High-reliability zircon separation for hunting the oldest material on Earth: An automatic zircon separator with image-processing/microtweezers-manipulating system and double-step dating

Despite the recent development in radiometric dating of numerous zircons by LA-ICPMS, mineral separation still remains a major obstacle, particularly in the search for the oldest material on Earth. To improve the efficiency in zircon separation by an order of magnitude, we have designed/developed a new machine-an automatic zircon separator(AZS). This is designed particularly for automatic pick-up of100 μm-sized zircon grains out of a heavy mineral fraction after conventional separation procedures. The AZS operates in three modes:(1) image processing to choose targeted individual zircon grains out of all heavy minerals spread on a tray,(2) automatic capturing of the individual zircon grains with microtweezers, and(3) placing them one-by-one in a coordinated alignment on a receiving tray. The automatic capturing was designed/created for continuous mineral selecting without human presence for many hours. This software also enables the registration of each separated zircon grain for dating, by recording digital photo-image, optical(color) indices, and coordinates on a receiving tray. We developed two new approaches for the dating; i.e.(1) direct dating of zircons selected by LA-ICPMS without conventional resin-mounting/polishing,(2) high speed U-Pb dating, combined with conventional sample preparation procedures using the new equipment with multiple-ion counting detectors(LA-MIC-ICPMS).With the first approach, Pb-Pb ages obtained from the surface of a mineral were crosschecked with the interior of the same grain after resin-mounting/polishing. With the second approach, the amount of time required for dating one zircon grain is ca. 20 s, and a sample throughput of 150 grains per hour can be achieved with sufficient precision(ca. 0.5%).We tested the practical efficiency of the AZS, by analyzing an Archean Jack Hills conglomerate in Western Australia with the known oldest(4.3 Ga) zircon on Earth. Preliminary results are positive; we were able to obtain more than 194 zircons that are over 4.0 Ga out of ca. 3800 checked grains, and 9 grains were over 4300 Ma with the oldest at 4371 ± 7 Ma. This separation system by AZS, combined with the new approaches, guarantees much higher yield in the hunt for old zircons.     

Degradation and utilization of protein derived from <Emphasis Type="Italic">Pseudomonas aeruginosa</Emphasis> by marine microbial community

Microbial degradation and utilization of proteins derived from bacterial detritus were investigated in a microcosm experiment using Pseudomonas aeruginosa detritus as a substrate. To assess the effects of natural marine microbial communities on degradation and utilization of protein derived from P. aeruginosa cells, four microcosms were prepared: natural seawater (containing the natural microbial community) with P. aeruginosa detritus (N+Pa), autoclaved seawater with P. aeruginosa detritus (A+Pa), natural seawater (N) and autoclaved seawater (A) without adding anything as a control. The numbers of total and growing bacterial cells, protease activity, and transition of P. aeruginosa proteins were monitored in the four microcosms. Changes in the numbers of total and growing bacterial cells and protease activities indicated that bacterial detritus significantly stimulated the microbial community in the microcosms. Both the surviving P. aeruginosa in A+Pa and natural microbial community in N+Pa microcosms were able to degrade and utilize P. aeruginosa detritus; however, the community in N+Pa including various microbes maintained high activity longer, indicating that diversity is an important factor in keeping the community active. Even under the very high protease activity in N+Pa, 39-kDa and 48-kDa proteins from P. aeruginosa remained in the microcosm during the entire experiment (150 days). Immunoblotting suggested the 48-kDa protein was an intact molecule of OprP, which had been detected from the dissolved fraction of natural seawater in previous studies. This result suggests that the protein molecules that had been detected from natural seawater actually had a high tolerance to microbial degradation.     

U–Pb zircon geochronology of the North Pole Dome adamellite in the eastern Pilbara Craton

Supracrustal rocks around the North Pole Dome area, Western Australia, provide valuable information regarding early records of the evolution of crustal processes, surface environments, and biosphere. Owing to the occurrence of the oldest known microfossils, the successions at the North Pole Dome area have attracted interest from many researchers. The Paleoarchean successions (Warrawoona Group) mainly comprise mafic‐ultramafic greenstones with intercalated cherts and felsic lavas. Age constraints on the sediments have been mainly based on zircon U–Pb geochronology. However, many zircon grains have suffered from metamictization and contain anomalously high contents of common Pb, which makes interpretation of the U–Pb data complicated. In order to provide more convincing chronological constraints, an U–Pb Concordia age is widely accepted as the best estimate. Most zircons separated from two adamellites also suffered from severe metamictization. In our analyses, less metamictized domains were selected using a pre‐ablation technique in conjunction with elemental mapping, and then their U–Pb isotopic compositions were determined with a laser ablation inductively coupled plasma mass spectrometry. Most analyzed domains contained certain amounts of common Pb (²⁰⁴Pb/²⁰⁶Pb > 0.000 1), whereas three and five U–Pb data points with less common Pb (²⁰⁴Pb/²⁰⁶Pb < 0.000 1) were obtained. These U–Pb datasets yielded U–Pb Concordia ages of ca 3 445 Ma and 3 454 Ma, respectively. These ages represent the timing of the adamellite intrusion, and constrain the minimum depositional age of the Warrawoona Group. In addition, a single xenocrystic zircon grain showed a ²⁰⁷Pb/²⁰⁶Pb age of ca 3 545 Ma, supporting the idea that the sialic basement of the Pilbara Craton existed prior to 3 500 Ma. The in situ U–Pb zircon dating combined with the pre‐ablation technique has the potentials to identify non‐metamictized parts and to yield precise and accurate geochronological data even from partially metamictized zircons.     

Energy build-up and release mechanisms in solar and auroral flares

The 80 and 160 MHz radioheliograph observations of a flare-associated outburst in June 1973 reveal three distinctive type IV continuum sources: a flare continuum, an isolated moving continuum and a storm continuum, in that time sequence. The observed characteristics of the three sources and their relations in space and time are described. The observed characteristics and the interrelations between the three continuum sources are explained in terms of energetic electrons trapped and accelerated in, and ejecta moving through, an arch-like magnetic field. It is also suggested that the isolated moving source - in the present event and in similar ones - is probably quite independent of the MHD shock wave which is responsible for the preceding type II burst.     

Adsorption of extracellular proteases in seawater onto filters during size fractionation

In aquatic environments extracellular enzymes are bound to microbial cells or exist in a free and adsorbed state. Various filters have been used to fractionate these enzymatic activities, but enzymes may be readily adsorbed onto some materials, and such adsorption can induce errors in the estimation of enzymatic activity. In this study we examined three filters to determine the most suitable filter for fractionation when estimating proteolytic enzyme activity in seawater. We found that the polycarbonate Nuclepore membrane, widely used for size fractionation because of its pore-size accuracy, was the most favorable for this purpose, even though it adsorbed slightly more enzymes than the low-protein-binding polyethersulfone membrane. We also found that trypsin-and chymotrypsin-type enzymes were more easily adsorbed than aminopeptidases.     

Seismological evidence for compositional variations at the base of the mantle transition zone under Japan Islands

Temperature and water content anomalies just above the 660-km discontinuity under the Japan Islands are estimated from seismological observations. Two sets of seismological observations of P-wave velocity perturbations and depth variations of the 660-km discontinuity are used, which are (1) long-wavelength (~ 500 km) variations from seismic tomography based on a grid parameterization and waveform analysis of ScS reverberations and (2) moderate-wavelength (~ 150 km) variations from seismic tomography with a block parameterization and receiver-function analysis. To estimate temperature and water content anomalies, partial derivatives of velocity and depth variations with respect to temperature and water content determined by mineral physics studies are used. Under Southwest Japan, low temperature and high water-content anomalies are obtained from both sets of seismological observations, which have already been found by a previous study. Under Northeast Japan, however, there are discrepancies between the results estimated from the two data sets, possibly due to the different resolution scales. This discrepancy is dismissed when examining distributions of the temperature and water content anomalies. These anomalies can be grouped into two geographical locations, one group under Southwest Japan, the other under Northeast Japan. The two groups are clearly separated and exhibit common features in both sets of the seismological observations. The grouping is interpreted by a compositional difference between the subducting Pacific slab under Southwest Japan and a normal mantle under Northeast Japan.     

http://dx.doi.org/10.1016/j.gsf.2016.10.001

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.