Cima Volcanic Field Research Articles

Melt inclusions in spinel from a composite mantle xenolith

Composite mantle xenoliths from the Cima Volcanic Field (CA, USA) contain a variety of melt (now glassy) inclusions hosted within mantle phases. The compositions and textures of these melt inclusions have the potential to constrain their trapping processes, melt sources, and the rates of ascent of their parent xenoliths. Here we focus on unusual spinel-hosted melt inclusions from one composite xenolith, reporting glass and daughter mineral compositions and textures and attempting to reconstruct inclusion bulk compositions. The xenolith contains spinel-hosted melt inclusions in its harzburgite, olivine-websterite and lherzolite layers; there are none in its orthopyroxenite layer.The glass compositions and reconstructed bulk compositions of the partly-crystallized inclusions correspond to alkaline intermediate melts, mostly trachyandesites. Such melts are most likely to be generated and trapped by vapor-undersaturated phlogopite or amphibole dehydration melting to an assemblage of liquid + spinel + olivine ± pyroxenes. We modeled the near-liquidus phase relations of the inclusion bulk compositions and noted the closest approach of each inclusion to simultaneous saturation with spinel and either phlogopite or amphibole, resulting in estimated trapping pressures of ∼0.5–1.5 GPa and temperatures of ∼1000–1100 °C. The large size of the hosting spinel grains suggests a slow process associated with these breakdown reactions, probably thinning of the lithosphere and steepening of the geotherm during regional extension.A linear correlation between the vesicle area and inclusion area (as proxies for volume) suggests an in-situ exsolution process from melts of relatively uniform volatile initial contents, consistent with trapping of vapor-undersaturated melts that later exsolve vapor during cooling and daughter crystal growth. A negative correlation between the glass content in melt inclusions and the size of the inclusion itself suggests a control on the degree of crystallinity with the size. There appears to be a two-stage cooling history captured by the inclusions, forming first prismatic daughter crystals and large round vesicles at the wall of the inclusion, followed by quenching to form a mat of fine crystallites and small vesicles in most inclusions. We connect the final quench to rapid ascent of the xenolith in its host melt, which also triggered partial breakdown of remaining amphibole to fine glassy symplectites.

Origin of low Mg# hawaiites carrying peridotite xenoliths from the Cima volcanic field, California, USA: Evidence of rapid magma mixing during ascent along intersecting fractures

AbstractThe Cima volcanic field, in the southern Basin and Range province (California, USA), includes >70 eruptive units over the last 8 m.y. The youngest (≤1 Ma) are low Mg# (≥56) hawaiites derived from an asthenospheric mantle source. The Cima hawaiites, and adjacent Dish Hill basanites, are known for carrying large mantle xenoliths, which precludes stalling in a crustal reservoir. This raises the question of how low Mg# hawaiites, which cannot be in equilibrium with peridotite mantle, formed and differentiated while carrying dense, mantle xenoliths. Several hypotheses are evaluated and the only one shown to be viable is mixing between high-MgO basanite (with entrained mantle xenoliths and sparse olivine phenocrysts) and low-MgO mugearite liquids, which formed by partial melting of mafic lower crust under relatively dry and reducing conditions. Multiple lines of evidence, including the presence of mantle xenoliths in hawaiites, diffusion-limited growth textures in olivine and clinopyroxene, and notably thin Fe-rich rims on high-MgO olivine crystals (inherited), indicate magma mixing must have occurred rapidly (days or less) during ascent to the surface along intersecting fractures, and not in a stalled crustal reservoir. Abundant evidence points to clinopyroxene growth immediately after mixing, and application of clinopyroxene-melt barometry constrains the depth of mixing to the lower and middle crust (0.8–0.4 GPa). Results from olivinemelt thermometry/hygrometry (~1196 °C and ~1.4 wt% H2O) applied to a basanite from Dish Hill carrying 5–20 cm mantle xenoliths leads to calculated ascent velocities ≥0.3–4.9 km/h, enabling ascent through the 36 km thick crust in ≤7–119 h.

The potential of phosphorus in clinopyroxene as a geospeedometer: Examples from mantle xenoliths

We investigate the potential to use concentrations and zoning patterns of phosphorus (P) in clinopyroxene as indicators of the rates of igneous and metasomatic processes, comparable to recent applications of P in olivine but applicable to more evolved rocks and lower temperatures of crystallization. Few high-P pyroxenes have been previously reported, and none have been analyzed in detail for the mechanism of P enrichment or the implications for mineral growth kinetics. Here, we report the discovery and characteristics of exotic phosphorus-rich secondary clinopyroxene in glassy pockets and veins in composite mantle xenoliths from the Cima Volcanic Field (California, USA) and the Middle Atlas Mountains (Morocco, West Africa). These glass-bearing xenoliths preserve evidence of melt infiltration events and the contrasting behavior of P in their pyroxene crystals constrains the different rates of reaction and extents of equilibration that characterized infiltration in each setting. We report optical petrography and chemical analysis of glasses and minerals for major elements by electron microprobe microanalyzer and trace elements by laser-ablation Inductively Coupled Plasma Mass Spectrometry. The Cima Volcanic Field specimen shows one end-member behavior, with unzoned P-rich clinopyroxene in a melt pocket. We attribute this occurrence to a slow crystallization process that occurred after the melt temperature reached near-equilibrium with the host rock and during which the P concentration in the melt was buffered by apatite saturation. In the Morocco xenolith, by contrast, clinopyroxene exhibits zonation with P increasing all the way to the rim, in contact with the glass. We ascribe this feature to a rapid growth process in which excess P was incorporated into the growing clinopyroxene from a diffusive boundary layer. We demonstrate quantitative agreement between the enrichment of P and other trace elements and their expected diffusion and partitioning behavior during rapid growth. We suggest that P has not been widely reported in clinopyroxene in large part because it has rarely been looked for and that its analysis offers considerable promise as a kinetic indicator both in xenoliths and volcanic rocks.

Emplacement dynamics and timescale of a Holocene flow from the Cima Volcanic Field (CA): Insights from rheology and morphology

We present a rheological and morphological study of a Holocene lava flow emitted by a monogenetic cinder cone in the Cima Volcanic Field, eastern California. Our field observations focused on surface morphology, which transitions from smooth core extrusions near the vent to jagged ‘a’ā blocks over the majority of the flow, and on channel and levée dimensions. We collected airborne photogrammetry data and used it to generate a digital elevation model. From this, the total flow volume was estimated and surface roughness was quantified in terms of standard deviation of the real surface (5cm resolution) from the software-generated 1m-average plane. Sample textural analyses revealed that the near-vent portion of the flow is significantly more crystalline (ϕxtal=0.95±0.04) than the main flow body (ϕxtal=0.66±0.11). The rheology of Cima lavas was determined experimentally by concentric cylinder viscometry between 1550°C and 1160°C, including the first subliquidus rheology measurements for a continental intraplate trachybasaltic lava. The experimentally determined effective viscosity increases from 54Pa·s to 1361Pa·s during cooling from the liquidus (~1230°C) to 1160°C, where crystal fraction is 0.11. The lava viscosity over this range is still lower than most basaltic melts, due to the high alkali content of Cima lavas (~6wt% Na2O+K2O). Monte Carlo simulations were used to account for and propagate experimental uncertainties, and to determine which rheological model (Bingham, power law, or Herschel-Bulkley) provides the best-fit of the obtained rheological data. Results suggest that Bingham and Herschel-Bulkley models are statistically indistinguishable from each other, and that both fit the data better than a power law model. By combining field observations and experimental results, we reconstructed the eruption temperature and few days-long emplacement history of the Cima flow.

Phosphorus zoning as a recorder of crystal growth kinetics: application to second-generation olivine in mantle xenoliths from the Cima Volcanic Field

Composite mantle xenoliths from the Cima Volcanic Field (CA, USA) contain glassy veins that cross-cut lithologic layering and preserve evidence of lithospheric melt infiltration events. Compositions and textures of minerals and glasses from these veins have the potential to place constraints on the rates and extents of reaction during infiltration. We studied glass-bearing regions of two previously undescribed composite xenoliths, including optical petrography and chemical analysis for major and trace elements by electron probe microanalysis and laser-ablation inductively coupled plasma mass spectrometry. The petrogenetic history of each vein involves melt intrusion, cooling accompanied by both wall-rock reaction and crystallization, quench of melt to a glass, and possibly later modifications. Exotic secondary olivine crystals in the veins display concentric phosphorus (P)-rich zoning, P-rich glass inclusions, and zoning of rapidly diffusing elements (e.g., Li) that we interpret as records of rapid disequilibrium events and cooling rates on the order of 10 °C/h. Nevertheless, thermodynamic modeling of the diversity of glass compositions recorded in one of the samples demonstrates extensive reaction with Mg-rich olivine from the matrix before final quench. Our results serve as a case study of methods for interpreting the rates and processes of lithospheric melt-rock reactions in many continental and oceanic environments.

Multi-stage history of compound mantle xenoliths from Western USA: Implications for metasomatic processes in the deep mantle.

The compound mantle xenoliths from Cima Volcanic Field and Chino Valley (Western U.S.A.) represent outstanding candidates to illustrate the processes that occur prior to their delivery to the surface by alkali-basaltic volcanism. The xenoliths share characteristics like pyroxene zonation, amphibole breakdown and formation of glass and armalcolite. Their petrogenetic evolution involved partial melting of the silicate minerals, infiltration of reactive melts and dissociation of minerals en route to the surface, suggesting that these rocks followed multi-stage histories that initiated deep in the mantle (>1.0 GPa) and proceeded during a very short period of time.

Post-eruptive sediment transport and surface processes on unvegetated volcanic hillslopes – A case study of Black Tank scoria cone, Cima Volcanic Field, California

Conical volcanic edifices that are made up from lapilli to block/bomb pyroclastic successions, such as scoria cones, are widespread in terrestrial and extraterrestrial settings. Eruptive processes responsible for establishing the final facies architecture of a scoria cone are not well linked to numerical simulations of their post-eruptive sediment transport. Using sedimentological, geomorphic and 2D fragment morphology data from a 15-ky-old scoria cone from the Cima Volcanic Field, California, this study provides field evidence of the various post-eruptive sediment transport and degradation processes of scoria cones located in arid to semi-arid environments. This study has revealed that pyroclast morphologies vary downslope due to syn-eruptive granular flows, along with post-eruptive modification by rolling, bouncing and sliding of individual particles down a slope, and overland flow processes. The variability of sediment transport rates on hillslopes are not directly controlled by local slope angle variability and the flank length but rather by grain size, and morphological characteristics of particles, such as shape irregularity of pyroclast fragments and block/lapilli ratio. Due to the abundance of hillslopes degrading in unvegetated regions, such as those found in the Southwestern USA, granulometric influences should be accounted for in the formulation of sediment transport laws for geomorphic modification of volcanic terrains over long geologic time.

Dating desert pavements – First results from a challenging environmental archive

Desert pavements are widespread landforms of arid environments. They consist of a monolayer of clasts at the surface, associated with an underlying unit of eolian fines. In some situations, buried desert pavements can be observed, which is interpreted as a change in the environmental conditions. Therefore, it is believed that desert pavements represent important paleoenvironmental sediment archives, especially for arid environments, where natural archives of past environments are rare. To better understand the formation process of desert pavements and to enable the paleoenvironmental interpretation of these valuable sediment archives, reliable chronologies are of crucial importance. Thus, OSL dating was applied to samples from well-developed desert pavements in two different study areas, the Cima Volcanic Field, eastern Mojave Desert, USA, and the desert of northeastern Badia, Jordan. To test the suitability of the sediments for OSL dating, the luminescence characteristics of the fine- and coarse-grain quartz fraction are described and compared. Finally, first OSL ages are presented.

From Las Vegas to Long Beach: Desert Pedology, Land Use, and Wild Lands Tour

On 30 Oct.–1 Nov. 2014, the Victorville MLRA Soil Survey Office; Dr. Bob Graham (University of California–Riverside, UCR), and Dr. Brenda Buck (University of Nevada–Las Vegas, UNLV) hosted the Desert Pedology, Land Use, and Wild Lands—Las Vegas to Long Beach two-day tour. The tour was planned and executed by Leon Lato and the rest of the Victorville MLRA Soil Survey, Region 8 team with a special thanks to Carrie Ann Houdeshell who originally initiated the effort before she transferred to her modelling position in Region 2. From Las Vegas, NV to the outskirts of the Los Angeles Basin, CA, there is a wide range of desert landscape, remote wilderness, unique vegetation, and soil formation. This area represents isolation and preservation and, at the same time, promises for the future of open space for renewable energy production, food and fiber production, watershed renovation, and alternative uses for new minerals for green infrastructure and modern technology. The tour started in Las Vegas at the University of Nevada with a lecture and lab tour from NCSS cooperator Dr. Brenda Buck. Her lecture on “Naturally Occurring Asbestos: Potential for Human Exposure, Southern Nevada, USA” was illuminating and a good start to understanding the issues of dust in the Mojave Desert and the southwestern U.S. to humans and the difficulties of studying the subject in an urbanizing environment. The group travelled on to California to the Mojave National Preserve (MNP). Dr. Mandy Williams of UNLV and Dustin Detweiler, NRCS ecological site specialist, started the field discussions with examples of biocrusts, ecological site descriptions, and the function of dust in these systems. Dr. Daniel Hirmas, Kansas University, explained his Ph.D. UCR thesis of Eolian deposits on mountains, and Leon Lato, NRCS soil survey project leader, Mojave National Preserve, led the discussion of the playa soil of Soda Dry Lake. The group stayed at the Desert Studies Center in Zzyzx, CA (named by Dr. Springer, owner of this famous 1930s health spa as the “last” word in health). On the second day, the group visited the basalt flows in the Cima volcanic field with Dr. Yvonne Katzenstein, which illustrated the potential of 1 to 3 m of dust (eolian fines) inflating the landscape over measured geologic time. A highlight of the tour was the taxonomic discussion and illustration of the new master V horizon by Dr. Robert Graham, UCR. This site shows how the V horizon developed from eolian deposits under a desert pavement surface. The vesicular pores and platy structure of the silt loam surface (V horizon) contrasts strongly with the gravelly-sandy material below. The final stop was at the eastern side of the Kelso Dunes with an explanation by Dr. Katherine Kendrick, USGS, on desert dust and dune processes. Pre-meeting SSSA Pedology Tour 2014 group picture on the eastern edge of the Kelso Dune field in MNP in an area stabilized by big galleta grass. Playa soil on Soda Dry Lake looking south at the Desert Studies Center in Zzyzx, CA—coarse-loamy, mixed, superactive, hyperthermic Oxyaquic Udorthents. The tour brought together students, soil scientists, and soil taxonomists from China, Iraq, Syria, India, Europe, Canada, Australia, Africa, and throughout the U.S. Those who attended came away with a clearer understanding of the importance of the V master horizon and dust in desert environments as well as congenial memories of NRCS's work in the region. They enjoyed the spectacular weather including some overnight rain and relatively light desert breezes for the trip. Tour participants scrutinizing samples of the V horizon in MNP. Desert pavement with a V horizon in MNP—sandy-skeletal, mixed, thermic Typic Haplocalcids.

Field demonstration of an instrument performing automatic classification of geologic surfaces.

This work presents a method with which to automate simple aspects of geologic image analysis during space exploration. Automated image analysis on board the spacecraft can make operations more efficient by generating compressed maps of long traverses for summary downlink. It can also enable immediate automatic responses to science targets of opportunity, improving the quality of targeted measurements collected with each command cycle. In addition, automated analyses on Earth can process large image catalogs, such as the growing database of Mars surface images, permitting more timely and quantitative summaries that inform tactical mission operations. We present TextureCam, a new instrument that incorporates real-time image analysis to produce texture-sensitive classifications of geologic surfaces in mesoscale scenes. A series of tests at the Cima Volcanic Field in the Mojave Desert, California, demonstrated mesoscale surficial mapping at two distinct sites of geologic interest.

The growth and erosion of cinder cones in Guatemala and El Salvador: Models and statistics

Morphologic data for 147 cinder cones in southern Guatemala and western El Salvador are compared with data from the San Francisco volcanic field, Arizona (USA), Cima volcanic field, California (USA), Michoácan–Guanajuato volcanic field, Mexico, and the Lamongan volcanic field, East Java. The Guatemala cones have an average height of 110 +/− 50 m, an average basal diameter of 660 +/− 230 m and an average top diameter of 180 +/− 150 m. The general morphology of these cones can be described by their average cone angle of slope (24 +/− 7), average height-to-radius ratio (0.33 +/− 0.09) and their flatness (0.24 +/− 0.18). Although the mean values for the Guatemalan cones are similar to those for other volcanic fields (e.g., San Francisco volcanic field, Arizona; Cima volcanic field, California; Michoácan–Guanajuato volcanic field, Mexico; and Lamongan volcanic field, East Java), the range of morphologies encompasses almost all of those observed worldwide for cinder cones. Three new 40Ar/ 39Ar age dates are combined with 19 previously published dates for cones in Guatemala and El Salvador. There is no indication that the morphologies of these cones have changed over the last 500–1000 ka. Furthermore, a re-analysis of published data for other volcanic fields suggests that only in the Cima volcanic field (of those studied) is there clear evidence of degradation with age. Preliminary results of a numerical model of cinder cone growth are used to show that the range of morphologies observed in the Guatemalan cinder cones could all be primary, that is, due to processes occurring at the time of eruption.

Assessing the potential for luminescence dating of basalts

The possibility of dating basalt using luminescence was tested on four samples with independent age control from Cima volcanic field, California, with the ultimate aim of assessing whether the technique could be used to date sediments on the surface of Mars. Previous analysis of these samples had demonstrated that the infrared stimulated luminescence (IRSL) signal is most suitable for dating as it showed the lowest fading rate among various luminescence signals. In this study, changes in equivalent dose as a function of preheat are described. The ages for the two youngest Cima samples agree with the independent ages based on cosmogenic nuclide measurements (12.0 ± 0.8 ka). In the two older samples (dated to 320 and 580 ka by K–Ar), the luminescence behaviour is more complex and the form of the IRSL decay curve is seen to vary with dose. Mathematical fitting is used to isolate two components and their intensities are used to produce dose response curves. The slower component yields a larger equivalent dose. However, even using this component and after correction for fading, the ages obtained for the older samples are younger than the K–Ar ages.

Surface control of desert pavement pedologic process and landscape function, Cima Volcanic field, Mojave Desert, California

Desert pavement is a distinctive feature widespread across arid lands of the world. It plays a dynamic role in geomorphic, hydrologic, and ecologic processes. Where desert pavement predominates, infiltration is limited and rainfall is delivered as runoff to nearby bare ground areas where shrubs cluster. Desert pavement surfaces may appear monotonously flat and barren, but we have found, instead, that they are a complex association of landscape and hydrologic elements governed by their surface characteristics. Previously, we identified six unique surface mosaic types that accurately capture the subtle, but distinct, variations in surface clast arrangements for a desert pavement landscape formed on a single-aged basalt flow in the Mojave Desert. We now report that these surface mosaics predict the spatial distribution of fundamental desert vegetation and soil characteristics. Characteristics of soil morphology and texture, the leaching depth of soluble salts, percent plant cover, and shrub species diversity are remarkably consistent for each mosaic type across a 580,000-year-old basalt flow even when measured >1 km apart. Hydrologic character is distinctly different between desert pavement and bare ground regions and vegetation distributions reflect the spatially heterogeneous soil moisture. Where desert shrubs cluster on the three bare ground surface mosaics, leaching is deep, removing most soluble salts to below the 50-cm depth. Where shrubs are absent or few, on the three desert pavement mosaics, leaching depths are shallow, with soluble salt depth distributions as well as desert shrub percent cover precisely controlled by the percent clast cover of the surface.

Blind testing of rock varnish microstratigraphy as a chronometric indicator: results on late Quaternary lava flows in the Mojave Desert, California

Rock varnish is a manganiferous dark coating ubiquitous in desert landscapes. To test the validity of varnish microstratigraphy as a chronometric indicator, varnish samples were collected from radiometrically dated and undated late Quaternary lava flows in Amboy, Cima, and Pisgah volcanic fields (AVF, CVF, PVF) in the Mojave Desert of California, western United States. Varnish microstratigraphies show a replicable layering sequence that appears to record regional climate changes that likely correspond in time to the Younger Dryas and Heinrich events in the North Atlantic region. Microstratigraphic patterns on these volcanic fields match patterns found in varnishes from other western US sites with available radiometric age constraints. Based on this regional chronology, varnishes from the A flow, H flow, and a stone pavement surface in the Cima volcanic field were estimated to be 16.5–24, 74–85, and 74–85 ka, respectively; these ages are consistent with previously published cosmogenic 3He ages of 18–20, 72–74, and 80–85 ka for these geomorphic surfaces. Varnishes from the I flow at Cima yielded a puzzling age estimate of 39 ka, which is consistent with an older 3He age of 37±6 ka reported for the I flow, but inconsistent with a younger 3He age of 31±7 ka and a cosmogenic 36Cl age of 27±1.3 ka for the same flow. Reinterpretation of the original varnish age data, with knowledge of then available field mapping results of the I flow, suggests that the I cone is polycyclic and different flow units were probably unintentionally sampled in the field. The revised varnish ages of 30 and 39 ka for the I flow thus may be in good agreement with their corresponding 3He and 36Cl ages. In a blind test of the method, varnishes from the Phase 1 flow at Pisgah, an unnamed flow (called here the I′ flow) at Cima, and the Amboy flow were estimated to be 24–30, 46–60, and 74–85 ka, respectively; these ages agree well with 36Cl ages of 22.5±1.3, 46±2, and 79±5 ka reported for the same flows by Phillips [Geomorphology (2002).]. These test results provide convincing evidence that varnish microstratigraphy, once radiometrically calibrated, can be used as a valid dating tool to estimate surface exposure ages of desert landforms in the western US drylands.

Pedogenesis of Vesicular Horizons, Cima Volcanic Field, Mojave Desert, California

An existing model of desert pavement formation suggests desert pavement clasts rise vertically on an accreting eolian mantle and the underlying vesicular horizon coevolves with pavement formation. Results presented here support this model and provide a mechanism whereby eolian material is transported from the ground surface to ped interiors, thereby increasing the thickness of the vesicular horizon underlying desert pavements. Basaltic desert pavements in the Cima Volcanic Field are underlain by a vesicular horizon with strong coarse columnar and strong medium platy soil structure. Peds collected from three sites along a topographic gradient were subsampled into seven ped domains to quantify physical, chemical, and micromorphological properties within peds and along the topographic transect. Ped interiors have up to 40% clay and 12% CaCO3 whereas sediments adhering to ped sides have <7% clay and 2% CaCO3 Argillans and siltans lining platy surfaces in ped centers and bottoms indicate desert dust is translocated vertically along the macropores between peds and horizontally along platy boundaries to ped interiors. Once soils develop a strong columnar and platy structure, translocation to ped interiors is enhanced. Calibrated radiocarbon ages between 5440 and 5045 BP for Av ped centers suggest enhanced dust flux and pedogenesis occurred during the drier middle Holocene, an inference supported by luminescence dating and correlations with regional eolian chronologies.

Pedogenesis of Vesicular Horizons, Cima Volcanic Field, Mojave Desert, California

An existing model of desert pavement formation suggests desert pavement clasts rise vertically on an accreting eolian mantle and the underlying vesicular horizon coevolves with pavement formation. Results presented here support this model and provide a mechanism whereby eolian material is transported from the ground surface to ped interiors, thereby increasing the thickness of the vesicular horizon underlying desert pavements. Basaltic desert pavements in the Cima Volcanic Field are underlain by a vesicular horizon with strong coarse columnar and strong medium platy soil structure. Peds collected from three sites along a topographic gradient were subsampled into seven ped domains to quantify physical, chemical, and micromorphological properties within peds and along the topographic transect. Ped interiors have up to 40% clay and 12% CaCO3 whereas sediments adhering to ped sides have <7% clay and 2% CaCO3 Argillans and siltans lining platy surfaces in ped centers and bottoms indicate desert dust is translocated vertically along the macropores between peds and horizontally along platy boundaries to ped interiors. Once soils develop a strong columnar and platy structure, translocation to ped interiors is enhanced. Calibrated radiocarbon ages between 5440 and 5045 BP for Av ped centers suggest enhanced dust flux and pedogenesis occurred during the drier middle Holocene, an inference supported by luminescence dating and correlations with regional eolian chronologies.

Seismic anisotropy under extended crust: evidence from upper mantle xenoliths, Cima Volcanic Field, California

A model of P-wave velocity anisotropy and shear-wave splitting for upper mantle under extended crust is derived from comparisons of seismic field measurements and observed xenolith anisotropies. Seismic velocity measurements and anisotropies calculated from petrofabric measurements of 15 upper mantle xenoliths from the Cima Volcanic Field, Eastern Mojave Desert, California show average maximum anisotropies of 6.9% for V p and 4.5% for V s. Xenolith compressional wave velocities corrected for upper mantle temperature and pressure agree well with field observations in the vicinity of the Cima Volcanic Field. The velocities calculated from the olivine fabrics suggest that upper mantle foliations beneath the region are horizontal, and lineations trend ENE. The fabrics are believed to originate from early Cenozoic extension in the area, which had the same trend as the lineations. The unusually high upper mantle temperatures in this region apparently have not significantly affected upper mantle fabrics.

Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle

Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridótite and anhydrous and hydrous pyroxenite, some cut by igneous‐textured pyroxenite‐gabbro veins and dikes and some by veins of amphibole ± plagioclase. Igneous‐textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma‐mantle wall rock reactions, episodic intrusion and infiltration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (εNd = −5.7 to +3.4; 87Sr/86Sr = 0.7051 – 0.7073; 206Pb/204Pb = 19.045 – 19.195) and the igneous‐textured xenoliths (εNd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 – 0.7036 with one carbonate‐affected outlier at 0.7054; and 206Pb/204Pb = 18.751 – 19.068), so that they cannot be related. The igneous‐textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have εNd of +5.1 to +9.3; 87Sr/86Sr of 0.7028 – 0.7050, and 206Pb/204Pb of 18.685 – 21.050. The igneous‐textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous‐textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and distinct sources. Sm‐Nd two‐mineral “isochrons” yield apparent ages for petrographically identical rocks believed to be coeval ranging from ∼0 to 113±26 Ma, indicating the unreliability of dating these rocks with this method. Amphibole and plagioclase megacrysts are isotopically like the host basalts and probably originate by mechanical breakup of veins comagmatic with the host basaltic rocks. Unlike other Basin and Range localities, Cima Cr‐diopside group isotopic compositions do not overlap with those of the host basalts.

Eolian sediments generated by anthropogenic disturbance of playas: human impacts on the geomorphic system and geomorphic impacts on the human system

In many of the Earth's arid and semiarid lands, saline lakes, playas, and similar landforms are disturbed as a result of human activity. Diversion and/or consumptive use of surface or groundwaters has created the effect of a climate change in numerous drainage basins, resulting in the desiccation of lakes and reactivation of eolian processes at many locations. Playas are natural sites for extensive eolian activity because of the deposition of clastic and chemical sediments in basins by surface water (via fluvial transport) and groundwater (via efflorescence). Wind erosion and deposition of playa sediments has had a major role in the development of landforms and sedimentary units in the present (lunette fields worldwide; Simpson Desert, Australia) and geological past, from the Triassic (Mercia Mudstone, England) to the Quaternary (Lahontan Basin and Cima Volcanic Field, USA). Anthropogenic disturbance or desiccation of playa systems has resulted in the eolian transport of sand (e.g. Lop Nor, China; Konya Basin, Turkey; Rajasthan, India; Kappakoola, Australia; several sites in West Africa) and/or dust (e.g. Aral Sea, Kazakhstan/Uzbekistan; Old Wives Lake, Canada; Kara Bogaz Gol, (ex-)USSR; Lake Texcoco, Mexico; Owens (dry) Lake, Mono Lake and other playas, USA). Typically, this is accomplished by abstraction of water and/or removal of vegetation from terminal lake basins. An extensive review of the literature documents many examples and/or potential examples of such phenomena in numerous nations. The reactivation of eolian processes from closed basins produces air pollution in the form of fugitive dust (naturally occurring compounds released into the atmosphere by human actions), and has significant environmental and economic impacts on human activities in the surrounding areas. Restoration or mitigation of degraded land on or surrounding playas has been accomplished at Lake Texcoco, Kara Bogaz Gol and the Konya Basin, and is being actively implemented at Mono Lake, Owens (dry) Lake and the Aral Sea.

Magmatic infiltration and melting in the lower crust and upper mantle beneath the Cima volcanic field, California

Xenoliths of lower crustal and upper mantle rocks from the Cima volcanic field (CVF) commonly contain glass pockets, veins, and planar trains of glass and/or fluid inclusions in primary minerals. Glass pockets occupy spaces formerly occupied by primary minerals of the host rocks, but there is a general lack of correspondence between the composition of the glass and that of the replaced primary minerals. The melting is considered to have been induced by infiltration of basaltic magma and differentiates of basaltic magma from complex conduits formed by hydraulic fracturing of the mantle and crustal rocks, and to have occurred during the episode of CVF magmatism between ∼7.5 Ma and present. Variable compositions of quenched melts resulted from mixing of introduced melts and products of melting of primary minerals, reaction with primary minerals, partial crystallization, and fractionation resulting from melt and volatile expulsion upon entrainment of the xenoliths. High silica melts ( >∼60% SiO2) may result by mixing introduced melts with siliceous melts produced by reaction of orthopyroxene. Other quenched melt compositions range from those comparable to the host basalts to those with intermediate Si compositions and elevated Al, alkalis, Ti, P, and S; groundmass compositions of CVF basalts are consistent with infiltration of fractionates of those basalts, but near-solidus melting may also contribute to formation of glass with intermediate silica contents with infiltration only of volatile constituents.