Abstract Oxygen 3‐isotope ratios of magnetite and carbonates in aqueously altered carbonaceous chondrites provide important clues to understanding the evolution of the fluid in the asteroidal parent bodies. We conducted oxygen 3‐isotope analyses of magnetite, dolomite, and breunnerite in two sections of asteroid Ryugu returned samples, A0058 and C0002, using a secondary ion mass spectrometer (SIMS). Magnetite was analyzed by using a lower primary ion energy that reduced instrumental biases due to the crystal orientation effect. We found two groups of magnetite data identified from the SIMS pit morphologies: (1) higher δ 18 O (from 3‰ to 7‰) and ∆ 17 O (~2‰) with porous SIMS pits mostly from spherulitic magnetite, and (2) lower δ 18 O (~ −3‰) and variable ∆ 17 O (0‰–2‰) mostly from euhedral magnetite. Dolomite and breunnerite analyses were conducted using multi‐collection Faraday cup detectors with precisions ≤0.3‰. The instrumental bias correction was applied based on carbonate compositions in two ways, using Fe and (Fe + Mn) contents, respectively, because Ryugu dolomite contains higher amounts of Mn than the terrestrial standard. Results of dolomite and breunnerite analyses show a narrow range of ∆ 17 O; 0.0‰–0.3‰ for dolomite in A0058 and 0.2‰–0.8‰ for dolomite and breunnerite in C0002. The majority of breunnerite, including large ≥100 μm grains, show systematically lower δ 18 O (~21‰) than dolomite (25‰–30‰ and 23‰–27‰ depending on the instrumental bias corrections). The equilibrium temperatures between magnetite and dolomite from the coarse‐grained lithology in A0058 are calculated to be 51 ± 11°C and 78 ± 14°C, depending on the instrumental bias correction scheme for dolomite; a reliable temperature estimate would require a Mn‐bearing dolomite standard to evaluate the instrumental bias corrections, which is not currently available. These results indicate that the oxygen isotope ratios of aqueous fluids in the Ryugu parent asteroid were isotopically heterogeneous, either spatially, or temporary. Initial water ice accreted to the Ryugu parent body might have ∆ 17 O > 2‰ that was melted and interacted with anhydrous solids with the initial ∆ 17 O < 0‰. In the early stage of aqueous alteration, spherulitic magnetite and calcite formed from aqueous fluid with ∆ 17 O ~ 2‰ that was produced by isotope exchange between water (∆ 17 O > 2‰) and anhydrous solids (∆ 17 O < 0‰). Dolomite and breunnerite, along with some magnetite, formed at the later stage of aqueous alteration under higher water‐to‐rock ratios where the oxygen isotope ratios were nearly at equilibrium between fluid and solid phases. Including literature data, δ 18 O of carbonates decreased in the order calcite, dolomite, and breunnerite, suggesting that the temperature of alteration might have increased with the degree of aqueous alteration.
The accretion rate of micrometeorites in the last glacial period was estimated from the concentrations of micrometeorites in the blue ice around the Yamato Mts. in Antarctica. The samples from this study were collected from the five sampling points (M03, K02, K11, J09 and J10) in the blue ice. The blue ice was melted and filtered, and the micrometeorites were handpicked from the collected "glacial sands". The weight of the micrometeorites in the blue ice was estimated from the abundance of recovered micrometeorites and the solar noble gas concentrations in the "residue" after handpicking. The age of the blue ice from the K area was estimated to be 27–33 kyr before present based on oxygen isotope data. The estimated accretion rate to the whole Earth ranges from 5300 × 103kg/a to 16000 × 103kg/a. However, the lower end of this range probably represents lower limits due to possible loss of solar noble gases during long residence in the glacier ice. Hence, we estimate that the accretion rate of micrometeorites 27–33 kyr before present to be in the range between (11000 ± 6600) × 103kg/a and (16000 ± 9100) × 103kg/a. These results, as well as the other estimates, suggest that the accretion rate of micrometeorites in the last glacial period was comparable to that in the present. Micrometeorite k]accretion rate k]Antarctica k]last glacial periods k]noble gas k]interplanetary dust particle
Abstract— Phosphates in martian meteorites are important carriers of trace elements, although, they are volumetrically minor minerals. PO 4 also has potential as a biomarker for life on Mars. Here, we report measurements of the U‐Th‐Pb systematics of phosphates in the martian meteorite ALH 84001 using the Sensitive High Resolution Ion MicroProbe (SHRIMP) installed at Hiroshima University, Japan. Eleven analyses of whitlockites and 1 analysis of apatite resulted in a total Pb/U isochron age of 4018 ± 81 Ma in the 238 U/ 206 Pb‐ 207 Pb/ 206 Pb‐ 204 Pb/ 206 Pb 3‐D space, and a 232 Th‐ 208 Pb age of 3971 ± 860 Ma. These ages are consistent within a 95% confidence limit. This result is in agreement with the previously published Ar‐Ar shock age of 4.0 ± 0.1 Ga from maskelynite and other results of 3.8–4.3 Ga but are significantly different from the Sm‐Nd age of 4.50 ± 0.13 Ga based on the whole rock and pyroxene. Taking into account recent studies on textural and chemical evidence of phosphate, our result suggests that the shock metamorphic event defines the phosphate formation age of 4018 ± 81 Ma, and that since then, ALH 84001 has not experienced a long duration thermal metamorphism, which would reset the U‐Pb system in phosphates.
In the first paper, the present author has attacked the same problem using the data given in A. Schmidt's paper, but some important points were remained untouched there. These point are such as those of the assumption of the variation of the magnetic force with height, the error accompanied by the calculation and the recalculation based on the new and precise data of the magnetic force. In this paper these important problems are discussed with an additional notice on the physical interpretation of the current.(a) In the first paper, the present author adopted the following expression after the wireless research made by Appleton and Builder:This expression is the same as that which shows the distribution of the magnetic force near the earth's surface when a magnet was placed at the centre of the earth. Then the values of the magnetic force on the earth's surface calculated by the above assumption are considerably different from the observed ones. The expression which characterizes, without serious error, the distribution of the magnetic force on the earth's surface was already given by Gauss and others. This expression may afford us a criterion of the expression used in the first paper.According to Gauss, the components of the magnetic force are expressed byIf we put r=R+h, we haveThe observation shows us that the second term of the right hand side of this expression is of the order of 1/10 or so as compared with the first term. This means thatThe expression (1) is able to be used if h/R is of the order of 1/10, namely if we consider h to be of the order of several hundred kilometers.Therefore the horizontal electric current calculated in the first paper is the average current in the region extending from the earth's surface to the upper atmosphere of several hundred kilometers of height, generally including the ionosphere.(b) The values of the magnetic force adopted in the first paper were based on those given in A. Schmidt's paper. Therefore the original value may have a considerable error of 100γ or so. Thus the average value _??_0 or _??_0 obtained by the graphical method will be accompanied by the error of 500γ or so. Thus the expressions 2α_??_0+Z0A-Z0B and 2 sin θβ_??_0+Z0A'-Z0B', each of them is of the order of 1, 000γ, will have, the error less than 500γ. Thereforethe calculated value of the horizontal electric current may generally lie out of the error.(c) The above discussions show us that the treatment in the first paper is not meaningless. Hence the author recalculated the horizontal current according to the data given by the magnetic surveying in 1922.In this paper the average values as to the latitude were calculated with the following result.The horizontal electric currents flow generally to the direction of the east and west as is tabulated in Table 1 and schematically given in Fig. 2. (pp. 61, 62.)The comparison of the current calculated from the data given in A. Schmidt's paper with those obtained in 1922 is graphed in Fig. 1, showing that the average features of the current are very similar.(d) It is concluded that the current is directed to east in the northern part than 30°N and in the region between the equator and 50°S. In the other part of the earth the current flows westwardly. The intensity of the current is of the order of 1amp/km2 and it is somewhat great at the polar caps.As in the upper atmosphere there exist the ionosphere with considerably deep distribution, the current of the same order of the above calculated value will be flowing in the ionosphere.
Radiometric ages of detrital zircons in psammitic schists from the Nagasaki, Kurume, Konoha and Kiyama areas, northern Kyushu, were obtained from 238U/206Pb ratio and isotopic compositions of Pb using a Sensitive High Resolution Ion Microprobe (SHRIMP II). Zircons from the Nagasaki, Kurume and Konoha areas show bimodal age distribution with peaks at ca. 1900 Ma and 250 Ma. It is suggested from this study that the older zircons were derived from Proterozoic landmass and the Korean Peninsula. Zircons from the Kiyama metamorphic rock show a different pattern with ages concentrated at 380-590 Ma. Such zircons are rare in rock samples from the Nagasaki, Kurume and Konoha areas, indicating that Kiyama rocks had a different origin than those from the other three areas. The youngest zircons from the Kiyama, Nagasaki, Kurume and Konoha areas show ages of 382±28 Ma, 238±13 Ma, 249±13 Ma, and 175±4 Ma, respectively. These data mark the upper age limit of their deposition. Since a continuous igneous activity occurred during the period from 300 to 170 Ma in Far East Asia, and the metamorphic age has been close to the zircon age of each area, these youngest ages for the Nagasaki, Kurume and Konoha areas are considered nearly contemporary to the depositional ages. An evaluation of the nature of metamorphism and available ages suggest the possibility that the Nagasaki metamorphic rocks as well as the schist from the Kurume area belong to the Suo zone of the Sangun belt, whereas the metamorphic rocks in the Konoha area may belong to the Ryoke belt or Suo zone of the Sangun belt.
We performed rare-earth element (REE) geochemistry and U-Pb geochronology on apatites in metasediments from the ∼3.8 Ga Isua supracrustal belt (ISB) and Akilia Island, West Greenland, together with stepwise combustion isotopic investigation of carbon and nitrogen for the apatite-bearing quartz-magnetite BIF of uncontested sedimentary origin from northeastern ISB. Ion microprobe analyses reveal that apatites in psammitic schist from the ISB show a U-Pb isochron of 1.5 ± 0.3 Ga. This age is similar to those of Akilia apatite and the Rb-Sr age of 1.6 Ga for the pegmatitic gneiss in the Isukasia area in literature, suggesting a late (∼1.5 Ga) metamorphic event (≤400°C). Pb isotopic ratios of apatite in the quartz-magnetite BIF are also affected by the late metamorphic event around 1.5 Ga. Chondrite-normalized REE patterns of apatites in the BIF show flat patterns with a significant positive Eu anomaly, suggesting hydrothermal influence; this is consistent with a primary depositional origin. In contrast with the quartz-magnetite BIF, apatites in the psammitic schist from the ISB and those in the Akilia BIF show different REE patterns, which resemble those of apatites from secondary mafic and felsic rocks, respectively. Carbon isotopic ratios for the quartz-magnetite BIF by stepwise combustion suggest that two components of reduced carbon are present. One is released below 1000°C (mainly 200-400°C; lowtemperature carbon = LTC), and the other above 1000°C (high-temperature carbon = HTC). δ13C values of the LTC are about -24‰. The LTC is clearly contaminant incorporated after metamorphism, because such a low-temperature component could not have survived the ≤400°C metamorphic event. On the other hand, δ13C values of the HTC are -30‰ for one aliquot and -19‰ for another. The HTC is probably sequestered within magnetite in the BIF, because the decrepitation temperature of magnetite is ∼1200°C. The HTC could not exist within quartz and apatite (decrepitation temperatures: 400-600°C and 600-800°C, respectively), or along grain boundaries. Because the magnetite is concordant with bedding surfaces, it is plausible that the HTC was incorporated in the magnetite during diagenesis. Thus, HTC is the most important candidate for primary carbon preserved in the BIF. δ13C values of HTC cannot be explained as those of Isua carbonate. On the other hand, that the very low δ13C values (-30‰), negative δ15N values (-3‰), and low C/N elemental ratios (86) for the >1000°C fraction of one aliquot are comparable to those of kerogen in Archean metasediments. Therefore, despite the presence of secondary carbon (i.e., LTC), the BIF is suggested to possibly contain highly 13C-depleted kerogenous material, which is unlikely to have been incorporated after metamorphism. Although carbon isotopic change of the kerogenous material due to metamorphic effects cannot be precisely determined from the present data, this study shows that further analysis of magnetite from the Isua BIF is a key to the search for the early life.
