Earth & Planetary Science

Research Frontiers 2023

Curved post-garnet phase boundary andits implications for mantle dynamics
Recent seismic tomography studies have revealed
that slabs stagnate at 660–1000 km depths in many
subduction zones [1] and that plumes from thelowermost mantle become invisible at a depth above
1000 km [2]. One explanation for this is that a phasetransition below the 660-km discontinuity decelerates
slab subduction and accelerates plume upwelling in
the upper part of the lower mantle. To achieve this,the sign of the Clapeyron slope must change withtemperature because a positive Clapeyron slope
enhances mantle convection (plume acceleration),whereas a negative slope impedes it (slab stagnation).
Garnet, a major mantle mineral, transforms intoa bridgmanite-bearing assemblage at a depth of~700 km. This is called the post-garnet transition andonly occurs in the upper part of the lower mantle.
Thus, determining the Clapeyron slope of the postgarnet transition is essential for understanding thedynamics of the upper part of the lower mantle.
The most accurate and precise method todetermine the Clapeyron slope of a mineral phasetransition is in situ X-ray diffraction with a multianvilpress. Our recent studies highlighted two potential
problems in accurately and precisely determining
phase boundaries, namely the pressure change
during heating and the sluggish kinetics of theinvestigated phase transitions [3,4]. An inevitable rapid
pressure change upon heating hampers the accurate
determination of the transition pressures. Althoughphase stability must be determined by observing

pairs of normal and reversed reactions, this has
not been confirmed in most previous studies. Thus,
most previous studies have misinterpreted the phaserelationships.
In this study, we have rigorously determined theboundary of the post-garnet transition in Mg3Al2Si3O12(pyrope to bridgmanite +corundum transition), a major
component of garnet, was determined using multianvil
techniques combined with in situ X-ray diffraction
using the Kawai-type multianvil press SPEED-Mk.II at
SPring-8 BL04B1 [5]. The strategy for establishing thephase boundary was as follows: (1) phase stability wascontinuously determined from the lowest temperatureapplied after sufficient annealing to release mostdeviatoric stresses, avoiding substantial pressurechanges during phase identification; (2) the low(pyrope) and high-pressure (bridgmanite + corundum)
phases always coexisted during the experiments tomonitor the direction of the reaction; and (3) the peakintensity change with time was monitored using X-ray
diffraction patterns of the sample taken under thesame conditions to identify a stable phase (Fig. 1).
These experimental treatments resulted in a precise
and accurate phase identification without rapid
pressure changes or sluggish kinetics.
The phase relations in the Mg3Al2Si3O12 systemwere determined at 25.5–26.5 GPa and 1350 –
2100 K (Fig. 2). The transition pressure changed
nonlinearly with increasing temperature, resulting ina downward boundary shape. The Clapeyron slope

2530

35

40

211(Brg)
440+404(Gt)
202(Brg, Cor)
113(Brg)
122(Brg)
611+532+325+116(Gt)620+602+206(Gt)004+220(Brg)103 (Brg)

45

Energy (keV)

023(Brg)

431+413+314(Gt)
120(Brg)
210(Brg)

224+422(Gt)

112(Brg)
332+323(Gt)
200(Brg) +110(Cor)
020(Brg)

400+004(Gt)411(Gt)111(Brg)
321+312+213(Gt)012(Cor)
012(Brg)

Intensity (arb. units)

T = 166
16666 ± 5 K
Press loadload = 11 MN

420+402+240(Gt)

1st XRD pattern → 2nd XRD pattern (Brg+Cor ↓ +Gt ↑ )

5055

60

Fig. 1. An example of the accurate determination of phase stability between garnet(Gt) and bridgmanite (Brg) plus corundum (Cor) at 25.93(5) GPa and 1666 K. The
Gt peaks are stronger in the second diffraction (green) than in the first (purple),whereas those of Brg and Cor are weaker, indicating that Gt is stable.

96Research Frontiers 2023

Mg3Al2Si3O1227

essential in forming the nonlinear boundary. Based on
previously reported heat capacity data, we proposed
an increase in the pyrope heat capacity at > 1300 K.
This can be explained by the possible disorder among
the crystallographic sites in the VIIIMg 3VIAl 2IVSi 3O 12
pyrope, as in the VIIIMg3VI[Mg,Si]IVSi3O12 majorite.
The curved post-garnet boundary explains thedynamics in the upper part of the lower mantle. In
a cold subducting region, the post-garnet boundary
has a negative slope (Fig. 3), which could stagnatea slab or decelerate the slab motion after crossing the660-km discontinuity. This may result in a tomographic
snapshot of the apparent slab stagnation below the660-km discontinuity [1]. Slabs stagnating below
the 660-km discontinuity are further promoted to
subduct into deeper regions after being heated by
the surrounding mantle because the Clapeyron slopeof the post-garnet transition approaches zero withan increase in the average temperature (Fig. 3). In a
hot upwelling region, the post-garnet boundary has apositive slope, which can accelerate plume upwelling(Fig. 3), resulting in plume thinning and reduced
visibility in seismic tomography. This scenario may
explain why tomographic images show indistinct plumetraces below a depth of 1000 km [2]. The two typesof seismic observations of slab stagnation and plume
upwelling can be consistently interpreted based on thecurved boundary of the post-garnet transition.

760
740

Bridgmanite + Corundum

Depth (km)Pressure (GPa)287202625700

Garnet680

2414001600

1800

Temperature (K)

2000

2200

Fig. 2. Phase relations of the post-garnet
transition in Mg3Al2Si3O12. The blue and redcircles indicate the stable phases of garnet
and bridgmanite plus corundum, respectively.

had a negative value of −1.5 MPa/K at 1400–1800 K,
and a positive value of +2.5 MPa/K at 1900–2100 K.
A change in the slope from positive to negative
was first discovered during the phase transitions ofmantle minerals. The Clapeyron slope is definedas the ratio of the entropy change (ΔS tr ) to thevolume change (ΔV tr ) associated with the phasetransition: dP/dT = ΔS tr /ΔV tr. As ΔV tr is negativeand only changes slightly with temperature, ΔS tr isHigher TSlab660-km discontinuity

Pgt transition

Stagnation by
negative Pgt slope

Acceleration bypositive Pgt slopes

Ambient mantle

Warming-up &re-subduction by~zero Pgt slope

Hot plume

Lower TFig. 3. Mantle dynamics in the upper part of the lower mantle controlled by phase transitions. The bold
solid line is the post-garnet (Pgt) transition boundary determined in this study. Blue arrows show the positive
buoyancies in a slab caused by the negative slopes of the post-spinel and Pgt transitions. Pink arrows indicate
the positive buoyancies in a plume by the positive slopes of the Pgt transition. The main phase transition in aplume around a depth of 660-km is the Pgt transition. The arrow sizes express buoyancy magnitudes.

Takayuki Ishii a,* and Tomoo Katsura bab

Institute for Planetary Materials, Okayama University
Bayerisches Geoinstitut, University of Bayreuth, Germany*Email: takayuki.ishii@okayama-u.ac.jp

References[1] Y. Fukao and M. Obayashi: J. Geophys. Res. SolidEarth 118 (2013) 5920.
[2] M. L. Rudolph et al.: Science 350 (2015) 1349.
[3] T. Ishii et al.: Nat. Geosci. 12 (2019) 869.
[4] A. Chanyshev et al.: Nature 601 (2022) 69.
[5] T. Ishii, D. J. Frost, E. J. Kim, A. Chanyshev, K. Nishida,B. Wang, R. Ban, J. Xu, J. Liu, X. Su, Y. Higo, Y. Tange, H.-K.
Mao and T. Katsura: Nat. Geosci. 16 (2023) 828.
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