Mercury (Hg) in meteorites: variations in abundance, thermal release profile, mass-dependent and mass-independent isotopic fractionation

摘要

We have measured the concentration, isotopic composition and thermal releaseprofiles of Mercury (Hg) in a suite of meteorites, including both chondritesand achondrites. We find large variations in Hg concentration between differentmeteorites (ca. 10 ppb to 14'000 ppb), with the highest concentration orders ofmagnitude above the expected bulk solar system silicates value. From thepresence of several different Hg carrier phases in thermal release profiles(150 to 650 {\deg}C), we argue that these variations are unlikely to be mainlydue to terrestrial contamination. The Hg abundance of meteorites shows nocorrelation with petrographic type, or mass-dependent fractionation of Hgisotopes. Most carbonaceous chondrites show mass-independent enrichments in theodd-numbered isotopes 199Hg and 201Hg. We show that the enrichments are notnucleosynthetic, as we do not find corresponding nucleosynthetic deficits of196Hg. Instead, they can partially be explained by Hg evaporation andredeposition during heating of asteroids from primordial radionuclides andlate-stage impact heating. Non-carbonaceous chondrites, most achondrites andthe Earth do not show these enrichments in vapor-phase Hg. All meteoritesstudied here have however isotopically light Hg ({\delta}202Hg = ~-7 to -1)relative to the Earth's average crustal values, which could suggest that theEarth has lost a significant fraction of its primordial Hg. However, the lateaccretion of carbonaceous chondritic material on the order of ~2%, which hasbeen suggested to account for the water, carbon, nitrogen and noble gasinventories of the Earth, can also contribute most or all of the Earth'scurrent Hg budget. In this case, the isotopically heavy Hg of the Earth's crustwould have to be the result of isotopic fractionation between surface anddeep-Earth reservoirs.