Yemane Asmerom

Strontium isotopic variations of Neoproterozoic seawater: Implications for crustal evolution

We report high precision Sr isotopic data on carbonates from the Neoproterozoic Shaler Group, Victoria Island, Northwest Territories, Canada. Lithostratigraphic correlations with the relatively well-dated Mackenzie Mountains Supergroup constrain Shaler deposition to approximately 770-880 Ma, a range corroborated by 723 +/- 3 Ma lavas that disconformably overlie Shaler carbonates and by Late Riphean microfossils within the section. Samples with low 87Rb/86Sr ratios (<0.01) were selected for Sr isotopic analysis. Delta 18O, Mn, Ca, Mg, and Sr data were used to recognize altered samples. The altered samples are characterized by high Mn/Sr (> or = 2) and variable delta 18O; most are dolomites. The data indicate that between ca. 790-850 Ma the 87Sr/86Sr ratio of seawater varied between 0.70676 and 0.70561. The samples show smooth and systematic variation, with the lowest 87Sr/86Sr value of 0.70561 at ca. 830 Ma. The low 87Sr/86Sr ratio of carbonates from the lower parts of our section is similar to a value reported for one sample from the Adrar of Mauritania (approximately 900 Ma), West African Craton. Isotopic ratios from the upper part of the Shaler section are identical to values from the lower part of the Neoproterozoic Akademikerbreen Group, Spitsbergen. Although a paucity of absolute age determinations hinders attempts at the precise correlation of Neoproterozoic successions, it is possible to draw a broad outline of the Sr isotopic composition of seawater for this period. Indeed, the Sr isotope data themselves provide a stratigraphic tool of considerable potential. Data from this study and the literature are used to construct a curve of the 87Sr/86Sr ratio of Neoproterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in seawater for the period 790-850 Ma. The Sr isotope composition of seawater reflects primarily the balance between continental Sr input through river input and mantle input via hydrothermal circulation of seawater through mid-ocean ridges. Coupling of Nd and Sr isotopic systems allows us to model changes in seafloor spreading rates (or hydrothermal flux) and continental erosion. The Sr hydrothermal flux and the erosion rate (relative to present-day value) are modeled for the period 500-900 Ma. The results indicate that the hydrothermal flux reached a maximum value at ca. 830 Ma. In contrast, a large peak in erosion rate is indicated at ca. 570 Ma. The peaks in hydrothermal flux and erosion rate are most likely related to developments in the Pan-African and related orogenic events, whose initial development is characterized by production of juvenile crust during supercontinental break up and rifting. The time ca. 570 Ma is characterized by continent-continent collision and production of recycled crust. Sr isotope data from Proterozoic carbonates offer a valuable resource for understanding large-scale crust dynamics.

The Pb isotopic evolution of the Earth: inferences from river water suspended loads

Pb and U concentrations and Pb isotopic variations in the suspended loads of major rivers draining regions with a range of ages of crustal formation are reported. The206Pb/204Pb,207Pb/204Pb and208Pb/204Pb ratios are positively correlated with87Sr/86Sr and negatively correlated witheNd values in the samples. A coherent negative correlation of the Pb isotope ratios witheNd values allows us to estimate the average Pb isotope composition of the upper continental crust at206Pb/204Pb= 19.32 ± 0.28,207Pb/204Pb= 15.76 ± 0.09 and208Pb/204Pb= 39.33 ± 0.39. The206Pb/204Pb ratios are positively correlated with their Nd model ages, reflecting the higher upper continental crust μ values relative to mantle μ values. The upper continental crust, as well as the MORB and OIB sources, plot to the right of the geochron. Assuming that the lower crust is the missing low-μ reservoir, we have calculated a range of possible lower crust Pb isotope compositions similar to many granulite terrains. Overall, the upper continental crust has somewhat higher207Pb/204Pb than MORB or OIB, while206Pb/204Pb overlaps with the MORB values and is less radiogenic than some OIB values. Typical measured μ values for the suspended loads of large rivers are ∼ 4, while the μ values of their crustal source rocks (based on the Pb isotope composition) are ∼ 12, allowing for up to 65% of U from continental weathering being carried in the dissolved load to the oceans. The near-overlap of the206Pb/204Pb values of MORB and continental crust is consistent with the suggestion thatU is recycled into the mantle through altered oceanic crust. The exception to these trends are the samples with the oldest Nd model ages (∼ 3.5 Ga). The Pb isotope ratios show drastic deviation from the Pb and Sr correlation trend suggesting an inherent low μ value in the mantle source of the early Archean continental crust.

Variations in magma source regions during large-scale continental extension, Death Valley region, western United States

A tubular connector sleeve has a radially outwardly projecting inwardly open groove formed at one end which is defined by a radially outwardly projecting wall and a radially inwardly projecting flange at the end of the sleeve. A portion of the flange is cut away to provide access to the groove. A connector tube has a radially outwardly projecting flange at one end and the outside diameter of this flange is generally equal to the inside diameter of the connector sleeve so that the flange on the connector tube can extend inside the connector sleeve. A spiral lock ring is wound through the opening in the sleeve flange until the entire spiral lock ring is in the groove. The wall length of the spiral lock ring is greater than the depth of the groove so a portion of the ring wall extends radially inside the sleeve far enough beyond the internal surface of the sleeve so the radially outwardly projecting flange at the end of the connector tube abuts the portion of the spiral lock ring wall to prevent separation between the connector sleeve and the connector tube.

Permian-Triassic plutonism and tectonics, Death Valley region, California and Nevada

Significant contractional structures that deform Permian rocks but predate an Early Triassic overlap sequence are recognized within the Cordilleran orogen, western United States. Thrusting in the Death Valley region of the orogen, however, has been regarded as Middle Triassic or younger and thus kinematically distinct. We present new isotopic age limits on two posttectonic stocks that intrude major structures of the Death Valley thrust belt. The stocks are no younger than Middle Triassic, but are likely Late Permian in age, consistent with stratigraphic and structural data suggesting that thrusting predates the overlap sequence. We hypothesize that Permian shortening may have affected more than 700 km of the Cordilleran orogen at the same time arc activity began within cratonic North America but prior to Early Triassic emplacement of the structurally higher Sonomian arc terrane.