Archaean plate tectonics revisited 1. Heat flow, spreading rate, and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents

Abbott, D. H.; Hoffman, Sarah

doi:10.1029/tc003i004p00429

Cited by 153 publications

(44 citation statements)

References 62 publications

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“…Several authors have recognised that in the Archean, hotter, smaller and shallowly subducting plates would favour slab melting over slab dehydration wedge melting, accounting for the prevalence of high-Al intrusive TTG suites and Type 1 extrusive rhyolites (adakites) (Abbott and Homan 1984;Drummond and Defant 1990;Martin 1994).…”

Section: Discussionmentioning

confidence: 99%

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

Hollings

Kerrich

2000

Contrib Mineral Petrol

128

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The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east±west along the southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the Birch±Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of ma®c, intermediate and felsic composition. Tholeiitic basalts range compositionally from Mg# 59±26 evolving continuously to greater REE contents (La 2±19 ppm; Th/La pm~1 ), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Yb n 1±8), high Th/Nb pm ratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium andesites. The two suites are interpreted as a paired arc±back arc sequence. A third group of Nb-enriched basalts (NEB; Nb 9±18 ppm) extend to extremely high TiO 2 , Ta, P 2 O 5 , Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas Zr/ Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but¯at trace element patterns and is interpreted as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean high-Al, high-La/Yb n tonalites; they possess Yb £ 0.4 ppm, Y £ 6 ppm and Sc £ 8 ppm, with La/Yb n of 19±30 and Zr/Sm of 50±59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere, often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched source that subsequently melts during induced mantle convection. The Archean adakite±NEB association erupted during development of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the Birch±Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs.

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Section: Discussionmentioning

confidence: 99%

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

Hollings

Kerrich

2000

Contrib Mineral Petrol

128

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“…In modern oceans, sediments are on average approximately 500-1;000 m thick (45), and the near-surface geothermal gradient is about 25°C∕km. If the Earth's heat flow in the early Archaean was approximately three times greater than at present (46,47), then a thick package of seafloor sediments on the Archaean ocean floor could isotopically exchange with seawater-derived pore waters at temperatures approaching 70°C, as on average, the temperature gradient of the Earth's crust will scale linearly with heat flow. A decline in Earth's heat flow with geologic time could explain the progressive increase in δ 18 O of cherts and carbonates, although we note that such a decrease in heat flow over geologic time has been disputed (48,49).…”

Section: Models For a Global Hydrogen Budgetmentioning

confidence: 99%

Isotope composition and volume of Earth’s early oceans

Pope

Bird

Rosing

2012

Proc. Natl. Acad. Sci. U.S.A.

129

145

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Oxygen and hydrogen isotope compositions of Earth's seawater are controlled by volatile fluxes among mantle, lithospheric (oceanic and continental crust), and atmospheric reservoirs. Throughout geologic time the oxygen mass budget was likely conserved within these Earth system reservoirs, but hydrogen's was not, as it can escape to space. Isotopic properties of serpentine from the approximately 3.8 Ga Isua Supracrustal Belt in West Greenland are used to characterize hydrogen and oxygen isotope compositions of ancient seawater. Archaean oceans were depleted in deuterium [expressed as δD relative to Vienna standard mean ocean water (VSMOW)] by at most 25 AE 5‰, but oxygen isotope ratios were comparable to modern oceans. Mass balance of the global hydrogen budget constrains the contribution of continental growth and planetary hydrogen loss to the secular evolution of hydrogen isotope ratios in Earth's oceans. Our calculations predict that the oceans of early Earth were up to 26% more voluminous, and atmospheric CH 4 and CO 2 concentrations determined from limits on hydrogen escape to space are consistent with clement conditions on Archaean Earth. Both interpretations have been questioned based on the susceptibility of chemical sediments to diagenetic alteration over geologic time (2, 4), and for the latter, a lack of geologic evidence for the extreme greenhouse gas concentrations required to sustain such high temperatures under a less luminous young Sun (9).Minerals formed by metasomatic reactions between seawater and oceanic lithosphere provide an alternative proxy for early ocean chemistry (10-14). Here we report hydrogen and oxygen isotope compositions of 114 silicate mineral separates from the ca. 3.8 Ga Isua Supracrustal Belt (ISB) of West Greenland, including metamorphosed basalts, gabbros, and ultramafic rocks that represent fragments of Eoarchaean oceanic crust (see Fig. S1, Table S2). Minerals formed during heterogeneous late-stage CO 2 -or meteoric-dominated metamorphic overprinting are distinguished by characteristic trends in their coupled δD and δ 18 O values (Fig. S2). In contrast, serpentine minerals formed by reaction of seawater with ultramafic rocks (peridotites) preserved in a low-strain enclave of the ISB (15) define a different isotopic trend. Here we use these serpentines to constrain D∕H and 18 O∕ 16 O ratios of the Eoarchaean ocean. Serpentine Isotope GeochemistrySerpentine-group minerals are hydrous magnesian silicates commonly formed by hydrothermal infiltration of seawater into ultramafic oceanic crust, hydration of the mantle wedge by slabderived fluids, or fluid-rock interaction in continentally emplaced ultramafic lithologies (16)(17)(18). Serpentinized oceanic peridotites are dominantly composed of low-temperature (<300°C) polymorphs lizardite and chrysotile, and to a lesser extent antigorite, which forms at temperatures >250°C (17). In modern oceanic serpentinites, antigorites have δD between −46 and −30‰ (Figs. 1 and 2A, filled squares), values that indicate equilibrium with modern-...

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“…The flux during the Archean may have been larger or smaller. Greater heat loss from the Archean mantle (Turcotte 1980) was likely accommodated at least in part by more rapid seafloor spreading and ocean crust creation (Abbott & Ho↵man 1984). An increase in the formation of crust implies an equivalent increase in the exposure of unaltered basalt to serpentinizing conditions at the seafloor.…”

Section: Pre-biotic Hydrogen Cycling: Hydrogen Sourcesmentioning

confidence: 99%

A theory of atmospheric oxygen

Laakso

Schrag

2017

Geobiology

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There is no direct geologic record of the level of free oxygen in the atmosphere over Earth history. Indirect proxy records have led to a canonical view of atmospheric pO 2 , according to which the atmosphere has passed through three stages. During the first of these periods, corresponding roughly to the Archean eon, pO 2 was less than 0.001% present atmospheric levels (PAL). Oxygen levels rose abruptly around 2.4 billion years ago, a transition referred to as the "Great Oxidation Event" (GOE). This event marks the beginning of the second phase in the history of oxygen, corresponding roughly to the Proterozoic eon, during which pO 2 was in the range of 1% to 10% PAL. Between the latest Neoproterozoic and the early Phanerozoic eon, oxygen rose again, beginning the final stage in the history of oxygen, characterized by essentially modern levels of pO 2 .The processes governing this evolution of the atmosphere are poorly understood. The biogeochemical cycles of redox-sensitive species in the ocean and atmosphere, including oxygen, carbon, iron, and sulfur, must somehow stabilize pO 2 on billion-year time scales, much longer than the residence time of the individual species, and yet also allow pO 2 to achieve equilibrium at widely divergent levels at di↵erent points in time. Only with a clear understanding of these steady-state processes can we understand how pO 2 will respond to the changes in biogeochemical cycling that may have driven the two major oxidation events.In this thesis we use a model of biogeochemical cycling and laboratory experiments to exiii plore the processes that stabilize pO 2 at di↵erent levels over Earth history. We find that a suite of negative feedbacks, including the oxygen-sensitivity of organic carbon burial, allow the stability of oxygen at modern levels. These feedbacks leave pO 2 very insensitive to most aspects of the biogeochemical system, such that stable, Proterozoic levels of pO 2 can only be explained by a smaller supply of phosphorus to the biosphere at that time. Experimental results show that inorganic scavenging processes, which compete with biology for phosphorus, may be more e↵ective in low-oxygen environments due to di↵erences in iron-redox cycling.We explore redox dynamics in the Archean by coupling our biogeochemical model to a hydrogen escape calculation that incorporates the e↵ects of changing oxygen levels on thermosphere composition and temperature. We find that the Archean was characterized by several di↵erent steady states of oxygen, each corresponding to a di↵erent stage in the evolution of life. Furthermore, interactions between the cycles of carbon, oxygen, iron and calcium give rise to a previously unrecognized positive feedback. Our model results show that this feedback allows Archean pO 2 to increase rapidly to a new steady state at Proterozoic levels, given a large enough perturbation. The high levels of atmospheric carbon dioxide following a Snowball Earth glacial event do act as such a trigger in our simulations, providing a hypothesis for the appa...

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Archaean plate tectonics revisited 1. Heat flow, spreading rate, and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents

Abstract: A simple model which relates the rate of seafloor creation and

Cited by 153 publications

References 62 publications

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

Isotope composition and volume of Earth’s early oceans

A theory of atmospheric oxygen

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