Secular change in lifetime of granitic crust and the continental growth: A new view from detrital zircon ages of sandstones

U-Pb ages of detrital zircons were newly dated for 4 Archean sandstones from the Pilbara craton in Australia, Wyoming craton in North America, and Kaapvaal craton in Africa. By using the present results with previously published data, we compiled the age spectra of detrital zircons for 2.9, 2.6, 2.3,1.0, and0.6 Ga sandstones and modern river sands in order to document the secular change in age structure of continental crusts through time. The results demonstrated the following episodes in the history of continental crust：（1） low growth rate of the continents due to the short cycle in production/destruction of granitic crust during the Neoarchean to Paleoproterozoic（2.9-23 Ga）,（2） net increase in volume of the continents during Paleo-to Mesoproterozoic（2.3-1.0 Ga）, and（3） net decrease in volume of the continents during the Neoproterozoic and Phanerozoic（after 1.0 Ga）. In the Archean and Paleoproterozoic, the embryonic continents were smaller than the modern continents, probably owing to the relatively rapid production and destruction of continental crust. This is indeed reflected in the heterogeneous crustal age structure of modern continents that usually have relatively small amount of Archean crusts with respect to the post-Archean ones. During the Mesoproterozoic, plural continents amalgamated into larger ones comparable to modern continental blocks in size. Relatively older crusts were preserved in continental interiors, whereas younger crusts were accreted along continental peripheries.In addition to continental arc magmatism, the direct accretion of intra-oceanic island arc around continental peripheries also became important for net continental growth. Since 1.0 Ga, total volume of continents has decreased, and this appears consistent with on-going phenomena along modern active arc-trench system with dominant tectonic erosion and/or arc subduction. Subduction of a huge amount of granitic crusts into the mantle through time is suggested, and this requires re-consideration of the mantle composition and heterogeneity.

Growth, Duplication and Lateral Mutual Compressive Deformation of Akouemma hemisphaeria on the Seafloor of Okondja Basin at 2.2 Ga (Gabon)

A colony of macro-fossils Akouemma hemisphaeria has been described in the Paleoproterozoic sedimentary basin of Okondja, Gabon. These fossils are classified into two groups according to their spheroidal or elongated forms. The spheroidal shapes are similar, have a tripartite structure with two hemispheres and a median disc and gradually pass to the elongated forms. These elongated forms have a pronounced bipartite tendency to two “hemispheres” separated by a median surface, and often have several ovoid “pieces” attached. The elongated specimens show both lateral growth marks and signs of fission. Growth marks are characterized by unidirectional homogeneous side elongations and lateral bud-like protuberances. The signs of fission are marked by circular furrows perpendicular to the direction of elongation, called “constriction furrows” with varying depths depending on the degree of fission of the specimen and internal vertical “division planes”. All of these ovoid and elongated specimens have undergone significant initial deformations due mainly to mutual lateral compressions in tabular beds. The Akouemma hemisphaeria macro-organisms, which were primitive probably sessile organisms, lived on the seafloor. They provide the oldest known record of macro-organisms on Earth having vegetative growth and asexual reproduction by budding, lateral elongation and fission. Their mutual lateral deformations would result from their growth.

Chronological constraints on the Paleoproterozoic Francevillian Group in Gabon

The Francevillian Group in Gabonese Republic was recently established as a typical sedimentary sequence for the Paleoproterozoic.However,its age is rather poorly constrained,mainly based on Rb-Sr and Nd-Sm datings.This study reports new zircon data obtained from Chaillu massif and N＇goutou complex,which constrain the protolith age of the basement orthogneisses and the igneous age of an intrusive granite,respectively.Most zircons from the orthogneisses are blue and exhibit oscillatory zoning in cathode-luminescence images.Zircons with lower common lead abundances tend to be distributed close to the concordia curve.Two age clusters around 2860 Ma and 2910 Ma are found in zircons plotted on the concordia curve.Based on the Th/U ratios of zircons,these ages correspond to the protolith ages of the orthogneisses,and the zircons are not metamorphic in origin.Syenites and granites were collected from the N＇goutou complex that intrudes into the FA and FB units of the Francevillian Group.The granitoids exhibit chemical composition of A-type granite affinity.Half of zircons separated from the granite are non-luminous,and the remaining half exhibit obscure internal textures under cathode-luminescence observation.All zircon grains contain significant amounts of common lead;the lead isotopic variability is probably attributed to the mixing of two components in the zircons.The zircon radiogenic ^（207）Pb/^（206）Pb ratio is 0.13707 ± 0.0010.corresponding to a ^（207）Pb/^（206）Pb age of 2191 ± 13 Ma.This constrains the minimum depositional age of the FA and FB units.Furthermore,the FB unit consists of manganese-rich carbonate rocks and organic carbon-rich black shales with macroscopic fossils.Based on our age constraints,these organisms appeared in the study area just after the last Paleoproterozoic Snowball Earth event,in concert with global scale oxidation event encompassing the Snowball Earth.

Surface Force Analysis of Pyrite (FeS2): Its Reactivity to Amino Acid Adsorption

It is well known that mineral surfaces play an important role as catalysts for abiotic polymerization of amino acids to form peptides, which are the main components of the first self-replicating system. Understanding the mechanism behind the adsorption of simple amino acids on mineral surfaces is a topic of great interest not only in field of prebiotic evolution and but also in many other branches of material sciences. Various clay minerals have been suggested for studying how organic molecules were first synthesized in a prebiotic “inorganic” environment. Among them, pyrite (FeS2) is one of the most potential minerals as it possesses a highly reactive surface to drive molecular adsorption in prebiotic chemistry reactions. Recent theoretical experiments suggest that amino acids are adsorbed on the pyrite surface depending on its surface structures. However, these results have not been tested experimentally, and the exact mechanism of the specific interactions on this mineral has not been fully resolved yet at the molecular level. In this work, through quantitative force analysis with atomic force microscope (AFM) in which a single amino acid residue was mounted on the tip apex of AFM probe, we were able to find the reaction sites and study the interaction forces between the amino acid and the pyrite surface. Our results of Raman spectroscopic studies and force measurements with a well-designed AFM probe demonstrated for the first time that pyrite provided higher adsorption probabilities of amino acid residues for the chemical reactions at surfaces.