R-3 Repository :: Browsing by Author "Leeman, William P."

Browsing by Author "Leeman, William P."

Now showing 1 - 17 of 17

Boron metasomatism in the Alta stock contact aureole, Utah
(1995) Woodford, Darrell Todd; Leeman, William P.
The effects of fluid evolution and infiltration in the Alta stock contact aureole, Utah, were studied by evaluating chemical additions and depletions in igneous, replacement skarn, and carbonate whole-rock samples. Of the major and trace elements studied, boron proved to be the most interesting tracer of fluid, due in part to extensive borate mineralization (ludwigite and kotoite) in carbonates near the stock contact, and in part to the physical-chemical properties of boron which make it a useful element for isotopic studies and for spatial distribution analysis via alpha track mapping. Boron is enriched throughout the Alta aureole and is hosted by a variety of metamorphic and metasomatic silicate minerals (forsterite and humite group are the most significant). Aureole whole-rock B concentrations increase toward the contact, indicating that B originated in the stock and migrated down-temperature in exsolved magmatic fluids. Boron stable isotope measurements of carbonates, skarns and mineral separates range from $-$7.2 to +1.6$\perthous\ \delta\sp{11}$B. A value of ${-}6.0\perthous$ for a B-enriched igneous sill falls within this range and also indicates that the stock is the source of aureole B-enrichments.
Geochemistry and regional correlation of pre-tertiary volcanic rocks in West-Central Nevada
(1983) Seidensticker, C. Michael; Oldow, John S.; Leeman, William P.; Lallemant, Hans G. Avé
Isolated exposures of late Paleozoic and Mesozoic volcanic rocks in the Walker Lake region of west-central Nevada are tectonically disrupted, and their interrelationships are uncertain. Recent work has resulted in the recognition of several formations: The Pennsylvanian Shamrock, Permian Black Dyke, Triassic Pamlico, and Cretaceous Gold Range. Each of these formations is characterized, with particular emphasis on lithology, petrography, chemistry, and age. Outcrops of pre-Tertiary volcanic rocks of unknown age are similarly characterized, and several units are thereby correlated with known formations. Geochemical data confirm the presumption that the volcanic rocks formed at a convergent plate boundary. Upper Paleozoic volcanic rocks were created in an island arc system which underwent major deformation during the Early Triassic Sonoma orogeny, whereas Mesozoic volcanic rocks were erupted along an active continental margin and deposited in a back arc basin of the Mesozoic Sierran arc system.
Geology and stratigraphy of the Mount Bennett Hills, and the origin of west-central Snake River Plain rhyolites
(1990) Honjo, Norio; Leeman, William P.
Voluminous mid-Miocene (8-13 Ma) rhyolitic ash-flows and lava flows (Idavada volcanics) are exposed continuously along the northern (Mount Bennett Hills or MBH) and southern margins of the west-central Snake River Plain (SRP). These rhyolites unconformably rest on Cretaceous granitic rocks of Idaho batholith and Eocene Challis volcanics and are locally overlain by (or intercalated with) minor Tertiary basalt flows and fluvial and lacustrine sediments. At both margins, the rhyolites dip gently and thicken toward the SRP. NW-to EW-trending normal faults drop the western SRP forming a graben-like structure where the inferred source vents of the rhyolites are buried by younger basalt flows (Pliocene to Recent) and sediments. The stratigraphy of Idavada volcanics in the MBH and evaluation of correlations with those exposed in the adjacent areas suggest that the rhyolites exposed in the northern margin of the west-central SRP are different from the southern margin counterparts and that the majority of west-central SRP rhyolites were erupted almost concurrently with the formations of the normal faults that formf"d the western SRP graben. Idavada volcanics range in composition from quartz latite to high-silica rhyolite and are characterized by anhydrous mineral assemblages (Plagioclase ±Sanidine ±Quartz + Augite + Pigeonite ± Hypersthene + Fe-Ti Oxides ± Fayalite + accessory Zircon and Apatite), suggesting that the rhyolites were formed under dry conditions. Pyroxene temperatures of Davidson and Lindsley (1985) range from 800 (high-silica rhyolites) to lOOO°C (quartz latites). Major and trace element, and isotopic (87Sr/86Sr = 0.709-0.713; 143Nd/144Nd = 0.51219-0.51230) compositions of the rhyolites suggest that the rhyolites may have formed by partial melting (caused by intrusions of hotspot-related basaltic magmas into the lower crust) of sialic lower crust with significant Juvenile component under dry conditions. Major and trace element variations observed in the west-central SRP rhyolites can be explained by differences in residence times which resulted in variable degrees of fractional crystallization. Variations of isotopic compositions and estimated magmatic temperatures of the rhyolites suggest that higher temperature rhyolitic magmas assimilated more Archean upper crustal material than lower temperature ones.
Glacial marine geology of the George V-Adelie continental shelf, East Antarctica
(1980) Domack, Eugene Walter; Anderson, John B.; Casey, Richard E.; Leeman, William P.
Minéralogie and textural data examined in this study suggest that glacial ice derived from the present day Cook Ice Shelf extended to the edge of the d'Urville Sea continental shelf. Based upon diatom floras and sedimentary relationships, this event took place within the last 18 years. As part of this glacial maximum, basal tills and glacial marine sediments were deposited over an irregular subglacial surface. Extensive redeposition of eroded material took place in the middle and outer portions of the continental shelf. Retreat of glacial ice was relatively rapid and was associated with widespread deposition of a thin residual glacial marine unit and turbidity current deposits in the far western and eastern parts of the region. Today, sedimentation on the continental shelf of the d'Urville Sea is controlled by biogenic and physical oceanographic processes. Deposition of ice-rafted detritus from icebergs undoubtably occurs but is relatively insignificant. Relict glacial and glacial marine deposits are being reworked on the outer and eastern portions of the shelf. This reworking is associated with an impinging Circumpolar Deep Water Mass and a westerly flowing geostrophic current. Deeper, inner portions of the continental shelf are not exposed to these currents and relatively rapid deposition of siliceous muds is occuring in these areas. Depostion of such laminated muds may be seasonal and is most likely related to the production of Saline Shelf Water.
Investigation on the generation of primitive basalts in the southern Washington Cascades
(2004) Lewis, Jared Fairhurst; Leeman, William P.
An investigation of the petrography, mineralogy and chemistry of primitive basalts found in a E-W transect of southern Washington and northern Oregon provides constraints concerning the thermal structure and composition of the mantle wedge beneath the Cascadia subduction zone. The Cascades represents a warm, slowly subducting end-member of subduction zone types. Chemical and petrographical characteristics of basalts from a transect across southern Washington representing at least six variants of primitive melts can be recognized. Estimates of magma temperature and depth based on mineral and whole-rock chemistry provide estimations on the Cascadia subduction zone's thermal structure. Mineral chemistry and whole-rock trace element chemistry are further used to interpret the mantle chemistry and the processes involved in producing subduction zone basalts. The results can be explained by multiple sources found within the subduction zone mantle, melting at various depths and temperatures.
Magma and Mineral Composition Response to Increasing Slab-Derived Fluid Flux: Nevado de Longaví Volcano, Southern Chilean Andes
(Frontiers Media S.A., 2022) Sellés, Daniel; Dungan, Michael; Langmuir, Charles; Rodríguez, Ana Carolina; Leeman, William P.
Nevado de Longaví volcano (NLV), in the Southern-Central Chilean Andes, has erupted during the Holocene magmas with compositions that are in several ways atypical for the region. These characteristics include elevated La/Yb ratios in evolved magmas, in an area of only moderately thick crust, coupled with low concentrations of K, Th, and other incompatible elements and elevated ratios of fluid-mobile (B, Cs, Li, Sb) to fluid-immobile elements. Samples have an unusual mafic mineralogy dominated by amphibole. The petrology of the Holocene products of NLV have been related to the influence of an oceanic transform fault (Mocha Fracture Zone; MFZ) that supplies the mantle wedge with unusually high amounts of fluids via dehydration of serpentinite bodies hosted by the subducted oceanic lithosphere. Because the trace of this transform fault is oblique to the convergence vector, its position along the arc has varied through time, as has the magnitude of its influence on the nature of the magmas erupted at NLV. The whole-rock and mineral chemistry of volcanic products from NLV, tied to a simplified stratigraphy, documents the secular changes in the magmatic system as the oceanic fault approached its current position. Magmas erupted ∼1–0.6 Ma are relatively low in water (as inferred from mineralogy and chemical proxies) and reduced (NNO-1 to NNO+0.5), and are similar to compositions found in neighboring volcanoes. From 0.25 Ma to the present, magmas are water-rich and oxidized (NNO-0.5 to NNO+1.7). In the intervening 0.6–0.25 Ma, mafic magmatism acquired a transient crustal component, which we identify as subducted sediment melts, on the basis of radiogenic isotopes and Pb, Th, and U abundances. Fluids released from serpentinite in the fracture zone were rich in Li, B, Sb, Cs and Ba, but not in K, Th, U and Sr. The fluid addition led to enhanced melting, particularly hydrous magmas that stabilized amphibole early during fractionation, higher oxygen fugacities, and distinctive chemical compositions.
Major and trace element compositions and strontium, neodymium, and lead isotopic ratios of late Cenozoic mafic lavas from the northern Basin and Range
(1992) Lum, Clinton Chew Lun; Leeman, William P.
During the late Cenozoic mafic lavas were emplaced contemporaneously with Basin and Range extension near Battle Mountain, Nevada. They display wide ranges in major and trace elements contents and Sr, Nd, and Pb ratios. The temporal and compositional characteristics of the basalts indicate that the effects of crustal contamination on the compositions of the younger lavas has decreased relative to the older lavas. Assimilation and fractional crystallization (AFC) modeling suggests that their compositional heterogeneity cannot be attributed solely to crustal contamination, and instead most likely reflects their derivation from a heterogeneous mantle source region. Regional variations in the trace element ratios (Rb/La and Ba/Zr) and Sr, Nd and Pb isotopic compositions of mafic lavas from the western U.S. indicate they were derived from a heterogeneous lithospheric mantle. Crustal contamination and subduction related processes appear to be unable to reproduce the range in $\sp{87}$Sr/$\sp{86}$Sr and low Rb/La of the lavas. Therefore, the variation in the basalts' isotopic ratios most likely reflects their derivation from a heterogeneous mantle source region. Plots of $\sp{87}$Sr/$\sp{86}$Sr versus $\sp{207}$Pb/$\sp{204}$Pb or $\sp{207}$Pb/$\sp{207}$Pb clearly illustrates the compositional characteristics of their source regions. The Snake River Plain and southern Sierran Province basalts display elevated $\sp{87}$Sr/$\sp{86}$Sr and varied Pb isotopic ratios indicating they were derived from a heterogeneous ancient lithospheric source with high time integrated Rb/Sr and U/Pb ratios. The low $\sp{87}$Sr/$\sp{86}$Sr and Pb isotopic ratios of the suggest the Colorado Plateau-Eastern Transition Zone basalts were derived from a heterogeneous ancient lithospheric source with low time integrated Rb/Sr and U/Pb ratios. Basalts from the Basin and Range, Oregon Plateau, and northern Sierran Province display a positive correlation between their $\sp{87}$Sr/$\sp{86}$Sr versus $\sp{208}$Pb/$\sp{204}$Pb or $\sp{207}$Pb/$\sp{204}$Pb ratios, which appears to reflect two component mixing between accreted depleted oceanic-type mantle and an enriched mantle component.
Petrology and geochemistry of intermediate rocks in gabbro-granite contact zones, Wichita Province, Oklahoma
(1981) Kennedy, Jerry Wilson; Powell, Benjamin N.; Leeman, William P.; Baker, Donald R.
The investigation of intermediate rocks in two granite-gabbro contact zones in the Wichita province of southwestern Oklahoma has established petrologic and geochemical constraints which suggest intrusive relationships exist between the major silicic and basic plutonic rocks. Field studies of the Poko Mountain area in the eastern portion of the province indicate the contact of the younger granite with the underlying gabbro is discordant and is characterized by the presence of gabbro xenoliths within the granite. Whole-rock and trace element chemistry disclose the intermediate rock found highest in the gabbro section at this exposure is more fractionated than the contact phase of the granite. Negative Eu anomalies demonstrate the granite has experienced strong plagioclase fractionation. The gabbro, however, shows no complimenatry positive Eu patterns. In addition, the plagioclases of the gabbro exhibit reverse cryptic variation with the most calcic plagioclase (An67) being found in the intermediate rock immediately below the granite contact. Petrogenesis of the intermediate rock at Poko Mountain is therefore attributed primarily to crystal fractionation of the gabbro and secondarily to alkali metasomatism from the overlying granite. Examination of the Twin Mountains outcrop in the western part of the province reveals the contact between granite and intermediate rock is knife-sharp. The intermediate rock, originally an anorthositic cumulate, is volumetrically composed of approximately 8% plagioclase but is now identified by a negative Eu anomaly. Contact thermal alteration and resorption of plagioclase within the intermediate rock are evident and indicate disequilibrium. The data suggest the development of the intermediate rock at this locale is due solely to hydrothermal contamination of the basic rock by alkali metasomatism from the adjacent granite. The evidence thus suggests that at neither outcrop are the granites a product of differentiation of the basic rocks. ' Most probably the granites originated by partial melting of the Precambrian granitic basement by upwelling basic magmas during the rifting associated with the evolution of the southern Oklahoma aulacogen.
Petrology and geochemistry of plagiogranite and related basic rocks of the Canyon Mountain ophiolite complex, Oregon
(1980) Gerlach, David Christian; Lallemant, Hans G. Avé; Leeman, William P.; Schwarzer, Rudy R.
The Canyon Mountain complex, located in the Blue Mountains of eastern Oregon, is part of a belt of Paleozoic and Mesozoic rocks believed to represent former island arc and oceanic crust. Stratigraphic relations and radiometric dating have confirmed the age of the complex to be Permian. The Canyon Mountain complex is similar in many ways to other typical ophiolites, although some problematical differences are apparent. Sheeted basalt dikes normal to cumulate layering, and pillow basalts are absent. Basaltic intrusions in the upper portion of the complex cure mostly in the form of sills. A large volume of plagiogranite is present at Canyon Mountain as well. Plagiogranites are concentrated along the contact of high-level hornblende gabbros and diabase with the overlying keratophyre host rocks. Diabase and basalt both predate and intrude the plagiogranites. The field relations, and major- and trace element chemistry support a model by which the plagiogranites were derived by lowpressure partial melting of hydrothermally-altered basic rocks. It is also possible that some of the plagiogranites could be late-stage differentiated liquids. The major and trace element chemistry of the gabbros, diabases, and basalts afford evidence that the ophiolite complex was an open system during much of its development. The evidence does not support a simple model of fractional crystallization of a basaltic magma, but repeated influxes of new basic magmas, each possibly produced by partial melting of a mantle source already depleted in LIL elements. Different mantle source lithologies or more complex melting models may be necessary to explain the comparatively greater depletion in both LREE and HREE in certain samples of basalts. The Canyon Mountain ophiolite was probably created as a result of short-lived magmatism representing incipient spreading in a marginal or back-arc basin. The keratophyres may represent volcanic rocks derived from an adjacent island arc; these may be correlative with arc volcanics of Lower Permian age in the Seven Devils terrane to the north, in which case the Central Melange terrane enclosing the Canyon Mountain complex is also exotic. These terranes, and possibly the Huntington Arc terrane as well, were created at a great distance from the North American continent and were accreted in the Late Jurassic based on structural, stratigraphic, and paleomagnetic data.
A Study of Cathodoluminescence and Trace Element Compositional Zoning in Natural Quartz from Volcanic Rocks: Mapping Titanium Content in Quartz
(Microscopy Society of America, 2012) Leeman, William P.; MacRae, Colin M.; Wilson, Nick C.; Torpy, Aaron; Lee, Cin-Ty A.; Student, James J.; Thomas, Jay B.; Vicenzi, Edward P.
Subduction zone thermal models: Numerical modeling and data analysis
(2002) Huang, Saijin; Leeman, William P.
Temperature distribution in subduction zones is a key factor in understanding magma generation, earthquake occurrence, metamorphic reactions, and geochemical element recycling. In this thesis, I have developed a new numerical method to model subduction zone thermal structures with real slab geometries. The new method couples the finite-difference method with the finite-element method to solve the conduction-convection heat transfer system involved in a subduction zone. The mantle and wedge convection is simulated with the finite-element method with the incoming slab convergence rate imposed as a boundary condition at the slab surface. The heat transfer problem is solved with the finite-difference method. It uses a staggered-grid discretization approach so that the effect of the mantle and wedge convection on the thermal model can be accurately taken into account. With material averaging, the simulation method can easily handle a curved slab with a simple, structured grid. The thermal structures that I have calculated for the east Aleutian subduction zone show that a two-segment slab model can overestimate the slab surface temperature at 100 km by up to 90°C for a 75-km thick overriding lithosphere. The thermal structures of ten subduction zones around the Pacific Rim have been generated including the east Aleutians, the Cascades, the Central and South America, the northeast Japan, and Mariana subduction zones. I have shown that the predicted slab surface temperature at 100 km is correlated with B/Zr ratio. As the slab surface temperature increases, the B/Zr ratio decreases systematically for two different levels of B-enrichment (5 ppm and 10 ppm). The results show that the slab tip temperatures have little correlation with the slab lengths and depths.
The effect of metamorphism on the trace element composition of subducted oceanic crust and sediment
(1993) Moran, Ann Elizabeth; Leeman, William P.
Metasedimentary and metabasaltic rocks of the Pelona schist of Sierra Pelona, southern California, preserve a relatively high P/T inverted metamorphic gradient (Graham and Powell, 1984) and provide an opportunity to study possible compositional changes in an oceanic slab progressively metamorphosed under P-T conditions similar to those in shallow parts of some subduction zones. Progressive metamorphism resulted in continuous major element compositional change in plagioclase, white mica, amphibole, and epidote. Variations in whole-rock compositions do not correlate with increasing metamorphic grade and largely appear to reflect protolith heterogeneity. Loss of H$\sb2$O-rich fluid during chlorite breakdown reactions largely accounts for the decrease in weight percent LOI and H content with increasing metamorphic grade. Comparison with unmetamorphosed equivalents and lower temperature, high-pressure metamorphic rocks (including metabasaltic samples from the Shuksan schist and the Franciscan Complex) suggest that As and Sb may be lost from metasedimentary rocks and that B may be lost from metabasaltic rocks at temperatures less than 450$\sp\circ$C. Trace element analyses obtained by ion microprobe for minerals in the Pelona, Catalina, and Shuksan schists document the mineral residencies of trace elements and the redistribution of trace elements among minerals as modal abundances vary. Boron, Ba, Li, Rb, and Cs are strongly concentrated in micas. In samples with coexisting white mica and biotite, higher concentrations of B, Ba, and Sr are observed in white mica, relative to Li, Rb, and Cs, which preferentially substitute into biotite. Cesium and Rb partition in a relatively constant ratio between white mica and biotite. Strontium is strongly concentrated in epidote, but is increasingly incorporated into white mica and plagioclase as the modal abundance of epidote decreases and as plagioclase compositions become more calcic. Similarly, Li appears to be repartitioned into amphibole as chlorite decreases in modal abundance. These observations support a model of gradual release of fluid-mobile trace elements during progressive metamorphism (as from subducted slabs). Prograde continuous reactions involving mica may particularly control the mobility of alkali and alkaline earth elements enriched in arc magmas and provide a mechanism for fractionating incompatible element ratios (e.g. B/Cs) during subduction zone metamorphism.
The fluid phase in the granulite facies: evidence from the Adirondack Mountains, N. Y
(1984) Lamb, William M.; Valley, John W.; Lallemant, Hans G. Avé; Leeman, William P.
The fluid phase plays an important role in many crustal processes, such as metamorphism, metasomatism, and partial melting, yet little is known about the fluid phase in the deep crust. Early workers assumed that H2O and CO2 were relatively constant with P(H2O) = P(lithostatic) in non-carbonate metamorphic rocks while P(CO2) = P(lithostatic) in carbonates (Turner, 1948). The granulite facies may be an important exception to this traditional view of metamorphic fluids as the lack of hydrous minerals suggests that this may be a metamorphic regime where the fugacity of water is low. Phase equilibria, when applied to fluid buffering reactions, can provide useful information concerning the composition and movement of fluids during the granulite facies metamorphism. The Adirondack Mountains, New York, have been chosen for a study of deep crustal fluids as pressures and temperatures of metamorphism are well known and the area last equilibrated during a single pervasive metamorphism. Two mineral assemblages which buffered the fugacity of H2O (fH2) during the granulite facies metamorphism have been located near the Oregon Dome anorthosite in the Adirondack Mountains, New York. The first assemblage involves the breakdown of amphibole to orthopyroxene, clinopyroxene, quartz and H2O. This assemblage buffered fH2O to low values, with XH2O approximately equal to .1. Another assemblage, which is potentially very useful in the granulite facies, is the reaction of phlogopite and quartz to enstatite, sanidine, and H2O. There are, however, a number of uncertainties which must be evaluated before this assemblage can be applied to calculate fl^O. These include: 1) disagreement between various experiments in the P-T placement of this reaction, 2) a lack of understanding of the relationship of this reaction to more Fe-rich analogues, a problem which is in part due to uncertainties in the determination of the distribution coefficient (KQ) for Mg-Fe between biotite and orthopyroxene, and 3) the possible effects of tetrahedral order or disorder in trioctahedral micas. In spite of these uncertainties this reaction can be applied to calculate the fugacity of water, which is low is these rocks with the average XH2O equal to .1 +/- .1. Two samples contain graphite in addition to an H2O buffering mineral assemblage, making it possible to estimate the fugacity of six fluid species, CO2, H2O, CH4, CO, O2 and H2, if it is assumed that the sum of the partial pressures of these six fluid species is equal to the lithostatic pressure (French, 1966; Ohmoto and Kerrick, 1977). Such calculations indicate that if there was a free fluid phase then-CO2 was the dominant fluid species in these rocks during granulite-facies metamorphism. The other five fluid species were minor. One of the graphite-bearing assemblages is located less then 6 meters from a mineral assemblage which buffered the fugacity of CO2 to low values. This shows that CO2 was not a pervasive fluid in large quantities, but may have been the dominant fluid species in certain rock types. The results of this study indicate that the fugacities of various fluid species can be highly variable and sharp gradients in fluid compositions may exist during granulite-facies metamorphism. Any fluid movement which may have existed in the deep crust would, therefore, be channelized rather than pervasive.
The mineralogy and phase chemistry of silicic tephras erupted from Mount St. Helen's volcano, Washington
(1980) Smith, Diane R.; Leeman, William P.; Powell, Benjamin N.; Schwarzer, Rudy R.
The Recent eruptive history of Mount St. Helens volcano in Washington includes numerous explosive eruptions which produced voluminous and widespread silicic tephra layers. Many tephra sets have "been defined on the "basis of ferromagnesian phenocryst assemblages and carbon-l4 ages (Mullineaux et al., 1975) and provide an excellent record of explosive activity through time. They are useful time-stratigraphic markers as they are interbedded with other sedimentary and volcanic units in areas of the Pacific Northwest. Investigations of petrography, modal abundances of heavy minerals and phase compositions (determined by electron microprobe) were utilized in an effort to define parameters to unambiguously identify individual tephras. The characteristics which are most distinctive of different tephras include orthopyroxene compositions, iron-titanium oxide compositions, equilibration temperatures and oxygen fugacities obtained with magnetite-ilmenite geothermometry and geobarometry, and ferromagnesian silicate assemblages as defined by optical and x-ray diffraction techniques. No single characteristic is in itself distinctive and several characteristics should be used in identification of unknown tephras. The wide compositional ranges for silicate minerals, variable modal abundances of heavy minerals, and observed ranges of oxide equilibration temperatures and oxygen fugacities suggest that some of the magmas may have been chemically or physically heterogeneous. Iron-titanium oxide equilibration temperatures and oxygen fugacities correlate with mineral assemblage and composition. High temperatures and oxygen fugacities correspond with assemblages of hornblende + cummingtonite + orthopyroxene (relatively low in FeO) and lower temperatures and oxygen fugacities correspond with hornblende + orthopyroxene (relatively enriched in FeO). The Mount St. Helens cummingtonite-bearing tephras show higher temperatures and oxygen fugacities of equilibration of associated iron-titanium oxides compared to other cummingtonite-bearing silicic pumice deposits. Temporal trends of temperature and ferromagnesian silicate assemblages suggest that some of the tephras may be related, but limited geochemical data and biased sampling of only silicic tephras and not other products erupted during the same time intervals do not allow strong conclusions regarding petrogenetic relations.
The origin and evolution of lavas from Haleakala Crater, Hawaii
(1988) West, Howard Bruce; Leeman, William P.
Sr, Nd, and Pb isotope systematics of lavas from the Maui Volcanic Complex (MVC) are consistent with a three-component petrogenetic mixing model. MVC shield-building (SB) lavas define linear trends on isotope-isotope plots, consistent with two-component mixing between primitive (PM) and enriched (EM) mantle components. The two-component (PM-EM) Hawaiian plume source is variable in composition during production of tholeiite magmas even within a single shield. Sr and Pb isotopic ratios of Haleakala post shield-building (PSB) lavas define a strong positively correlated array that deviates from the SB array towards an unradiogenic end-member. The PSB array may therefore result from time- and volume-dependent binary mixing between Hawaiian plume melts and a depleted (DM) mantle (i.e. MORB source) component. Several trace element ratios in Haleakala PSB lavas are correlated with isotopic compositions but not with major and trace element contents, and therefore appear to reflect changes in source composition. Trace element mixing systematics for these lavas indicate that the DM component must be a melt. The inferred PM component has chondritic ratios for several trace elements, consistent with it representing primitive mantle. The EM component may represent a part of the Hawaiian plume source that was either metasomatized or metasomatically scavenged. Alkalic cap lavas exposed in the northwest wall of Haleakala Crater display systematic, upsection geochemical variations indicative of the repetitive intrusion of discrete magma batches. Magma batches are separated by geochemical discontinuities characterized by abrupt upsection increases in incompatible element contents and commensurate drops in compatible element contents. In contrast, lava compositions within magma batches vary upsection progressively, and geochemical variations are opposite to those observed for interbatch discontinuities. Together, these geochemical variations are interpreted as resulting from the cyclic operation of a dynamic, evolving, open system magma chamber. Interbatch transitions appear to reflect periods of eruptive quiessence characterized by low magma recharge rates and relatively high degrees of crystal fractionation. Intrabatch variations appear to represent eruptive periods characterized by relatively high recharge rates, low degrees of crystal frationation, and progressive mixing of evolved rest magma with more primitive recharge magma.
Trace element geochemistry of basalts from the Oregon Cascade Range: Implications for magma generation and possible variations in the sub-arc mantle
(1998) Barker, Sharon Lindsay; Leeman, William P.
Analyses of Oregon Cascades basalts reveal two groups of High Cascades (9-0 Ma) lavas: (1) "enriched" basalts with high Ba, Sr, Ba/Nb, and Ba/La, and (2) "depleted" basalts with flat rare earth element patterns and relatively low Sr and Ba. Fluid mobile elements, such as B, are depleted in High Cascades basalts. The decoupling of B and Ba presents an interesting problem for volcanic arc magma generation, since both elements are believed to be controlled through fluid input from the subducting slab. Petrogenetic modeling reveals that most Cascade magmas can be derived by uniform. partial melting of variably metasomatized primitive or NMORB-source mantle or by varying degrees of melting of a mineralogically inhomogeneous metasomatized mantle. The decoupling of B and Ba may result from shallow dehydration reactions in the slab which release B into the overlying mantle, while Ba is retained in the slab and released during subsequent dehydration reactions.
U-Pb LA-ICP-MS Zircon Dating of Crustal Xenoliths: Evidence of the Archean Lithosphere Beneath the Snake River Plain
(MDPI, 2024) Leeman, William P.; Vervoort, Jeffrey D.; DuFrane, S. Andrew
New U-Pb zircon ages are reported for granulite facies crustal xenoliths brought to the surface by mafic lavas in the Snake River Plain. All samples yield Meso-to-Neoarchean ages (2.4–3.6 Ga) that significantly expand the known extent of the Archean Wyoming Craton at least as far west as the west-central Snake River Plain. Most zircon populations indicate multiple growth episodes with complexity increasing eastward, but they bear no record of major Phanerozoic magmatic episodes in the region. To extrapolate this work further west to the inferred craton boundary, zircons from southwestern Idaho batholith granodiorites were also analyzed. Although most batholith zircons record Cretaceous formation ages, all samples have zircons with inherited cores—with some recording Proterozoic ages (approaching 2 Ga). These data enhance our perspectives regarding lithosphere architecture beneath southern Idaho and adjacent areas and its possible influence on Cenozoic magmatism associated with the Snake River Plain–Yellowstone “melting anomaly”.

Browsing by Author "Leeman, William P."

Results Per Page

Sort Options