H. Baadsgaard

Precise K-Ar, 40Ar/39Ar, Rb-Sr and U/Pb mineral ages from the 27.5 Ma fish canyon tuff reference standard

The accuracy of ages measured using the 40Ar/39Ar technique is affected by uncertainties in the age of radiation fluence-monitor minerals. At present, there is lack of agreement about the ages of certain minerals used as fluence monitors. The accuracy of the age of a standard may be improved if the age can be measured using different decay schemes. This has been done by measuring ages on minerals from the Oligocene Fish Canyon Tuff (FCT) using the K–Ar, 40Ar/39Ar, Rb–Sr and U/Pb methods. K–Ar and 40Ar/39Ar total fusion ages of sanidine, biotite and hornblende yielded a mean age of 27.57±0.36 Ma. The weighted mean 40Ar/39Ar plateau age of sanidine and biotite is 27.57±0.18 Ma. A biotite–feldspar Rb–Sr isochron yielded an age of 27.44±0.16 Ma. The U–Pb data for zircon are complex because of the presence of Precambrian zircons and inheritance of radiogenic Pb. Zircons with 207Pb/235U<0.4 yielded a discordia line with a lower concordia intercept of 27.52±0.09 Ma. Evaluation of the combined data suggests that the best age for FCT is 27.51 Ma.

Timing of late Archaean terrane assembly, crustal thickening and granite emplacement in the Nuuk region, southern West Greenland

A breakthrough in understanding the structural and metamorphic evolution of Archaean gneiss complexes occurred with the recognition that the Nuuk region of southern West Greenland comprised separate terranes assembled in the late Archaean. From northwest to southeast these are: the Akia (3220-2970 Ma), Akulleq (3870-3600 and 2820 Ma) and Tasiusarsuaq (2920-2860 Ma) terranes. The minimum time of assembly is recorded by the first event common to all component terranes. Using SHRIMP UPb zircon geochronology the oldest events common to all terranes (including emplacement of crustally derived granites, contemporaneous metamorphism and anatexis) have been dated at 2710–2725 Ma. In the Akulleq terrane areas where in situ diatexite formed, abundant granitoid sheets were intruded and common growth of metamorphic zircon (mostly low ThU) in most lithologies occurred. In the Akia and Tasiusarsuaq terranes there was only intrusion of a lesser number of ∼ 2720 Ma granitoid sheets, because metamorphic zircons of that age have not been found. The ∼ 2720 Ma event is interpreted as marking, or shortly following, terrane assembly, when the diverse components of the Akulleq terrane were tectonically juxtaposed with the other two. This new documentation of metamorphic and associated igneous events within an accreting cratonic region is an illustration that the stabilisation of extensive areas of Archaean gneisses can be due to accretionary tectonics long after the individual components were first formed.

Geochronology and isotopic variation of the early Archaean Amitsoq gneisses of the Isukasia area, southern West Greenland

Abstract Relatively unaltered samples of the Amitsoq gneiss complex in the Isukasia area were identified and analysed despite the effects of metamorphism and local Proterozoic hydrothermal alteration and contamination. The parents of the three lithologies that make up the Amitsoq gneisses of the area were emplaced over about 320 Ma. Taking a weighted average of dates from U-Pb on zircons, Rb-Sr on WR, Sm-Nd on WR and Pb-Pb on WR, the most probable ages of intrusion are: grey gneisses at 3690 ± 50 Ma, white gneisses at 3590 ± 40 Ma and pegmatitic gneisses at 3370 ± 60 Ma. The Rb-Sr isochron initial 87 Sr 86 Sr ratios for the grey, white and pegmatitic gneisses are 0.7006 ± 1, 0.7016 ± 1 and 0.8248 ± 3, respectively. The corresponding average ϵ Nd ( T ) values are +1.0, +0.7 and −0.2. Since the REE chemistry of the rocks that were the direct parents of the grey gneisses is not known, the +1.0 average ϵ Nd ( T ) of the grey gneisses need not imply generation from a depleted mantle. The Sr initial ratio and ϵ Nd ( T ) values for the white and pegmatitic gneisses are compatible with consecutive derivation at two discrete intervals by partial melting of crust mainly comprised of rocks similar to the grey gneisses. The progressive evolution of ϵ Nd with T for the Amitsoq gneisses in the early Archaean shows that values of ϵ Nd ( T ) near zero in granitoids do not necessarily indicate derivation from the “undepleted” mantle (CHUR).

Mineral isotopic age relationships in the polymetamorphic Amîtsoq gneisses, Godthaab district, West Greenland

Abstract U-Th-Pb, Pb-Pb, Rb-Sr and K-Ar radiometric relationships in the minerals from six selected Amitsoq gneiss samples reveal a complicated history of variable mineral response to polymetamorphism. K-Ar dates on biotite range from 2170 to 3220 m.y. (excess argon present), on hornblende from 2340 to 2510, and on a single muscovite at 1670 m.y. Rb-Sr whole rock results give an apparent isochron of at least 4065 m.y., but this result is likely fortuitous from a small sample selection since Pb-Pb whole rock analyses give ~ 3600 m.y. and the zircons in these rocks yield a concordia-discordia intersection at 3600 m.y. Rb-Sr mineral analyses generally give a confusing and variable pattern of isotopic relationships; but hornblende, K-feldspar, apatite, allanite and sphene appear to have last responded to metamorphism at 2200–2600 m.y. Rb-Sr in biotite, epidote and, in part, plagioclase have been affected by an event at ~ 1550 m.y. U-Th-Pb data from sphene, apatite and allanite give almost concordant dates at 2500–2600 m.y. soul 207 Pb 204 Pb vs soul 206 Pb 204 Pb plots yield two separate lines for apatite (slope age 2435 m.y.) and for sphene + allanite (slope age 2530 m.y.), indicating apatite to have a different (less-radiogenic) ‘initial’ Pb than that for sphene and allanite. A similar pattern is found for the soul 208 Pb 204 Pb vs soul 207 Pb 204 Pb plot for sphene and apatite. The Pb-isotopic composition of the feldspars is very homogeneous and the least-radiogenic of all components, pointing towards a homogeneous parent material for the now lithologically diverse Amitsoq gneisses. Using a 0 = 9.307, b 0 = 10.294, C 0 = 29.476, t 0 = 4.56 b.y., ω = 6.9 and soul 232 Th 204 Pb = 27.1 ; the feldspars give a model Pb age of 3500–3600 m.y. by either U-derived or Th-derived Pb. The segregation of the present Amitsoq gneisses from the homogeneous parent material was apparently accompanied by a U and Th loss with preservation or enrichment of Pb at ~ 3600 m.y. ago. No consistent treatment of the present U-Th-Pb data will produce viable data indicating an age > 3600 m.y. for the parent materials of the Amitsoq gneiss. Petrographie observations generally concur with radiometric results and permit the postulation of the reaction: Hbl + K-feld→ biotite + epidote + sodic plag, to account for some of the effects of the latest metamorphism. The total internal radiometric evidence indicates three major metamorphic events affected the Amitsoq gneisses close to 3600, 2500 and 1550 m.y.

Alteration and metamorphism of Amitsoq gneisses from the Isukasia area, West Greenland: Recommendations for isotope studies of the early crust

Early Archaean gneisses (3400–3700 Ma) in the Isukasia area of West Greenland have been subjected to a series of local and regional metamorphic processes. Metasomatic alteration accompanied the intrusion of the early Archaean white gneisses (~3600 Ma) into the grey gneisses (~3700 Ma), and resulted in local formation of altered rocks. Pb-Pb isotope results on whole rock and feldspar from the gneisses reveal a major rearrangement of Pb isotopes at about 2600 Ma together with some local third stage Pb changes at a later time. This 2600 Ma event is also shown by Rb-Sr data for phengites in the ~3400 Ma Pegmatitic gneiss sheets and can be correlated with metamorphism accompanying the intrusion of a late Archaean pegmatite (~2600 Ma) swarm. Local hydrothermal alteration by fluids emanating from Proterozoic faults and fractures is petrographically recognised in and near major faults and lithological boundaries. This fault-controlled alteration, though only locally significant, may correlate with low grade thermal metamorphism reflected in a 1600 Ma Rb-Sr mineral isochron on biotites from both altered and unaltered gneisses. Oxygen isotope results show that this alteration probably was associated with influx of a low-δ18O fluid, perhaps meteoric water. Selection of the least altered Amitsoq gneisses can be made on the basis of petrographic and field criteria, and furnishes the best material to study original geochemical and isotopic systematics in these early Archaean rocks.

Potassium-Argon Dates of Biotites from Cordilleran Granites

Biotites from 20 plutons of the Cordillera of North America, principally from British Columbia, have been dated using the potassium-argon method. Samples were selected from well-known intrusive masses in an attempt to compare physically measured dates with relative age established by geological mapping; most dates confirm the geological age interpretations. The Cordillera underwent granitic intrusion or orogeny at five times separated by long periods of quiescence. The earliest orogeny, of Devonian (Acadian) age, occurred 350-360 million years ago and resulted in intrusion of plutons as widely separated as the Ice River syenite of the Rocky Mountains and the Fitton granite of the northern Yukon. Early Mesozoic orogeny resulted in the emplacement of the upper Triassic or lower Jurassic Guichon (186 m.y.) and Topley (163 m.y.) batholiths of central British Columbia. The major Cordilleran intrusion occurred 95–100 million years ago, in about the middle of the Cretaceous, with emplacement of the Coast Range batholith and early phases of the Nelson batholith of British Columbia, and the Cassiar and Itsi batholiths of Yukon. In late Cretaceous time, 80 million years ago, the Bayonne pluton, British Columbia, and Boulder batholith and Marysville stock of Montana were emplaced. During Rocky Mountain orogeny (Eocene), 50–60 million years ago, late phases of the Nelson batholith and a number of plutons such as Coryell were emplaced. Eighteen million years ago, in the Miocene, small granitic bodies intruded the Cascade Range; these are believed to be the youngest exposed batholithic rocks in North America.

Determination of the 87Rb decay constant

The decay constant 87Rb has been redetermined by measuring the amount of radiogenic 87Sr produced over a period of 19 years, in 20 g samples of purified RbClO4, using isotope dilution techniques. The rubidium sample was spiked with 84Sr and the nanogram quantities of strontium separated by coprecipitation with Ba(NO3)2. Analyses were carried out on a 25cm, 90° sector mass spectrometer equipped with a Spiraltron electron multiplier. Measurement of three independent ratios permitted continuous monitoring of the ion beam fractionation. The average of nine determinations gives a value for the decay constant of 1.419(±0.012) × 10−11 yr−1 (2σ). [τ12 = 4.89(±0.04) × 1010yr.]

The U-Th-Pb systematics of zircons from the type Nûk gneisses, Godthåbsfjord, West Greenland

Abstract U-Pb analyses were made on zircons separated from different lithologies within the Nuk gneisses taken from Godthab (Nuuk) townsite and from the south-west shore of Bjorneoen. Size fractions of the zircons from each rock yield linear discordia which give apparent crystallisation ages for the parents of the Nuk gneisses of 2890–3065 Myr. This spread of ages is not a result of multiple intrusion, but more likely results from different degrees of ‘diffusion’ and metamorphic episodic lead loss by the zircons. Since U-Pb and Th-Pb discordance are similar, loss of radiogenic lead (and not U or Th-loss/gain) by radiation-damage diffusion and through metamorphism is the main factor causing discordance of the U-Pb dates. The results show that the discordia line given by the analyses of fractions of a single zircon population may not give a true crystallisation age when the rock has been strongly metamorphosed. This interpretation is supported by Rb-Sr whole rock analyses of the same rocks, in which the Nuk townsite rocks scatter about a reference isochron of 2840 Myr, while the Bjorneoen samples give an isochron (MSWD = 0.55) of 3076 ± 27 Myr and the initial ratio at 0.70205 ± 0.00009. The zircon results do not rule out the possibility that more than one metamorphic episode affected the original Nuk gneiss parents prior to 2520 Myr (Qorqut granite intrusion). A date of 2660 Myr was obtained from an aplite intruded during a period of plutonic activity that included the formation of a linear belt of intense ductile deformation that extends through south-western Bjorneoen and Godthab town.

The leakage of radiogenic argon from sanidine

Abstract The presence of unaltered sanidine in stratigraphically well-defined bentonite clay horizons offers an opportunity to date a sedimentary horizon by the Ar 40 K 40 method. It has been demonstrated that radiogenic argon losses of from 15–40 per cent may take place from microcline and orthoclase, and an evaluation of the argon retentivity of sanidine is necessary. The effects of variable temperature and grain-size in short-term (days) tests of argon leakage from pure sanidines were investigated. The results indicate that the radiogenic argon is quantitatively retained at temperatures below 400°C except, possibly, for the finest grain-sizes (− 325 mesh) and for sanidines with lattice irregularities. Argon loss above 400°C and below the melting temperature appears to involve the loss of at least two separate argon components for structurally inhomogeneous sanidine. Bentonitic sanidine (of homogeneous structure) showed only simple diffusion loss of argon. Argon loss from feldspars is discussed in terms of diffusion and lattice discontinuities. To test possible long term (millions of years) low-temperature losses of radiogenic argon from bentonitic sanidine, seven biotite-sanidine pairs, taken from bentonite clay horizons and ranging in age from ~65 to ~450 million years, were sampled and analysed. The sets of Ar 40 K 40 values obtained are the same within analytical error (± 5 per cent) over the entire range of ages and show that bentonitic sanidine retains radiogenic argon as well as comparable biotite. The data appear to indicate that fresh sanidine obtained from volcanic bentonite horizons retains radiogenic argon sufficiently well to yield reliable Ar 40 K 40 ages.

Source identification of lead found in tissues of sailors from the Franklin Arctic Expedition of 1845

Abstract Atomic absorption analysis of recently discovered human remains from a 19 century British Arctic expedition indicates lead levels consistent with lead intoxication. Levels up to 30 times higher than those found in modern exposed individuals indicate that the effects of lead may have contributed to the loss of the entire expedition. Lead isotope ratio analysis by mass spectrometry demonstrates that the lead found in the human tissues originated from soldered food cans supplied to the expedition.