Geology topics

Robert J. Bodnar

Publications and source records attributed to Robert J. Bodnar.

9 recordsLinked to original sources

USGS2024

Insights into magma storage depths and eruption controls at Kīlauea Volcano during explosive and effusive periods of the past 500 years based on melt and fluid inclusions

Kīlauea Volcano experiences centuries-long cycles of explosive and effusive eruptive behavior, but the relation, if any, between these eruptive styles and changing conditions in the magma plumbing system remains poorly known. We analyze olivine-hosted melt and fluid inclusions to determine magma storage depths during the explosive-era Keanakākoʻi Tephra eruptions (∼1500–1840 CE) and compare these results to modern effusive-era Kīlauea eruptions (1959 Kīlauea Iki, 1960 Kapoho, 2018 lower East Rift Zone). We find that shallow (1–3 km) magma storage has persisted for centuries at Kīlauea, spanning both explosive and effusive periods. In contrast, mid-crustal zones of magma storage shallowed over time, from 5 to 8 km during the Keanakākoʻi sequence to 3–5 km during the modern effusive period. Melt and fluid inclusions in high-forsterite olivine (Fo 86–89 ) trapped at shallow depths indicate that high-temperature magmas (1200 to ∼1300 °C) commonly reach depths of ≤3 km. CO 2 -rich fluid inclusions are present in olivine from all investigated Kīlauea eruptions but are larger and much more abundant in Keanakākoʻi units, which we interpret as indicating that a greater volume fraction of exsolved CO 2 -rich fluid was present in pre-eruptive Keanakākoʻi melts. Increased amounts of CO 2 -rich fluids in the Keanakākoʻi-era magmas would have increased magma buoyancy and driven rapid magma ascent, thereby increasing eruption energy and enhancing near-surface magma-water interactions compared to the current effusive period.

Hawaii↗

USGS2021

Formation of miarolitic-class, segregation-type pegmatites in the Taishanmiao batholith, China: The role of pressure fluctuations and volatile exsolution during pegmatite formation in a closed, isochoric system

The Taishanmiao granitic batholith, located in the Eastern Qinling Orogen in Henan Province, China, contains numerous small (mostly tens of centimeters in maximum dimension) bodies exhibiting textures and mineralogy characteristics of simple quartz and alkali feldspar pegmatites. Analysis of melt inclusions (MI) and fluid inclusions (FI) in pegmatitic quartz, combined with Rhyolite-MELTS modeling of the crystallization of the granite, have been applied to develop a conceptual model of the physical and geochemical processes associated with the formation of the pegmatites. These results allow us to consider the formation of the Taishanmiao pegmatites within the context of varios models that have been proposed for pegmatite formation. Field observations and geochemical data indicate that the pegmatites represent the latest stage in the crystallization of the Taishanmiao granite and occupy ≤4 vol% of the syenogranite phase of the batholith. Results of Rhyolite-MELTS modeling suggest that the pegmatite-forming melts can be produced through continuous fractional crystallization of the Taishanmiao granitic magma, consistent with the designation of the pegmatites as a miarolitic class, segregation-type pegmatites rather than the more common intrusive-type of pegmatite. The mineral assemblage predicted by Rhyolite-MELTS after ~96% of the original granite-forming melt had crystallized consists of ~51 vol% alkali feldspar, 34 vol% quartz, 14 vol% plagioclase, 0.1 vol% biotite, and 1 vol% magnetite, similar to the alkali feldspar + quartz dominated mineralogy of the pegmatites. Moreover, the modeled residual melt composition following crystallization of ~96% of the original melt is similar to the composition of homogenized MI in quartz within the pegmatite. Rhyolite-MELTS predicts that the granite-forming melt remained volatile-undersaturated during crystallization of the batholith and contained ~6.3 wt% H 2 O and ~500 ppm CO 2 after ~96% crystallization when the pegmatites began to develop. The Rhyolite-MELTS prediction that the melt was volatile-undersaturated at the time the pegmatites began to form, but became volatile-saturated during the early stages of pegmatite formation, is consistent with the presence of some inclusion assemblages consisting of only MI, while others contain co-existing MI and FI. The relationship between halogen (F and Cl) and Na abundances in MI is also consistent with the interpretation that the very earliest stages of pegmatite formation occurred in the presence of a volatile-undersaturated melt and that the melt became volatile saturated as crystallization progressed. We propose a closed system, isochoric model for the formation of the pegmatites. Accordingly, the Taishanmiao granite crystallized isobarically at ~3.3 kbar, and the pegmatites began to form at ~734 °C and ~ 3.3 kbar, after ~96% of the original granitic melt had crystallized. During the final stages of crystallization of the granite, small pockets of the remaining residual melt became isolated within the enclosing granite and evolved as constant mass (closed), constant volume (isochoric) systems, similar to the manner in which volatile-rich melt inclusions in igneous phenocrysts evolve during post-entrapment crystallization under isochoric conditions. As a result of the negative volume change associated with crystallization, pressure in the pegmatite initially decreases as crystals form, and this leads to volatile exsolution from the melt phase. The changing PTX conditions produce a pressure-induced “liquidus deficit” that is analogous to liquidus undercooling and results in crystal growth as required to return the system to equilibrium PTX conditions. Owing to the complex closed system, isochoric PVTX evolution of the melt-crystal-volatile system, the pressure does not decrease rapidly or monotonically during pegmatite formation but, rather, gradually fluctuates such that at some stages in the evolution of the pegmatite the pressure is decreasing while at other times the pressure increases as the system cools to maintain mass and volume balance. This behavior, in turn, leads to alternating episodes of precipitation and dissolution that serve to coarsen (ripen) the crystals to produce the pegmatitic texture. The evolution of the pegmatitic melt described here is analogous to that which has been well-documented to occur in volatile-rich MI that undergo closed system, isochoric, post-entrapment crystallization.

American Mineralogist↗

USGS2019

Shift in the Raman symmetric stretching band of N2, CO2, and CH4 as a function of temperature, pressure, and density

The Raman spectra of pure N 2 , CO 2 , and CH 4 were analyzed over the range 10 to 500 bars and from −160°C to 200°C (N 2 ), 22°C to 350°C (CO 2 ), and −100°C to 450°C (CH 4 ). At constant temperature, Raman peak position, including the more intense CO 2 peak ( ν +), decreases (shifts to lower wave number) with increasing pressure for all three gases over the entire pressure and temperature ( PT ) range studied. At constant pressure, the peak position for CO 2 and CH 4 increases (shifts to higher wave number) with increasing temperature over the entire PT range studied. In contrast, N 2 first shows an increase in peak position with increasing temperature at constant pressure, followed by a decrease in peak position with increasing temperature. The inflection temperature at which the trend reverses for N 2 is located between 0°C and 50°C at pressures above ~50 bars and is pressure dependent. Below ~50 bars, the inflection temperature was observed as low as −120°C. The shifts in Raman peak positions with PT are related to relative density changes, which reflect changes in intermolecular attraction and repulsion. A conceptual model relating the Raman spectral properties of N 2 , CO 2 , and CH 4 to relative density (volume) changes and attractive and repulsive forces is presented here. Additionally, reduced temperature-dependent densimeters and barometers are presented for each pure component over the respective PT ranges. The Raman spectral behavior of the pure gases as a function of temperature and pressure is assessed to provide a framework for understanding the behavior of each component in multicomponent N 2 -CO 2 -CH 4 gas systems in a future study.

Journal of Raman Spectroscopy↗

USGS2015

Fluid inclusion chemistry of adularia-sericite epithermal Au-Ag deposits of the southern Hauraki Goldfield, New Zealand

Microthermometry, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and Raman spectroscopy have been used to determine the temperature, apparent salinity, and composition of individual fluid inclusions in adularia-sericite Au-Ag epithermal veins from the Karangahake, Martha, Favona, and Waitekauri deposits, southern Hauraki goldfield, New Zealand. Quartz veins contain colloform to crustiform bands that alternate with coarse-grained quartz and amethyst. The ore mineralization occurs only in colloform to crustiform bands. Analyses of individual fluid inclusions by LA-ICP-MS identify Na as the most abundant cation, together with variable concentrations of K, Ca, Rb, Sr, Sb, and As. Rare inclusions have detectable Li, Al, and Ba concentrations, although recorded Al concentrations with values up to 231 ppm in Al-free quartz may reflect an accidentally captured mineral phase rather than fluid itself. The Na content ranges from ~260 to 10,200 ppm for inclusions in quartz and ~9,700 to 13,700 ppm for inclusions in amethyst. Antimony is the second most commonly detected element in both quartz- and amethyst-hosted inclusions; this element is also detected in the host mineral. Concentrations of Sb and As range from 0.3 to 988 ppm and from 3.33 to 418 ppm, respectively, and are most commonly detected in inclusions from the Karangahake and Martha deposits. The poor correlation between the Na content with either Sb or As suggests that Sb and As were transported as neutral hydroxyl complexes of Sb(OH)3 and As(OH)3. Both Au and Ag occur at concentrations that are less than their respective detection limits (ppm). Geochemical modeling of the microthermometric and LA-ICP-MS data obtained from individual fluid inclusions suggests that fluids responsible for the quartz deposition were neutral to alkaline and that adiabatic boiling is the most effective mechanism for both gold and silica precipitation. The presence of single-phase vapor-only fluid inclusions in some mineralized samples indicates that local flashing may have contributed to deposition of Au and Ag. Assuming adiabatic boiling under hydrostatic pressure, samples from the Karangahake deposit (Maria vein) were deposited from low-salinity fluids (<3.9 wt % NaCl equiv) at temperatures between 225° and 262°C and at depths of 270 to 575 m below the former water table. The average deep reservoir fluid temperature estimated from the Na/K geothermometer is 287°C, and the steam loss during boiling ranges between 8 and 17%. Fluid inclusions in quartz from the Martha deposit trapped dilute fluids with salinity less than 1.7 wt % NaCl equiv. The coexisting liquid-rich (homogenization temperature, Th = 189°–225°C) and vapor-rich inclusions (Th = 205°–243°C) suggest formation at depths of 200 to 400 m below the water table. According to the Na/K geothermometer, the deep reservoir fluid temperature was near 295°C, and the steam loss during boiling ranged between 15 and 23%. Pseudosecondary inclusions in amethyst display salinity around 4.0 wt % NaCl equiv and homogenization temperatures between 218° and 241°C. Secondary inclusions are slightly more dilute (3.2–4.2 wt % NaCl equiv), with homogenization temperatures between 213° and 242°C. Fluid inclusions in quartz from the Waitekauri deposit homogenize from 210° to 265°C and contain less than 1.2 wt % NaCl equiv. A thin quartz vein that occurs between the Jubilee and Scotia deposits contains coexisting liquid- and vapor-rich inclusions; their homogenization temperatures indicate a formation depth of 300 m below the former water table. The calculated deep reservoir fluid temperature is around 283°C and the steam loss is estimated to be between 13 and 18%. LA-ICP-MS analyses show that in some cases different fluid inclusion assemblages (FIAs) within a single sample trapped fluids with variable chemistries. These differences likely reflect modification of a single parent fluid through mineral dissolution and precipitation, water/rock interactions, boiling and vapor loss, conductive cooling, and mixing.

Economic Geology↗

USGS2015

Statistical analysis of soil geochemical data to identify pathfinders associated with mineral deposits: An example from the Coles Hill uranium deposit, Virginia, USA

Soil geochemical anomalies can be used to identify pathfinders in exploration for ore deposits. In this study, compositional data analysis is used with multivariate statistical methods to analyse soil geochemical data collected from the Coles Hill uranium deposit, Virginia, USA, to identify pathfinders associated with this deposit. Elemental compositions and relationships were compared between the collected Coles Hill soil and reference soil samples extracted from a regional subset of a national-scale geochemical survey. Results show that pathfinders for the Coles Hill deposit include light rare earth elements (La and Ce), which, when normalised by their Al content, are correlated with U/Al, and elevated Th/Al values, which are not correlated with U/Al, supporting decoupling of U from Th during soil generation. These results can be used in genetic and weathering models of the Coles Hill deposit, and can also be applied to future prospecting for similar U deposits in the eastern United States, and in regions with similar geological/climatic conditions.

Virginia↗

USGS2013

Temporal and spatial distribution of alteration, mineralization and fluid inclusions in the transitional high-sulfidation epithermal-porphyry copper system at Red Mountain, Arizona

Red Mountain, Arizona, is a Laramide porphyry Cu system (PCD) that has experienced only a modest level of erosion compared to most other similar deposits in the southwestern United States. As a result, the upper portion of the magmatic–hydrothermal system, which represents the transition from shallower high-sulfidation epithermal mineralization to deeper porphyry Cu mineralization, is well preserved. Within the Red Mountain system, alteration, mineralization and fluid inclusion assemblages show a systematic distribution in both time and space. Early-potassic alteration (characterized by the minerals biotite and magnetite) is paragenetically earlier than late-potassic alteration (K-feldspar–anhydrite) and both are followed by later phyllic (sericite–pyrite) alteration. Advanced argillic alteration (pyrophyllite–alunite–other clay minerals) is thought to be coeval with or postdate phyllic alteration. Minerals characteristic of advanced argillic alteration are present in the near surface. Phyllic alteration extends to greater depths compared to advanced argillic alteration. Early-potassic and late-potassic alteration are only observed in the deepest part of the system. Considerable overlap of phyllic alteration with both early-potassic and late-potassic alteration zones is observed. The hypogene mineralization contains 0.4–1.2% Cu and is spatially and temporally related to the late-potassic alteration event. Molybdenum concentration is typically < 300 ppm but positive anomalies (between 600 and 1200 ppm) occur, and typically correlate with the zones of higher Cu grades. Silver and Au range up to 50 ppm and 1 ppm, respectively, and mostly occur in the deeper parts of the system. Individual assays of up to 18 ppm Au and 274 ppm Ag in the shallower part of the system are interpreted to be associated with areas of highly focused fluid flow (i.e., breccias and thick veins). A near-surface, discontinuous chalcocite blanket is represented by scattered Cu anomalies within the mixed oxide/sulfide zone and its discontinuous nature may reflect differential permeability along fractures and faults. In the deepest part of the system, an early generation of low-to-moderate density and salinity liquid + vapor inclusions with opaque daughter minerals is followed in time by halite-bearing inclusions that also contain opaque daughter minerals indicating that an early intermediate-density magmatic fluid evolved to a high-density, high-salinity mineralizing fluid. The increase in density and salinity of fluids with time observed in the deeper parts of the system may be the result of immiscibility (“boiling”) of the earlier magmatic fluids or may reflect the compositional evolution of fluids that exsolved from the magma. Trails of inclusions consisting of only vapor-rich inclusions are common in the shallow parts of the system, and are associated with advanced argillic alteration, suggesting that intense boiling (“flashing”) occurred at (or below) this level. Fluid inclusion assemblages consisting of coexisting vapor-rich and halite-bearing inclusions are observed in samples extending from the surface to the upper part of the late-potassic zone, indicating that fluid immiscibility occurred within this depth interval.

Arizona↗

USGS2012

Bulk rock composition and geochemistry of olivine-hosted melt inclusions in the Grey Porri Tuff and selected lavas of the Monte dei Porri volcano, Salina, Aeolian Islands, southern Italy

The Aeolian Islands are an arcuate chain of submarine seamounts and volcanic islands, lying just north of Sicily in southern Italy. The second largest of the islands, Salina, exhibits a wide range of compositional variation in its erupted products, from basaltic lavas to rhyolitic pumice. The Monte dei Porri eruptions occurred between 60 ka and 30 ka, following a period of approximately 60,000 years of repose. The bulk rock composition of the Monte dei Porri products range from basaltic-andesite scoria to andesitic pumice in the Grey Porri Tuff (GPT), with the Monte dei Porri lavas having basaltic-andesite compositions. The typical mineral assemblage of the GPT is calcic plagioclase, clinopyroxene (augite), olivine (Fo 72−84 ) and orthopyroxene (enstatite) ± amphibole and Ti-Fe oxides. The lava units show a similar mineral assemblage, but contain lower Fo olivines (Fo 57−78 ). The lava units also contain numerous glomerocrysts, including an unusual variety that contains quartz, K-feldspar and mica. Melt inclusions (MI) are ubiquitous in all mineral phases from all units of the Monte dei Porri eruptions; however, only data from olivine-hosted MI in the GPT are reported here. Compositions of MI in the GPT are typically basaltic (average SiO 2 of 49.8 wt %) in the pumices and basaltic-andesite (average SiO 2 of 55.6 wt %) in the scoriae and show a bimodal distribution in most compositional discrimination plots. The compositions of most of the MI in the scoriae overlap with bulk rock compositions of the lavas. Petrological and geochemical evidence suggest that mixing of one or more magmas and/or crustal assimilation played a role in the evolution of the Monte dei Porri magmatic system, especially the GPT. Analyses of the more evolved mineral phases are required to better constrain the evolution of the magma.

Grey Porri Tuff;Monte Dei Porri;Salina;Aeolian Isl↗

USGS2010

Porphyry copper deposit model

This report contains a revised descriptive model of porphyry copper deposits (PCDs), the world's largest source (about 60 percent) and resource (about 65 percent) of copper and a major source of molybdenum, gold and silver. Despite relatively low grades (average 0.44 percent copper in 2008), PCDs have significant economic and societal impacts due to their large size (commonly hundreds of millions to billions of metric tons), long mine lives (decades), and high production rates (billions of kilograms of copper per year). The revised model describes the geotectonic setting of PCDs, and provides extensive regional- to deposit-scale descriptions and illustrations of geological, geochemical, geophysical, and geoenvironmental characteristics. Current genetic theories are reviewed and evaluated, knowledge gaps are identified, and a variety of exploration and assessment guides are presented. A summary is included for users seeking overviews of specific topics.

Scientific Investigations Report↗

USGS2009

Thermodynamic model for uplift and deflation episodes (bradyseism) associated with magmatic-hydrothermal activity at the Campi Flegrei (Italy)

Campi Flegrei (CF) is a large volcanic complex located west of the city of Naples, Italy. Repeated episodes of bradyseism (slow vertical ground movement) near the town of Pozzuoli have been documented since Roman times. Bradyseismic events are interpreted as the consequence of aqueous fluid exsolution during magma solidification on a slow timescale (10 3 –10 4 yr) superimposed upon a shorter (1–10 yr) timescale for the episodic expulsion of fluid from a deep (~ 3–5 km) lithostatically-pressured low-permeability reservoir to an overlying hydrostatic reservoir. Cycles of inflation and deflation occur during short duration transient events when connectivity is established between deep and shallow hydrothermal reservoirs. The total seismic energy released (4 × 10 13 J) during the 1983–1984 bradyseismic crisis is consistent with the observed volume change (uplift) and consistent with the notion that seismic failure occurs in response to the shear stress release induced by volume change. Fluid transport and concomitant propagation of hydrofractures as fluid expands from lithostatic to hydrostatic pressure during decompression leads to ground surface displacement. Fluid decompression occurs along the fluid isenthalp (Joule–Thompson expansion) during transient periods of reservoir connectivity and leads to mineral precipitation. Each kilogram of fluid precipitates about 3 × 10 − 3 kg of silica along a typical decompression path along the isenthalp. Mineral precipitation modifies the permeability and acts to reseal connection paths thereby isolating lithostatic and hydrostatic reservoirs ending one bradyseism phase and beginning another. Crystallization and exsolution of the magmatic fluid generates ≈ 7 × 10 15 J of mechanical ( P Δ V ) energy, and this is sufficient to accomplish the observed uplift at CF. Although magma emplacement is the ultimate origin of bradyseism, fresh recharge of magma is not a prerequisite. Instead, short to intermediate timescale phenomena associated with fluid decompression and expansion in the crust with large variations in permeability, including permeability anisotropy, are the key elements at CF.

Naples↗