Effects of Compositional Variation of Basalt on Subducting Slabs over Time

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) Å3, respectively, which are smaller than bm in modern MORB (170.0(1) Å3). 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.