Brent D. Turrin

Intercalibration of astronomical and radioisotopic time

The 40Ar/39Ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in Earths history, but the accuracy of this method is limited by the accuracy with which the ages of neutron-fluence monitors (dating standards) are known. Calibrating the ages of standards by conventional means has proved difficult and contentious. The emerging astronomically calibrated geomagnetic polarity time scale (APTS) offers a means to calibrate the ages of 40Ar/39Ar dating standards that is independent of absolute isotopic abundance measurements. Seven published 40Ar/39Ar dates for polarity transitions, nominally ranging from 0.78 to 3.40 Ma, are based on the Fish Canyon sanidine standard and can be compared with APTS predictions. Solving the 40Ar/39Ar age equation for the age of the Fish Canyon sanidine that produces coincidence with the APTS age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27.78 to 28.09 Ma, with an inverse variance-weighted mean of 27.95 ± 0.18 Ma. Normalized residuals are minimized at an age of 27.92 Ma, indicating the robustness of the solution.

Late Cenozoic landscape evolution on lava flow surfaces of the Cima volcanic field, Mojave Desert, California

Landscape evolution in the eastern Mojave Desert is recorded by systematic changes in Pliocene to latest Pleistocene volcanic land-forms that show discrete periods of eolian deposition, surface stabilization, drainage-network expansion, and erosion on basaltic lava flows. These processes are documented by K-Ar dating in conjunction with morphometric, sedimentologic, pedologic, and geophysical studies. Lava-flow surfaces are composed of constructional bedrock highs and accretionary eolian mantles with overlying stone pavements. The stratigraphy of these mantles records episodic, climatically induced influxes of eolian fines derived from playa floors and distal piedmont regions. The relative proportions of mantle and exposed bedrock vary with flow age, and flows between 0.25 and 0.75 m.y. old support the most extensive eolian mantle and pavement reflecting landscape stability. Drainage networks evolve on flows by (1) rapid initial extension, (2) maximum extension and elaboration, and (3) abstraction of drainage. Increases in bedrock exposures and erosion of the eolian mantle on flows >0.70 m.y. old coincide with maximum drainage extension and significant changes in soil and hydrologic properties within this mantle. Increasing the content of pedogenic clay and CaCO 3 causes the accretionary mantle9s permeability to decrease; decreased mantle permeability promotes increased runoff, surface erosion, and drainage development. In the late Cenozoic landscape evolution of lava flows, four major stages reflect variations in landscape stability that are controlled by the impact of episodic influxes of eolian fines and increasing soil-profile development on infiltration-runoff properties of the flow surfaces.

High-3He plume origin and temporal-spatial evolution of the Siberian flood basalts

An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a 40Ar/39Ar plateau age of 253.3 � 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed 3He/4He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-3He SFBP plume with a suboceanic-type upper mantle beneath Siberia.

Element fluxes from the volcanic front of Nicaragua and Costa Rica

10 10 kg/m/Myr) and central Costa Rica (2.4 � 10 10 kg/m/Myr) is greatly reduced from previous estimates and now within the range of error estimates. We estimate the subducted component of flux for Cs, Rb, Ba, Th, U, K, La, Pb, and Sr by subtracting estimated mantle-derived contributions from the total element flux. An incompatible element-rich OIB source for the Cordillera Central segment in Costa Rica makes the subducted element flux there highly sensitive to small changes in the modeled mantle-derived contribution. For the other three segments studied, the estimated errors in concentrations of highly enriched, subductionderived elements (Cs, Ba, K, and Pb) are less than 26%. Averaged over the time of the current episode of volcanism, the subduction-derived fluxes of Cs, Ba, K, Pb, and Sr are not significantly different among the four segments of the Central American volcanic front in Nicaragua and Costa Rica. The subductionderived fluxes of Th and La appear to increase to the SE across Nicaragua and Costa Rica, but the estimated errors in their subduction-derived concentrations are very high, making this variation questionable. The lack of change in the fluxes of Cs, Ba, K, Pb, and Sr argues that the well-defined regional variation in Ba/La is the result of changes in the mode or mechanics of fluid delivery into the mantle wedge, not the total amounts of fluids released from the slab. Concentrated or focused fluids in Nicaragua lead to high degrees of melting. Diffuse fluids in Costa Rica cause lower degrees of melting. Components: 12,742 words, 11 figures, 5 tables.

The rift-to-drift transition in the North Atlantic: A stuttering start of the MORB machine?

We report U-Pb and 39Ar-40Ar measurements on plutonic rocks recovered from the Ocean Drilling Program (ODP) Legs 173 and 210. Drilling revealed continental crust (Sites 1067 and 1069) and exhumed mantle (Sites 1070 and 1068) along the Iberia margin and exhumed mantle (Site 1277) on the conjugate Newfoundland margin. Our data record a complex igneous and thermal history related to the transition from rifting to seafloor spreading. The results show that the rift-to-drift transition is marked by a stuttering start of MORB-type magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by a low degree of decompression melting due to tectonic delocalization of deformation. Such “off-axis” magmatism might be a common process in (ultra-) slow oceanic spreading systems, where “magmatic” and “tectonic” spreading varies in both space and time.

40Ar/39Ar ages from the rhyolite of Alder Creek, California: Age of the Cobb Mountain Normal-Polarity Subchron revisited

New 40 Ar/ 39 Ar age determinations on sanidine from the rhyolite of Alder Creek, California, indicate a 1.186 ±0.006 Ma age for the Cobb Mountain Normal-Polarity Subchron. The new age is statistically older (α = 0.05) than the previously reported K-Ar age (1.12 ±0.02 Ma) and agrees with the age suggested by the astronomical polarity time scale. Incomplete extraction of radiogenic 40 Ar ( 40 Ar*) from the sanidine is the most likely reason for the disparity between the 40 Ar/ 39 Ar and K-Ar ages. Because the Cobb Mountain subchron is a worldwide, short-duration event, and because no widely used interlaboratory 40 Ar/ 39 Ar standard younger than 27 Ma exists, we propose that sanidine from the rhyolite of Alder Creek be considered for use as a new Quaternary 40 Ar/ 39 Ar mineral standard.

The geology of Damavand volcano, Alborz Mountains, northern Iran

Damavand volcano, located in northern Iran, is a large (>400 km^3) composite cone that is currently dormant; it shows fumarolic activity near the summit but no evidence of eruption in the past 1000 yr. The volcano represents an isolated focus of magmatism of uncertain tectonic affinity, although geophysical and geochemical constraints point toward a local hotspot/plume origin, possibly associated with lithospheric delamination, rather than any association with subduction. New (U-Th)/He and ^(40)Ar/^(39)Ar geochronological constraints indicate that the present cone (Young Damavand) has been constructed over ∼600 k.y. on an older, eroded edifice of indistinguishable composition (younger than 1.8 Ma). Damavand activity has been characterized by the eruption of radially directed trachyandesite lava flows, almost exclusively from summit vents. Limited pyroclastic activity has yielded thin fallout pumice lapilli layers and a few pyroclastic flows. Only one significant pyroclastic event is recognized in the remnants of a welded ignimbrite, ponded and preserved along the Haraz River drainage. Relatively short periods of volcanic eruptive activity were interspersed with longer periods of erosion in which volcanic products were transported, particularly as hyperconcentrated flows, into the surrounding drainage systems to be further reworked into epiclastic deposits. Occasional catastrophic events punctuated this interplay between volcanism and erosion. At least one sector collapse is signified by the presence of a large debris avalanche deposit, and the regional drainage systems appear to have been frequently dammed by incursions of volcanic material.

Laschamp Excursion at Mono Lake

The Laschamp Geomagnetic Excursion (ca. 41 ka) and a related increase of cosmogenic nuclides provides a global tie point among sedimentary and ice core records. In the Wilson Creek Formation, Mono Lake, California, the Laschamp Excursion has not been recognized although the so-called Mono Lake excursion was found in the section with an estimated age of about 28 14 C ka. However, our reevaluation of the age of the Mono Lake excursion at its type locality using new 14 C dates on carbonates and 40 Ar/ 39 Ar sanidine dates on ash layers yields an estimate of 38^41 ka. This chronology and the absence of a second excursion in the Wilson Creek Formation suggest that the distinct paleomagnetic feature with negative inclinations at Mono Lake is correlative with the Laschamp Excursion. fl 2002 Elsevier Science B.V. All rights reserved.

Degradation of Quaternary cinder cones in the Cima volcanic field, Mojave Desert, California

Basaltic cinder cones in the Cima volcanic field record a detailed history of progressive erosion in the arid environment of the eastern Mojave Desert. These cones range in age from ∼0.015 m.y. to 1.09±0.08 m.y., as dated by radiocarbon or K-Ar analyses of the youngest lava flows from each cone. Cone heights range from 50 to 155 m, and basal widths range from 400 to 915 m; on younger cones, height/width ratios average 0.17, and crater-width/cone-width ratios average 0.42. The degradational morphology of these cones displays several trends that are closely related to cone age. (1) Crater-width/cone-width ratios decrease from 0.48 on the youngest cone to 0.21 on the oldest cone with a preserved crater. (2) Mean maximum side slopes (Tan Sc) decrease from 0.575 on the youngest cone to an average of 0.41 on the oldest cones studied. (3) Debris-apron–height/cone-height ratios increase from <0.10 on the youngest cone to an average of 0.34 on the oldest cones. (4) Cone drainage evolves from irregularly spaced rills and gullies on the youngest cone to regularly spaced gullies on 0.20- to 0.35-m.y.-old cones to valleys as much as 110 m wide and 10 m deep on cones 0.59 m.y. old and older. These trends form the basis of an empirical model of cinder-cone degradation in arid environments. This model documents (1) an erosional loss of ∼15% of cone volume during the first million years; (2) progressive decline of cone slope (at an average rate of 0.006°/103 yr) and cone height (at an average rate of 2.25 cm/103 yr); (3) initial rapid stripping of the loose cinder mantle from upper-cone slopes accompanied by rapid debris-apron formation; and (4) a gradual transition, between 0.25 and 0.6 m.y., from relatively uniform stripping of upper slopes to localized fluvial dissection of both cone slopes and debris apron. This transition is apparently controlled by a concomitant change from diffuse subsurface drainage within the pervious cinder mantle to concentrated surface flow across the heterogeneous assemblage of agglutinate layers, dikes, and ponded flows of the cone interior.

K-Ar dating of the Cima volcanic field, eastern Mojave Desert, California: Late Cenozoic volcanic history and landscape evolution

Thirty-four K-Ar ages and supporting paleomagnetic measurements from flows of the Cima volcanic field provide a detailed volcanic history and a temporal basis for analysis of evolving landforms. The Cima field has undergone three periods of volcanic activity, spanning late Miocene through latest Pleistocene time: (1) 7.6 to 6.5, (2) 4.5 to 3.6, and (3) 1.0 to at least 0.015 m.y. ago. These precisely dated sequences afford a unique opportunity to quantify the long-term effects of pedogenesis and erosion on volcanic landforms in an arid environment. Morphologic and pedologtc data indicate that the most stable geomorphic surfaces in the field occur on flows between 0.25 and 0.75 m.y. old. Younger flows are dominated by eolian aggradation and outcrop rubbling, and older flows are dominated by surface runoff and fluvial dissection. A process-response model involving progressive pedogenesis and a subsequent shift from infiltration to surface runoff is proposed to explain this temporal variation in land-surface stability.