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L.W. Snee

Publications and source records attributed to L.W. Snee.

At least 37 records · Page 2Linked to original sources
USGS1999

Chronology of polyphase extension in the Windermere Hills, northeast Nevada

Fission-track and 40 Ar/ 39 Ar dating and chemical correlation of volcanic strata exposed in the Windermere Hills and northern Pequop Mountains, northeast Nevada, indicate a protracted, polyphase history of Tertiary (late Eocene–late Miocene) extension along the northern margin of a major Cordilleran metamorphic core complex. Early extension is recorded by a west-tilted half graben filled with early Oligocene (34.79 ± 0.18–39.18 ± 0.12 Ma) sedimentary rocks in the eastern Windermere Hills above the low-angle Black Mountain detachment fault. The early Oligocene half graben conformably overlies a widespread suite of late Eocene (39.18 ± 0.12–40.38 ± 0.06 Ma) calc-alkaline volcanic rocks, reflecting a temporal link between early extension at a high structural level and the end of the ignimbrite flare-up. These strata are cut by east-west–striking normal faults, which are exposed along, and parallel to, the northern margin of the metamorphic complex. Available age data (e.g., between 14.93 ± 0.08 and 34.79 ± 0.18 Ma) permit the interpretation that the east-west–striking faults formed at the same time as, or after, large-magnitude unroofing of high-grade rocks. We interpret the east-west–striking faults to accommodate differential uplift of greenschist-grade metamorphic rocks in the upper crust, above a lateral ramp in a west-northwest–directed mylonitic shear zone. Subsequent extension in the Windermere Hills is defined by deep, rapidly filled half grabens of middle Miocene (<7.42 ± 2.0 to 14.93 ± 0.08 Ma) age that unconformably overlie older faults and synextensional deposits. These are the youngest half grabens in the region and are inferred to be initiated by extensional stresses imparted to the base of the lithosphere by a laterally spreading mantle plume (e.g., the Yellowstone hotspot) located in southeastern Oregon at this time.

Nevada↗
USGS1998

Geochemical and geochronological constraints on the genesis of Au-Te deposits at Cripple Creek, Colorado

The Cripple Creek district (653 metric tons (t) of Au) consists of Au-Te veins and disseminated gold deposits that are spatially related to alkaline igneous rocks in an Oligocene intrusive complex. Vein paragenesis includes quartz-biotite-K feldspar-fluorite-pyrite followed by base metal sulfides and telluride minerals. Disseminated deposits consist of microcrystalline native gold with pyrite that are associated with zones of pervasive adularia.New 40 Ar/ 39 Ar dates indicate that there was a complex magmatic and hydrothermal history. Relatively felsic rocks (tephriphonolite, trachyandesite, and phonolite) were emplaced into the complex over about 1 m.y., from 32.5 + or - 0.1 (1Sigma ) to 31.5 + or - 0.1 Ma. A younger episode of phonolite emplacement outside of the complex is indicated by an age of 30.9 + or - 0.1 Ma. Field relationships suggest that at least one episode of mafic and ultramafic dike emplacement occurred after relatively more felsic rocks and prior to the main gold mineralizing event. Only a single whole-rock date for mafic phonolite (which indicated a maximum age of 28.7 Ma) was obtained. However, constraints on the timing of mineralization are provided by paragenetically early vein minerals and K feldspar from the disseminated gold pyrite deposits. Early vein minerals (31.3 + or - 0.1-29.6 + or - 0.1 Ma) and K feldspar (29.8 + or - 0.1 Ma) from the Cresson disseminated deposit, together with potassically altered phonolite adjacent to the Pharmacist vein (28.8 and 28.2 + 0.1 Ma), suggest there was a protracted history of hydrothermal activity that began during the waning stages of phonolite and early mafic-ultramafic activity and continued, perhaps intermittently, for at least 2 m.y.Estimated whole-rock delta 18 O values of the alkaline igneous rocks range from 6.4 to 8.2 per mil. K feldspar and albite separates from igneous rocks have lead isotope compositions of 206 Pb/ 204 Pb = 17.90 to 18.10, 207 Pb/ 204 Pb = 15.51 to 15.53, and 208 Pb/ 204 Pb = 38.35 to 38.56. These isotopic compositions, together with major and trace element data, indicate that the phonolitic magmas probably evolved by fractional crystallization of an alkali basalt that assimilated lower crustal material. Upper crustal contamination of the magmas was not significant The 206 Pb/ 204 Pb compositions of vein galenas almost entirely overlap those of phonolites suggesting a genetic relationship between alkaline magmatism and mineralization. However, a trend toward higher 207 Pb/ 204 Pb (15.57-15.60) and a 208 Pb/ 204 Pb ratios (38.94-39.48) of some galenas suggests a contribution to the ore fluid from surrounding Early Proterozoic rocks, probably through leaching by mineralizing fluids. Limited stable isotope compositions of quartz, K feldspar, and biotite from this and previous studies support a largely magmatic origin for the early vein fluids.It is suggested that three features were collectively responsible for generating alkaline magmas and associated mineral deposits: (1) the timing of magmatism and mineralization, which coincided with the transition between subduction-related compression and extension related to continental rifting; (2) the location of Cripple Creek at the junction of four major Precambrian units and at the intersection of major northeast-trending regional structures with northwest-trending faults, which served as conduits for magmas and subsequent hydrothermal fluids; and (3) the complex magmatic history which included emplacement of relatively felsic magmas followed by successively more mafic magmas with time.

Economic Geology↗
USGS1998

Geology of the Sierra de Fiambala, northwestern Argentina: Implications for Early Palaeozoic Andean tectonics

Field mapping in conjunction with structural, metamorphic, and geochronological data document the tectono-thermal history of exhumed deep crustal rocks in the Sierra de Fiambalá, NW Argentina. The range consists of two structural blocks distinguished by different metasedimentary sequences and different grades of metamorphism. Orthogneiss and paragneiss in the northern structural block may have a Precambrian history. Greenschist- to amphibolite-facies metamorphism, intrusion, and injection migmatization affected all rocks at 540–550 Ma. A subsequent event in the Late Cambrian to Ordovician ( c. 515 to 470 Ma) involved amphibolite- to granulite-facies metamorphism, mafic intrusion, and deformation, followed by cooling through mid-Palaeozoic time. The emplacement of Carboniferous (325–350 Ma) post-tectonic granites caused reheating and retrogression that was strongest toward the northeast part of the range. The Cambrian, Ordovician, and Carboniferous events in the Sierra de Fiambalá were of regional extent as indicated by temporal correlations with events reported for other deep crustal rocks of the northern Sierras Pampeanas. Correlations between periods of intrusion and high-grade metamorphism in the northern Sierras Pampeanas and volcanic-sedimentary events in the adjacent supracrustal exposures confirm that rocks in the northern Sierras Pampeanas formed at deep (10–25 km) structural levels in the early Palaeozoic continental margin of Gondwana.

Geological Society Special Publication↗
USGS1998

Paleomagnetism of the Miocene intrusive suite of Kidd Creek: Timing of deformation in the Cascade arc, southern Washington

Paleomagnetic study of the intrusive suite of Kidd Creek in the southern Washington Cascades (23 sites in dikes and sills) was undertaken to help determine if these rocks are comagmatic and whether they postdate regional folding of the volcanic arc. Fission track and 40 Ar- 39 Ar age determinations indicate an age of ∼12.7 Ma (middle Miocene) for these rocks. The similarity of normal-polarity characteristic directions for most samples corroborate the available geochemical data indicating that these rocks are most likely comagmatic. Reversed-polarity directions for samples from four sites, however, show that emplacement of Kidd Creek intrusions spanned at least one reversal of the geomagnetic field. The paleomagnetic directions for the dikes and sills fail a fold test at the 99% confidence level indicating that the Kidd Creek rocks postdate regional folding. The mean in situ direction also indicates that the Kidd Creek and older rocks have been rotated 22°±6° clockwise about a vertical or near-vertical axis from the expected Miocene direction. Compression and regional folding of the Cascade arc in southern Washington therefore had ended by ∼12 Ma prior to the onset of deformation resulting in rotation of these rocks.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Evidence for Proterozoic and late Cretaceous-early Tertiary ore-forming events in the Coeur d'Alene district, Idaho and Montana

New 40 Ar/ 39 Ar age spectra on sericite and lead isotope data on tetrahedrite, siderite, galena, bournonite, and stibnite, together with previously published isotopic, geochemical, and geologic studies provide evidence for two major vein-forming events in the Coeur d'Alene district and surrounding areas of the Belt basin. The data suggest that the zinc- and lead-rich veins (e.g., Bunker Hill and Star-Morning mines) formed in the Proterozoic (1.0 Ga), whereas the silver-rich veins (e.g., Silver belt mines), antimony veins (e.g., U.S. Antimony mine), and gold-bearing quartz veins (Murry subdistrict) formed in Late Cretaceous to early Tetitary time.

Economic Geology↗
USGS1998

Geological setting and petrogenesis of symmetrically zoned, miarolitic granitic pegmatites at Stak Nala, Nanga Parbat - Haramosh Massif, northern Pakistan

Miarolitic granitic pegmatites in the Stak valley in the northeast part of the Nanga Parbat - Haramosh Massif, in northern Pakistan, locally contain economic quantities of bi- and tricolored tourmaline. The pegmatites form flat-lying sills that range from less than 1 m to more than 3 m thick and show symmetrical internal zonation. A narrow outer or border zone of medium-to coarse-grained oligoclase - K-feldspar - quartz grades inward to a very coarse-grained wall zone characterized by K-feldspar - oligoclase - quartz - schorl tourmaline. Radiating sprays of schorl and flaring megacrysts of K-feldspar (intermediate microcline) point inward, indicating progressive crystallization toward the core. The core zone consists of variable mixtures of blocky K-feldspar (intermediate microcline), oligoclase, quartz, and sparse schorl or elbaite, with local bodies of sodic aplite and miarolitic cavities or "pockets". Minor spessartine-almandine garnet and lo??llingite are disseminated throughout the pegmatite, but were not observed in the pockets. The pockets contain well-formed crystals of albite, quartz, K-feldspar (maximum microcline ?? orthoclase overgrowths), schorl-elbaite tourmaline, muscovite or lepidolite, topaz, and small amounts of other minerals. Elbaite is color-zoned from core to rim: green (Fe2+- and Mn2+-bearing), colorless (Mn2+-bearing), and light pink (trace Mn3+). Within ???10 cm of the pegmatites, the granitic gneiss wallrock is bleached owing to conversion of biotite to muscovite, with local quartz and albite added. Schorl is disseminated through the altered gneiss, and veins of schorl with bleached selvages locally traverse the wallrock up to 1 m from the pegmatite contact. The schorl veins can be traced into the outer part of the wall zone, which suggests that they formed from aqueous fluids derived during early saturation of the pegmatite-forming leucogranitic magma rich in H2O, F, B, and Li. Progressive crystallization resulted in a late-stage sodic magma and abundant aqueous fluids. Two late stages of volatile escape are recognized: the first stage caused pressure-quenching of the last magma, which produced aplite and caused albitization (An3 to An8) of earlier crystallized K-feldspar and oligoclase. The second stage, released during the rupture of miarolitic cavities, produced platy albite ("cleavelandite," An1) locally associated with F-rich moscovite and elbaite. Albitization is likely due to cooling of alkali-fluoride-dominated fluids at less than 2 kbar pressure. The pegmatites are derived from Himalayan leucogranitic magma emplaced prior to 5 Ma into granulitic gneiss that was at 300?? to 550??C and 1.5 to 2 kbar. The pegmatites were emplaced during uplift of the Haramosh Massif, since they cross-cut ductile normal faults but are cut by brittle normal faults. Economically important pink tourmaline mineralization formed in pockets concentrated near the crest of a broad antiform, as a result of trapping of late magmatic aqueous fluids that had become Fe-poor owing to the prior crystallization of schorl.

Canadian Mineralogist↗
USGS1997

Rapid extension in an Eocene volcanic arc: Structure and paleogeography of an intra-arc half graben in central Idaho

A study of extension, volcanism, and sedimentation in the middle Eocene Panther Creek half graben in central Idaho shows that it formed rapidly during an episode of voluminous volcanism. The east-southeast-tilted Panther Creek half graben developed across the northeast edge of the largest cauldron complex of the Challis volcanic field and along the northeast-trending Trans-Challis fault zone. Two normal fault systems bound the east side of the half graben. One fault system strikes northeast, parallel to the Trans-Challis fault zone, and the other strikes north to northwest. The geometry of the basin-fill deposits shows that movement on these two normal fault systems was synchronous and that both faults controlled the development of the Panther Creek half graben. Strikes of the synextension volcanic and sedimentary rocks are similar throughout the half graben, whereas dips decrease incrementally upsection from as much as 60?? to less than 10??. Previous K-Ar dates and a new 40Ar/39Ar plateau date from the youngest widespread tuff in the basin suggest that most of basin formation spanned 3 m.y. between about 47.7 Ma and 44.5 Ma. As much as 6.5 km of volcanic and sedimentary rocks were deposited during that time. Although rates of extension and subsidence were very high, intense volcanic activity continually filled the basin with ash-flow tuffs, outpacing subsidence and sedimentation, until the end of basin development. After the abrupt end of Challis volcanism, locally derived pebble to boulder conglomerate and massive, reworked ash accumulated in the half graben. These sedimentary rocks make up a small part of the basin fill in the Panther Creek half graben and were derived mainly from Proterozoic metasedimentary rocks uplifted in the footwall of the basin. The east-southeast tilt of the sedimentary rocks, their provenance and coarse grain size, and the presence of a gravity slide block derived from tilted volcanic rocks in the hanging wall attest to continued tectonism during conglomerate deposition. Provenance data from the sedimentary rocks imply that the highland in the footwall of the Panther Creek half graben was never thickly blanketed by synex-tension volcanic rocks, despite intense volcanic activity. Analysis of the Panther Creek half graben and other intra-arc rift basins supports previous interpretations that relative rates of volcanism and subsidence control the proportion of volcanic rocks deposited in intra-arc rifts.

Idaho↗
USGS1996

Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagnetic and 40Ar/39Ar study

Paleomagnetic results from Early Proterozoic metabasite sills and Middle Proterozoic diabase dikes from the southern Highland Mountains of southwestern Montana give well-defined, dual-polarity magnetizations that are statistically identical to those from a small Late Cretaceous pluton that cuts the dikes. The concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the Proterozoic rocks were remagnetized, probably during Late Cretaceous time. Paleomagnetic, rock magnetic, and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low-Ti magnetite. Combining virtual geomagnetic poles from metabasite sills, diabase dikes, and the Late Cretaceous pluton, we obtain a paleomagnetic pole at 85.5°N, 310.7°E ( K = 19.9, A 95 = 9.1°, N = 14 sites) that is similar to a reference pole from the 74 Ma Adel Mountain Volcanics of western Montana. Biotite and hornblende 40 Ar/ 39 Ar isotopic dates from host basement gneiss and a hornblende from a remagnetized metabasite sill yield ages of ca. 1800 Ma; these dates probably record cooling of the southern Highland Mountains following high-grade metamorphism at 1.9–1.8 Ga. The gneiss and metabasite age spectra show virtually no evidence of disturbance, indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems. Thus, the overprint magnetization of the Highland Mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby Late Cretaceous plutons. Remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of Late Cretaceous hydrothermal fluids related to epithermal mineralization. The absence of significant disturbance to the 40 Ar/ 39 Ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may go unrecognized and undated, even if 40 Ar/ 39 Ar dating is applied.

Montana↗
USGS1996

The Border Ranges fault system in Glacier Bay National Park, Alaska: Evidence for major early Cenozoic dextral strike-slip motion

The Border Ranges fault system of southern Alaska, the fundamental break between the arc basement and the forearc accretionary complex, is the boundary between the Peninsular–Alexander–Wrangellia terrane and the Chugach terrane. The fault system separates crystalline rocks of the Alexander terrane from metamorphic rocks of the Chugach terrane in Glacier Bay National Park. Mylonitic rocks in the zone record abundant evidence for dextral strike-slip motion along north-northwest-striking subvertical surfaces. Geochronologic data together with regional correlations of Chugach terrane rocks involved in the deformation constrain this movement between latest Cretaceous and Early Eocene (~50 Ma). These findings are in agreement with studies to the northwest and southeast along the Border Ranges fault system which show dextral strike-slip motion occurring between 58 and 50 Ma. Correlations between Glacier Bay plutons and rocks of similar ages elsewhere along the Border Ranges fault system suggest that as much as 700 km of dextral motion may have been accommodated by this structure. These observations are consistent with oblique convergence of the Kula plate during early Cenozoic and forearc slivering above an ancient subduction zone following late Mesozoic accretion of the Peninsular–Alexander–Wrangellia terrane to North America.

Alaska↗
USGS1996

40Ar/39Ar geochronology and paleomagnetism of Independence volcano, Absaroka volcanic supergroup, Beartooth mountains, Montana

Independence volcano is a major volcanic complex in the lower part of the Absaroka Volcanic Supergroup (AVS) of Montana and Wyoming. Recently reported Rb–Sr mineral dates from the complex give apparent ages of 91 and 84 Ma, whereas field relationships and the physical and compositional similarity of the rocks with other dated parts of the AVS indicate an Early to Middle Eocene age for eruption and deposition. To resolve the conflict between age assignments based on stratigraphic correlations and Rb–Sr dates, we report new paleomagnetic data and 40 Ar/ 39 Ar dates for Independence volcano. Paleomagnetic data for the stock and an andesite plug that cuts the stock are well grouped, of reverse polarity, and yield a virtual geomagnetic pole that is essentially identical to Late Cretaceous and Tertiary reference poles. The reverse polarity indicates that the magnetization of these rocks is probably younger than the Cretaceous normal superchron, or less than about 83.5 Ma. Hornblende from a volcanic breccia near the base of the volcanic pile gives a 40 Ar/ 39 Ar age of 51.57 Ma, whereas biotites from a dacite sill and a granodiorite stock that forms the core of the volcano give dates that range from 49.96 to 48.50 Ma. These dates record the age of eruption and intrusion of these rocks and clearly show that the age of Independence volcano is Early to Middle Eocene, consistent with stratigraphic relations. We suggest that the Rb–Sr mineral dates from the Independence stock and related intrusions are unreliable.

Montana, Wyoming↗
USGS1996

Emerald mineralization and metasomatism of amphibolite, khaltaro granitic pegmatite - Hydrothermal vein system, Haramosh Mountains, Northern Pakistan

Emerald mineralization is found within 0.1- to 1-m-thick hydrothermal veins and granitic pegmatites cutting amphibolite within the Nanga Parbat - Haramosh massif, in northern Pakistan. The amphibolite forms a sill-like body within garnet-mica schist, and both are part of a regional layered gneiss unit of Proterozoic (?) age. The 40Ar/39Ar data for muscovite from a pegmatite yield a plateau age of 9.13 ?? 0.04 Ma. Muscovite from mica schist and hornblende from amphibolite yield disturbed spectra with interpreted ages of 9 to 10 Ma and more than 225 Ma, respectively, which indicate that peak Tertiary metamorphism reached 325 to 550??C prior to 10 Ma. Pegmatites were emplaced after peak metamorphism during this interval and are older than pegmatites farther south in the massif. At Khaltaro, simply zoned albite-rich miarolitic pegmatites and hydrothermal veins containing various proportions of quartz, albite, tourmaline, muscovite, and beryl are associated with a 1- to 3-m-thick heterogeneous leucogranite sill, that is locally albitized. The pegmatites likely crystallized at 650 to 600??C at pressures of less than 2 kbar. Crystals of emerald form within thin (<30 cm) veins of quartz and tourmaline-albite, and more rarely in pegmatite, near the contacts with altered amphibolite. The emerald-green coloration is produced by Cr and Fe. The Cr and total Fe contents, expressed as Cr2O3 and Fe2O3, respectively, decrease systematically from emerald (>0.20, 0.54-0.89 wt%), to pale blue beryl (<0.07, 0.10-0.63%), to colorless beryl (<0.07, 0.07-0.28%). The amphibolite is metasomatized in less than 20-cm-wide selvages that are symmetrically zoned around veins or pegmatites. A sporadic inner zone containing F-rich biotite, tourmaline, and fluorite, with local albite, muscovite, quartz, and rare beryl, gives way to an intermediate zone containing biotite and fluorite with local plagioclase and quartz, and to an outer zone of amphibolite containing sparse biotite and local quartz. The inner and intermediate zones experienced gains of K, H, F, B, Li, Rb, Cs, Be, Ta, Nb, As, Y and Sr, and losses of Si, Mg, Ca, Fe, Cr, V and Sc. The outer alteration zone has gained F, Li, Rb, Cs, and As. Oxygen isotope analyses of igneous and hydrothermal minerals indicate that a single fluid of magmatic origin with ??18OH2O = 8??? produced the pegmatite-vein system and hydrothermal alteration at temperatures between 550 and 400??C. The formation of emerald results from introduction of HF-rich magmatic-hydrothermal fluids into the amphibolite, which caused hydrogen ion metasomatism and released Cr and Fe into the pegmatite-vein system.

Canadian Mineralogist↗
USGS1993

Thermochronology of the Cornubian batholith in southwest England: Implications for pluton emplacement and protracted hydrothermal mineralization

The metalliferous ore deposits of southwest England are associated with biotite-muscovite granites that intruded upper Paleozoic sediments and volcanic rocks at the end of the Hercynian Orogeny. The hydrothermal mineralization can be subdivided into four stages: 1. exoskarns 2. high-temperature tin and tungsten oxide-bearing sheeted greisen bordered veins and Sn-bearing tourmaline veins and breccias 3. polymetallic quartz-tourmaline-chlorite-sulfide-fluorite-bearing fissure veins, which represent the main episode of economic mineralization 4. late-stage, low-temperature polymetallic fluorite veins. U-Pb dating of monazite and xenotime and 40 Ar 39 Ar "> 40 Ar 39 Ar dating of muscovite were used to determine emplacement ages and cooling times for individual plutons within the Cornubian batholith, as well as separate intrusive phases within the plutons. In addition, 40 Ar 39 Ar "> 40 Ar 39 Ar ages from hornblende and secondary muscovite and Sm-Nd isochron ages from fluorite were employed to determine the relationship between pluton emplacement and different stages of mineralization. The U-Pb ages indicate that granite magmatism was protracted from ~300 Ma down to ~275 Ma with no evidence of a major hiatus. There is no systematic relation between the age of a pluton and its location within the batholith. The U-Pb ages for separate granite phases within a single pluton are resolvable and indicate that magma emplacement within individual plutons occurred over periods of as much as 4.5 myrs. Felsic porphyry dike emplacement was coeval with plutonism, but continued to ~270 Ma. The geochronologic data suggest that the Cornubian batholith originated from repeated melting events over 30 myrs and was formed by a series of small coalescing granitic bodies. Cooling rates of the main plutons are unrelated to emplacement age, but decrease from the southwest to the northeast from ~210°C myr −1 to ~60°C myr −1 with a mean of 100°C myr −1 . These slow cooling rates appear to reflect the addition of heat from multiple intrusive episodes. The mineralization history is distinct for each pluton and ranges from coeval with, to up to 40 myrs younger than the cooling age for the host pluton. Stage 2 mineralization is broadly synchronous with the emplacement of granite magmas, is dominated by fluids expelled during crystallization, and may be repeated by the emplacement of younger magmas within the same pluton. Sm-Nd isochrons for fluorite from stage 3 polymetallic mineralization give ages of 259 ± 7, 266 ± 3 and 267 ± 12 Ma, postdating stage 2 mineralization by up to 25 myrs within the same deposit. The similarity in age of the main polymetallic mineralization hosted by the oldest and youngest plutons, suggests that this stage of mineralization is unlikely to be related to hydrothermal circulation driven by the emplacement and cooling of the host granite. The mineralization is more likely the product of regional hydrothermal circulation driven by heat from the emplacement and crystallization of younger buried pulses of magma.

Cornubian Batholith↗
USGS1991

40Ar/39Ar systematics and argon diffusion in amber: Implications for ancient earth atmospheres

Argon isotope data indicate retained argon in bulk amber (matrix gas) is radiogenic [ 40 Ar/ 39 Ar ≃32o] than the much more abundant surface absorbed argon [ 40 Ar/ 39 Ar ≃295.5]. Neutron-induced 39 Ar is retained in amber during heating experiments to 150° -250°C, with no evidence of recoiled 39 Ar found after irradiation. A maximum permissible volume diffusion coefficient of argon in amber (at ambient temperature) D≤1.5 x 10 −17 cm 2 S −1 is calculated from 39 Ar retention. 40 Ar/ 39 Ar age calculations indicate Dominican Republic amber is ≃ 45 Ma and North Dakota amber is ≃ 89 Ma, both at least reasonable ages for the amber based upon stratigraphic and paleontological constraints and upon the small amount of radiogenic 40 Ar. To date, over 300 gas analyses of ambers and resins of Cretaceous to Recent age that are geographically distributed among fifteen noted world locations identify mixtures of gases in different sites within amber (Berner and Landis, 1988). The presence of multiple mixing trends between compositionally distinct end-members gases within the same sample and evidence for retained radiogenic argon within the amber argue persuasivley against rapid exchange by diffusion of amber-contained gases with moder air. Only gas in primary bubbles entrapped between successive flows of tree resin has been interpreted as original “ancient air”, which is an O 2 -rich end-member gas with air-like N 2 /Ar ratios. Gas analyses of these primary bubbles indicate atmospheric O 2 levels in the Late Cretaceous of ≃ 35%, and that atmospheric O 2 dropped by early Tertiary time to near a present atmospheric level of 21% O 2 . A very low argon diffusion coefficient in amber persuasively argues for a gas in primary bubbles trapped in amber being ancient air (possibly modified only by O 2 reaction with amber).

North Dakota↗