Co-rich sulfides in mantle peridotites from Penghu Islands, Taiwan: Footprints of Proterozoic mantle plumes under the Cathaysia Block

摘要

Abundant primary sulfides occur as inclusions in silicates and as discrete grains in mantle-derived spinel Iherzolite xenoliths from Miocene intraplate basalts on the Penghu Islands, Taiwan, which is located at the southeastern margin of Cathaysia Block. These sulfides are dominantly mixtures of Fe-rich and Ni-rich monosulfide solid solutions (MSS), with minor pentlandite, millerite and chalcopyrite, and are considered to represent sulfide melts crystallized at high temperatures (>900℃). Some sulfides from the Tungchiyu (TCY) islet (37 out of 118 grains) have remarkably high Co contents resulting in subchondritic Ni/Co ratios (<21; 5-20, median = 12), distinct from the superchondritic values (Ni/Co = 48-157, median = 83) typical of mantle sulfides worldwide. The Co-rich nature of the TCY sulfides is considered to be a primary characteristic as no secondary processes can be identified to account for the feature. They are similar to Ni-Co-rich sulfides from Lac de Gras, Slave Craton (Aulbach et al. (2004) Chemical Geology 208, 61-88) interpreted as being derived from the lower mantle. Experimental studies suggest that the sulfide melt/silicate melt partition coefficient of Ni becomes lower than that of Co at pressures greater than 28 GPa, similar to recent estimates of the magma ocean conditions. Os model ages of the TCY Co-rich sulfides reveal four episodes of generation: 2.0,1.7,1.4 and 0.8 Ga; this is consistent with the age pattern of all Penghu sulfides, indicating significant lithosperic mantle formation, melt extraction or metasomatic events at these time periods. These events closely correspond to the global 1.9-Ga superplume event related to the assembly of the Nena/Columbia supercontinent, a minor 1.7-Ga superplume event in SW Laurentia prior to breakup of Nena/Columbia, the 1468 Ma Moyie event in the Belt Basin region in western Laurentia and the ～0.8 Ga breakup of Rodinia, with which the Cathaysia Block was associated at various stages during its Proterozoic evolution (Li et al. (2008) Precambrian Research 160, 179-210 and references therein). Olivine in a peridotite sample from the TCY locality has distinctly high ~3He/~4He (11 R_A), whereas other peridotites from the KP and TCY localities have ~3He/~4He ~6.7 R_A, lower than MORB. The high ~3He/~4He further suggests that materials from the deep mantle have interacted with the host peridotite of Co-rich sulfides. We thus propose that the Co-rich sulfide melts may have been trapped in the lower mantle during core-mantle differentiation and then transported to shallow depths by mantle plumes that entrained lower mantle materials at several different time periods. This study provides the first substantial evidence from the lithosperic mantle beneath the Cathaysia Block to support the activity of mantle plumes related to the breakup of the supercontinents Nena/Columbia and Rodinia in Proterozoic time.