Decoupled oscillatory and O-isotope zonation in high pressure low temperature garnet: records of heterogeneous fluid transfer processes

2021 ◽  
Author(s):  
Freya R. George ◽  
Daniel R. Viete ◽  
Janaína Ávila ◽  
Gareth G. E. Seward
Keyword(s):  
High Pressure ◽  
Grain Boundary ◽  
Short Wavelength ◽  
Subduction Zones ◽  
Isotope Composition ◽  
Oscillatory Zoning ◽  
Garnet Growth ◽  
Spatial Covariance ◽  
O Isotope ◽  
Fluid Transfer

<p>High pressure garnet porpyhroblasts formed in subduction zones serve as a witness to an integrated history of fluid flow, deformation, metamorphic reaction, and exhumation processes. Seemingly ubiquitous within garnet from a heterogeneous suite of eclogite and blueschist units is primary oscillatory elemental zoning—rhythmic, short wavelength (< 10 µm) concentric fluctuations concentrated near the rims of porphyroblasts—which has been documented using a combined major element X-ray mapping and trace element LA-ICP-MS mapping approach. This oscillatory zoning must reflect some fundamental petrogenetic process operating during subduction zone metamorphism. While longer length scale (> 50 µm) oscillations have been interpreted to reflect rock-wide P–T changes during physical cycling through the subduction channel, these short wavelength oscillations have typically been interpreted to reflect changes in the effective grain boundary chemistry induced by fluid fluxing during mineral growth.</p><p>Here, we present secondary ion mass spectrometry (SIMS) O-isotope data across the oscillatory zoning in garnet from six subduction settings. A lack of spatial covariance between the elemental and δ<sup>18</sup>O records is inconsistent with the interpretation that oscillatory zoning is directly linked to infiltration of chemically and isotopically distinct fluids. However, in most samples, vascillations in δ<sup>18</sup>O of < 2 ‰ (over 20–50 µm) in the mantle and rim, coupled with < 1 ‰ net core-to-rim change may point to the predominance of: (a) an internally-controlled grain boundary fluid and relatively stagnant fluid conditions, with grain boundaries that may experience transient opening, heterogeneous and locally-derived fluid fluxing, and then re-sealing, or (b) a rock-buffered oxygen isotope composition during garnet growth between  450 ˚C and 550 ˚C. However, several samples exhibit a systematic 2.5–4 ‰ change in δ<sup>18</sup>O across oscillatory major and trance element zoning, accompanied by a 2–3 mol% decrease in andradite content. This change, outside that predicted via closed system crystallization and fractionation, is suggested to reflect the relatively uncommon and sudden transient passage of a reduced external fluid. While this dataset does not reveal the mystery of the oscillatory zoning, it demonstrates spatial and temporal heterogeneity of fluid transfer in subduction zones.</p>

scholarly journals Decoupling of inorganic and organic carbon during slab mantle devolatilisation

2022 ◽  
Vol 13 (1) ◽  
Author(s):  
P. Bouilhol ◽  
B. Debret ◽  
E. C. Inglis ◽  
M. Warembourg ◽  
T. Grocolas ◽  
...  
Keyword(s):  
High Pressure ◽  
Organic Carbon ◽  
Subduction Zones ◽  
Inorganic Carbon ◽  
Mantle Wedge ◽  
Isotope Composition ◽  
Sr Isotope ◽  
Ultra High Pressure ◽  
Isotope Analyses ◽  
Inorganic And Organic

AbstractSerpentinites are an important sink for both inorganic and organic carbon, and their behavior during subduction is thought to play a fundamental role in the global cycling of carbon. Here we show that fluid-derived veins are preserved within the Zermatt-Saas ultra-high pressure serpentinites providing key evidence for carbonate mobility during serpentinite devolatilisation. We show through the O, C, and Sr isotope analyses of vein minerals and the host serpentinites that about 90% of the meta-serpentinite inorganic carbon is remobilized during slab devolatilisation. In contrast, graphite-like carbonaceous compounds remain trapped within the host rock as inclusions within metamorphic olivine while the bulk elemental and isotope composition of organic carbon remains relatively unchanged during the subduction process. This shows a decoupling behavior of carbon during serpentinite dehydration in subduction zones. This process will therefore facilitate the transfer of inorganic carbon to the mantle wedge and the preferential slab sequestration of organic carbon en route to the deep mantle.

Oscillatory and stepwise compositional zoning in high pressure–low temperature garnets: records of transient and spatially-variable fluid-fluxing during subduction?

2020 ◽  
Author(s):  
Freya R. George ◽  
Daniel R. Viete ◽  
Janaína Ávila ◽  
Gareth G.E. Seward
Keyword(s):  
Mass Spectrometry ◽  
High Pressure ◽  
Low Temperature ◽  
Grain Boundary ◽  
Inductively Coupled Plasma ◽  
Metamorphic Rock ◽  
Major Element ◽  
Oscillatory Zoning ◽  
Compositional Zoning ◽  
Rock Record

<p>During subduction, devolatization reactions within the downgoing slab release significant volumes of fluid. Once released, the fate of such fluids remains unclear; they may either stagnate such that local rock systems remain undrained, or fluids may be mobilized over large length scales, draining the dehydrating rock volume. The fact that there is evidence from the metamorphic rock record to support both open- and closed-system fluid behavior demonstrates that permeability in deep crystalline metamorphic rock is both spatially and temporally heterogeneous. Prograde eclogitic veins greater than cm-scale are volumetrically scarce in the high pressure–low temperature (<em>HP–LT</em>) rock record, suggesting that either transient channelized flow is incredibly efficient and thus necessitates negligible grain boundary transfer and a low intact rock permeability, or that a large proportion of fluid migration to the subduction interface may be via more elusive grain boundary mechanisms.</p><p>Major element electron microprobe maps of <em>HP–LT</em> garnets from metabasic rocks of the Urals, Russia, As Sifa, Oman, and Syros, Greece, variably reveal short-wavelength and concentric oscillatory zoning in the outer rim region. Oscillatory zoning in most garnets is accompanied by homogeneous core-to-rim aluminum content. However, in samples from As Sifa and Syros, the onset of near-rim major element oscillatory zoning is concomitant with a rimwards step increase in Al content. Secondary ion mass spectrometry (SIMS) O-isotope analyses across rhythmic zoning in samples from each setting are used to assess the hypothesis that this sharp, stepwise change in garnet chemistry reflects a period of localized open system fluid-fluxing behavior, superimposed on a history of an otherwise stagnant fluid within an impermeable grain boundary network. In such a case, coupled oscillatory zoning in major and trace elements—as revealed by laser ablation–inductively coupled plasma­–mass spectrometry (LA–ICP–MS) mapping—may point to pulsed <em>P–T</em> fluctuations, variable partitioning behavior, local kinetic effects associated with metamorphic reaction/dehydration, or changes in redox state serving as a driver for the development of this characteristic <em>HP–LT</em> geochemical garnet zoning.</p>

scholarly journals Up the down escalator: the exhumation of (ultra)-high pressure terranes during on-going subduction

2012 ◽  
Vol 4 (1) ◽  
pp. 745-781 ◽  
Author(s):  
C. J. Warren
Keyword(s):  
High Pressure ◽  
Subduction Zone ◽  
Continental Crust ◽  
Subduction Zones ◽  
Crustal Material ◽  
Time And Space ◽  
Ultra High Pressure ◽  
Tectonic Environment ◽  
Tectonic Style ◽  
Weak Material

Abstract. The exhumation of high and ultra-high pressure rocks is ubiquitous in Phanerozoic orogens created during continental collisions, and is common in many ocean-ocean and ocean-continent subduction zone environments. Three different tectonic environments have previously been reported, which exhume deeply buried material by different mechanisms and at different rates. However it is becoming increasingly clear that no single mechanism dominates in any particular tectonic environment, and the mechanism may change in time and space within the same subduction zone. In order for buoyant continental crust to subduct, it must remain attached to a stronger and denser substrate, but in order to exhume, it must detach (and therefore at least locally weaken) and be initially buoyant. Denser oceanic crust subducts more readily than more buoyant continental crust but exhumation must be assisted by entrainment within more buoyant and weak material such as serpentinite or driven by the exhumation of structurally lower continental crustal material. Weakening mechanisms responsible for the detachment of crust at depth include strain, hydration, melting, grain size reduction and the development of foliation. These may act locally or may act on the bulk of the subducted material. Metamorphic reactions, metastability and the composition of the subducted crust all affect buoyancy and overall strength. Subduction zones change in style both in time and space, and exhumation mechanisms change to reflect the tectonic style and overall force regime within the subduction zone. Exhumation events may be transient and occur only once in a particular subduction zone or orogen, or may be more continuous or occur multiple times.

Grain boundary effect on the β-boron electrical transport properties at high pressure

2010 ◽  
Vol 97 (17) ◽  
pp. 174101 ◽  
Author(s):  
Ming Li ◽  
Jie Yang ◽  
Karim Snoussi ◽  
Lixin Li ◽  
Huixin Wang ◽  
...  
Keyword(s):  
High Pressure ◽  
Grain Boundary ◽  
Transport Properties ◽  
Electrical Transport ◽  
Boundary Effect ◽  
Electrical Transport Properties ◽  
Grain Boundary Effect

scholarly journals Iron and Carbon Isotope Constraints on the Formation Pathway of Iron-Rich Carbonates within the Dagushan Iron Formation, North China Craton

Minerals ◽  
2021 ◽  
Vol 11 (1) ◽  
pp. 94
Author(s):  
Xiaoxue Tong ◽  
Kaarel Mänd ◽  
Yuhao Li ◽  
Lianchang Zhang ◽  
Zidong Peng ◽  
...  
Keyword(s):  
Iron Reduction ◽  
Isotope Composition ◽  
Iron Formation ◽  
Banded Iron Formations ◽  
Organic C ◽  
Formation Pathway ◽  
Wide Range ◽  
Dissimilatory Iron Reduction ◽  
O Isotope ◽  
Iron Formations

Banded iron formations (BIFs) are enigmatic chemical sedimentary rocks that chronicle the geochemical and microbial cycling of iron and carbon in the Precambrian. However, the formation pathways of Fe carbonate, namely siderite, remain disputed. Here, we provide photomicrographs, Fe, C and O isotope of siderite, and organic C isotope of the whole rock from the ~2.52 Ga Dagushan BIF in the Anshan area, China, to discuss the origin of siderite. There are small magnetite grains that occur as inclusions within siderite, suggesting a diagenetic origin of the siderite. Moreover, the siderites have a wide range of iron isotope compositions (δ56FeSd) from −0.180‰ to +0.463‰, and a relatively negative C isotope composition (δ13CSd = −6.20‰ to −1.57‰). These results are compatible with the reduction of an Fe(III)-oxyhydroxide precursor to dissolved Fe(II) through microbial dissimilatory iron reduction (DIR) during early diagenesis. Partial reduction of the precursor and possible mixing with seawater Fe(II) could explain the presence of siderite with negative δ56Fe, while sustained reaction of residual Fe(III)-oxyhydroxide could have produced siderite with positive δ56Fe values. Bicarbonate derived from both DIR and seawater may have provided a C source for siderite formation. Our results suggest that microbial respiration played an important role in the formation of siderite in the late Archean Dagushan BIF.

Jilin Zircon – A New Natural Reference Material for Microbeam U-Pb Geochronology and Hf-O Isotopic Analysis

2021 ◽  
Author(s):  
Tao Luo ◽  
Qiuli Li ◽  
Xiaoxiao Ling ◽  
Yang Li ◽  
Chuan Yang ◽  
...  
Keyword(s):  
Reference Material ◽  
Isotope Composition ◽  
Isotopic Analysis ◽  
The Matrix ◽  
O Isotope

Zircon U-Pb geochronology and Hf-O isotope composition can provide important information on geological events. The matrix-matched reference material is routinely used to yield accurate and precise zircon U-Pb ages and...

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