Geology topics

W.A. Bohrson

Publications and source records attributed to W.A. Bohrson.

7 recordsLinked to original sources

USGS2010

Variations in the fine-scale composition of a central Pacific ferromanganese crust: Paleoceanographic implications

A 47- to 60-mm-thick Fe-Mn crust from Horizon Guyot (water depth 1800–1780 m), central Pacific, was used to evaluate the potential of crusts as recorders of Neogene paleoceanographic and paleoclimatic conditions. The chemical composition was determined by microprobe for 16 elements from a polished thin section. Three analyses were made per millimeter and averaged to give the composition of each millimeter. The age of the crust was determined by measuring the strontium isotope composition of the crust and comparing it with the Tertiary seawater curve. The crust represents 18.5 m.y. of growth of Fe and Mn oxyhydroxides. The crust is composed of alternating botryoidal and laminated layers. The botryoidal layers formed during the same time intervals that widespread Neogene deep-sea hiatuses were forming in bottom sediments. The botryoidal layers represent growth during times of intensified deepwater flow, whereas the laminated intervals represent more quiescent conditions. The correspondence between the botryoidal layers and the Neogene hiatuses is so strong that we were able to choose a variable growth rate model over a constant growth rate model for the crust. Chemical changes in the crust take two forms. The first is represented by broad changes in the composition defined chiefly by fourth-order polynomial fits to the chemical profiles of each element with depth in the crust. The second is high-frequency changes in composition. The broader changes occurred primarily at about 15, 11.5, 7.4, 6.4, 5.2, and 4.6 Ma, which may correlate with major changes in paleoceanographic circulation and development of ice caps at the poles. The periods of the high-frequency changes may reflect climatic changes that resulted from orbital forcing. These high-frequency changes may correspond to the high-order eccentricity periods of 3.47, 2.04, and 1.31 m.y.

Paleoceanography and Paleoclimatology↗

USGS2008

Multiple plagioclase crystal populations identified by crystal size distribution and in situ chemical data: Implications for timescales of magma chamber processes associated with the 1915 eruption of Lassen Peak, CA

Products of the 1915 Lassen Peak eruption reveal evidence for a magma recharge–magma mixing event that may have catalyzed the eruption and from which four compositional members were identified: light dacite, black dacite, andesitic inclusion, and dark andesite. Crystal size distribution, textural, and in situ chemical (major and trace element and Sr isotope) data for plagioclase from these compositional products define three crystal populations that have distinct origins: phenocrysts (long axis > 0·5 mm) that typically have core An contents between 34 and 36 mol %, microphenocrysts (long axis between 0·1 and 0·5 mm) that have core An contents of 66–69, and microlites (long axis < 0·1 mm) with variable An core contents from 64 to 52. Phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling. Based on textural, compositional, and isotopic data for the magma represented by the dacitic component, magma recharge was not an important process until just prior to the 1915 eruption. Average residence times for phenocrysts are in the range of centuries to millennia. Microphenocrysts formed in a hybrid layer that resulted from mixing between end-member reservoir dacite and recharge magma of basaltic andesite composition. High thermal contrast between the two end-member magmas led to relatively high degrees of undercooling, which resulted in faster crystal growth rates and acicular and swallowtail crystal habits. Some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution–precipitation, seen in both crystal textures and rim compositions. Average microphenocryst residence times are of the order of months. Microlites may have formed in response to decompression and/or syn-eruptive degassing as magma ascended from the chamber through the volcanic conduit. Chemical distinctions in plagioclase microlite An contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions. We suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population. Melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing; as a consequence, melt and microlites among these three products have similar compositions. The results of this study highlight the integrated use of crystal size distribution, textural, and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies.

Journal of Petrology↗

USGS2006

Phase equilibria constraints on the chemical and physical evolution of the campanian ignimbrite

The Campanian Ignimbrite is a > 200 km3 trachyte-phonolite pyroclastic deposit that erupted at 39.3 ?? 0.1 ka within the Campi Flegrei west of Naples, Italy. Here we test the hypothesis that Campanian Ignimbrite magma was derived by isobaric crystal fractionation of a parental basaltic trachyandesitic melt that reacted and came into local equilibrium with small amounts (5-10 wt%) of crustal rock (skarns and foid-syenites) during crystallization. Comparison of observed crystal and magma compositions with results of phase equilibria assimilation-fractionation simulations (MELTS) is generally very good. Oxygen fugacity was approximately buffered along QFM+1 (where QFM is the quartz-fayalite-magnetite buffer) during isobaric fractionation at 0.15 GPa (???6 km depth). The parental melt, reconstructed from melt inclusion and host clinopyroxene compositions, is found to be basaltic trachyandesite liquid (51.1 wt% SiO2, 9.3 wt% MgO, 3 wt% H2O). A significant feature of phase equilibria simulations is the existence of a pseudo-invariant temperature, ???883??C, at which the fraction of melt remaining in the system decreases abruptly from ???0.5 to < 0.1. Crystallization at the pseudo-invariant point leads to abrupt changes in the composition, properties (density, dissolved water content), and physical state (viscosity, volume fraction fluid) of melt and magma. A dramatic decrease in melt viscosity (from 1700 Pa s to ???200 Pa s), coupled with a change in the volume fraction of water in magma (from ??? 0.1 to 0.8) and a dramatic decrease in melt and magma density acted as a destabilizing eruption trigger. Thermal models suggest a timescale of ??? 200 kyr from the beginning of fractionation until eruption, leading to an apparent rate of evolved magma generation of about 10-3 km3/year. In situ crystallization and crystal settling in density-stratified regions, as well as in convectively mixed, less evolved subjacent magma, operate rapidly enough to match this apparent volumetric rate of evolved magma production. ?? Copyright 2007 Oxford University Press.

Journal of Petrology↗

USGS2006

Chapter 13 Petrogenesis of the Campanian Ignimbrite: Implications for crystal-melt separation and open-system processes from major and trace elements and Th isotopic data

The Campanian Ignimbrite is a large-volume trachytic to phonolitic ignimbrite that was deposited at ≈39.3 ka and represents one of a number of highly explosive volcanic events that have occurred in the region near Naples, Italy. Thermodynamic modeling using the MELTS algorithm reveals that major element variations are dominated by crystal-liquid separation at 0.15 GPa. Initial dissolved H 2 O content in the parental melt is ∼3 wt.% and the magmatic system fugacity of oxygen was buffered along QFM+1. Significantly, MELTS results also indicate that the liquid line of descent is marked by a large change in the proportion of melt (from 0.46 to 0.09) at ∼884°C, which leads to a discontinuity in melt composition (i.e., a compositional gap) and different thermodynamic and transport properties of melt and magma across the gap. Crystallization of alkali feldspar and plagioclase dominates the phase assemblage at this pseudo-invariant point temperature of ∼884°C. Evaluation of the variations in the trace elements Zr, Nb, Th, U, Rb, Sm, and Sr using a mass balance equation that accounts for changing bulk mineral-melt partition coefficients as crystallization occurs indicates that crystal-liquid separation and open-system processes were important. Th isotope data yield an apparent isochron that is ∼20 kyr younger than the age of the deposit, and age-corrected Th isotope data indicate that the magma body was an open system at the time of eruption. Because open-system behavior can profoundly change isotopic and elemental characteristics of a magma body, these Th results illustrate that it is critical to understand the contribution that open-system processes make to magmatic systems prior to assigning relevance to age or timescale information derived from such systems. Fluid-magma interaction has been proposed as a mechanism to change isotopic and elemental characteristics of magma bodies, but an evaluation of the mass and thermal constraints on such a process suggests large-scale interaction is unlikely. In the case of the magma body associated with the Campanian Ignimbrite, the most likely source of the open-system signatures is assimilation of partial melts of compositionally heterogeneous basement composed of cumulates and intrusive equivalents of volcanic activity that has characterized the Campanian region for over 300 kyr.

Developments in Volcanology↗

USGS2001

New constraints on the pyroclastic eruptive history of the Campanian volcanic Plain (Italy)

The &sim; 150 km 3 (DRE) trachytic Campanian Ignimbrite, which is situated north-west of Naples, Italy, is one of the largest eruptions in the Mediterranean region in the last 200 ky. Despite centuries of investigation, the age and eruptive history of the Campanian Ignimbrite is still debated, as is the chronology of other significant volcanic events of the Campanian Plain within the last 200–300 ky. New 40 Ar/ 39 Ar geochronology defines the age of the Campanian Ignimbrite at 39.28 ± 0.11 ka, about 2 ky older than the previous best estimate. Based on the distribution of the Campanian Ignimbrite and associated uppermost proximal lithic and polyclastic breccias, we suggest that the Campanian Ignimbrite magma was emitted from fissures activated along neotectonic Apennine faults rather than from ring fractures defining a Campi Flegrei caldera. Significantly, new volcanological, geochronological, and geochemical data distinguish previously unrecognized ignimbrite deposits in the Campanian Plain, accurately dated between 157 and 205 ka. These ages, coupled with a xenocrystic sanidine component > 315 ka, extend the volcanic history of this region by over 200 ky. Recent work also identifies a pyroclastic deposit, dated at 18.0 ka, outside of the topographic Campi Flegrei basin, expanding the spatial distribution of post-Campanian Ignimbrite deposits. These new discoveries emphasize the importance of continued investigation of the ages, distribution, volumes, and eruption dynamics of volcanic events associated with the Campanian Plain. Such information is critical for accurate assessment of the volcanic hazards associated with potentially large-volume explosive eruptions in close proximity to the densely populated Neapolitan region.

Mineralogy and Petrology↗

USGS1995

Chemically diverse, sporadic volcanism at seamounts offshore southern and Baja California

Compositions of lavas from seven small to medium-sized seamounts, between lat 34.0°N and 30.5°N offshore southern and Baja California, include low-K 2 O tholeiitic, transitional, and mildly to moderately alkalic basalt and their differentiates. The low-K 2 O tholeiites resemble primitive (>9% MgO) mid-oceanic-ridge basalt (MORB) with low incompatible element abundances and very depleted, concave-downward, chondrite-normalized rare-earth-element (REE) patterns and lower 87 Sr/ 86 Sr and higher 143 Nd/ 144 Nd ratios than typical MORB from the East Pacific Rise. The seamounts with these MORB-like lavas are inferred to have formed at or near the spreading center. Transitional and mildly to moderately alkalic basalts have higher abundances of incompatible elements and steeper slopes for chondrite-normalized REE patterns with light REE enrichment up to 150 times chondrites. The alkalic compositions indicate more variably enriched mantle sources than those of most seamounts presently located near the East Pacific Rise, but the compositions are within the mantle array defined by other ocean-island basalts. Volcanic rocks from the upper part of Rocas Alijos, a much larger and morphologically more complex edifice than the northern seamounts, located offshore central Baja California at lat ∼25°N, are all highly differentiated trachyte and trachyandesite. Based on 40 Ar/ 39 Ar laser fusion techniques, MORB-like lava from one of the northern edifices is as old as the underlying oceanic crust (>20 Ma), indicating that it originated at a spreading center. Other seamount lava ages are much younger than the oceanic crust on which they reside, ranging from 16.8 ± 0.3 to <7 Ma for some of the northern seamounts to 270 ± 16 ka for the trachyte from Rocas Alijos. Similar highly evolved lavas cap fossil spreading centers like Guadalupe and Socorro Islands, but Rocas Alijos, based on magnetic anomalies, is not an abandoned spreading center but may instead have formed on a leaky transform fault. Some of the seamounts with transitional and alkalic lavas may have formed as part of a short, age-progressive chain formed by a short-lived mantle plume. Many others, aligned along abandoned spreading centers or faults and fracture zones which are abundant in the tectonically complex region offshore southern and peninsular California, may have resulted from upwelling mantle diapirs in response to localized extension. Some of the episodes of volcanism appear to have been contemporaneous with volcanism in the continental borderland and coastal southern California, suggesting linkage between extension along the continental margin and the seamount province farther offshore. The data available for the abundant volcanic edifices of varying sizes, shapes, and orientations in this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism, tapping variably enriched, heterogeneous mantle, which occurred sporadically from early Miocene to late Pleistocene.

Baja California↗

USGS1986

Preliminary geologic map of Kiholo quadrangle, Hawaii

No abstract available.

Hawaii↗