# Coupled strontium-calcium isotopes in Archean anorthosites reveal a late start for mantle depletion

## TL;DR

This study uses isotopic analysis of ancient rocks to show that large-scale continental crust formation on Earth started later than previously thought.

## Contribution

The paper presents the most unradiogenic terrestrial Sr isotope ratio yet, refining models of early Earth evolution.

## Key findings

- Plagioclase from 3.73 Ga rocks shows the most unradiogenic 87Sr/86Sr ratio measured on Earth.
- The data support Sr isotope homogenization between Earth and Moon during the giant impact at ~4.515 Ga.
- A depleted mantle signature in Sr isotopes appears post-3.5 Ga, indicating late continental growth.

## Abstract

Voluminous felsic continental crust is, as far as we know, unique to Earth, yet the timescales of its earliest growth remain debated. Archean mantle evolution is complementary to crustal growth but is largely unconstrained for strontium isotopes due widespread Rb-Sr disturbance. Here, we perform high spatial resolution radiogenic Sr and Ca isotope measurements in magmatic plagioclase megacrysts from 3.7–2.8 Ga Archean anorthosites and leucogabbros. We report mantle-like Ca isotope signatures and the most unradiogenic terrestrial 87Sr/86Sr ratio yet measured on Earth, in plagioclase (87Sr/86Srinitial = 0.700050 ± 0.000017, 95% confidence interval) from the 3.73 Ga Manfred Complex of the Narryer Terrane, Western Australia. These data are consistent with 87Sr/86Sr homogenisation between the Earth and Moon (87Sr/86Srinitial ≈ 0.699061) at ca. 4.515 Ga during the Moon-forming giant impact. The appearance of a depleted mantle signature in the terrestrial strontium record post-3.5 Ga suggests that extensive continental growth began relatively late.

High spatial resolution analyses of plagioclase within 3.7–2.8-billion-year-old anorthosites are used to refine strontium isotope models of Earth’s early evolution and imply that large-scale continental growth began relatively late in Earth history.