Gail Mahood - Academia.edu (original) (raw)

Papers by Gail Mahood

Geological Society of America Bulletin, Oct 16, 2009

We undertook a 40 Ar/ 39 Ar study of young mafi c and silicic lavas at Mammoth Mountain and the L... more We undertook a 40 Ar/ 39 Ar study of young mafi c and silicic lavas at Mammoth Mountain and the Long Valley caldera (eastcentral California) to better understand the frequency of these eruptions and the magmatic plumbing system that drives them. Our results show that most of Mammoth Mountain, a lava-dome complex straddling the southwestern topographic rim of the caldera, consists of trachydacite lavas erupted at ca. 68 ka. These ages and new 29-and 41-ka ages for trachydacite lavas in the northwest quadrant of the caldera indicate that these silicic lavas are considerably younger than previously thought. Mafi c lavas vented widely in the western third of the caldera in the past 190,000 years, suggesting that this area has not been underlain by large bodies of silicic magma during this interval, as such magma would have prevented the rise of the denser basaltic magma. We identify four eruptive sequences over the past 190,000 years: the western moat sequence (~190-160 ka), the Mammoth sequence (~120-58 ka), the northwest caldera sequence (~41-29 ka), and the Inyo chain sequence (~9 ka-present). In each eruptive sequence mafi c and silicic lavas erupted contemporaneously from spatially associated vents. This suggests that intrusion of alkali basalt into the shallow crust led to the silicic eruptions. If the seismic unrest and deformation of the past three decades is a result of basalt injected beneath Mammoth Mountain and perhaps the western third of the caldera, then there is the possibility of spatially associated small-volume silicic eruptions, which would typically be considerably more explosive. In the past 40,000 years, eruptions have occurred along a N-S linear trend less than 10 km wide, limiting the zone subject to volcanic hazards. Our data bear on Pleistocene glaciation in the region. Ages of 162 ± 2 ka and 99 ± 1 ka for bracketing mafi c lava fl ows better constrain the age of the Casa Diablo till. Our results provide equivocal support for a suggested anticorrelation between volcanism and glaciation for the past 800,000 years in eastern California (Glazner et al., 1999).

Geofisica Internacional, 1985

Geological Society of America Bulletin, Apr 1, 1996

Zoning patterns of mineral, rock, and isotopic compositions in the Hall Canyon pluton in southeas... more Zoning patterns of mineral, rock, and isotopic compositions in the Hall Canyon pluton in southeastern California are used to determine and discuss the petrologic processes that operate in magma chambers that solidify to form peraluminous plutons. The pluton is biotite-muscovite granodiorite at lower levels and grades to a 150-m-thick upper zone of muscovite granite, capped by a 10- to 20-m-thick zone of pegmatite, aplite, and quartz veins at the roof of the pluton. Mineral and whole-rock compositions are essentially homogeneous in the >600 m of exposed lower zone, whereas the thinner upper zone/roof zone is systematically zoned to more evolved compositions roofward. Compositions of garnet, plagioclase, and muscovite in the upper zone/roof zone track bulk-rock compositions and, in the case of muscovite, range beyond those that have generally been considered magmatic, suggesting that compositional criteria for distinguishing magmatic from secondary muscovite cannot be drawn without consideration of bulk-rock composition. Initial ϵ Nd values of −18 to −19 for the pluton indicate that the magma sources were entirely intracrustal and lower Proterozoic in age. The lower zone could represent melt formed by dehydration melting of a biotite-rich metaigneous or metapsammitic source rock. Upper zone/roof zone rocks are too evolved to have been unmodified crustal melts. Even the least evolved was a product of fractional crystallization, and further in situ fractionation of the upper zone/roof zone produced compositional zoning that defines whole-rock Sr and Nd isochron ages of ca. 72 Ma. Slight differences in initial isotopic ratios indicate that lower zone magma and magma parental to the upper zone/roof zone were derived from different source rocks and remained separate during most of the subsequent fractionation of the upper zone/roof zone. Melt separation was inefficient, leaving behind melt-rich mushes that formed rocks that do not bear strong chemical signatures of being cumulate rocks. We suggest that the upper zone/roof zone was static while it fractionated by loss of residual liquid upward, whereas the thicker lower zone may have been capable of convection that stirred the magma as it crystallized, preventing it from becoming zoned. Late in the crystallization of the pluton, meter-scale layering of the granite formed when biotite muscovite granitic magma was injected as sills into the partly crystalline margin of the chamber near the upper zone–lower zone boundary. We suggest that peraluminous plutons are more heterogeneous isotopically and usually have less regular zoning patterns than metaluminous plutons, because the small effective heights of the magma bodies inhibit convection. This results, in part, from the sill-like form of many peraluminous plutons, but also from melt production rates in zones of purely intracrustal melting that are so low that there is significant cooling of magma in a chamber between arrival of successive melt batches. If resident magma has crystallized to the point of “lock-up,” it cannot mix with new arrivals and does not contribute to the height of magma capable of convection. Systems in which melting is largely the result of advection of heat by mantle-derived basalt might be expected to have larger magma supply rates, because the volume of partial melt produced is not so strictly limited by the abundance of hydrous phases and by the approach of quartz and feldspar to cotectic proportions in the source, and because some of the mafic magma is incorporated into the chamber. Convection would be favored by the resulting greater thickness of magma capable of flow.

Contributions to Mineralogy and Petrology, Oct 1, 1988

High-silica rhyolites of the Sierra La Primavera, a late Pleistocene center near Guadalajara, are... more High-silica rhyolites of the Sierra La Primavera, a late Pleistocene center near Guadalajara, are extremely Sr-poor (0.3-1.3 ppm), yet (with one exception) values of 87Sr/86Srl are relatively low at 0.7041-0.7048. Values of 143Nd/lr for all the rhyolites are (within errors) identical to a basalt at 0.5129. These surprisingly primitive values, along with feldspar 8180 of +6.6%0, are consistent with an origin by fractional crystallization of mantle-derived basalt. However, absence of the large volume of associated intermediate rocks that would be expected if the 40 km 3 of erupted rhyolite were produced mainly by fractional crystallization suggests alternative processes involving partial melting of Mesozoic or Tertiary mafic intrusive rocks (or lower-crustal metamorphic equivalents). The latter interpretation is preferred, especially in light of comparative data for other North American, Cenozoic, high-silica rhyolites. Isotopic compositions correlate with basement age, but generally lie between values for associated basalts and the underlying crust. Nearly all can be interpreted as containing both a young mantle-derived component and a crustal component, probably derived by partial melting at intermediate to deep levels of the crust. No matter what the proportions of mantle-and crust-derived material in parental magmas, the extremely low concentrations of Sr and Ba in the highsilica rhyolites require extensive fractional crystallization of feldspar-rich assemblages after parental liquids attain rhyolitic compositions. At La Primavera, contamination by shallow roof rocks

Nature Communications, Aug 16, 2017

The omnipresence of lithium-ion batteries in mobile electronics, and hybrid and electric vehicles... more The omnipresence of lithium-ion batteries in mobile electronics, and hybrid and electric vehicles necessitates discovery of new lithium resources to meet rising demand and to diversify the global lithium supply chain. Here we demonstrate that lake sediments preserved within intracontinental rhyolitic calderas formed on eruption and weathering of lithium-enriched magmas have the potential to host large lithium clay deposits. We compare lithium concentrations of magmas formed in a variety of tectonic settings using in situ trace-element measurements of quartz-hosted melt inclusions to demonstrate that moderate to extreme lithium enrichment occurs in magmas that incorporate felsic continental crust. Cenozoic calderas in western North America and in other intracontinental settings that generated such magmas are promising new targets for lithium exploration because lithium leached from the eruptive products by meteoric and hydrothermal fluids becomes concentrated in clays within caldera lake sediments to potentially economically extractable levels.

Journal of Volcanology and Geothermal Research, Oct 1, 1980

The Sierra La Primavera volcanic complex consists of late Pleistocene comenditic lava flows and d... more The Sierra La Primavera volcanic complex consists of late Pleistocene comenditic lava flows and domes, ash-flow tuff, air-fall pumice, and caldera-lake sediments. The earliest lavas were erupted about 120,000 years ago, and were followed approximately 95,000 years ago by the eruption of about 20 km3 of magma as ash flows that form the compositionally-zoned Tala Tuff. Collapse of the roof zone of the magma chamber led to the formation of a shallow ll-km-diameter caldera. It soon filled with water, forming a caldera lake in which sediment began to collect. At about the same time, two central domes erupted through the middle of the lake and a "giant pumice horizon", an important stratigraphic marker, was deposited. Shortly thereafter ring domes erupted along two parallel arcs: one along the northeast portion of the ring fracture, and the other crossing the middle of the lake. All these events occurred during a period of approximately 5000-10,000 years. Sedimentation continued and a period of volcanic quiescence was marked by the deposition of some 30 m of fine-grained ashy sediments virtually free from pumice lapilli. Approximately 75,000 years ago, a new group of ring domes erupted at the southern margin of the lake. These domes are lapped by only lo-20 m of sediments, as uplift resulting from renewed insurgence of magma brought an end to the lake. This uplift culminated in the eruption, beginning approximately 60,000 years ago, of aphyric lavas along a southern arc. The youngest of these lavas erupted approximately 20,000-30,000 years ago. The four major fault systems in the Sierra La Primavera are related to caldera collapse or to uplift caused by the insurgence of the southern arc magma. Steam vents and largedischarge 65" C hot springs are associated with the faulting. Calculated equilibrium temperatures of the geothermal fluids are-17O"C, but temperatures in excess of 240°C have been encountered in an exploratory drill hole. A seismic survey showed attenuation of both S and P waves within the caldera, P waves attenuated more severely than S waves. The greatest attenuation is associated with an area of steam vents, and the rapid lateral variations in attenuation suggest that they are produced by a shallow geothermal system rather than by underlying magma.

Journal of Geophysical Research, 1984

Strongly peralkaline volcanic complexes tend to be shieldlike due to coalescence of low-viscosity... more Strongly peralkaline volcanic complexes tend to be shieldlike due to coalescence of low-viscosity lavas from numerous vent areas and blanketing by pyroelastic flows and falls. Dense welding and rheomorphism commonly mask the nature of pyroelastic units; welded pumice falls are common. Relatively low eruptive columns produce poorly sorted pumice falls that are difficult to distinguish from topographymantling pyroelastic flows. Welded fall deposits are characterized by multiple, fine-scale, welding reversals, welding zones that correspond with stratification, and by flattening of fiamme parallel to underlying slopes, whereas welded topography-mantling ignimbrites may have fine-grained basal layers, lithie lenses,:•imbricate fiamme, and variations in crystal content and fiamme size with topographic position. Calderas are relatively small, typically 3-9 km in diameter, with little evidence for single-stage subsidence in excess of a few hundred meters. Many strongly peralkaline volcanic centers show evidence of two or more nested calderas, and partial reactivation of older ring fractures is common. Replenishment of magma reservoirs beneath calderas can promote dilatant reactivation of older subsidence structures, resulting in multicyclic collapses that reuse these structures. In several centers, the caldera-forming unit is zoned from pantellerite to more crystal-rich trachyte. Following collapse, trachyte lavas commonly erupt from a central vent on the caldera floor, building a cone that nearly fills the caldera. Reestablishment of isostatic equilibrium may be accomplished both by eruption of these lavas and by flow of trachytic and more-marie magma into the root zone beneath the volcano. Resurgent doming sensu strictu has not been documented in strongly peralkaline systems; at Pantelleria and Mount Suswa, however, the central portions of the cauldron blocks were uplifted and tilted. Magma reservoirs that feed explosive eruptions generally are shallow, as indicated by pyroelastic units that contain lithie fragments of the volcanic edifice (commonly including cognate syenite) but lack subjacent crustal lithologies. In addition, the overwhelming predominance of alkali feldspar as a phenocryst phase and the scarcity of quartz are consistent with pantellerite liquidus relations only at pressures •100 MPa. Pantellerite and trachyte shields may represent the mature stage of continental rift volcanism; once the flux of mantle-derived magmas has been sufficient to generate a large mass of felsic liquid that can act as a density filter, marie magmas rarely reach the surface.

Earth and Planetary Science Letters, Sep 1, 1989

The high-silica rhyolite lavas of Glass Mountain, California, provide a detailed record of the ev... more The high-silica rhyolite lavas of Glass Mountain, California, provide a detailed record of the evolution of the Long Valley magmatic system during the 1.4 m.y. prior to the catastrophic eruption of the voluminous zoned rhyohtic Bishop Tuff at 0.73 Ma. The older lavas of Glass Mountain are extremely evolved, and were erupted from 2.1 to 1.2 Ma with 87Sr/86Sr of 0.707-0.739 and eNd of-3 to-4, whereas the younger lavas are slightly less evolved and were erupted between 1.2 and 0.79 Ma with 87Sr/86Sr of 0.706-0.707 and cNd close to-1, essentially identical to the Sr and Nd isotopic compositions of the Bishop Tuff. Neither the older nor younger lavas display a relationship between the isotopic compositions of Sr and Nd. The Pb isotopic compositions are effectively uniform at 206Pb/204 Pb = 19.13-19.17. A group of older lavas that outcrop in a northwest-trending band near the topographic rim of the caldera collectively define an apparent Rb-Sr isochron age (I) of 2.09 + 0.06 Ma with an intercept of "Sr/"Sr = 0.7060 + 3, whereas analyzed older lavas outcropping outboard of this band define an isochron of 1.90 + 0.02 Ma ((87Sr/86 Sr), = 0.7063 f 4). These isochron ages are identical to the K-Ar ages for oldest erupted rhyolites in the two regions, respectively. The younger lavas away from the caldera define an apparent Rb-Sr isochron age (t) of 1.14 * 0.08 Ma with the same

AGU Fall Meeting Abstracts, Dec 1, 2018

IOP conference series, Oct 1, 2008

Revista Mexicana De Ciencias Geologicas, 1979

Geofisica Internacional, 1986

Geological Society of America Bulletin, Jun 23, 2017

Earth and Planetary Science Letters, Jul 1, 1996

Single-crystal laser-probe "OAr/39Ar dating of 133 grains of sanidine and plagioclase has enabled... more Single-crystal laser-probe "OAr/39Ar dating of 133 grains of sanidine and plagioclase has enabled us to resolve the eruption ages of the Upper Basin Member rhyolites-the lava flows and related tuffs that erupted within the Yellowstone Caldera shortly after its collapse 630 ky ago on eruption of the Lava Creek Tuff. Two lavas and a tuff that erupted from the eastern ring-fracture zone yield an eruptive age of 481 f 8 ka, whereas two flows from the western ring-fracture zone yield eruptive ages of 516 f 7 and 198 f 8 ka. Most of the units contain old xenocrysts, explaining why previous attempts at dating these earliest post-caldera units by the conventional K-Ar method yielded poorly resolved and, in some cases, anomalous ages. The tuff shows the most severe contamination. Grains from a single pumice lapilli in the tuff show as large an age range as those from bulk vitrophyre, indicating that the xenocrysts were incorporated in the magma prior to its near-surface explosive fragmentation. Diffusion calculations indicate that the xenocrysts could not have remained in the magma for more than a few years without degassing and giving ages indistinguishable from the phenocrysts. Thus, the contamination represented by the xenocrysts probably occurred during fracturing and conduit propagation, rather than during caldera collapse, which took place more than 100 ky earlier. The apparent ages of xenocrysts and their compositions as determined by electron microprobe suggest that the Eocene Absaroka volcanics are the main contaminant, with a single xenocryst probably coming from Precambrian basement rocks. Most of the xenocrysts are difficult to distinguish optically or chemically from feldspar phenocrysts, illustrating the necessity of single-crystal analysis to date many young volcanic rocks accurately.

Contributions to Mineralogy and Petrology, 1991

Glass Mountain, California, consists of > 50 km 3 of high-silica rhyolite lavas and associated py... more Glass Mountain, California, consists of > 50 km 3 of high-silica rhyolite lavas and associated pyroclastic deposits that erupted over a period of > 1 my preceding explosive eruption of the Bishop Tuff and formation of the Long Valley caldera at 0.73 Ma. These "minimum-melt" rhyolites yield Fe-Ti-oxide temperatures of 695-718~ and contain sparse phenocrysts of plagioclase + quartz + magnetite + apatite + sanidine, biotite, ilmenite, allanite, and zircon. Incompatible trace elements show similar or larger ranges within the Glass Mountain suite than within the Bishop Tuff, despite a much smaller range of major-element concentrations, largely due to variability among the older lavas (erupted between 2.1 and 1.2 Ma). Ratios of the most incompatible elements have larger ranges in the older lavas than in the younger lavas (1.2-0.79 Ma), and concentrations of incompatible elements span wide ranges at nearly constant Ce/Yb, suggesting that the highest concentrations of these elements are not the result of extensive fractional crystallization alone; rather, they are inherited from parental magmas with a larger proportion of crustal partial melt. Evidence for the nature of this crustal component comes from the presence of scarce, tiny xenocrysts derived from granitic and greenschist-grade metamorphic rocks. The wider range of chemical and isotopic compositions in the older lavas, the larger range in phenocryst modes, the eruption of magmas with different compositions at nearly the same time in different parts of the field, and the smaller volume of individual lavas suggest either that more than one magma body was tapped during eruption of the older lavas or that a single chamber tapped by all lavas was small enough that the composition of its upper reaches was easily affected by new additions of crustal melts. We interpret the relative chemical, mineralogical, and isotopic homogeneity of the younger Glass Mountain lavas as reflecting eruptions from a large, integrated magma chamber. The small number of eruptions between 1.4 and 1.2 Ma may have allowed

Geology, 1998

INTRODUCTION Voluminous silicic eruptions produce large ignimbrites and widespread fallout deposi... more INTRODUCTION Voluminous silicic eruptions produce large ignimbrites and widespread fallout deposits. These pumice and ash layers provide instantaneous chronostratigraphic markers important in constraining the timing of glaciations and magnetic reversals and in correlating stratigraphy over large areas. For example, distal ashes from the three climactic caldera-forming eruptions of the Yellowstone Plateau, designated Pearlette types B, S, and O ashes (corresponding with the Huckleberry Ridge, Mesa Falls, and Lava Creek Tuffs; Izett, 1981), have been widely used in dating Quaternary landscapes, flora, and fauna throughout the central United States (Naeser et al., 1973), as well as glaciations in the Yellowstone area (Pierce, 1979; Richmond, 1986). Contamination by older material is a major problem in the isotopic dating of both effusive and explosive young volcanic rocks, especially in distal ashes reworked by wind or water. An effective technique for identifying and correcting for contamination in age dating is the singlecrystal 40 Ar/ 39 Ar method. This high-precision technique allows the elimination of anomalously old ages, which is impossible in bulk-sample techniques (van den Bogaard et al.