Boron isotope record of peak metamorphic ultrahigh-pressure and retrograde fluid-rock interaction in white mica (Lago di Cignana, Western Alps)

This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid–rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 µg/g B) occur in peak metamorphic phengite from a garnet–phengite quartzite. The B isotopic composition is variable (d11B?=?-?10.3 to?-?3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid–rock interaction during peak metamorphic conditions. This interpretation is supported by fluid–rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in d11B and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive d11B values (d11B?=?20?±?5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 µg/g and d11B values that cluster around 0% (d11B?=?-?5.0 to?+?3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid–rock interaction with low-to-moderate (<?3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones

A limited role for metasomatized sub-arc mantle in the generation of boron isotope signatures of arc volcanic rocks

Metasomatized sub-arc mantle is often regarded as one of the mantle reservoirs enriched in fluid mobile elements (FME, e.g. B, Li, Cs, As, Sb, Ba, Rb, Pb) which, when subject to wet melting, will contribute to the characteristic FME-rich signature of arc volcanic rocks. Evidence of wet melts in the sub-arc mantle wedge is recorded in metasomatic amphibole-, phlogopite- and pyroxene-bearing veins in ultramafic xenoliths recovered from arc volcanoes. Our new B and δ11B study of such veins in mantle xenoliths from Avachinsky and Shiveluch volcanoes, Kamchatka arc, indicates that slab-derived FME, including B and its characteristically high δ11B, are delivered directly to a melt that experiences limited interaction with the surrounding mantle before eruption. The exceptionally low B contents (from 0.2 to 3.1 µg g-1) and low δ11B (from -16.6 to +0.9 ‰) of mantle xenolith vein minerals are, instead, products of fluids and melts released from the isotopically light subducted and dehydrated altered oceanic crust (AOC) and, to a lesser extent, from isotopically heavy serpentinite. Therefore, melting of amphibole-, and phlogopite-bearing veins in metasomatized mantle wedge cannot alone produce the characteristic FME geochemistry of arc volcanic rocks, which require a comparatively large, isotopically heavy and B-rich serpentinite-derived fluid component in their source

Volatiles and Intraplate Magmatism: a Variable Role for Carbonated and Altered Oceanic Lithosphere in Ocean Island Basalt Formation

Recycling of material at subduction zones has fundamental implications for melt composition and mantle rheology. Ocean island basalts (OIBs) sample parts of the mantle from variable depths that have been diversely affected by subduction zone processes and materials, including the subducted slab, metasomatising melts and fluids. Resultant geochemical differences are preserved at a variety of scales from melt inclusions to whole rocks, from individual islands to chains of islands. Here we examine a global dataset of ocean island basalt compositions with a view to understanding the connection between silica-saturation, olivine compositions, and halogens in glass and olivine-hosted melt inclusions to reveal information regarding the mantle sources of intraplate magmatism. We find that minor elements incorporated into olivine, although informative, cannot unambiguously discriminate between different source contributions, but indicate that none of the OIB analysed here are derived solely from dry peridotite melting. Nor can differences in lithospheric thickness explain trace element variability in olivine between different ocean islands. We present new halogen (F, Cl, Br/Cl, I/Cl) data along with incompatible trace element data for the global array and encourage measurement of fluorine along with heavier halogens to obtain better insight into halogen cycling. We suggest that Ti-rich silica-undersaturated melts require a contribution from carbonated lithosphere, either peridotite or eclogite and are an important component sampled by ocean island basalts, together with altered oceanic crust. These results provide new insights into our understanding of mantle-scale geochemical cycles, and also lead to the potential for the mantle transition zone as an underestimated source for observed volatile and trace-element enrichment in ocean island basalts

Boron Isotopes as a Tracer of Subduction Zone Processes

This chapter reviews recycling of boron (B) and its isotopes in subduction zones. It discusses the profound changes in B concentrations and B isotope ratios of various materials involved in convergent margin evolution, in particular highlighting the fate and evolution of progressively dehydrating subducting slabs and the behavior of B during burial and subsequent metamorphism. We review various models used to parameterize these complex and often poorly understood processes and critically evaluate the available data from the literature. We proceed by reviewing B isotope data from mafic arc volcanic rocks and explore systematic variations with subduction zone geometry as well as familiar geochemical tracers of subduction processes. Finally, the role of serpentinisation in the mantle wedge is discussed in the light of new geochemical and petrological insights on the importance of serpentinites and subduction erosion. We provide recommendations for further research on B isotopes in subduction zones and directions where we think this exciting stable isotope tracer may make the biggest impact

The behaviour of nitrogen during subduction of oceanic crust: insights from in situ SIMS analyses of high-pressure rocks

Understanding the Earth’s geological nitrogen (N) cycle requires an understanding of how N behaves during dehydration of subducted crust. We present the first in situ measurements of N in silicate minerals by secondary ion mass spectrometry, focusing on high pressure rocks representing subducted oceanic crust. We investigate the distribution of N between mineral phases, and combine analyses of N with other trace and major elements in order to constrain the behaviour of N during fluid-rock interaction. The data confirm that white mica (phengite, paragonite) is the primary host for N, containing up to 320 µg/g, whereas minerals including clinopyroxene, amphibole and epidote contain < 5 µg/g N. Chlorite can also contain N (up to 83 µg/g) and may play a previously unrecognised role in subduction zone N cycling. Bulk rock N concentrations estimated from mineral N concentrations and mineral modes are consistent with N concentrations measured by bulk combustion, which confirms that most N is hosted within silicate minerals and not along grain boundaries or in fluid inclusions. Bulk rock N contents correlate with K2O (N/K2O = 19.3 ± 2.0). Using N/K2O ratios and the average K2O of altered oceanic crust, the flux of N subducted in oceanic crust is estimated to be 0.6 − 2.4 × 1011 21 g/yr, which is consistent with but at the lower end of previous estimates. The data were also used to investigate the behaviour of N during fluid-rock interaction. Open system fluid-rock interaction modelling was used to model the evolution of N, B and Li contents during fluid-rock interaction in phengite from a garnet-phengite quartzite. By comparison to data for B and Li, the phengite-fluid partition coefficient for N was estimated to be 0.1–1.5. Separately, the growth of paragonite during fluid-rock interaction in a blueschist was shown to sequester N from phengite and limit bulk N loss to the fluid. The stability of white mica during fluid-rock interaction is therefore critical in controlling the behaviour of N. Nitrogen addition from sediment-derived fluids appears to be an important process in subduction zone rocks. Mafic crust can act as a sink for this N if white mica is stable. This work provides the first natural constraints on the fluid-mineral partitioning behaviour of N at subduction zone conditions and emphasises the complexity of N mobility within subduction zones, with redistribution between different phases and lithologies being important

Cadmium and phosphate in coastal Antarctic seawater: Implications for Southern Ocean nutrient cycling

Cadmium is a biologically important trace metal that co-varies with phosphate (PO43- or Dissolved Inorganic Phosphate, DIP) in seawater. However, the exact nature of Cd uptake mechanisms and the relationship with phosphate and other nutrients in global oceans remain elusive. Here, we present a time series study of Cd and PO43- from coastal Antarctic seawater, showing that Cd co-varies with macronutrients during times of high biological activity even under nutrient and trace metal replete conditions. Our data imply that Cd/PO43- in coastal surface Antarctic seawater is higher than open ocean areas. Furthermore, the sinking of some proportion of this high Cd/PO43- water into Antarctic Bottom Water, followed by mixing into Circumpolar Deep Water, impacts Southern Ocean preformed nutrient and trace metal composition. A simple model of endmember water mass mixing with a particle fractionation of Cd/P (αCd-P) determined by the local environment can be used to account for the Cd/PO43- relationship in different parts of the ocean. The high Cd/PO43- of the coastal water is a consequence of two factors: the high input from terrestrial and continental shelf sediments and changes in biological fractionation with respect to P during uptake of Cd in regions of high Fe and Zn. This implies that the Cd/PO43- ratio of the Southern Ocean will vary on glacial-interglacial timescales as the proportion of deep water originating on the continental shelves of the Weddell Sea is reduced during glaciations because the ice shelf is pinned at the edge of the continental shelf. There could also be variations in biological fractionation of Cd/P in the surface waters of the Southern Ocean on these timescales as a result of changes in atmospheric inputs of trace metals. Further variations in the relationship between Cd and PO43- in seawater arise from changes in population structure and community requirements for macro- and micronutrients. © 2008 Elsevier B.V. All rights reserved

Boron recycling in the mantle: Evidence from a global comparison of ocean island basalts

Radiogenic and noble gas isotopes have been integral for demonstrating the existence, source, and age of (geo) chemical reservoirs in the mantle, yet, the volatile element composition of the Earth’s interior remains poorly characterized. Boron isotopes have the potential to constrain the processes that generate distinct mantle reservoirs, as they fractionate strongly at the surface of the Earth and during subduction but are little perturbed during high-temperature mantle processes, and so can inform our understanding of mantle volatile cycling. Here, we present the largest, internally consistent, high-precision B isotope dataset from ocean island basalt (OIB) glasses and olivine-hosted melt inclusions measured by Secondary Ion Mass Spectrometry (SIMS) to date, including new data derived from the Pitcairn Islands, Tristan da Cunha, St. Helena, Ascension Island, the MacDonald (Ra) Seamount, and Fogo (Cape Verde Islands) in addition to previously published data from La Réunion, La Palma (Canary Islands), Iceland, and Hawai’i. This dataset allows a comparison of ocean island basalts that contain heterogeneous recycled components (e.g., Pitcairn Islands) to those with primordial components (e.g., La Réunion) in their sources. We focus on basaltic glass and melt inclusions (>6 wt% MgO) as they are least affected by shallow differentiation and assimilation processes. We find that our new OIB data show a limited spread in average δ11B (−5.9 ± 3.0‰ to −10.8 ± 0.7‰), which is smaller compared to previous OIB studies. These data generally overlap with mid-ocean ridge basalts (MORB; −7.1 ± 0.9‰) and display lighter values when compared to mafic arc magmas (∼−9 to +20‰). Importantly, some trace element enriched OIB endmembers display lighter δ11B values and lower B/P and B/Zr, indicative of a source with lower B concentrations relative to the primordial mantle. This suggests that the deeper mantle is becoming increasingly B-depleted over time because boron is effectively stripped from recycled lithologies during subduction and slab dehydration. In addition, the results highlight the decoupling of B isotopes from radiogenic (Sr, Pb) isotopes providing a new perspective on volatile recycling

Investigating ocean island mantle source heterogeneity with boron isotopes in melt inclusions

Recycling of the lithosphere via subduction drives the trace element and isotopic heterogeneity of the mantle, yet, the inventory of volatile elements in the diverse array of mantle reservoirs sampled at ocean islands remains uncertain. Boron is an ideal tracer of volatile recycling because it behaves similarly to volatiles during high-temperature geochemical reactions and carries a distinctive isotope signature into the mantle, but is subsequently little-influenced by degassing on return to the surface. Furthermore, B-rich recycled lithologies will have a strong influence on typical upper mantle compositions characterized by low B concentrations (<0.2 μg/g and δ11B −7.1 ± 0.9‰). Here, we present and compare the B abundances and isotope compositions, together with the volatile element contents (H₂O, CO₂, and Cl) of basaltic glasses and olivine-hosted melt inclusions from two different ocean island localities (La Palma, Canary Islands, and Piton de Caille, La Réunion Island). Our results suggest that olivine hosted melt inclusions are protected from contamination during ascent and provide more robust estimates of primary mantle source δ11B than previous bulk rock studies. We find that the δ11B of the La Réunion samples (−7.9 ± 0.5‰ (2σ)) overlaps with the recently defined MORB datum, indicating that the depleted upper-mantle and ‘primitive mantle’ reservoirs are indistinguishable with respect to δ11B, or that B concentrations are sufficiently low that they are diluted by partial melting in the uppermost mantle. In contrast, the La Palma samples, notable for their radiogenic Pb isotope ratios, are characterized by δ11B values that are distinctly isotopically lighter (−10.5 ± 0.7‰ (2σ)) than La Réunion or MORB. We suggest these isotopically light values are derived from significantly dehydrated recycled materials preserved in the La Palma mantle source region, in keeping with their lower B/Zr and H₂O/Ce. This work therefore provides strong new support for subduction zone processing as a mechanism for generating radiogenic Pb isotopic signatures and volatiles heterogeneities in the mantle

Rapid pre-eruptive mush reorganisation and atmospheric volatile emissions from the 12.9 ka Laacher See eruption, determined using apatite

Magma is commonly thought to be stored as a crystal-rich mush within vertically extensive, crustal storage regions. A key unknown is how to remobilise and erupt such crystal-rich material, and whether the growth of gas bubbles within the mush could promote remobilisation. In order to investigate this, we need improved constraints on the timing of volatile saturation in magmas. The mineral apatite represents a potentially useful record of pre-eruptive magmatic volatiles, but data interpretation is complex because exchange reactions control the volatile partitioning. Model solutions are therefore non-unique. Here, we present a numerical forward modelling program with a sensitivity analysis function, which addresses non-uniqueness by identifying alternative sets of starting parameters that match a target compositional trend through a population of apatite crystals. The model is applied to a new dataset of volatiles in apatite from the 12.9 ka Laacher See eruption, Eifel volcanic region, Germany. The results indicate that the magma was initially strongly volatile-undersaturated and became saturated through progressive crystal fractionation. Apatite crystals are not in volatile or trace element equilibrium with their carrier melts, indicating dispersal of crystals into different chemical environments. Consideration of apatite diffusivities suggests that this reorganisation occurred shortly before eruption. Our modelling results also allow us to constrain directly the amount of pre-eruptive magmatic vapour emitted during the explosive eruption, highlighting the importance of considering the behaviour of halogens during magma storage. Overall, our approach confirms the value of measuring apatite volatile contents and highlights the potential of this method to provide quantitative constraints on magmatic evolution and storage conditions