Os, Sr, Nd, and Pb isotope systematics of southern African peridotite xenoliths: Implications for the chemical evolution of subcontinental mantle
TL;DR: Isotope analyses of Os, Sr, Nd, and Pb elements were caried out on twelve peridotite xenoliths from the Jagersfontein, Letseng-la-terae, Thaba Patsoa, Mothae, and Premier kimberlites of southern Africa, to investigate the timing and the nature of melt extraction from the continental lithosphere and its relation to the continent formation and stabilization.
About: This article is published in Geochimica et Cosmochimica Acta.The article was published on 1989-07-01. It has received 688 citations till now. The article focuses on the topics: Peridotite & Craton.
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TL;DR: A survey of the dimensions and composition of the present continental crust is given in this paper, where it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons).
Abstract: A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post-Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present-day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond-hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction-related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post-Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.
3,656 citations
TL;DR: In this paper, a pyrolitic GPa from a mantle enriched in SiO2 relative to pyrolite was made from 3 to 7 GPa in piston-cylinder and multi-anvil apparatus.
Abstract: However, it can be a residue of komatiite melt extraction at >7 Melting experiments on fertile peridotite KR4003, a ‘pyrolitic’ GPa from a mantle enriched in SiO2 relative to pyrolite. composition, were made from 3 to 7 GPa in piston-cylinder and multi-anvil apparatus. Temperature gradients across the sample were minimized (<25°C), and the compositions of all phases were determined. Modal abundances of coexisting phases were calculated by mass balance, and the results were used to determine phase
1,270 citations
TL;DR: The continental crust has an andesitic bulk composition, which cannot have been produced by the basaltic magmatism that dominates sites of present-day crustal growth as mentioned in this paper.
Abstract: The continental crust has an andesitic bulk composition, which cannot have been produced by the basaltic magmatism that dominates sites of present-day crustal growth—at both convergent margins and within plates. These observations suggest that there may have been a different mode of continental crust generation in Archaean times, and may point to delamination of the lower crust as an important recycling process.
1,189 citations
TL;DR: Camiguin is a small volcanic island located 12 km north of Mindanao Island in southern Philippines as mentioned in this paper, which is the most likely source of Camiguin and most CMA magmas is the mantle wedge metasomatized by fluids dehydrated from a subducted slab.
Abstract: Camiguin is a small volcanic island located 12 km north of Mindanao Island in southern Philippines. The island consists of four volcanic centers which have erupted basaltic to rhyolitic calcalkaline lavas during the last ∼400 ka. Major element, trace element and Sr, Nd and Pb isotopic data indicate that the volcanic centers have produced a single lava series from a common mantle source. Modeling results indicate that Camiguin lavas were produced by periodic injection of a parental magma into shallow magma chambers allowing assimilation and fractional crystallization (AFC) processes to take place. The chemical and isotopic composition of Camiguin lavas bears strong resemblance to the majority of lavas from the central Mindanao volcanic field confirming that Camiguin is an extension of the tectonically complex Central Mindanao Arc (CMA). The most likely source of Camiguin and most CMA magmas is the mantle wedge metasomatized by fluids dehydrated from a subducted slab. Some Camiguin high-silica lavas are similar to high-silica lavas from Mindanao, which have been identified as “adakites” derived from direct melting of a subducted basaltic crust. More detailed comparison of Camiguin and Mindanao adakites with silicic slab-derived melts and magnesian andesites from the western Aleutians, southernmost Chile and Batan Island in northern Philippines indicates that the Mindanao adakites are not pure slab melts. Rather, the CMA adakites are similar to Camiguin high-silica lavas which are products of an AFC process and have negligible connection to melting of subducted basaltic crust.
881 citations
TL;DR: The Re-Os isotope sytem, based on the long-lived β− transition of 187Re to 187Os, has matured to wide use in cosmochemistry and high-temperature geochemistry as discussed by the authors.
Abstract: ▪ Abstract The Re-Os isotope sytem, based on the long-lived β− transition of 187Re to 187Os, has matured to wide use in cosmochemistry and high-temperature geochemistry. The siderophilic/chalcophilic behavior of Re and Os is different from that of the elements that comprise most other long-lived radiogenic isotope systems. Magmatic iron meteorites (IIIAB, IIAB, IVA, and IVB) have Re-Os isochrons that indicate asteroidal core crystallization within the first 10–40 million years of Solar System evolution. Rocks from Earth's convecting mantle show generally chondritic Re/Os evolution throughout Earth history that is explained by the addition of highly siderophile elements to the mantle after core formation via late accretion. Oceanic basalts have Os-isotope systematics that improve the detailed geological interpretation of extant mantle components. Some portions of ancient subcontinental lithospheric mantle are severely depleted in Re and have correspondingly subchondritic 187Os/188Os, indicating long-term i...
860 citations
References
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TL;DR: In this article, Pollack et al. constructed a global map of lithospheric thickness based on the regional variation of surface heat flow, geotherms, and lithosphere thickness, and identified the lid as synonymous with the lithosphere.
969 citations
881 citations
TL;DR: In this article, a revised andesite model is used to obtain total crustal composition, assuming that the upper crust, one-third of the total, is derived from intracrustal partial melting.
Abstract: The composition of the present-day upper crust, inferred from the uniformity of sedimentary rock r.e.e. (rare earth element) patterns, is close to that of granodiorite. A revised ‘andesite’ model is used to obtain total crustal composition. The lower crust is the composition remaining, assuming that the upper crust, one-third of the total, is derived from intracrustal partial melting. The upper-crustal r.e.e. pattern has pronounced Eu depletion (Eu/Eu* = 0.64), the lower-crustal pattern has Eu enrichment (Eu/Eu* = 1.17) and the total crust has no Eu anomaly relative to chondritic abundances. The Eu depletion in the upper crust is attributed to retention of Eu in plagioclase in the lower crust. Because plagioclase is not stable below 40 km (> 10 kbar), the anomaly is intracrustal in origin. The Archaean upper crust has a different r.e.e. pattern to that of the present-day upper crust, being lower in total r.e.e., and La/Yb ratios, and lacking an Eu anomaly. These data are used to infer the Archaean upper-crustal composition, which resembles that of the present-day total crust, except that Ni and Cr contents are higher. The Archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites. The latter have steep light r.e.e.-enriched-heavy r.e.e.-depleted patterns, consistent with equilibration with garnet and hence probable mantle derivation. There is little reason to suppose that the Archaean lower crust was different in composition from the upper crust, except locally where partial melting episodes occurred. The r.e.e. evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early Archaean, a massive increase (to about 70% of the total crust) between about 3000 and 2500 Ma B.P. and a slow increase until the present day. The change from Archaean to post-Archaean r.e.e. patterns in the upper crust is not isochronous, but is reflected in the sedimentary rock r.e.e. patterns at differing times in different continents. On the basis of a model composition for the mantle, 36% of the potassium, 30% of uranium, 15% of lanthanum and 3 % of ytterbium are concentrated in the present continental crust. This enrichment is related to ionic size and valency differences from common mantle cations (e.g. Mg, Fe). Pre-3.9 Ga B.P. crusts were obliterated by meteorite bombardment. No geochemical evidence exists for primordial anorthositic, sialic or mafic crusts.
788 citations