With the experience using the novel in situ technique to analyze mantle sulfides for their platinum group elements (PGEs) compositions and Os isotope ratios, I further extend the application to mantle peridotites collected from central Mongolia, peri-Lake Baikal in Russian Siberia, Russian Far East (Central Asia Orogenic Belt; CAOB), southern Ethiopia (Arabian-Nubian Shield) and southern Kyrgyz Tienshan. Such results provide new insights to characterize ancient continental lithospheric mantle (CLM) and their geochemical evolution and to understand behavior of the HSE and Os isotopes in sulfides and platinum group minerals (PGMs) in CLM.
The convective upper mantle is one of the major geochemical reservoirs in the Earth, which is supposed to be homogeneous due to its convective character. Recently more studies, however, demonstrated that substantial heterogeneity in terms of certain isotopic composition (e.g., Pb and Os isotopes) does exist in the convective upper mantle. Ophiolites represent fragments of upper mantle and oceanic crust that are direct witnesses of upper mantle compositions. To study ophiolitic peridotites formed at different geological time and ultramafic peridotites from Archean greenstone belts will add significant new information on formation, evolution and modification of the mantle, especially from the views on ancient upper mantle. To tackle difficulty to find scarce sulfides and PGMs in refractory peridotites in ophiolites and Archean greenstone belt, chemical separation and analytical methodology of whole-rock Os isotope and PGE concentration analyses had been set up at IES. It has been applied to ophiolitic peridotites from Guleman and Kizidag in SE Turkey and Dunzhugu ophiolites in Russian Siberia, as well as Archean Olondo greenstone belt.
With respect to the research topics, sound results achieved from the representative researches are summarized as below.
(1) No excessive crustal growth in the Central Asian Orogenic Belt: Further evidence from field relationships and isotopic data:
Evidence for the existence of ancient sub-crustal mantle lithosphere (SCLM) in the CAOB is still comparatively rare, and the mechanism of its preservation in an accretionary orogen is still poorly understood. Our discovery, in recent years, of several additional terranes from off-cratonic settings (Tariat, Mongolia; Vitim, Khamar Daban range (KDR) and Sviyaginsky volcano, Russia where such old SCLM occurs prevailingly. Recent studies have shown that volumes of ancient depleted material can survive in the convecting asthenospheric mantle for long periods of time so that the use of Os model ages of mantle xenoliths to constrain the age of lithospheric mantle events should be approached with caution. However, it would be a remarkable coincidence if sulfides derived from randomly selected fragments of refractory material in the convecting asthenospheric mantle would combine to give such a systematic correlation as shown in the above sections. Moreover, some of the ancient Os model ages are from apparently residual sulfide phases with subchondritic 187Re/188Os and 187Os/188Os ratios. To interpret these sulfides as being derived from depleted material residing within the asthenospheric mantle, it would be necessary to quantitatively melt the older sulfides, transport them into the SCLM and deposit them again without modifying their isotopic systematics. This seems to be an unlikely scenario. We therefore prefer the simplest interpretation of these data, namely the sulfide Os model ages in the Tariat, Vitim Khamar Daban Range and Sviyaginsky peridotites record major events (i.e., melt extraction) that affected the underlying SCLM. Comparing the lithospheric mantle domains from the above regions as revealed by Os model ages, with ancient microcontinents at least Mesoproterozoic in age and predating formation of the CAOB significantly diminishes the volume of new juvenile crust generated during the orogeny. Although significant mantle involvement during evolution of the CAOB has been summarized in previous chapters, the extent of ancient continental material may be larger than previously estimated. The results have been incorporate as one focus review paper invited by Gondwana Research and was published in the volume 50, page 135-166 in 2017.
(2) Prevailing ancient (Archean-Proterozoic) SCLM beneath the Phanerozoic CAOB with dominantly juvenile crust:
In situ analyses of sulfides for Os isotope ratios is an invaluable tool for untangling the metasomatic history of the sulfides and their source rocks. Both TMA from the least-disturbed sulfides (187Re/188Os<0.07) and TRD from higher Re/Os sulfides without later introduction/loss of Os, yield the oldest model ages of the Archean-Proterozoic time and younger Neoproterozoic-Cambrian peak ages in mantle peridotites from Tariat in central Mongolia, Kharma Darban Ridge in Russian Siberia and Sviyaginsky in Russian Far East. The oldest events recognized in these SCLM are consistent with those recorded in overlying crust of the Tarvagatay Terrane, Slyudyanka Terrane and Khanka Massif. Younger sulfide Os ages may mark the commencement of the Central Asia Orogeny since the Neoproterozoic and involvement of mantle as suggested by Senger et al. (1993) and Jahn (2004). These could be the first results showing ancient root beneath the CAOB. Compiling with Mesoproterozoic Os model ages (up to 2.0 Ga) from the Vitim region in Russian Siberia, ancient lithospheric mantle domains are prevailing in the CAOB, which might diminish extents of juvenile crustal growth in the Orogen as expected before. The results have been published as one paper in the Central Asia special volume of Journal of Asian Earth Sciences in 2013 (v62, p37-50) and another one is in Terra Nova in 2015 (v27, p277–284).
(3) Influence of Ryukyu subduction system to magma genesis of Northern Taiwan Volcanic Zone and Middle Okinawa Trough from Boron isotopes observation.
Boron (B) is an excellent geochemical tracer for investigating crustal recycling processes at convergent margins, due to its high fluidmobility under high P–T conditions, distinct elemental abundances and isotopic compositions in the mantle wedge and subducting slabs. The Northern Taiwan Volcanic Zone (NTVZ), wherein the nature of magma genesis has long been a topic of debate, is located at the rear side of the Okinawa Trough (OT), an atypical back-arc rift in the Ryukyu subduction system. In this study, B and B isotopes (δ11B) were measured in 19 volcanic rocks collected from the NTVZ and the middle Okinawa Trough (MOT) to assess the influence of the Ryukyu subduction system on magma genesis. The δ11B data also provide insight into the nature of the different subduction components. A three-end-member mixing model suggests that MOT and NTVZ magmas may have incorporated subduction fluids derived from altered oceanic crust or serpentinite, whereas the onshore volcanoes in the NTVZ, and in particular in the Tsaolingshan, may have involved a higher proportion of sediment component. These results have been published in Lithos, 260, p 242-252 in 2016.
(4) Analytical technique development:
In addition to research achievements above, I also devoted substantial time to develop laboratory facilities and new analytical methods established at IES, Academia Sinica after joining here since late-2005. To fulfill my principal research interests, in-situ (laser-ablated) elemental and isotopic analyses and precise geo-analytical chemistry to study basalt and mantle geochemistry and mantle dynamics, including in situ zircon U-Pb dating, in situ zircon Hf isotopic analysis and in situ determination of elemental concentration on single silicate minerals, have been developed and operated routinely since 2010. After then, the IES became strengthened as a novel, world-class geochemical facility with high-precision isotope analysis in solution samples, as well as in situ microanalysis of trace elements and isotopic ratios in minerals and rocks. These state-of-the-art facilities and methodology at IES have attracted numerous research cooperation worldwide working on significant topics: (only listed since 2014),
· In situ Hf isotopes of gabbro-granitoid complexes in the Panxi region of the Emeishan LIP, China
· In situ Hf isotope of jadeitites (Japan and Guatemala): their genesis and protoliths
· Ages of the Gonzha Group, Argun Terrane and the Uril Formation of the Amur Group, Lesser Khingan Terrane in Russia inferred from in situ U-Pb and Lu-Hf data
· Origin of silicic volcanic rocks in the Early Permian Panjal Traps, Kashmir, India
· U–Pb ages and Hf isotopic constraints on sources of Slyudyanskiy meta-terrigeneous rocks, Russia
· Hf isotopic constraints on Triassic arc magmatism, Carboniferous ophiolite and mafic dyke swarm in the Qiangtang area, northern Tibet, China
· U-Pb ages and Hf isotopes of detrital zircons from the Truong Son Belt, central Vietnam and alkaline silicic rocks in Phan Si Pan-Tu Le region, N. Vietnam
· Hf isotopes of meta- and diatexite migmatites in Higo metamorphic terrane, Kyushu, and rodingite in Nagasaki metamorphic rocks in Nomo Peninsula, Nagasaki, Japan
· U–Pb ages of detrital zircons from Neoproterozoic Placers on Erementau–Niyaz Massif, Kazakhstan
· In situ elemental comp. of Neogene marine ash layers derived from the Galápagos hotspot, Ecuador
· In situ elemental comp. of the Pleistocene, phonolitic Cão Grande Formation on Santo Antão, Cape Verde
· Geochemistry, zircon U-Pb age and Hf isotopes of the North Muya block granitoids
All above results have been published on international high-profile journals including Geology, Geochemistry-Geophysics-Geosystems, Gondwana Research, Precambrian Research. There are 28 papers published since 2014 using the methodology that I developed at IES (see applicant’s publication list above).
(5) De-barcoding ancient mantle: geochemical investigation of ophiolites in Central Asia Orogenic Belt (CAOB):
My current working project and also parts of future plan aim to understand heterogeneity and geochemical evolution of ancient (~1000-250 Ma) convective upper mantle from the view of points on composition of ophiolites and chromitites in them. Ophiolites represent fragments of upper mantle and oceanic crust that were incorporated into continental margins during continent-continent and arc-continent collisions, ridge-trench interactions, and/or subduction-accretion events. The convective upper mantle is one of the major geochemical reservoirs in the Earth, which is supposed to be homogeneous due to its convecting character. Recently more studies, however, demonstrated that substantial heterogeneity in terms of certain isotopic composition (e.g., Pb and Os isotopes) does exist in the convective upper mantle. The Central Asian Orogenic belt (CAOB) is selected to be the targeted region. I will apply several approaches to provide geochemical data including Sr-Nd-Hf-Pb-Os isotope compositions related to the origin and evolution of ophiolites and chromitites in them. The principal aims are: (1) to determine precise ages of ophiolites by applying laser U-Pb dating on zircons in gabbroic rocks in the ophiolites; (2) to characterize convective upper mantle and their geochemical evolution; (3) to resolve potential causes of heterogeneity in the convective upper mantle; (4) to provide new information to understand behavior of the HSE and Os isotopes in sulfides and PGMs from ophiolites; (5) to understand the timing order of accretion and to properly reconstruct tectonic units in the CAOB.