Research interests:

 

1.     Arc volcanism and metasomatic processes in subduction zones.

2.     Mantle xenoliths composition and geodynamics of the subcontiental lithospheric mantle (SCLM).

3.     In-situ highly siderphile elements (HSE) and Re-Os isotopic analyses for mantle sulfides.

4.     Petrology and geochemistry of basaltic, potassic, ultrapotassic rocks in orogenic zones and their tectonic implications.

 

Current research:

1.         In situ Re-Os isotope analysis for mantle sulfides from Southern Ethiopia: lithospheric mantle evolution beneath the East African Rift:

Through collaboration with Japanese geochemists, mantle sulfides in spinel lherzolites hosted by Quaternary alkali basalts from NE of the Turkana Depression, Southern Ethiopia were offered for the in situ Re-Os isotope analysis. The preliminary results reveal the presence of Proterozoic subcontinental lithospheric mantle (SCLM) beneath the continental rift setting in East Africa. Most of the sulfides have subchondritic 187Os/188Os (<0.129). A large range in 187Re/188Os (0.003-0.809) suggests recent addition of Re, perhaps reflecting the Paleogene mantle plume activity, which not only caused the East Africa Rift but also significantly perturbed the SCLM in the region. Sulfides with low 187Re/188Os (<0.075) yield similar TMA and TRD model ages of 1.1±0.2 Ga, interpreted as the depletion age of the SCLM beneath the region. Re-Os mixing lines defined by sulfides in single samples give an initial 187Os/188Os (0.1184) consistent with formation of some volumes of the SCLM at ~1.3 Ga. TRD model ages of sulfides can provide minimum estimates for the SCLM age and record later metasomatic events. All model ages of the sulfides suggest a main SCLM depletion age at 1.1 Ga with a later metasomatic event at 0.4-0.5 Ga. A few older ages (1.5-1.8 Ga) suggest the presence of older parts of the SCLM, but no Archean model ages were found. The SCLM depletion age of 1.1 Ga is consistent with the known Meso-Neoproterozoic crustal evolution event of the East African Orogen, and the 0.4-0.5 Ga may be related to closing stages of the Paleozoic Pan-Africa orogeny. The sulfide Re-Os data show that Proterozoic SCLM has survived the extensive continental rifting due to the mantle plume.

 

2.         PGE fractionation during metasomatism inferred from distinct groups of mantle sulfides from Penghu Islands:

Abundant primary sulfides occur as inclusions in silicates and as discrete grains in spinel lherzolite xenoliths from Miocene intraplate basalts on the Penghu Islands, Taiwan. These sulfides are mixtures of Fe-rich and Ni-rich monosulfide solid solutions (MSS), pentlandite, millerite and chalcopyrite. Two distinct groups, the Fe-Cu-rich Kueipi (KP) sulfides and the Ni-Co-rich Tungchiyu (TCY) sulfides, commonly have low Platinum group element (PGE) contents and similar PGE patterns typical of sulfide liquids. The KP sulfides have relatively more fractionated PGE patterns and show fractionation of Os from Ir, coupled with a unique negative correlation between 187Re/188Os and 187Os/188Os. The sulfides were introduced into the shallow peridotites by metasomatic processes; the most likely metasomatic agent is a hydrous fluid. Another metasomatic agent: silicate melt, which can not fractionate I-PGE.

 

3.         Origin of unique Co-rich mantle sulfides from Penghu Islands and its relationship with Neoproterozoic mantle plume causing breakup of Rodinia:

The Ni-Co-rich TCY sulfide liquid has subchondritic Ni/Co ratios (< 21), and requires formation under the pressure-temperature conditions of the lower mantle to achieve appropriate metal-silicate partition coefficients for Ni and Co. Similar high-Co sulfides from the Slave Craton, Canada are interpreted as derived from the lower mantle. The Os model ages of these sulfides show that only TCY sulfides recorded a 0.8~0.9 Ga event in both TMA and TRD model ages, whereas the KP sulfides did not have these model ages. Thus, this event may reflect transport the sulfides with subchondritic Ni/Co ratios from depth, which are only shown in TCY sulfides. Li et al. (1999) have suggested that a mantle plume caused the breakup of Rodinia on the South China Block at 0.83 Ga, which can also be tracked on Australia and Laurentia. A mantle plume originating from the lower mantle is a plausible candidate capable of carrying the Co-rich sulfide melt from depth. The Os model ages of the TCY sulfides therefore may represent the first isotopic evidence from the lithospheric mantle for the proposed mantle plume that led to the breakup of Rodinia. The occurrence of these unique Co-rich sulfides in the Penghu Islands may shed light on the involvement of the deeper mantle in geodynamic processes in East Asia.

 

4.         Proterozoic mantle lithosphere beneath the extended margin of the South China block: In situ Re-Os evidence:

The Os isotope compositions of sulfides in mantle xenoliths from the Penghu Islands, Taiwan Strait (Fig. 1 and Photo 1), reveal the presence of Proterozoic SCLM beneath the highly extended southeast margin of the South China block. These sulfides have recently undergone three types of disturbance in their Os isotope systematics: (1) addition of Re with no apparent addition of Os, or with only lithospheric Os with low 187Os/188Os ratios (Fig. 2C and 2D); (2) addition of Re, and of Os with an isotope composition near the present-day PUM (Fig. 2A); (3) addition of radiogenic Os, but little or no Re  (Fig. 2B). The highly radiogenic Os in disturbance type 3 could be derived from lithospheric sources such as pyroxenites or subducted basalts, and the transporting medium may have been an oxidizing fluid derived from the Mesozoic subducting slab beneath the area. Despite the Os disturbance, TRD model ages of sulfides provide minimum estimates for the age of the SCLM (Fig. 3). Both TRD model ages for individual sulfides and model ages estimated from the initial 187Os/188Os ratios of Re-Os mixing lines require that some volumes of the SCLM formed prior to 2.3–1.9 Ga. Later events in the SCLM may be recorded by TRD model ages of 1.5–1.2 Ga and ca. 0.9 Ga. The events recognized in the SCLM are consistent with those known in the crust of the mainland South China block. The significant sulfide Os age data lead to the recognition of relict Proterozoic lithospheric mantle domains beneath the South China Block. The mixture of ages and rock types suggests that during the stretching of the lithosphere (the rigid Earth layer above the convecting mantle), the ancient continental root was partly disrupted and replaced by younger material, but stayed attached to the lower crust. The results are directly relevant to current debates about the geodynamics of continent formation and breakup, and the long-term evolution of the upper mantle. This study resulted in the first dating of mantle domains in that region, and the result was fast tracked by Geology published in the issue of August 2003.

 

Fig. 1: A simplified tectonomagmatic map of the South China and Taiwan region. Red areas mark outcrops of late Cenozoic intraplate basalts. Stars mark localities of mantle xenoliths: the yellow one indicates xenoliths in this study. The inset shows detail sample localities in Penghu Islands.

 

Photo 1: The port at the Tungchiyu Islet where sulfide-abundant mantle xenoliths are found. Traveling by a yacht is the best way to transport in-between 64 islets of the Penghu Islands.

 

Figure 2: Plots of 187Os/188Os vs. 187Re/188Os for sulfides. Error bars shown in A-D are ± 2s. A: Sulfides from Kueipi and Tungchiyu show positive correlation trends. B: Sulfides from Kueipi show negative correlation trends. C: Other sulfides from Kueipi peridotites. D: Sulfides from Kueipi and Tungchiyu pyroxenites. Solid symbols represent enclosed sulfides; open symbols represent interstitial sulfides. PUM—primitive upper-mantle values (Meisel et al., 2001).

Figure 3: Re depletion ages (TRD) of Penghu sulfides.

5. Geochemical characteristics of mantle-derived xenoliths Penghu Islands, Taiwan Strait: Implications for mantle composition and processes:

The xenoliths are dominantly spinel peridotites with minor spinel pyroxenites. Most peridotites have fine to coarse-grained porphyroclastic or equigranular microstructures; some are foliated, and many show textural disequilibrium with clinopyroxene exsolution lamellae in orthopyroxenes, spinel exsolution in clinopyroxenes, and strained clinopyroxene grains coexisting with strain-free clinopyroxene neoblasts. These peridotites range from relatively fertile compositions, with 10-20 vol.% clinopyroxene (cpx), to depleted compositions with <5 vol.% cpx. Whereas rare Kueipi xenoliths (Photo 2) contain amphibole and/or apatite, most Tungchiyu xenoliths (Photo 3 and 4) contain amphibole, reflecting modal metasomatism. They also show complex episodes of fluid inclusions in ortho- and clinopyroxene pophyroblasts, indicating at least two different episodes of metasomatism. Zoning of orthopyroxenes from high-Ca cores to low-Ca rims suggests some of these peridotites represent upwelling of deeper mantle materials. Their major-element data show that some of the lithospheric mantle beneath this region is quite refractory. The Fo contents of olivines in spinel peridotites range from 89.0 to 91.7, but most fall between 90.0 and 91.0; mg# of olivine is correlated with cr# (0.11~0.55) in spinel. It suggests that some of the lithospheric mantle beneath this region may possibly be Proterozoic, as old as mantle xenoliths from several nearby mainland localities (Fig. 4). At least two types of metasomatism are reflected in the trace element patterns of clinopyroxenes. High La/Yb and low Ti/Eu ratios indicate carbonatitic metasomatism (Fig. 5), which the GEMOC database shows to be typical for many extensional settings worldwide. The other metasomatic signature is characteristic of silicate melt interaction. Apparently this result is consistent with petrographic observation for these mantle xenoliths.

 

Photo 2: Xenolith outcrop at the locality Keuipi, Penghu Main Island. A disconformity is shown between the lava hosting mantle xenoliths and underlying Tertiary sedimentary rocks.

 

Photo 3: The pyroclastic flow layer which hosts sulfide-abundant xenoliths on the Tungchiyu Islet, Penghu Islands.

 

Photo 4: Abundant and variable mantle xenoliths in the pyroclastic flow on the Tungchiyu Islet, Penghu Islands.

 

Figure 4: Plots of Mg-number versus modal olivine (%) of peridotites with sulfides from Penghu Islands. Peridotites from nearby Minxi, Niutoushan and Qilin, inland southeastern China are also plotted for comparison (Xu et al., 2000). Oceanic trend cited from Boyd (1989). Phanerozoic, Proterozoic and Archean areas are from Griffin et al. (1998).

Figure 5: (La/Yb)N vs Ti/Eu ratios of clinopyroxenes from Penghu Islands peridotites. Modified from Coltorti et al. (1999).

 

6.         Geotherm and crust-mantle boundary from geochemical characteristics of garnet-bearing xenoliths in Penghu islands, Taiwan Strait: The equilibration temperatures calculated from mineral compositions range from 880°C to 1250°C. Combining the reported data of crustal and garnte-bearing xenoliths, with equilibrium temperature result estimated from this study, a local geotherm is proposed that is similar to that beneath eastern Australia (Fig. 6).

Fig 6: Penghu geotherm constrained by crustal, spinel-bearing and garnet-bearing xenoliths.

 

7.     Composition of mantle sulfides from Penghu Islands:

Some spinel peridotites and pyroxenites contain sulfide inclusions large enough to be laser-probed (50 micron). Nearly all sulfide grains from the Kueipi locality consist of interfingered Ni-rich and Fe-rich monosulfide solid solutions (MSS), with an outer rim of chalcopyrite. Those from the Tungchiyu islet mostly have Fe-rich MSS cores, surrounded by Ni-rich MSS and a Co-rich rim. These sulfides have high bulk cobalt contents (up to 8 wt.%) and Ni/Co ratios lower than the chondritic value. Trace-element patterns of the Tungchiyu sulfides are flat and similar to that of sulfide liquid, or MSS crystallized from such liquid after separation from residual MSS. Similar high-Co sulfides from the Slave Craton, Canada are interpreted as derived from the lower mantle. The occurrence of these unique Co-rich sulfides in the Penghu Islands may shed light on the involvement of the deeper mantle in geodynamic processes in East Asia.

8.     Composition of the SCLM in East Asia: implication to origin of the enriched mantle components

Combining geochemistry of the host Miocene basalts, to reveal the role of the SCLM in producing EM1/EM2 signatures in basalts and origin of the EM components, and evolution history of the SCLM in the extensional setting in eastern China since Cenozoic.