Unlocking the secrets of the Archean cratonic mantle through garnet Lu-Hf geochronology

The sub-continental lithospheric mantle (SCLM) of Archean cratons represents the depleted and buoyant residue left behind after crust extraction. The history of the SCLM is notably complex in all cratons, often recording multiple episodes of melting and metasomatism. Garnet is a prime target for studying this history, as it provides thermobarometric constraints and hosts incompatible trace elements that can help identify melting and refertilization. Placing the rich geological record of mantle garnet in time is crucial for resolving cratonic evolution. Robust age constraints from garnet have nevertheless been difficult to obtain. Isolating enough analyte material for Lu-Hf or Sm-Nd chronometry is challenging for small xenoliths of highly depleted mantle rock. Age estimates are typically based on external or two-point isochrons with limited statistical robustness and geological interpretability. Moreover, chronometer systematics are principally not well constrained for the conditions and processes of the mantle. The question of which assemblages and chemical features of the Archean SCLM are actually of the Archean age is often left unanswered. To address this, we used ultralow-blank Lu-Hf chronometry, in concert with trace element analysis, on a targeted analysis of texturally and compositionally different mantle xenoliths from three Archean cratons (Slave, Kaapvaal, and Siberian Cratons).

The samples analyzed in this study represent a variety of garnet-bearing lithologies: clinopyroxene-rich fertile lherzolite, harzburgite (both granular and sheared), and orthopyroxenite with pyrope in exsolution lamellae. These samples were chosen, as they capture various stages of mantle evolution, from initial melting and subsolidus equilibration to shearing and metasomatic re-equilibration. We were able to obtain multi-point internal Lu-Hf isochrons for all lithologies, including those with extremely depleted compositions. The Lu-Hf ages span the history of the SCLM, from the Mesoarchean to the ages of kimberlite eruption. The oldest ages were obtained from lithologies depleted in Ca and clinopyroxene, i.e., exsolved orthopyroxenites and harzburgites from the Kaapvaal and Siberian Cratons. Lherzolites provided younger ages corresponding to metasomatic events, some of which could be linked to synchronous magmatic episodes in the overlying crust.

The data show that compositional and geochronological signatures in garnet can be retained on billion-year time scales. Static and dynamic recrystallization, and metasomatism – rather than temperature alone – control these signatures in garnet. The exsolution of pyrope in Ca-depleted Kaapvaal and Siberian orthopyroxenites is now confirmed to have occurred in the Archean. The geochemistry and petrology of these particular samples thus can constrain the P-T evolution that led to the development of the early continents.