Calibration of the interlaboratory 40Ar39Ar dating standard, MMhb-1

Samson, Scott D.; Alexander, E. Calvin

doi:10.1016/0168-9622(87)90025-x

Cited by 468 publications

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“…Vertical and horizontal variations in neutron flux in the irradiated package were monitored by 8 to 12 standards distributed along the length of each vial; the geometry of the package was such that each unknown sample was adjacent to at least one standard. The flux monitor used was hornblende MMHb-1, which has an assigned K-Ar age of 520.4 + 1.7 Ma [Samson and Alexander, 1987]. Corrections for reactor-produced interfering reactions were made using argon isotopes of K, SO, and CaF• irradiated in each package.…”

Section: Resultsmentioning

confidence: 99%

Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

Harlan¹,

Geissman²

1998

J. Geophys. Res.

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Section: Resultsmentioning

confidence: 99%

Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

Harlan¹,

Geissman²

1998

J. Geophys. Res.

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“…[ [Cebula et al, 1986] and McClure Mountain (MMhb-1) hornblende (520.4 Ma) [Samson and Alexander, 1987]. Monitors were analyzed by total fusion of 1 -5 crystals with an Ar-ion laser.…”

Section: Discussionmentioning

confidence: 99%

Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from⁴⁰Ar/³⁹Ar and (U‐Th)/He thermochronology

et al. 2002

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[1] High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperaturetime paths inferred from 40 Ar/ 39 Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar 40 Ar release spectra, and (U-Th)/He thermochronology of zircon and apatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling (<1°C/m.y.) from Jurassic times until the late Miocene or early Pliocene. The regional extent and consistency of thermal histories during this time period suggest the presence of a stable thermal structure and imply that regional denudation rates were low (<0.1 mm/yr for nominal continental geotherms). Beginning in the late Miocene or early Pliocene, these samples experienced a pronounced cooling event (>30°-50°C/m.y.) coincident with exhumation from inferred depths of $8 -10 km, at denudation rates of 1 -2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period ($3°C/m.y.), suggesting limited exhumation (1 -2 km). However, these samples record a slight increase in cooling rate (from <1 to $3°C/m.y.) at some time during the middle Tertiary; the tectonic significance of this change remains uncertain. Regardless, late Cenozoic denudation in this region appears to have been markedly heterogeneous, with the highest rates of exhumation focused at the topographic front of the plateau margin. We infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable Sichuan Basin and thus marks the onset of deformation related to the development of the Tibetan Plateau in this region. The present margin of the plateau adjacent to and north of the Sichuan Basin is apparently no older than the late Miocene or early Pliocene ($5 -12 Ma).

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“…Corrections for radiation-produced interfering isotopes of argon were made by applying measured production ratios for the interfering isotopes of argon produced in pure CaF 2 and K 2 SO 4 irradiated along with the samples (Appendix I). Neutron flux during irradiation was monitored using hornblende standard MMhb-1, which has a K-Ar age of 520.4 ± 1.7 Ma (Samson and Alexander, 1987).…”

Section: Methodsmentioning

confidence: 99%

Geochronology and geology of late Oligocene through Miocene volcanism and mineralization in the western San Juan Mountains, Colorado

Bove¹,

Hon²,

Budding³

et al. 2000

Open-File Report

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Twenty-five new 40 Ar/ 39 Ar ages from volcanic rocks and veins in the western San JuanMountains clarify relationships between volcanism and mineralization in this classic area. Five calc-alkaline ash-flow sheets erupted from caldera sources (Ute Ridge, Blue Mesa, Dillon Mesa, Sapinero Mesa, and Crystal Lake Tuffs) from 28.6 to 27.6 Ma. This is a much more restricted time interval than previously thought and indicates that the underlying batholith rose and evolved very rapidly beneath the western San Juan Mountains. The new ages and geologic relations constrain the timing of joint resurgence of the Uncompahgre and San Juan calderas to between 28.2 and 27.6 Ma. The collapse of the Silverton caldera produced a set of strong ring fractures that intersected with graben faults on the earlier resurgent dome to produce the complex set of structures that localized the mid-Miocene epithermal gold veins.Later calc-alkaline monzonitic to quartz monzontic plutons solidified at 26.5-26.0 Ma as the underlying batholith rose through its volcanic cover. A new age from lavas near Uncompahgre Peak supports earlier interpretations that these lavas were fed by nearby 26 Ma monzonite intrusions. Nearly all of these intrusions are associated with subeconomic Mo and Cu mineralization and associated alteration, and new ages of 26.40 and 25.29 Ma from the Ute-Ulay and Lilly veins in the Lake City region show that some of the most important silver and 2 base-metal veins were temporally and poss ibly genetically connected to these plutons. In addition, the Golden Fleece telluride vein cuts all of the post-Uncompahgre caldera volcanics in the area and is probably temporally related to this cycle, though its age of 27.5 ± 0.3 Ma was determined by less precise U/Pb methods.The 22.9 Ma Lake City caldera collapsed within the older Uncompahgre caldera structure but is petrologically unrelated to the older calc-alkaline activity. The distinctive suite of high-silica rhyolite tuff and alkaline resurgent intrusions indicates that it is closely related to the early stages of bimodal high-silica rhyolite-alkali basalt volcanism that accompanied the onset of extensional tectonism in the region. Both 40 Ar/ 39 Ar ages and paleomagnetic data confirm that the entire caldera sequence formed in less than 330,000 years. Only weak quartz vein mineralization is present in the center of the caldera, and it appears to be related to leaching of metals from the intracaldera tuffs above the resurgent intrusion. Massive alunitization and weak Mo and Cu mineralization along the eastern ring fracture are associated with calc-alkaline lavas and stocks related to late stages of the caldera cycle. These calc-alkaline stocks also appear to be genetically and temporally linked to a radial pattern of barite-precious metal veins on the northeastern margin of the Lake City caldera.

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Calibration of the interlaboratory 40Ar39Ar dating standard, MMhb-1

Cited by 468 publications

References 5 publications

Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from⁴⁰Ar/³⁹Ar and (U‐Th)/He thermochronology

Geochronology and geology of late Oligocene through Miocene volcanism and mineralization in the western San Juan Mountains, Colorado

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Calibration of the interlaboratory 40Ar39Ar dating standard, MMhb-1

Cited by 468 publications

References 5 publications

Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from40Ar/39Ar and (U‐Th)/He thermochronology

Geochronology and geology of late Oligocene through Miocene volcanism and mineralization in the western San Juan Mountains, Colorado

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Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from⁴⁰Ar/³⁹Ar and (U‐Th)/He thermochronology