SECULAR TRENDS IN THE MELTING DEPTHS OF MANTLE PLUMES - EVIDENCE FROMHFSE REE SYSTEMATICS OF ARCHEAN HIGH-MG LAVAS AND MODERN OCEANIC BASALTS/

Al-depleted komatiites and associated Fe-tholeiites from Boston Townsh ip, Abitibi belt (similar to 2.7 Ga), Canada, are characterized by the conjunction of low Al2O3/TiO2 (4-5) ratios, low Sc and Y contents, an d HREE depletion [(Gd/Yb)(n) = 1.7-2.3], with negative P, Zr and Hf an omalies 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 (similar to 2.7 Ga), Westem 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-c hondritic 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 an d REE systematics between the two late Archean high-Mg lavas are inter preted as partial melting of mantle plumes with different residual min erals 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 E HMT may have been generated either by low-degree partial melting with a olivine + orthopyroxene + clinopyroxene + garnet residue at pressure s <6 GPa, or more advanced melting with pyrope garnet in the residue a t pressures between 6 and 14 GPa. Comparison of HFSE/REE systematics b etween Archean high-Mg lavas and modem OIB indicates that most modem O IB 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 h ave occurred at greater depths than that of modem plumes. HFSE/REE sys tematics of plume-derived volcanic rocks records decreasing melting de pths of mantle plume over time, consistent with the secular cooling of the Earth.