Growth Rate Of Continental Crust Research Articles

Growth rate of continental crust in the northeast margin of the North China Craton: Constraints from the U-Pb dating and Lu-Hf isotopes of detrital zircons from the Laoha River

1.2 Ga (McCulloch and Bennett, 1994; Condie, 1998, 2000). The fastest continental crust growth period of the North China Craton is ~2.7 Ga (Wu et al., 2005; Yang et al., 2009; Geng et al., 2012; Peng et al., 2012, 2013; Wang et al., 2012, 2013; Zhao and Zhai, 2013; Zheng et al., 2013; Bao et al., 2013). The other hypothesis is that the mass of continental crust that formed between 4.0 Ga and 4.5 Ga was similar to today’s and has been a steady-state since then with continental crust being recycling into the mantle as fast as it forms (Armstrong and Harmon, 1981; Armstrong, 1991; Wang et al., 2009; Yang et al., 2009). In recent years, how to quantitatively estimate the growth rate of continental crust is an enigmatic issue (Iizuka et al., 2010; Dhuime et al., 2012). With the development of ICP-MS technology, the U–Pb dating and Lu–Hf isotopic compositions of detrital zircons from fluGrowth rate of continental crust in the northeast margin of the North China Craton: Constraints from the U–Pb dating and Lu–Hf isotopes of detrital zircons from the Laoha River

Increased loss of continental crust during supercontinent amalgamation

The volume of Earth's continental crust depends on the rate of addition of continental crust from the mantle compared to the rate of continental loss back to the mantle, which at present is roughly balanced. Models for the growth rate of continental crust vary, with isotope data suggesting various episodes of increased growth rate throughout Earth's history; these episodes have been correlated with the supercontinent cycle, but may be a consequence of preferential preservation of continental crust during these cycles. The global balance between addition and loss of continental crust is controlled by: 1) the extent of internal orogens versus exterior orogens, with the latter favouring continental addition, and 2) the balance between exterior orogens in retreating mode versus those in advancing mode, with the latter favouring continental loss. A greater balance of continental addition versus loss should exist during supercontinent break-up, due to a high magmatic flux in retreating accretionary orogens, whereas the amalgamation of supercontinents should involve increased continental loss due to increased sediment subduction and tectonic erosion. Zircon U–Pb and Hf isotopes provide insight to models of crustal growth rate since they sample the continental crust at their time of formation. Using the distribution of data within εHf(t)-time space of a global zircon database, it is demonstrated that the data are in accord with the concept of increased continental loss during supercontinent amalgamation. Periods featuring increased continental addition relative to continental loss, and hence increased continental crust growth rate, occur at ~1.7–1.2Ga, ~0.85–0.75Ga, and ~0.45–0.35Ga, and follow the formation of the Columbia (Nuna), Rodinia and Gondwana supercontinents respectively. Distinct increases in continental loss compared to continental addition, i.e. decreased continental growth rate, occur at ~1.0–0.9Ga, and ~0.6–0.55Ga, correlating with the periods of Rodinia and Gondwana amalgamation respectively. Formation of Pangea by introversion rather than extroversion, means that continental addition in exterior orogens was concurrent with continental loss in interior orogens; a similar process may have been responsible for formation of the Columbia supercontinent. Peaks in the compilation of U–Pb zircon ages correlate with the timing of supercontinent amalgamation, and are likely to be a consequence of preferential preservation of continental crust during this part of the supercontinent cycle.

Controls on the heterogeneous distribution of mineral deposits through time

Abstract Mineral deposits exhibit heterogeneous distributions, with each major deposit type showing distinctive, commonly unique, temporal patterns. These reflect a complex interplay between formational and preservational forces that, in turn, largely reflect changes in tectonic processes and environmental conditions in an evolving Earth. The major drivers were the supercontinent cycle and evolution from plume-dominated to modern-style plate tectonics in a cooling Earth. Consequent decrease in the growth rate of continental crust, and change from thick, buoyant sub-continental lithospheric mantle (SCLM) in the Precambrian to thinner, negatively buoyant SCLM in the Phanerozoic, led to progressive decoupling of formational and preservational processes through time. This affected the temporal patterns of deposit types including orogenic gold, porphyry and epithermal deposits, volcanic hosted massive sulphide (VHMS), palaeoplacer Au, iron oxide, copper gold (IOCG), platinum group elements (PGE), diamond and probably massive sulphide SEDEX deposits. Sedimentary mineral deposits mined for redox-sensitive metals show highly anomalous temporal patterns in which specific deposit types are restricted to particular times in Earth history. In particular, palaeoplacer uranium, banded iron formation (BIF) and BIF-associated manganese carbonates that formed in the early Precambrian do not reappear in younger basins. The most obvious driver is progressive oxidation of the atmosphere, with consequent long-term changes in the hydrosphere and biosphere, the latter influencing the temporal distribution and peak development of deposits such as Mississippi Valley types (MVT), hosted in biogenic sedimentary rocks.

Supercontinents, superplumes and continental growth: the Neoproterozoic record

Abstract Between 1300 and 500 Ma the Neoproterozoic supercontinent Rodinia aggregated (1300–950 Ma), broke up (850–600 Ma) and a new supercontinent, Pannotia-Gondwana, formed (680–550 Ma). Only c. 11% of the preserved continental crust was produced during this 800 Ma time interval and most of this crust formed as arcs, chiefly continental margin arcs. At least 50% of juvenile continental crust produced between 750 and 550 Ma is in the Arabian-Nubian Shield and in other terranes that formed along the northern border of Amazonia and West Africa. An additional 20% occurs in Pan-African orogens within Amazonia, and c. 16% in the Adamastor and West African orogens. The growth rate of continental crust between 1350 and 500 Ma was similar or less than the average rate of continental growth during the Phanerozoic of 1 km 3 /a, and this low rate characterizes both formation and breakup stages of the supercontinents. The low rates of continental growth during the Neoproterozoic may be due to the absence of a superplume event associated with either Rodinia or Pannotia-Gondwana. If supercontinent breakup is required to produce a superplume event, perhaps by initiating catastrophic collapse of lithospheric slabs at the 660 km seismic discontinuity, the absence of a Meso-proterozoic-Neoproterozoic superplume event may mean that a Palaeoproterozoic supercontinent did not fully breakup prior to aggregation of Rodinia.

Evolution of the continents and the atmosphere inferred from Th-U-Nb systematics of the depleted mantle

Temporal evolution of depleted mantle thorium-uranium-niobium systematics constrain the amount of continental crust present through Earth's history (through the niobium/thorium ratio) and date formation of a globally oxidizing atmosphere and hydrosphere at approximately 2.0 billion years ago (through the niobium/uranium ratio). Increase in the niobium/thorium ratio shows involvement of hydrated lithosphere in differentiation of Earth since approximately 3.8 billion years ago. After approximately 2.0 billion years ago, the decreasing mantle thorium/uranium ratio portrays mainly preferential recycling of uranium in an oxidizing atmosphere and hydrosphere. Net growth rate of continental crust has varied over time, and continents are still growing today.

Use and abuse of crustal accretion calculations

Recent attempts to calculate the average growth rate of continental crust for the Late Proterozoic shield of Arabia and Nubia are subject to large geological uncertainties, and widely contrasting conclusions result from dissimilar boundary conditions. The four greatest sources of divergence are (1) the extent of 620-920 Ma arc-terrane crust beneath Phanerozoic cover; (2) the extent of pre-920 Ma continental crust within the arc terranes; (3) the amount of postaccretion magmatic addition and erosion; and (4) the aggregate length and average life span of Late Proterozoic magmatic-arc systems that formed the Arabian-Nubian Shield. Calculations restricted to the relatively well known Arabian segment of the Arabian-Nubian Shield result in average crustal growth rates and arc accretion rates comparable to rates for modern arc systems, but we recognize substantial uncertainty in such results. Critical review of available geochemical, isotopic, and geochronological evidence contradicts the often stated notion that intact, pre-920 Ma crust is widespread in the eastern Arabian Shield. Instead, the arc terranes of the region apparently were contaminated with sediments derived, in part, from pre-920 Ma crust. Available geologic and radiometric data indicate that the Arabian-Nubian Shield and its Pan-African extensions constitute the greatest known volume of arc-accreted crust on Earth that formed in the period 920-620 Ma. Thus, the region may truly represent a disproportionate share of Earth9s crustal growth budget for this time period.

Sm/Nd constraints on the growth rate of continental crust

The time constants are calculated which rule the evolution of Nd isotopes in a model Earth composed of the continental crust, a depleted upper mantle and a pristine lower mantle and in which the upper mantle is not constrained to grow or not to grow at the expense of the lower mantle. The growth rate of the continental crust is calculated to be 1.8 ± 0.7 km 3/ yr, with 2.5 ± 0.5 km 3/yr of new crust addition from the mantle and 0.8 ± 0.5 km 3/yr of sediment entrainment at subduction zones. The characteristic times for Nd evolution in the depleted mantle and continental crust imply that the Sm-Nd system evolves far from steady state.

Fate of sediments on the descending plate at convergent margins

The evolution of the continents and of continental crust is strongly dependent on the trajectory of the sedimentary cover on the descending oceanic lithosphere at arc systems. Although direct calculations of accretion are not reliable, indirect evidence strongly suggests that most of the sediment cover is either accreted or underplated (subcreted) to the upper plate. This evidence includes the increased thickening of accretionary prism beneath parts of the inner trench slope that cannot be explained by deformation within the prism and by protolith composition both in subophiolitic metamorphics and in blueschist terrains. That a small fraction of this sediment cover is transported to depths of at least 100 km is demonstrated in several ways. Flux calculations of mass and selected elements through arc systems require addition of a few tens of metres of sediment to the arc magmas. Global correlations of variations between arc magma characteristics and regional geological parameters show: (1) a strong correlation between silica content of average arc magmas and thickness or maturity of crust in the upper plate, attributed to upper plate contamination; (2) regional variations in 87 Sr/ 86 Sr and Pb isotopic ratios of arc basalts that correlate spatially with isotopic ratios in the non-calcareous components of pelagic sediments. This correlation is argued to reflect the variation of terrigenous material in the basal pelagics that are involved in magma production. Subduction of continental crust to depths of 100 km, either as part of the descending plate or by tectonic erosion of the upper plate is not supported by these correlations. Recycling rates of continental crust by accretion and subcretion and of mantle by subduction of oceanic lithosphere in contemporary arcs are very large compared with the growth rate of continental crust, which appears similar to the magma production rates in arc systems. This present production rate of continental crust is very much smaller than that during early Earth history, and is compatible with Phanerozoic ocean freeboard changes. In addition, average SiO 2 and K 2 O contents of contemporary arc magmas are much lower than those estimated for mean continental crust, leading to the conclusion that the magmas being produced at active arcs cannot be used as a model for the development of most of the Earth’s continental areas.