Noble gas degassing from meteorites as inferred from 40Ar-39Ar analytical data

\u27Retention rate of radiogenic ^Ar through a collisional event\u27 (f_r) was estimated on the basis of ^Ar-^Ar analytical data for each meteorite reported. \u27The amount of trapped ^Ar\u27 (^Ar_T) seems to be roughly correlated with the determined Ar retention rate, \u27f_r\u27, for samples with f_r of less than 0.5. When we extend this correlation to f_x=1,the original amount of trapped ^Ar in ordinary chondrites is estimated to be in the order of 10^cm^3 STP/g. Even meteorites with no clear sign of impact often show lower amounts of trapped ^Ar, suggesting that the loss of trapped components might have occurred in a stage of meteorite formation for these meteorites. The amounts of trapped ^Ar and ^Kr seem to be correlated with those of trapped ^Xe, reflecting also the degree of equilibrium state of meteorites. Meteorites with signs of impact show lower amounts of trapped heavy noble gases. Such a correlation might be explained by degassing processes through diffusion at relatively high temperatures for prolonged periods of time

40Ar-39Ar analyses of a lumar meteorite (Yamato-86032) and a few LL3 and LL4 chondrites from Antarctica

^Ar-^Ar analyses were performed for a lunar meteorite (Yamato-86032), two LL3 (Y-790448,Allan Hills-764) and one LL4 (Y-74442) chondrites from Antarctica. The lunar meteorite Y-86032 shows a scattered ^Ar-^Ar age spectrum with anomalously high ages in the higher temperature fractions, indicating a shock effect on the analyzed sample. Among the three LL chondrites analyzed, the sample Y-790448 indicates a plateau ^Ar-^Ar age of 4521±28Ma in the lower temperature fractions (600-800℃). The other two LL chondrites show inverse staircase spectra in the higher temperature fractions. In the LL chondrite group, un-equilibrated chondrites seem to show older ^Ar-^Ar ages compared with equilibrated ones, which may be related to the differences in the thermal history of each portion of their parent body

^<40>Ar-^<39>Ar Dating of Terrestrial and Exterrestrial Materials : Basalts from the Japan Sea Floor, Deccan Plateau and Meteorites from Antarctica

Through a series of ^Ar-^Ar dating for terrestrial and exterrestrial samples, the results for basalts from the Japan Sea floor, Deccan Plateau and for meteorites from Antarctica give significant information concerning their formation histories. ^Ar-^Ar ages of basalts from the Japan Sea floor give a constraint that the Japan Sea floor was formed at least 20 Ma. ^Ar-^Ar ages for dyke samples from the Deccan Plateau indicate a possibility for the occurrence of a little younger ages than that of the main plateau formations by a few million years. Meteorites collected from Antarctica show some variable ^Ar-^Ar ages reflecting their different thermal histories

40Ar-39Ar analyses of Yamato-75097 (L6) chondrite from Antarctica

^Ar-^Ar analyses were performed on the host phase of an H-clast bearing chondrite Yamato-75097 (L6). The result indicates a typical U-shaped age spectrum, suggesting that the serious degassing event occurred at about 490Ma or slightly younger age for this meteorite. Since the meteorite shows a sign of shock, this event might have been a collisional event. It is estimated that more than 99% of radiogenic ^Ar was degassed during this event. For this degassing, it requires prolonged time of at least a few hours even at 1200℃, and a longer time is required if the ambient temperature is lower and the diffusion mainly controls the degassing process

40Ar-39Ar analyses of Y-74063 and ALH-78230: Consortium study onunique meteorites from Antarctica

As part of a consortium study on unique meteorites from Antarctica, ^Ar-^Ar analyses were performed for two meteorites Y-74063 and ALH-78230,which are related to lodranite and the Acapulco meteorite. Y-74063,93 shows an ^Ar-^Ar plateau age of 4556±53 Ma, containing about 10^cm^3STP/g trapped ^Ar. ALH-78230,55 also indicates an ^Ar-^Ar plateau age of 4531±23 Ma in the higher temperature fractions, but the lower temperature fractions suggest the occurrence of a later degassing event around 400-500 Ma. Trapped ^Ar is about 10^cm^3STP/g. These results indicate that the two meteorites Y-74063 and ALH-78230 experienced different thermal histories

Correction to “Noble gas signatures of abyssal gabbros and peridotites at an Indian Ocean core complex”

Author Posting. © American Geophysical Union 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 5 (2004): Q02010, doi:10.1029/2004GC000695

40Ar-39Ar analyses of Juvinas fragments

Three fragments of the Juvinas (eucrite) taken from the main mass with a size of about 30cm were analyzed by the ^Ar-^Ar method in order to examine differences of secondary thermal effects. The brecciated matrix shows a scattered pattern in the ^Ar-^Ar age spectrum, indicating excess ^Ar in the higher temperature fractions. Two other fragments show no definite plateau ages but indicate similar age patterns. A probable disturbance event (s) around 4-4.1 Ga is indicated in the 975-1050℃ fractions. Thus, Juvinas shows evidence of secondary thermal events, probably due to impacts

Radiometric age of lava flows of the Enrei ｆormations in central Japan (1).

Two samples from lava flows of the upper member of the Lower Enrei formations exposed at the top of Utsukushi-ga-hara volcano are dated approximately at 1.3 × 10v K-A years. The dates of these lava flows which are magnetized reversely and eastward are placed in the middle part of the Matuyama reversed epoch. The Ina formations equivalent of the Lower member of the Lower Enrei formations suggest that during this period climatic deterioration had already taken place, so that most parts of the Enrei formations should be younger than the base of the Pleistocene.Article信州大学理学部紀要 1(2): 93-96(1967)departmental bulletin pape

40Ar-39Ar analysis of phlogopite in the Horoman Peridotite Complex, Hokkaido, Japan and implications for its origin

金沢大学理工研究域自然システム学系40Ar-39Ar analysis of phlogopite separated from a plagioclase lherzolite of the Horoman Peridotite Complex, Hokkaido, Japan, has yielded a plateau age of 20.6 ± 0.5 Ma in an environment where the metamorphic fluid was characterized by an almost atmospheric Ar isotopic ratio. The age spectrum is slightly saddle-shaped, implying some incorporation of excess 40Ar during the formation of the phlogopite at a depth. As the phlogopite has been inferred to have formed in veins and/or interstitials during exhumation of the peridotite body, metasomatic fluids, to which ground- and sea water might have contributed, were probably involved in the formation of phlogopite in the crustal environment. A total 40Ar-39Ar age of 129 Ma of a whole rock sample of the plagioclase lherzolite, from which the phlogopite was separated and is representative of the main lithology of the Horoman Peridotite Complex, indicates the occurrence of excess 40Ar. Hence, the age has no geological meaning

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