TY - JOUR
T1 - Transition metals in komatiitic olivine
T2 - Proxies for mantle composition, redox conditions, and sulfide mineralization potential
AU - Locmelis, Marek
AU - Arevalo, Ricardo D.
AU - Puchtel, Igor S.
AU - Fiorentini, Marco L.
AU - Nisbet, Euan G.
PY - 2019/8/1
Y1 - 2019/8/1
N2 - We present the results of a comprehensive study on the concentrations of first-row transition elements (FRTE: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), as well as Ga and Ge, in liquidus olivine from 2.7-3.5 Ga old Al-undepleted and Al-depleted komatiites from the Kaapvaal and Zimbabwe Cratons in southern Africa, the Yilgarn Craton in Australia, and the Superior Craton in Canada. The sample set includes komatiites that remained sulfur-undersaturated upon emplacement, as well as komatiites that reached sulfide saturation owing to assimilation of crustal sulfur.All olivine grains display concentrations of Mn, Zn, Ge, Co, Fe, Mg, and Ni similar to the Bulk Silicate Earth (BSE) values, with significant negative anomalies in Sc, Ti, V, Cr, Ga, and Cu. Olivine from the studied Al-depleted komatiites displays on average higher 100xGa/Sc ratios (>5) than olivine from Al-undepleted komatiites (Olivine from the Paleo-Archean (3.5-3.3 Ga) komatiites displays overall higher V/Sc ratios (V/Sc = 2.1 +/- 0.96; 2 S.D.) than olivine from their Neo-Archean (2.7 Ga) counterparts (V/Sc = 1.0 +/- 0.81, 2 S.D.). Vanadium and Sc behave similarly during partial melting of the mantle and are similarly compatible in majorite garnet. However, V is redox-sensitive and its compatibility in olivine increases as the system becomes less oxidized, whereas Sc is redox-insensitive. We argue that olivine from the studied Paleo-Archean komatiites crystallized from more reduced magmas than their Neo-Archean counterparts. Elevated Fe/Mn ratios in olivine from Paleo-Archean komatiites mimic the V/Sc signatures and are interpreted to reflect that Fe2+ is more compatible in olivine than Fe3+. These results imply that V/Sc and Fe/Mn in komatiitic olivine may potentially provide insight into the evolution of the oxidation state of the Archean mantle. Additional studies that integrate the chemistry of komatiitic olivine with those of relict interstitial glass and melt/fluid inclusions are encouraged to fully understand and quantify the potential of FRTE in olivine as a proxy for the oxidation state of the mantle sources of komatiite magmas.
AB - We present the results of a comprehensive study on the concentrations of first-row transition elements (FRTE: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), as well as Ga and Ge, in liquidus olivine from 2.7-3.5 Ga old Al-undepleted and Al-depleted komatiites from the Kaapvaal and Zimbabwe Cratons in southern Africa, the Yilgarn Craton in Australia, and the Superior Craton in Canada. The sample set includes komatiites that remained sulfur-undersaturated upon emplacement, as well as komatiites that reached sulfide saturation owing to assimilation of crustal sulfur.All olivine grains display concentrations of Mn, Zn, Ge, Co, Fe, Mg, and Ni similar to the Bulk Silicate Earth (BSE) values, with significant negative anomalies in Sc, Ti, V, Cr, Ga, and Cu. Olivine from the studied Al-depleted komatiites displays on average higher 100xGa/Sc ratios (>5) than olivine from Al-undepleted komatiites (Olivine from the Paleo-Archean (3.5-3.3 Ga) komatiites displays overall higher V/Sc ratios (V/Sc = 2.1 +/- 0.96; 2 S.D.) than olivine from their Neo-Archean (2.7 Ga) counterparts (V/Sc = 1.0 +/- 0.81, 2 S.D.). Vanadium and Sc behave similarly during partial melting of the mantle and are similarly compatible in majorite garnet. However, V is redox-sensitive and its compatibility in olivine increases as the system becomes less oxidized, whereas Sc is redox-insensitive. We argue that olivine from the studied Paleo-Archean komatiites crystallized from more reduced magmas than their Neo-Archean counterparts. Elevated Fe/Mn ratios in olivine from Paleo-Archean komatiites mimic the V/Sc signatures and are interpreted to reflect that Fe2+ is more compatible in olivine than Fe3+. These results imply that V/Sc and Fe/Mn in komatiitic olivine may potentially provide insight into the evolution of the oxidation state of the Archean mantle. Additional studies that integrate the chemistry of komatiitic olivine with those of relict interstitial glass and melt/fluid inclusions are encouraged to fully understand and quantify the potential of FRTE in olivine as a proxy for the oxidation state of the mantle sources of komatiite magmas.
KW - Komatiite
KW - olivine
KW - Archean mantle evolution
KW - first row transition elements
KW - oxygen fugacity
KW - laser ablation ICP-MS
KW - TRACE-ELEMENT VARIATIONS
KW - WESTERN-AUSTRALIA
KW - OXIDATION-STATE
KW - MELT INCLUSIONS
KW - ARCHEAN MANTLE
KW - VOISEYS BAY
KW - EARLY EARTH
KW - GEOCHEMISTRY
KW - SULFUR
KW - NI
U2 - 10.2138/am-2019-6914
DO - 10.2138/am-2019-6914
M3 - Article
SN - 0003-004X
VL - 104
SP - 1143
EP - 1155
JO - American Mineralogist
JF - American Mineralogist
IS - 8
ER -