Evidence of a Suducting Crustal Slab From EoArchean Sulfur Isotopes

Multiple sulfur isotope signature of early Archean oceanic crust, Isua (SW-Greenland)

Authors:

Siedenberg et al

Abstract:

The Archean sulfur cycle was different from the present-day cycle, as the emission of volcanogenic sulfurous gases was the dominant process in the anoxic environment of the early Earth.This emitted sulfur exhibits mass-independently fractionated sulfur isotopes (MIF-S), resulting from photochemical reactions in the atmosphere, and it differs substantially from unfractionated sulfur in the mantle. So far, the main focus of multiple sulfur analyses (32S32S, 33S33S, 34S34S and 36S36S) was placed on the sedimentary part of the Archean sulfur cycle. In order to constrain the magmatic part of the sulfur cycle, we analyzed the sulfur isotopic composition of oceanic crustal rocks from the ca. 3.7–3.8 Ga Isua Supracrustal Belt (ISB). Differently altered samples were taken from two units:(1) the Undifferentiated Amphibolites (UA) and (2) the younger Amphibolites with Boninitic affinity (AB). The mean values are:δ34SCRS=+0.01±0.65‰δ34SCRS=+0.01±0.65‰ (values range from −0.87 to 1.37‰; CRS = chromium-reducible sulfur),Δ33SCRS=+0.02±0.12‰Δ33SCRS=+0.02±0.12‰ (values range from −0.17 to 0.26‰), Δ36SCRS=-0.47±0.06‰Δ36SCRS=-0.47±0.06‰ (values range from −0.56 to −0.38‰). Thus, the mean isotope values support the assumption that the sulfur isotopic signature reflects the expected near-zero signature of their mantle origin. However, differences in Δ33SCRSΔ33SCRS values are discernible and non-zero suggesting that different sources are contributing to the isotopic signature. An influence of alteration is excluded for all samples as different alteration-sensitive geochemical parameters do not show any correlation with the multiple sulfur isotope signatures. Further, it is unlikely that the small magnitudes in Δ33SCRSΔ33SCRS are generated by microbial mass-dependent processes because of the narrow range of δ34SCRSδ34SCRS values. Possible sources contributing an atmospheric MIF-S signature include seawater sulfate (negative Δ33SCRSΔ33SCRS values) through hydrothermal circulation, the assimilation of ocean floor sediments during the ascent of the melt and/or a mantle source contamination by subducted oceanic slab.

Subduction or sagduction in Scotland's Archean ultramafic–mafic bodies

Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland

Authors:

Johnson et al

Abstract:

The Lewisian Complex of NW Scotland is a fragment of the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in the late Neoarchean to Palaeoproterozoic. Within the granulite facies central region of the mainland Lewisian Complex, discontinuous belts composed of ultramafic–mafic rocks and structurally overlying garnet–biotite gneiss (brown gneiss) are spatially associated with steeply-inclined amphibolite facies shear zones that have been interpreted as terrane boundaries. Interpretation of the primary chemical composition of these rocks is complicated by partial melting and melt loss during granulite facies metamorphism, and contamination with melts derived from the adjacent migmatitic TTG host rocks. Notwithstanding, the composition of the layered ultramafic–mafic rocks is suggestive of a protolith formed by differentiation of tholeiitic magma, where the ultramafic portions of these bodies represent the metamorphosed cumulates and the mafic portions the metamorphosed fractionated liquids. Although the composition of the brown gneiss does not clearly discriminate the protolith, it most likely represents a metamorphosed sedimentary or volcano-sedimentary sequence. For Archean rocks, particularly those metamorphosed to granulite facies, the geochemical characteristics typically used for discrimination of paleotectonic environments are neither strictly appropriate nor clearly diagnostic. Many of the rocks in the Lewisian Complex have ‘arc-like’ trace element signatures. These signatures are interpreted to reflect derivation from hydrated enriched mantle and, in the case of the TTG gneisses, partial melting of amphibolite source rocks containing garnet and a Ti-rich phase, probably rutile. However, it is becoming increasingly recognized that in Archean rocks such signatures may not be unique to a subduction environment but may relate to processes such as delamination and dripping. Consequently, it is unclear whether the Lewisian ultramafic–mafic rocks and brown gneisses represent products of plate margin or intraplate magmatism. Although a subduction-related origin is possible, we propose that an intraplate origin is equally plausible. If the second alternative is correct, the ultramafic–mafic rocks and brown gneisses may represent the remnants of intracratonic greenstone belts that sank into the deep crust due to their density contrast with the underlying partially molten low viscosity TTG orthogneisses.

A new Synthesis of Archaean Crustal Evolution in West Africa

Archaean crustal evolution in West Africa: A new synthesis of the Archaean geology in Sierra Leone, Liberia, Guinea and Ivory Coast

Author:

Rollinson

Abstract:

A new synthesis of the geology and geochronology of the little-known Archaean rocks in Sierra Leone, Liberia, Guinea and Ivory Coast is presented in order to better understand the processes of Archaean crustal evolution in this region, and to attempt to interpret these data in the light of our current understanding of Archaean crustal evolution. In addition, this study seeks to identify those aspects of Archaean crustal evolution which are currently not known in this area and which need to become the subject of future studies, given the economic importance of this region in terms of the mineral deposits hosted in the Archaean rocks. These include greenstone-belt hosted iron ore, lode gold, chromite and columbite–tantalite and younger diamondiferous kimberlites intrusive into Archaean felsic gneisses.

The new results show that this cratonic nucleus comprises of four main geological units:

(1)  The oldest crust is made up of 3.5–3.6 Ga TTG (tonalite–trondhjemite–granodiorite) gneisses. These only outcrop in the east of the craton in Guinea but their presence is indicated elsewhere in the central part of the craton though xenocrystic zircon cores in younger rocks.

(2) The major rock type found throughout the craton is 3.26–2.85 Ga TTG gneiss. In detail these magmas are thought to have formed in two episodes one between 3.05–3.26 Ga and the other between 2.85–2.96 Ga. The presence of inherited zircons in the younger suite indicate that this event represents the partial reworking of the older gneisses. 3.4 Ga eclogite xenoliths in kimberlite derived from the sub-continental lithospheric mantle are thought to be the restite after the partial melting of a basaltic protolith in the production of the TTG magmas.

(3)  Supracrustal rocks form linear belts infolded into the TTG gneisses and metamorphosed to amphibolite and granulite grade. They are of different sizes, contain a variety of lithological sequences and may be of several different ages. The larger supracrustal belts in Sierra Leone contain a thick basalt-komatiite sequence derived by the partial melting of two different mantle sources, unconformably overlain by a sedimentary formation. They are seen as an important resource for gold, iron-ore, chromite and columbite–tantalite.

(4) A suite of late Archaean granitoids formed by the partial melting of the TTG gneisses in a craton wide deformation-metamorphic-partial melting event at 2800 ± 20 Ma. This thermal event is thought to be responsible for the stabilisation of the craton.

This new synthesis highlights major geological and geochronological similarities between the Archaean rocks of Sierra Leone, Liberia, Guinea and Ivory Coast and those in the Reguibat Shield in the northern part of the West African Craton suggesting that the two regions were once more closely related.

Evidence of NeoArchean/PaleoProterozoic Crustal Reworking

Detrital zircon U-Pb ages and Hf isotopes of the metasedimentary rocks from the Jiaodong Group, Jiaobei terrane: Provenance and implications for the crustal evolution in the North China Craton

Authors:

Shan et al

Abstract:

Clastic sedimentary rocks can be a very powerful tool to gain insights into the crustal evolution history of many terranes. Here we present detrital zircon U-Pb ages and Hf isotopic data for the metasedimentary rocks in the Jiaobei terrane in the North China Craton (NCC), with a view to constrain the provenance of the protolith and depositional age, and to discuss implications for crustal evolution. The metasedimentary rocks in this study are similar to the sedimentary sequence of the Jiaodong Group in age populations of detrital zircons, petrology and mineralogy, and metamorphism and deformation, indicating that the studied samples belong to the sedimentary sequence of the Jiaodong Group. Detrital zircon U-Pb ages and Hf isotopic data of the metasedimentary rocks from the Jiaodong Group show similarity to those of the zircons from the Meoarchean-late Neoarchean granitoid gneisses in the Jiaobei terrane, suggesting that the Meoarchean-late Neoarchean granitoid gneisses in the Jiaobei terrane possibly provided the major source for the metasedimentary rocks of the Jiaodong Group. In combination with the previous studies, the detrital zircon populations in the Jiaobei terrane can place a good constraint on the depositional age of the Jiaodong Group. The youngest U-Pb age population of the magmatic detrital zircons (peak at 2489 Ma) is considered to constrain the maximum depositional age of the Jiaodong Group to 2.49 Ga. A major phase of metamorphism (late Paleoproterozoic) has been identified in the metasedimentary rocks of the Jiaodong Group, which mirrors the high-pressure granulite-facies regional metamorphism in the Jiaobei terrane. The magmatic detrital zircon Hf isotopes suggest that the zircons crystallized in magmas formed from the mixing of juvenile crustal components, including magmas derived from the depleted mantle, with ancient crustal materials. Therefore, our data suggest juvenile crustal growth and contemporaneous crustal reworking during the Neoarchean-Paleoproterozoic transition in the Jiaobei terrane.

PaleoArchean Crust in South Africa With Evidence of Subduction

Chronology of the oldest supracrustal sequences in the Palaeoarchaean Barberton Greenstone Belt, South Africa and Swaziland

Authors:

Kröner et al

Abstract:

Zircon age data for felsic metavolcanic rocks of the Sandspruit and Theespruit formations, the two oldest supracrustal components in the Palaeoarchaean Barberton Greenstone Belt, show that these two successions are time-equivalent and constitute one single volcanic event at ca. 3530 Ma. The Sandspruit felsic rocks are ubiquitously metasomatized, intensely deformed and intruded by, and tectonically interlayered with, ca. 3450 Ma granitoid sills that are probably part of the Theespruit Pluton. One metasomatized Sandspruit sample contains abundant metamorphic zircons with a weighted mean 207Pb/206Pb age of 3220.1±1.6 Ma, reflecting a widespread metamorphic event in parts of the eastern Kaapvaal craton in South Africa and Swaziland.

Several samples of felsic metavolcanic rocks of the Theespruit Formation confirm a previously established magmatic emplacement age of ca. 3530 Ma, but slightly older rocks up to 3552 Ma were found in the easternmost exposure of the Theespruit sequence near the South African/Swaziland border and may represent a lower lithostratigraphic level than exposed farther west.

Hf-in-zircon isotopic data for most felsic metavolcanic rocks confirm earlier results suggesting that these rocks predominantly originated from melting of a felsic continental basement, possibly related to the oldest, ca. 3660-3550 Ma components of the Ancient Gneiss Complex in Swaziland. However, several Sandspruit samples also suggest that a juvenile source was involved in their generation, perhaps a mafic underplate. We see no evidence in the geochemistry and isotopic signatures of felsic volcanic rocks of the Sandspruit and Theespruit formations for partial melting of a metabasaltic protolith and for Palaeoarchean oceanic crust that formed in connection with subduction. We rather favour a plateau-type setting on older continental crust.

Does a new Type of Hydrothermal Vent Explain how the Earth's Crust Cooled?

The first discovery of a new type of hydrothermal vent system in a decade helps explain the long observed disconnect between the theoretical rate at which the Earth's crust is cooling at seafloor spreading ridge flanks, and actual observations. It could also help scientists interpret the evidence for past global climates more accurately.

This discovery has been made by scientists at the National Oceanography Centre (NOC) and the University of Southampton using a combination of robot-subs and remotely operated vehicles operated by the NOC.

Dr Bramley Murton, who co-supervised this research, published today in Nature Communications, said "This will really improve our understanding of how the Earth's interior cools. Theory has long predicted that there must be more cooling in certain locations on the Earth's crust than we could account for using the known mechanisms....and this new class of hydrothermal vent system may account for that difference."

What makes these hydrothermal vent systems different is that the source of heat driving them comes from hot rock pushed towards the seabed by low angle faults, called tectonic spreading centres, rather than volcanic heat from magma chambers. Dr Murton has been involved in research that discovered tectonic seafloor spreading centres at a number of sites across the ocean floor.

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Evidence of Crustal Growth and Reworking in the EoArchean and PaleoArchean

Zircon U-Pb-Lu-Hf-O isotopic evidence for ≥ 3.5 Ga crustal growth, reworking and differentiation in the northern Tarim Craton

Authors:

Ge et al

Abstract:

Continental crust was largely generated before 2.5 Ga through mafic-ultramafic and TTG (tonalite-trondhjemite-granodiorite) magmatism, but it is contentious when did such primitive crust evolve into mature granodioritic to granitic composition similar to modern upper crust. Here we present zircon U-Pb-Lu-Hf-O isotopic data for late Paleoproterozoic metasedimentary rocks (Xingditag Group) in the Kuruktag area, northern Tarim Craton, NW China. CL-imaging reveals core-rim structures for most zircons from a garnet-bearing paragneiss and a semi-pelitic schist, whereas two quartzites are dominated by metamorphic zircons. SHRIMP and/or LA-ICP-MS U-Pb dating yielded a range of detrital ages from ca. 2.0 – 3.5 Ga for the zircon cores and a consistent metamorphic age of ca. 1.93 Ga for the rims for the paragneiss and schist. However, zircons from the two quartzites mainly record a ca. 1.85 Ga metamorphic event; detrital zircons are rare or absent. These data confirm that the Xingditag Group was deposited after ca. 2.0 Ga and was metamorphosed at ca. 1.93 and/or 1.85 Ga. Importantly, the ca. 2.0 – 3.5 Ga concordant detrital zircons exhibit low initial 176Hf/177Hf ratios (as low as 0.28045) and high δ18O values (6.6 – 11.4‰). These values are interpreted as recording primary magmatic features of the basement rocks in the northern Tarim Craton, because: 1) the dominantly prismatic or fragmentary morphology, oscillatory zoning and moderate Th/U ratios of the detrital zircons indicate a local provenance dominated by igneous rocks; and 2) the within-grain and overall heterogeneities argue against Hf and O isotopic resetting during metamorphism. Linear regressions of the initial 176Hf/177Hf values of these detrital zircons yield a remarkably consistent 176Lu/177Hf ratio of 0.01 for the oldest (TDM2 = 3.9 and 3.7 Ga) and youngest (TDM2 = 2.8 Ga) crustal components. These observations suggest that significant amounts of felsic continental crust may have been formed, altered and reworked as early as ca. 3.5 Ga, marking crustal differentiation and maturation during the Paleoarchean. Hafnium crustal model ages reveal that the oldest crustal component in the northern Tarim Craton may have been generated before ca. 3.9 Ga, much earlier than previously thought.

Evidence the Earth's Crust is VERY Hydrated From Brazil

A University of Alberta diamond scientist has found the first terrestrial sample of a water-rich gem that yields new evidence about the existence of large volumes of water deep beneath the Earth.

An international team of scientists led by Graham Pearson, Canada Excellence Research Chair in Arctic Resources at the U of A, has discovered the first-ever sample of a mineral called ringwoodite. Analysis of the mineral shows it contains a significant amount of water—1.5 per cent of its weight—a finding that confirms scientific theories about vast volumes of water trapped 410 to 660 kilometres beneath the Earth, between the upper and lower mantle.

"This sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area," said Pearson, a professor in the Faculty of Science, whose findings were published March 13 in Nature. "That particular zone in the Earth, the transition zone, might have as much water as all the world's oceans put together."

Ringwoodite is a form of the mineral peridot, believed to exist in large quantities under high pressures in the transition zone. Ringwoodite has been found in meteorites but, until now, no terrestrial sample has ever been unearthed because scientists haven't been able to conduct fieldwork at extreme depths.

Pearson's sample was found in 2008 in the Juina area of Mato Grosso, Brazil, where artisan miners unearthed the host diamond from shallow river gravels. The diamond had been brought to the Earth's surface by a volcanic rock known as kimberlite—the most deeply derived of all volcanic rocks.

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Evidence of the Hadean Crust Found

Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography

Authors:

Valley et al

Abstract:

The only physical evidence from the earliest phases of Earth’s evolution comes from zircons, ancient mineral grains that can be dated using the U–Th–Pb geochronometer. Oxygen isotope ratios from such zircons have been used to infer when the hydrosphere and conditions habitable to life were established. Chemical homogenization of Earth’s crust and the existence of a magma ocean have not been dated directly, but must have occurred earlier. However, the accuracy of the U–Pb zircon ages can plausibly be biased by poorly understood processes of intracrystalline Pb mobility. Here we use atom-probe tomography to identify and map individual atoms in the oldest concordant grain from Earth, a 4.4-Gyr-old Hadean zircon with a high-temperature overgrowth that formed about 1 Gyr after the mineral’s core. Isolated nanoclusters, measuring about 10 nm and spaced 10–50 nm apart, are enriched in incompatible elements including radiogenic Pb with unusually high 207Pb/206Pb ratios. We demonstrate that the length scales of these clusters make U–Pb age biasing impossible, and that they formed during the later reheating event. Our tomography data thereby confirm that any mixing event of the silicate Earth must have occurred before 4.4 Gyr ago, consistent with magma ocean formation by an early moon-forming impact4 about 4.5 Gyr ago.

What is the Crustal Structure Under Southern Tibet?

Mapping crustal structure beneath southern Tibet: Seismic evidence for continental crustal underthrusting

Authors:

Xu et al

Abstract:

Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~ 50 km to 80 km beneath the Himalaya terrane, and locally reaches ~ 85 km beneath the Indus-Yalung suture. It remains at ~ 80 km depth across the Lhasa terrane, and shallows to ~ 70 km depth under the Qiangtang terrane. An intra-crustal interface at ~ 60 km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong-Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30 km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.

Evidence of Crustal Evolution in the Early NeoProterozoic of China

Early Neoproterozoic crustal evolution in northern Yili Block: Insights from migmatite, orthogneiss and leucogranite of the Wenquan metamorphic complex in the NW Chinese Tianshan

Authors:

Wang et al

Abstract:

The northern part of the Yili Block, located in the northwest of the Chinese Tianshan, extends westward to joint with the Aktau-Junggar domain in Kazakhstan, and is one of the major continental constituents of the Central Asian Orogenic Belt (CAOB). The nature and tectonic significance of the basement of the continental domains are important for understanding the continental evolution and geodynamic processes of the CAOB. We investigated the Wenquan metamorphic complex (WMC) that represents the basement and metamorphosed sedimentary cover of the northern Yili Block. The WMC is mainly composed of gneissic S-type granite, migmatite associated with amphibolite, paragneiss, micaschist, quartzite, marble, and intruding leucogranitic dykes. The migmatites display banded, schlieren and ptygmatic structures containing rootless, lensoid or layered leucosome and leucocratic sills, which are indicative of in situ partial melting of the country rocks and variable degrees of melt migration. Geochemical data suggest that the protoliths of the orthogneiss and gneissic K-granite belong to peraluminous S-type granites characterized by wide range of I(Sr) values (0.68324–0.72365), low ɛNd(t) values (−1.9 to −4.4) and two-stage Nd model ages (tMD-2: 1.56–1.82 Ga). The S-type granites might have derived from both clay-poor and clay-rich source rocks that are probably meta-sedimentary and meta-volcanic rocks in the WMC. The leucogranites show relatively narrow range of I(Sr) values (0.70273–0.70419), more negative ɛNd(t) values (−6.9 to −9.1) and more consistent tMD-2 ages (1.51–1.55 Ga). The leucosome and leucocratic sills show variable Sr and Nd isotopic compositions probably due to localized and different degree of partial melting. Our new zircon SHRIMP and LA-ICPMS U–Pb ages, together with previously published data indicate that the migmatization occurred at 926–909 Ma, the gneissic S-type granites emplaced during 919–862 Ma, and the leucogranites emplaced in the period of 909–845 Ma. Inherited old zircons were found in all dated samples and may have come from their source rocks. Association of geochronologically overlapped and genetically linked migmatites, gneissic S-type granites and leucogranitic rocks is interpreted as the results of anatexis of the upper crustal rocks during an early Neoproterozoic tectonic/metamorphic event that needs to be further studied in the northern Yili Block. Such early Neoproterozoic migmatization and S-type granitic magmatism are considered as an important episode of reworking and cratonization of the continental crust of the Yili and adjacent continental blocks in Central Asia.