Effects of Compositional Variation of Basalt on Subducting Slabs over Time

Abstract:

In much of Earth history, basaltic crust has been injected into the mantle due to its high density. In Archean, downwelling basaltic crust is thought to be much more mafic (Mg/Si=0.72) than that of the modern mid-ocean ridge basalt (MORB) (Mg/Si=0.25). This difference in composition may result in different behaviors of basaltic crust in the mantle with time. We have studied the two compositions of ancient basalt: x=0.8 (Mg/Si=0.72) and x=0.4 (Mg/Si=0.54). The former represents a composition at 3.5 Ga. The glass starting materials were mixed with gold (10 wt%) for internal pressure scale and loaded into the laser-heated diamond-anvil cell (LHDAC) together with a Ne or Ar medium. We have conducted in-situ X-ray diffraction at APS up to 34 GPa and 2200 K. The recovered samples have been analyzed using energy-dispersive X-ray spectroscopy (EDS) in aberration-corrected electron microscopy (ACEM) at ASU. The previous study of the modern MORB reported the post-garnet transition occurs at 26-27 GPa and 1800 K. The same transition was observed at a similar pressure range within 2 GPa for both compositions. The post-spinel transition was observed in x=0.8 at 24±2 GPa, while it does not exist in x=0.4 and modern MORB due to the absence of ringwoodite. At 34 GPa and 2200 K, mineralogy of x=0.8 consists of 82% of bridgmanite (bm), 16% of Ca perovskite (CaPv), and 2% of stishovite (stv), while the modern MORB consists of 30% of stv, 26% of CaPv, 23% of bm, and 22% of aluminous phases (in mole fraction). The mineralogy of x=0.4 is similar to x=0.8 with different proportions, but still different from that of modern MORB. The unit-cell volumes of quenched bm at 1 bar and 300 K for x=0.8 and 0.4 were 166.05(5) and 165.69(6) ų, respectively, which are smaller than bm in modern MORB (170.0(1) ų). At high pressures, the unit-cell volume of CaPv is greater in the x=0.4 and 0.8 compositions. The compositions of bm and CaPv in x=0.8 show less Al, but more Mg than those found in modern MORB. The zero-pressure density for x=0.8 is estimated to be 4.30(2) g/cm³ which is greater than that of 4.25 g/cm³ in modern MORB. Our new experimental results suggest that the Archean basaltic crust may be more negatively buoyant than modern MORB, making them viable candidates for the lower-mantle heterogeneities.