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Marvin A. Lanphere

Publications and source records attributed to Marvin A. Lanphere.

At least 37 records · Page 2Linked to original sources
USGS2002

Mesozoic thermal history and timing of structural events for the Yukon-Tanana Upland, east-central Alaska: 40Ar/39Ar data from metamorphic and plutonic rocks

We present new 40 Ar/ 39 Ar ages for hornblende, muscovite, and biotite from metamorphic and plutonic rocks from the Yukon–Tanana Upland, Alaska. Integration of our data with published 40 Ar/ 39 Ar, kinematic, and metamorphic pressure ( P ) and temperature ( T ) data confirms and refines the complex interaction of metamorphism and tectonism proposed for the region. The oldest metamorphic episode(s) postdates Middle Permian magmatism and predates the intrusion of Late Triassic (215–212 Ma) granitoids into the Fortymile River assemblage (Taylor Mountain assemblage of previous papers). In the eastern Eagle quadrangle, rapid and widespread Early Jurassic cooling is indicated by ~188–186 Ma 40 Ar/ 39 Ar plateau ages for hornblende from plutons that intrude the Fortymile River assemblage, and for metamorphic minerals from the Fortymile River assemblage and the structurally underlying Nasina assemblage. We interpret these Early Jurassic ages to represent cooling resulting from northwest-directed contraction that emplaced the Fortymile River assemblage onto the Nasina assemblage to the north as well as the Lake George assemblage to the south. This cooling was the final stage of a continuum of subduction-related contraction that produced crustal thickening, intermediate- to high- P metamorphism within both the Fortymile River assemblage and the structurally underlying Lake George assemblage, and Late Triassic and Early Jurassic plutonism in the Fortymile River and Nasina assemblages. Although a few metamorphic samples from the Lake George assemblage yield Jurassic 40 Ar/ 39 Ar cooling ages, most yield Early Cretaceous 40 Ar/ 39 Ar ages: hornblende ~135–115 Ma, and muscovite and biotite ~110–108 Ma. We interpret the Early Cretaceous metamorphic cooling, in most areas, to have resulted from regional extension and exhumation of the lower plate, previously tectonically thickened during Early Jurassic and older convergence.

Alaska↗
USGS2000

Duration of sedimentation of Creede Formation from 40 Ar/ 39 Ar ages

The Oligocene Creede Formation was deposited in the moat of the Creede caldera, which formed as a result of eruption of ythe Snowshoe Mountains Tuff. The Creede Formation in the two moat drill holes contains ash layers that are considered fallout tuffs derived from Fisher Dacite volcanoes that were erupting during accumulation of the Creede Formation. The duration of sedimentation of the Creede Formation could hnot be determinted directly by measuring the ages of the ash layers because 40 Ar/ 39 Ar ages of biotite from the asj layers do not stack in the correct stratigraphic order, indicating that the ash layers have been contaminated by biotite from older units. The duration of sedimentation is constrained by the ages of volcanic unites that stratigraphically bracket the Creede Formation. Pooling all ages for the underlyinh Snowshoe Mountain Tuff yields an age of 26.92 ± 0.07 Ma for the unit. The age of the stratigraphically highest lavas of Fisher Dacite, which overlie the Creede Formation, is 26.26 ± 0.04 Ma. The two limits give a maximum duration for sedimentation of the Creede Formation of 0.66 m.y. Using the ages of older Fisher Dacite lavas, on which some beds of the Creede Formation were deposited, a more realistic maximum duration of 0.34 m.y. for sedimentation of the Creede Formation can be determined.

Colorado↗
USGS2000

Recognition of primary and diagenetic magnetizations to determine the magnetic polarity record and timing of deposition of the moat-fill rocks of the Oligocene Creede Caldera, Colorado

Sedimentary and volcaniclastic rocks of the Oligocene Creede Formation fill the moat of the Creede caldera, which formed at about 26.9 Ma during the eruption of the Snowshoe Mountain Tuff. Paleomagnetic and rock magnetic studies of two cores (418 and 703 m long) that penetrated the lower half of the Creede Formation, in addition to paleomagnetic and isotopic dating studies of stratigraphically bracketing volcanic units, provide information on the age and the time span of sedimentation of the caldera fill. Normal polarity magnetization are found in Snowshoe Mountain Tuff beneath the moat sediments; in detrital-magnetite-bearing graded tuffs near the bottom of the moat fill; in an ash-fall deposit about 200 m stratigraphically about the top of core 2; and in postcaldera lava flows of the Fisher Dacite that overlie the Creede Formation. Normal polarity also characterizes detrital-magnetite-bearing tuff and sandstone unites within the caldera moat rocks that did not undergo severe sulfidic alteration. The combination of initially low magnitude of remanent magnetization and the destructive effects of subsequent diagenetic sulfidization on detrital iron oxides results in a poor paleomagnetic record for the fine-grained sedimentary rocks of the Creede Formation. these fine-grained rocks have either normal or revered polarity magnetizations that are carried by magnetite and/or maghemite. Many more apparent reversals are found that can be accommodated by any geomagnetic polarity time scale over the interval spanned by the ages of the bracketing extrusive rocks. Moreover, opposite polarity magnetization are found in specimens separated by only a few centimeters, without intervening hiatuses, and by specimens in several tuff beds, each of which represents a single depositional event. These polarity changes cannot, therefore, be attributed to detrital remanent magnetization. Many polarity changes are apparently related to chemical remanent magnetizations carried by postdepositional magnetite and maghemite that formed in rocks in which most or all detrital megnetic iron oxide was destroyed. Incipient oxidation of early diagenetic pyrite may have normal polarity Snowshoe Mountain Tuff (26.89 ± 0.0 Ma, 1 δ) and on the normal polarity postcaldera Fisher lava flows (as young as 26.23 ± 0.05 Ma, 1 δ) indicate that deposition of the Creede Formation spanned about 340-660 k.y. The intermittently defined normal polarity magnetization for the caldera-fill sequence, compared with different versions of the geomagnetic polarity time scale, is consistent with the shorter time span.

Colorado↗
USGS2000

Neogene geomorphic and climatic evolution of the central San Juan Mountains, Colorado: K/Ar age and stable isotope data on supergene alunite and jarosite from the Creede mining district

K/Ar age determinations or supergene alunite and jarosite, formed during Neogene weathering of the epithermal silver and base-metal ores of the Creede mining district, have been combined with geologic evidence to estimate the timing of regional uplift of the southern Rocky Mountains and related canyon cutting. In addition, oxygen and hydrogen isotopic studies suggest climate changes in the central San Juan Mountains during the past 5 m.y. Alunite [ideally (K,Na)Al 3 (SO 4 ) 2 (OH) 6 ] and jarosite [ideally KFe 3 (SO 4 ) 2 (OH) 6 ] can be dated by K/Ar or 40 Ar/ 39 Ar techniques and both contain OH and SO 4 sites that enable four stable isotope analyses (δD, δ 18 O OH , and δ 34 S) to be made. This supergene alunite and jarosite formed by weathering of sulfide-rich ore bodies may record the evolution of the chemical and hydrologic processes affecting ancient oxidized acid ground water, as well as details of climate history and geomorphic evolution. Fine-grained (1-10 μm) supergene alunite and jarosite occur in minor fractures in the upper, oxidized parts of the 25 Ma sulfide-bearing veins of the Creede mining district, and jarosite also occurs in adjacent oxidized Ag-bearing clastic sediments. K/Ar ages for alunite range from 4.8 to 3.1 Ma, and for jarosite range from 2.6 to 0.9 Ma. The δD values for alunite and jarosite show opposite correlations with elevation, and values for jarosite correlate with age. Calculated δD H2O values of alunite fluids approach but are larger than those of present-day meteoric water. Calculated δD H2O values for jarosite fluids are more variable; the values of the youngest jarosites are lowest and are similar to those of present-day meteoric water in the district. The narrow δD-δ 18 O SO4 values of alunites reflects oxidation of sulfide below the water table. The greater range in these values for jarosites reflects oxidation of sulfide under vadose conditions. The ages of alunite mark the position of the paleo-water table at the end of a period of moderate erosion from ca. 25 to 5 Ma that exposed the tops of the ore bodies to oxidation. The younger jarosite formed in the vadose zone during or following subsequent canyon cutting related to regional uplift of the southern Rocky Mountains, The δD values suggest that climates in the area were similar to those of the present day prior to regional uplift but went through a warm period before returning to present conditions during or after regional uplift. The results of this study indicate that the combined stable and radiogenic isotope analysis of supergene alunite and jarosite has broad application in understanding climate and geomorphic evolution of selected areas.

Colorado↗
USGS2000

Revised age of the Rockland tephra, northern California: Implications for climate and stratigraphic reconstructions in the western United States: Reply

Sarna-Wojcicki suggests that the 40 Ar/ 39 Ar age of 614 ± 8 ka for the Rockland tephra ( Lanphere et al., 1999 ) is too old because of the presence of xenocrystic plagioclase. The Rockland tephra is indeed full of lithic debris that can be seen at any well-exposed outcrop of ash-flow or air-fall deposits. In order to minimize contamination, we selected pumice lumps known to be representative of the Rockland magma. Plagioclase in the pumice lumps forms a homogeneous population of weakly zoned phenocrysts. By contrast, plagioclase from the intermediate to silicic volcanic lithic fragments in the Rockland tephra typically displays complex zoning, variable resorption patterns and strong discontinuities. Such plagioclase would be readily recognizable if present in the Rockland magma. We saw no petrographic evidence that the Rockland magma was contaminated with xenocrystic plagioclase. The pumice lumps were carefully cleaned before mineral separations were made.

California↗
USGS2000

First-principles calibration of 38Ar tracers :implications for the ages of 40Ar/39Ar fluence monitors

We describe the analytical procedure for first-principles calibrations of 38Ar tracers in the U.S. Geological Survey (USGS) Laboratory in Menlo Park, California. The ages of fluence monitors determined in the USGS laboratory and those reported in the literature differ by nearly 2 percent. However, the radiogenic-40Ar content of the primary mineral standard on which these ages are based has never been verified by first-principles measurement in other laboratories.

Professional Paper↗
USGS1999

Chapter 1: Previous research

Santorini has fascinated and stimulated explorers and scholars since ancient times. Jason and the Argonauts were apparently visitors to the islands and described a giant called Talos. Molten metal flowed from his feet and he threw stones at them. The island is perhaps best known for the paroxysmal eruption that took place in the Late Bronze Age at the height of the Minoan civilization that dominated Crete and the Aegean region. The legend of Atlantis, in which a whole city sank beneath the sea in a single day and night, is plausibly based on the effects of this eruption on the Minoan Civilization. The geographer Strabo described the eruption of 197 bc in the following way: ... for midway between Thera and Therasia fires broke forth from the sea and continued for four days, so that the whole sea boiled and blazed, and the fires cast up an island which was gradually elevated as though by levers and consisted of burning masses... This introduction gives a brief synopsis of research on the volcano since this dramatic chronicle was written. Research on Santorini has contributed substantial advances not only in the understanding of Santorini itself but to general principles in volcanology and petrology. three centres for lava eruptions on Therasia and northern Thera: the Peristeria, Simandiri and Skaros-Therasia Volcanoes.

Santorini↗
USGS1999

Chapter 2: Geological and tectonic setting of Santorini

Santorini lies in an area of complex extensional and subduction-related tectonics in a continental environment (Jackson 1994). The region has had a long geological history with large changes occurring during the Alpine orogeny and in the Late Tertiary as a consequence of post-collisional extension and major re-adjustments of the plate boundaries. The Aegean region is thought to be moving towards the southwest where the Aegean microplate overrides the Eastern Mediterranean sea floor. The collision has created the Hellenic trench to the south of Crete where the eastern Mediterranean sea floor is subducting beneath the Aegean Sea at 5-6 cm a-1. Santorini is one of several Quaternary volcanoes that define the present day active volcanic arc related to this subduction zone (Fig. 2.1). We here provide a synopsis of the broader geological and tectonic context of Santorini within the Aegean area. The geology, isotope geochemistry and geochronology of the crystalline basement of the southern Aegean is of particular relevance as it is likely that the Santorini magmas have interacted extensively with the continental crust. Geochemical evidence presented in Chapters 6 and 7 confirms this prediction. We also give a synopsis of the Quaternary Aegean Island Arc, of which Santorini is the most active centre.

Santorini↗
USGS1999

Chapter 3: Development of the Santorini volcanic field in space and time

Santorini is one of the largest Quaternary volcanic centres of the Aegean Region. The caldera cliffs preserve well-exposed sequences of lavas and pyroclastic deposits, which record the long development of the volcano in space and time. These include the products of 12 major explosive eruptions and the dissected remains of several ancient lava shields, stratovolcanoes, and lava-dome complexes. The former existence of multiple eruptive centres scattered over the present-day islands shows that Santorini is best considered as a volcanic field , which probably also continues under the sea (Heiken & McCoy 1984). Santorini is best known for the Minoan eruption of the late Bronze Age (Bond & Sparks 1976; Heiken & McCoy 1984; Sparks & Wilson 1990), but some of the previous explosive eruptions may have been as large (Druitt et al . 1989). The occurrence of repeated explosive eruptions has triggered formation of at least four large calderas, such that the present-day caldera is a composite structure (Druitt & Francaviglia 1992). Santorini is potentially one of the most dangerous volcanoes in Europe, having had numerous eruptions in historic times, some of them with significant explosive components (Fytikas et al . 1990 a ).

Santorini↗
USGS1999

Chapter 4: Compositional zoning and petrology of the Thera pyroclastics

Compositional zoning is a common feature of pyroclastic deposits erupted from calderas worldwide (Smith 1979; Hildreth 1981; Bacon & Druitt 1988; De Silva 1991; Feeley & Davidson 1994). Compositionally zoned tuffs provide a geologically instantaneous snapshot of the vertical stratigraphy and density stratification in the chamber immediately before eruption. Magma bodies are commonly zoned in density, with light, relatively cool, silicic magma overlying hotter, more mafic magma. Vertical zonation can arise by a number of processes, including crystallization and convective fractionation on the chamber.

Santorini↗
USGS1999

Chapter 5: Cumulate nodules in the Thera pyroclastics

Gabbroic and dioritic nodules are common in some horizons of the Thera pyroclastics. These nodules contain pyroxene as the main ferromagnesian phase in contrast to the early Akrotiri centre, which contain nodules with abundant hornblende (Nicholls 1971a; Pichler & Kussmaul 1972). Calcsilicate nodules and gabbroic nodules have also been recognized in Santorini lavas (Fouqu6 1879; Nicholls 1971a). Previous studies of Santorini volcanic rocks indicate that crystal fractionation has played an important role in generating the compositional range basalt-andesite-dacite-rhyodacite (Nicholls 1971a; Mann 1983; Huijsmans 1985; Barton & Huijsmans 1986). This interpretation is also consistent with much of the petrological data presented in Chapter 4 and the geochemical data and models presented in Chapter 6. However, disequilibrium phenocryst assemblages in some lavas (Huijsmans 1985; Chapter 4), banded pumice clasts in many pyroclastic deposits (Druitt et al. 1989; Chapters 4), isotopic disequilibria in phenocrysts (Pyle et al. 1988; Chapters 6 and 7) and complex zoning patterns in plagioclase crystals (Stamatelopoulou-Seymour et al. 1990) indicate that magma mixing has also occurred. In addition, trace element and isotopic abundances can be interpreted in terms of assimilation of continental crust during fractionation (Barton et al. 1983; Stamatelopoulou-Seymour et al. 1990; Chapter 6). The nodules provide information on petrogenetic processes that have influenced the evolution of Santorini magmas. Their petrology is documented in this chapter and compared with that of associated juvenile ejecta. The nodules are identified as cumulates from andesitic and dacitic magmas, con- firming the importance of fractional crystallization in the petrogenesis of Santorini magmas. They also demonstrate that mafic plutonic rocks can be formed from intermediate to silicic magmas.

Santorini↗
USGS1999

Chapter 6 Geochemistry, isotopic composition and petrogenetic modelling of the Thera pyroclastics

This chapter examines the major element, trace element, and isotope geochemistry of the Thera pyroclastics, and places it in the context of the geochemical evolution of the whole volcanic field. Geochemical studies to date have been undertaken predominantly on the lavas, and the pyroclastic succession has been relatively neglected. The data on the pyroclastics are taken from Druitt (1983), Mellors (1988), Pyle (19906), and Edwards (1994). Wholerock analyses and sample descriptions are given in Appendix 2. Most analyses are of a single pumice or scoria lump, although some are combinations of several small clasts and a few are compositionally banded. The rock classification scheme used throughout the memoir is shown in Fig. 6.1. Previous isotopic studies on Santorini are also largely confined to the lavas, and no detailed studies had been carried out on the pyroclastic rocks. In this chapter we present new 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb/ 204 Pb and δ 18 О isotopic data on 40 samples of the Thera pyroclastics. We also include some measurements of hydrogen isotopes. Methods of geochemical and isotopic analysis are described in Appendix 1. This chapter also presents some quantitative models of fractional crystallization and assimilation of crustal rocks. The geochemical data and models point towards the importance of crystal fractionation, magma mixing and assimilation of continental crust in magma genesis at Santorini. The chapter also documents temporal changes in geochemical features of Santorini magmas.

Santorini↗
USGS1999

Chapter 7: Studies of short-lived radionuclides in Santorini volcanics

Disequilibria between the short-lived radioactive nuclides 226 Ra, 230 Th and 238 U in young volcanic rocks can be used both for dating the time of crystallisation of a rock, and as isotopic tracers of the sources of magmas and the chemical evolution of volcanic systems. The principles of these methods have been reviewed extensively elsewhere (Condomines et al . 1988; Gill et al . 1992; Gill & Condomines 1992; Macdougall 1995). In this chapter, we present new 238 U- 230 Th- 226 Ra disequilibria data for the Thera pyroclastics and for selected lavas. These data complement the radiogenic isotopic data presented in Chapter 6 on the compositional evolution through time of the pyroclastic ejecta. In particular, we find that the ( 230 Th/ 232 Th) isotopic ratio corrected to the time of eruption varies in a systematic way with time through the second cycle of the Thera pyroclastics and variations in this ratio are correlated with the variations of 87 Sr/ 86 Sr, 143 Nd/ 144 Nd and Pb isotopes.

Santorini↗
USGS1999

Chapter 8: The evolution of Santorini

There now follows a synthesis of the main results of this study. We interpret the observations in terms of the interplay between magmatic, tectonic and volcanic processes and place these processes within a global context. Santorini has displayed a wide variety of igneous and volcanic phenomena over a period of at least 650 000 years of almost continuous and focused volcanism. Perhaps the most intriguing issues are the causes of volcanic and magmatic cyclicity, the secular variations in magma composition and the way in which magmatic processes have influenced the style of volcanism.

Santorini↗
USGS1999

Appendix 1: Analytical methods and errors

Thirty eight K-Ar and eight 40Ar/39Ar high-precision age determinations were made at the US Geological Survey, Menlo Park, on a total of 22 rocks from the entire volcanic field. Duplicate or triplicate determinations were carried out on 14 samples in order to improve analytical precision. All ages were measured on whole-rock samples selected after thin-section examination. Most of the samples meet the usual criteria for whole-rock dating (Mankinen © Dal-rymple 1972), but some contain minor amounts of glass and a few samples are very glassy. The samples selected for dating were crushed to 0.5-lmm (-18 to +35 mesh). For K-Ar dating aliquots weighing c. 25 g were used for the Ar measurements. A 10 g aliquot was ground to -200 mesh and splits of the powder were used for K20 measurements, which were made in duplicate on each of two separate splits of sample powder by flame photometry after lithium metaborate fusion and dissolution (Ingamells 1970). Ar analyses were by isotope-dilution mass spectrometry using a high-purity (>99.9%) 38Ar tracer and techniques and equipment described previously (Dalrymple & Lanphere 1969). All samples for Ar extraction were baked overnight at 280°C. Mass analyses were done on a 22.68 cm radius, multiple-collector mass spectrometer with a nominal 90° sector magnet, using automated data collection (Stacey etal 1981). Errors given for the calculated K-Ar ages of individual measurements are estimates of the standard deviation of analytical precision. The errors were calculated using formulae derived by Cox & Dalrymple (1967) and Dalrymple & Lanphere (1969).

Geological Society, London, Memoirs↗
USGS1999

Appendix 2: Tabulated analytical data

Details of analytical methods and errors are given in Appendix 1. bd means below detection and na not analysed. Samples numbers which begin with the letter S are from Druitt (1983) and those that begin with LS are from Edwards (1994). Sample numbers which begin with the number 8 are either from Mellors (1988) or Pyle (1990 b ). Subunits of each major tuff of the Thera pyroclastics are denoted by upper case letters (A to D). These are listed in Table 3.4. In the case of the minor sequences (Appendix 2.3), the sequence abbreviations (M6 to M8) are shown in Fig. 3.17 and the individual unit abbreviations (CD1 to CD10) are from Edwards (1994).

Geological Society, London, Memoirs↗
USGS1999

Digital Geologic Map of the Nevada Test Site and Vicinity, Nye, Lincoln, and Clark Counties, Nevada, and Inyo County, California

This digital geologic map of the Nevada Test Site (NTS) and vicinity, as well as its accompanying digital geophysical maps, are compiled at 1:100,000 scale. The map compilation presents new polygon (geologic map unit contacts), line (fault, fold axis, metamorphic isograd, dike, and caldera wall) and point (structural attitude) vector data for the NTS and vicinity, Nye, Lincoln, and Clark Counties, Nevada, and Inyo County, California. The map area covers two 30 x 60-minute quadrangles-the Pahute Mesa quadrangle to the north and the Beatty quadrangle to the south-plus a strip of 7.5-minute quadrangles on the east side-72 quadrangles in all. In addition to the NTS, the map area includes the rest of the southwest Nevada volcanic field, part of the Walker Lane, most of the Amargosa Desert, part of the Funeral and Grapevine Mountains, some of Death Valley, and the northern Spring Mountains. This geologic map improves on previous geologic mapping of the same area (Wahl and others, 1997) by providing new and updated Quaternary and bedrock geology, new geophysical interpretations of faults beneath the basins, and improved GIS coverages. Concurrent publications to this one include a new isostatic gravity map (Ponce and others, 1999) and a new aeromagnetic map (Ponce, 1999).

Open-File Report↗