The maximum depth of the subduction channel in modern subduction zones

The subduction channel is located directly above a downgoing oceanic plate and forms by dehydration of this plate. The ascending water-rich fluids react with the mantle to hydrous minerals such as chlorite and amphibole. This process rheologically weakens the mantle and reduces its density so that an upwards-directed mass flow is continuously generated as long as the oceanic plate is subducted. However at great depth, the fluids ascending from the subducting plate do not produce hydrous minerals anymore due to too high pressure-temperature (P-T) conditions. Thus, the question arises how high can these conditions become in order to still generate such hydrous minerals in the mantle. To answer this question, thermodynamic modelling was undertaken with PERPLE_X using different data sets of Holland and Powell (1998, 2011), corresponding solid-solution models for relevant minerals, and the bulk-rock composition of a common lherzolite + 2.5 wt% H₂O. In addition, results of experiments at high pressure on the P-T stability of hydrous minerals such as chlorite were considered.

Under the assumption of a relatively steeply and fast dipping oceanic plate, the geothermal gradient at the interface between this plate and the overlying mantle wedge should be below 7.5 °C/km (100 km = 3.2 GPa). At such low gradients, that are common in modern subduction zones, chlorite is the only (nominally) hydrous mineral in the lherzolite considered because amphibole shows an upper pressure limit, for example 2.3 GPa using model cAmph(G), in the calculation results. Calculations with the data set of Holland and Powell (1998) lead to results at pressures >3 GPa, which are, due to the used equation-of-state for minerals, incompatible with experimental results, whereas the results produced with the more recent data set (Holland and Powell, 2011) are compatible. Along gradients of 7.5, 5, and 3.5 °C/km, chlorite decomposes to form garnet in lherzolite at about 740 (3.15 GPa), 660 (4.3 GPa), and 570 °C (5.3 GPa), respectively. These temperatures are 60-80 °C lower than calculated for the reaction of chlorite + enstatite = forsterite + pyrope + H₂O in the system MgO-Al₂O₃-SiO₂-H₂O.

The aforementioned P-T conditions limit the subduction channel towards great depths, which should be less than 160 km (5.2 GPa) even at very low thermal gradients, and are compatible with peak P-T conditions of many eclogites exhumed in the subduction channel from the surface of the downgoing oceanic plate. A few exceptions were reported which suggest exhumation of eclogite from depths > 200 km (e.g., Ye et al., 2000). The reason for these greater depths could be another exhumation mechanism. However, a misinterpretation of so-called exsolution lamellae in eclogitic minerals, taken as evidence for unusual mineral compositions and, thus, depths > 200 km, is more likely (see Liu and Massonne, 2022).

Holland, T.J.B., Powell, R., 1998. J. Metamorph. Geol. 16, 309-343.

Holland, T.J.B., Powell, R., 2011. J. Metamorph. Geol. 29, 333–383.

Liu, P., Massonne, H.-J., 2022. J. Metamorph. Geol., doi: 10.1111/jmg.12649

Ye, K., et al., 2000. Nature 407, 734–736.