Intergranular trace elements in mantle xenoliths from Russian Far East: Example for mantle metasomatism by hydrous melt

金沢大学理工研究域自然システム学系Based on both major and trace element chemistry, the occurrence of the intergranular component in mantle-derived xenoliths from far eastern Russia has been constrained. Whole-rock trace element measurements of one xenolith show apparent negative anomalies in Ce, Th, and high field strength elements on normalized trace element patterns. The trace element pattern of the whole rock differs from those of constituent minerals, indicating that the anomalies in the whole rock are attributable to the presence of an intergranular component. That assumption was confirmed using in situ analysis of trace elements in the intergranular substance and melt inclusion using laser ablation inductively coupled plasma-mass spectrometry. Both the intergranular component and the melt inclusions have identical trace element patterns, which mean that these materials are a cognate metasomatizing agent. The anomalies are regarded as mantle metasomatism related to an aqueous fluid. Hydrous minerals were observed on the wall of the melt inclusions using micro-Raman spectroscopy, indicating that the melt inclusions contained a large amount of water. Thus, this study reveals a trace element composition of a hydrous metasomatizing agent in the mantle. © 2009 The Authors Journal compilation © 2009 Blackwell Publishing Asia Pty Ltd

U-Th radioactive disequilibrium and ESR dating of a barite-containing sulfide crust from South Mariana Trough

A block of sulfide crust collected from an active hydrothermal mound in an Archaean site (12°56.4′N, 143°37.9′E; depth ca. 3000 m) of the South Mariana Trough was dated using both 230Th/234U disequilibrium and electron spin resonance (ESR) methods to establish the growth duration. Eight subsamples from the sulfide crust were separated further into magnetic and non-magnetic fractions using a Franz isodynamic separator. Thirteen sulfide samples, soluble in nitric acid, yielded 230Th/234U ages of 0.3–2.2 ka. The magnetic fractions had significantly lower Th/U ratios, which enabled age determinations as precise as ±2% (2σ). The age distribution obtained for the section of sulfide crust analyzed is consistent with deposition of sulfide minerals from the upper surface of the crust to the inner side. The 230Th/234U ages of the sulfide minerals were compared with ESR ages of barites separated from 12 subsamples of the same sulfide crust. ESR ages of 0.27–1.3 ka show a spatial pattern broadly resembling that observed in 230Th/234U dating method. While there are some significant offsets, these results illustrate the potential of the two methods for use in investigation of the evolutional history of a hydrothermal system

ESR, 226Ra-210Pb and 228Ra-228Th Dating of Barite in Sea-Floor Hydrothermal Sulfide Deposits in the Okinawa Trough

ESR (electron spin resonance) dating method has beenapplied to calcite, aragonite, hydroxyapatite, quartz andgypsum. Recently it was shown that this method is alsopractically applicable to barite, especially those in sea-floorhydrothermal sulfide deposits (Takamasa et al., 2013).ESR, 226Ra-210Pb, and 228Ra-228Th ages were determined forbarite crystals extracted from hydrothermal sulfide depositstaken at hydrothermal fields at the Okinawa Trough. The agesrange 4.1 to 16000 years where the ages obtained by the threemethods coincide in some of the samples while the 228Ra-228Thages are the youngest and ESR ages, the oldest and the 226Ra- 210Pb ages in between in the other samples. The samples withyounger ESR ages show younger 226Ra-210Pb ages and thosewith older ESR ages show older 226Ra-210Pb ages with nodetection of 228Ra. The inconsistency of the ages between thesemethods would be explained by the mixture of the baritecrystals with younger and older ages, formed by severalhydrothermal activities. The order of ages of the 5hydrothermal fields would be arranged, from young to old asfollows; Yoron Hole field, Daiyon-Yonaguni Knoll field,Hatoma Knoll field, being nearly equal to Iheya North Knollfield．Goldschmidt201

ESR dating of barite in sulphide deposits formed by the sea-floor hydrothermal activities

Barite is a mineral newly found to be practically useful for electron spin resonance (ESR) dating of sulphide deposits formed bythe sea-floor hydrothermal activities. The recent studies for the properties of the ESR dating signal in barite are summarised inthe present paper as well as the formulas for corrections for accurate dose-rate estimation are developed including the dose-rateconversion factors, shape correction for gamma-ray dose and decay of 226Ra. Although development of the techniques for ESRdating of barite has been completed, further comparative studies with other dating techniques such as U –Th and 226Ra –210Pbdating are necessary for the technique to be widely used

Dating Hydrothermal Minerals in Active Hydrothermal Fields in the Southern Mariana Trough

Ages of hydrothermal mineral samples were determined by 230Th/234U radioactive disequilibrium dating technique. Sulfidesamples were collected from four active sites in the mostsouthern region of the Mariana Trough; Snail site (located onthe spreading axis), Archaean site (located on a mound 1.5 kmdistant from the axis), Pika site (located on an off-axis knoll4.9 km distant from the axis) and Urashima site (located on thenorthern slope of the same knoll). Samples were collected fromactive hydrothermal vents and inactive sulfide spires duringdive expeditions of YK10-10 cruise using the submersibleSHINKAI6500. In addition, massive sulfide ores were coredby shallow drilling using a BMS (Benthic Multi-coringSystem) during the TAIGA10 cruise.Sulfide ores collected from the hydrothermal mound of theArchaean site range in age from < 100 to 3520 years old. Thegrowth rate of the massive sulfide ore body is calculated to be0.12 - 1.5 mm/year based on results of the core samples. Theseresults suggest that a few thousand years of continuoushydrothermal activity would be required for formation ofmassive ore deposit that overlays the hydrothermal mound 50-100 m high and 250-300 m in diameter.Sulfide ores collected from the two hydrothermal fieldslocated on the off-axis knoll (Pika and Urashima sites) are upto 9000 years old. When combined with geophysical evidencefor crustal velocity anomaly, this mineralization is consideredto be in the late-stage of the hydrothermal activity, and relatedto a cooling magma body.Goldschmidt　201