scholarly journals Potassium isotopic heterogeneity in subducting oceanic plates

2020 ◽  
Vol 6 (49) ◽  
pp. eabb2472
Author(s):  
Yan Hu ◽  
Fang-Zhen Teng ◽  
Terry Plank ◽  
Catherine Chauvel
Keyword(s):  
Oceanic Crust ◽  
High Precision ◽  
Western Pacific ◽  
Isotopic Heterogeneity ◽  
Analytical Precision ◽  
Diagenetic Alteration ◽  
Continental Weathering ◽  
Oceanic Plates

Oceanic crust and sediments are the primary K sinks for seawater, and they deliver considerable amounts of K to the mantle via subduction. Historically, these crustal components were not studied for K isotopes because of the lack of analytical precision to differentiate terrestrial variations. Here, we report a high-precision dataset that reveals substantial variability in oceanic plates and provides further insights into the oceanic K cycle. Sixty-nine sediments worldwide yield a broad δ41K range from −1.3 to −0.02‰. The unusually low values are indicative of release of heavy K during continental weathering and uptake of light K during submarine diagenetic alteration. Twenty samples of altered western Pacific crust from ODP Site 801 display δ41K from −0.60 to −0.05‰, averaging at −0.32‰. Our results indicate that submarine alteration of oceanic plates is essential for generating the high-δ41K signature of seawater. These regionally varying subducting components are heterogeneous K inputs to the mantle.

High-precision triple oxygen isotope analysis of Archean and Proterozoic carbonates

2020 ◽  
Author(s):  
Oliver Jäger ◽  
Jakub Surma ◽  
Nina Albrecht ◽  
Chris S. Marien ◽  
Wanli Xiang ◽  
...  
Keyword(s):  
Oxygen Isotope ◽  
High Precision ◽  
Isotope Ratio ◽  
Sea Water ◽  
Isotope Analysis ◽  
Isotopic Shift ◽  
Equilibrium Conditions ◽  
Mass Spectrom ◽  
Diagenetic Alteration ◽  
Gas Source

<p>Oxygen isotopes are a widely used tracer in the field of paleoceanography and provide unique information on mineral formation and environmental conditions. Carbonate sediments record a shift in δ<sup>18</sup>O of 10 to 15‰ from the Archean towards higher values in the Phanerozoic. Three different scenarios are suggested to explain this observation: (I) hot Archean oceans, (II) depletion of <sup>18</sup>O in Archean oceans compared to present day and (III) diagenetic alteration of the primary isotopic signature [1]. Recent advances in high-resolution gas source isotope ratio mass spectrometry provide a new tool that may allow to decipher the origin of this isotopic shift observed in the early rock record. We performed high-precision <sup>18</sup>O/<sup>16</sup>O and <sup>17</sup>O/<sup>16</sup>O measurements on oxygen ion fragments (<sup>16</sup>O<sup>+</sup>, <sup>17</sup>O<sup>+</sup>, <sup>18</sup>O<sup>+</sup>) generated in the ion source from CO<sub>2</sub> gas [2]. Isobaric interferences on m/z=17 (<sup>16</sup>OH<sup>+</sup>) and m/z=18 (H<sub>2</sub><sup>16</sup>O<sup>+</sup>) are separated by means of high mass resolution. The CO<sub>2</sub> gas is first liberated from carbonate samples by orthophosphoric acid digestion and then analyzed on a <em>Thermo Scientific Ultra</em> dual-inlet gas source isotope ratio mass spectrometer [3]. By adding the dimension of <sup>17</sup>O/<sup>16</sup>O to the classical<sup> 18</sup>O/<sup>16</sup>O system, equilibrium trajectories of carbonates that are defined by the equilibrium fractionation factor (<sup>18</sup>a<sub>eq</sub>) and the triple isotope fractionation exponent (θ) can be predicted as a function of temperature. Minerals that were altered by or formed in meteoric water can be distinguished from those that precipitated in equilibrium with ambient sea water. Therefore, triple oxygen isotope analysis of carbonates does not only hold the potential for a new single-phase paleothermometer, but may also be used to trace the origin of carbonates. Here, we present high-precision triple oxygen isotope data for carbonates from the Pilbara and the Kaapvaal cratons that cover nearly one billion years from the Paleoarchean to the Paleoproterozoic. Marine carbonates from the Phanerozoic complement the dataset. The carbonates were formed in different marine settings, from shallow marine stromatolites to carbonates grown in the interstitial space of basaltic pillows. Phanerozoic carbonates record equilibrium conditions with modern sea water at moderate temperatures. The majority of Precambrian carbonates plot below the predicted equilibrium curve in the δ’<sup>18</sup>O-Δ‘<sup>17</sup>O space and do not reflect equilibrium conditions with modern sea water at elevated temperatures that were proposed for the Archean oceans. Modeling the triple oxygen isotope composition of carbonates in equilibrium with sea water, that is depleted in <sup>18</sup>O also cannot explain the observed isotopic shift. Further modeling of post-depositional alteration suggests that most carbonates interacted and re-equilibrated with meteoric waters at variable water-rock ratios and temperatures.</p><p>[1] Shields and Veizer, 2002, Geochem., Geophy., Geosyst., 10.1029/2001GC000266<br>[2] Getachew et al., 2019, Rapid Commun. Mass. Spectrom., 10.1002/rcm.847<br>[3] Eiler et al., 2013, Int. J. Mass. Spectrom., 335, 45-56.</p>

How feasible was subduction in the Archean?

2014 ◽  
Vol 51 (3) ◽  
pp. 286-296 ◽  
Author(s):  
Andrew Hynes
Keyword(s):  
Heat Flow ◽  
Partial Melting ◽  
Oceanic Crust ◽  
Flexural Rigidity ◽  
Oceanic Lithosphere ◽  
Low Density ◽  
Theoretical Argument ◽  
Asthenospheric Mantle ◽  
Depleted Mantle ◽  
Oceanic Plates

As the asthenospheric mantle rises at oceanic spreading centres, it undergoes partial melting, producing oceanic crust and depleted mantle, both of which have lower intrinsic density than the asthenospheric mantle from which they were derived. With a warmer asthenosphere in the Archean, these effects are enhanced, leading to the possibility that subduction was no longer feasible. I investigate the density of the oceanic crust and underlying mantle for a mantle with temperatures 200 °C higher than today, using models of the chemistry of melting and the mineralogy of the ensuing rocks. For the melting model used, crustal thicknesses are 21 km and the depth to which the mantle is partially melted is 114 km, compared with 7 and 54 km for a comparable model of modern Earth. Two thermal-evolution models for Archean oceanic lithosphere are examined. One assumes twice the heat flow into the base of the plates, which severely restricts the depths to which the plates can cool with age. A second assumes the plates can cool to the depth to which the asthenosphere undergoes partial melting, resulting in heat flow into the base of the plates only 1.3 times as large as today. With the first model, oceanic plates do not become denser than an equivalent column of asthenosphere. With the second, they do after ∼50 Ma of cooling. In both cases, however, the cooling is sufficient to provide a significant driving force for the initiation of subduction because the sole requirement for a subduction-initiation drive is that the cooled lithosphere be denser than the column of differentiated asthenosphere that would replace it. This, combined with the low flexural rigidity of Archean plates, makes the initiation of subduction probably slightly easier than it is today. The relatively low density of oceanic plates results in lower slab pull, but this effect is counterbalanced both by the likelihood that some of the low-density crust may have been delaminated, and by the effect of passage of the thicker crust through the eclogite transition. Given our present knowledge of Archean thermal conditions, there does not appear to be a compelling theoretical argument against efficient subduction processes at that time.

Highly permeable and layered Jurassic oceanic crust in the western Pacific

1993 ◽  
Vol 119 (1-2) ◽  
pp. 71-83 ◽  
Author(s):  
Roger L. Larson ◽  
Andrew T. Fisher ◽  
Richard D. Jarrard ◽  
Keir Becker
Keyword(s):  
Oceanic Crust ◽  
Western Pacific ◽  
The Western Pacific

Seismic Imaging, Arc Magamtism and Megathrust Earthquake under the Western Pacific Subduction Zone

2020 ◽  
Author(s):  
Zhi Wang ◽  
Jian Wang
Keyword(s):  
Partial Melting ◽  
Subduction Zone ◽  
Oceanic Crust ◽  
Mantle Wedge ◽  
Western Pacific ◽  
Arc Magmatism ◽  
S Wave ◽  
Slab Melting ◽  
Megathrust Earthquake ◽  
The Western Pacific

<p>Arc magmatism and megathrust earthquake occurrence in a subduction zone are deemed to attribute to many factors, including structural heterogeneities, fluid contents, temperature, depth of subducting oceanic crust, and etc. However, how these factors affect them is unclear. The extensive arc magmatism observed on the island arcs is considered to be an indicator on chemical exchange between the wedge mantle and the surface in a subduction zone. Megathrust earthquake frequently occurrence is also considered to be attributed to the slab melting and associated interplate coupling of the subducting plate. The Western Pacific subduction zone is regarded as one of the best Laboratory for seismologists to examine these processes due to the densest seismic networks recording numerous earthquakes. Some of the previous studies suggest that the island-arc magmatism is mainly contributed to the melting of peridotite in the mantle wedge due to fluids intrusion from the dehydration process associated with the subducting oceanic crust. They further argued that the oceanic plate could only provide water to the overlying mantle wedge for arc magmatism, but not melt itself due to that it is too cold to melt at a depth between 100 and 200km. However, some of other studies revealed that the hydrated basalt derived from the mid-ocean ridge will be melted with high T and water saturated on the upper interface of the sbuducting plate in the mantle wedge. We consider that the three-dimensional (3-D) P- and S- wave velocity (Vp, Vs) and Poisson’s ratio (σ) structures of the subduction zone, synthesized from a large-quantity of high-quality direct P-, and S-wave arrival times of source-recive pairs from the well located suboceanic events with sP depth phase data could provide a compelling evidence for structural heterogeneity, highly hydrated and serpentinized forearc mantle and dehydrated fluids in the subduction zones. In this study, we combined seismic velocities and Poisson’s ratio images under the entire-arc region of the Western Pacific subduction zone to reveal their effects on megathrust earthquake generation and arc magmatism. We find that a ~10 km-thick low-velocity layer with high-V and high-Poisson’s ratio anomalies is clearly imaged along the upper interface of the subducting Pacific slab. This distinct layer implies partial melting of the oceanic crust due to the deep-seated metamorophic reactions depending on the source of fluids and temperature regime. Such a process could refertilize the overlying mantle wedge and enrich the peridotite sources of basalts under the island arc. Significant low-V and high-Poisson’s ratio anomalies were observed in the mantle wedge along the volcanic front, indicating melting or partial melting of peridotite-rich mantle and then yield tholeiitic magma there. The present study demonstrates that the combined factors of fluid content, mineral composition and thermal regime play a crucial role in both slab melting and arc-magmatism under the Western Pacific subduction zone.</p>

Sign in / Sign up

Export Citation Format

Share Document