Isotopic Boundary Research Articles

New Sr, Nd, and Pb isotopic data from plutons in the northern Great Basin: Implications for crustal structure and granite petrogenesis in the hinterland of the Sevier thrust belt

The influence of tectonic setting and age on the variation of isotopic signatures of granitic plutons in the northern Great Basin has, in general, not been apparent from previous investigations. Although Elison et al. pointed out isotopic differences between Jurassic and younger plutons near the 0.706 Sr isopleth, and Farmer and DePaolo noted that there might be a difference between Cenozoic vs. Mesozoic plutonic rocks in the eastern part of the northern Great Basin, neither of these studies revealed the remarkable correlation between isotopic signature, age, and tectonic setting shown by our expanded Sr, Nd, and Pb isotopic data base. Jurassic-Early Cretaceous plutons in the northern Great Basin have a limited range of Sr and Nd isotopic values that cluster near bulk earth. All but one of these plutons have ϵ Nd values less negative than -7 despite their location both to the west and east of the ϵ Nd = -7 line. Construction of Sr 0.706 and ϵ Nd = -7 isotopic boundaries is virtually impossible for plutons of this age range. In contrast, Upper Cretaceous peraluminous granites east of the ϵ Nd = -7 line have very negative ϵ Nd values and high initial Sr ratios, and they appear to represent essentially pure crustal melts. The data favor a model that equates generation of these plutons via crustal thickening associated with the Sevier thrust belt. Cenozoic plutons appear to be mixtures of mantle and crustal reservoirs, and their isotopic systematics, along with those of the Late Cretaceous age plutonic suite, define a previously unrecognized, approximately east-west-trending crustal boundary between predominantly Archean crust to the north and predominantly Proterozoic crust to the south. The isotopic data from the Jurassic-Early Cretaceous plutonic suite do not reflect the presence of this boundary, suggesting that the isotopic systematics of this plutonic suite may not have been controlled by the same variations in crustal and/or mantle lithospheric structure at depth.

Proterozoic Geochronologic and Isotopic Boundary in NW Arizona

U-Pb ages determined from zircon, sphene, and apatite in conjunction with Pb isotopic analyses of alkali feldspar establish a regional geochronological framework and constrain the location of a major north-trending Proterozoic crustal boundary in northwestern Arizona. Two regions west of the boundary (Hualapai Mountains and Lost Basin Range-Garnet Mountain) are characterized by complex U-Pb zircon systematics, evidence for inheritance of an older zircon component (1.8-2.3 Ga), and elevated {sup 207}Pb/{sup 204}Pb from feldspars compared to the east. Although the discordia patterns are complex, supracrustal rocks are interpreted to be ca. 1.73 Ga and are intruded by plutonic rocks ca. 1.70 Ga. Deformation is younger than ca. 1.70 Ga foliated granites and older than the 1,682 {plus minus} 4 Ma Garnet Mt. monzogranite. The rocks in one area east of the boundary (Cottonwood Cliffs) are characterized by relatively simple U-Pb zircon systematics, no evidence for inheritance of any older component, and feldspar {sup 207}/{sup 204}Pb near model mantle values. Supracrustal rocks are older than 1.73 Ga, as they are intruded by a 1,730 {plus minus} 9 Ma foliated granodiorite. Timing of deformation is constrained by the late syn-kinematic Valentine granite dated at 1,713 {plus minus} 12 Ma. Mineral ages indicate thatmore » the rocks on either side of the boundary had different cooling histories and inferentially, different uplift histories. West of the boundary, the cooling history is inferred from minerals separated from an amphibolite: metamorphic zircon is 1,687 +13/{minus}8 Ma, sphene is 1,660 {plus minus} 5 Ma, hornblende has a {sup 40}Ar/{sup 39}Ar age of 1,552 {plus minus} 5 Ma, and apatite has a U-Pb age of 1,520 {plus minus} 45 Ma. East of the boundary an amphibolite has sphene with an age of 1,670 {plus minus} 11 Ma and apatite with an age of 1,630 {plus minus} 8 Ma.« less

Seismic expression of Quarternary climatic cycles in the peri-platform carbonate ooze of the northern Bahamas

Stratigraphic, physical, and acoustic properties of peri-platform carbonate sediments in an 18-m-long piston core (core 2218, 655 m) from Northwest Providence Channel, Bahamas, have been investigated. The sediments range in age from recent to about 1 m.y. old. There is a close correlation between changes of compressional wave velocity and Quaternary climate as indicated by δ 18 O. Velocity correlates negatively to aragonite content, water content, and porosity, whereas it correlates positively to grain size, saturated bulk density, and observed degree of cementation. Changes in the degree of cementation exert a particularly strong influence on velocity variations. All parameters correlate to Quaternary climate/sea-level fluctuations. During glacial/low-sea-level stages, the aragonite content, water content, water content, and porosity decrease, but grain size, saturated bulk density, and the degree of cementation increase; all of these increase velocity. In general, water content, porosity, and the aragonite content decrease slightly with core depth, but grain size, saturated bulk density, degree of cementation, and velocity increase slightly. Two processes may contribute to these trends: (1) shallow subsurface diagenesis upon burial and (2) variations of both the interglacial production of bank-top sediments and the intensity of glacial sea-floor diagenesis. Major changes in velocity are associated with transitions from glacial to interglacial stages. Acoustic reflectors correlate one-to-one with the velocity changes occurring at isotopic stage boundaries 7-8, 9-10, and 11-12. At the site of core 2218, deeper reflectors have an interference origin because velocity changes are more closely spaced. The reflectors that correlate with isotopic stage boundaries can be used to study the detailed history of Quaternary peri-platform carbonate deposition and place it into the context of global climate/sea-level cycles.

Isotope evidence of a mantle convection boundary at the Australian-Antarctic Discordance

Basalts from the Southeast Indian Ridge south of Australia form two geographically and isotopically distinct groups that show affinities with either Indian Ocean or Pacific/Atlantic Ocean isotope compositions. The data raise the possibility that there is a sharp boundary between the Indian and Pacific Ocean isotope provinces. The isotope boundary occurs over less than 200km within the Australian–Antarctic Discordance, a technically complex region believed to overlie a zone of downwelling mantle flow.

Total internal reflection on isotopic interface: a case for isotopic fiber optics

The feasibility of isotopic engineering in a form of isotopic boundary for light-confinement applications is assessed. It is estimated that isotopic structuring alone is capable of providing the light-confinement aperture of a same (or almost the same) order of magnitude as conventional fiber optics.

Light bending and light confinement at isotopic boundaries—possibility of isotopic fiber optics

Light bending and light confinement at isotopic boundaries—possibility of isotopic fiber optics

Strontium isotope variations in lower tertiary-quaternary volcanic rocks from the Kurile island arc

Average 87Sr/86Sr ratios for lavas from Quaternary and Pleistocene volcanoes of the Kurile island arc, NW Pacific, decrease from 0.7035 in the south to 0.7032 in the north. The northern Kuriles are characterised by K2Oricher volcanics and by an older crust. Varying ratios show no simple relation to crustal thickness or geochemical indicators of crustal contamination. This is thought to reflect the immature character of the crust — its simatic composition, low Rb/Sr ratios and youthfulness. Older lavas from the Kuriles (Lower Tertiary, Miocene) have similar or slightly higher 87Sr/86Sr ratios; some have suffered slight alteration and possibly crustal contamination. Quaternary volcanics from the Kurile and Aleutian arcs have the lowest 87Sr/86Sr ratios of all circum-Pacific arcs and this may be ascribed to (a) the isotopic individuality of the landward North American plate and/or (b) the high degree of mechanical coupling between the Pacific and North American plates reducing the amount of subducted 87Sr-rich sediments and seawater. An isotopic boundary between island arcs is located in central Hokkaido. The primary basaltic magmas of the Kuriles were derived from mantle recently contaminated by radiogenic Sr. Subsequent fractionation to andesites and dacites occurred by closed-system fractional crystallization.