Phase relations and phase transitions in subducted slabs

Subducting oceanic lithosphere shows marked vertical stratification and can be generalized as having a top thin layer (~ 1 km) of terrigenous and pelagic sediments, covering layers of basaltic-gabbroic oceanic crust of about 6 km thick overlying thicker layers (~ 50-100 km) of melt depleted harzburgite (5-20 km) and followed by more fertile lherzolite (Fig. 1.6).

During subduction a significant section of the sedimentary layer may be scraped off the slab to form a fore-arc accretionary wedge. A thermal and rheological boundary layer must also form most likely within the less depleted lherzolite material to decouple the asthenospheric mantle from the subducting lithosphere. A slab approaching the 660 km seismic discontinuity can be reasonably simplified as comprising a basaltic crust (MORB) and underlying harzburgite rocks (Irifune and Tsuchiya, 2015). The compositions of harzburgite and MORB are different from pyrolite (Table 1.2), leading as a result to different mineralogies at high pressure and high temperature conditions corresponding to the Earth’s mantle.

23 Figure 1.6 Schematic sections of the subducting oceanic lithosphere (modified from Ringwood, 1991).

Table 1.2 Representative chemical compositions of pyrolite, MORB and Harzburgite.

Pyrolite Harzburgite MORB

SiO2 44.5 43.6 50.4

TiO2 0.2 - 0.6

Al2O3 4.3 0.7 16.1

Cr2O3 0.4 0.5 -

FeO 8.6 7.8 7.7

MgO 38.0 46.4 10.5

CaO 3.5 0.5 13.1

Na2O 0.4 - 1.9

K2O 0.1 - 0.1

Notes: Pyrolite, Sun (1982); Harzburgite, Michael and Bonatti (1985); MORB, Green (1979).

24

orthopyroxene and ~ 5 vol.% garnet. Thus, the mineralogy of harzburgite is dominated by the phase transformations in olivine described in section 1.3.1 and orthopyroxene.

Orthopyroxene transforms to clinopyroxene at ~ 10 GPa and then into garnet at transition zone conditions. At ~ 19-22 GPa, ringwoodite (~ 89 vol.%) coexists with garnet (~ 8 vol.%) and a minor amount of stishovite (~ 3 vol.%). Due to the low Al content, the (Mg,Fe)SiO3

pyroxene component is not able to be totally incorporated into garnet at high pressures, and an additional phase, akimotoite ((Mg,Fe)SiO3 with an ilmenite-type structure) (~ 6-10 vol.%) coexists with ringwoodite (~ 82 vol.%) and majoritic garnet (~ 7-12 vol.%) at pressure above 22 GPa (~ 600 km) (Irifune and Ringwood, 1987a; Ringwood, 1991). This assemblage transforms to bridgmanite (~ 75 vol.%) plus ferropericlase (~ 25 vol.%) near the 660 km discontinuity (Irifune and Ringwood, 1987a; Ringwood, 1991). Because the transformation of akimotoite to bridgmanite occurs at lower pressures compared with ringwoodite, the bridgmanite stability field will be shifted to shallower depth compared to a pyrolitic composition (Irifune and Ringwood, 1987a). Although no experimental data is available at pressures higher than 26 GPa, the phase relations in harzburgite at lower mantle conditions can be inferred based on changes in the two constituent phases—bridgmanite and ferropericlase which have been extensively studied in other bulk compositions (Irifune and Tsuchiya, 2015). The mineral assemblages of harzburgite as a function of depth are shown in Figure 1.5c.

1.3.2.2 Mid-ocean ridge basalt (MORB)

Phase transitions in basaltic compositions as illustrated in Figure 1.5b are quite different from those expected in pyrolitic and harzburgitic compositions due to the higher Al, Ca and

25 Na and lower Mg contents. At the uppermost mantle conditions, MORB is comprised of clinopyroxene, garnet and an additional Si phase, i.e. coesite (~ 10 vol.%) (Irifune and Ringwood, 1987b, 1993). At pressures between 4-10 GPa, the relative proportions of garnet and pyroxene only change a little (<10 vol.%). Above 10 GPa, coesite transforms to stishovite (St) and Ca-rich clinopyroxene progressively dissolves into garnet (Irifune and Ringwood, 1987b, 1993). At transition zone conditions (14-15 GPa), Ca-rich clinopyroxene is entirely dissolved into garnet, forming a garnetite assemblage (majorite garnet+small amount of St) (Irifune and Ringwood, 1987b, 1993). CaSiO3 perovskite (Ca-Pv) begins to exsolve from majorite (Mj) at ~ 20 GPa (Akaogi, 2007; Irifune and Ringwood, 1987b) and a mineral assemblage of Mj (~ 74 vol.%) +St (~ 10 vol.%) +Ca-Pv (~ 16 vol.%) is observed at 24 GPa (Hirose et al., 1999; Irifune and Ringwood, 1993). The assemblage progressively changes to an assemblage of bridgmanite (~ 40 vol.%), CaSiO3 perovskite (~ 22 vol.%), stishovite (~ 20 vol.%) and an Al-rich phase (hexagonal or calcium-ferrite (CF)/ calcium-titanite (CT) structures, ~ 18 vol.%) over a wide pressure range from ~ 24 to 27 GPa (Akaogi, 2007; Hirose et al., 1999; Irifune and Ringwood, 1993; Irifune and Tsuchiya, 2015; Ono et al., 2001; Trønnes, 2010). Because only garnet is involved in the transformation to bridgmanite, the stability field of bridgmanite is shifted to greater depth compared with pyrolitic compositions. This assemblage of Brg+Ca-Pv+St+Al-phase is stable in the upper part of the lower mantle. Stishovite transforms to a CaCl2-type structure at 62 GPa (~ 1500 km) (Hirose et al., 2005; Ono et al., 2002) which further transforms to α-PbO2-type structure at ~ 120 GPa (~ 2600 km) (Hirose et al., 2005; Murakami et al., 2003). The most abundant mineral, bridgmanite, then undergoes the phase transition to the CaIrO3-type post-perovskite phase above 110 GPa at 2500 K (Hirose et al., 2005).

When the oceanic lithosphere subducts into the mantle, a density contrast between MORB and the surrounding pyrolite, due to their different mineralogy, is expected. At transition zone conditions, the garnetite facies of MORB consisting of majorite, stishovite and CaSiO3 perovskite, are denser than the mineral assemblage of the surrounding pyrolite mantle. However, a density crossover is expected to occur at 660-720 km depth due to the slow garnet to bridgmanite phase transition. Therefore, the oceanic crust may be

26 gravitationally trapped at this depth. Nevertheless, MORB will be denser than pyrolite at depths greater than ~ 720 km and throughout almost the entire region of the lower mantle once bridgmanite and Ca perovskite are formed. As a result, if slabs accumulate to a sufficient thickness at the top of the lower mantle, they may have the chance to sink into the deeper lower mantle (Akaogi, 2007; Hirose et al., 1999).

Due to the slow solid-state homogenization processes in the mantle, equilibration between cold subducted slabs and the surrounding mantle is only expected to occur at lengths scales of the order of meters (Holzapfel et al., 2005). Therefore, the mantle may be a disequilibrium mechanical mixture of different rock types with varying length scales. In fact, geophysical observations have confirmed the existence of small-scale heterogeneities in the mantle that scatter seismic waves, which may be attributed to recycled oceanic crust based on the size of the scatter (Frost, 2008; Kaneshima and Helffrich, 1999, 2003; Vinnik et al., 2001). Moreover, the cold subducting slabs are also far from being in thermal equilibrium with the surrounding mantle although the temperature difference may decrease with depth. The unique chemical compositions and temperature of the subducted oceanic lithosphere would give rise to distinct seismic velocities and transport properties compared to the surrounding pyrolite mantle, which may have considerable geodynamic significance.