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CURATOR
A pinboard by
Morgan Williams

PhD Candidate, Research School of Earth Sciences, Australian National University

PINBOARD SUMMARY

I investigate how hydrous fluids change the chemisty of oceanic rocks throughout their lifetime

I investigate how hydrous fluids change the chemistry of oceanic rocks from their formation at mid-oceanic ridges through to their destruction at convergent plate boundaries. Fluids facilitate the transport of both heat and mass over a wide range o geological conditions. The oceanic crust dominates the surface of our planet (oceans cover approximately 70% its area), and through subduction it contributes to large-scale geochemical recycling at convergent plate margins. The hydration of oceanic crust on the seafloor allows large volumes of water to be incorporated. Seafloor hydrothermal systems facilitate the formation of seafloor ore deposits, and can provide environments for unique seafloor microbial communities. Hydrated oceanic crust is eventually dragged into the mantle within subduction zones underneath buoyant continental plates. Water-bearing minerals formed on the seafloor are destabilized by increasing pressure and temperature. Through metamorphism to form less hydrous rocks, water-rich fluids are given off into surrounding rocks and the overlying mantle - where it lowers the melting point and induces melting. This is why volcanoes form along convergent plate boundaries near the ocean, and how continents can grow. Clearly, the incorporation of water into oceanic crust has a significant influence on the morphology of our planet's surface.

To understand the cycling of fluids in the oceanic crust from start to end, I'm investigating recently hydrated rocks drilled from the seafloor (serpentinites) and rocks collected from high in the Western Italian Alps which have been through subduction but survived (serpentinites, altered sediments and altered basalts). These rocks have all been altered more than once, and to pull out as much information as possible, I use targeted microscale analysis of individual minerals associated with each of the major alteration stages to reconstruct mineral chemistry through time. Rock chemistry is broadly reflective of formation conditions, and this can be used to build a model of when, where and how rocks have been altered. By constructing 'geochemical histories', we can better understand the processes controlling why the surface of our planet looks the way it does. This is particularly relevant for the formation of ore deposits (both on the seafloor and above subduction zones) and potentially the evolution of the first life on earth (and potentially on other planets!).

8 ITEMS PINNED

Slab-derived halogens and noble gases illuminate closed system processes controlling volatile element transport into the mantle wedge

Abstract: Halogen and noble gas systematics are powerful tracers of volatile recycling in subduction zones. We present halogen and noble gas compositions of mantle peridotites containing H2O-rich fluid inclusions collected at volcanic fronts from two contrasting subduction zones (the Avacha volcano of Kamchatka arc and the Pinatubo volcano of Luzon arcs) and orogenic peridotites from a peridotite massif (the Horoman massif, Hokkaido, Japan) which represents an exhumed portion of the mantle wedge. The aims are to determine how volatiles are carried into the mantle wedge and how the subducted fluids modify halogen and noble gas compositions in the mantle. The halogen and noble gas signatures in the H2O-rich fluids are similar to those of marine sedimentary pore fluids and forearc and seafloor serpentinites. This suggests that marine pore fluids in deep-sea sediments are carried by serpentine and supplied to the mantle wedge, preserving their original halogen and noble gas compositions. We suggest that the sedimentary pore fluid-derived water is incorporated into serpentine through hydration in a closed system along faults at the outer rise of the oceanic, preserving Cl/H2O and 36Ar/H2O values of sedimentary pore fluids. Dehydration–hydration process within the oceanic lithospheric mantle maintains the closed system until the final stage of serpentine dehydration. The sedimentary pore fluid-like halogen and noble gas signatures in fluids released at the final stage of serpentine dehydration are preserved due to highly channelized flow, whereas the original Cl/H2O and 36Ar/H2O ratios are fractionated by the higher incompatibility of halogens and noble gases in hydrous minerals.

Pub.: 25 Oct '16, Pinned: 24 Aug '17

Mobility of major and trace elements in the eclogite-fluid system and element fluxes upon slab dehydration

Abstract: The equilibrium between aqueous fluids and allanite-bearing eclogite has been investigated to constrain the effect of temperature (T) and fluid composition on the stability of allanite and on the mobility of major and trace elements during the dehydration of eclogites. The experiments were performed at 590 - 800 °C and 2.4 – 2.6 GPa, and fluids were sampled as synthetic fluid inclusions in quartz using an improved entrapment technique. The concentrations and bulk partition coefficients were determined for a range of major (Mg, Ca, Na, Fe, Al, Ti) and 16 trace elements as a function of T and fluid composition. The results reveal a significant effect of T on element partitioning between the fluids and the solid mineral assemblage. The partition coefficients increase by more than an order of magnitude for most of the major and trace elements, and several orders of magnitude for light rare-earth elements (LREE) from 590 to 800 °C. The addition of various ligand species into the fluid at 700 °C results in distinctive trends on element partitioning. The concentrations and corresponding partition coefficients of most of the elements are enhanced upon addition of NaF to the fluid. In contrast, NaCl displays a nearly opposite effect by suppressing the solubilities of major elements and consequently affecting the mobility of trace elements that form stable complexes with alkali(alumino)-silicate clusters in the fluid, e.g. high field strength elements (HFSE). The results further suggest that fluids in equilibrium with orthopyroxene and/or diopsidic clinopyroxene are peralkaline (ASI ∼ 0.1 - 0.7), whereas fluids in equilibrium with omphacitic pyroxene are more peraluminous (ASI ∼ 1.15). Therefore, natural aqueous fluids in equilibrium with eclogite at about 90 km depth will be slightly peraluminous in composition. Another important finding of this study is the relatively high capacity of aqueous fluids to mobilize LREE, which may be even higher than that of hydrous melts.

Pub.: 01 Nov '16, Pinned: 24 Aug '17

Ocean floor and subduction record in the Zermatt-Saas rodingites, Valtournanche, Western Alps

Abstract: Multiscale structural analysis and petrological modelling were used to establish the pressure-peak mineral assemblages and pressure–temperature (P–T) conditions recorded in the rodingites of the upper Valtournanche portion of the oceanic Zermatt-Saas Zone (ZSZ; Western Alps, northwestern Italy) during Alpine subduction. Rodingites occur in the form of deformed dykes and boudins within the hosting serpentinites. A field structural analysis showed that rodingites and serpentinites record four ductile deformation stages (D1–D4) during the Alpine cycle, with the first three stages associated with new foliations. The most pervasive fabric is S2 that is marked by mineral assemblages in serpentinite indicating pressure-peak conditions, involving mostly serpentine, clinopyroxene, olivine, Ti-clinohumite and chlorite. Three rodingite types can be defined: epidote-bearing, garnet–chlorite–clinopyroxene-bearing and vesuvianite-bearing rodingite. In these, the pressure-peak assemblages coeval with S2 development involve: (i) epidoteII + clinopyroxeneII + Mg-chloriteII + garnetII ± rutile ± tremoliteI in the epidote-bearing rodingite; (ii) Mg-chloriteII + garnetII clinopyroxeneII ± vesuvianiteII ± ilmenite in the garnet–chlorite–clinopyroxene-bearing rodingite; (iii) vesuvianiteII + Mg-chloriteII + clinopyroxeneII + garnetII ± rutile ± epidote in vesuvianite-bearing rodingite. Despite the pervasive structural reworking of the rodingites during Alpine subduction, the mineral relicts of the pre-Alpine ocean floor history have been preserved and consist of clinopyroxene porphyroclasts (probable igneous relicts from gabbro dykes) and Cr-rich garnet and vesuvianite (relicts of ocean floor metasomatism). Petrological modelling using thermocalc in the NCFMASHTO system was used to constrain the P–T conditions of the S2 mineral assemblages. The inferred values of 2.3–2.8 GPa and 580–660 °C are consistent with those obtained for syn-S2 assemblages in the surrounding serpentinites. Multiscale structural analysis indicates that some ocean floor minerals remained stable under eclogite facies conditions suggesting that minerals such as vesuvianite, which is generally regarded as a low-P phase, could also be stable in favourable chemical systems under high-P/ultra-high-pressure (HP/UHP) conditions. Finally, the reconstructed P–T–d–t path indicates that the P/T ratio characterizing the D2 stage is consistent with cold subduction as estimated in this part of the Alps. The estimated pressure-peak values are higher than those previously reported in this part of ZSZ, suggesting that the UHP units are larger and/or more abundant than those previously suggested.

Pub.: 05 Sep '16, Pinned: 24 Aug '17

Lithium, boron and chlorine as tracers for metasomatism in high-pressure metamorphic rocks: a case study from Syros (Greece)

Abstract: High-pressure metamorphic (HPM) rocks (derived from igneous protoliths) and their metasomatised rinds from the island of Syros (Greece) were analysed for their B and Cl whole-rock abundances and their H2O content by prompt-gamma neutron-activation analysis (PGNAA) and for their Li and Be whole-rock abundances by ICP-OES. In the HPM rocks, B /Be and Cl /Be ratios correlate with H2O contents and appear to be controlled by extraction of B and Cl during dehydration and prograde metamorphism. In contrast, samples of the metasomatised rinds show no such correlation. B /Be ratios in the rinds are solely governed by the presence or absence of tourmaline, and Cl /Be ratios vary significantly, possibly related to fluid inclusions. Li/Be ratios do not correlate with H2O contents in the HPM rocks, which may in part be explained by a conservative behaviour of Li during dehydration. However, Li abundances exceed the vast majority of published values for Li abundances in fresh, altered, or differentiated oceanic igneous rocks and presumably result from metasomatic enrichment of Li. High Li concentrations and highly elevated Li/Be ratios in most metasomatised samples demonstrate an enrichment of Li in the Syros HP mélange during fluid infiltration. This study suggests that B and Cl abundances of HPM meta-igneous rocks can be used to trace prograde dehydration, while Li concentrations seem to be more sensitive for retrograde metasomatic processes in such lithologies.

Pub.: 20 Nov '08, Pinned: 24 Aug '17

Halogens and noble gases in Mathematician Ridge meta-gabbros, NE Pacific: implications for oceanic hydrothermal root zones and global volatile cycles

Abstract: Six variably amphibolitised meta-gabbros cut by quartz–epidote veins containing high-salinity brine, and vapour fluid inclusions were investigated for halogen (Cl, Br, I) and noble gas (He, Ne, Ar, Kr, Xe) concentrations. The primary aims were to investigate fluid sources and interactions in hydrothermal root zones and determine the concentrations and behaviours of these elements in altered oceanic crust, which is poorly known, but has important implications for global volatile (re)cycling. Amphiboles in each sample have average concentrations of 0.1–0.5 wt% Cl, 0.5–3 ppm Br and 5–68 ppb I. Amphibole has Br/Cl of ~0.0004 that is about ten times lower than coexisting fluid inclusions and seawater, and I/Cl of 2–44 × 10−6 that is 3–5 times lower than coexisting fluid inclusions but higher than seawater. The amphibole and fluid compositions are attributed to mixing halogens introduced by seawater with a large halogen component remobilised from mafic lithologies in the crust and fractionation of halogens between fluids and metamorphic amphibole formed at low water–rock ratios. The metamorphic amphibole and hydrothermal quartz are dominated by seawater-derived atmospheric Ne, Ar, Kr and Xe and mantle-derived He, with 3He/4He of ~9 R/Ra (Ra = atmospheric ratio). The amphibole and quartz preserve high 4He concentrations that are similar to MORB glasses and have noble gas abundance ratios with high 4He/36Ar and 22Ne/36Ar that are greater than seawater and air. These characteristics result from the high solubility of light noble gases in amphibole and suggest that all the noble gases can behave similarly to ‘excess 40Ar’ in metamorphic hydrothermal root zones. All noble gases are therefore trapped in hydrous minerals to some extent and can be inefficiently lost during metamorphism implying that even the lightest noble gases (He and Ne) can potentially be subducted into the Earth’s mantle.

Pub.: 24 Oct '15, Pinned: 09 Aug '17

Oxygen isotope record of oceanic and high-pressure metasomatism: a P–T–time–fluid path for the Monviso eclogites (Italy)

Abstract: Fluids are considered a fundamental agent for chemical exchanges between different rock types in the subduction system. Constraints on the sources and pathways of subduction fluids thus provide crucial information to reconstruct subduction processes. The Monviso ophiolitic sequence is composed of mafic, ultramafic and minor sediments that have been subducted to ~80 km depth. In this sequence, both localized fluid flow and channelized fluids along major shear zones have been documented. We investigate the timing and source of the fluids that affected the dominant mafic rocks using microscale U–Pb dating of zircon and oxygen isotope analysis of mineral zones (garnet, zircon and antigorite) in high-pressure rocks with variable degree of metasomatic modification. In mafic eclogites, Jurassic zircon cores are the only mineralogical relicts of the protolith gabbros and retain δ18O values of 4.5–6 ‰, typical of mantle melts. Garnet and metamorphic zircon that grew during prograde to peak metamorphism display low δ18O values between 0.2 and 3.8 ‰, which are likely inherited from high-temperature alteration of the protolith on the sea floor. This is corroborated by δ18O values of 3.0 and 3.6 ‰ in antigorite from surrounding serpentinites. In metasomatized eclogites within the lower shear zone, garnet rim formed at the metamorphic peak shows a shift to higher δ18O up to 6 ‰. The age of zircons in high-pressure veins and metasomatized eclogites constrains the timing of fluid flow at high pressure at around 45–46 Ma. Although the oxygen data do not contradict previous reports of interaction with serpentinite-derived fluids, the shift to isotopically heavier oxygen compositions requires contribution from sediment-derived fluids. The scarcity of metasediments in the Monviso sequence suggests that such fluids were concentrated and fluxed along the lower shear zone in a sufficient amount to modify the oxygen composition of the eclogitic minerals.

Pub.: 29 Oct '15, Pinned: 09 Aug '17