Geology topics

M.H. Reed

Publications and source records attributed to M.H. Reed.

8 recordsLinked to original sources

USGS2012

Composition and origin of rhyolite melt intersected by drilling in the Krafla geothermal field, Iceland

The Iceland Deep Drilling Project Well 1 was designed as a 4- to 5-km-deep exploration well with the goal of intercepting supercritical hydrothermal fluids in the Krafla geothermal field, Iceland. The well unexpectedly drilled into a high-silica (76.5 % SiO 2 ) rhyolite melt at approximately 2.1 km. Some of the melt vesiculated while extruding into the drill hole, but most of the recovered cuttings are quenched sparsely phyric, vesicle-poor glass. The phenocryst assemblage is comprised of titanomagnetite, plagioclase, augite, and pigeonite. Compositional zoning in plagioclase and exsolution lamellae in augite and pigeonite record changing crystallization conditions as the melt migrated to its present depth of emplacement. The in situ temperature of the melt is estimated to be between 850 and 920 °C based on two-pyroxene geothermometry and modeling of the crystallization sequence. Volatile content of the glass indicated partial degassing at an in situ pressure that is above hydrostatic (~16 MPa) and below lithostatic (~55 MPa). The major element and minor element composition of the melt are consistent with an origin by partial melting of hydrothermally altered basaltic crust at depth, similar to rhyolite erupted within the Krafla Caldera. Chondrite-normalized REE concentrations show strong light REE enrichment and relative flat patterns with negative Eu anomaly. Strontium isotope values (0.70328) are consistent with mantle-derived melt, but oxygen and hydrogen isotope values are depleted (3.1 and −118 ‰, respectively) relative to mantle values. The hydrogen isotope values overlap those of hydrothermal epidote from rocks altered by the meteoric-water-recharged Krafla geothermal system. The rhyolite melt was emplaced into and has reacted with a felsic intrusive suite that has nearly identical composition. The felsite is composed of quartz, alkali feldspar, plagioclase, titanomagnetite, and augite. Emplacement of the rhyolite magma has resulted in partial melting of the felsite, accompanied locally by partial assimilation. The interstitial melt in the felsite has similar normalized SiO 2 content as the rhyolite melt but is distinguished by higher K 2 O and lower CaO and plots near the minimum melt composition in the granite system. Augite in the partially melted felsite has re-equilibrated to more calcic metamorphic compositions. Rare quenched glass fragments containing glomeroporphyritic crystals derived from the felsite show textural evidence for resorption of alkali feldspar and quartz. The glass in these fragments is enriched in SiO 2 relative to the rhyolite melt or the interstitial felsite melt, consistent with the textural evidence for quartz dissolution. The quenching of these melts by drilling fluids at in situ conditions preserves details of the melt–wall rock interaction that would not be readily observed in rocks that had completely crystallized. However, these processes may be recognizable by a combination of textural analysis and in situ analytical techniques that document compositional heterogeneity due to partial melting and local assimilation.

Contributions to Mineralogy and Petrology↗

USGS2008

Trace elements in hydrothermal quartz: Relationships to cathodoluminescent textures and insights into vein formation

High-resolution electron microprobe maps show the distribution of Ti, Al, Ca, K, and Fe among quartz growth zones revealed by scanning electron microscope-cathodoluminescence (SEM-CL) from 12 hydrothermal ore deposits formed between ???100 and e1750 ??C. The maps clearly show the relationships between trace elements and CL intensity in quartz. Among all samples, no single trace element consistently correlates with variations in CL intensity. However in vein quartz from five porphyry-Cu (Mo-Au) deposits, CL intensity always correlates positively with Ti concentrations, suggesting that Ti is a CL activator in quartz formed at >400 ??C. Ti concentrations in most rutile-bearing vein quartz from porphyry copper deposits indicate reasonable formation temperatures of <750 ??C using the TitaniQ geothermometer. Titanium concentrations of <10 ppm in all veins that formed at temperatures <350 ??C suggest broad correlation between Ti concentrations and temperature of quartz precipitation. In quartz from most deposits formed at <350 ??C, bimodal Al concentrations exist wherein some growth bands contain <50 ppm and others contain >2000 ppm, but in high-temperature quartz, Al concentrations are consistently in the range of several hundred ppm. Aluminum concentrations in quartz refl ect the Al solubility in hydrothermal fluids, which is strongly dependent on pH. Aluminum concentrations in quartz therefore reflect fluctuations in pH that may drive metal-sulfide precipitation in hydrothermal systems. ?? 2008 The Geological Society of America.

Geology↗

USGS2006

Incorporation of seawater into mid-ocean ridge lava flows during emplacement

Evidence for the interaction between seawater and lava during emplacement on the deep seafloor can be observed in solidified flows at a variety of scales including rapid quenching of their outer crusts and the formation of lava pillars through the body of the flow. Recently, an additional interaction, incorporation of heated seawater (vapor) into the body of a flow, has been proposed. Large voids and vesicles beneath the surface crusts of mid-ocean ridge crest lobate and sheet lava flows and lava drips found within those cavities have been cited as evidence for this interaction. The voids resulting from this interaction contribute to the high porosity of the shallow ocean crust and play an important role in crustal permeability and hydrothermal circulation at mid-ocean ridges, and thus it is important to understand their origin. We analyze lava samples from the fast-spreading East Pacific Rise and intermediate-spreading Galapagos Spreading Center to characterize this process, identify the source of the vapor, and investigate the implications this would have on submarine lava flow dynamics. We find that lava samples that have interacted with a vapor have a zone of increased vesicularity on the underside of the lava crust and a coating of precipitate minerals (i.e., crystal fringe) that are distinct in form and composition from those crystallized from the melt. We use thermochemical modeling to simulate the reaction between the lava and a vapor and find that only with seawater can we reproduce the phase assemblage we observe within the crystal fringes present in the samples. Model results suggest that large-scale contamination of the lava by mass exchange with the vapor is unlikely, but we observe local enrichment of the lava in Cl resulting from the incorporation of a brine phase separated from the seawater. We suggest that high eruption rates are necessary for seawater incorporation to occur, but the mechanism by which seawater enters the flow has yet to be resolved. A persistent vapor phase may be important in inhibiting the collapse of lava flow roofs during natural waxing and waning of lava levels during emplacement allowing lava pathways to be maintained during long lived eruptions. In addition, we illustrate the potential for a persistent vapor layer to increase local flow rates within submarine flows by up to a factor of three, thereby influencing how lava is distributed across the ridge crest. ?? 2006 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2006

Intensity of quartz cathodoluminescence and trace-element content in quartz from the porphyry copper deposit at Butte, Montana

Textures of hydrothermal quartz revealed by cathodoluminescence using a scanning electron microscope (SEM-CL) reflect the physical and chemical environment of quartz formation. Variations in intensity of SEM-CL can be used to distinguish among quartz from superimposed mineralization events in a single vein. In this study, we present a technique to quantify the cathodoluminescent intensity of quartz within individual and among multiple samples to relate luminescence intensity to specific mineralizing events. This technique has been applied to plutonic quartz and three generations of hydrothermal veins at the porphyry copper deposit in Butte, Montana. Analyzed veins include early quartz-molybdenite veins with potassic alteration, pyrite-quartz veins with sericitic alteration, and Main Stage veins with intense sericitic alteration. CL intensity of quartz is diagnostic of each mineralizing event and can be used to fingerprint quartz and its fluid inclusions, isotopes, trace elements, etc., from specific mineralizing episodes. Furthermore, CL intensity increases proportional to temperature of quartz formation, such that plutonic quartz from the Butte quartz monzonite (BQM) that crystallized at temperatures near 750 ??C luminesces with the highest intensity, whereas quartz that precipitated at ???250 ??C in Main Stage veins luminesces with the least intensity. Trace-element analyses via electron microprobe and laser ablation-ICP-MS indicate that plutonic quartz and each generation of hydrothermal quartz from Butte is dominated by characteristic trace amounts of Al, P, Ti, and Fe. Thus, in addition to CL intensity, each generation of quartz can be distinguished based on its unique trace-element content. Aluminum is generally the most abundant element in all generations of quartz, typically between 50 and 200 ppm, but low-temperature, Main Stage quartz containing 400 to 3600 ppm Al is enriched by an order of magnitude relative to all other quartz generations. Phosphorous is present in abundances between 25 and 75 ppm, and P concentrations in quartz show little variation among quartz generations. Iron is the least abundant of these elements in most quartz types and is slightly enriched in CL-dark quartz in pyrite-quartz veins with sericitic alteration. Titanium is directly correlated with both temperature of quartz precipitation, and intensity of quartz luminescence, such that BQM quartz contains hundreds of ppm Ti, whereas Main Stage quartz contains less than 10 ppm Ti. Our results suggest that Ti concentration in quartz is controlled by temperature of quartz precipitation and that increased Ti concentrations in quartz may be responsible for increased CL intensities.

American Mineralogist↗

USGS2004

Geochemical models of metasomatism in ultramafic systems: Serpentinization, rodingitization, and sea floor carbonate chimney precipitation

In a series of water-rock reaction simulations, we assess the processes of serpentinization of harzburgite and related calcium metasomatism resulting in rodingite-type alteration, and seafloor carbonate chimney precipitation. At temperatures from 25 to 300??C (P = 10 to 100 bar), using either fresh water or seawater, serpentinization simulations produce an assemblage commonly observed in natural systems, dominated by serpentine, magnetite, and brucite. The reacted waters in the simulations show similar trends in composition with decreasing water-rock ratios, becoming hyper-alkaline and strongly reducing, with increased dissolved calcium. At 25??C and w/r less than ???32, conditions are sufficiently reducing to yield H2 gas, nickel-iron alloy and native copper. Hyperalkalinity results from OH- production by olivine and pyroxene dissolution in the absence of counterbalancing OH- consumption by alteration mineral precipitation except at very high pH; at moderate pH there are no stable calcium minerals and only a small amount of chlorite forms, limited by aluminum, thus allowing Mg2+ and Ca2+ to accumulate in the aqueous phase in exchange for H+. The reducing conditions result from oxidation of ferrous iron in olivine and pyroxene to ferric iron in magnetite. Trace metals are computed to be nearly insoluble below 300??C, except for mercury, for which high pH stabilizes aqueous and gaseous Hg??. In serpentinization by seawater at 300??C, Ag, Au, Pd, and Pt may approach ore-forming concentrations in sulfide complexes. Simulated mixing of the fluid derived from serpentinization with cold seawater produces a mineral assemblage dominated by calcite, similar to recently discovered submarine, ultramafic rock-hosted, carbonate mineral deposits precipitating at hydrothermal vents. Simulated reaction of gabbroic or basaltic rocks with the hyperalkaline calcium- and aluminum-rich fluid produced during serpentinization at 300??C yields rodingite-type mineral assemblages, including grossular, clinozoisite, vesuvianite, prehnite, chlorite, and diopside. ?? 2004 Elsevier Ltd.

Geochimica et Cosmochimica Acta↗

USGS2001

Magmatic gas scrubbing: Implications for volcano monitoring

Despite the abundance of SO2(g) in magmatic gases, precursory increases in magmatic SO2(g) are not always observed prior to volcanic eruption, probably because many terrestrial volcanoes contain abundant groundwater or surface water that scrubs magmatic gases until a dry pathway to the atmosphere is established. To better understand scrubbing and its implications for volcano monitoring, we model thermochemically the reaction of magmatic gases with water. First, we inject a 915??C magmatic gas from Merapi volcano into 25??C air-saturated water (ASW) over a wide range of gas/water mass ratios from 0.0002 to 100 and at a total pressure of 0.1 MPa. Then we model closed-system cooling of the magmatic gas, magmatic gas-ASW mixing at 5.0 MPa, runs with varied temperature and composition of the ASW, a case with a wide range of magmatic-gas compositions, and a reaction of a magmatic gas-ASW mixture with rock. The modeling predicts gas and water compositions, and, in one case, alteration assemblages for a wide range of scrubbing conditions; these results can be compared directly with samples from degassing volcanoes. The modeling suggests that CO2(g) is the main species to monitor when scrubbing exists; another candidate is H2S(g), but it can be affected by reactions with aqueous ferrous iron. In contrast, scrubbing by water will prevent significant SO2(g) and most HCl(g) emissions until dry pathways are established, except for moderate HCl(g) degassing from pH < 0.5 hydrothermal waters. Furthermore, it appears that scrubbing will prevent much, if any, SO2(g) degassing from long-resident boiling hydrothermal systems. Several processes can also decrease or increase H2(g) emissions during scrubbing making H2(g) a poor choice to detect changes in magma degassing. We applied the model results to interpret field observations and emission rate data from four eruptions: (1) Crater Peak on Mount Spurr (1992) where, except for a short post-eruptive period, scrubbing appears to have drastically diminished pre-, inter-, and post-eruptive SO2(g) emissions, but had much less impact on CO2(g) emissions. (2) Mount St. Helens where scrubbing of SO2(g) was important prior to and three weeks after the 18 May 1980 eruption. Scrubbing was also active during a period of unrest in the summer of 1998. (3) Mount Pinatubo where early drying out prevented SO2(g) scrubbing before the climactic 15 June 1991 eruption. (4) The ongoing eruption at Popocate??petl in an arid region of Mexico where there is little evidence of scrubbing. In most eruptive cycles, the impact of scrubbing will be greater during pre- and post-eruptive periods than during the main eruptive and intense passive degassing stages. Therefore, we recommend monitoring the following gases: CO2(g) and H2S(g) in precursory stages; CO2(g), H2S(g), SO2(g), HCl(g), and HF(g) in eruptive and intense passive degassing stages; and CO2(g) and H2S(g) again in the declining stages. CO2(g) is clearly the main candidate for early emission rate monitoring, although significant early increases in the intensity and geographic distribution of H2S(g) emissions should be taken as an important sign of volcanic unrest and a potential precursor. Owing to the difficulty of extracting SO2(g) from hydrothermal waters, the emergence of > 100 t/d (tons per day) of SO2(g) in addition to CO2(g) and H2S(g) should be taken as a criterion of magma intrusion. Finally, the modeling suggests that the interpretation of gas-ratio data requires a case-by-case evaluation since ratio changes can often be produced by several mechanisms; nevertheless, several gas ratios may provide useful indices for monitoring the drying out of gas pathways. Published by Elsevier Science B.V.

Journal of Volcanology and Geothermal Research↗

USGS1996

Mount St. Augustine volcano fumarole wall rock alteration: Mineralogy, zoning, composition and numerical models of its formation process

Intensely altered wall rock was collected from high-temperature (640 °C) and low-temperature (375 °C) vents at Augustine volcano in July 1989. The high-temperature altered rock exhibits distinct mineral zoning differentiated by color bands. In order of decreasing temperature, the color bands and their mineral assemblages are: (a) white to grey (tridymite-anhydrite); (b) pink to red (tridymite-hematite-Fe hydroxide-molysite (FeCl 3 ) with minor amounts of anhydrite and halite); and (c) dark green to green (anhydrite-halite-sylvite-tridymite with minor amounts of molysite, soda and potash alum, and other sodium and potassium sulfates). The alteration products around the low-temperature vents are dominantly cristobalite and amorphous silica with minor potash and soda alum, aphthitalite, alunogen and anhydrite. Compared to fresh 1986 Augustine lava, the altered rocks exhibit enrichments in silica, base metals, halogens and sulfur and show very strong depletions in Al in all alteration zones and in iron, alkali and alkaline earth elements in some of the alteration zones. To help understand the origins of the mineral assemblages in altered Augustine rocks, we applied the thermochemical modeling program, GASWORKS, in calculations of: (a) reaction of the 1987 and 1989 gases with wall rock at 640 and 375 °C; (b) cooling of the 1987 gas from 870 to 100 °C with and without mineral fractionation; (c) cooling of the 1989 gas from 757 to 100 °C with and without mineral fractionation; and (d) mixing of the 1987 and 1989 gases with air. The 640 °C gas-rock reaction produces an assemblage consisting of silicates (tridymite, albite, diopside, sanidine and andalusite), oxides (magnetite and hercynite) and sulfides (bornite, chalcocite, molybdenite and sphalerite). The 375 °C gas-rock reaction produces dominantly silicates (quartz, albite, andalusite, microcline, cordierite, anorthite and tremolite) and subordinate amounts of sulfides (pyrite, chalcocite and wurtzite), oxides (magnetite), sulfates (anhydrite) and halides (halite). The cooling calculations produce: (a) anhydrite, halite, sylvite; (b) Cu, Mo, Fe and Zn sulfides; (c) Mg fluoride at high temperature (> 370 °C); (d) chlorides, fluorides and sulfates of Mn, Fe, Zn, Cu and Al at intermediate temperature (170–370 °C); and (e) hydrated sulfates, liquid sulfur, crystalline sulfur, hydrated sulfuric acid and water at low temperature (< 170°C). The volcanic gas-air mixing calculation produces major amounts of Na and K sulfates, minor amounts of hematite and trace amounts (< 1%) of anhydrite at log gas/air (Ig/a) ratios > 0.41 (> 628°C). This is followed by precipitation of sulfates of Fe, Cu. Pb. Zn and Al at Ig/a ratios between 0.31 and -0.4 (>628-178°C). At a lg/r ratio of ≤ -0.4 (178°C). anhydrous sulfates are replaced by their hydrated forms and hygroscopic sulfuric acid forms. At these low g/a ratios. hydrated sulfuric acid becomes the dominant phase in the system. Comparison of the thermochemical modeling results with the natural samples suggests that the alteration assemblages include: (1) minerals that precipitate from direct cooling of the volcanic gas; (2) phases that form by volcanic gases mixing with air: and (3) phases that form by volcanic gas-air-rock reaction. A complex interplay of the three processes produces the observed mineral zoning. Another implication of the numerical simulation results is that most of the observed incrustation and sublimate minerals apparently formed below 700°C.

Journal of Volcanology and Geothermal Research↗

USGS1987

Volatilization, transport and sublimation of metallic and non-metallic elements in high temperature gases at Merapi Volcano, Indonesia

Condensates, silica tube sublimates and incrustations were sampled from 500–800°C fumaroles and lava samples were collected at Merapi Volcano, Indonesia in Jan.–Feb., 1984. With respect to the magma, Merapi gases are enriched by factors greater than 10 5 in Se, Re, Bi and Cd; 10 4 –10 5 in Au, Br, In, Pb and W; 10 3 –10 4 in Mo, Cl, Cs, S, Sn and Ag; 10 2 –10 3 in As, Zn, F and Rb; and 1–10 2 in Cu, K, Na, Sb, Ni, Ga, V, Fe, Mn and Li. The fumaroles are transporting more than 10 6 grams/day ( g d "> gd ) of S, Cl and F; 10 4 –10 6 g/d of Al, Br, Zn, Fe, K and Mg; 10 3 –10 4 g d "> gd of Pb, As, Mo, Mn, V, W and Sr; and less than 10 3 g d "> gd of Ni, Cu, Cr, Ga, Sb, Bi, Cd, Li, Co and U. With decreasing temperature (800-500°C) there were five sublimate zones found in silica tubes: 1) cristobalite and magnetite (first deposition of Si, Fe and Al); 2) K-Ca sulfate, acmite, halite, sylvite and pyrite (maximum deposition of Cl, Na, K, Si, S, Fe, Mo, Br, Al, Rb, Cs, Mn, W, P, Ca, Re, Ag, Au and Co); 3) aphthitalite (K-Na sulfate), sphalerite, galena and Cs-K. sulfate (maximum deposition of Zn, Bi, Cd, Se and In; higher deposition of Pb and Sn); 4) Pb-K chloride and Na-K-Fe sulfate (maximum deposition of Pb, Sn and Cu); and 5) Zn, Cu and K-Pb sulfates (maximum deposition of Pb, Sn, Ti, As and Sb). The incrustations surrounding the fumaroles are also chemically zoned. Bi, Cd, Pb, W, Mo, Zn, Cu, K, Na, V, Fe and Mn are concentrated most in or very close to the vent as expected with cooling, atmospheric contamination and dispersion. The highly volatile elements Br, Cl, As and Sb are transported primarily away from high temperature vents. Ba, Si, P, Al, Ca and Cr are derived from wall rock reactions. Incomplete degassing of shallow magma at 915°C is the origin of most of the elements in the Merapi volcanic gas, although it is partly contaminated by particles or wall rock reactions. The metals are transported predominantly as chloride species. As the gas cools in the fumarolic environment, it becomes saturated with sublimate phases that fractionate from the gas in the order of their equilibrium saturation temperatures. Devolatilization of a cooling batholith could transport enough acids and metals to a hydrothermal system to play a significant role in forming an ore deposit. However, sublimation from a high temperature, high velocity carrier gas is not efficient enough to form a large ore deposit. Re, Se, Cd and Bi could be used as supporting evidence for magmatic fluid transport in an ore deposit.

Geochimica et Cosmochimica Acta↗