Phase transition and density of subducted MORB crust in the lower ...

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240 K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 pressure studies on end-member compositions suggest that these minerals may undergo structural phase transitions at greater depths. Phase relation and crystal chemistry in MORB bulk composition, however, are not yet well understood under the deep lower mantle conditions [1,7]. Both experimental and theoretical studies showed that pure MgSiO3 perovskite transforms to a CaIrO3-type post-perovskite phase near the base of the mantle (e.g., [8–10]). MgSiO 3rich perovskite includes large amounts of FeO, Fe 2O 3, and Al 2O 3 in MORB bulk composition, which could significantly affect the stability of postperovskite phase. Pure SiO2 phase transforms from stishovite to CaCl2-type structure [11] and further to a-PbO2-type structure [12] in the lower mantle. Phase transition from tetragonal or orthorhombic to cubic structure in CaSiO 3 perovskite has been reported in both pure [13] and Al-bearing compositions [14]. Funamori et al. [15] demonstrated that CaFe 2O 4type MgAl2O4 transforms to CaTi2O4-type structure above 40 GPa, suggesting that phase transition of CaFe2O4-type Al-phase in MORB composition possibly occurs in the lower mantle. Here we determined the phase transition boundaries in MORB composition up to the condition of core–mantle boundary region, based on X-ray diffraction measurements insitu at high-pressure and -temperature. Phase transitions in subducted MORB crust may be the causes of seismic anomalies observed locally but in a wide range of depths in the lower mantle (e.g., [16–20]). Subduction of basaltic crust gives rise to a strong chemical heterogeneity in the mantle. The amount of basaltic component that has ever subducted into the Earth’s interior over the geological time possibly sums up to several tens percent of the whole mantle [21]. The fate of basaltic crust may be controlled by the density relationship with the surrounding mantle. Previous studies suggested that basaltic crust is buoyant in the transition zone at 660- to 720-km depth [2,5,6,22] and possibly in the lower mantle below 1500–2000-km depth [4,23]. In this paper, we determined the chemical composition of each constituent mineral by using analytical TEM. The density of MORB crust in the lower mantle was calculated from measured volume and chemical composition data. Results indicate that the subducted MORB crust is denser than the surrounding below 720-km depth to the bottom of the mantle. 2. Experimental and analytical procedures Starting material was a glass (runs #1 to #4) or gel (runs #5 to #7) (Table 1). Both are chemically identical with a composition of normal MORB (Table 2). The same glass starting material was used in our previous studies [2,5]. High P–T conditions were generated using laser-heated diamond anvil cell (LHDAC) techniques. The sample was mixed with a fine powder of gold (~10 wt.%) that served as an internal pressure standard and a laser absorber. The sample mixture (~20-Am thick) was loaded into a rhenium gasket with an initial thickness of ~50 Am, being sandwiched by pure basalt layers that was not mixed with gold (~15-Am thick for both sides). About 50-Am area was heated with a focused multimode continuous wave Nd:YAG laser using double-side heating technique, which minimizes both radial and axial temperature gradient [24]. The laser beam was not scanned on the sample. Angle-dispersive X-ray diffraction measurements were conducted in-situ at high-pressure and -temperature at BL10XU of SPring-8. A monochromatic incident X-ray beam with a wavelength of 0.41328 or 0.41688 A˚ was collimated to 20-Am in diameter. X-ray diffraction spectra were obtained on an imaging plate with exposure time of 1 to 10 min. Twodimensional X-ray diffraction image was integrated as a function of two-theta in order to give a conventional one-dimensional diffraction profile. The diffraction patterns of the sample were repeatedly collected during heating and after quenching to room temperature. The uncertainty in temperature within 20-Am area from which X-ray diffraction was collected could be F10% [14]. Pressure was determined by applying the equation of state (EOS) of gold, using two to four diffraction lines. Pressure determination strongly depends on the choice of EOS (e.g., [7]). We primarily used EOS recently proposed by Tsuchiya [25]. Tsuchiya’s gold scale is practically useful because it is consistent with platinum pressure scale at 300 K [26]. Pressures calculated by EOS of gold proposed by Jamieson et al. [27] were also presented in Table 1, which predicts larger thermal pressure than Tsuchiya’s EOS. The pressure uncertainties shown in Table 1 are derived mainly from large errors in temperature to apply P–V–T EOS.
Table 1 Volumes of coexisting minerals Run P (GPa) Au by Tsuchiya [25] P (GPa) Au by Jamieson et al. [27] K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 241 T (K) Unit-cell volumes (A˚ 3 ) MgPv [Z =4] MgPP [Z =4] St (rutile-type SiO2) [Z =2] CaCl 2-type SiO2 [Z =2] a-PbO 2-type SiO2 [Z =4] CaPv [Z =1] CF [Z =4] #1 55.4 (14) 56.4 (17) 1890 145.68 (21) 41.64 (4) 39.62 (1) 205.27 (66) 49.4 (13) 50.6 (16) 1750 147.94 (10) 42.09 (1) 40.12 (1) 208.02 (12) 39.7 (3) 38.8 (3) 300 147.96 (14) 42.21 (3) 40.25 (4) 207.86 (75) #2 59.6 (16) 60.8 (19) 2140 144.86 (12) 41.27 (3) 39.22 (1) 202.65 (65) 56.6 (15) 57.9 (18) 2060 146.31 (15) 41.70 (2) 39.59 (2) 203.93 (116) 47.1 (3) 45.6 (2) 300 146.00 (9) 41.51 (6) 39.49 (5) 204.30 (36) #3 70.6 (15) 70.9 (19) 2170 139.99 (9) 40.43 (16) 38.29 (1) 197.09 (35) 60.3 (3) 57.3 (3) 300 140.07 (7) 40.57 (5) 38.19 (6) 197.38 (15) 73.2 (12) 72.4 (15) 1770 138.75 (8) 39.97 (6) 37.93 (1) 195.43 (22) 68.6 (12) 68.3 (15) 1770 139.94 (12) 40.44 (12) 38.22 (0) 197.29 (5) 62.0 (3) 58.8 (3) 300 139.39 (13) 40.31 (6) 38.11 (5) 196.08 (21) #4 132.0 (18) 124.4 (23) 2130 124.61 (8) 74.81 (13) 34.30 (2) 175.91 (33) 122.2 (14) 110.2 (12) 300 124.09 (14) 74.71 (15) 34.25 (4) 176.11 (49) #5 85.0 (12) 83.3 (16) 1980 135.83 (21) 39.06 (11) 37.09 (1) 190.99 (28) 78.3 (8) 72.9 (7) 300 134.29 (8) 39.20 (8) 36.94 (5) 189.72 (18) 81.4 (9) 75.5 (8) 300 133.95 (12) 38.85 (6) 36.82 (5) 188.78 (19) 100.0 (24) 96.5 (27) 2060 131.81 (19) 38.04 (13) 36.17 (4) 186.24 (28) 86.4 (12) 79.8 (10) 300 132.88 (14) 38.50 (8) 36.35 (5) 186.83 (15) #6 112.6 (25) 108.0 (29) 2240 128.93 (8) 77.38 (17) 35.46 181.82 (57) 104.3 (12) 95.0 (10) 300 128.52 (6) 76.89 (8) 35.41 (4) 181.47 (28) 104.3 (13) 95.0 (11) 300 128.43 (10) 76.64 (2) 35.25 (4) 181.44 (21) 106.9 (17) 97.2 (14) 300 128.62 (10) 76.51 (5) 35.16 (5) 180.34 (61) #7 129.9 (17) 123.1 (22) 2290 125.30 (6) 75.19 (9) 34.22 (2) 177.42 127.0 (24) 120.5 (28) 2280 125.65 (15) 75.66 (13) 34.63 (3) 178.32 (62) 125.2 (13) 118.7 (18) 2170 126.18 (8) 75.92 (10) 34.73 (1) 178.62 (34) 116.4 (12) 105.2 (10) 300 125.50 (6) 75.48 (9) 34.64 (3) 177.62 (24) 118.7 (9) 107.2 (7) 300 125.40 (15) 75.24 (8) 34.53 (2) 177.72 (27) 132.3 (11) 118.9 (10) 300 122.76 (7) 73.79 (14) 33.91 (2) 173.75 (28) Numbers in parenthesis indicate uncertainties in the last digits. MgPv, MgSiO3-rich perovskite; MgPP, MgSiO3-rich post-perovskite phase; St, stishovite; CaPv, CaSiO3 perovskite; CF, CaFe2O4-type Al-phase. Table 2 Chemical compositions of starting material and coexisting phases at 60 and 113 GPa Sample SM 60 GPa (run #2) 113 GPa (run #6) MgPv St CaPv CF MgPP a-PbO2-SiO2 CaPv CF N a 7 6 3 9 7 6 3 4 SiO2 49.64 41.5 (24) 96.6 (11) 50.3 (21) 28.7 (26) 36.5 (22) 87.4 (18) 47.1 (24) 30.7 (13) TiO2 1.64 2.4 (5) – 0.3 (2) 1.6 (7) 2.9 (8) – 0.7 (6) 0.5 (1) Al2O3 14.88 13.7 (19) 3.4 (11) 2.9 (6) 35.5 (29) 12.1 (15) 12.6 (18) 4.2 (30) 37.1 (32) FeO* 11.43 20.1 (12) – 0.8 (6) 10.5 (28) 23.2 (27) – 2.7 (18) 9.6 (8) MgO 8.51 21.3 (32) – 1.6 (4) 9.0 (26) 19.5 (19) – 1.7 (7) 8.2 (12) CaO 10.55 1.0 (6) – 42.9 (9) 2.2 (23) 0.9 (9) – 43.6 (16) 1.7 (13) Na2O 2.90 – – 0.8 (4) 12.5 (12) 5.0 (14) – – 12.1 (11) K2O 0.12 – – 0.4 (4) – – – – – Proportion (wt.%) 35 (5) 17 (2) 23 (3) 25 (4) 38 (3) 23 (2) 23 (2) 16 (3) Numbers in parenthesis indicate one standard deviation in the last digits. SM, starting material [2]; MgPv, MgSiO3-rich perovskite; St, stishovite; CaPv, CaSiO3 perovskite; CF, CaFe2O4-type Al-phase; MgPP, MgSiO3-rich post-perovskite phase. a Number of analyses.
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