Zircons reveal the history of fluctuations in oxidation state of crustal magmatism and supercontinent cycle
SCIENCE CHINA PRESS
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ZIRCONS, A MINERAL NEARLY AS OLD AS EARTH ITSELF, IS A TIME KEEPER, AND ALSO PROVIDES A CHEMICAL WINDOW INTO MANY GEOLOGICAL PHENOMENA, SUCH AS OXIDATION STATE. BY DETERMINING THE OXIDATION LEVELS OF THE MAGMAS THAT FORMED THESE DETRITAL ZIRCONS, SCIENTISTS ARE ABLE TO DEDUCE THE ONSET OF CRUST TO MANTLE RECYCLING, WEATHERING, AND THE SUPERCONTINENT CYCLE.
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This study is led by Dr. Rui Wang and his PhD student Shao-chen Wu (Institute of Earth Sciences, China University of Geosciences, Beijing), Dr. Roberto Weinberg and Dr. Peter Cawood (Monash University), and Dr. William Collins (Curtin University).
Zircons, a mineral nearly as old as Earth itself, crystalize when magmas (molten rocks) cool and can be found in trace quantities in magmatic rocks. The formation of magmas constitutes the mountains in the Earth. Through interactions with water and atmosphere, the mountains break down into sediments. Zircons are so durable and resistant to weathering and erosion that they rarely go away, and therefore this mineral in sediments (so call “detrital zircons”) holds the greatest insight into the history of the Earth. Zircon enriches with U (U-Pb dating) is a time keeper, and also provides a chemical window into many geological phenomena, such as oxidation state.
The team uses a new method of Loucks et al (2020) for determining the oxidation state of granitic magmas that uses ratios of Ce, U, and Ti in zircon to track oxidation state change of crustal magmas through Earth history. The calculation does not require ionic charge to be known, nor is determination of crystallization temperature, pressure, or parental melt composition required.
“Previous methods include Ce/Ce* and Eu/Eu* oxybarometers, but each has limitations related to temperature, pressure, host rock chemical compositional variations, or precision of REE elements needed to measure the Ce/Ce* and Eu/Eu* anomalies.” Bob Loucks from Western Australia says.
This improved oxybarometer allows a more confident evaluation of the variation in oxidation state, which can now be interpreted in terms of global tectonic changes with time. By determining the oxidation levels of the magmas that formed these detrital zircons, scientists are able to deduce the onset of crust to mantle recycling, weathering, and the supercontinent cycle.
The key point is that rocks that lay at the Earth's surface can be carried back down to deep in the Earth's mantle (hundreds to thousands of km below the surface. Our data shows that not only has this happening today but could have been going on for billions of years. Looking at zircons from the early Earth, 3-billion-year-old zircons, to those formed today we have found that the redox state of the magmas in which they formed. The oxidation state (expressed as ΔFMQ) of the detrital zircons rise at ~3.5 billion years followed by a consistent average ΔFMQ > 0 over the last 3 billion years, suggesting recycling of oceanic lithosphere back into the mantle in what eventually became established as subduction zones. It shows that the lower limit of redox state dropped dramatically at 2.6 billion years ago, marking the formation of well-defined continents and the burial of oceanic rocks back into the deep mantle of the Earth. Further to that we found a cyclicity of the redox patterns: every 600 million years or so, continents come together to form supercontinents, like Gondwana, Rodinia, Nura, and Superia.
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Science Bulletin
Zircon age histogram and redox variations with the supercontinent amalgamation intervals.
Moving average of ΔFMQ of the igneous-derived and the sediment-derived zircon (proportion below than 10% is not shown) with the supercontinent amalgamation intervals. The more reduced magmas associated with sedimentary sources with a ca. 600 Ma cyclicity, became established at 2.6 billion years.
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©Science China Press
Formation of a new subduction zone with hot or cold incipient channel
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COMPARISONS OF DEFINITIONS, FAVORABLE CONDITIONS AND NATURAL CASES BETWEEN HOT AND COLD SUBDUCTION INITIATION
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This study is led by Prof. Zhong-Hai Li (University of Chinese Academy of Sciences). The present solid Earth is actually active, with new plates generating in the mid-ocean ridges and some old plates sinking back into the interior through subduction zones. Subduction is thus a key process of the tectonics and geodynamics of the Earth. However, the formation mechanism of a new subduction zone, i.e. subduction initiation (SI), is widely debated. “Comparing to the long-term mature subduction, its initiation is more like an “instantaneous” process with limited geological records. Furthermore, these records experience erosion and modification by the later subduction. Consequently, the remnant geological records are rare, which plays as a major barrier for the better understanding of subduction initiation process.” Li says.
Geologists tried to decipher the SI process through analyzing the characteristic rock records. The most widely studied, characteristic magmatic record is the forearc rock sequence (forearc basalt – boninite – arc tholeiites) in the Izu-Bonin-Mariana (IBM) subduction zone. In addition, the SSZ-type (Supra-Subduction-Zone) ophiolites, e.g. in the Troodos (Cyprus) and Semail (Oman), have comparable petrological and geochemical characteristics with the IBM forearc sequence. Thus, it is further proposed that the SSZ-ophiolite could be generated during subduction initiation. Another type of geological record for SI is the metamorphic sole, which normally emplaces accompanying with the SSZ-ophiolite. All these magmatic and metamorphic records point to a high temperature and low pressure condition for SI. Then, “the question is whether the occurrence of all the subduction initiation in nature requires such a critical condition with rather high temperature at shallow depths.” Li says.
In the present-day ocean, there are several early-stage subduction zones with differential geological records, for example, the Puysegur subduction zone to the south of New Zealand. This SI process lacks the typical magmatic and metamorphic records. Instead, the geological records include the responses of structural deformation and sedimentary evolution. Similarly, there are a series of young oceanic subduction zones in the western Pacific, e.g. the Negro subduction zone in the Sulu Sea, the north Sulawesi and Cotabato subduction zones in the Celebes Sea. The thermal conditions in these incipient subduction channels should be colder, at least lower than the required temperature for the generation of ophiolite and metamorphic sole.
“It thus indicates that the extremely high temperature condition at shallow depths, for the generation of naturally observed ophiolite and metamorphic sole, only represents the high temperature end-member of subduction initiation, but cannot be used as the diagnostics for all the SI.” Li says and he further proposes two contrasting regimes for subduction initiation, i.e. the hot versus cold end-members, as shown in Figure 1. The hot SI regime is more “traditional”, with the geological records of magmatic and metamorphic rocks which have been regarded as the typical responses of SI and even as the diagnostics for deciphering paleo-SI cases in the orogens. In contrast, the cold SI regime lacks such kind of magmatic and metamorphic records, and thus attracts less attention in observational studies, but does occur for many subduction zones.
“Consequently, the SSZ-ophiolite and metamorphic sole are only the typical records of hot SI, but are not necessarily generated in the cold SI regime. Thus, we cannot use such specific rock records to judge the occurrence of SI or not; instead, multiple geological responses should be combined together to get a full view of this puzzling issue.” Li says.
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See the article:
Hot versus cold subduction initiation
https://doi.org/10.1093/nsr/nwae012
JOURNAL
National Science Review
DOI
Pre-Cryogenian stratigraphy, palaeontology, and paleogeography of the Tibetan Plateau and environs
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IN THE FIGURE CAN BE SEEN THE POSSIBLE PALEOGEOGRAPHIC LOCATIONS OF THE TERRANES IN THE TIBETAN PLATEAU AND ENVIRONS. IND=INDIA; AU=AUSTRALIA; ANT=ANTARCTIC; KAL=KALAHARI; SF=SAN FRANCISCO; CON=CONGO; LAU=LAURENTIA; RIO=RIO DE LA PLATA; AMZ=AMAZONIA; BAL=BALTICA; W.AF=WEST AFRICA; SIB=SIBERIA; ANS=ARABIAN-NUBIAN SHIELD; SC=SOUTH CHINA; AFB=ALBANY-FRASER OROGEN; EGB=EASTERN GHATS OROGEN; KB-IB=KIBARAN AND IRUMIDE OROGENS; G=GREENVILLE OROGEN.
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This study is led by Dr. Pei-yuan Hu and Qing-guo Zhai (Institute of Geology, Chinese Academy of Geological Sciences). In this study, the characteristics of pre-Cryogenian sedimentation, paleontology, magmatism, and metamorphism in the Tibetan Plateau and its surrounding areas are systematically summarized. Based on existing data, the records of pre-Cryogenian sedimentation and paleontology are mainly concentrated in the Meso-Neoproterozoic, with relatively few records from the Paleoproterozoic or earlier. The oldest geological record is the Hadean detrital zircons in the metamorphosed sedimentary rocks of the Himalaya and Qamdo areas (ca. 4.0 Ga). The Tibetan Plateau and surrounding areas preserve records related to the formation and evolution of the Kenor supercraton, and the Columbia, Rodinia, and Gondwana supercontinents. Pre-Cryogenian basements can be divided into three types: Tarim-, Yangtze-, and Lhasa-type.
The Tarim-type basement includes the southern Tarim terrane and the northern edge of the Indian continent, which are characterized by (1) magmatic-metamorphic records of the Columbian assembly and detrital zircon age peaks at ca. 2.0-1.8 Ga; (2) the absence of magmatic-metamorphic records and detrital zircon age peaks related to the Rodinia assembly (ca. 1.3-0.9 Ga); and, (3) the development of Cryogenian diamictite, prior to which no large-scale volcanic activity has been observed.
The Yangtze-type basement terranes include the western Yangtze terrane and the western Qinling-Qilian-Kunlun terrane, which are characterized by: (1) widespread development of Cryogenian diamictite, beneath which large-scale volcanic activity related to the global climate cooling can be observed; (2) magmatic-metamorphic records related to Rodinia assembly (ca. 1.1-1.0 Ga) and arc-related magmatism during ca. 1000-750 Ma; and, (3) abundant Meso-Neoproterozoic stromatolite and micropaleoflora fossils.
The Lhasa terrane is located at the core of the Tibetan Plateau and has distinct differences in its pre-Cryogenian basement compared to other terranes in the plateau and adjacent areas. These differences are mainly reflected in: (1) the development of early Neoproterozoic continental rift sedimentary records (ca. 900 Ma); and (2) the presence of a strong 1.2-1.1 Ga detrital zircon age peak in Precambrian to Paleozoic sedimentary strata, accompanied by contemporaneous magmatic-metamorphic records.
The above-mentioned results suggest that the pre-Cryogenian material in the Tibetan Plateau and its surrounding areas have played an important role in studying the formation and evolution of early supercontinents on Earth. Integrating previous studies with this contribution, the Tarim- and Yangtze-type basement terranes are interpreted as having a paleogeographic affinity with the northern margins of the Australian and Indian continents and the Lhasa-type basement possibly came from the African continent.
See the article:
Pre-Cryogenian stratigraphy, palaeontology, and paleogeography of the Tibetan Plateau and environs
https://doi.org/10.1007/s11430-022-1127-8
JOURNAL
Science China Earth Sciences
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