Determination of Rb-Sr age for separated chondrules from the Allegan chondrite

Rb-Sr isotopic study as well as petrologic investigations have been done on sixteen individul chondrule samples separated from the Allegan (H5) chondrite. The Rb-Sr chondrule isochron does not define a very precise age (4.42±0.17 Ga) and 87Sr/86Sr initial ratio (0.6997±0.0014) due to the very small variation in Rb/Sr ratio for the chondrules, whereas the model ages give a much more speciffic value (4.42±0.02 Ga). These ages may probably be indicative of the time of primary process (es) (e.g. chondrule formation) rather than secondary (metamorphic reheating) or tertiary (shock heating) processes in and/or on the H-chodrite parent body (or bodies). Chemical compositions of oliveines (Fa=17.5mol%, mean deviation (MD)=1.3%) and orthopyoxenes (Fs=15.4mol%, MD=3.2%) are moderately homogenized among the chondrules, while certains chemically uneguilibrated features are also distinctly preserved in this chondrite. Homogeneity of the mineral compositions, therefore, could not have been caused by thermal metamorphism but were probably inherited from processes prior to aggregation of the components. The overall implications of the chondrule isochron and model ages in relation to the petrological features and in comparison with Rb-Sr data of other chondrites of different chemical groups and petrologic types are discussed

Rb-Sr and Sm-Nd mineral isochron ages of a pegmatitic gneiss from Oku-iwa Rock, Lutzow-Holm Complex, East Antarctica

Oku-iwa Rock is located in the transitional zone between granulite- and amphibolite-facies metamorphic zones in the Lutzow-Holm Complex (LHC), East Antarctica. Hornblende biotite (HB) gneiss widely outcrops in the northern part of this area. Sm-Nd and Rb-Sr mineral isochron ages obtained from pegmatitic HB gneiss are 578±36Ma and 431±14Ma with initial isotopic ratios of 0.511892±0.000040 and 0.70718±0.00038, respectively. The former mineral isochron age is consistent with the Rb-Sr whole rock isochron age of the HB gneiss (583Ma). The latter age is close to the previously published Rb-Sr mineral isochron age of granitic rocks (418Ma) in Oku-iwa Rock, implying that these ages represent the time when the rocks cooled down to the closure temperature of biotite. The granodioritic precursor formed at 674Ma was gradually cooled down and its temperature reached 700°C at 583Ma based on a previous report. Temperature of the HB gneiss rose at 578Ma by injection of source material for the pegmatitic HB gneiss and rose again at 529Ma (SHRIMP U-Pb zircon age) by regional metamorphism. At 485Ma, the temperature of the gneisses rose for the third time by intrusion of granitic rocks. After the intrusion, the constituent rocks of Oku-iwa Rock gradually cooled by uplifting and the temperature reached 310°C at 431Ma

Isotopic Analysis of Rb and Sr Using a Full Automatic Thermal Ionization Mass Spectrometer

Analytical method for strontium isotope ratios and rubidium and strontium concentrations has been established using a full automatic thermal ionization mass spectrometer. The machine is a modified model "MAT 260" of Varian MAT LTD. Each of simultaneously loaded thirteen samples is successively analysed full automatically following to a specific controling program which has been selected as being most suitable for each sample. However, the most characteristic feature of this machine compared to other types is computer controled peak jumping and peak centering before measurment of the signal of each peak. By this the accuracy of isotopic measurement has been surprisingly improved. Analytical procedures are described in detail which include decomposition of samples, separation of Rb and Sr, loading samples on filament and mass spectrometry. Accuracy and reproducibility of isotope analyses are excellent : 43 separate analyses of standard sample NBS 987 over one year gave a mean (87)Sr/(86)Sr ratio of 0.710238 (normalized to the (88)Sr/(86)Sr ratio of 8.375209) with a value of　20δ of 0.000008. The ratio obtained is slightly　higher than the value of 0.71014 given by NBS,　but it is almost identical to the mean of recently　reported twenty analyses. Our determinations for Rb and Sr concentrations of standard samples are as follows : JG-1, Rb 184.4 ppm, Sr 185.5 ppm; JB-l, Rb 41.5Ppm, Sr 448.4 ppm , each of which is near the mean of reported values for corresponding element of the sample (ANDO et al., 1974). Concentrations of Rb and Sr in pure water and regents used in chemical treatement of samples were also measured; a possible error due to the contamination is negligible for most geochemical samples

Pan-African adakitic rocks from the Sør Rondane Mountains, East Antarctica

The Sør Rondane Mountains (22°E -28°E ) are located in eastern Dronning Maud Land, East Antarctica, and consist of amphibolite to granulite facies metamorphic rocks and various intrusive rocks. Peak metamorphism took place during the Pan-African event. Considering the mode of occurrence and previously reported age data, felsic magma activity started at the waning stage of the metamorphism. In this study, we report on the petrogenesis of discordantly intruding post-tectonic pyroxene tonalite (Px tonalite) with adakitic composition. In addition, the petrogenesis of posttectonic biotite granite (Bt granite), of which the geochemical features are similar to high-K adakite in the collision zone, are reevaluated with reference to the Px granite. The initial Sr isotopic ratios (SrI) corrected to 525Ma of Px tonalite and Bt granite represent limited values (0.7037-0.7052) that are included within the SrI of the Proterozoic metatonalite corrected to the same age (525Ma). These Sr isotopic signatures combined with geochemical modeling suggest that chemical variation of Px tonalite and Bt granite can be explained by partial melting of the Proterozoic metatonalite with various degrees of melting, leaving garnet as a residual phase, subsequent to fractional crystallization and/or accumulation of specific phases. Taking the tectonic situation and geochemical signature into account, Px tonalite and Bt granite from the Sør Rondane Mountains are correlated to the collision zone adakite and represent the post-collision magmatism in the suture zone during the Gondwana formation of the Pan-African event

Geochronological evidence for multistage-metamorphic events in ultrahigh-temperature granulites from central Highland Complex, Sri Lanka

The ultrahigh-temperature (UHT)granulite from the central Highland Complex,Sri Lanka preserves histories of several metamorphic events.The application of geothermobarometries on sapphirine-garnet-orthopyroxene-sillimanite-quartz bearing granulites confirms that metamorphism occurred under the UHT condition. We report a middle Proterozoic age of 1478+-58 Ma from the internal isochron using a garnet core,whole rock and felsic fraction (quartz in the Sm-Nd system from the same rock samples.As this granulite preserves several stages of orthopyroxene formation,the measured isotope composition of orthopyroxene give mixed values.The reference isochron plotted with orthopyroxene and whole rock give an age of 550 Ma.The present results can be interpreted to mean that the middle Proterozoic metamorphic event could have been the thermal peak.During the uplift stage,this granulite might have remetamorphosed at younger stages along with the adjacent rocks

Geochemistry of the pre/syn-metamorphic granite in the Ongul Islands, East Antarctica

Latest Proterozoic to Early Paleozoic pre/syn- and post-metamorphic granites occur in the Lutzow-Holm Complex (LHC), East Antarctica. The pre/syn-metamorphic granites in the Ongul Islands consist of biotite hornblende (BH) granite and garnet biotite hornblende (GBH) granite. The Rb-Sr whole rock isochron age of 580±23Ma with an initial ^(87)Sr/^(86)Sr ratio of 0.70784±0.00059 is obtained from the BH granite. This age is slightly older than SHRIMP U-Pb zircon and CHIME monazite metamorphic ages (520-550Ma) from the complex. The BH granite has lower aluminum saturation index than the GBH granite. The pre/syn-metamorphic granites have a wide variation of εSr and εNd values at 580m.y. before the present, and the BH granite has lower εSr_(580Ma) and higher εNd_(580Ma) values than the GBH granite. One end of the variations in the ε diagram is close to the values of the mafic to intermediate metamorphic rocks in the island; the other is close to those of the old continental crust. These geochemical and isotopic features suggest that the PSMGs were originated by mixing between magma derived from mafic to intermediate metamorphic rocks and old continental crust

Rb-Sr, Sm-Nd ages of the Phalaborwa Carbonatite Complex, South Africa

We analyzed Rb-Sr and Sm-Nd isotopic compositions of whole-rock and minerals from syenite, biotite gneiss xenolith, dolerite dyke, phoscorite, and carbonatite from the Phalaborwa Carbonatite Complex located in northeastern Transvaal, South Africa. Syenite does not give significant Rb-Sr and Sm-Nd whole-rock and mineral isochron ages. Dolerite gives an Rb-Sr whole-rock and mineral isochron age of 2062±74Ma. This age overlaps with the timing of the magmatism of the Phalaborwa Carbonatite Complex. Biotite gneiss and phoscorite do not also give significant Rb-Sr and Sm-Nd isochrons. The Rb-Sr whole-rock and mineral isochron of phoscorite, however, gives an age of 2013±93Ma. The age is clearly the cooling age of this complex. Carbonatites are divided into two groups, having low and high initial Sr isotopic ratios. This coordinates with the result of S.C. Eriksson (Carbonatites, ed. by K. Bell, 1989). In addition, these groups indicate different initial Nd isotopic ratios. These suggest that carbonatite and related rocks were formed by mixing of two source magmas having different Sr and Nd isotopic compositions

Rb-Sr age of K-rich LL-chondrite Yamato-74442

Yamato-74442, a brecciated LL-group chondrite, consists of fragments with igeous texture, chondrules, mineral fragments, and host. The fragments involve K-rich fragments as well as normal ones. Typically, both fragments consist mainly of euhedral porphyritic olivines and a few orthopyroxenes in a glassy grundmass. EPMA analyses for these fragments indicate that olivines and orthoptroxenes have a range of Fe/(Fe+Mg) ratio of 27.19～31.33 and 20.10～26.91 mol%, respectively. There is no significant difference in these rations between K-rich (Na2O/K2O<1) and normal framents. The K2O contents of groundmass for K-rich fragments range from 0.51 to 4.89 wt% and that of the normal fragments is 0.38～090 wt%. K2O contents of groundmass vary from fragment to fragment but are rather homegeneous in a single fragment. Rb-Sr isotopic determination on eighteen K-rich fragments (1.5 to 9.5mg in weight) gave an internal isochron age of 4.514±0.022 (2σ) Ga (λ87Rb=1.42×10-11y-1) and initial 87Sr/86Sr ratio of 0.7016±0.0024. This age is somewhat old compared with a whole rock isochron age of LL chondrites (4.493±0.18 Ga; Minster and Allegre, 1981). From a comoarison of the present result with others so far reported, it is inferred that the alkali differentiation as observed in Yamato-74442 probably occurred prior to or during a process of accumulation from LL chondrite parent material in the early solar nebula.

Geochemistry and preliminary Sr-Nd isotopic data on the Neoproterozoic granitoids from the Bantoum area, west Cameroon: evidence for a derivation from a Paleoproterozoic to Archaean crust

The Bantoum area in west Cameroon is composed of migmatitic gneisses associated with parallel strips of amphibolites,quartz-monzonites,biotite-granites, two-mica leucogranites and granitic dikes.Quartz-monzonites are metaluminous (A/CNK=0.8-0.9)I-type,biotite-granites are peraluminous (A/CNK=1.0-1.10)I-type, leucogranites are peraluminous (A/CNK=1.14)S-type granitoids.All are hyper-potassic rocks defining a calc-alkaline trend.Quartz-monzonites gave an Rb-Sr isochron age of 720+-61 Ma assumed to be a mixing age.The thermometry estimated from major elements and zircon saturation indicate that the biotite-granites crystallized from high temperature melts (812-866゜C) whereas leucogranites crystallized from low temperature melts (719-745゜C). The trace element distribution diagrams are characterized by an enrichment in LILE and LREE (5<La_N/Sm_N<17),with negative Nb,Ta,Sr and Ti anomalies. Model initial ^87Sr/^86 Sr ratios (620 Ma)are 0.707614-0.708363 for quartz-monzonites,0.711242-0.713784 for biotite-granites,and 0.715835 for leucogranites.They have highly negative ε_Nd (620 Ma)(-19~-11) and T_DM model ages ranging from 1.9 to 2.9 Ga. These geochemical and isotopic features imply that the granites are generated at different temperatures and from different crustal materials;they are the witnesses of the recycling of a Paleoproterozoic to Archean crust with minor inputs of juvenile magmas during the Pan-African orogeny. Chemical similarities between gneisses and some biotite-granites suggest that the partial melting of these gneisses may have contributed to the formation of granites

Geochemistry of syenite of the Phalaborwa Carbonatite Complex, South Africa

We surveyed the Spitskop syenite pipe,one of the satellite bodies of the Phalaborwa Carbonatite Complex located in northeastern Transvaal,South Africa. This pipe is composed of the inner cumulus syenite and outer ring syenite.The brecciation zone between these syenites includes many blocks of syenite,pyroxenite, melanocratic rock,biotite gneiss and granitic rocks.Dolerite dykes intruded into the plug and brecciation zone.Fine-and coarse-grained syenites,melanocratic rock, alkali-feldspar granite and dolerite were collected from the brecciation zone of this pipe.The whole-rock and mineral chemistry suggests that syenites and melanocratic rocks of the brecciation zone were derived from the inner cumulus syenite magma. These rocks do not indicate any clear isochron.It may be a result of mixing of various rocks at the brecciation stage