From Mike Cotterill
You described Rita Cabral’s surprising discovery of sulphur isotopes that formed under intense light (27 April, p 14), yet were found in volcanic ocean basalts only 20 million years old.
She concludes they were derived from oceanic crust that was at the surface 2.45 billion years ago when Earth lacked an ozone sunscreen and sank via subduction before rising again. But it is difficult to see how such ancient subducted crust could have regained buoyancy and risen from the mantle, so there could be an alternative explanation,
Is it possible the isotopes could be from a large basalt achondrite? This type of meteorite comes from the crust of a planetesimal, or mini-planet, heated sufficiently by impacts, radioactive decay and gravitational compression to split into layers similar to those within Earth. Basalts on a planetesimal surface, lacking an atmosphere, would clearly have been exposed to intense light.
Freshwater, Isle of Wight, UK
