09:45 〜 10:00
[SCG45-04] Highly refractory dunite formation in Gibbs Island and Bruce Bank, and its role in the evolution of the circum-Antarctic continent
キーワード：Re-Os isotope system、Continental crust、flux melting
The continental margin is of profound importance as it records continental growth by steady-state accretion of orogenic magmas and following rifting. Intense mantle melting due to hydrous fluid input is expected to simultaneously stimulate the continental growth and lower intrinsic density of the mantle than more fertile mantle, which in turn isolate the continental lithosphere from the convective mantle. Gibbs Island (South Shetland Islands) and Bruce Bank located between South America and the Antarctic Peninsula, are tectonic windows into the Earth’s interior in the circum-Antarctic region. To investigate the continental growth of Antarctica, we present geochemical data of 8 dunites from Gibbs Island and 1 dunite from Bruce Bank, including Re-Os isotope and highly siderophile element compositions.
The dunites are severely suffered from serpentinization of antigorite + brucite or lizardite (loss on ignition = LOI ranging from 3 to 34 wt%), but contain primary euhedral to subhedral chromian spinels with or without spherical olivine inclusions. The chromian spinels rarely form lens-shaped aggregates. A dunite from Gibbs Island contains fresh olivine grains filling a fracture of chromian spinel with low LOI (3 wt%), indicating a deserpentinization origin from serpentinized dunite. The dunites show highly depleted bulk major-element composition: Mg/Si = 1.4–1.6 and Al/Si = 0.004–0.01 for the dunites from Gibbs Island, and Mg/Si = 0.66 and Al/Si = 0.008 for the dunite from Bruce Bank. The positive correlation in Re/Ir and LOI space corroborates Re input during latest serpentinization process. The 187Os/188Os of the dunites vary significantly (0.1188–0.1445).
Phanerozoic Re-depletion (melt-depletion) ages of ca. <470 Ma are recorded in the dunites, except for a dunite from Gibbs Island with Proterozoic Re-depletion age of ca. 1.2 Ga. Since there exists serpentinization-related perturbation of Re, they provide minimum time estimates for melt depletion events. The early Paleozoic magmatism is inferred to have occurred during Ross orogeny in response to plate convergence along the margin of the eastern Gondwana, whereas the Mesoproterozoic Re-depletion age is consistent with the Re-depletion ages of mantle xenoliths and Nd model ages of granites and orthogneisses in Marie Byrd Land, West Antarctica. The petrographic characteristics of the chromian spinels with highly depleted nature of the dunites corroborate peridotite-melt reaction and subsequent melt mixing in a subduction setting. Thus, a simple interpretation of the Re-depletion ages of the dunites is that “residual” lithospheric mantle of Antarctica that experienced multiple fluid-assisted melting was incorporated into the continental outskirts during continental break-up. Widespread existence of the dunites in Gibbs Island and Bruce Bank may be attributed to pervasive removal of the continental lithospheric mantle at continental margin and possible incorporation into the oceanic convection system. This process reconciles with the existence of refractory mantle domains in the oceanic mantle lithosphere.
The dunites are severely suffered from serpentinization of antigorite + brucite or lizardite (loss on ignition = LOI ranging from 3 to 34 wt%), but contain primary euhedral to subhedral chromian spinels with or without spherical olivine inclusions. The chromian spinels rarely form lens-shaped aggregates. A dunite from Gibbs Island contains fresh olivine grains filling a fracture of chromian spinel with low LOI (3 wt%), indicating a deserpentinization origin from serpentinized dunite. The dunites show highly depleted bulk major-element composition: Mg/Si = 1.4–1.6 and Al/Si = 0.004–0.01 for the dunites from Gibbs Island, and Mg/Si = 0.66 and Al/Si = 0.008 for the dunite from Bruce Bank. The positive correlation in Re/Ir and LOI space corroborates Re input during latest serpentinization process. The 187Os/188Os of the dunites vary significantly (0.1188–0.1445).
Phanerozoic Re-depletion (melt-depletion) ages of ca. <470 Ma are recorded in the dunites, except for a dunite from Gibbs Island with Proterozoic Re-depletion age of ca. 1.2 Ga. Since there exists serpentinization-related perturbation of Re, they provide minimum time estimates for melt depletion events. The early Paleozoic magmatism is inferred to have occurred during Ross orogeny in response to plate convergence along the margin of the eastern Gondwana, whereas the Mesoproterozoic Re-depletion age is consistent with the Re-depletion ages of mantle xenoliths and Nd model ages of granites and orthogneisses in Marie Byrd Land, West Antarctica. The petrographic characteristics of the chromian spinels with highly depleted nature of the dunites corroborate peridotite-melt reaction and subsequent melt mixing in a subduction setting. Thus, a simple interpretation of the Re-depletion ages of the dunites is that “residual” lithospheric mantle of Antarctica that experienced multiple fluid-assisted melting was incorporated into the continental outskirts during continental break-up. Widespread existence of the dunites in Gibbs Island and Bruce Bank may be attributed to pervasive removal of the continental lithospheric mantle at continental margin and possible incorporation into the oceanic convection system. This process reconciles with the existence of refractory mantle domains in the oceanic mantle lithosphere.