Vinai K. Rai

Noble gases in ureilites: cosmogenic, radiogenic, and trapped components

Abstract Abundances and isotopic compositions of Ne (in bulk samples only), Ar, Kr, and Xe have been investigated in 6 monomict, 3 polymict, and the diamond-free ureilite ALH78019 and their acid-resistant, C-rich residues. Isotopic ratios of Kr and Xe are very uniform and agree with data for ureilites from the literature. The measured ratio 38 Ar/ 36 Ar showed large variations due to an experimental artifact. This is shown to be connected to the pressure dependence of the instrumental mass discrimination, which for ureilites with their low abundance of 40 Ar is different from that of the usual air standard. This observation necessitates a reassessment for the recently reported 36 Ar excesses due to possible decay of extinct 36 Cl in the Efremovka meteorite. Trapped 22 Ne in the range of (1.4–2.5) × 10 −8 cc STP/g is present in bulk ureilites. A Ne three-isotope plot for polymict ureilites indicates the presence of solar Ne. 21 Ne-based cosmic ray exposure ages for the 10 ureilites studied range from 0.1 Ma (for ALH78019) to 46.8 Ma (for EET83309) All ureilites may have started with nearly the same initial elemental ratio ( 132 Xe/ 36 Ar) 0 , established in the nebula during gas trapping into their carbon carrier phases (diamond, amorphous C) by ion implantation. Whereas diamonds are highly retentive, amorphous C has suffered gas loss due to parent body metamorphism. The correlation of the elemental ratios 132 Xe/ 36 Ar and 84 Kr/ 36 Ar along the mass fractionation line could be understood as a two-component mixture of the unaffected diamond gases and the fractionated (to varying degrees) gases from amorphous C. In this view, the initial ratio ( 132 Xe/ 36 Ar) 0 is a measure of the plasma temperature in the nebula at the formation location of the carbon phases. Its lack of correlation with Δ 17 O (a signature of the silicate formation location) indicates that carbon phases and silicates formed independently in the nebula, and not from a carbon-rich magma The elemental ratios 132 Xe/ 36 Ar and 84 Kr/ 36 Ar in carbon-rich acid residues show a decreasing trend with depth (inferred from carbon consumption during combustion), which can be interpreted as a consequence of the ion implantation mechanism of gas trapping that leads to greater depth of implantation for lighter mass ion The similarity between trapped gases in phase Q in primitive chondrites and the C phases in ureilites—for both elemental and isotopic compositions—strongly suggests that phase Q might also have received its noble gases by ion implantation from the nebula. The slight differences in the elemental ratios can be explained by a plasma temperature at the location of phase Q gas loading that was about 2000 K lower than for ureilite C phases. This inference is also consistent with the finding that the trapped ratio 129 Xe/ 132 Xe (1.042 ± 0.002) in phase Q is slightly higher, compared to that of ureilite C phases (1.035 ± 0.002), as a consequence of in situ decay of 129 I, and becomes observable due to higher value of I/Xe in phase Q as a result of ion implantation at about 2000 K lower plasma temperature.

Nitrogen components in ureilites

Abundances and isotopic compositions of nitrogen and argon have been investigated in bulk samples as well as in acid-resistant C-rich residues of a suite of ureilites consisting of six monomict (Havero, Kenna, Lahrauli, ALH81101, ALH82130, LEW85328), three polymict (Nilpena, EET87720, EET83309), and the diamond-free ureilite ALH78019. Nitrogen in bulk ureilites varies from 6.3 ppm (in ALH 78019) to ∼55 ppm (in ALH82130), whereas C-rich acid residues have ∼65 to ∼530 ppm N, showing approximately an order of magnitude enrichment, compared with the bulk ureilites, somewhat less than trapped noble gases. Unlike trapped noble gases that show uniform isotopic composition, nitrogen shows a wide variation in δ15N values within a given ureilite as well as among different ureilites. The variations observed in δ15N among the ureilites studied here suggest the presence of at least five nitrogen components. The characteristics of these five N components and their carrier phases have been identified through their release temperature during pyrolysis and combustion, their association with trapped noble gases, and their carbon (monitored as CO + CO2 generated during combustion). Carrier phases are as follows: 1) Amorphous C, as found in diamond-free ureilite ALH78019, combusting at ≤500°C, with δ15N = –21‰ and accompanied by trapped noble gases. Amorphous C in all diamond-bearing ureilites has evolved from this primary component through almost complete loss of noble gases, but only partial N loss, leading to variable enrichments in 15N. 2) Amorphous C as found in EET83309, with similar release characteristics as component 1, δ15N ≥ 50‰ and associated with trapped noble gases. 3) Graphite, as clearly seen in ALH78019, combusting at ≥700°C, δ15N ≥ 19‰ and devoid of noble gases. 4) Diamond, combusting at 600–800°C, δ15N ≤ –100‰ and accompanied by trapped noble gases. 5) Acid-soluble phases (silicates and metal) as inferred from mass balance are expected to contain a large proportion of nitrogen (18 to 75%) with δ15N in the range –25‰ to 600‰. Each of the ureilites contains at least three N components carried by acid-resistant C phases (amorphous C of type 1 or 2, graphite, and diamond) and one acid-soluble phase in different proportions, resulting in the observed heterogeneity in δ15N. In addition to these five widespread components, EET83309 needs an additional sixth N component carried by a C phase, combusting at <700°C, with δ15N ≥ 153‰ and accompanied by noble gases. It could be either noble gas–bearing graphite or more likely cohenite. Some excursions in the δ15N release patterns of polymict ureilites are suggestive of contributions from foreign clasts that might be present in them. Nitrogen isotopic systematics of EET83309 clearly confirm the absence of diamond in this polymict ureilite, whereas the presence of diamond is clearly indicated for ALH82130. Amorphous C in ALH78019 exhibits close similarities to phase Q of chondrites. The uniform δ15N value of −113 ± 13 ‰ for diamond from both monomict and polymict ureilites and its independence from bulk ureilite δ15N, Δ17O, and %Fo clearly suggest that the occurrence of diamond in ureilites is not a consequence of parent body–related process. The large differences between the δ15N of diamond and other C phases among ureilites do not favor in situ shock conversion of graphite or amorphous C into diamond. A nebular origin for diamond as well as the other C phases is most favored by these data. Also the preservation of the nitrogen isotopic heterogeneity among the carbon phases and the silicates will be more consistent with ureilite formation models akin to “nebular sedimentation” than to “magmatic” type.

Temporal variations in 87Sr/86Sr and ɛNd in sediments of the southeastern Arabian Sea: Impact of monsoon and surface water circulation

Sr and Nd isotopic composition of silicate fractions of sediments have been measured in two well dated gravity cores from the eastern Arabian Sea archiving a depositional history of ∼29 and ∼40 ka. The 87 Sr/ 86 Sr and e Nd in the northern core (SS-3104G; 12.8°N, 71.7°E) ranges from 0.71416 to 0.71840 and −8.8 to −12.8; these variations are limited compared to those in the southeastern core (SS-3101G; 6.0°N, 74.0°E), in which they vary from 0.71412 to 0.72069 and −9.0 to −15.2 respectively. This suggests that the variation in the relative proportions of sediments supplied from different sources to the core SS-3104G are limited compared to core SS-3101G. The 87 Sr/ 86 Sr and e Nd profiles of SS-3101G exhibit two major excursions, ca. 9 ka and 20 ka, coinciding with periods of Holocene Intensified Monsoon Phase (IMP) and the Last Glacial Maximum (LGM) respectively with more radiogenic 87 Sr/ 86 Sr and lower e Nd during these periods. These excursions have been explained in terms of changes in the erosion patterns in the source regions and surface circulation of the Northern Indian Ocean resulting from monsoon intensity variations. The intensification of North-East (NE) monsoon and associated strengthening of the East Indian Coastal Current in southwest direction during LGM transported sediments with higher 87 Sr/ 86 Sr and lower e Nd from the western Bay of Bengal to the Arabian Sea. In contrast, enhanced South-West (SW) monsoon at ∼9 ka facilitated the transport of sediments from the northern Arabian Sea, particularly Indus derived, to the southeastern Arabian Sea. This study thus highlights the impact of monsoon variability on erosion patterns and ocean surface currents on the dispersal of sediments in determining the Sr and Nd isotopic composition of sediments deposited in the eastern Arabian Sea during the last ∼40 ka.

Provenance of the Late Quaternary sediments in the Andaman Sea: Implications for monsoon variability and ocean circulation

We present a geochemical and Sr-Nd isotopic study on a sediment core collected from the Andaman Sea in an attempt to reconstruct the Late Quaternary weathering and erosion patterns in the watersheds of the river systems of Myanmar and understand their controlling factors. Age control is based on nine radiocarbon dates and δ18O stratigraphy. The rate of sedimentation was strongly controlled by fluctuations of the monsoon. We identify three major sediment provenances: (1) the Irrawaddy catchment, (2) the western slopes of the Indo-Burman-Arakan (IBA) mountain ranges and the Andaman Islands, and (3) the catchments of Salween and Sittang and the Bengal shelf, with the first two contributing 30–60% of the material. Enhanced contributions from juvenile sources and corresponding positive shifts of δ18O are observed at seven time periods (11–14, 20–23, 36, 45, 53, 57, and 62 ka) of which five are synchronous with cooling of the northern hemisphere, suggesting a link between the changes in sediment provenances and the shifting of the locus of the summer monsoon, southward from the Himalayas, without substantial reduction in intensity. Our data, and that from other cores in the region suggest that an eastward moving surface current disperses sediments, derived from the Bengal shelf and western margin of Myanmar, from the eastern Bay of Bengal into the western Andaman Sea and that its strength has increased since the LGM. The existence of this current during the LGM implies that the Andaman Sea and the Bay of Bengal were well connected during the last glacial period.

Rare earth elements and (87)Sr/(86)Sr isotopic characterization of Indian Basmati rice as potential tool for its geographical authenticity.

The increasing demand for premium priced Indian Basmati rice (Oryza sativa) in world commodity market causing fraudulent activities like adulteration, mislabelling. In order to develop authentication method for Indian Basmati rice, (87)Sr/(86)Sr ratios and REEs composition of Basmati rice, soil and water samples were determined and evaluated their ability as geographical tracer in the present study. In addition, the possible source of Sr in rice plant has also been examined. Basmati rice samples (n=82) showed (87)Sr/(86)Sr ratios in the range 0.71143-0.73448 and concentrations of 10 REEs (La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Yb) in ppb levels. Statistical analysis showed strong correlation between (87)Sr/(86)Sr ratios of rice, silicate and carbonate fractions of soil. Good correlation and closeness of (87)Sr/(86)Sr of rice with water indicate its uptake in rice from water. Rice grown in southern Uttar Pradesh contains higher (87)Sr/(86)Sr compared to other region of Indo-Gangetic Plain due to higher (87)Sr/(86)Sr of the Ganga compared to other rivers. (87)Sr/(86)Sr ratios can be used as a tracer for differentiating Indian Basmati rice from the other country originated rice samples.

Isotope signature study of the tea samples produced at four different regions in India

India ranks second in the world for tea production and is well known for Darjeeling tea, which has great demand in the world market due to its unique flavor. In the present study, the combination of isotopic composition of Sr (as 87Sr/86Sr) and C (as δ13C) was studied as geographic tracing signatures for Indian tea samples grown in different regions. Authentic tea leaves as well as soil samples were collected from different tea producing regions; namely, Assam, Darjeeling, Munnar and Kangra, which are geographically distinct from one another. Isotopic analyses were performed by Multi-Collector Inductively Coupled Plasma Mass Spectrometry and Elemental Analyzer-Isotope Ratio Mass Spectrometry for Sr and C, respectively. On the basis of Sr isotopic data, Darjeeling tea samples were found to be more radiogenic than the other tea samples, with changes in the 87Sr/86Sr ratio being an excellent geographic indicator. Variations in δ13C proved to be an excellent geographic indicator for Munnar and Kangra teas. The 87Sr/86Sr values were statistically identical in both the soil and the tea. Principal Component Analysis (PCA), using the combination of 87Sr/86Sr, δ13C and strontium concentration data, was used to effectively differentiate among different tea producing regions.

Tracing the Vedic Saraswati River in the Great Rann of Kachchh

The lost Saraswati River mentioned in the ancient Indian tradition is postulated to have flown independently of the Indus River into the Arabian Sea, perhaps along courses of now defunct rivers such as Ghaggar, Hakra and Nara. The persistence of such a river during the Harappan Bronze Age and the Iron Age Vedic period is strongly debated. We drilled in the Great Rann of Kachchh (Kutch), an infilled gulf of the Arabian Sea, which must have received input from the Saraswati, if active. Nd and Sr isotopic measurements suggest that a distinct source may have been present before 10 ka. Later in Holocene, under a drying climate, sediments from the Thar Desert probably choked the signature of an independent Saraswati-like river. Alternatively, without excluding a Saraswati-like secondary source, the Indus and the Thar were the dominant sources throughout the post-glacial history of the GRK. Indus-derived sediment accelerated the infilling of GRK after ~6 ka when the Indus delta started to grow. Until its complete infilling few centuries ago, freshwater input from the Indus, and perhaps from the Ghaggar-Hakra-Nara, probably sustained a productive marine environment as well as navigability toward old coastal Harappan and historic towns in the region.

Lithology, monsoon and sea-surface current control on provenance, dispersal and deposition of sediments over the Andaman continental shelf

Sediments deposited on the Northern and Eastern Andaman Shelf along with a few sediments from the Irrawaddy and the Salween Rivers are studied for their elemental, Sr and Nd concentrations and their isotope composition to identify their sources, constrain their transport pathways and assess the factors influencing the erosion in the catchment and their dispersal and deposition over the Andaman Shelf region. Major elemental compositions of the shelf sediments suggest mafic lithology such as ophiolites and ultrabasic rocks in the Irrawaddy drainage and over Indo – Burman – Arakan (IBA) ranges as their dominant source. 87Sr/86Sr ratios in sediments of the Northern and Eastern Andaman Shelf range between 0.712245 and 0.742183 whereas, eNd varies from -6.29 to -17.25. Sediments around Mergui have the highest 87Sr/86Sr and the lowest eNd values. Sr and Nd isotope composition of these sediments along with that in the potential sources suggest four major sources of these sediments to the Andaman Shelf, (i) the Irrawaddy River, (ii) the Salween River, (iii) Rivers draining the IBA ranges and (vi) Rivers draining the Western/Central granitic ranges of the Southern Myanmar and Western Thailand such as the Tavoy and the Tanintharyi Rivers. Erosion in the catchment is controlled by the precipitation and topography. Intensely focused precipitation over the higher relief of the western slopes of the IBA and western/central granitic ranges causes higher erosion over this mountainous region, supplying huge sediments through the Kaladan, Irrawaddy, Salween, and the Tanintharyi Rivers to the western Myanmar Shelf, Northern, and Eastern Andaman Shelves respectively. The majority of the sediments produced in the drainage are delivered to the shelf during the south-west monsoon which is dispersed eastward by sea-surface circulation from the mouth of the Irrawaddy Rivers towards the Gulf of Martaban and further southward. The Andaman Shelf receives very little sediment, if any, from the Ganga-Brahmaputra Delta. Higher erosion over the Western/Central granitic belt of the Southern Myanmar and Western Thailand and its importance in delivering sediments to the Eastern Andaman Shelf around the Mergui Archipelago are highlighted for the first time in this study.