Secular trends in the melting depths of mantle plumes: Evidence from HFSE/REE systematics of Archean high-Mg lavas and modern oceanic basalts

Al-depleted komatiites and associated Fe-tholeiites from Boston Township, Abitibi belt (∼ 2.7 Ga), Canada, are characterized by the conjunction of low Al2O3/TiO2 (4–5) ratios, low Sc and Y contents, and HREE depletion [(Gd/Yb)n = 1.7–2.3], with negative P, Zr and Hf anomalies relative to MREE on primitive-mantle-normalized diagrams (P/Nd = 23–36; Zr/Sm = 13–18; Hf/Sm = 0.3–0.5). Enriched high-Mg tholeiites (EHMT) from the Norseman area ( ∼ 2.7 Ga), Western Australia, are characterized by low Al2O3/TiO2 (4.0–4.7), fractionated HREE [(Gd/Yb)n = 2.3–2.5], and negative P anomalies (P/Nd = 21–26), but near-chondritic Zr/Sm (23–27 vs. 25) and Hf/Sm (0.6–0.8 vs. 0.7) ratios. In both areas there are tholeiites with flat REE patterns derived from a distinct mantle source than the komatiites and EHMT. Different HFSE and REE systematics between the two late Archean high-Mg lavas are interpreted as partial melting of mantle plumes with different residual minerals at different depths; Boston Township Al-depleted komatiites may have been derived from partial melting of a plume with majorite garnet in the residue at pressures between 14 and 24 GPa, whereas Norseman EHMT may have been generated either by low-degree partial melting with a olivine + orthopyroxene + clinopyroxene + garnet residue at pressures < 6 GPa, or more advanced melting with pyrope garnet in the residue at pressures between 6 and 14 GPa. ison of HFSE/REE systematics between Archean high-Mg lavas and modern OIB indicates that most modern OIB are featured by low Al2O3/TiO2 (4–7), P/Nd (50–60) and fractionated HREE [(Gd/Yb)n = 2–3], but near-chondritic Zr/Sm (24–25) and Hf/Sm (0.6–0.7) ratios, similar to Norseman EHMT, consistent with low-degree partial melting with garnet in the residue at low pressures. Partial melting of Archean plumes which generated Al-depleted komatiites may have occurred at greater depths than that of modern plumes. HFSE/REE systematics of plume-derived volcanic rocks records decreasing melting depths of mantle plume over time, consistent with the secular cooling of the Earth.