TY - JOUR

T1 - Persistent mildly supra-chondritic initial Hf in the Lewisian Complex, NW Scotland

T2 - Implications for Neoarchean crust-mantle differentiation

AU - Whitehouse, Martin J.

AU - Kemp, Anthony I.S.

AU - Petersson, Andreas

N1 - Publisher Copyright: © 2022 The Authors

PY - 2022

Y1 - 2022

N2 - The composition of the mantle source from which continental crust is ultimately derived informs both the timing and rate of continental growth. This composition is, however, poorly constrained during the first billion or so years of Earth's evolution. Typically in zircon Hf isotope studies, a linearly evolving depleted mantle back-calculated from modern MORB-source mantle to chondrite at 4.57 Ga has been used to calculate model ‘crust formation’ ages, which are themselves used in crustal growth models. Yet, an increasing number of studies of Eo- to Paleoarchean continental crust suggest crust extraction from a relatively undifferentiated mantle. Zircon Hf isotope studies of ca. 2.9 Ga gneisses from the Neoarchean Lewisian Gneiss Complex of Scotland have previously revealed initial Hf isotope compositions that suggest derivation from such an undepleted mantle source. Here, we present combined zircon U-Pb and Lu-Hf isotope data from seventeen representative grey gneiss samples from across the Lewisian of the Scottish mainland and the Outer Hebridean archipelago, ranging in age from 3.1 to 2.7 Ga. Regardless of location and exact age, initial Hf isotope signatures are predominantly near- to slightly supra-chondritic. Several models are investigated to explain this observation, our favoured one involving episodic extraction of the grey gneiss precursors from a mildly depleted mantle that began to diverge from chondritic composition at ca. 3.5 Ga. Samples from the southern Outer Hebrides record slightly more radiogenic initial Hf signatures, consistent with a possible terrane boundary along the Paleoproterozoic South Harris shear zone. This study provides further confirmation that domains of Hadean to Paleoarchean, and possibly even locally Mesoarchean, mantle remained chondritic with respect to Hf isotopes. Initiation of a depleted mantle source at this later time and its limited divergence from chondritic mantle, even into the Neoarchean as suggested here, has implications for crustal growth models based on detrital zircon Hf model ages. On a regional basis, the signatures recorded in our data are consistent with contemporaneous gneisses across Fennoscandia and East Greenland and the mildly depleted mantle source of continental crust furthermore persists into the Mesoproterozoic.

AB - The composition of the mantle source from which continental crust is ultimately derived informs both the timing and rate of continental growth. This composition is, however, poorly constrained during the first billion or so years of Earth's evolution. Typically in zircon Hf isotope studies, a linearly evolving depleted mantle back-calculated from modern MORB-source mantle to chondrite at 4.57 Ga has been used to calculate model ‘crust formation’ ages, which are themselves used in crustal growth models. Yet, an increasing number of studies of Eo- to Paleoarchean continental crust suggest crust extraction from a relatively undifferentiated mantle. Zircon Hf isotope studies of ca. 2.9 Ga gneisses from the Neoarchean Lewisian Gneiss Complex of Scotland have previously revealed initial Hf isotope compositions that suggest derivation from such an undepleted mantle source. Here, we present combined zircon U-Pb and Lu-Hf isotope data from seventeen representative grey gneiss samples from across the Lewisian of the Scottish mainland and the Outer Hebridean archipelago, ranging in age from 3.1 to 2.7 Ga. Regardless of location and exact age, initial Hf isotope signatures are predominantly near- to slightly supra-chondritic. Several models are investigated to explain this observation, our favoured one involving episodic extraction of the grey gneiss precursors from a mildly depleted mantle that began to diverge from chondritic composition at ca. 3.5 Ga. Samples from the southern Outer Hebrides record slightly more radiogenic initial Hf signatures, consistent with a possible terrane boundary along the Paleoproterozoic South Harris shear zone. This study provides further confirmation that domains of Hadean to Paleoarchean, and possibly even locally Mesoarchean, mantle remained chondritic with respect to Hf isotopes. Initiation of a depleted mantle source at this later time and its limited divergence from chondritic mantle, even into the Neoarchean as suggested here, has implications for crustal growth models based on detrital zircon Hf model ages. On a regional basis, the signatures recorded in our data are consistent with contemporaneous gneisses across Fennoscandia and East Greenland and the mildly depleted mantle source of continental crust furthermore persists into the Mesoproterozoic.

KW - Archean

KW - Crustal evolution

KW - Geochronology

KW - Hafnium isotopes

KW - Lewisian Complex

KW - Zircon

U2 - 10.1016/j.chemgeo.2022.121001

DO - 10.1016/j.chemgeo.2022.121001

M3 - Journal article

AN - SCOPUS:85133428550

VL - 606

JO - Chemical Geology

JF - Chemical Geology

SN - 0009-2541

M1 - 121001

ER -