Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga.Among the more important of these are the following:(1)increasing proportion of basalts with"arc-like"mantle sources;(2)an inc...Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga.Among the more important of these are the following:(1)increasing proportion of basalts with"arc-like"mantle sources;(2)an increasing abundance of basalts derived from enriched(EM)and depleted(DM)mantle sources;(3)onset of a Great Thermal Divergence in the mantle;(4)a decrease in degree of melting of the mantle;(5)beginning of large lateral plate motions;(6)appearance of eclogite inclusions in diamonds;(7)appearance and rapid increase in frequency of collisional orogens;(8)rapid increase in the production rate of continental crust as recorded by zircon age peaks;(9)appearance of ophiolites in the geologic record,and(10)appearance of global LIP(large igneous province)events some of which correlate with global zircon age peaks.All of these changes may be tied directly or indirectly to cooling of Earth's mantle and corresponding changes in convective style and the strength of the lithosphere,and they may record the gradual onset and propagation of plate tectonics around the planet.To further understand the changes that occurred between 3 and 2 Ga,it is necessary to compare rocks,rock associations,tectonics and geochemistry during and between zircon age peaks.Geochemistry of peak and inter-peak basalts and TTGs needs to be evaluated in terms of geodynamic models that predict the existence of an episodic thermal regime between stagnant-lid and plate tectonic regimes in early planetary evolution.展开更多
Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time.Four types of mantle are characterized from incompatible element distributions in basalts a...Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time.Four types of mantle are characterized from incompatible element distributions in basalts and komatiites:depleted,hydrated,enriched and mantle from which komatiites are derived.Our most important observation is the recognition for the first time of what we refer to as a Great Thermal Divergence within the mantle beginning near the end of the Archean,which we ascribe to thermal and convective evolution.Prior to 2.5 Ga,depleted and enriched mantle have indistinguishable thermal histories,whereas at 2.5-2.0 Ga a divergence in mantle magma generation temperature begins between these two types of mantle.Major and incompatible element distributions and calculated magma generation temperatures suggest that Archean enriched mantle did not come from mantle plumes,but was part of an undifferentiated or well-mixed mantle similar in composition to calculated primitive mantle.During this time,however,high-temperature mantle plumes from dominantly depleted sources gave rise to komatiites and associated basalts.Recycling of oceanic crust into the deep mantle after the Archean may have contributed to enrichment of Ti,Al,Ca and Na in basalts derived from enriched mantle sources.After 2.5 Ga,increases in Mg~# in basalts from depleted mantle and decreases in Fe and Mn reflect some combination of growing depletion and cooling of depleted mantle with time.A delay in cooling of depleted mantle until after the Archean probably reflects a combination of greater radiogenic heat sources in the Archean mantle and the propagation of plate tectonics after 3 Ga.展开更多
The combined use of Hf, Nd and Sr isotopes is more useful in understanding the supercontinent cycle than the use of only Hf isotopic data from detrital zircons. Sr and Nd seawater isotopes, although not as precise as ...The combined use of Hf, Nd and Sr isotopes is more useful in understanding the supercontinent cycle than the use of only Hf isotopic data from detrital zircons. Sr and Nd seawater isotopes, although not as precise as εNdand εHfdistributions, also record input from ocean ridge systems. Unlike detrital zircons where sources cannot be precisely located because of crustal recycling, both the location and tectonic setting often can be constrained for whole-rock Nd isotopic data. Furthermore, primary zircon sources may not reside on the same continent as derivative detrital zircons due to supercontinent breakup and assembly. Common to all of the isotopic studies are geographic sampling biases refecting outcrop distributions, river system sampling, or geologists, and these may be responsible for most of the decorrelation observed between isotopic systems. Distributions between 3.5 and 2 Ga based on εHfmedian values of four detrital zircon databases as well as our compiled εNddatabase are noisy but uniformly distributed in time, whereas data between 2 and 1 Ga data are more tightly clustered with smaller variations. Grouped age peaks suggest that both isotopic systems are sampling similar types of orogens. Only after 1 Ga and before 3.5 Ga do we see wide variations and signifcant disagreement between databases, which may partially refect variations in both the number of sample locations and the number of samples per location. External and internal orogens show similar patterns in εNdand εHfwith age suggesting that both juvenile and reworked crustal components are produced in both types of orogens with similar proportions. However, both types of orogens clearly produce more juvenile isotopic signatures in retreating mode than in advancing mode. Many secular changes in εHfand εNddistributions correlate with the supercontinent cycle. Although supercontinent breakup is correlated with short-lived decreasing εHfand εNd( 100 Myr) for most supercontinents, there is no isotopic evidence for the breakup of the Paleoproterozoic supercontinent Nuna. Assembly of supercontinents by extroversion is recorded by decreasing εNdin granitoids and metasediments and decreasing εHfin zircons, attesting to the role of crustal reworking in external orogens in advancing mode. As expected, seawater Sr isotopes increase and seawater Nd isotopes decrease during supercontinent assembly by extroversion. Pangea is the only supercontinent that has a clear isotopic record of introversion assembly, during which median εNdand εHfrise rapidly for 100 Myr. Although expected to increase, radiogenic seawater Sr decreases(and seawater Nd increases) during assembly of Pangea, a feature that may be caused by juvenile input into the oceans from new ocean ridges and external orogens in retreating mode. The fact that a probable onset of plate tectonics around 3 Ga is not recorded in isotopic distributions may be due the existence of widespread felsic crust formed prior to the onset of plate tectonics in a stagnant lid tectonic regime, as supported by Nd and Hf model ages.展开更多
Ophiolites are fragments of oceanic lithosphere that are produced at spreading centers at ocean ridges,back arc basins,or forearcs during subduction initiation,and are key indicators of plate tectonics.Although it is ...Ophiolites are fragments of oceanic lithosphere that are produced at spreading centers at ocean ridges,back arc basins,or forearcs during subduction initiation,and are key indicators of plate tectonics.Although it is widely agreed that ophiolites are remnants of oceanic crust and associated depleted mantle preserved in orogens,the recognition of ophiolites and their tectonic significance is still a topic of discussion and disagreement.We propose that ophiolites can be recognized in the geologic record by some combination of genetically related pillow basalt,layered gabbro,sheeted dykes,podiform chromite,harzburgite or/and dunite.Mafic igneous rocks have either ocean-ridge basalt or immature oceanic arc basalt chemical compositions.Using a scoring system of 1–11 for ophiolite confidence level,scores of8 are considered confident,6–8 probable,and<6 questionable or unlikely ophiolites.Most ophiolites with scores6 are<900 Ma.The oldest confident ophiolite(score of 8)is the Zunhua ophiolite in eastern China at 2550 Ma,and the oldest well documented sheeted dykes occur in the Jormua and Purtuniq ophiolites at 2000–1950 Ma.Ophiolites do not become geographically widespread until after 900 Ma,and most ophiolites of all ages formed in a forearc(subduction initiation)tectonic setting.If ophiolite production requires plate tectonics,subduction must have begun at least locally by 2700 Ma but did not become widespread until after 2000 Ma.The abundance of ophiolites after 900 Ma may reflect better preservation of subduction-related ophiolites,or to an increasing global network of interconnected plates.Ophiolite frequency peaks in the geologic record partially reflect geographic regions where ophiolites have been extensively studied rather than monitoring the production rate of ophiolites,but the scarcity of>900 Ma ophiolites is probably real.展开更多
文摘Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga.Among the more important of these are the following:(1)increasing proportion of basalts with"arc-like"mantle sources;(2)an increasing abundance of basalts derived from enriched(EM)and depleted(DM)mantle sources;(3)onset of a Great Thermal Divergence in the mantle;(4)a decrease in degree of melting of the mantle;(5)beginning of large lateral plate motions;(6)appearance of eclogite inclusions in diamonds;(7)appearance and rapid increase in frequency of collisional orogens;(8)rapid increase in the production rate of continental crust as recorded by zircon age peaks;(9)appearance of ophiolites in the geologic record,and(10)appearance of global LIP(large igneous province)events some of which correlate with global zircon age peaks.All of these changes may be tied directly or indirectly to cooling of Earth's mantle and corresponding changes in convective style and the strength of the lithosphere,and they may record the gradual onset and propagation of plate tectonics around the planet.To further understand the changes that occurred between 3 and 2 Ga,it is necessary to compare rocks,rock associations,tectonics and geochemistry during and between zircon age peaks.Geochemistry of peak and inter-peak basalts and TTGs needs to be evaluated in terms of geodynamic models that predict the existence of an episodic thermal regime between stagnant-lid and plate tectonic regimes in early planetary evolution.
基金funding from the European Research Council(ERC StG 279828)
文摘Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time.Four types of mantle are characterized from incompatible element distributions in basalts and komatiites:depleted,hydrated,enriched and mantle from which komatiites are derived.Our most important observation is the recognition for the first time of what we refer to as a Great Thermal Divergence within the mantle beginning near the end of the Archean,which we ascribe to thermal and convective evolution.Prior to 2.5 Ga,depleted and enriched mantle have indistinguishable thermal histories,whereas at 2.5-2.0 Ga a divergence in mantle magma generation temperature begins between these two types of mantle.Major and incompatible element distributions and calculated magma generation temperatures suggest that Archean enriched mantle did not come from mantle plumes,but was part of an undifferentiated or well-mixed mantle similar in composition to calculated primitive mantle.During this time,however,high-temperature mantle plumes from dominantly depleted sources gave rise to komatiites and associated basalts.Recycling of oceanic crust into the deep mantle after the Archean may have contributed to enrichment of Ti,Al,Ca and Na in basalts derived from enriched mantle sources.After 2.5 Ga,increases in Mg~# in basalts from depleted mantle and decreases in Fe and Mn reflect some combination of growing depletion and cooling of depleted mantle with time.A delay in cooling of depleted mantle until after the Archean probably reflects a combination of greater radiogenic heat sources in the Archean mantle and the propagation of plate tectonics after 3 Ga.
文摘The combined use of Hf, Nd and Sr isotopes is more useful in understanding the supercontinent cycle than the use of only Hf isotopic data from detrital zircons. Sr and Nd seawater isotopes, although not as precise as εNdand εHfdistributions, also record input from ocean ridge systems. Unlike detrital zircons where sources cannot be precisely located because of crustal recycling, both the location and tectonic setting often can be constrained for whole-rock Nd isotopic data. Furthermore, primary zircon sources may not reside on the same continent as derivative detrital zircons due to supercontinent breakup and assembly. Common to all of the isotopic studies are geographic sampling biases refecting outcrop distributions, river system sampling, or geologists, and these may be responsible for most of the decorrelation observed between isotopic systems. Distributions between 3.5 and 2 Ga based on εHfmedian values of four detrital zircon databases as well as our compiled εNddatabase are noisy but uniformly distributed in time, whereas data between 2 and 1 Ga data are more tightly clustered with smaller variations. Grouped age peaks suggest that both isotopic systems are sampling similar types of orogens. Only after 1 Ga and before 3.5 Ga do we see wide variations and signifcant disagreement between databases, which may partially refect variations in both the number of sample locations and the number of samples per location. External and internal orogens show similar patterns in εNdand εHfwith age suggesting that both juvenile and reworked crustal components are produced in both types of orogens with similar proportions. However, both types of orogens clearly produce more juvenile isotopic signatures in retreating mode than in advancing mode. Many secular changes in εHfand εNddistributions correlate with the supercontinent cycle. Although supercontinent breakup is correlated with short-lived decreasing εHfand εNd( 100 Myr) for most supercontinents, there is no isotopic evidence for the breakup of the Paleoproterozoic supercontinent Nuna. Assembly of supercontinents by extroversion is recorded by decreasing εNdin granitoids and metasediments and decreasing εHfin zircons, attesting to the role of crustal reworking in external orogens in advancing mode. As expected, seawater Sr isotopes increase and seawater Nd isotopes decrease during supercontinent assembly by extroversion. Pangea is the only supercontinent that has a clear isotopic record of introversion assembly, during which median εNdand εHfrise rapidly for 100 Myr. Although expected to increase, radiogenic seawater Sr decreases(and seawater Nd increases) during assembly of Pangea, a feature that may be caused by juvenile input into the oceans from new ocean ridges and external orogens in retreating mode. The fact that a probable onset of plate tectonics around 3 Ga is not recorded in isotopic distributions may be due the existence of widespread felsic crust formed prior to the onset of plate tectonics in a stagnant lid tectonic regime, as supported by Nd and Hf model ages.
文摘Ophiolites are fragments of oceanic lithosphere that are produced at spreading centers at ocean ridges,back arc basins,or forearcs during subduction initiation,and are key indicators of plate tectonics.Although it is widely agreed that ophiolites are remnants of oceanic crust and associated depleted mantle preserved in orogens,the recognition of ophiolites and their tectonic significance is still a topic of discussion and disagreement.We propose that ophiolites can be recognized in the geologic record by some combination of genetically related pillow basalt,layered gabbro,sheeted dykes,podiform chromite,harzburgite or/and dunite.Mafic igneous rocks have either ocean-ridge basalt or immature oceanic arc basalt chemical compositions.Using a scoring system of 1–11 for ophiolite confidence level,scores of8 are considered confident,6–8 probable,and<6 questionable or unlikely ophiolites.Most ophiolites with scores6 are<900 Ma.The oldest confident ophiolite(score of 8)is the Zunhua ophiolite in eastern China at 2550 Ma,and the oldest well documented sheeted dykes occur in the Jormua and Purtuniq ophiolites at 2000–1950 Ma.Ophiolites do not become geographically widespread until after 900 Ma,and most ophiolites of all ages formed in a forearc(subduction initiation)tectonic setting.If ophiolite production requires plate tectonics,subduction must have begun at least locally by 2700 Ma but did not become widespread until after 2000 Ma.The abundance of ophiolites after 900 Ma may reflect better preservation of subduction-related ophiolites,or to an increasing global network of interconnected plates.Ophiolite frequency peaks in the geologic record partially reflect geographic regions where ophiolites have been extensively studied rather than monitoring the production rate of ophiolites,but the scarcity of>900 Ma ophiolites is probably real.
