skip to main content


Title: Thermoelastic Properties of Eclogitic Garnets and Omphacites: Implications for Deep Subduction of Oceanic Crust and Density Anomalies in the Upper Mantle
Abstract

Synchrotron‐based high‐pressure/high‐temperature single‐crystal X‐ray diffraction experiments to ~24 GPa and 700 K were conducted on eclogitic garnets (low‐Fe: Prp28Alm38Grs33Sps1and high‐Fe: Prp14Alm62Grs19Adr3Sps2) and omphacites (low‐Fe: Quad57Jd42Ae1and high‐Fe: Quad53Jd27Ae20), using an externally heated diamond anvil cell. Fitting the pressure‐volume‐temperature data to a third‐order Birch‐Murnaghan equation of state yields the thermoelastic parameters including bulk modulus (KT0), its pressure derivative (KT0), temperature derivative ((∂KT/∂T)P), and thermal expansion coefficient (αT). The densities of the high‐Fe and low‐Fe eclogites were then modeled along typical geotherms of the normal mantle and the subducted oceanic crust to the transition zone depth (550 km). The metastable low‐Fe eclogite could be a reason for the stagnant slabs within the upper range of the transition zone. Eclogite would be responsible for density anomalies within 100–200 km in the upper mantle of Asia.

 
more » « less
Award ID(s):
1722969
NSF-PAR ID:
10460791
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geophysical Research Letters
Volume:
46
Issue:
1
ISSN:
0094-8276
Page Range / eLocation ID:
p. 179-188
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Identifying and locating the geochemical and geophysical heterogeneities in the Earth’s interior is one of the most important and challenging tasks for the deep Earth scientists. Subducted oceanic crust metamorphizes into the dense eclogite in the upper mantle and is considered as a major cause of geochemical and geophysical heterogeneities in the deep Earth. In order to detect eclogitic materials inside the Earth, precise measurements of the high pressure‐temperature single‐crystal elasticity of major minerals in eclogite are thus exceedingly important. Omphacite, a Na,Al‐bearing clinopyroxene, constitutes up to 75 vol% of eclogite. In the present study, we performed the first high pressure‐temperature single‐crystal elasticity measurements of omphacite using Brillouin spectroscopy. Utilizing the finite‐strain approach, we obtained the following thermoelastic parameters for omphacite:KS0’ = 4.5(1),G0’ = 1.53(5), ∂KS0/∂T = −0.029(5) GPa/K, ∂G0/∂T = −0.013(5) GPa/K, withKS0 = 123(3) GPa,G0 = 74(2) GPa, andρ0 = 3.34(1) g/cm3. We found that the seismic velocities of undeformed eclogite are similar to pyrolite at the depths of 200–300 and 410–500 km, thus eclogite is seismically invisible at these depths. Combined with the lattice‐preferred orientations of the omphacite in naturally deformed eclogites, we also modeled seismic anisotropy of eclogite at various pressure‐temperature conditions. A 10 km thick subducted eclogitic crust can result in ∼0.2 s shear wave splitting in the Earth’s upper mantle.

     
    more » « less
  2. Abstract

    Single‐crystal X‐ray diffraction and Brillouin spectroscopy experiments were performed on a natural Cr‐pyrope (Prp71.0Alm12.6Sps0.7Grs3.5Uvr12.2) at high pressure and high temperature up to 11.0 GPa and 800 K. Fitting the collected data to the third‐order finite strain equation yields bulk modulus (KS0), shear modulus (G0), their pressure ((∂KS/∂P)Tand (∂G/∂P)T) and temperature ((∂KS/∂T)Pand(∂G/∂T)P) derivatives,KS0 = 167.7(8) GPa,G0 = 91.5(5) GPa, (∂KS/∂P)T = 4.3(1), (∂G/∂P)T = 1.4(1), (∂KS/∂T)P = 0.0175(1) GPa/K and (∂G/∂T)P = 0.0073(1) GPa/K. Using the obtained results, we examined whether the elastic properties of the Cr‐pyrope can be accurately calculated from those of endmembers including pyrope, almandine, grossular, and uvarovite assuming a linear relationship between elastic properties and composition (end‐member model). The results indicate that the end‐member model provides a sufficient approximation for the elastic properties of Cr‐pyrope in calculating the density and velocity of the subcontinental lithospheric mantle (SCLM). We modeled the densities and velocities of three typical types of SCLM (Archon, Proton, and Tecton) in order to investigate how the variation of chemical composition influences the SCLM. We obtained that the compositional change from the Archon to the Tecton increases the density of the SCLM significantly, which can be an important prerequisite for SCLM delamination. However, the compositional variation only slightly changes the velocity of the SCLM and the change is within the uncertainty of the calculation. Moreover, in comparison to the velocity,ρ/VPandρ/VSare much more sensitive to the compositional change of the SCLM.

     
    more » « less
  3. Abstract

    Eclogite thermobarometry is crucial for constraining the depths and temperatures to which oceanic and continental crust subduct. However, obtaining the pressure and temperature (P–T) conditions of eclogites is complex as they commonly display high‐variance mineral assemblages, and the mineral compositions only vary slightly withP–T. In this contribution, we present a comparison between two independent and commonly used thermobarometric approaches for eclogites: conventional thermobarometry and forward phase‐equilibrium modelling. We assess how consistent the thermobarometric calculations are using the garnet–clinopyroxene–phengite barometer and garnet–clinopyroxene thermometer with predictions from forward modelling (i.e. comparing the relative differences between approaches). Our results show that the overall mismatch in methods is typically ±0.2–0.3 GPa and ±29–42°C although differences as large as 80°C and 0.7 GPa are possible for a few narrow ranges ofP–Tconditions in the forward models. Such mismatch is interpreted as the relative differences among methods, and not as absolute uncertainties or accuracies for either method. For most of the investigatedP–Tconditions, the relatively minor differences between methods means that the choice in thermobarometric method itself is less important for geological interpretation than careful sample characterization and petrographic interpretation for derivingP–Tfrom eclogites. Although thermobarometry is known to be sensitive to the assumedXFe3+of a rock (or mineral), therelativedifferences between methods are not particularly sensitive to the choice of bulk‐rockXFe3+, except at high temperatures (>650°C, amphibole absent) and for very large differences in assumedXFe3+(0–0.5). We find that the most important difference between approaches is the activity–composition (a–x) relations, as opposed to the end‐member thermodynamic data or other aspects of experimental calibration. When equivalenta–xrelations are used in the conventional barometer,Pcalculations are nearly identical to phase‐equilibrium models (ΔP < 0.1). To further assess the implications of these results for real rocks, we also evaluate common mathematical optimizations of reaction constants used for obtaining the maximumP–Twith conventional thermobarometric approaches (e.g. using the highestaGrs2 × aPrp in garnet and Si content in phengite, and the lowestaDi in clinopyroxene). These approaches should be used with caution, because they may not represent the compositions of equilibrium mineral assemblages at eclogite facies conditions and therefore systematically biasP–Tcalculations. Assuming method accuracy, geological meaningfulPmaxat a typical eclogite facies temperature of ~660°C will be obtained by using the greatestaDi,aCel, andaPrp and lowestaGrs andaMs; garnet and clinopyroxene with the lowest Fe2+/Mg ratios may yield geological meaningfulTmaxat a typical eclogite facies pressure of 2.5 GPa.

     
    more » « less
  4. Abstract

    Omphacite is a major mineral phase of eclogite, which provides the main driving force for the slab subduction into the Earth's interior. We have measured the single‐crystal elastic moduli of omphacite at high pressures for the first time up to 18 GPa at ambient temperature using Brillouin spectroscopy. A least squares fit of the velocity‐pressure data to the third‐order finite strain equation of state yieldsKS0′ = 4.5 (3),G0′ = 1.6 (1) withρ0 = 3.34 (1) g/cm3,KS0 = 123 (3) GPa, andG0 = 74 (2) GPa. In addition, the synchrotron single‐crystal X‐ray diffraction data have been collected up to 18 GPa and 700 K. The fitting to Holland‐Powell thermal‐pressure equation of state yieldsKT0′ = 4.6 (5) andα0 = 2.7 (8) × 10−5 K−1. Based on the obtained thermoelastic parameters of omphacite, the anisotropic seismic velocities of eclogite are modeled and compared with pyrolite between 200 and 500 km. The largest contrast between the eclogite and pyrolite in terms of seismic properties is observed between ~310 and 410 km.

     
    more » « less
  5. Abstract

    Jeffbenite (Mg3Al2Si3O12) is a tetragonal phase found in so far only in superdeep diamonds, and its thermoelastic parameters are a prerequisite for determining entrapment pressures as it is regarded as a potential indicator for superdeep diamonds. In this study, the thermoelastic properties of synthetic Fe3+‐jeffbenite were measured up to 33.7 GPa and 750 K. High‐temperature static compression data were fitted, giving (∂KT0/T)P = −0.0107 (4) GPa/K andαT = 3.50 (3) × 10−5 K−1. The thermoelastic properties and phase stability are applied to modeling isomekes, orP‐Tpaths intersecting possible conditions of entrapment in diamond. We calculate that under ideal exhumation, jeffbenite entrapped at mantle transition zone conditions will exhibit a high remnant pressure at 300 K (Pinc) of ∼5.0 GPa. Elastic geobarometry on future finds of jeffbenite inclusions can use the new equation of state to estimate entrapment pressures for this phase with still highly uncertain stability field in the mantle.

     
    more » « less