Ian McDougall

Diffusion of 40Ar in biotite: Temperature, pressure and compositional effects

Abstract The measured radiogenic 40Ar loss from sized biotite (56% annite) samples following isothermalhydrothermal treatment have provided model diffusion coefficients in the temperature interval 600°C to 750°C, calculated on the assumption that Ar transport proceeds parallel to cleavage. These data yield an array on an Arrhenius plot with a slope corresponding to an activation energy 47.0 ± 2 kcal-mol−1 and a frequency factor of 0.077+0.21−0.06 cm2-sec−1. Together with previous diffusion data for micas in the annitephlogopite series, our results indicate a strong compositional effect, with increasing Fe Mg ratio corresponding to an increase in diffusivity. An effective diffusion radius of about 150 μm for biotite is inferred from the experimental data which compares favorably with that estimated from geological studies. A pressure effect on activation energy corresponding to an activation volume of about 14 cm3-mol−1 is observed. These data yield closure temperature estimates for this biotite composition cooling at rates of 100°C-Ma−1, 10°C-Ma−1 and 1°C-Ma−1 of 345°C, 310°C and 280°C, respectively. 40 Ar 39 Ar age-spectrum analysis of a hydrothermally treated biotite yields a complex release pattern casting doubt on the general usefulness of such measurements for geochronological purposes.

Stratigraphic placement and age of modern humans from Kibish, Ethiopia.

In 1967 the Kibish Formation in southern Ethiopia yielded hominid cranial remains identified as early anatomically modern humans, assigned to Homo sapiens . However, the provenance and age of the fossils have been much debated. Here we confirm that the Omo I and Omo II hominid fossils are from similar stratigraphic levels in Member I of the Kibish Formation, despite the view that Omo I is more modern in appearance than Omo II. 40Ar/39Ar ages on feldspar crystals from pumice clasts within a tuff in Member I below the hominid levels place an older limit of 198 ± 14 kyr (weighted mean age 196 ± 2 kyr) on the hominids. A younger age limit of 104 ± 7 kyr is provided by feldspars from pumice clasts in a Member III tuff. Geological evidence indicates rapid deposition of each member of the Kibish Formation. Isotopic ages on the Kibish Formation correspond to ages of Mediterranean sapropels, which reflect increased flow of the Nile River, and necessarily increased flow of the Omo River. Thus the 40Ar/39Ar age measurements, together with the sapropel correlations, indicate that the hominid fossils have an age close to the older limit. Our preferred estimate of the age of the Kibish hominids is 195 ± 5 kyr, making them the earliest well-dated anatomically modern humans yet described.

Potassium-Argon Ages from Lavas of the Hawaiian Islands

Thirty six samples of lava from seven of the Hawaiian volcanoes have been dated by the K-Ar method. The results show that the order of extinction of the volcanoes, excluding minor activity after the main shield-building phase was completed, occurred along the island chain from northwest to southeast. All the lavas are late Pliocene and Pleistocene, excepting the Mauna Kuwale trachyte, which is early to middle Pliocene. The exposed parts of the volcanoes dated were built in less than 0.5 m.y., with the exception of the Waianae volcano of West Oahu, which was active over more than 0.7 m.y., and Kauai, which was active over more than 1.8 m.y. The thin veneer of alkali lavas of the main shield-building phase generally is less than 0.2 m.y. younger than the underlying “primitive” basaltic lavas, of which the Hawaiian volcanoes are mainly built.

40Ar/39Ar dating of white micas from an Alpine high-pressure metamorphic belt on Naxos (Greece): the resetting of the argon isotopic system

Overprinting of white micas from high pressure, low to medium temperature (M1) metamorphic assemblages in pelitic schists on Naxos during subsequent thermal dome (M2) metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome. The original (M1) minerals are phengites (Si4+=6.7–7.0) and the overprinting minerals are muscovites (Si4+=6.0–6.45). 40Ar/39Ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra, characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between. These upward-convex age spectra are shown to be caused by mixing of two generations of micas, each of which has a different age spectrum and argon release pattern. Seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages. Further, the upward-convex age spectra have been used to trace the isotopic signature of phengites toward increasing M2 metamorphic grade, and suggest that as long as phengites can be observed in the rocks upward-convex age spectra occur. On Naxos, crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas. However, there is also good evidence for argon loss by volume diffusion from phengites. Simple diffusion calculations suggest that the M2 metamorphism was caused by a shortlived heat source.

Excess40Ar in metamorphic rocks from Broken Hill, New South Wales: implications for40Ar/39Ar age spectra and the thermal history of the region

40Ar/39Ar age spectrum analyses of samples from Broken Hill, New South Wales, indicate that the region has experienced a complex thermal history following high-grade metamorphism, 1660 Ma ago. The terrain cooled slowly (∼3°C Ma−1) until about 1570 Ma ago, when the temperature fell below about 500°C. Following granitoid emplacement ∼1500 Ma ago, the region remained relatively cold until affected by a thermal pulse 520±40Ma ago, causing temperatures to rise to∼350°C in some places. During this event, accumulated40Ar was released from minerals causing a significant Ar partial pressure to develop. Laboratory Ar solubility data combined with the40Ar/39Ar age spectra gives a local estimate of this partial pressure of ∼10−4atm. The region finally cooled below 100°C about 280 Ma ago. 40Ar/39Ar age spectrum analyses of hornblende, plagioclase and clinopyroxene containing excess40Ar are characterized by saddle-shaped age spectra. Detailed analysis of plagioclase samples reveals a complex diffusion behaviour, which is controlled by exsolution structures. This effect, in conjunction with the presumed different lattice occupancy of excess40Ar with respect to radiogenic40Ar, appears to be responsible for the saddle-shaped age spectra.

Linear volcanism in French Polynesia

Abstract The island chains of French Polynesia form subparallel line segments whose southeasterly extensions are perpendicular to the East Pacific Rise, the site of present sea-floor spreading in the eastern Pacific Ocean. Samples collected from island members of the Society and Austral Islands chains are used, together with previously reported age determinations for the Marquesas and Pitcairn-Gambier Islands, in a geochronological study of the southeastward migration of volcanism in each of those four lineaments. The suggestion from geomorphologic evidence that island ages increase to the northwest within each island chain, is confirmed by KAr whole-rock ages. The linear volcanism which built the islands of French Polynesia began in the Miocene and continues today. Rates of migration of volcanism are calculated from the nearly linear relationship between average island ages and distance from the southeast ends of the four island lineaments. The four rates are indistinguishable, within limits of detection, at 11 ± 1 cm/year. These rates are consistent with the model of rigid Pacific plate movement over four fixed sources of volcanism, be they dynamic as in the hot spot/plume models or passive as in models of propagating lithospheric fractures. If it is accepted that these volcanic sources trace the motion of the lithosphere over the mantle and thus define the “absolute” frame of reference for plate movement, Pacific plate motion may be fixed to the geometry and volcanic migration rates of French Polynesia. This allows calculation of the absolute motion of all other plates, providing an accurate relative motion model is known (Minster et al., 1974). Such a calculation predicts that Africa is virtually stationary and that the Mid-Atlantic Ridge and East Pacific Rise are moving slowly to the west.

New hominin genus from eastern Africa shows diverse middle Pliocene lineages

Most interpretations of early hominin phylogeny recognize a single early to middle Pliocene ancestral lineage, best represented by Australopithecus afarensis, which gave rise to a radiation of taxa in the late Pliocene. Here we report on new fossils discovered west of Lake Turkana, Kenya, which differ markedly from those of contemporary A. afarensis, indicating that hominin taxonomic diversity extended back, well into the middle Pliocene. A 3.5 Myr-old cranium, showing a unique combination of derived facial and primitive neurocranial features, is assigned to a new genus of hominin. These findings point to an early diet-driven adaptive radiation, provide new insight on the association of hominin craniodental features, and have implications for our understanding of Plio–Pleistocene hominin phylogeny.

Investigations of an intrusive contact, northwest Nelson, New Zealand—I. Thermal, chronological and isotopic constraints

Abstract In the Nelson area of New Zealand, intrusion of the Separation Point Batholith 114 Ma ago caused thermal metamorphism in the adjacent Devonian Rameka Gabbro that allowed radiogenic 40Ar to diffuse partially out of hornblendes in the gabbro. Comparison of mineral ages in the batholith (obtained by the K-Ar, 40 Ar 39 Ar , Rb-Sr, U-Pb and fission track methods) with closure temperature estimates for the various isotopic systems has allowed thermal histories of several samples to be resolved. These curves indicate that, following emplacement, the batholith cooled at an exponentially decreasing rate reaching 100°C by about 75 Ma ago. Oxygen isotope measurements on minerals from the gabbro and granodiorite give calculated equilibrium water compositions of +7 and +9 of δ18OSMOW respectively. Measured porosities from samples of the gabbro yield a model permeability of 3 × 10−17 m12. These results confirm that heat transfer from the batholith to the gabbro was predominantly by conduction. A numerical heat flow model has been developed for the region, and shows that samples in the gabbro near the contact reached ~600°C. whereas 5 km away from the contact, the samples were only raised ~ 50°C above the ambient temperature. Excess 40Ar was identified as causing a significant scatter in the K-Ar ages of hornblendes from the gabbro. Though obscured by this effect, these data suggest a crystallization age of the Rameka Gabbro of 367 Ma, which is consistent with geological evidence.

U-Th-He dating of apatite: A potential thermochronometer

Abstract We found a gem quality crystal of Durango fluorapatite to have a 4He content consistent with complete retention of radiogenic helium since its formation at about 31 Ma. Isothermal heating and step-heating analysis reveal 4He loss to occur systematically by volume diffusion at low temperatures. The linear, low-temperature portion of the diffusion data yields an activation energy of 38.5 ± 8.1 kcal/mol and a frequency factor of In ( D 0 a 2 ) = 16.4 ± 2.8 sec −1 , corresponding to a closure temperature of 105°C ± 30°C (cooling rate 10°C/m.y.). It appears that U-Th-He dating of apatite might represent a useful new thermochronometer with a range similar to that of fission-track dating of apatite. From these results, we infer that a number of the too-young U-Th-He dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low-temperature thermal history of their host rocks

Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya

Sites in eastern Africa have shed light on the emergence and early evolution of the genus Homo. The best known early hominin species, H. habilis and H. erectus, have often been interpreted as time-successive segments of a single anagenetic evolutionary lineage. The case for this was strengthened by the discovery of small early Pleistocene hominin crania from Dmanisi in Georgia that apparently provide evidence of morphological continuity between the two taxa. Here we describe two new cranial fossils from the Koobi Fora Formation, east of Lake Turkana in Kenya, that have bearing on the relationship between species of early Homo. A partial maxilla assigned to H. habilis reliably demonstrates that this species survived until later than previously recognized, making an anagenetic relationship with H. erectus unlikely. The discovery of a particularly small calvaria of H. erectus indicates that this taxon overlapped in size with H. habilis, and may have shown marked sexual dimorphism. The new fossils confirm the distinctiveness of H. habilis and H. erectus, independently of overall cranial size, and suggest that these two early taxa were living broadly sympatrically in the same lake basin for almost half a million years.