Oxygen isotope (δ18O, Δ′17O) insights into continental mantle evolution since the Archean

Abstract

Oxygen isotopic ratios are largely homogenous in the bulk of Earth’s mantle but are strongly fractionated near the Earth’s surface, thus these are robust indicators of recycling of surface materials to the mantle. Here we document a subtle but significant ~0.2‰ temporal decrease in δ18O in the shallowest continental lithospheric mantle since the Archean, no change in Δ′17O is observed. Younger samples document a decrease and greater heterogeneity of δ18O due to the development and progression of plate tectonics and subduction. We posit that δ18O in the oldest Archean samples provides the best δ18O estimate for the Earth of 5.37‰ for olivine and 5.57‰ for bulk peridotite, values that are comparable to lunar rocks as the moon did not have plate tectonics. Given the large volume of the continental lithospheric mantle, even small decreases in its δ18O may explain the increasing δ18O of the continental crust since oxygen is progressively redistributed by fluids between these reservoirs via high-δ18O sediment accretion and low-δ18O mantle in subduction zones.

A corundum reference material for oxygen isotope analysis by secondary ionization mass spectrometry

“…Sessions at UO included fluorination of 16 HD-LR1 single fragments with a mass of 0.6–1.5 mg and analysis of gas using both O 2 ( n = 2) or CO 2 ( n = 14) methods. Analyses were normalized using in-house UOG garnet (δ 18 O = +6.52‰), 19 UWG2 (δ 18 O = +5.80‰), 20 San Carlos Olivine (SCO, δ 18 O = +5.25‰, Δ′ 17 O = −0.051‰) 19 and high-δ 18 O standard Stevens Klint Flint (SKFS, δ 18 O = +33.93‰, Δ′ 17 O = −0.114‰). 21 Repeatability for these silicate reference materials is ±0.1‰ for δ 18 O and ±0.01‰ for Δ′ 17 O; the observed larger scatter for HD-LR1 relative to the silicate values is accounted for by stating an overall error based on the standard deviation of corundum analyses.…”

“…9 Sessions at UO included uorination of 16 HD-LR1 single fragments with a mass of 0.6-1.5 mg and analysis of gas using both O 2 (n = 2) or CO 2 (n = 14) methods. Analyses were normalized using in-house UOG garnet (d 18 20 San Carlos Olivine (SCO, 19 and high-d 18 O standard Stevens Klint Flint (SKFS, 21 Repeatability for these silicate reference materials is ±0.1& for d 18 O and ±0.01& for D ′17 O; the observed larger scatter for HD-LR1 relative to the silicate values is accounted for by stating an overall error based on the standard deviation of corundum analyses. A scale compression correction of 0.5&/18 = 0.038& was applied to match data obtained by CO 2 and O 2 methods of light and heavy references and unknowns.…”