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USGS1996

Organic geochemistry applied to environmental assessments of Prince William Sound, Alaska, after the Exxon Valdez oil spill—a review

Organic geochemistry played a major role in the environmental assessments conducted following the Exxon Valdez oil spill, which occurred on March 24, 1989, and released about 258,000 bbls (41 million liters) of Alaska North Slope crude oil into Prince William Sound. Geochemical analyses of more than 15,000 sediment, tar, and biological samples and about 5000 water samples provide the largest database yet collected on oil-spill chemistry, and we review the results here. The marine environment of the Sound has a complex background of petrogenic, pyrogenic, and biogenic hydrocarbons from natural and anthropogenic sources. Geochemical evaluation of the fate and effects of the spilled oil required that this oil and its residues be distinguished from the background. A variety of molecular and isotopic techniques were employed to identify various hydrocarbon sources and to distinguish quantitatively among mixed sources in the samples. Although the specific criteria used to distinguish multiple sources in the region affected by the Exxon Valdez spill are not necessarily applicable to all spill situations, the principles that governed their selection are. Distributions of polycyclic aromatic hydrocarbons (PAH) and dibenzothiophenes distinguish Exxon Valdez oil and its weathered residues from background hydrocarbons in benthic sediments. Ratios of C 2 -dibenzothiophene C 2 -phenanthrene "> C 2 -dibenzothiopheneC 2 -phenanthrene and C 3 -dibenzothiophene C 3 -phenanthrene "> C 3 -dibenzothiopheneC 3 -phenanthrene were particularly useful. Carbon isotopes and terpane distributions distinguished Exxon Valdez residues found on shorelines from tars from other sources. Diesel and diesel soot were identified by the absence of alkylated chrysenes and a narrow distribution of n -alkanes, whereas pyrogenic products were distinguished by the dominance of 4- to 6-ring PAH over 2- to 3-ring PAH and by the dominance of non-alkylated over alkylated homologues of each PAH series. The presence of 18α(H)-oleanane in benthic sediments, coupled with its absence in Exxon Valdez oil and its residues, confirm another petrogenic source. Results of geochemical studies suggest that the petrogenic component in the background of benthic sediments is derived from oil seeps in the eastern Gulf of Alaska. In 1990 and 1991, Exxon Valdez residues, generally forming a small increment to the pre-spill background, were found to be only sporadically distributed in some shallow, near shore sediments adjacent to shorelines that had been heavily oiled in 1989. In 1994, occurrences of Exxon Valdez tars on shoreline surfaces were rare, although residues could be found buried in shoreline sediments at some isolated locations along the spill path where they were protected from wave action. Spilled oil residues collected 16 months after the spill were degraded, on average, by nearly 50%. Shoreline residues from sources other than the spill were also identified and are widespread throughout the Sound. These residues include (1) geochemically distinct tars and oils imported from California oil fields to Alaska for fuel and construction purposes prior to the discovery of the Cook Inlet and North Slope oil fields, (2) diesel and diesel soot, and (3) more highly refined products. Of the more than 2700 chemical analyses of biological samples of higher life forms (fish, birds, and mammals) about 150 (6%) indicate recognizable residues of Exxon Valdez oil, which were identified by their distribution of polycyclic aromatic hydrocarbons (PAH). Most of these samples (138) were collected in 1989 and most were associated with external surfaces or the gastrointestinal tract. Rarely do internal tissues or fluids contain recognizable fingerprints of spilled oil. This observation includes samples from marine mammals that were visibly oiled externally. Other hydrocarbon sources, including diesel and a non-petroleum artifact that occurs when concentrations of individual PAH are at or near their method detection limit, are also identified in biological samples.

Alaska↗

USGS1995

The role of palynology in paleoecological analyses of Tertiary coals

Pollen, spores, and other plant microfossils are important constituents of Tertiary coals, both as petrographic components (the maceral sporinite) and as distinctive indicators of the nature of the depositional environment in which the coal-forming peat accumulated. Palynological analyses, which have been conducted on Tertiary coals since the 1930's, play an increasingly important role in modern paleoecological analyses of coals. Among the most important factors controlling the petrographic composition and facies characteristics of coal deposits are the types and relative abundances of plants that composed the peat. At present, wetland environments in which peat accumulates (mires) range from marshes and bogs to forested swamps; each is inhabited by distinctive plant communities. These plant communities are associated with varied conditions of climate, water depth, chemistry, and nutrient supply, and the floras can be indicative of these conditions. In the Tertiary Period, plant communities of mires were also highly diverse and specialized (especially as compared with those of the Carboniferous). The dominant species in some Paleocene mire floras were gymnosperms, but continuing a trend begun in Late Cretaceous time, angiosperms became increasingly prominent through the Tertiary. The diverse: angiosperm-dominated mire floras produced coals that vary significantly in facies and hence in quality. Such contrasts are evident in contemporaneous coals from different paleolatitudes, as well as among, coals of different ages within the Tertiary, as exemplified by selected deposits in the western United. States. The palynofloras of these coals reflect the floristic composition of the plant communities of the ancient mires.

International Journal of Coal Geology↗

USGS1988

Preliminary observations of streamflow generation during storms in a forested Piedmont watershed using temperature as a tracer

Variations in streamwater temperature at the outlet of a 41-ha forested watershed at Panola Mountain in the Georgia Piedmont indicate that the initial rapid hydrologic response is caused by a combination of groundwater discharge and channel interception of rainwater. A storm in May 1986 caused a rapid increase in discharge that was accompanied by a decrease in streamwater temperature and a rise in the water table level adjacent to the stream. The higher water table provided the hydraulic gradient necessary to increase the discharge of colder groundwater to the stream. Storms that occurred under very dry antecedent conditions in July 1986 and June 1987 caused a rapid hydrologic response but no change in water table level, indicating the response was caused by channel interception of rainwater. This conclusion was supported by increases in streamwater temperature in the June storm and by chemical changes in the July storm. When rainfall is sufficient, flow in the ephemeral part of the stream in the catchment headwaters generates a second and larger discharge peak that reflects the chemistry and temperature of runoff from a 3-ha granite outcrop in the headwaters; sulfate concentration and temperature increase and alkalinity decreases relative to prestorm conditions. The initial response, however, results from channel interception and groundwater discharge. Rapid rises in the water table level during some storms suggest that macropore flow may play a major role in the hydrologic response of the watershed to rainstorms.

Journal of Contaminant Hydrology↗

USGS1987

The chemical, physical and structural properties of estuarine ice in Great Bay, New Hampshire

The purpose of this study was to provide general information on the chemical, physical and structural properties of estuarine ice and show how it compares with sea ice found at higher latitudes in order to determine whether the ice in Great Bay can be used as an analog in the study of arctic sea ice. Ice cores and water samples were collected during the 1983–1984 winter season at Adams Point in Great Bay, New Hampshire. Concentrations of chloride, nitrogen (as nitrate and nitrite), bromide, phosphate, sulfate and silicate were determined for samples chosen on the basis of identifiable stratigraphic layers (i.e. bubble size and shape, sediment layers, etc.). Similarities between ice formation in Great Bay and those in the arctic regions include the nature of the freezing process and the ice types produced. In addition, the distribution and concentration of chemical constituents were found to be similar to those observed in arctic sea ice. Factors affecting the chemistry of the ice in Great Bay include rainfall during the freezing season, the presence of sediment layers in the ice cores, the nature of incorporation of brine into the crystal structure of the ice and the drainage of brine.

New Hampshire↗

USGS1996

Effects of agricultural practices and vadose zone stratigraphy on nitrate concentration in ground water in Kansas, USA

Differences in nitrate-N concentrations in,around water in Kansas can be explained by variations in agricultural practices and vadose-zone stratigraphy. In northwestern Kansas, past use of a local stream for tailwater runoff from irrigation and high fertilizer applications for sugar-beet farming resulted in high nitrate-N concentrations (12-60 mg L-1; in both soil and ground water. Nitrogen isotope values from the soil and ground water range from +4 to +8? which is typical for a fertilizer source. In parts of south-central Kansas, the use of crop rotation and the presence of both continuous fine-textured layers and a reducing ground-water chemistry resulted in ground-water nitrate-N values of < 3 mg L-1. The effects of denitrification in the vadose zone and ground water are indicated by enriched ?? 15N values of +10 to +15?. At a site study, irrigated continuous corn was grown on sandy soils with discontinuous fine-textured layers. Here, nitrate-N concentrations were often > 10 mg L-1; in both soil and grounwater. Nitrogen isotope values of +3 to +7? indicate a fertilizer source. Crop rotation decreased nitrate-N values in the shallow ground water (9 m). However, deeper ground water showed increasing nitrate-N concentrations as a result of past farming practices.

Conference Paper↗

USGS1991

Pliocene-climate history of the western United States derived from lacustrine ostracodes

Pliocene sediments from several sites in western North America contain ostracodes indicating deposition in lacustrine and wetland settings. The ostracodes offer a means of reconstructing the aquatic paleoenvironment. Because water temperature, chemistry, and lake volume are coupled to climate, reconstruction of these parameters provides a direct insight into Pliocene climate. The site ages were determined from tephrochronology, paleomagnetics, and associated mammals. The morphology of many ostracode species also provides direct information about the paleoenvironment in which they lived. During the Pliocene (about 3.5-2.5 Ma) some species have unusually ornate carapace morphology indicative of large geologically stable lakes, which must have required a stable climate to sustain them. North American Pliocene climate changed from a modern-like state 4.5-3.5 Ma to a period with greater precipitation and less evaporation than today, 3.5-2.5 Ma. This wetter period, inferred from the large geologically long-lived lakes, implies a stable atmospheric circulation pattern. The stable circulation pattern collapsed around 2.5 Ma and climate returned to a modern-like situation.

western United States↗

USGS1975

The Pikes Peak batholith, Colorado front range, and a model for the origin of the gabbro-anorthosite-syenite-potassic granite suite

This study of the Pikes Peak batholith includes the mineralogy and petrology of quartz syenite at West Creek and of fayalite-bearing and fayalite-free biotite granite near Mount Rosa; major element chemistry of the batholith; comparisons with similar postorogenic, intracratonic, sodic to potassic intrusives; and genesis of the batholith. The batholith is elongate in plan, 50 by 100 km, composite, and generally subalkalic. It was emplaced at shallow depth 1,040 m. y. ago, sharply transects its walls and may have breached its roof. Biotite granite and biotite—hornblende granite are predominant; quartz syenite, fayalite granite and riebeckite granite are present in minor amounts. Fayalite-bearing and fayalite-free quartz syenite, fayalite-biotite granite and riebeckite granite show a well-defined sodic differentiation trend; the less sodic fayalite-free granites exhibit a broader compositional range and no sharp trends. Crystallization was largely at P H2O < P total ; P H2O approached P total only at late stages. Aplite residual to fayalite-free biotite granite in the north formed at about 1,500 bars, or 5 km depth. Feldspar assemblages indicate late stages of crystallization at about 720°C. In the south ilmenite and manganian fayalite indicate f O2 of 10 −17 or 10 −18 bars. Biotite and fayalite compositions and the ‘granite minimum’ imply completion of crystallization at about 700°C and 1,500 bars. Nearby fayalite-free biotite granite crystallized at higher water fugacity. All types of syenite and granite contain 5–6% K 2 O through a range of SiO 2 of 63–76%. Average Na 2 O percentages in quartz syenite are 6.2, fayalite granite 4.2, and fayalite-free granite 3.3 MgO contents are low, 0.03–0.4%; FeO averages 1.9–2.5%. FeO/Fe 2 O 3 ratios are high. Fluorine ranges from 0.3 to 0.6%. The Pikes Peak intrusives are similar in mode of emplacement, composition, and probably genesis to rapakivi intrusives of Finland, the Younger Granites of Nigeria, Cape Ann Granite and Beverly Syenite, Mass., and syenite of Kungnat, Greenland, among others — allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence. A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K 2 O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na 2 O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid — mostly olivine, clinopyroxene and plagioclase.

Colorado↗

USGS1992

Charnockites and granites of the western Adirondacks, New York, USA: A differentiated A-type suite

Granitic rocks in the west-central Adirondack Highlands of New York State include both relatively homogeneous charnockitic and hornblende granitic gneisses (CG), that occur in thick stratiform bodies and elliptical domes, and heterogeneous leucogneisses (LG), that commonly are interlayered with metasedimentary rocks. Major- and trace-element geochemical analyses were obtained for 115 samples, including both types of granitoids. Data for CG fail to show the presence of more than one distinct group based on composition. Most of the variance within the CG sample population is consistent with magmatic differentiation combined with incomplete separation of early crystals of alkali feldspar, plagioclase, and pyroxenes or amphibole from the residual liquid. Ti, Fe, Mg, Ca, P, Sr, Ba, and Zr decrease with increasing silica, while Rb and K increase. Within CG, the distinction between charnockitic (orthopyroxene-bearing) and granitic gneisses is correlated with bulk chemistry. The charnockites are consistently more mafic than the hornblende granitic gneisses, although forming a continuum with them. The leucogneisses, while generally more felsic than the charnockites and granitic gneisses, are otherwise geochemically similar to them. The data are consistent with the LG suite being an evolved extrusive equivalent of the intrusive CG suite. Both CG and LG suites are metaluminous to mildly peraluminous and display an A-type geochemical signature, enriched in Fe, K, Ce, Y, Nb, Zr, and Ga and depleted in Ca, Mg, and Sr relative to I- and S-type granites. Rare earth element patterns show moderate LREE enrichment and a negative Eu anomaly throughout the suite. The geochemical data suggest an origin by partial melting of biotite- and plagioclase-rich crustal rocks. Emplacement occurred in an anorogenic or post-collisional tectonic setting, probably at relatively shallow depths. Deformation and granulite-facies metamorphism with some partial melting followed during the Ottawan phase of the Grenville Orogeny, yielding the present migmatitic granitic and charnockitic gneisses.

New York↗

USGS1970

Chemical indicators of subsurface temperature applied to hot spring waters of Yellowstone National Park, Wyoming, U.S.A.

Under favorable conditions the chemistry of hot springs may give reliable indications of subsurface temperatures and circulation patterns. These chemical indicators can be classified by the type of process involved: Indicator Dominant Process The silica geothermometer Solution-precipitation Alkali ratios Ion exchange Cl/(HCO 3 + CO 3 ) ratio Rock alteration by dissolved CO 2 Relative concentration of Partitioning owing to subsur- volatiles in spring water face boning All these indicators have certain limitations. The silica geothermometer gives results independent of the local mineral suite and gas partial pressures, but may be affected by dilution. Alkali ratios are strongly affected by the local mineral suite and the formation of complex ions. Carbonate-chloride ratios are strongly affected by subsurface P CO2 . The relative concentration of volatiles can be very misleading in high-pressure liquid systems. In Yellowstone National Park most thermal waters issue from hot, shallow aquifers with pressures in excess of hydrostatic by 2 to 6 bars and with large flows (the flow of hot spring water from the Park is greater than 4000 liters per second). These conditions should be ideal for the use of chemical indicators to estimate aquifer temperatures. In five drill holes aquifer temperatures were within 2°C of that predicted from the silica content of nearby hot springs; the temperature level off at a lower value than predicted in only one hole, and in four other holes drilling was terminated before the predicted aquifer temperature was reached. The temperature-Na/K ratio relationship does not follow any published experimental or empirical curve for water-feldspar or water-clay reactions. We suspect that ion exchange reactions involving zeolites in the Yellowstone rocks result in higher Na/K ratios at given temperatures than result from feldspar or clay reactions. Comparison of SiO 2 and Cl/(HCO 3 + CO 3 ) suggest that because of higher subsurface P CO2 in Upper Geyser Basin a given Cl/(HCO 3 + CO 3 ) ratio there means a higher temperature than in Lower Geyser Basin. No correlation was found in Yellowstone Park between the subsurface regions of highest temperature and the relative concentration of volatile components such as boron and ammonia.

Wyoming↗

USGS1979

Gases in steam from Cerro Prieto geothermal wells with a discussion of steam/gas ratio measurements

As part of a joint USGS-CFE geochemical study of Cerro Prieto, steam samples were collected for gas analyses in April, 1977. Analyses of the major gas components of the steam were made by wet chemistry (for H2O,CO2,H2S and NH3) and by gas chromatography (He,H2,Ar,O2,N2 and hydrocarbons). The hydrocarbon gases in Cerro Prieto steam closely resemble hydrocarbons in steam from Larderello, Italy and The Geysers, California which, although they are vapor-dominated rather than hot-water geothermal systems, also have sedimentary aquifer rocks. These sedimentary geothermal hydrocarbons are characterized by the presence of branched C4-6 compounds and a lack of unsaturated compounds other than benzene. Relatively large amounts of benzene may be characteristic of high-temperature geothermal systems. All hydrocarbons in these gases other than methane most probably originate from the thermal metamorphosis of organic matter contained in the sediments. ?? 1979.

Geothermics↗

USGS1979

Geochemical evidence of drawdown in the Cerro Prieto geothermal field

Some wells of the Cerro Prieto geothermal field have undergone changes in the chemistry of fluids produced which reflect reservoir processes. Pressure decreases due to production in the southeastern part of the field have produced both drawdown of lower chloride fluids from an overlying aquifer and boiling in the aquifer with excess steam reaching the wells. These reservoir changes are indicated by changes in fluid chloride concentrations, Na/K ratios and measured enthalpies and by comparisons of aquifer fluid temperatures and chloride concentrations calculated from enthalpy and chemical measurements. Fluid temperatures have not been greatly affected by this drawdown because heat contained in the rock was transferred to the fluid. When this heat is exhausted, fluid temperatures may drop rapidly. ?? 1979.

Geothermics↗

USGS1981

The origin of the Cerro Prieto geothermal brine

The Cerro Prieto geothermal brine may have originated from mixing of Colorado River water with seawater evaporated to about six times its normal salinity. This mixture circulated deeply and was heated by magmatic processes. During deep circulation, Li, K, Ca, B, SiO 2 and rare alkalis were transferred from rock minerals to the water, and Mg, SO 4 , and a minor quantity of Na were transferred to the rock. Similar alteration of seawater salt chemistry has been observed in coastal geothermal systems and produced in laboratory experiments. After heating and alteration the brine was further diluted to its present range of composition. Oxygen isotopes in the fluid are in equilibrium with reservoir calcite and have been affected by exploitation-induced boiling and dilution.

Geothermics↗

USGS1988

KAr ages, chemical composition and geothermal significance of cenozoic basalt near the Jordan rift

Late Cenozoic mafic lavas crop out locally along the Jordan rift. Some of these lavas are spatially associated with thermal springs, and this association has prompted some workers to hypothesize that the hot water derives its thermal energy from the shallow, still hot intrusive roots of the volcanic rocks. However, all of the volcanic rocks appear to represent mantle-derived mafic magma that rose rather quickly to the Earth's surface, without filling crustal reservoirs within which differentiation would have produced evolved, derivative products. Moreover, the lavas are too old and of too small a volume to represent the surface expression of an active reservoir of magma within the crust. These interpretations of the volcanic geology are consistent with conclusions drawn from the chemistry of the thermal water; the water has equilibrated with host rocks at no more than 110°C, probably at depths of 2–3 km. Thus, thermal springs along the Jordan rift appear to reflect heating during circulation through a regional regime of average crustal heat flow (Galanis et at. , 1986). The magmatic activity may only be a second or third order contributor to this heat flow.

Geothermics↗

USGS1991

A review of the hydrogeologic-geochemical model for Cerro Prieto

With continued exploitation of the Cerro Prieto, Mexico, geothermal field, there is increasing evidence that the hydrogeologic model developed by Halfman and co-workers presents the basic features controlling the movement of geothermal fluids in the system. In mid–1987 the total installed capacity at Cerro Prieto reached 620 MW c , requiring a large rate of fluid production (more than 10,500 tonnes/hr of a brine-steam mixture; August 1988). This significant mass extraction has led to changes in reservoir thermodynamic conditions and in the chemistry of the produced fluids. Pressure drawdown has caused an increase in cold water recharge in the southern and western edges of the field, and local and general reservoir boiling in parts of the geothermal system. After reviewing the hydrogeologic and geochemical models of Cerro Prieto, the exploitation-induced cold water recharge and reservoir boiling (and plugging) observed in different areas of the field, are discussed and interpreted on the basis of these models and schematic flow models that describe the hydrogeology.

Geothermics↗

USGS1986

Geochemical and fluid zonation in the skarn environment at the tomboy-minnie gold deposits, Lander County, Nevada

The Tomboy—Minnie gold deposits are related to the middle Tertiary porphyry copper system centered at Copper Canyon. Gold-silver ores in the deposits occur mostly in a pyrrhotite- and pyrite-rich basal 30-m-thick sequence of altered calcareous conglomerate belonging to the Middle Pennsylvanian Battle Formation. The entire mineralized system contained at least 3.3 million troy oz gold before large-scale mining operations began. Alteration in the Tomboy—Minnie deposits includes actinolite- and chlorite-dominant assemblages, in marked contrast to the skarn, potassic, and phyllic assemblages characterizing the copper-gold-silver deposits of the system. Introduction of gold occurred penecontemporaneously with replacement of early diopside-alteration assemblages by actinolite and chlorite. Metals are zoned strongly in the Copper Canyon system: the West and East ore bodies occur in a copper-gold-silver zone that is followed outward by a gold-silver zone which includes the Tomboy deposit and in turn, is succeeded by a lead-zinc-silver zone. Locations of drill holes that have Au/Ag assay ratios of ⩾ 1 clearly outline the Tomboy—Minnie deposits within an area of rocks with Au/Ag ratios of ≈ 0.5. Fluid-inclusion studies suggest wide variations in temperature and chemistry prevailed in the fluids associated with mineralization at the Tomboy. Early fluids associated with diopsidequartz assemblages probably were dominantly CaCl 2 -rich brines and were boiling at temperatures higher than 500°C. These fluids were progressively enriched in sodium and potassium over time, and during the hydrosilicate stages, temperatures probably ranged from 320 to 500°C at the time actinolite formed, and from 220 to 320°C at the time chlorite was dominant. Sulfur isotopic data suggest that sulfur, mostly from a magmatic or deep-seated crustal source, was transported by hydrothermal fluids as aqueous H 2 S with a δ 34 S of about . 4 ± 1‰ ">

Journal of Geochemical Exploration↗

USGS1995

A summary of the effects of mining and related activities on the sediment-trace element geochemistry of Lake Coeur d'Alene, Idaho, USA

During 1989 and 1990 a series of 12 gravity cores, and 150 surface grab samples were collected in Lake Coeur d'Alene, Idaho to determine trace element concentrations, partitioning and surface and subsurface distribution patterns in the bed sediments of the lake. In addition, selected subsamples from one core were analyzed for 117 Cs activity to begin to establish a trace element geochemical history for the lake. The intent was to try and relate the trace element concentrations and distributions in the sediment column to past and present mining and mining related activities in the area. Substantial portions of the surface and near-surface sediments in Lake Coeur d'Alene are markedly enriched in Ag, As, Cd, Hg, Pb, Sb and Zn, and somewhat enriched in Cu, Fe and Mn. Surface distribution patterns, as well as variations in the thickness of the trace element-rich subsurface sediments, indicate that the source of much of this enriched material is the Coeur d'Alene River. The similarity between the trace element-rich surface and subsurface sediments with respect to: their location, their bulk chemistry, and their trace element partitioning indicate that the sources and/or concentrating mechanisms causing the trace element enrichment in the lake sediments probably have been the same throughout their depositional history. An estimated 75 million metric tons of trace element-rich sediments have been deposited on or in the lakebed. Based on a Mt. St. Helens' ash layer from the 1980 eruption, ages estimated from 137 Cs activity, and the presence of 80 discernible and presumably annual layers in a core collected near the Coeur d'Alene River delta, indicate that the deposition of trace element-rich sediments began, at least in the Coeur d'Alene River delta, some time between 1895 and 1910, dates consistent with the onset of mining and ore-processing activities that began in the area in the 1880's.

Journal of Geochemical Exploration↗

USGS1982

Huge landslide blocks in the growth of piton de la fournaise, La réunion, and Kilauea volcano, Hawaii

Piton de la Fournaise, on the island of La Réunion, and Kilauea volcano, on the island of Hawaii, are active, basaltic shield volcanoes growing on the flanks of much larger shield volcanoes in intraplate tectonic environments. Past studies have shown that the average rate of magma production and the chemistry of lavas are quite similar for both volcanoes. We propose a structural similarity — specifically, that periodic displacement of parts of the shields as huge landslide blocks is a common mode of growth. In each instance, the unstable blocks are within a rift-zone-bounded, unbuttressed flank of the shield. At Kilauea, well-documented landslide blocks form relatively surficial parts of a much larger rift-zone-bounded block; scarps of the Hilina fault system mark the headwalls of the active blocks. At Fournaise, Hilina-like slump blocks are also present along the unbuttressed east coast of the volcano. In addition, however, the existence of a set of faults nested around the present caldera and northeast and southeast rift zones suggests that past chapters in the history of Fournaise included the slumping of entire rift-zone-bounded blocks themselves. These nested faults become younger to the east southeast and apparently record one of the effects of a migration of the focus of volcanism in that direction. Repeated dilation along the present set of northeast and southeast rift zones, most recently exemplified by an eruption in 1977, suggests that the past history of rift-zone-bounded slumping will eventually be repeated. The record provided by the succession of slump blocks on Fournaise is apparently at a relatively detailed part of a migration of magmatic focus that has advanced at least 30 km to the east-southeast from neighboring Piton des Neiges, an extinct Pliocene to Pleistocene volcano. ?? 1982.

Journal of Volcanology and Geothermal Research↗

USGS1991

Fluid inclusions and preliminary studies of hydrothermal alteration in core hole PLTG-1, Platanares geothermal area, Honduras

The Platanares geothermal area in western Honduras consists of more than 100 hot springs that issue from numerous hot-spring groups along the banks or within the streambed of the Quebrada de Agua Caliente (brook of hot water). Evaluation of this geothermal area included drilling a 650-m deep PLTG-1 drill hole which penetrated a surface mantling of stream terrace deposits, about 550 m of Tertiary andesitic lava flows, and Cretaceous to lower Tertiary sedimentary rocks in the lower 90 m of the drill core. Fractures and cavities in the drill core are partly to completely filled by hydrothermal minerals that include quartz, kaolinite, mixed-layer illite-smectite, barite, fluorite, chlorite, calcite, laumontite, biotite, hematite, marcasite, pyrite, arsenopyrite, stibnite, and sphalerite; the most common open-space fillings are calcite and quartz. Biotite from 138.9-m depth, dated at 37.41 Ma by replicate 40Ar/39 Ar analyses using a continuous laser system, is the earliest hydrothermal mineral deposited in the PLTG-1 drill core. This mid-Tertiary age indicates that at least some of the hydrothermal alteration encountered in the PLTG-1 drill core occurred in the distant past and is unrelated to the present geothermal system. Furthermore, homogenization temperatures (Th) and melting-point temperatures (Tm) for fluid inclusions in two of the later-formed hydrothermal minerals, calcite and barite, suggest that the temperatures and concentration of dissolved solids of the fluids present at the time these fluid inclusions formed were very different from the present temperatures and fluid chemistry measured in the drill hole. Liquid-rich secondary fluid inclusions in barite and caicite from drill hole PLTG-1 have Th values that range from about 20??C less than the present measured temperature curve at 590.1-m depth to as much as 90??C higher than the temperature curve at 46.75-m depth. Many of the barite Th measurements (ranging between 114?? and 265??C) plot above the reference surface boiling-point curve for pure water assuming hydrostatic conditions; however, the absence of evidence for boiling in the fluid inclusions indicates that at the time the minerals formed, the ground surface must have been at least 80 m higher than at present and underwent stream erosion to the current elevation. Near-surface mixed-layer illite-smectite is closely associated with barite and appears to have formed at about the same temperature range (about 120?? to 200??C) as the fluid-inclusion Th values for barite. Fluid-inclusion Th values for calcite range between about 136?? and 213??C. Several of the calcite Th values are significantly lower than the present measured temperature curve. The melting-point temperatures (Tm) of fluid-inclusion ice yield calculated salinities, ranging from near zero to as much as 5.4 wt. % NaCl equivalent, which suggest that much of the barite and calcite precipitated from fluids of significantly greater salinity than the present low salinity Platanares hot-spring water or water produced from the drill hole.

Journal of Volcanology and Geothermal Research↗