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M.A. Clynne

Publications and source records attributed to M.A. Clynne.

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

Magmatic interactions as recorded in plagioclase phenocrysts of Chaos Crags, Lassen Volcanic Center, California

The silicic lava domes of Chaos Crags in Lassen Volcanic National Park contain a suite of variably quenched, hybrid basaltic andesite magmatic inclusions. The inclusions represent thorough mixing between rhyodacite and basalt recharge liquids accompanied by some mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement. 87Sr/86Sr ratios (~0.7037-0.7038) of the inclusions are distinctly lower than those of the host rhyodacite (~0.704-0.7041), which are used to fingerprint the origin of mineral components and to monitor the mixing and mingling process. Chemical, isotopic, and textural characteristics indicate that the inclusions are hybrid magmas formed from the mixing and undercooling of recharge basaltic magma with rhyodacitic magma. All the host magma phenocrysts (biotite, plagioclase, hornblende and quartz crystals) also occur in the inclusions, where they are rimmed by reaction products. Compositional and strontium isotopic data from cores of unresorbed plagioclase crystals in the host rhyodacite, partially resorbed plagioclase crystals enclosed within basaltic andesite inclusions, and partially resorbed plagioclase crystals in the rhyodacitic host are all similar. Rim 87Sr/86Sr ratios of the partially resorbed plagioclase crystals in both inclusions and host are lower and close to those of the whole-rock hybrid basaltic andesite values. This observation indicates that some crystals originally crystallized in the silicic host, were partially resorbed and subsequently overgrown in the hybrid basaltic andesite magma, and then some of these partially resorbed plagioclase crystals were recycled back into the host rhyodacite. Textural evidence, in the form of sieve zones and major dissolution boundaries of the resorbed plagioclase crystals, indicates immersion of crystals into a hotter, more calcic magma. The occurrence of partially resorbed plagioclase together with plagioclase microlites and olivine crystals reflects disaggregation of inclusions and mingling of this material into the silicic host. These processes are commonplace in some orogenic magma systems and may be elucidated by isotopic microsampling and analysis of the plagioclases crystallizing from them.

Journal of Petrology↗
USGS1999

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

The Rockland tephra is an important stratigraphic marker for climate and stratigraphic reconstructions over a broad area of the western United States. New 40 Ar/ 39 Ar ages are as much as 200 k.y. older than previous cogenetic zircon fission-track ages, which range from 400 to 560 ka. Incremental-heating 40 Ar/ 39 Ar analyses on two splits of plagioclase from a proximal ash flow of the Rockland tephra in the Lassen Peak area, California, yield an average age-spectrum-plateau age of 614 ± 8 ka and an isochron age of 611 ± 11 ka. Our new age for the Rockland tephra is compatible with an 40 Ar/ 39 Ar analysis of plagioclase from the basaltic andesite of Hootman Ranch that directly overlies the Rockland tephra. A plateau age of 565 ± 29 ka, an isochron age of 572 ± 39 ka, and transitional directions of remanent magnetization suggest an age for the basaltic andesite of Hootman Ranch as ca. 570 ka. Correlation of the Rockland tephra with its suspected distal ash in sedimentary sections at widely scattered localities has made the ash an extremely valuable stratigraphic tool. Our new age for the Rockland tephra requires significant revision of many recent climate-based analyses in the western United States. In particular, the best ages for the Rockland tephra (614 ka) and the Lava Creek B ash (660 ka) and their association with oxygen isotopic stages 16 and 15 will allow enhanced understanding of mid-Pleistocene pluvial and interpluvial events in the western United States.

Geology↗
USGS1998

The petrogenesis of felsic calc-alkaline magmas from the southernmost Cascades, California: Origin by partial melting of basaltic lower crust

The majority of felsic rocks from composite centers in the southernmost Cascades have geochemical and Sr, Nd and Pb isotopic ratios that suggest derivation by partial melting of lower crust that is compositionally similar to calc-alkaline basalts observed in the region. Only a few felsic rocks have δ 18 O and Pb isotopic compositions that indicate interaction with the upper crust. Mineralogical and geochemical differences among the felsic magmas result primarily from melting under variable f (H 2 O) and temperature conditions. Partial melting under low f (H 2 O) and high temperature conditions leaves an amphibole-poor residuum, and produces magmas that have orthopyroxene as the most abundant ferromagnesian phenocryst, relatively low silica contents, and straight rare earth element patterns. Partial melting under higher f (H 2 O) and lower temperature conditions leaves an amphibole-rich residuum, and produces magmas that have amphibole ± biotite phenocrysts, relatively high silica contents, and pronounced middle rare earth element depletions. These conclusions are consistent with published thermal models that suggest that reasonable volumes of basaltic magma emplaced beneath large composite centers in the southernmost Cascades can serve as the heat source for melting of the lower crust. Melting of the lower crust under variable f (H 2 O) conditions is likely to result from differences in the H 2 O contents of these basaltic magmas.

Journal of Petrology↗
USGS1997

Gravity anomalies, Quaternary vents, and Quaternary faults in the southern Cascade Range, Oregon and California: Implications for arc and backarc evolution

Isostatic residual gravity anomalies in the southern Cascade Range of northern California and southern Oregon are spatially correlated with broad zones of Quaternary magmatism as reflected by the total volume of Quaternary volcanic products, the distribution of Quaternary vents, and the anomalously low teleseismic P wave velocities in the upper 30 km of crust. The orientation of Quaternary faults also appears to be related to gravity anomalies and volcanism in this area, trending generally north-south within the magmatic regions and northwest-southeast as they enter the neighboring amagmatic zones to the north and south. The relationship between gravity anomalies, vent density, and fault orientations may indicate in a broad sense the strength of the middle and upper crust. The southern Cascade Range occupies a transition zone where horizontal stress is transferred from the northwest-southeast dextral shear of the Walker Lane belt to the east-west extension characteristic of the Cascade arc in central Oregon. Faulting along north-south strikes in the volcanically active areas indicates the east-west extensional stresses in thermally weakened crust, whereas northwest faulting between the volcanically active areas reflects the northwest trending, right lateral shear strain of the Walker Lane belt. The segmentation of the arc reflected in Quaternary magmatism may be caused by differential extension behind crustal blocks of the forearc rotating clockwise with respect to North America. In this view the volcanic centers at Mount Shasta, Medicine Lake volcano, and Lassen Peak in northern California are situated along the southern parts of the trailing edges of two distinct segments of the forearc where additional extension is implied by their differential clockwise rotation.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Comagmatic A-type granophyre and rhyolite from the Alid volcanic center, eritrea, northeast Africa

Granophyric blocks within late-Pleistocene pyroclastic flow ejecta from the Alid volcanic center, northeast Africa, are the rapidly crystallized, intrusive equivalent of pumice from the pyroclastic flow. Phenocryst compositions and geochemical characteristics of the pumice and granophyre are virtually identical. Silicate melt inclusions and other geochemical and geological constraints reveal those processes leading to development of the granophyric texture. Rhyolitic (A-type) magma with ∼2.6 wt % dissolved H 2 O and a temperature near 870°C was intruded to within 2–4 km of the surface, causing deformation and structural doming of shallow marine and subaerial strata. Eruptions of crystal-poor rhyolite from this shallow magma chamber caused degassing, which forced undercooling and consequent granophyric crystallization of some of the magma remaining in the intrusion. The most recent eruption from Alid excavated the crystallized granitic wall of the magma chamber, bringing the granophyric clasts to the surface.

Journal of Petrology↗
USGS1997

Primitive magmas at five Cascade volcanic fields: Melts from hot, heterogeneous sub-arc mantle

Major and trace element concentrations, including REE by isotope dilution, and Sr, Nd, Pb, and O isotope ratios have been determined for 38 mafic lavas from the Mount Adams, Crater Lake, Mount Shasta, Medicine Lake, and Lassen volcanic fields, in the Cascade arc, northwestern part of the United States. Many of the samples have a high Mg# [100Mg/(Mg + FeT) > 60] and Ni content (>140 ppm) such that we consider them to be primitive. We recognize three end-member primitive magma groups in the Cascades, characterized mainly by their trace-element and alkali-metal abundances: (1) High-alumina olivine tholeiite (HAOT) has trace element abundances similar to N-MORB, except for slightly elevated LILE, and has Eu/Eu* > 1. (2) Arc basalt and basaltic andesite have notably higher LILE contents, generally have higher SiO2 contents, are more oxidized, and have higher Cr for a given Ni abundance than HAOT. These lavas show relative depletion in HFSE, have lower HREE and higher LREE than HAOT, and have smaller Eu/Eu* (0.94-1.06). (3) Alkali basalt from the Simcoe volcanic field east of Mount Adams represents the third end-member, which contributes an intraplate geochemical signature to magma compositions. Notable geochemical features among the volcanic fields are: (1) Mount Adams rocks are richest in Fe and most incompatible elements including HFSE; (2) the most incompatible-element depleted lavas occur at Medicine Lake; (3) all centers have relatively primitive lavas with high LILE/HFSE ratios but only the Mount Adams, Lassen, and Medicine Lake volcanic fields also have relatively primitive rocks with an intraplate geochemical signature; (4) there is a tendency for increasing 87Sr/86Sr, 207Pb/204Pb, and ??18O and decreasing 206Pb/204Pb and 143Nd/144Nd from north to south. The three end-member Cascade magma types reflect contributions from three mantle components: depleted sub-arc mantle modestly enriched in LILE during ancient subduction; a modern, hydrous subduction component; and OIB-source-like domains. Lavas with arc and intraplate (OIB) geochemical signatures were erupted close to HAOT, and many lavas are blends of two or more magma types. Pre-eruptive H2O contents of HAOT, coupled with phase-equilibrium studies, suggest that these magmas were relatively dry and last equilibrated in the mantle wedge at temperatures of ~1300 degrees C and depths of ~40 km, virtually at the base of the crust. Arc basalt and basaltic andesite represent greater extents of melting than HAOT, presumably in the same general thermal regime but at somewhat lower mantle separation temperatures, of domains of sub-arc mantle that have been enriched by a hydrous subduction component derived from the young, relatively hot Juan de Fuca plate. The primitive magmas originated by partial melting in response to adiabatic upwelling within the mantle wedge. Tectonic extension in this part of the Cascade arc, one characterized by slow oblique convergence, contributes to mantle upwelling and facilitates eruption of primitive magmas.

Canadian Mineralogist↗
USGS1997

Olivine and chromian spinel in primitive calc-alkaline and tholeiitic lavas from the southernmost cascade range, California: A reflection of relative fertility of the source

Chromian spinel and coexisting olivine phenocrysts from a geochemically diverse suite of primitive tholeiitic and calc-alkaline basalts and magnesian andesites from the Lassen region, in the southernmost Cascade Range, in California, show that the sub-arc mantle is zoned. Depleted calc-alkaline basalts and magnesian andesites erupt in the forearc region, and calc-alkaline basalts contain increasing abundances of incompatible elements toward the backarc. High-alumina olivine tholeiites erupt from the arc and backarc areas. Olivine from all these lavas displays a limited compositional range, from Fo86 to Fo91, and crystallized at high temperature, generally 1225-1275??C. Chromian spinel trapped in the olivine phenocrysts displays a large range of composition: Cr# values span the range 9-76. Excess Al in the spinel relative to that in 1-atm spinel suggests that it crystallized at elevated pressure. The phenocrysts in these lavas are in equilibrium with their host liquids. The full range of Cr# of the spinel compositions cannot be explained by differentiation or variable pressure, variations in f(O2), subsolidus equilibration or variations in degree of partial melting of a single peridotitic source. Rather, the systematic compositional differences among phenocrysts in these primitive lavas result from bulk chemical variability in their mantle sources. Correlations between spinel and host-rock compositions support the assertion that the geochemical diversity of Lassen basalts reflects the relative fertility of their mantle sources.

Canadian Mineralogist↗
USGS1997

The variable role of slab-derived fluids in the generation of a suite of primitive calc-alkaline lavas from the Southernmost Cascades, California

The compositional continuum observed in primitive calc-alkaline lavas erupted from small volcanoes across the southernmost Cascade arc is produced by the introduction of a variable proportion of slab-derived fluid into the superjacent peridotite layer of the mantle wedge. Magmas derived from fluid-rich sources are erupted primarily in the forearc and are characterized by Sr and Pb enrichment (primitive mantle-normalized Sr/P > 5.5), depletions of Ta and Nb, low incompatible-element abundances, and MORB-like Sr and Pb isotopic ratios. Magmas derived from fluid-poor sources are erupted primarily in the arc axis and behind the arc, and are characterized by weak enrichment in Sr [1.0 < (Sr/P)N < 1.3], weak depletions in Ta and Nb, higher incompatible-element abundances, and OIB-like Sr, Nd, and Pb isotopic ratios. Fluxing the mantle wedge above the subducting slab with H2O-rich fluid stabilizes amphibole and enriches the wedge peridotites in incompatible elements, particularly unradiogenic Sr and Pb. The hydrated amphibole-bearing portion of the mantle wedge is downdragged beneath the forearc, where its solidus is exceeded, yielding melts that are enriched in Sr and Pb, and depleted in Ta and Nb (reflecting both high Sr and Pb relative to Ta and Nb in the fluid, and the greater compatibility of Ta and Nb in amphibole compared to other silicate phases in the wedge). A steady decrease of the fluid-contributed geochemical signature away from the trench is produced by the progressive dehydration of the downdragged portion of the mantle wedge with depth, resulting from melt extraction and increased temperature at the slab-wedge interface. Inverse correlation between incompatible-element abundances and the size of the fluid-contributed geochemical signature is generated by melting of more depleted peridotites, rather than by significant differences in the degree of melting. High-(Sr/P)N lavas of the forearc are generated by melting of a MORB-source-like peridotite that has been fluxed with a greater proportion of slab-derived fluid, and low (Sr/P)N lavas of the arc axis are produced by melting of an OIB-source-like peridotite in the presence of a smaller proportion of slab-derived fluid. This study documents the control that a slab-derived fluid can have on incompatible element and isotopic systematics of arc magmas by 1) the addition of incompatible elements to the wedge, 2) the stabilization of hydrous phases in the wedge, and 3) the lowering of peridotite solidi.

Canadian Mineralogist↗
USGS1996

Thermal and mass implications of magmatic evolution in the Lassen volcanic region, California, and minimum constraints on basalt influx to the lower crust

We have analyzed the heat and mass demands of a petrologic model of basalt-driven magmatic evolution in which variously fractionated mafic magmas mix with silicic partial melts of the lower crust. We have formulated steady state heat budgets for two volcanically distinct areas in the Lassen region: the large, late Quaternary, intermediate to silicic Lassen volcanic center and the nearby, coeval, less evolved Caribou volcanic field. At Caribou volcanic field, heat provided by cooling and fractional crystallization of 52 km 3 of basalt is more than sufficient to produce 10 km of rhyolitic melt by partial melting of lower crust. Net heat added by basalt intrusion at Caribou volcanic field is equivalent to an increase in lower crustal heat flow of ∼7 mW m −2 , indicating that the field is not a major crustal thermal anomaly. Addition of cumulates from fractionation is offset by removal of erupted partial melts. A minimum basalt influx of 0.3 km 3 (km 2 Ma) −1 is needed to supply Caribou volcanic field. Our methodology does not fully account for an influx of basalt that remains in the crust as derivative intrusives. On the basis of comparison to deep heat flow, the input of basalt could be ∼3 to 7 times the amount we calculate. At Lassen volcanic center, at least 203 km 3 of mantle-derived basalt is needed to produce 141 km 3 of partial melt and drive the volcanic system. Partial melting mobilizes lower crustal material, augmenting the magmatic volume available for eruption at Lassen volcanic center; thus the erupted volume of 215 km 3 exceeds the calculated basalt input of 203 km 3 . The minimum basalt input of 1.6 km 3 (km 2 Ma) −1 is >5 times the minimum influx to the Caribou volcanic field. Basalt influx high enough to sustain considerable partial melting, coupled with locally high extension rate, is a crucial factor in development of Lassen volcanic center; in contrast, Caribou volcanic field has failed to develop into a large silicic center primarily because basalt supply there has been insufficient.

Journal of Geophysical Research B: Solid Earth↗
USGS1994

Late Quaternary normal faulting of the Hat Creek basalt, northern California

The Hat Creek fault is a major, young, north-striking, normal fault along the western boundary of extensional Basin and Range deformation in the Lessen region of northeastern California. Volcanic rocks of Quaternary and late Pliocene age are displaced a total of >500 m down to the west along west-facing, en echelon scarps now retreated to ∼35° slopes. Fresh, young scarps as much as 30 m high cut the Hat Creek Basalt (erupted between 15 and ∼40 ka) a few tens of meters west of the retreated scarps. Prior to the late 1980s, these young scarps were interpreted as lava slump scarps formed as the Hat Creek Basalt ponded against the older fault scarps and then drained away to the northwest. Numerous pieces of geologic evidence, however, show that the young scarps formed after the Hat Creek Basalt solidified and cooled and are true fault features formed by the youngest displacements of the Hat Creek fault. Structural details are remarkably well preserved along the series of left-stepping scarps cutting the Hat Creek Basalt. Near the central parts of individual segments, the fault is displayed as a single, vertical scarp. Near the ends of the segments, the scarp decreases in height and becomes a monodinal flexure on which the recent dip separation has been taken up by small-scale offset along columnar cooling joints in the basalt These monodinal flexures commonly rotate into east-west monoclines that join adjacent north-south segments. Displacement of outwash gravel overlying the Hat Creek Basalt shows that vertical separation on the Hat Creek fault has averaged ∼1.3 mm yr −1 for the past 15,000 yr. The Hat Creek fault thus represents a potential earthquake hazard, despite the low level and diffuse nature of modern seismidty in the region.

Geological Society of America Bulletin↗
USGS1982

The densities of halite-saturated WIPP-A and NBT-6 brines and their NaCl contents in weight percent, molal, and molar units from 20 to 100 degrees C

A series of density measurements has been performed at 30?, 50?, 70?, and 90?C for halite-undersaturated WIPP-A and NBT-6 brines with various NaCl contents approaching saturation. The densities of halite-saturated WIPP-A and NBT-6 brines were obtained by extrapolating these measured densities to halite saturation points. The maximum difference between the densities obtained in this Fashion and those calculated from the model of Potter and Haas is 0.015 g/cm3. The NaCl contents in halite-saturated WIPP-A and NBT-6 brines are reported in wt %, molal, and molar units from 20? to 100?C.

Open-File Report↗