Open AccessRole of recycled oceanic crust in the potassium and argon budget of the Earth: Toward a resolution of the “missing argon” problem
Author(s) -
Lassiter John C.
Publication year - 2004
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2004gc000711
Subject(s) - oceanic crust , geology , crust , mantle (geology) , basalt , silicate , geochemistry , continental crust , subduction , eclogite , adakite , volatiles , tectonics , chemistry , paleontology , organic chemistry
The K content of the Silicate Earth is commonly estimated at ∼250 ppm on the basis of K/U ratios of ∼12,000–13,000 in mid‐ocean ridge basalts and continental crust, and assuming a chondritic U content for the Earth. Given this K content, only ∼50% of the 40 Ar generated over Earth history from the decay of 40 K currently resides in the atmosphere. Because the upper mantle is thoroughly degassed and contains little 40 Ar, the “missing” Ar has long been taken as evidence for a gas‐rich (and therefore convectively isolated) lower mantle. An alternative solution of the “missing Ar” problem is that the K content of the Silicate Earth is substantially lower than 250 ppm. Previous estimates have failed to take into account the composition of recycled oceanic crust, which could comprise up to 6–9% of the mantle by mass and contain up to 40% of the Earth's U. Because K is more soluble than U in subduction‐generated hydrous fluids, recycled, dehydrated oceanic crust is characterized by extremely low K/U ratios. This is evidenced in the low K/U ratios of eclogites (∼1000), and in HIMU‐type basalts (∼4000), which are thought to derive from sources containing significant quantities of recycled crust. Ocean island basalts, which are generated from mantle containing varying amounts and types of recycled material, possess highly variable K/U ratios, but average K/U ratios from individual island chains are negatively correlated with 206 Pb/ 204 Pb, indicating that the low K/U in some ocean‐island basalts is a long‐lived source feature. Inclusion of a recycled crust reservoir in K and U mass balance calculations results in an estimate of the Bulk Silicate Earth K/U of ∼7000–9000, or a K content of ∼150–195 ppm. As a result, much of the “missing” 40 Ar thought to be stored in the deep mantle may not exist, with ∼60–90% of the Earth's 40 Ar residing in the atmosphere or continental crust. The remaining 40 Ar can be accommodated in the convecting mantle without isolation of a deep, undegassed reservoir.