Fluid-mediated element cycling in subducted oceanic lithosphere: The orogenic serpentinite perspective

Serpentinites are central to water (re)cycling in subduction zones and thus effect fluid-mediated element transfer between the hydrosphere and the Earth’s mantle and back into continental crust via calcalkaline magmatism. Diverse and often controversial models exist on the relevance of various source contributions to the budget of fluid mobile elements of hydrous peridotites and how these evolve during the subduction cycle. This work offers novel constraints on ongoing debates. We present a comprehensive bulk rock and silicate mineral major to trace element study covering the antigorite dehydration reaction based on Cerro del Almirez antigorite-serpentinites and chlorite-harzburgites, including the systematics of As, Sb, B, W, Li, In, Tl, Bi, Cd, and Sn - so far unavailable for Almirez, and there exist only few such data for orogenic serpentinites in general. We integrate these with reviewed literature data and develop a general model for the geochemical systematics of subducting hydrous slab mantle covering magmatic peridotite conditioning, element enrichment upon oceanic hydration, compositional evolution with progressive subduction to peak temperature antigorite dehydration, and retrograde metasomatism upon exhumation. Pre-hydration magmatic processes produce strong compositional variations on centimetre to metre to kilometre scales. Serpentinisation via seawater and sediment-equilibrated pore fluids produces highly variable fluidmobile element (FME) bulk rock enrichments in B, As, Sb, W, Cs, ±Li, ±Bi, ±Pb, ±U exceeding primitive mantle concentrations. Hydration enrichment numbers represent a novel concept introduced in this work to refine the extent of hydration-mediated FME enrichment. They represent the measured ratio of fluid-mobile element over a fluid-immobile element of closely comparable magmatic compatibility normalised to its corresponding primitive mantle abundance ratio. Hydration enrichment numbers are highest for Sb and As (up to 650) and lowest for Ba and Rb (down to 0.06) for Almirez data, quantifying fractions of minimal enrichment (values >1) and minimal prograde subduction loss (values <1). FME enrichment occurred primarily in ocean floor to trench to shallow forearc settings where sediment-equilibrated pore fluids are relevant, while addition from deeper sediment metamorphic dehydration fluids with progressive subduction is subordinate at best. Prominent fractions of As, Sb, B, Rb, Sr, Cs, Ba, Pb, Zn, Cl, Br, Li, Na, K, and Ca are then lost to the fluid upon serpentinite dehydration including the antigorite-out reaction. We find no evidence in support of significant fluid-mediated element addition (e.g., Th, U, Ta, Sr, Pb, Cs, Rb, Li) upon antigorite dehydration as has been postulated in literature by simple comparison between Atg-serpentinite and Chl-harzburgite coexisting at Almirez. Magmatic preconditioning prior to serpentinisation can account for the differences in HFSE between Atg-serpentinite and Chlharzburgite, while prominent FME addition upon retrogression as measured on retrograde serpentine and talc is demonstrated for Cs, Pb, Sr and Ba, thus adulterating bulk rock systematics for certain FME. This work concludes that oceanic serpentinisation dominates the FME imprint of subducting slab serpentinites and that progressive subduction goes along with successive FME loss. We propose that serpentinites from fluiddominated, highly hybridised and oxidised plate interface melange materials displaying spectacular FME enrichments with a sedimentary flavour are distinctly overrepresented in our sample record. Therefore, the combined data set of hydrous peridotites from Almirez and Erro Tobbio may offer a more representative compositional estimate of the bulk mass of subducted slab serpentinite, to be used in modelling of the geochemical impact of serpentinite-derived matter on fluid-mediated chemical cycling in subduction zones

δ13C tracing of dissolved inorganic carbon sources in major world rivers

δ13C tracing of dissolved inorganic carbon sources in major world river

Laser-ablation ICP-MS analysis of silicate and sulfide melt inclusions in an andesitic complex II: evidence for magma mixing and magma chamber evolution

Laser-ablation microanalysis of a large suite of silicate and sulfide melt inclusions from the deeply eroded, Cu-Au-mineralizing Farallón Negro Volcanic Complex (NW Argentina) shows that most phenocrysts in a given rock sample were not formed in equilibrium with each other. Phenocrysts in the andesitic volcano were brought together in dominantly andesitic—dacitic extrusive and intrusive rocks by intense magma mixing. This hybridization process is not apparent from macroscopic mingling textures, but is clearly recorded by systematically contrasting melt inclusions in different minerals from a given sample. Amphibole (and rare pyroxene) phenocrysts consistently contain inclusions of a mafic melt from which they crystallized before and during magma mixing. Most plagioclase and quartz phenocrysts contain melt inclusions of more felsic composition than the host rock. The endmember components of this mixing process are a rhyodacite magma with a likely crustal component, and a very mafic mantle-derived magma similar in composition to lamprophyre dykes emplaced early in the evolution of the complex. The resulting magmas are dominantly andesitic, in sharp contrast to the prominently bimodal distribution of mafic and felsic melts recorded by the inclusions. These results severely limit the use of mineral assemblages to derive information on the conditions of magma formation. Observed mineral associations are primarily the result of the mixing of partially crystallized magmas. The most mafic melt is trapped only in amphibole, suggesting pressures exceeding 350MPa, temperatures of around 1,000°C and water contents in excess on 6wt%. Upon mixing, amphibole crystallized with plagioclase from andesitic magma in the source region of porphyry intrusions at 250MPa, 950°C and water contents of 5.5wt%. During ascent of the extrusive magmas, pyroxene and plagioclase crystallized together, as a result of magma degassing at low pressures (150MPa). Protracted extrusive activity built a large stratovolcano over the total lifetime of the magmatic complex (>3m.y.). The mixing process probably triggered eruptions as a result of volatile exsolutio

Magma evolution and the formation of porphyry Cu-Au ore fluids: evidence from silicate and sulfide melt inclusions

Silicate and sulfide melt inclusions from the andesitic Farallón Negro Volcanic Complex in NW Argentina were analyzed by laser ablation ICPMS to track the behavior of Cu and Au during magma evolution, and to identify the processes in the source of fluids responsible for porphyry-Cu-Au mineralization at the 600Mt Bajo de la Alumbrera deposit. The combination of silicate and sulfide melt inclusion data with previously published geological and geochemical information indicates that the source of ore metals and water was a mantle-derived mafic magma that contained approximately 6wt.% H2O and 200ppm Cu. This magma and a rhyodacitic magma mixed in an upper-crustal magma chamber, feeding the volcanic systems and associated subvolcanic intrusions over 2.6million years. Generation of the ore fluid from this magma occurred towards the end of this protracted evolution and probably involved six important steps: (1) Generation of a sulfide melt upon magma mixing in some parts of the magma chamber. (2) Partitioning of Cu and Au into the sulfide melt (enrichment factor of 10,000 for Cu) leading to Cu and Au concentrations of several wt.% or ppm, respectively. (3) A change in the tectonic regime from local extension to compression at the end of protracted volcanism. (4) Intrusion of a dacitic magma stock from the upper part of the layered magma chamber. (5) Volatile exsolution and resorption of the sulfide melt from the lower and more mafic parts of the magma chamber, generating a fluid with a Cu/Au ratio equal to that of the precursor sulfide. (6) Focused fluid transport and precipitation of the two metals in the porphyry, yielding an ore body containing Au and Cu in the proportions dictated by the magmatic fluid source. The Cu/S ratio in the sulfide melt inclusions requires that approximately 4,000ppm sulfur is extracted from the andesitic magma upon mixing. This exceeds the solubility of sulfide or sulfate in either of the silicate melts and implies an additional source for S. The extra sulfur could be added in the form of anhydrite phenocrysts present in the rhyodacitic magma. It appears, thus, that unusually sulfur-rich, not Cu-rich magmas are the key to the formation of porphyry-type ore deposits. Our observations imply that dacitic intrusions hosting the porphyry-Cu-Au mineralization are not representative of the magma from which the ore-fluid exsolved. The source of the ore fluid is the underlying more mafic magma, and unaltered andesitic dikes emplaced immediately after ore formation are more likely to represent the magma from which the fluids were generated. At Alumbrera, these andesitic dikes carry relicts of the sulfide melt as inclusions in amphibole. Sulfide inclusions in similar dykes of other, less explored magmatic complexes may be used to predict the Au/Cu ratio of potential ore-forming fluids and the expected metal ratio in any undiscovered porphyry deposi

Crystallization and Breakdown of Metasomatic Phases in Graphite-bearing Peridotite Xenoliths from Marsabit (Kenya)

Mantle-derived xenoliths from the Marsabit shield volcano (eastern flank of the Kenya rift) include porphyroclastic spinel peridotites characterized by variable styles of metasomatism. The petrography of the xenoliths indicates a transition from primary clinopyroxene-bearing cryptically metasomatized harzburgite (light rare earth element, U, and Th enrichment in clinopyroxene) to modally metasomatized clinopyroxene-free harzburgite and dunite. The metasomatic phases include amphibole (low-Ti Mg-katophorite), Na-rich phlogopite, apatite, graphite and metasomatic low-Al orthopyroxene. Transitional samples show that metasomatism led to replacement of clinopyroxene by amphibole. In all modally metasomatized xenoliths melt pockets (silicate glass containing silicate and oxide micro-phenocrysts, carbonates and empty vugs) occur in close textural relationship with the earlier metasomatic phases. The petrography, major and trace element data, together with constraints from thermobarometry and fO2 calculations, indicate that the cryptic and modal metasomatism are the result of a single event of interaction between peridotite and an orthopyroxene-saturated volatile-rich silicate melt. The unusual style of metasomatism (composition of amphibole, presence of graphite, formation of orthopyroxene) reflects low P -T conditions (∼850-1000°C at < 1·5 GPa) in the wall-rocks during impregnation and locally low oxygen fugacities. The latter allowed the precipitation of graphite from CO2. The inferred melt was possibly derived from alkaline basic melts by melt-rock reaction during the development of the Tertiary-Quaternary Kenya rift. Glass-bearing melt pockets formed at the expense of the early phases, mainly through incongruent melting of amphibole and orthopyroxene, triggered by infiltration of a CO2-rich fluid and heating related to the magmatic activity that ultimately sampled and transported the xenoliths to the surfac

Simultaneous cathodoluminescence hyperspectral imaging and X-ray microanalysis

A facility has been developed to acquire hyperspectral cathodoluminescence (CL) images simultaneously with X-ray composition data. Based around an electron microprobe, the system uses a built-in Cassegrain microscope to efficiently couple emitted light directly into the entrance slit of an optical spectrograph. A cooled array detector allows the parallel acquisition of CL spectra, which are then built up into a multidimensional data-cube containing the full set of spectrally- and spatially-resolved information for later analysis. This setup has the advantage of allowing wavelength-dispersive X-ray (WDX) data to be recorded concurrently, providing a powerful technique for the direct comparison of luminescent and compositional properties of materials. The combination of beam and sample scanning thus allows the correlation of composition and luminescence inhomogeneities on length scales ranging from a few cm to sub-micron

Fluid and source magma evolution of the Questa porphyry Mo deposit, New Mexico, USA

Combined fluid inclusion microthermometry and microanalysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) are used to constrain the hydrothermal processes forming a typical Climax-type porphyry Mo deposit. Molybdenum mineralisation at Questa occurred in two superimposed hydrothermal stages, a magmatic-hydrothermal breccia and later stockwork veining. In both stages, texturally earliest fluids were single-phase, of low salinity (~7wt.% NaClequiv.) and intermediate-density. Upon decompression to ~300bar, they boiled off a vapour phase, leaving behind a residual brine (up to 45wt.% NaClequiv) at temperatures of ~420°C. The highest average Mo concentrations in this hot brine were ~500μg/g, exceeding the Mo content of the intermediate-density input fluid by about an order of magnitude and reflecting pre-concentration of Mo by fluid phase separation prior to MoS2 deposition from the brine. Molybdenum concentrations in brine inclusions, then, decrease down to 5μg/g, recording Mo precipitation in response to cooling of the saline liquid to ~360°C. Molybdenite precipitation from a dense, residual and probably sulphide-depleted brine is proposed to explain the tabular shape of the ore body and the absence of Cu-Fe sulphides in contrast to the more common Cu-Mo deposits related to porphyry stocks. Cesium and Rb concentrations in the single-phase fluids of the breccia range from 2 to 8 and from 40 to 65μg/g, respectively. In the stockwork veins, Cs and Rb concentrations are significantly higher (45-90 and 110-230μg/g, respectively). Because Cs and Rb are incompatible and hydrothermally non-reactive elements, the systematic increase in their concentration requires two distinct pulses of fluid exsolution from a progressively more fractionated magma. By contrast, major element and ore metal concentrations of these two fluid pulses remain essentially constant. Mass balance calculations using fluid chemical data from LA-ICPMS suggest that at least 25km3 of melt and 7Gt of deep input fluid were necessary to provide the amount of Mo contained in the stockwork vein stage alone. While the absolute amounts of fluid and melt are uncertain, the well-constrained element ratios in the fluids together with empirical fluid/melt partition coefficients derived from the inclusion analyses suggest a high water content of the source melt of ~10%. In line with other circumstantial evidence, these results suggest that initial fluid exsolution may have occurred at a confining pressure exceeding 5kbar. The source of the molybdenum-mineralising fluids probably was a particularly large magma chamber that crystallised and fractionated in the lower crust or at mid-crustal level, well below the shallow intrusions immediately underlying Questa and other porphyry molybdenum deposit

Aluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: gabbro-troctolitic cumulates subjected to lithospheric foundering

Whole rock major and trace element abundances in aluminous garnet-spinel websterite, sapphirine-bearing Mg-Al granulite and hibonite-bearing Ca-Al granulite xenoliths from the Chyulu Hills volcanic field, Kenya, suggest that the samples represent a meta-igneous suite linked by fractionation. The incompatible major element contents increase from the websterites to the Mg-Al granulites and further to the Ca-Al granulites. High bulk rock Mg#s and very low concentrations of most incompatible trace elements indicate that the rocks are cumulates rather than crystallized melts. Elevated Ni abundances, impoverishment in Cr and HFSE and high contents of normative plagioclase and olivine in the granulites indicate that their protoliths were similar to troctolite. The textures and metamorphic reaction paths recorded in the granulites suggest igneous emplacement in the crust and cooling from igneous to ambient crustal temperatures accompanied or followed by compression. For the websterite xenoliths, there is an apparent contradiction between the results of P-T calculations that suggest high P and T of crystallization of early generation pyroxenes and elevated P-T conditions during final equilibration (1.4-2.2GPa/740-980°C) on the one hand and the positive Eu anomaly that suggests shallow-level plagioclase accumulation on the other hand. This contradiction can be reconciled by a model of compression of a plagioclase-bearing (gabbroic) protolith to mantle depths where it recrystallized to an ultramafic assemblage, which requires foundering of dense lower crustal material into the mantl

Internal and External Fluid Sources for Eclogite-facies Veins in the Monviso Meta-ophiolite, Western Alps: Implications for Fluid Flow in Subduction Zones

To contribute to our understanding of the mechanisms and pathways of fluid movement through deeply subducted crust, we investigate high-pressure veins cutting eclogite-facies (∼2·0 GPa and ∼600°C) metagabbros of the Monviso Ophiolite, Italian Western Alps. The veins consist mainly of omphacite with minor garnet, rutile, talc and accessory zircon. Most of the vein minerals have major and trace element compositions that are comparable with the host-rock minerals, and vein and host-rock zircons have similar Hf isotopic compositions. These observations support the conclusions of previous studies that these veins largely formed from a locally sourced hydrous fluid during prograde or peak metamorphism. However, the bulk-rock Cr and Ni contents of the veins are significantly higher than those of the surrounding host eclogites. We also document distinct Cr-rich (up to weight per cent levels) zones in omphacite, garnet and rutile in some vein samples. Vein garnet and talc also have relatively high MgO and Ni contents. X-ray maps of vein garnet and rutile grains reveal complex internal zoning features, which are largely defined by micrometre-scale variations in Cr content. Some grains have concentric and oscillatory zoning in Cr, whereas others feature a chaotic fracture-like pattern. These Cr-rich zones are associated with high concentrations of Ni, B, As, Sb, Nb, Zr and high ratios of light rare earth elements (LREE) to middle REE (MREE) compared with low-Cr vein and host-rock minerals. Petrological and mass-balance constraints verify that the Cr-rich zones in the veins were not derived from internally sourced fluids, but represent precipitates from an external fluid. The external source that is consistent with the distinctive trace element characteristics of the vein components is antigorite serpentinite, which forms the structural basement of the high-pressure metagabbros. We propose at least two separate growth mechanisms for the Monviso veins. Most vein infillings were formed during progressive prograde metamorphism from locally derived fluid. Influx of the serpentinite-derived or other external fluid was transient and episodic and was probably achieved via brittle fractures, which preferentially formed along the pre-existing vein structures. The dehydration of serpentinite at high pressures in subduction zones may provide crucial volatiles and trace elements for arc magmas. Our results indicate that the movement of these fluids through subducted oceanic crust is likely to be highly channeled and transient so the progressive development of vein systems in mafic rocks may also be crucial for forming channelways for long-distance fluid flow at depth in subduction zone

Laser-ablation ICP-MS analysis of silicate and sulfide melt inclusions in an andesitic complex I: analytical approach and data evaluation

Quantitative microanalysis of entire silicate and sulfide melt inclusions by Excimer Laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICPMS) has been applied to extrusive and shallow intrusive rocks from the andesitic Farallón Negro Volcanic Complex (northwestern Argentina). Silicate melts are trapped in pyroxene, amphibole, plagioclase and quartz, and sulfide melts are trapped in amphibole. Details of the analytical approach and the quantification procedure are given and the results are evaluated to test the accuracy of the technique and the validity of the interpretation of analytical signals. Similar compositions of silicate melt inclusions trapped in truly co-precipitating minerals show that the quantification approach of melt inclusion compositions from LA-ICPMS signals through an internal standard is valid. This correspondence also shows that melt inclusions investigated in this study are not significantly influenced by the boundary layer around a growing crystal or by post-entrapment modifications. Post-entrapment diffusive re-equilibration only affected the Fe and Mg content of melt inclusions in mafic phases. Thus, melt inclusions are representative samples of the melt from which the host mineral crystallized, with regard to most major and trace elements. Sulfide melt inclusions (present as pyrrhotite with exsolution of Au and Cu in phases separated during cooling) were analyzed for their bulk Fe, Cu and Au content, and the abundance of these elements was quantified using a silicate glass as external standard. The validity of this calibration was tested by comparing electron microprobe analyses of Fe, Cu, Ni and Co in homogeneous sulfide minerals with LA-ICPMS results. Identical results within calculated uncertainty (one standard deviation of five to nine analyses, mostly between 1 and 5wt% RSD) demonstrate that for these elements, measured element ratios are independent of the matrix using our analytical setu