Clark E. Isachsen

The decay constant of 176Lu determined from Lu-Hf and U-Pb isotope systematics of terrestrial Precambrian high-temperature mafic intrusions

Abstract At present, there is large uncertainty in the decay constant of 176Lu needed for Lu–Hf isotopic studies. We have determined λ176Lu by cross-calibration of U–Pb and Lu–Hf isotopic systems on mineral fractions from the Proterozoic Karlshamn and Sorkka dolerites in Sweden and Finland. The dolerites crystallized at shallow depths from homogeneous, high-temperature magmas, carry olivine that is nearly 100% unaltered, and show no signs of post-magmatic isotopic disturbance. The Lu and Hf isotopic compositions of plagioclase, olivine, pyroxene, apatite, ilmenite and baddeleyite were determined by multicollector-inductively coupled plasma mass spectrometry (MC-ICPMS). Calibrating the Lu–Hf results against baddeleyite U–Pb dates of 954.1±1.2 and 1256.2±1.4 Ma for the dolerites yields a mean λ176Lu of 1.867±0.008×10−11 year−1. The pristine character of the rocks and the agreement of the λ176Lu values with those from other terrestrial data sets [E. Scherer et al., Science 293 (2001) 683–687] suggest that the true value of λ176Lu lies between 1.86 and 1.87×10−11 year−1. Calibration experiments on extraterrestrial samples give significantly higher (4–6%) values, a discrepancy that may be due to plotting of non-cogenetic samples on the same Lu–Hf isochron diagram, or may have other, as yet undetermined, causes. The result of this study also indicates that the Lu–Hf method is capable for dating the crystallization of mafic rocks. The high 176Lu/177Hf ratio in apatite suggests that intrusive ages can be determined at a precision of a few million years or better.

Isotopic and structural constraints on the location of the Main Central thrust in the Annapurna Range, central Nepal Himalaya

Five isotope-enhanced geologic transects in the southern Annapurna Range of central Nepal elucidate structural geometries near the Main Central thrust. Whole-rock Nd isotopes and U-Pb ages of detrital zircons unambiguously distinguish Greater Himalayan (hanging wall) and Lesser Himalayan (footwall) metasedimentary rocks. ϵ Nd (0) values for lower Lesser Himalayan rocks typically range from −20 to −26, whereas Greater Himalayan rocks usually have ϵ Nd (0) values of −19 to −12. Lower Lesser Himalayan rocks yield detrital zircons with an age peak at ca. 1880 Ma and no detrital zircons younger than ca. 1550 Ma. In contrast, Greater Himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca. 1050 Ma and no detrital zircons younger than ca. 600 Ma. The protolith boundary between Greater and Lesser Himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology. Field and microstructural observations indicate the presence of a top-to-the-south ductile shear zone superimposed on this boundary, confirming this shear zone as the Main Central thrust. No evidence exists for large-scale structural mixing of Greater and Lesser Himalayan rocks along the Main Central thrust in the Annapurna Range.

Sm^Nd dating of spatially controlled domains of garnet single crystals: a new method of high-temperature thermochronology

Ganguly and Tirone [Meteorit. Planet. Sci. 36 (2001) 167^175] recently presented a method of determining the cooling rates of rocks from the difference between the core and bulk ages of a crystal, as determined by a single decay system. Here we present the first application of the method using the core and bulk ages of garnet single crystals, according to the Sm^Nd decay system, in two rock samples with contrasting cooling rates, which can be constrained independently. The samples belong to the metamorphic core complex, Valhalla, British Columbia, and the mid-crustal magmatic arc exposure of the Salinian terrane, California. We have micro-sampled the garnet crystals over specific radial dimensions, and measured the Nd isotopes of these small sample masses, as NdO þ via solid source mass spectrometry, to determine the Sm^Nd age difference between the core and bulk crystals. Using a peak metamorphic P^T condition of 8 > 1 kbar, 820 > 30‡C [Spear and Parrish, J. Petrol. 37 (1996) 733^765], the core (67.3 > 2.3 Ma) and bulk (60.9 > 2.1 Ma) ages of the British Columbian garnet sample yield a cooling rate of 2^13‡C/Myr, which is in very good agreement with the cooling rates that we have derived by modeling the retrograde Fe^Mg zoning in the same garnet, and assuming the same peak metamorphic P^T condition. Considering earlier cooling rate data derived from closure temperature vs. age relation of multiple geochronological systems [Spear and Parrish, J. Petrol. 37 (1996) 733^ 765], a cooling rate of V15^20‡C/Myr seems most reasonable for the Valhalla complex. Diffusion kinetic analysis shows that the Sm^Nd core age of the selected garnet crystal could not have been disturbed during cooling. Consequently, the core age of the garnet crystal, 67.3 > 2.3 Ma, corresponds to the peak metamorphic age of the Valhalla complex. The Salinian sample, on the other hand, yields indistinguishable core (78.2 > 2.7 Ma) and bulk (77.9 > 2.9 Ma) ages, as expected from its fast cooling history, which can be constrained by the results of earlier studies. The Sm^Nd decay system in garnet has relatively high closure temperature (usually s 650‡C); therefore, the technique developed in this paper fills an important gap in thermochronology, since the commonly used thermochronometers are applicable only at lower temperatures. Simultaneous modeling of the retrograde Fe^Mg zoning in garnet, spatially resolved Sm^Nd ages of garnet single crystals, and resetting of the bulk garnet Sm^Nd age

Batholith emplacement at mid-crustal levels and its exhumation within an obliquely convergent margin

Abstract Emplacement of the central part of the Coast Mountains batholith of northern coastal British Columbia occurred within a regime characterized by oblique convergence between the Farallon/Kula and North American plates. We use new structural, kinematic and U–Pb isotopic data to show that the locations, geometry, and mechanisms of pluton emplacement within this batholith were controlled by displacements within a network of normal faults and transtensional shear zones. These data also show that the most active period of pluton emplacement, from ∼67 to ∼51 Ma, coincided with a change in style of deformation within the batholith. Prior to ∼67 Ma plutons were emplaced within an arc dominated by regional-scale contractional shear zones. In contrast, emplacement of 67–51 Ma plutons occurred in an arc increasingly dominated by normal faults with arc-parallel to oblique displacement and by sinistral transtensional shear zones. We have identified and mapped the structure of three plutonic complexes composed of 67 to 51 Ma plutons: the Khyex sill complex, Arden Lake plutonic complex and Quottoon plutonic complex. Shear-zone-controlled emplacement of plutons within the batholith accounts for the widely different orientations and structural features that characterize plutons within these three complexes. During and after this latest Cretaceous–Paleogene period of intense plutonic activity and accompanying deformation, the deep roots of the batholith were rapidly unroofed by ductile normal faulting prior to 50 Ma.

Behavior of zircon during high-pressure, low-temperature metamorphism Case study from the Internal Unit of the Sesia Zone (Western Italian Alps)

Zircon from two high-pressure, low-temperature orthogneisses (1-1.5 GPa and 550 ° C) from the Sesia Zone, Western Alps were examined by means of scanning electron, back-scattered and cathodoluminescence imaging and isotope dilution thermal ionization mass spectrometry to investigate U-Pb systematics of zircon in high-pressure, low-temperature terranes. The results of the U-Pb geochronology show a simple, two-stage mixing trend between crystallization of the leucogranitic protolith and eo-Alpine high-pressure metamorphism. The upper intercept ages of 435 ± 8 and 396 ± 21 Ma constrain the age of the protoliths and can be correlated to the zircon interiors that display strong cathodoluminescence. The lower intercept age of 68 ± 6.6 Ma dates the eo-Alpine high-pressure metamorphism and can be correlated with weakly luminescent, slightly zoned overgrowth seen in CL- and BSE-images, respectively. Two air-abraded single-grain analyses of apparently inherited zircons give a concordant age of 3.7 Ga and a discordant age of 2.4 Ga. Eo-Alpine zircon dissolution and overgrowth are characterized by internal and external embayed surfaces and multi-faceted crystal forms, respectively. A notable lack of evidence for pervasive fluid-rock interactions supports the view that dissolution of zircon was aided by local pore fluids of low salinity produced during metamorphic dehydration reactions of biotite and zoisite to form garnet. Precipitation of zircon overgrowths may have accompanied the H 2 O-consuming reaction of phengite formation from garnet.

Zircon U/Pb ages for gniess from Qianshan, Anhui

Zircon U/Pb ages were measured for Qianshan gneiss at the ultrahigh pressure metamorphic (UHPM) complex in the Southern Dabie Terrain (SDT). According to morphology, Th and U contents of eight fractions of zircon and data distribution on the discordia, it is indicated that they should be treated as metamorphic overgrowth mixing zircons, in which the cores mainly are magmatic and the outers were metamorphic. The intercept ages show that the gneiss underwent UHPM. There is no direct correlation for zircon between morphological feature and genesis.