Chlorine and fluorine partition coefficients and abundances in sub-arc mantle xenoliths (Kamchatka, Russia): Implications for melt generation and volatile recycling processes in subduction zones

2017 ◽  
Vol 199 ◽  
pp. 324-350 ◽  
Author(s):  
A. Bénard ◽  
K.T. Koga ◽  
N. Shimizu ◽  
M.A. Kendrick ◽  
D.A. Ionov ◽  
...  
Keyword(s):  
Partition Coefficients ◽  
Subduction Zones ◽  
Mantle Xenoliths ◽  
Melt Generation

Magnetite (U-Th-Sm)/He dating: analytical challenges and application

2021 ◽  
Author(s):  
Marianna Corre ◽  
Martine Lanson ◽  
Arnaud Agranier ◽  
Stephane Schwartz ◽  
Fabrice Brunet ◽  
...  
Keyword(s):  
Laser Ablation ◽  
Subduction Zones ◽  
Major Elements ◽  
Western Alps ◽  
Planetary Science ◽  
Mantle Xenoliths ◽  
Transform Faults ◽  
Geological Processes ◽  
History Of ◽  
Geochronological Constraints

<p>Magnetite (U-Th-Sm)/He dating method has a strong geodynamic significance, since it provides geochronological constraints on serpentinization episodes, which are associated to important geological processes such as ophiolite obductions, subduction zones, transform faults and fluid circulations. Although helium content that range from 0.1 pmol/g to 20 pmol/g can routinely be measured, the application of this dating technique however is still limited due to major analytical obstacles. The dissolution of a single magnetite crystal and the measurement of the U, Th and Sm present at the ppb level in the corresponding solution, remains highly challenging, especially because of the absence of magnetite standard. In order to overcome these analytical issues, two strategies have been followed, and tested on magnetite from high-pressure rocks from the Western Alps (Schwartz et al., 2020). Firstly, we purified U, Th and Sm (removing Fe and other major elements) using ion exchange columns in order to analyze samples, using smaller dilution. Secondly, we performed in-situ analyzes by laser-ablation-ICPMS. Since no solid magnetite certified standard is yet available, we synthetized our own by precipitating magnetite nanocrystals. The first quantitative results obtained by LA-ICP-MS using this synthetic material along with international glass standards, are promising. The laser-ablation technique overcomes the analytical difficulties related to sample dissolution and purification. It thus opens the path to the dating of magnetite (and also spinels) in various ultramafic rocks such as mantle xenoliths or serpentinized peridotites in ophiolites.</p><p>Schwartz S., Gautheron C., Ketcham R.A., Brunet F., Corre M., Agranier A., Pinna-Jamme R., Haurine F., Monvoin G., Riel N., 2020, Unraveling the exhumation history of high-press ure ophiolites using magnetite (U-Th-Sm)/He thermochronometry. Earth and Planetary Science Letters 543 (2020) 116359.</p>

Melting processes at convergent plate boundaries: from melt segregation, extraction to the formation of crustal magmatic systems

2020 ◽  
Author(s):  
Harro Schmeling
Keyword(s):  
Continental Crust ◽  
Subduction Zones ◽  
Two Phase Flow ◽  
Solidus Temperature ◽  
Plate Boundaries ◽  
Phase Flow ◽  
Two Phase ◽  
Convergent Plate Boundaries ◽  
Melt Generation ◽  
Open Question

<p><strong>Melting at convergent plate boundaries</strong></p><p>At divergent plate boundaries hot mantle upwelling is associated with abundant melt generation and volcanism. At convergent plate boundaries such as subduction zones and continental collision zones thick and cold plates feed mantle downwellings. Yet these "cold" regions also show abundant volcanic activity with mean volcanic output rates of almost similar order of magnitudes (White et al., 2006, G-cubed). Responsible melt generation mechanisms are addressed including a) volatile driven decrease of the solidus temperature, b) decompressional melting in the mantle wedge or in shallow asthenosphere associated with delamination, or c) increased radiogenic heating within thickened continental crust.     </p><p><strong>Melt transport mechanisms</strong></p><p>The above processes form partially molten regions. By which mechanism(s) does the melt segregate out of the melt source region and rise through the mantle or crust. The basic mechanism is two-phase flow, i.e. a liquid phase percolates through a solid, viscously deforming matrix. The corresponding equations and related issues such as compaction or effective matrix rheology are addressed. Beside simple Darcy flow, special solutions of the equations are addressed such as solitary porosity waves. Depending on the bulk to shear viscosity ratio of the matrix and the non-dimensional size of these waves, they show a variety of features: they may transport melt over large distances, or they show transitions from rising porosity waves to diapiric rise or to fingering. Other solutions of the equations lead to channeling, either mechanically or chemically driven. One open question is how do such channels transform into dykes which have the potential of rising through sub-solidus overburden. A recent hypothesis addresses the possibility that rapid melt percolation may reach the thermal non-equilibrium regime, i.e. the local temperature of matrix and melt may evolve differently.  Once dykes have been formed they may propagate upwards driven by melt buoyancy and controlled by the ambient stress field. As another magma ascent mechanism diapirism is addressed.  </p><p><strong>Modelling magmatic systems in thickened continental crust </strong></p><p>Once basaltic melts rise from subducting slabs, they may underplate continental crust and generate silicic melts. Early dynamic models (Bittner and Schmeling, 1995, Geophys. J. Int.) showed that such silicic magma bodies may rise to mid-crustal depth by diapirism. More recent approaches (e.g. Blundy and Annan, 2016, Elements) emplace sill intrusions into the crust at various levels and calculate the thermal and melting effects responsible for the formation of mush zones. Recently Schmeling et al. (2019, Geophys. J. Int.) self-consistently modelled the formation of crustal magmatic systems, mush zones and magma bodies by including two-phase flow, melting/solidification and effective power-law rheology. In these models melt is found to rise to mid-crustal depths by a combination of compaction/decompaction assisted two-phase flow, sometimes including solitary porosity waves, and diapirism. An open question in these models is whether or how dykes may self-consistently form to transport the melts to shallower depth. First models which combine the two-phase flow crustal models with elastic dyke-propagations models (Maccaferri et al., 2019, G-cubed) are promising.      </p><p>      </p>

Imaging Hales Discontinuity beneath India: Seismological and Petrological Model

2020 ◽  
Author(s):  
Jashodhara Chaudhury ◽  
Supriyo Mitra ◽  
Tapabrato Sarkar
Keyword(s):  
Subduction Zones ◽  
Receiver Function ◽  
Joint Inversion ◽  
Perfect Match ◽  
Mantle Peridotite ◽  
Mantle Xenoliths ◽  
P Wave ◽  
Phase Velocity Dispersion ◽  
Forward Calculation ◽  
Hales Discontinuity

<p>We model the depth and Vs structure of the Hales discontinuity (H-D) beneath Eastern Dharwar Craton (EDC) and Southern Granulite Terrain (SGT) using P-wave receiver function (P-RF) analysis and joint inversion with Rayleigh wave phase velocity dispersion. We calculate P-RFs at higher frequency (fmax 0.46 Hz), compared to previous studies, to show that the P-to-S converted phase from the H-D (Phs) is distinct from the crustal reverberations. The Phs at stations in the EDC arrive at ~10 s beneath GBA, and ~11 s beneath HYB. From joint inversion the H-D is modeled at 97 ± 5 km and 108 ± 5 km depth, with 5% and 3% Vs increase, beneath GBA and HYB, respectively. For KOD, in SGT, the Phs coincides with the mid-crustal PpSs+PsPs reverberation at most ray-parameters, causing destructive intereference. This explains the apparent absence of Phs in previous studies. We isolated P-RFs where Phs is distinct at ~10.5 s and model it at depth of 101 ± 5 km with Vs increase of 3%. We demonstrate through forward calculation that the spinel-garnet mineral transformation cannot explain the H-D Vs increase. From data of mantle xenoliths in the Wajrakarur kimberlite field, Southern India, we calculate Vs of mantle peridotite and eclogite, using published bulk rock compositions through Perple-X. At the H-D depth and temperature derived from Indian shield geotherm, we observed a perfect match to the Vs. We hypothesize that H-D marks the surface of a paleo-subducted eclogitic oceanic slab embeded within the upper mantle peridotite. Observations of mantle faults within the Canadian lithosphere, at similar depth, has been related to relict-subduction zones and therefore independently supports our model.</p>

Rheology and thermal structure of the lithosphere beneath the Hawaiian Ridge inferred from gravity data and models of plate flexure

2020 ◽  
Vol 222 (1) ◽  
pp. 207-224 ◽  
Author(s):  
Alexandra Pleus ◽  
Garrett Ito ◽  
Paul Wessel ◽  
L Neil Frazer
Keyword(s):  
Brittle Failure ◽  
Subduction Zones ◽  
Flexural Rigidity ◽  
Thermal Structure ◽  
Gravity Data ◽  
Mantle Xenoliths ◽  
Model Parameters ◽  
Plate Flexure ◽  
Hawaiian Ridge ◽  
Flow Laws

SUMMARY We examine the rheology and thermal structure of the oceanic lithosphere, expressed in situ by plate flexure beneath the Hawaiian Ridge, where volcanoes of variable sizes have loaded seafloor of approximately the same age, and thus where the lithosphere is expected to have had an approximately uniform age-dependent thermal structure at the time of loading. Shipboard and satellite-derived gravity, as well as multibeam bathymetry data are used in models of plate flexure with curvature-dependent flexural rigidity, the strength of which is limited, in the shallow lithosphere, by brittle failure, and in the deeper lithosphere, by low-temperature plasticity (LTP). We compute relative likelihoods and posterior probabilities for four model parameters: average crustal density ρc, friction coefficient for brittle failure ${\mu _f}$, a pre-exponential weakening factor F controlling the strength of LTP and lithospheric geotherm age t. Results show that if the lithosphere temperatures were as is expected for normal (t = ) 90-Myr-old seafloor at the time of volcano loading, the rheology must be significantly weaker than expected. Specifically, weak brittle strengths (μf ≤ 0.3) show relatively high probabilities for three of the six published LTP flow laws examined. Alternatively, moderate-to-large brittle strengths (μf ≥ 0.5) require all LTP flow laws to be substantially weakened with F = 102 to > 108 or, equivalently, activation energy reduced by 10–35 per cent. In contrast, if the lithosphere has been moderately reheated by the Hawaiian hotspot, represented by geotherms for t = 50–70 Myr, then the flow laws of Evans & Goetze, Raterron et al. and Krancj et al. require little or no weakening. Such modest thermal rejuvenation is allowed by heatflow constraints, supported by regional mantle seismic tomography imaging as well as compositions of mantle xenoliths, and reconciles previously noted discrepancies between the LTP strengths of lithosphere beneath Hawaii versus that entering the Pacific subduction zones.

Partition Coefficient Ratios and Tumour Perfusion Studied with 85mKr and 133Xe

1987 ◽  
Vol 26 (06) ◽  
pp. 253-257
Author(s):  
M. Mäntylä ◽  
J. Perkkiö ◽  
J. Heikkonen
Keyword(s):  
Partition Coefficient ◽  
Partition Coefficients ◽  
Regional Blood Flow ◽  
Blood Perfusion ◽  
Compartmental Analysis ◽  
Malignant Tumour ◽  
Mean Value ◽  
Perfusion Rate ◽  
Biological Variables ◽  
The Mean

The relative partition coefficients of krypton and xenon, and the regional blood flow in 27 superficial malignant tumour nodules in 22 patients with diagnosed tumours were measured using the 85mKr- and 133Xe-clearance method. In order to minimize the effect of biological variables on the measurements the radionuclides were injected simultaneously into the tumour. The distribution of the radiotracers was assumed to be in equilibrium at the beginning of the experiment. The blood perfusion was calculated by fitting a two-exponential function to the measuring points. The mean value of the perfusion rate calculated from the xenon results was 13 ± 10 ml/(100 g-min) [range 3 to 38 ml/(100 g-min)] and from the krypton results 19 ± 11 ml/(100 g-min) [range 5 to 45 ml/(100 g-min)]. These values were obtained, if the partition coefficients are equal to one. The equations obtained by using compartmental analysis were used for the calculation of the relative partition coefficient of krypton and xenon. The partition coefficient of krypton was found to be slightly smaller than that of xenon, which may be due to its smaller molecular weight.

Petrologic and geochemical role of serpentinite in subduction zones and plate interface domains

2015 ◽  
Vol 37 ◽  
pp. 61-64
Author(s):  
Marco Scambelluri ◽  
Enrico Cannaò ◽  
Mattia Gilio ◽  
Marguerite Godard
Keyword(s):  
Subduction Zones ◽  
Plate Interface
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