Accurate determination of pressure
is essential for laboratory experiments to contribute to the geophysical understandings of the deep interiors of Earth, other planets in the solar system, and exoplanets. As Orson Anderson demonstrated through his important work, thermal equations of state (EOS) of the standard materials can be used for estimating pressures from measurements of volume and temperature in high-pressure experiments [1
The development of third-generation synchrotron facilities in the late 1990s and early 2000s enabled in situ measurements of phase boundaries and physical properties, allowing mineral physicists to take advantage of the thermal EOS established by Anderson and others. However, early experiments found that results from different pressure scales and different experimental techniques differed by 2–3 GPa for the important phase boundaries near the 660-km discontinuity [2
]. Intense efforts have been made since then [7
], but the discrepancy for some important phase boundaries still remain unresolved [10
]. Other potential sources of the discrepancy have been investigated. For example, pressure effects on the thermocouple electromotive force (emf) calibration are important for large-volume press (LVP) experiments [12
]. Spectroradiometry has been the standard method for temperature measurements in laser heating, but optical effects from diamond anvils and thermal gradients could potentially introduce artifacts in the measured temperature [15
]. While the accuracies of spectroradiometry through LHDAC [19
] and thermocouple emf [13
] have been separately investigated, temperatures from spectroradiometry and thermocouples have never been cross examined at high pressure to our knowledge.
Comparing phase boundaries provides an opportunity to examine the pressure and temperature scales in high-pressure apparatuses. However, in such an effort, it is difficult to separate pressure effects and temperature effects [10
]. It is desirable to have a reference point in the
space for such comparison. At pressures between 20 and 24 GPa, the triple point exists between bridgmanite (Bm), akimotoite (Ak), and majorite (Mj) in MgSiO3
. Not only does understanding of the triple point provide a new opportunity to compare pressures and temperatures measured in different techniques but also the triple point itself is important in geophysics for understanding the origin of the seismic discontinuity structures near 660-km depths. Although the 660-km discontinuity has been related mainly to the post-spinel transition in Mg2
for many decades [20
], it has been well known that the phase boundaries in MgSiO3
can exist at depths very close to the 660-km discontinuity and therefore affect the complex discontinuity structures at the bottommost mantle transition zone [21
]. In this paper, we report the triple point between Bm, Ak, and Mj (BAM) in pure MgSiO3
measured in both LVP and LHDAC. We compare the pressure and temperature of the BAM triple point from those two separate measurements and discuss possible sources of discrepancy between LVP and LHDAC.
conditions of the Bm–Ak–Mj (BAM) triple point that we obtained are 19.9 ± 0.4 GPa and 2000 ± 50 K for LVP and 23.8 ± 0.6 GPa and 1990 ± 100 K for LHDAC (Figure 4
). Despite the fact that they can be biased by different systematic error sources (such as pressure effects on thermocouple emf calibration in LVP and optical effects on spectroradiometry through diamond anvils in LHDAC), a remarkable agreement was found in temperature for the triple point from both techniques. To our knowledge, this is the first direct cross examination of the two temperature measurement techniques. Such an agreement may not necessarily be applicable for pressures much lower or much higher than the range in which we conducted our measurements, i.e., 20–24 GPa, because some of the perceived systematic error sources could be pressure dependent, such as pressure effects on thermocouple emf calibration. However, for the pressure range of the mantle transition zone, our result provides important experimental confirmation for comparing temperature measurements from LVP and LHDAC experiments.
Our results suggest that pressure calibration is the most important issue to resolve in comparing LVP and LHDAC datasets of a 3.9 GPa difference. So far, LHDAC studies have reported systematically higher pressures for the phase boundaries in the mantle transition zone by 2–3 GPa compared with LVP studies [2
]. Our new results reported here also confirm the trend but with a greater magnitude. The difference is particularly important to resolve because the COMPRES 8/3 assembly and its pressure calibration [23
] have been widely used in high-pressure studies.
The calibration for the 8/3 assembly was conducted through in situ measurements using the tungsten EOS [27
] in Leinenweber et al. [23
]. For our LHDAC experiments, we chose to use the thermal EOS of Pt by Dorogokupets and Dewaele [29
]. This pressure scale is known to yield better agreements with the Au and MgO scales according to Ye et al. [11
] for a wide pressure range. In order to ensure the consistency between LHDAC and LVP results, it would be useful to examine the agreements between the W scale and the Pt scale at in situ high
. However, the high shear strength of tungsten can be a potential issue for the accurate determination on its EOS [38
]. Therefore, the COMPRES 8/3 assembly can be calibrated using other pressure standards. Pt, Au, and MgO could be good candidates as they have been used widely in LHDAC. However, despite the agreements over a larger pressure range, the MgO, Au, and Pt scales by Dorogokupets and Dewaele [29
] are different by ∼2 GPa at 20–40 GPa and high temperatures according to Ye et al. [11
]. Therefore, an important challenge still remains for the thermal EOS of important materials at the pressure range for the mantle transition zone. It is notable that some of the standard materials (particularly Au) have low melting temperatures compared with the mantle geotherm at pressures of
GPa, potentially causing significant anharmonic effects in their EOS or even abnormal premelting behaviors.
The seismic properties of the 660-km discontinuity are in general agreement with those of the post-spinel transition [40
]. Ishii et al. [37
] showed that the Ak–Bm boundary should be close to the post-spinel boundary within 1 GPa [37
]. Figure 4
shows that the Ak–Bm and Mj–Bm boundaries in MgSiO3
measured by LHDAC are closer to the
conditions expected for the 660-km discontinuity. Does this mean that LHDAC yields more reliable results for the location of the phase boundary? This approach is not desirable as the experimental methods should be able to address the question of whether the mantle phase boundaries are indeed the source of the 660-km discontinuity rather than the other way around.
Some former LVP studies have measured the Ak–Mj, Mj–Bm, and Ak–Bm boundaries in pure MgSiO3
and inferred the triple point [4
]. However, the reported
conditions for the boundaries and the triple point do not agree with each other: the discrepancy can be as large as 2 GPa in pressure and 300 K in temperature among the LVP measurements. In terms of temperature, our results are in best agreement with the most recent report by Ishii et al. [37
]. They reported stability of Ak up to 1973 K at 22.3 GPa, and the triple point in their phase diagram can be inferred to be 2035 ± 60 K, which is in agreement with our LVP and LHDAC results on the temperature of the Bm–Ak–Mj (BAM) triple point within 100 K. This agreement is encouraging in that at least the recent studies converge on the temperature of the BAM triple point within 100 K even between different high-pressure techniques (LVP and LHDAC) and between different temperature measurement techniques (thermocouple W5%Re-W26%Re in our LVP study; Pt/Pt-13%Rh in the LVP study by Ishii et al. [37
]; and spectroradiometry in our LHDAC study). As efforts are being made for enhancing the accuracy of thermocouple emf calibrations at high pressures [13
], it remains to be seen if future calibration work on the thermocouples used in this study and in Ishii et al. [37
] can find further improvement in the agreement.
The pressure inferred for the BAM triple point in Ishii et al. [37
] is between our LVP and LHDAC results, located approximately in the middle (Figure 4
). They calibrated pressures at high temperatures based on previously reported boundaries in Mg2
, and MgAl2
(see Ishii et al. [37
] for references) which are all different in pressure calculation methods. If we were to use the same Ak–Bm transition pressure point (at 1873 K and 22.3 GPa) that is used in Ishii et al. [37
] as an internal calibration point, our multi-anvil BAM triple point would lie at almost the same P
as that in Ishii et al. [37
]. Although it would depend on experimental setup, including the sample geometry and anvil materials, possible pressure change during heating is an important factor to consider for improving pressure estimation in LVP experiments [46
]. In order to further gain insight into the differences between the DAC and LVP pressures and to possibly close the gap, it would be worthwhile to make detailed measurements on the BAM triple point in situ using the same sample and Pt pressure standard that was used in the LHDAC.