Using perovskite to determine the pre-shallow level contamination magma characteristics of kimberlite

It remains difficult to obtain reliable geochemical signatures of uncontaminated kimberlite magma from bulk rock studies due to the combined effects of crustal assimilation and element mobility during post-emplacement alteration processes. Groundmass perovskite (CaTiO3), a typical accessory phase, from Orapa (Botswana) and Wesselton (South Africa) kimberlites has been used to evaluate the isotope and trace element composition of the pre-contamination magmas and the effects of shallow level contamination. In-situ trace element signatures of Orapa and Wesselton perovskite grains are broadly similar and unaffected by crustal contamination. Single grain Sr-87/Sr-86 isotope ratios of perovskite from Orapa (0.7030-0.7036) are less scattered than bulk rock analyses (0.7063-0.7156), which are variably affected by contamination and late stage alteration. Initial Sr-87/Sr-86 isotope ratios of perovskite (0.7044-0.7049) from Wesselton overlap with published whole rock studies on fresh hypabyssal kimberlites (0.7042-0.7047). The limited intra-kimberlite variation in Sr isotope ratios recorded by the perovskite are unlikely to be due to crustal contamination as the calculated liquid compositions in equilibrium with the perovskite analysed typically have &gt;1500 ppmSr, and most common crustal lithologies underlying these kimberlites have relatively low Sr contents and are not highly radiogenic. Calculated pre-shallow level contamination magma compositions for Orapa and Wesselton have significantly fractionated LREE and highly variable non-smooth trace element patterns. Initial Sr and Nd isotope ratios of both kimberlites fall on the mantle Nd-Sr array with enriched Sr and slightly depleted Nd signatures, similar to Group I kimberlites. Overall, the trace element and isotopic composition of Orapa and Wesselton kimberlites are similar to the reported Group I kimberlites from southern Africa, which are derived by very low degrees of partial melting from a LREE depleted metasomatised sub-continental lithospheric mantle (SCLM) source. (C) 2013 Elsevier B.V. All rights reserved.</p

Tectonics and crustal evolution

We thank the Natural Environment Research Council (grants NE/J021822/1 and NE/K008862/1) for funding.The continental crust is the archive of Earth's history. Its rock units record events that are heterogeneous in time with distinctive peaks and troughs of ages for igneous crystallization, metamorphism, continental margins, and mineralization. This temporal distribution is argued largely to reflect the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. Isotopic and elemental data from zircons and whole rock crustal compositions suggest that the overall growth of continental crust (crustal addition from the mantle minus recycling of material to the mantle) has been continuous throughout Earth's history. A decrease in the rate of crustal growth ca. 3.0 Ga is related to increased recycling associated with the onset of plate tectonics. We recognize five stages of Earth's evolution: (1) initial accretion and differentiation of the core/mantle system within the first few tens of millions of years; (2) generation of crust in a pre-plate tectonic regime in the period prior to 3.0 Ga; (3) early plate tectonics involving hot subduction with shallow slab breakoff over the period from 3.0 to 1.7 Ga; (4) Earth's middle age from 1.7 to 0.75 Ga, characterized by environmental, evolutionary, and lithospheric stability; (5) modern cold subduction, which has existed for the past 0.75 b.y. Cycles of supercontinent formation and breakup have operated during the last three stages. This evolving tectonic character has likely been controlled by secular changes in mantle temperature and how that impacts on lithospheric behavior. Crustal volumes, reflecting the interplay of crust generation and recycling, increased until Earth's middle age, and they may have decreased in the past ∼1 b.y.Publisher PDFPeer reviewe

Continental growth seen through the sedimentary record

This work was supported by the Natural Environment Research Council [NERC grant NE/K008862/1], the Leverhulme Trust [grant RPG-2015–422], and the Australian Research Council [grant FL160100168].Sedimentary rocks and detrital minerals sample large areas of the continental crust, and they are increasingly seen as a reliable archive for its global evolution. This study presents two approaches to model the growth of the continental crust through the sedimentary archive. The first builds on the variations in U-Pb, Hf and O isotopes in global databases of detrital zircons. We show that uncertainty in the Hf isotope composition of the mantle reservoir from which new crust separated, in the 176Lu/177Hf ratio of that new crust, and in the contribution in the databases of zircons that experienced ancient Pb loss(es), adds some uncertainty to the individual Hf model ages, but not to the overall shape of the calculated continental growth curves. The second approach is based on the variation of Nd isotopes in 645 worldwide fine-grained continental sedimentary rocks with different deposition ages, which requires a correction of the bias induced by preferential erosion of younger rocks through an erosion parameter referred to as K. This dimensionless parameter relates the proportions of younger to older source rocks in the sediment, to the proportions of younger to older source rocks present in the crust from which the sediment was derived. We suggest that a Hadean/Archaean value of K = 1 (i.e., no preferential erosion), and that post-Archaean values of K = 4–6, may be reasonable for the global Earth system. Models built on the detrital zircon and the fine-grained sediment records independently suggest that at least 65% of the present volume of continental crust was established by 3 Ga. The continental crust has been generated continuously, but with a marked decrease in the growth rate at ~ 3 Ga. The period from > 4 Ga to ~ 3 Ga is characterised by relatively high net rates of continental growth (2.9–3.4 km3 yr−1 on average), which are similar to the rates at which new crust is generated (and destroyed) at the present time. Net growth rates are much lower since 3 Ga (0.6–0.9 km3 yr−1 on average), which can be attributed to higher rates of destruction of continental crust. The change in slope in the continental growth curve at ~ 3 Ga is taken to indicate a global change in the way bulk crust was generated and preserved, and this change has been linked to the onset of subduction-driven plate tectonics. At least 100% of the present volume of the continental crust has been destroyed and recycled back into the mantle since ~ 3 Ga, and this time marks a transition in the average composition of new continental crust. Continental crust generated before 3 Ga was on average mafic, dense, relatively thin (< 20 km) and therefore different from the calc-alkaline andesitic crust that dominates the continental record today. Continental crust that formed after 3 Ga gradually became more intermediate in composition, buoyant and thicker. The increase in crustal thickness is accompanied by increasing rates of crustal reworking and increasing input of sediment to the ocean. These changes may have been accommodated by a change in lithospheric strength at around 3 Ga, as it became strong enough to support high-relief crust. This time period therefore indicates when significant volumes of continental crust started to become emergent and were available for erosion and weathering, thus impacting on the composition of the atmosphere and the oceans.PostprintPeer reviewe

Re-partitioning of Cu and Zn isotopes by modified protein expression

Cu and Zn have naturally occurring non radioactive isotopes, and their isotopic systematics in a biological context are poorly understood. In this study we used double focussing mass spectroscopy to determine the ratios for these isotopes for the first time in mouse brain. The Cu and Zn isotope ratios for four strains of wild-type mice showed no significant difference (δ65Cu -0.12 to -0.78 permil; δ66Zn -0.23 to -0.48 permil). We also looked at how altering the expression of a single copper binding protein, the prion protein (PrP), alters the isotope ratios. Both knockout and overexpression of PrP had no significant effect on the ratio of Cu isotopes. Mice brains expressing mutant PrP lacking the known metal binding domain have δ65Cu isotope values of on average 0.57 permil higher than wild-type mouse brains. This implies that loss of the copper binding domain of PrP increases the level of 65Cu in the brain. δ66Zn isotope values of the transgenic mouse brains are enriched for 66Zn to the wild-type mouse brains. Here we show for the first time that the expression of a single protein can alter the partitioning of metal isotopes in mouse brains. The results imply that the expression of the prion protein can alter cellular Cu isotope content

The role of continental lithosphere in the generation of the Karoo volcanic rocks: evidence from combined Nd-and Sr-isotope studies

143Nd/144Nd, 87Sr/86Sr, Sm and Nd analyses are reported on suites of Karoo volcanic rocks from the four sub-areas of Nuanetsi-north Lebombo, south Lebombo, the Central area, and north-west SWA/Namibia. Only seven (12%) of the samples analysed have positive ENd values similar to those found in the majority of recent mantle-derived rocks. Most of the rest have negative ENd (-1.0 to -17.1) and positive ESr, (+3.0 to +240) and thus must contain at least a contribution from source areas which were both old, and had lower Sm/Nd and higher Rh/Sr ratios than the bulk earth

Precise Re–Os ages of organic-rich mudrocks and the Os isotope composition of Jurassic seawater

Rhenium and osmium isotope and abundance data have been obtained on precisely-located samples from three suites of immature, organic-rich mudrocks from Jurassic coastal outcrops in England, The data provide accurate whole-rode ages of 207 +/- 12 Ma, 181 +/- 13 Ma and 155 +/- 4.3 Ma for suites of Hettangian, Toarcian (exaratum Subzone) and Kimmeridgian (sensu anglico, wheatleyensis Subzone) samples. These new Re-Os ages are indistinguishable, within the assigned analytical uncertainties, from interpolated depositional ages estimated from published geological timescales, and establish the importance of the Re-Os dating technique for chronostratigraphic studies. Early-diagenetic pyrite nodules possess levels of Re and Os which are similar to 1-2 orders of magnitude lower than in the enclosing organic-rich mudrocks, indicating that these elements had already been removed from sediment pore waters at the time of nodule formation. Thus the Re-Os isotope system in these organic-rich mudrocks has been closed since, or from very soon after, the time of sediment deposition. Because most of the Re (98+%) and Os (95-99.8+%) in the mudrocks is shown to be hydrogenous, the Os-187/Os-188((i)) of the samples is interpreted to be that of contemporaneous seawater. The data thereby provide the first estimates of the Os isotope composition of Jurassic seawater. During the earliest Jurassic (Hettangian), the seawater Os-187/Os-188 ratio was extremely unradiogenic (similar to 0.15); it had increased to similar to 0.8 at the end of the Early Jurassic (Toarcian) similar to 20 Ma later, while in the Late Jurassic (Kimmeridgian) the seawater Os-187/Os-188 ratio was similar to 0.59. The most likely explanation for the unradiogenic Os isotope composition of Hettangian seawater is that the contribution of unradiogenic Os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic Os from the continents at that time. This interpretation is in Line with observations suggesting that global weathering rates were low in the Hettangian, and that increased hydrothermal and volcanic activity preceded the break-up of Pangea. The Re/Os ratios of Hettangian mudrocks (and by inference, of contemporaneous seawater) are similar to those of mudrocks deposited at later times during the Jurassic, and argues against the unradiogenic Os in Hettangian seawater being derived from extraterrestrial meteoritic sources

Continental crustal volume, thickness and area, and their geodynamic implications

We appreciate support from Australian Research Council grant FL160100168 and Leverhulme Trust grants RPG-2015-422 and EM-2017-047\4.Models of the volume of continental crust through Earth history vary significantly due to a range of assumptions and data sets; estimates for 3 Ga range from 120% of present day volume. We argue that continental area and thickness varied independently and increased at different rates and over different periods, in response to different tectonic processes, through Earth history. Crustal area increased steadily on a pre-plate tectonic Earth, prior to ca. 3 Ga. By 3 Ga the area of continental crust appears to have reached a dynamic equilibrium of around 40% of the Earth's surface, and this was maintained in the plate tectonic world throughout the last 3 billion years. New continental crust was relatively thin and mafic from ca. 4–3 Ga but started to increase substantially with the inferred onset of plate tectonics at ca. 3 Ga, which also led to the sustained development of Earth's bimodal hypsometry. Integration of thickness and area data suggests continental volume increased from 4.5 Ga to 1.8 Ga, and that it remained relatively constant through Earth's middle age (1.8–0.8 Ga). Since the Neoproterozoic, the estimated crustal thickness, and by implication the volume of the continental crust, appears to have decreased by as much as 15%. This decrease indicates that crust was destroyed more rapidly than it was generated. This is perhaps associated with the commencement of cold subduction, represented by low dT/dP metamorphic assemblages, resulting in higher rates of destruction of the continental crust through increased sediment subduction and subduction erosion.PostprintPeer reviewe

SHORT EPISODES OF CRUST GENERATION DURING PROTRACTED ACCRETIONARY PROCESSES: EVIDENCE FROM CENTRAL ASIAN OROGENIC BELT, NW CHINA

continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (~540–270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at ~400 Ma and 280 Ma for the Altai segment, and ~430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (~20–40 Ma), and variations in magma volumes and in Nd–Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting.Accretionary orogens are major sites of generation of continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (~540–270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at ~400 Ma and 280 Ma for the Altai segment, and ~430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (~20–40 Ma), and variations in magma volumes and in Nd–Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting

Short episodes of crust generation during protracted accretionary processes

This study was supported by funding from the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (grant nos. XDB03010600 and XDB18020204), the National Natural Science Foundation of China (grant nos. 41202041, 41630208 and 41673033), the Key Program of the Chinese Academy of Sciences (QYZDJ-SSW-DQC026), the DREAM Program of China (No. 2016YFC0600407), talent project of Guangdong Province (2014TX01Z079), and GIG–CAS 135 project 135TP201601. PAC and CJH acknowledge support from the Natural Environment Research Council (grant NE/J021822/1). The senior author thanks the grant from the NSC, Taiwan, which supported his one-year academic visit at the NTU. This is contribution no. IS-2352 from GIG–CAS.Accretionary orogens are major sites of generation of continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (∼540–270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at ∼400 Ma and 280 Ma for the Altai segment, and ∼430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (∼20–40 Ma), and variations in magma volumes and in Nd–Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting. The Junggar segment is characterized by high and uniform Nd–Hf isotope ratios (εNd(t) = +5  to  +8; zircon εHf(t) = +10  to  +16) and it appears to have formed in an intra-oceanic arc system. In the Altai and Chinese Tianshan segments, the Nd–Hf isotope ratios (εNd(t) = −7  to  +8; zircon εHf(t) = −16  to  +16) are lower, although they increase with decreasing age of the rock units. The introduction of a juvenile component into the Chinese Tianshan and Altai granitic rocks appears to have occurred in continental arc settings and it reflects a progressive reduction in the contributions from old continental lower crust and lithospheric mantle. Within the long-lived convergent margin setting (over ∼200 Ma), higher volumes of magma, and greater contributions of juvenile material, were typically emplaced over short time intervals of ∼20–40 Ma. These intervals were associated with higher Nb/La ratios, coupled with lower La/Yb ratios, in both the mafic and granitic rocks, and these episodes of increased magmatism from intraplate-like sources are therefore thought to have been in response to lithospheric extension. The trace element and Nd–Hf isotope data, in combination with estimates of granitic magma volumes, highlight that crust generation rates are strongly non-uniform within long-lived accretionary orogens. The estimated crust generation rates range from ∼0.1 to ∼40 km3/km/Ma for the Paleozoic record of the CAOB, and only comparatively short (20–40 Ma) periods of elevated magmatic activity had rates similar to those for modern intra-oceanic and continental arcs.PostprintPeer reviewe