High Sr Ratios Research Articles

Design of a Linear Wavenumber Spectrometer for Line Scanning Optical Coherence Tomography with 50 mm Focal Length Cylindrical Optics.

Optical coherence tomography (OCT) has a wide range of uses in bioimaging and nondestructive testing. Larger bandwidth light sources have recently been implemented to enhance measurement resolution. Increased bandwidth has a negative impact on spectral nonlinearity in k space, notably in the case of spectral domain OCT (SD-OCT). This nonlinearity reduces the depth-dependent signal sensitivity of the spectrometers. A grating and prism combination is extensively used for linearizing. In an earlier study, we used a combination of the reflective grating and prism, as well as a cylindrical mirror with a radius of 180 mm, to achieve a high SR ratio with low nonlinearity. A creative design for a spectrometer with a cylindrical mirror of radius 50 mm, a light source with a center wavelength of 830 ± 100 nm (μm−1 − 6.756 μm−1 in k-space), and a grating of 1600 lines/mm is presented in this work. The design optimization is performed using MATLAB and ZEMAX. In the proposed design, the nonlinearity error reduced from 157 μm to 10.75 μm within the wavenumber range considered. The sensitivity research revealed that, with the new design, the SR ratio is extremely sensitive to the imaging optics’ angles. To resolve this, a spectrometer based on Grism is introduced. We present a Grism-based spectrometer with an optimized SR ratio of 0.97 and nonlinearity of 0.792 μm (). According to the sensitivity study, the Grism-based spectrometer is more robust.

Sr isotope-salinity modelling constraints on Quaternary Black Sea connectivity

Reconstruction of the Black Sea's connectivity history during the Quaternary is based on high-resolution isotope data and longer-term biotic records. These demonstrate that the Black Sea experienced rapidly changing hydrologic configurations involving, at different times, isolation from and/or connection with the Mediterranean and Caspian seas. Here, we simulate these connectivity scenarios using a numerical box model of Black Sea hydrography. This allows us to explore the magnitude of the different water fluxes and their consequences for Black Sea salinity and Sr isotope ratio; parameters that we can measure today and reconstruct in the past. We show that the model is able to replicate the present-day hydrography using new Sr isotopic data measured on modern Black Sea water (0.709143 ± 0.000008) and that the Black Sea has a homogenous Sr isotope ratio despite its strongly stratified water column. We then create a 1.2 Myr record of paired Sr isotope (ostracods) and palaeosalinity (dinoflagellate cysts) data, using samples from DSDP Site 379. We also provide the first constraints on the Sr isotope ratio of the Amu Darya River, a substantial fluvial system that previously fed the Caspian Sea and therefore influenced the Black Sea when the two basins were connected. Modelling the different connectivity scenarios and incorporating both new and previously published geochemical and biotic data indicates that influx from the Caspian Sea dominated Black Sea hydrography between 1.2 to around 0.5 Ma. In contrast, Mediterranean input has exerted increasing influence since 0.6 Ma, particularly during interglacial periods. The hypothesis that an interval of anomalously high Sr isotope ratios in the Black Sea during the late Saalian deglaciation (MIS 6) was caused by input from the Amu Darya is demonstrably unrealistic. Instead the high Sr ratios may have been driven by variations in glacial extent within the Dnieper catchment and the resultant erosion of Sr from granitic basement rocks. Our new combined model-data approach tests and refines interpretations of the glacio-hydrographic development of the Black Sea during the second half of the Quaternary.

Environmental upheavals of the Ediacaran period and the Cambrian “explosion” of animal life

The second half of the Ediacaran period began with a large impact – the Acraman impact in South Australia, which was accompanied by a negative δ13Ccarb anomaly and an extinction-radiation event involving acritarchs. A few million years later (∼570 Ma?) there was a second, deeper and longer-lived world-wide δ13Ccarb anomaly (the Shuram anomaly) which coincides with extinction of the acanthomorphic acritarchs. Wide distribution of the Shuram event is exemplified by stratigraphic sections from South Australia, Oman, southern California and South China. The widespread anomaly has been tentatively attributed to a marine impact. During recovery from the Shuram event the enigmatic Ediacaran biota achieved its zenith, only to be extirpated and replaced by a polyphyletic assemblage of shelly animals in what is known as the Cambrian “explosion”. This extinction-radiation cycle was preceded by glaciation, another δ13Ccarb excursion and the highest 87Sr/86Sr values known from marine carbonates. These high Sr ratios have been linked to weathering of extensive tracts of continental crust that were elevated during amalgamation of the supercontinent Gondwana. Introduction of essential nutrients to the oceans would have promoted biological production of oxygen and provided P and Ca for the important skeletonization that characterizes the Cambrian “explosion” and caused a quantum leap in the preservation potential of animal remains. Turbulent events of the last 50 million years of Precambrian time include three glaciations, two large impacts and a massive orogenic episode. These dramatic environmental upheavals are held responsible for three consecutive extinction-radiation cycles that culminated in the appearance of a diverse array of shelly fossils. Various lines of evidence suggest that the metazoans have deep roots so that they too may have been subjected to the environmental pressures of the late Ediacaran period clearly illustrated by acritarchs and the Ediacaran biota but the long-lived diversity of the metazoan population was “suddenly” revealed by the acquisition of biomineralization.

Mineralogy and Sr–Nd isotopes of SPM and sediment from the Mandovi and Zuari estuaries: Influence of weathering and anthropogenic contribution

Clay minerals and Sr–Nd isotopes of suspended particulate matter (SPM) and bottom sediment were investigated along transect stations of the Mandovi and Zuari estuaries, western India to determine the provenance and role of estuarine processes on their distribution. Kaolinite and illite, followed by minor goethite, gibbsite and chlorite were present in SPM and bottom sediment at all stations, both during monsoon and pre-monsoon. Smectite occurred in traces at river end stations but its contents increased downstream in both estuaries. Smectite contents were much higher in Zuari than in Mandovi estuary. The 87Sr/86Sr ratios and ɛNd of SPM were higher than those in hinterland rocks and laterite soils. The Sr ratios were highest at river end stations of both estuaries and decreased sharply seaward. The Sm/Nd ratios of SPM and sediment were close to that of iron ore material flushed into the estuaries. The mean ɛNd of SPM and sediment were similar in both estuaries. It is suggested that the smectite is formed in coastal plains and its distribution downstream is controlled by lithology and drainage basin of rivers. Abundant kaolinite and high Sr ratios reflect chemical weathering and lateritization of source rocks. Sr isotopic ratios along transects are influenced by changes in salinity, organic matter and turbidity. High and near identical ɛNd values along transect stations of both estuaries suggest that the Nd isotopic compositions are influenced by the lateritization of source rocks and anthropogenic contribution of ore material.

Strontium isotopes as tracers for quantifying mixing of groundwater in the alluvial plain of a coastal watershed, south-eastern Tanzania

Surface water and groundwater chemistry of the coastal aquifer systems in south-eastern Tanzania have been studied with a focus on isotopes and major ions, in order to assess the controlling hydrological and geochemical processes. 87Sr/86Sr isotope ratios have provided information about the hydrology that could not have been obtained by studying the physical and chemical characteristics of water quality alone. The data reveal a rather complex mixing pattern of various water source end-members that are subject to different hydro-geochemical processes, such as evaporation, weathering and other water–rock interactions, and seawater intrusion, which control the water quality. Data on 87Sr/86Sr ratio distribution and strontium (Sr2+) concentration delineate three types of groundwater in the coastal aquifer system:(1)water recharged by fresh precipitation/stream water infiltration and influenced by soil processes like weathering and evaporation,(2)groundwater subjected to rock–water interactions with different bedrock types, and(3)groundwater that is brackish due to seawater intrusion.Most of the groundwater data can be modeled as strontium-poor recharge water of meteoric origin. These waters are influenced to varying degrees by soil–water interactions in the surficial aquifer material. Elevated NO3− concentrations in a few shallow aquifer samples imply sewage infiltration from domestic wastewater. Data on the 87Sr/86Sr ratios did not appear to be affected by this anthropogenic influence. High Sr ratios (>0.711899) in the samples suspected of being contaminated by sewage as well as a couple of other shallow groundwater samples is likely to have been caused by weathering of surficial aquifer material. A lack of variation in Sr2+ content between the river water and groundwater samples reflects an efficient mixing of water during well discharge. The data also clearly demonstrate the influence of aerosol sources in rain water on the strontium isotope signatures.The results illustrate the strength of combining isotopic measurements with more conventional water chemistry data in order to obtain a better understanding of groundwater flow systems and the geochemical processes governing their chemistry.

Pillow lavas of the Sierra Leone test site, Mid-Atlantic Ridge, 5°–7°N: Sr-Nd isotope systematics, geochemistry, and petrology

Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the 87Sr/86Sr ratio between the basalts and their chilled glasses and less significant difference in ɛNd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear.

Clay minerals and Sr–Nd isotopes of the sediments along the western margin of India and their implication for sediment provenance

Mineralogy and Sr–Nd isotopic composition of the <2-μm fraction of the Holocene and late Pleistocene sediments from 12 gravity cores collected along the western margin of India were used to delineate the provenance and understand the response of isotopes with respect to the source rock composition. Holocene sediments show distinct clay mineral associations representing three major sources: illite, chlorite-rich sediments from the Indus, smectite-dominated sediments from the Deccan Traps, and kaolinite and smectite-enriched sediments from gneissic rocks. The mixing of Indus-derived sediment with the Deccan Traps-derived sediment or the Deccan Traps-derived sediment with gneissic rock-derived sediment occurs in the deeper water cores. Sr isotopes are very sensitive to weathering and laterization. Weathering does not influence Nd isotopes. The 87Sr/ 86Sr ratios of the Holocene sediments in shallow water cores are higher and fluctuated widely than those in deeper water cores. ϵ Nd correlates well with the clay mineral associations, indicating source rock composition or mixing of source sediments. Kaolinite-enriched sediments coupled with high Sr ratios and low ϵ Nd on the SW margin of India suggest intense chemical weathering in the hinterland. The influence of the sediment from the Ganges–Brahmaputra, transported by the Bay of Bengal waters during the NE monsoon surface current or SW monsoon underwater current, on the composition of the sediments of the SW Indian margin is insignificant. The late Pleistocene sediments reflect a decrease of hydrolytic processes resulting in decrease of smectite and Sr isotopic ratio. Clay minerals and ϵ Nd values suggest that the provenance and transport pathways of the late Pleistocene sediments are the same as those of the Holocene for each region.

Magmatism at the onset of back-arc basin spreading in the Okinawa Trough

The Okinawa Trough is a back-arc basin behind the Ryukyu Arc in the western Pacific. Near its southwestern end is Kueishantao located, an volcanic islet offshore northeastern Taiwan. Kueishantao is the last volcanic center at the southernmost end of the Okinawa Trough's spreading axis. We report the isotopic (Nd, Sr, O) and chemical compositions of fresh andesites from Kueishantao. These rocks reveal extremely low Nd isotopic values ( ϵ Nd = − 1.9 to − 5.2), very high Sr ratios ( 87Sr 86Sr > 0.705 ) and high δ 18O (between 7 and 8). Such a strong continental signature on Kueishantao can be explained by crustal contamination, most likely the magma resulting from a MORB-type magma assimilation with about 30% local continental crust materials (CC) and/or the thick overlying sediments during the onset of the “rifting” stage of the active back-arc basin. Basalts and dacites, which were dredged from the spreading axis in the central part of the Okinawa Trough (Iheya Deep and Aguni Deep, I&A), exhibit high ϵ Nd values (2.3–4.7) and low δ 18O values (< 6.6) (Honma et al., (1991)). This implies that less than 10% of continental crustal materials (CC) were involved in the magma generation. Based on the trace-element ratios ( Nb Zr vs. Ba Zr ) of I&A basaltic rocks, it suggests that the formation of I&A magma needs not only two components (MORB-type magma and CC) but also an island-arc basalt (IAB) component. Since the spreading of the Okinawa Trough is believed to have propagated southwestwards in the last 2 Ma, we suggest that the magma from the southern tip segment (Kueishantao) may result from the onset of the “rifting” stage, while the magma from the central segment (I&A region) may be generated in the more mature segment stepping in “drifting” stage in the back-arc basin.

Asian continental nature of 87Sr/ 86Sr ratios in north central Pacific sediments

Strontium isotopic compositions were determined for silicate detritus in 152 deep-sea and continental shelf surface sediment samples from the Pacific Ocean to trace their provenance. The major characteristics of the spatial variation of the Sr isotopes and the interpretation of this variation may be summarized as follows: (1) Sediments in the Pacific have a lower average 87Sr/ 86Sr ratio (0.7112) with a narrower range (0.703–0.724) than in the Atlantic (0.7166; 0.704–0.743). Many western Pacific volcanoes together with young continental crust supply material with low 87Sr/ 86Sr ratios to the Pacific, in contrast to the large old cratonic crust that supplies material with high Sr ratio to the Atlantic. (2) The 87Sr/ 86Sr ratio is high in the north central Pacific (0.711–0.719). The Izu-Ogasawara and Mariana arcs supply low 87Sr/ 86Sr material to the western Pacific, but farther to east the abundant supply of loess carried by middle-latitude westerlies from the Asian continent ( 87Sr/ 86Sr=ca.0.720) dominates the north central Pacific.

Early Proterozoic oceanic crust and the evolution of subcontinental mantle: Eclogites and related rocks from southern Africa

Seven eclogitic nodules from kimberlites in southern Africa have been studied in detail for whole-rock, major- and trace-element geochemistry, petrography, and mineral chemistry by electron microprobe; high-purity mineral separates from six of these samples have been analyzed for trace elements and for the isotopic composition of Sr, Nd, and oxygen. Three eclogite groups are recognized: group A eclogites have very high Mg/Fe, low Na in pyroxene, moderate δ18O (+4.7 to +5.3), and low 87Sr/86Sr and 143/144Nd. Olivine and enstatite may also be present as accessory phases in this group. Group B eclogites have moderate to high Mg/Fe ratios, high Na in pyroxene, low δ18O (+3.0 to +3.4), high 87Sr/86Sr and 143Nd/144Nd ratios, and extremely LREE-depleted/ HREE-enriched garnets with no Eu anomalies. Group C eclogites have low Mg/Fe ratios, high Na in pyroxene, and variable Sr-, Nd-, and O-isotope compositions. Accessory feldspar is present in one sample that may be of crustal origin. Mineral separates of garnet and pyroxene have positive Eu anomalies. The group A eclogites are too refractory to represent magma compositions and must have formed as cumulate dike rocks in the upper mantle that contain a minor trapped liquid component. This is supported by the presence of accessory olivine and enstatite, and by their KSm and KNd, which are similar to empirical pyroxene/garnet partition coefficients. The Group B eclogites are extremely depleted in incompatible elements and have ϵNdvalues 10x to 20x MORB. The high 87Sr/86Sr of these eclogites is not consistent with their strongly depleted LREE and high ϵNd, and cannot be primary—it must have been imposed on the protolith. The low δ18O of these eclogites cannot form by mantle fractionation processes and must also be inherited from the protolith. Both the Sr- and oxygen-isotope data are consistent with high- temperature hydrothermal alteration of a basaltic protolith, followed by partial melting to form the refractory compositions observed now. The hydrothermal fluid may have been sea water, but secondary enrichment of the protolith in Rb is required to generate the observed high Sr ratios. The high Na content of the pyroxenes supports a spilitic alteration event. The major- and trace-element characteristics of the group C eclogites are consistent with recrystallization of a cumulate gabbro protolith. One group C rock is probably a garnet granulite derived from the lower crust. The other may represent the plutonic portion of oceanic crust. A reconstructed whole-rock isochron for the three Bellsbank eclogites yields an age of 2.1 ± 0.1 b.y. This implies that plate-tectonic processes involving the generation and subduction of oceanic crust have been active since the early Proterozoic. The early Proterozoic ocean crust consisted of two components: a plutonic section of gabbro cumulates (group C eclogites) and a volcanic section of basalt (group B eclogites). The volcanic component has undergone substantial hydro-thermal alteration and subsequent partial melting to form a refractory residue; the plutonic component is less modified.

Provincial Variations in Cap Rock Source Materials of Gulf Coast Salt Domes: ABSTRACT

Isotopes of strontium, carbon, and oxygen are used to model hydrocarbon, brine, and meteoric fluid interactions during cap rock evolution. Provincial isotopic variations occur between older salt domes of the east Texas (ETx) and northern Louisiana (NLa) basins and the younger domes of the Texas-Louisiana (Tx-La) coastal basin. ETx and NLa cap rocks exhibit normal, mid-Jurassic seawater values (87Sr//86Sr = 0.7068 to 0.7076), very wide ^dgr13C ranges (-5 to -49 per mil PDB) and ^dgr18O values (-6 to -11 per mil PDB) that are slightly lighter than Tx-La (-4 to -10 per mil). Tx-La domes yield remarkably high 87Sr/86Sr ratios (0.7073 to 0.7100), and their ^dgr13C values (-8 to -41 per mil) have means which are 5 to 15 per mil heavier than ETx and NLa domes. Detailed studies of the Hockley dome (Tx-La basin) reveal chemical diversity not recognized in domes farther inland. Anhydrite from the salt stock (mid-Jurassic Louann evaporites) mixed with two separate strontium sources during calcite formation. Calcites near the dome's center formed from an intermediate Sr ratio fluid (87Sr/86Sr ^cong 0.7090), which, based on heavier than average ^dgr13C values, was enriched in CO2 relative to CH4; peripheral calcites evolved from a high Sr ratio fluid (87Sr/86Sr ^cong 0.7105) with a lower CO2/CH4 ratio. High 87Sr/86Sr ratios in other Tx-La anhydrite cap rocks compared with normal mid-Jurassic type values in ETx and NLa cap rocks suggest End_Page 297------------------------------ that the Tx-La basin was periodically isolated from normal seawater during Louann deposition; radiogenic fluids, derived either from local red beds or from meteoric waters, equilibrated with seawater prior to anhydrite precipitation. End_of_Article - Last_Page 298------------

Development of heterogeneities in the subcontinental Mantle: Evidence from the Ferrar Group, Antarctica

Tholeiitic lava flows (Kirkpatrick Basalts) and dolerite sills and dikes (Ferrar Dolerites) of the Jurassic Ferrar Group from Antarctica and dolerite sills from Tasmania, Australia are characterised by initial strontium isotope ratios ranging from 0.7089 to 0.7153. The mean and standard deviation of 85 analyses is 0.7115±0.0012. Some of the scatter in the initial 87Sr/86Sr ratios can be attributed to sample inhomogeneity, analytical uncertainties and sample alteration. The published major element data show well-defined trends that are consistent with an evolution by fractional crystallization. Recognition of a parental magma is difficult due to the fractionated nature of the rocks. Trace element analyses, particularly the rare earth elements (REE) support a differentiation model. Compared to mid-ocean ridge basalts, Ferrar Group rocks are enriched in light REE. Kirkpatrick Basalts from the central Transantarctic Mountains show significant correlations between initial 87Sr/86Sr ratios and major elements only for SiO2 and CaO. The general lack of strong correlation is the basis for rejecting the possibility of wholesale contamination by sialic material as a possible cause of the high 87Sr/86Sr ratios. Selective contamination of the basaltic magmas is a possibility and cannot be completely discounted. It would probably involve a fluid phase in order to transport and mix the light REE, Rb, 87Sr, and other elements. By analogy with selective contamination of ocean ridge basalts by sea water it is difficult to envisage a similar process acting on magma emplaced in a non-marine environment. Because of the elevated values of the initial 87Sr/86Sr ratios, their similar average value over 2,500 km and the large volume of magma involved (4× 105 km3) a mantle origin for the high Sr ratios is preferred. Models to account for the enrichment of Rb and light REE in the Antarctic mantle during or prior to the Jurassic include: 1. addition of continental material from a Palezoic Mesozoic subduction zone; 2. metasomatism of volatile elements from the lower mantle; and 3. evolution of a mantle with a high Rb/Sr ratio.