Showing papers by "Paul R. Renne published in 2006"

Asa Issie, Aramis and the origin of Australopithecus

Abstract: The origin of Australopithecus, the genus widely interpreted as ancestral to Homo, is a central problem in human evolutionary studies. Australopithecus species differ markedly from extant African apes and candidate ancestral hominids such as Ardipithecus, Orrorin and Sahelanthropus. The earliest described Australopithecus species is Au. anamensis, the probable chronospecies ancestor of Au. afarensis. Here we describe newly discovered fossils from the Middle Awash study area that extend the known Au. anamensis range into northeastern Ethiopia. The new fossils are from chronometrically controlled stratigraphic sequences and date to about 4.1-4.2 million years ago. They include diagnostic craniodental remains, the largest hominid canine yet recovered, and the earliest Australopithecus femur. These new fossils are sampled from a woodland context. Temporal and anatomical intermediacy between Ar. ramidus and Au. afarensis suggest a relatively rapid shift from Ardipithecus to Australopithecus in this region of Africa, involving either replacement or accelerated phyletic evolution.

Age Calibration of the Fish Canyon Sanidine 40Ar/39Ar Dating Standard Using Primary K- Ar Standards

Abstract: The 40 Ar/ 39 Ar dating technique requires the use of neutron fluence monitors (standards). Precise calibrations of these standards are crucial to decrease the uncertainties associated with 40 Ar/ 39 Ar dates. Optimal calibration of 40 Ar/ 39 Ar standards should be based on K/Ar standards having independent isotope dilution measurements of 40 K and 40 Ar*, based on independent isotope tracers (spikes) because this offers the possibility to eliminate random interlaboratory errors. In this study, we calibrate the widely used Fish Canyon sanidine (FCs) standard based on four primary K/Ar standards (GA-1550, Hb3gr, NL-25, and GHC-305) on which K and Ar* concentrations have been determined in different labs with independently calibrated tracers. We obtained a mean age of 28.03 ± 0.08 Ma (1 σ ; neglecting uncertainties of the 40 K decay constants) for FCs, based on the decay constant recommended by Steiger and Jager [Steiger R.H., Jager. E. 1977. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36 , 359–362.]. This age corresponds to a mean 40 Ar*/ 40 K value of (1.6407 ± 0.0047) × 10 −3 . We also discuss several criteria that prevent the use of previous calibrations of FCs based on other primary standards (LP-6, SB-3 and MMhb-1). The age of FCs obtained in this study is based on the 40 K decay constants of Steiger and Jager (1977) but we anticipate the imminent need for revision of the value and precision of the 40 K decay constants (representing the main source of uncertainties in 40 Ar/ 39 Ar dating). The 40 Ar*/ 40 K result of FCs obtained in this study allows therefore a rapid calibration of the age of FCs with uncertainties at the 0.29% level but perhaps more importantly this value is independent of any particular value of the 40 K decay constants and may be used in the future in conjunction with revised decay constants.

Eruptive history and petrologic evolution of the Albano multiple maar (Alban Hills, Central Italy)

Abstract: A comprehensive volcanological study of the Albano multiple maar (Alban Hills, Italy) using (i) 40Ar/39Ar geochronology of the most complete stratigraphic section and other proximal and distal outcrops and (ii) petrographic observations, phase analyses of major and trace elements, and Sr and O isotopic analyses of the pyroclastic deposits shows that volcanic activity at Albano was strongly discontinuous, with a first eruptive cycle at 69±1 ka producing at least two eruptions, and a second cycle with two peaks at 39±1 and 36±1 ka producing at least four eruptions. Contrary to previous studies, we did not find evidence of magmatic or hydromagmatic eruptions younger than 36±1 ka. The activity of Albano was fed by a new batch of primary magma compositionally different from that of the older activity of the Alban Hills; moreover, the REE and 87Sr/86Sr data indicate that the Albano magma originated from an enriched metasomatized mantle. According to the modeled liquid line of descent, this magma differentiated under the influence of magma/limestone wall rock interaction. Our detailed eruptive and petrologic reconstruction of the Albano Maar evolution substantiates the dormant state of the Alban Hills Volcanic District.

Effects of internal mineral structures on the magnetic remanence of silicate‐hosted titanomagnetite inclusions: An electron holography study

Abstract: [1] Titanomagnetite inclusions in pyroxene and plagioclase are carriers of stable magnetic remanence in some slowly cooled rocks such as gabbros, anorthosites, granulites, and diorites. Needle-shaped inclusions with average dimensions of 1 × 1 × >25 μm form epitaxially by exsolution from their host silicate. Close examination of clinopyroxene-hosted inclusions reveals an internal microstructure, which consists of magnetite (Fe3O4) prisms and ulvospinel (Fe2TiO4) lamellae that formed as a result of phase unmixing during initial cooling. This internal structure exerts a profound influence on the magnetic remanence properties of each inclusion, primarily by transforming it from a multidomain grain into an assemblage of magnetostatically interacting single-domain prisms. Here we use off-axis electron holography to image the magnetization states of individual prisms and the magnetostatic interactions between them. We show that the inclusions exhibit both single-domain and collective magnetic states that depend primarily on the shape anisotropies of individual magnetite prisms, magnetostatic interactions between closely spaced prism stacks, and the shape anisotropy of the needle itself. Prisms that are separated by thick ulvospinel lamellae show uniformly magnetized and/or vortex states. In contrast, closely spaced magnetite prisms behave as multipart vortices or as long composite columns, whose strong net magnetization may not be related directly to the orientation and shape of either the needle or the constituent prisms. The overall remanence direction recorded by clinopyroxene crystals containing finely exsolved inclusions is a reflection of both the inclusions' elongation directions and the prism arrangements within them.

The record of Miocene impacts in the Argentine Pampas

Abstract: Argentine Pampean sediments represent a nearly continuous record of deposition since the late Miocene (~10 Ma). Previous studies described five localized concentrations of vesicular impact glasses from the Holocene to late Pliocene. Two more occurrences from the late Miocene are reported here: one near Chasico (CH) with an 40Ar/39Ar age of 9.24 ± 0.09 Ma, and the other near Baha Blanca (BB) with an age of 5.28 ± 0.04 Ma. In contrast with andesitic and dacitic impact glasses from other localities in the Pampas, the CH and BB glasses are more mafic. They also exhibit higher degrees of melting with relatively few xenoycrysts but extensive quench crystals. In addition to evidence for extreme heating (>1700 °C), shock features are observed (e.g., planar deformation features [PDFs] and diaplectic quartz and feldspar) in impact glasses from both deposits. Geochemical analyses reveal unusually high levels of Ba (~7700 ppm) in some samples, which is consistent with an interpretation that these impacts excavated marine sequences known to be at depth. These two new impact glass occurrences raise to seven the number of late Cenozoic impacts for which there is evidence preserved in the Pampean sediments. This seemingly high number of significant impacts over a 10^6 km^2 area in a time span of 10 Myr is consistent with the number of bolides larger than 100 m expected to enter the atmosphere but is contrary to calculated survival rates following atmospheric disruption. The Pampean record suggests, therefore, that either atmospheric entry models need to be reconsidered or that the Earth has received an enhanced flux of impactors during portions of the late Cenozoic. Evidence for the resulting collisions may be best preserved and revealed in rare dissected regions of continuous, low-energy deposition such as the Pampas. Additionally, the rare earth element (REE) concentrations of the target sediments and impact melts associated with the Chasic event resemble the HNa/K australites of similar age. This suggests the possibility that those enigmatic tektites could have originated as high-angle, distal ejecta from an impact in Argentina, thereby accounting for their rarity and notable chemical and physical differences from other Australasian impact glasses.

Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range

Abstract: Late Miocene-Pliocene (8-3 Ma) olivine basalt lavas, dated in this study by the 40 Ar/ 39 Ar method, have been faulted and tilted on both the east and west sides of the Warner Range of NE California, which is itself a tilted block rising to 2960 m at its crest that is composed of Miocene-Oligocene lavas and volcaniclastic rocks. The late Miocene-Pliocene lavas, distinctively poor in K 2 O and rich in MgO, are called low-K olivine tholeiites and have a different mantle source region than that of the older subduction-related lavas of the main Warner Range. Hays Canyon Range (max. elev. 2400 m) lies to the east of the Wamer Range, and the broad Surprise Valley separates the two fault-bounded ranges. Middle Miocene (ca. 15 Ma) basic lavas, with a small easterly dip, cap the Hays Canyon Range and overlie Oligocene silicic ash-flow deposits and a basaltic andesite spatter volcano. Middle Miocene basic lavas also form the crest of the Warner Range and its westerly dip slope (-15°). Nearly horizontal basic lavas of the same age are also found on both sides of the Warner Range, and it is a plausible conclusion that these middle Miocene basalts were a contiguous group before faulting and uplift of the Warner Range. Derived estimates of uplift rates (∼1 mm/ yr) of the Warner Range indicate that uplift could have been initiated at ca. 4 Ma, a period of the most voluminous eruption of low-K olivine tholeiite lavas. If the slower Cretaceous exhumation rate of the Sierra Nevada (0.5-1.0 mm/yr) is applied to the total offset of the Warner Range (4270 m), and it did not vary with time, then the uplift of the Warner Range was initiated at ca. 8 Ma, which coincides with the age of the oldest low-K olivine tholeiite lava (8 Ma). Low-K olivine tholeiites require a hot shallow asthenospheric source, and it is the rise of this hot mantle that is presumed to have caused the uplift of the Warner Range. Whether or not the widespread eruption of small volumes of Pliocene low-K olivine tholeiites in central and eastern Oregon is associated with crustal uplift is unknown.

Geologic constraints on the existence and distribution of West Antarctic subglacial volcanism

Abstract: [1] Strong and abundant magnetic anomalies imaged beneath the West Antarctic Ice Sheet by aerogeophysical surveys have been interpreted as evidence of widespread Late Cenozoic basaltic volcanism, perhaps even a large igneous province. Petrological and geochemical composition of subglacial sediment samples from West Antarctica supports crustal provenance and does not provide positive evidence for the existence of the proposed mafic Late Cenozoic large igneous province. The only two identified basaltic pebbles, out of a total of >500 examined pebbles, are of Mesozoic to Cambrian age. We conjecture that the subglacial large igneous province does not exist or is blanketed by an at least Miocene age sedimentary drape.

Asa Issie, Aramis and the origin of

Abstract: The origin of Australopithecus, the genus widely interpreted as ancestral to Homo, is a central problem in humanevolutionary studies. Australopithecus species differ markedly from extant African apes and candidate ancestral hominidssuch as Ardipithecus, Orrorin and Sahelanthropus. The earliest described Australopithecus species is Au. anamensis, theprobable chronospecies ancestor of Au. afarensis. Here we describe newly discovered fossils from the Middle Awashstudy area that extend the known Au. anamensis range into northeastern Ethiopia. The new fossils are fromchronometrically controlled stratigraphic sequences and date to about 4.1–4.2million years ago. They include diagnosticcraniodental remains, the largest hominid canine yet recovered, and the earliest Australopithecus femur. These newfossils are sampled from a woodland context. Temporal and anatomical intermediacy between Ar. ramidus andAu. afarensis suggest a relatively rapid shift from Ardipithecus to Australopithecus in this region of Africa, involving eitherreplacement or accelerated phyletic evolution.

The record of Miocene impacts in the Argentine pampas. Meteorit Planet Sci

Abstract: Argentine Pampean sediments represent a nearly continuous record of deposition since the late Miocene (~10 Ma). Previous studies described five localized concentrations of vesicular impact glasses from the Holocene to late Pliocene. Two more occurrences from the late Miocene are reported here: one near Chasico (CH) with an 40Ar/39Ar age of 9.24 ± 0.09 Ma, and the other near Baha Blanca (BB) with an age of 5.28 ± 0.04 Ma. In contrast with andesitic and dacitic impact glasses from other localities in the Pampas, the CH and BB glasses are more mafic. They also exhibit higher degrees of melting with relatively few xenoycrysts but extensive quench crystals. In addition to evidence for extreme heating (>1700 °C), shock features are observed (e.g., planar deformation features [PDFs] and diaplectic quartz and feldspar) in impact glasses from both deposits. Geochemical analyses reveal unusually high levels of Ba (~7700 ppm) in some samples, which is consistent with an interpretation that these impacts excavated marine sequences known to be at depth. These two new impact glass occurrences raise to seven the number of late Cenozoic impacts for which there is evidence preserved in the Pampean sediments. This seemingly high number of significant impacts over a 10^6 km^2 area in a time span of 10 Myr is consistent with the number of bolides larger than 100 m expected to enter the atmosphere but is contrary to calculated survival rates following atmospheric disruption. The Pampean record suggests, therefore, that either atmospheric entry models need to be reconsidered or that the Earth has received an enhanced flux of impactors during portions of the late Cenozoic. Evidence for the resulting collisions may be best preserved and revealed in rare dissected regions of continuous, low-energy deposition such as the Pampas. Additionally, the rare earth element (REE) concentrations of the target sediments and impact melts associated with the Chasic event resemble the HNa/K australites of similar age. This suggests the possibility that those enigmatic tektites could have originated as high-angle, distal ejecta from an impact in Argentina, thereby accounting for their rarity and notable chemical and physical differences from other Australasian impact glasses.