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J. E. Nielson

Publications and source records attributed to J. E. Nielson.

17 recordsLinked to original sources
USGS2012

Age of the Peach Springs Tuff, southeastern California and western Arizona

Sanidine separates from pumice of the early Miocene Peach Springs Tuff are concordantly dated at 18.5±0.2 Ma by two isotopic techniques. The Peach Springs Tuff is the only known unit that can be correlated between isolated outcrops of Miocene strata from the central Mojave Desert of southeastern California to the western Colorado Plateau in Arizona, across five structural provinces, a distance of 350 km. Thus the age of the Peach Springs Tuff is important to structural and paleogeographic reconstructions of a large region. Biotite and sanidine separates from bulk samples of the Peach Springs Tuff from zones of welding and vapor-phase alteration have not produced consistent ages by the K-Ar method. Published ages of mineral separates from 17 localities ranged from 16.2 to 20.5 Ma. Discordant 40 Ar/ 39 Ar incremental release spectra were obtained for one biotite and two of the sanidine separates. Ages that correspond to the last gas increments are as old as 27 Ma. The 40 Ar/ 39 Ar incremental release determinations on sanidine separated from blocks of Peach Springs Tuff pumice yield ages of 18.3±0.3 and 18.6±0.4 Ma. Laser fusion measurements yield a mean age of 18.51±0.10. The results suggest that sanidine and biotite K-Ar ages older than about 18.5 Ma are due to inherited Ar from pre-Tertiary contaminants, which likely were incorporated into the tuff during deposition. Sanidine K-Ar ages younger than 18 Ma probably indicate incomplete extraction of radiogenic 40 Ar, whereas laser fusion dates of biotite and hornblende younger than 18 Ma likely are due to postdepositional alteration. Laser fusion ages as high as 19.01 Ma on biotite grains from pumice suggest that minerals from pre-Tertiary country rocks also were incorporated in the magma chamber.

Journal of Geophysical Research Solid Earth↗
USGS2012

Tertiary basin development and tectonic implications, Whipple Detachment System, Colorado River Extensional Corridor, California and Arizona

Colorado River extensional corridor. In the Mohave Mountains and Aubrey Hills of Arizona and the eastern Whipple Mountains of California near Parker Dam, these deposits comprise four unconformity-bounded sequences composed of locally derived epiclastic and volcanic rocks and the Peach Springs Tuff. The three older sequences represent syntectonic units that were deposited coeval with detachment faulting, and the fourth is interpreted to be postextensional. The sequences are correlated between four fault-bounded regions, which are the remnants of four different depositional basins. Similar sequences can be correlated over broad areas of the extensional corridor despite the general lack of widespread units. The basins developed in about the same positions, relative to each other and to volcanic sources, as they occupy at present. This is shown by gradational changes of pre-Tertiary rock types between regions, systematic variations in the abundance of magmatic units, and correlative volcanic units that occur in two adjacent regions. The basins formed in the early Miocene from segmentation of the upper crust into blocks bounded by high-angle faults that trended both parallel and perpendicular to the direction of extension and which were terminated at middle crustal depths by a low-angle detachment fault. Extreme rotation of one large crustal block, which constitutes the central Mohave Mountains, is recorded by a major unconformity in the lower Miocene section of one basin. Because coeval sections that formed in adjoining basins do not record this rotation, the underlying crustal blocks must have been separated by transfer faults that allowed them to rotate independently. These proposed transfer faults are represented at present by major faults with trends that parallel the direction of extension on the Whipple detachment system.

Arizona, California↗
USGS1999

Geologic map of the Mohave Mountains area, Mohave County, western Arizona

Introduction The Mohave Mountains area surrounds Lake Havasu City, Arizona, in the Basin and Range physiographic province. The Mohave Mountains and the Aubrey Hills form two northwest-trending ranges adjacent to Lake Havasu (elevation 132 m; 448 ft) on the Colorado River. The low Buck Mountains lie northeast of the Mohave Mountains in the alluviated valley of Dutch Flat. Lowlands at Standard Wash separate the Mohave Mountains from the Bill Williams Mountains to the southeast. The highest point in the area is Crossman Peak in the Mohave Mountains, at an elevation of 1519 m (5148 ft). Arizona Highway 95 is now rerouted in the northwestern part of the map area from its position portrayed on the base map; it now also passes through the southern edge of the map area. Geologic mapping was begun in 1980 as part of a program to assess the mineral resource potential of Federal lands under the jurisdiction of the U.S. Bureau of Land Management (Light and others, 1983). Mapping responsibilities were as follows: Proterozoic and Mesozoic rocks, K.A. Howard; dikes, J.K. Nakata; Miocene section, J.E. Nielson; and surficial deposits, H.G. Wilshire. Earlier geologic mapping includes reconnaissance mapping by Wilson and Moore (1959). The present series of investigations has resulted in reports on the crystalline rocks and structure (Howard and others, 1982a), dikes (Nakata, 1982), Tertiary stratigraphy (Pike and Hansen, 1982; Nielson, 1986; Nielson and Beratan, 1990), surficial deposits (Wilshire and Reneau, 1992), tectonics (Howard and John, 1987; Beratan and others, 1990), geophysics (Simpson and others, 1986), mineralization (Light and McDonnell, 1983; Light and others, 1983), field guides (Nielson, 1986; Howard and others, 1987), and geochronology (Nakata and others, 1990; Foster and others, 1990).

Arizona↗
USGS1993

Actualistic models of mantle metasomatism documented in a composite xenolith from Dish Hill, California

Major and trace-element whole rock and mineral variations in composite hornblendite-peridotite xenolith Ba-2-1, from Dish Hill, CA, are due to a single event of metasomatism in the mantle. The hornblendite is the crystallized selvage of a dike conduit charged with incompatible-element-enriched hydrous mafic magma. The magma infiltrated the refractory peridotite wallrock, reacted with its constituent minerals, and simultaneously deposited amphibole. The systematic data from this study show considerable variation in isotopic values and trace elements. These data provide insight into a mantle process that was defined previously from samples without context, lacking evidence about the number or source of metasomatic events. In the contact zone of Ba-2-1, peridotite is enriched in Fe, Ti, CO 2 ) and H 2 O; clinopyroxene and amphibole also are enriched in Fe and Ti, but clinopyroxene appears slightly depleted in CaO. Compared to chondrites, peridotite, clinopyroxene, and probably amphibole are enriched in light rare earth (LREE cn ) and other incompatible trace elements. Values of 87 Sr 86 Sr "> 87 Sr 86 Sr and 143 Nd 144 Nd "> 143 Nd 144 Nd in the contact zone are close to isotopic equilibrium with the dike. Whole rock and constituent clinopyroxene compositions change to those of refractory peridotite with distance from the contact. These compositional variations were modelled using Gresens' equation for whole-rock major and minor elements, and calculations for isotopic ratios and REEs, which emulate the effects of Chromatographic fractionation. The choice of endmembers was restricted to compositions actually present in mantle samples from Dish Hill. Model results indicate that: 1. the variations can be explained as the result of a single metasomatic event, probably a single pulse of previously fractionated liquid; 2. the ratio of total interacting liquid to peridotite was at least 1:3 by weight in the contact zone; and 3. the composition of the metasomatic liquid changed progressively as it infiltrated beyond that zone. The small distance over which variations occur is due to the small amount of liquid that infiltrated. Only in the contact zone was peridotite wallrock saturated by a liquid composition similar to the dike. Comparison of the Ba-2-1 data with those of another xenolith from Dish Hill suggests that the compositional variations of mantle metasomatism result from both the compositional contrast between the metasomatizing liquid and wallrock and the relative abundances of each. Compositional and volumetric variations of mantle partial melts and their fractionates, and repeated events of melting and reaction in contiguous mantle, can create broad ranges of metasomatic “signatures” from the same process.

California↗
USGS1993

Magma transport and metasomatism in the mantle: a critical review of current geochemical models

Conflicting geochemical models of metasomatic interactions between mantle peridotite and melt all assume that mantle reactions reflect chromatographic processes. Examination of field, petrological, and compositional data suggests that the hypothesis of chromatographic fractionation based on the supposition of large-scale percolative processes needs review and revision. Well-constrained rock and mineral data from xenoliths indicate that many elements that behave incompatibly in equilibrium crystallization processes are absorbed immediately when melts emerge from conduits into depleted peridotite. After reacting to equilibrium with the peridotite, melt that percolates away from the conduit is largely depleted of incompatible elements. Continued addition of melts extends the zone of equilibrium farther from the conduit. Such a process resembles ion-exchange chromatography for H2O purification, rather than the model of chromatographic species separation. -from Authors

American Mineralogist↗
USGS1991

Metasomatic oxidation of upper mantle periodotite

Examination of Fe3+ in metasomatized spinel peridotite xenoliths reveals new information about metasomatic redox processes. Composite xenoliths from Dish Hill, California possess remnants of magmatic dikes which were the sources of the silicate fluids responsible for metasomatism of the peridotite part of the same xenoliths. Mo??ssbauer spectra of mineral separates taken at several distances from the dike remnants provide data on Fe3+ contents of minerals in the metasomatized peridotite. Clinopyroxenes contain 33% of total iron (FeT) as Fe3+ (Fe3+/FeT=0.33); orthopyroxenes contain 0.06-0.09 Fe3+/FeT; spinels contain 0.30-0.40 Fe3+/FeT; olivines contain 0.01-0.06 Fe3+/FeT; and metasomatic amphibole in the peridotite contains 0.85-0.90 Fe3+/FeT. In each mineral, Fe3+ and Fe2+ cations per formula unit (p.f.u.) decrease with distance from the dike, but the Fe3+/FeT ratios of each mineral do not vary. Clinopyroxene, spinel, and olivine Fe3+/FeT ratios are significantly higher than in unmetasomatized spinel peridotites. Metasomatic changes in Fe3+/FeT ratios in each mineral are controlled by the oxygen fugacity of the system, but the mechanism by which each phase accommodates this ratio is affected by crystal chemistry, kinetics, rock mode, fluid composition, fluid/rock ratio, and fluid-mineral partition coefficients. Ratio increases in pyroxene and spinel occur by exchange reactions involving diffusion of Fe3+ into existing mineral grains rather than by oxidation of existing Fe2+ in peridotite mineral grains. The very high Fe3+/FeT ratio in the metasomatic amphibole may be a function of the high Fe3+/FeT of the metasomatic fluid, crystal chemical limitations on the amount of Fe3+ that could be accommodated by the pyroxene, spinel, and olivine of the peridotite, and the ability of the amphibole structure to accommodate large amounts of 3 + valence cations. In the samples studied, metasomatic amphibole accounts for half of the bulk-rock Fe2O3. This suggests that patent metasomatism may produce a greater change in the redox state of mantle peridotite than cryptic metasomatism. Comparison of the metasomatized samples with unmetasomatized peridotites reveals that both Fe2+ and Fe3+ cations p.f.u. were increased during metasomatism and 50% or more of iron added was Fe3+. With increasing distance from the dike, the ratio of added Fe3+ to added Fe2+ increases. The high Fe3+/FeT of amphibole and phlogopite in the dikes and in the peridotite, and the high ratios of added Fe3+/added Fe2+ in pyroxenes and spinel suggest that the Fe3+/FeT ratio of the metasomatic silicate fluid was high. As the fluid perolated through and reacted with the peridotite, Fe3+ and C-O-H volatile species were concentrated in the fluid, increasing the fluid Fe3+/FeT. ?? 1991 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1991

Intrinsic Nd, Pb, and Sr isotopic heterogeneities exhibited by the Lherz Alpine Peridotite Massif, French Pyrenees

Optically clear, acid-leached Cr-diopside, Al-augite, pargasitic amphibole, and phlogopite from peridotite and ultramafic dikes of the Lherz massif (French Pyrenees) exhibit chemical and isotopic heterogeneities indicative of a complex history. The data cover a wide range on the 143 Nd/ 144 Nd vs. 87 Sr/ 86 Sr covariation diagram, encompassing much of the mantle array and a large area below the array. Pb-isotopic signatures cover a wide range on Pb-Pb correlation diagrams, with several samples falling in the mid-ocean ridge basalt (MORB) field, but with others defining trends towards the enriched mantle II (EM II) and high-mu (HIMU) endmembers of Zindler & Hart (1986). Peridotites display the largest range in isotopic compositions, which formed in response to variable degrees of partial melting and subsequent enrichment during the intrusion of the ultramafic dikes. The ultra-mafic dikes represent at least four generations. Dike isotopic compositions range from ‘depleted’ to ‘enriched’, relative to Bulk Earth, reflecting the average composition of their source regions and subsequent melt-wall rock interactions. No simple isochronous relationships are observed on the 143 Nd/ 144 Nd vs. 147 Sm/ 144 Nd diagram for any lithologic group, demonstrating that the rock assemblages were not created by simple mixing of two homogeneous endmembers. Some ultramafic dikes, which represent low-temperature melt fractions, are enriched in the heavy isotopes of Sr, Nd, and Pb; any magmas derived from partial melting of these dikes are likely to have enriched isotopic signatures, particularly when the degrees of partial melting are only a few per cent.

Pyrenees↗
USGS1986

Correlation of the Peach Springs Tuff, a large-volume Miocene ignimbrite sheet in California and Arizona

The Peach Springs Tuff is a distinctive early Miocene ignimbrite deposit that was first recognized in western Arizona. Recent field studies and phenocryst analyses indicate that adjacent outcrops of similar tuff in the central and easten Mojave Desert may be correlative. This proposed correlation implies that outcrops of the tuff are scattered over an area of at least 35 000 km 2 from the western Colorado Plateau to Barstow, California, and that the erupted volume, allowing for posteruption crustal extension, was at least several hundred cubic kilometres. Thus, the Peach Springs Tuff may be a regional stratigraphic marker, useful for determining regional paleogeography and the time and extent of Tertiary crustal extension.

Geology↗