrations however suggests that the decomposition of gas hydrate is as fast as its formation. Such a state of gas hydrates can be attributed to a relatively low methane supply transported by the saline formation water at present. Our findings based on pore fluid composition corroborate previous inferences along Vestnesa Ridge that fluids sustaining seepage have migrated from great depths and that the variable gaseous and aqueous phases through the gas hydrate stability zone control the distributions of authigenic carbonates and gas hydrates.

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.