Peter Schiffman

Hydrothermal-flow regime and magmatic heat source of the Cerro Prieto geothermal system, Baja California, Mexico

Abstract Our detailed three-dimensional model of the natural flow regime of the Cerro Prieto geothermal field, before steam production began, is based on patterns of hydrothermal mineral zones and light stable isotopic ratios observed in rock samples from more than 50 deep wells, together with temperature gradients, wireline logs and other data. At the level so far penetrated by drilling, this hydrothermal system was heated by a thermal plume of water close to boiling, inclined at 45°, rising from the northeast and discharging to the west. To the east a zone of cold water recharge overlies the inclined thermal plume. Fission track annealing studies show that the reservoir reached 170°C only 10 4 years ago. Oxygen isotope exchange data indicate that a 12 km 3 volume of rock subsequently reacted with three times its volume of water hotter than 200°C. Averaged over the duration of the heating event this would require a flow velocity through a typical cross-section of the reservoir of about 6 m/year. The heat in storage in that part of the reservoir hotter than 200°C and shallower than 3 km depth is equivalent to that which would be released by the cooling of about 1 or 2 km 3 of basalt or gabbro magma. Although this is an extensional tectonic environment of ‘leaky’ transform faulting in which repeated intrusions of basalt magma are likely, for simplicity of computation we have modelled possible heat sources as simple two dimensional basalt intrusions of various sizes, shapes and locations. We have calculated a series of two-dimensional convective heat transfer models, with different heat sources and permeability distributions. The models which produce the best fit for the temperature distributions observed in the field today have in common a heat source which is a funnel-shaped basalt intrusion, 4 km wide at the top, emplaced at a depth of 5–6 km about 40,000–50,000 years ago. Although such forward modelling does not provide a unique solution of the thermal history, it does suggest that the igneous intrusion which supplied the heat to the Cerro Prieto system must be young, large and close. Modelling the hydrothermal system is useful in guiding geothermal resource evaluation; in turn we hope that modelling of the magmatic system will stimulate the search for the parent magma chamber by geophysical means, and eventually by deep drilling.

Alteration of hyaloclastites in the HSDP 2 Phase 1 Drill Core 1. Description and paragenesis

This is the publishers version. Copyright 2003 by Chinese Geophysical Society. All rights reserved.

Acid-fog deposition at Kilauea volcano: A possible mechanism for the formation of siliceous-sulfate rock coatings on Mars

On the summit of Kilauea volcano, sulfur dioxide, which is continuously emitted from Halemaumau crater and rapidly sequestered into sulfuric-acid–rich aerosol entrained in the prevailing trade winds, is subsequently precipitated as acid fog immediately downwind from Kilauea caldera in the Kau Desert. The characteristic pH of surface tephra deposits is <4.0 in Sand Wash, a region of nearly continuous, acidic aerosol fallout immediately southwest of the caldera. Vertical exposures of unconsolidated tephras of the Keanakakoi Ash found within fissures and small, dry gullies are coated with thin rock coatings of amorphous silica and jarosite. These rock coatings are formed via an evaporative mechanism whereby acidic pore fluids, circulating in the upper few meters within the highly porous tephra, are wicked toward the walls of the gullies. Geochemical modeling of the rock coating formation process implies that the sulfate formation via evaporation occurs subsequent to minimal interaction of acidic pore fluids with the basaltic tephra. This also suggests that the cycle from acid-fog fallout to precipitation of the siliceous-sulfate rock coatings must occur quite rapidly. Acid-fog deposition of sulfate and silica at Kilauea may provide one mechanism for the origin of jarosite-bearing outcrops on Mars.

Origin of a rhyolite that intruded a geothermal well while drilling at the Krafla volcano, Iceland

Magma fl owed into an exploratory geothermal well at 2.1 km depth being drilled in the Krafl a central volcano in Iceland, creating a unique opportunity to study rhyolite magma in situ in a basaltic environment. The quenched magma is a partly vesicular, sparsely phyric, glass containing ~1.8% of dissolved volatiles. Based on calculated H 2 O-CO 2 saturation pressures, it degassed at a pressure intermediate between hydrostatic and lithostatic, and geothermometry indicates that the crystals in the melt formed at ~900 °C. The glass shows no signs of hydrothermal alteration, but its hydrogen and oxygen isotopic ratios are much lower than those of typical mantle-derived magmas, indicating that this rhyolite originated by anhydrous mantle-derived magma assimilating partially melted hydrothermally altered basalts.

Oceanic Pillow Lavas and Hyaloclastites as Habitats for Microbial Life Through Time – A Review

This chapter summarizes research undertaken over the past 15 years upon the microbial alteration of originally glassy basaltic rocks from submarine environments. We report textural, chemical and isotopic results from the youngest to the oldest in-situ oceanic crust and compare these to data obtained from ophiolite and greenstone belts dating back to c. 3.8 Ga. Petrographic descriptions of the granular and tubular microbial alteration textures found in (meta)-volcanic glasses from pillow lavas and volcanic breccias are provided and contrasted with textures produced by abiotic alteration (palagonitization). The geological setting in particular the degree of deformation and metamorphism experienced by each study site is documented in outcrop photographs, geological maps and stratigraphic columns (where possible). In addition, X-ray mapping evidence and carbon isotopic data that are consistent with a biogenic origin for these alteration textures is explained and a model for their formation is presented. Lastly, the petrographic observations and direct radiometric dating techniques that have been used to establish the antiquity and syngenicity of these microbial alteration textures are reviewed.

Active metasomatism in the Cerro Prieto geothermal system, Baja California, Mexico: A telescoped low-pressure, low-temperature metamorphic facies series

In the Cerro Prieto geothermal field, carbonate-cemented, quartzofeldspathic sediments of the Colorado River delta are being actively metasomatized into calc-silicate metamorphic rocks by reaction with alkali chloride brines between 200/sup 0/ and 370/sup 0/C, low fluid and lithostatic pressures, and low oxygen fugacities. Petrologic investigations of drill cores and cutting from over 50 wells in this field identified a prograde series of calc-silicate mineral zones which include as index minerals: wairakite, epidote, prehnite, and clinopyroxene. Associated divariant mineral assemblages are indicative of a very low pressure/temperature metamorphic facies series which encompasses the clay-carbonate, zeolite, greenschist, and amphibolite facies. This hydrothermal metamorphic facies series, which is becoming increasingly recognized in other active geothermal systems, is characterized by temperature-telescoped dehydration and decarbonation mineral equilibria. Its equivalent should now be sought in fossil hydrothermal systems.

ALTERATION OF BASALT HYALOCLASTITE AT THE OFF-AXIS SEA CLIFF HYDROTHERMAL FIELD, GORDA RIDGE

Abstract The Sea Cliff hydrothermal field on the northern segment of the Gorda Ridge is situated along a rift-bounding normal fault about 2.6 km east of the neovolcanic zone and approximately 300 m above the spreading axis. The structural setting of this hydrothermal field differs from that of most other active seafloor hydrothermal sites investigated to date, which are typically situated in the neovolcanic zone. Mineralization occurs in basaltic talus covering a large normal-fault scarp. Hydrothermal crusts cover much of the seafloor in the area of the active hydrothermal field. These crusts form by extensive alteration of basaltic hyaloclastite in a zone of mixing between ascending hydrothermal fluid and entrained seawater. The initial stage of alteration is magnesian metasomatism of both crystalline basalt and basaltic glass, converting the rock to Mg-rich smectite and smectite/chlorite. Further alteration removes nearly all cations and ultimately leads to silicification. Preservation of basaltic texture in the silicified rocks provides evidence that even such sparingly soluble elements as A1 and Ti have been removed. Oxygen-isotopic ratios of the altered rocks constrain initial alteration to temperatures near 220°C, close to the maximum measured vent temperatures of 247°C. Silicification proceeded to much lower temperatures, and most amorphous silica deposition occurred at temperatures below 100°C. Sulfur-, strontium-, and lead-isotopic data all indicate a predominantly basaltic source, with important contributions from seawater but no significant contribution from sedimentary sources. Comparison with ophiolite-hosted massive sulfide deposits shows that the structural setting, alteration sequence, and depositional environment are all similar and suggests that mineralization and replacement of basaltic breccia in the Sea Cliff hydrothermal field likely occur in the subsurface beneath a capping layer of silicified hyaloclastite.

Very low-temperature alteration of sideromelane in hyaloclastites and hyalotuffs from Kilauea and Mauna Kea volcanoes: Implications for the mechanism of palagonite formation

Three petrographically distinct styles of altered glasses in two hyaloclastites and one hyalotuff were studied. The texture and chemistry of these samples were investigated using electron probe microanalysis, scanning electron microscopy and transmission electron microscopy in order to understand better the mechanism by which alteration of sideromelane and formation of palagonite occurred in these samples. The results show that clay minerals (primarily smectites) are present in three different microenvironments: (1) coating the surfaces of glass and crystals or vesicle walls; (2) as a relatively heterogeneous, but well crystallized, replacement product (i.e. reddened smectite grain replacement or RSGR) of glass or; (3) as a relatively homogeneous, amorphous to poorly crystalline replacement product (i.e. palagonite). Both the grain size and composition of these smectite-like materials vary considerably.Crystalline smectites occur in both hyaloclastites and have an intermediate composition between the two end-members nontronite and saponite. This composition could correspond to a mechanical intergrowth and/or an interstratification of two different smectites: one dioctahedral (i.e. nontronite) and one trioctahedral (i.e. saponite or stevensite) or simply to a true di-trioctahedral smectite. The coating smectite appears to have precipitated by a paragenetically-early, dissolution-precipitation mechanism prior to the formation of the RSGR. The high Ti content found in RSGR is attributable to an amorphous Ti-rich material which is intergrown with smectite and which behaves as a sink for immobile elements and those not included in smectite.Palagonite from both hyaloclastites and hyalotuff is poorly to non-crystalline and more aluminous than the coating smectites. Palagonite from the hyalotuff has an Fe-rich montmorillonite-like composition. The TEM images show a 30–50 nm thick leached layer formed by selective (non-stoichiometric) dissolution that takes place in the fracture domain. The hydration and replacement of glass during the palgonitization process is accompanied by the loss of Fe, Mg and Ca with a concomitant gain of Al. Both palagonites (from hyaloclastites and hyalotuff) show smilar textural and chemical characteristics.

Oxygen isotope evidence for submarine hydrothermal alteration of the Del Puerto ophiolite, California

Abstract The oxygen isotope compositions and metamorphic mineral assemblages of hydrothermally altered rocks from the Del Puerto ophiolite and overlying volcaniclastic sedimentary rocks at the base of the Great Valley sequence indicate that their alteration occurred in a submarine hydrothermal system. Whole rock δ18O compositions decrease progressively down section (with increasing metamorphic grade): +22.4‰ (SMOW) to +13.8 for zeolite-bearing volcaniclastic sedimentary rocks overlying the ophiolite; +19.6 to +11.6 for pumpellyite-bearing metavolcanic rocks in the upper part of the ophiolites volcanic member; +12.3 to +8.1 for epidote-bearing metavolcanic rocks in the lower part of the volcanic member; +8.5 to +5.7 for greenschist facies rocks from the ophiolites plutonic member; +7.6 to +5.8 for amphibolite facies or unmetamorphosed rocks from the plutonic member. Modelling of fluid-rock interaction in the Del Puerto ophiolite indicates that the observed pattern of upward enrichment in whole rock δ18O can be best explained by isotopic exchange with discharging18O-shifted seawater at fluid/rock mass ratios near 2 and temperatures below 500°C.18O-depleted plutonic rocks necessarily produced during hydrothermal circulation were later removed as a result of tectonism. Submarine weathering and later burial metamorphism at the base of the Great Valley sequence cannot by itself have produced the zonation of hydrothermal minerals and the corresponding variations in oxygen isotope compositions. The pervasive zeolite and prehnite-pumpellyite facies mineral assemblages found in the Del Puerto ophiolite may reflect its origin near an island arc rather than deep ocean spreading center.

Characterization of alteration products in tephra from Haleakala, Maui : A visible-infrared spectroscopy, mössbauer spectroscopy, XRD, EMPA and TEM study

Altered basaltic tephra from Haleakala, Maui, are characterized using multiple techniques in order to identify the minerals formed under a variety of conditions and to understand the soil formation processes here. We collected samples that are representative of typical bulk weathered material in the crater, as well as solfataric alteration in a hydrothermal environment. For this study X-ray diffraction, electron probe microanalysis, scanning electron microscopy and transmission electron microscopy are coupled with spectroscopic techniques including Mössbauer, visible-infrared reflectance, transmission infrared, and thermal infrared emission spectroscopies to analyze these samples. The unaltered tephra are composed of feldspar, glass, pyroxene and olivine. Observed alteration products include Fe oxides, phyllosilicates and sulfates, as well as SAED amorphous Al-Si-bearing material. These samples are potential analogs for altered volcanic material on Mars as the pedogenic influences and contact with plants and animals are minimal. Results from this study may help to determine spectral signatures of these samples that could be used for identification on Mars of the minerals observed here.