Minoru Utada

Bio: Minoru Utada is an academic researcher from University of Tokyo. The author has contributed to research in topics: Hydrothermal circulation & Chlorite. The author has an hindex of 10, co-authored 32 publications receiving 544 citations.

Further investigations of a conversion series of dioctahedral mica/smectites in the shinzan hydrothermal alteration area, northeast japan

Abstract: A complete conversion series for mica/smectites was found in a hydrothermal alteration envelope around Kuroko-type ore deposits at the Shinzan area, Akita Prefecture, Northeast Japan. The minerals are an alteration product of volcanic glass of Miocene age and are commonly associated with zeolites and silica minerals. Degrees of ordering of interstratification of the minerals change discontinuously from Reichweite g = 0 (100–55% expandable layers) to g = 1 (45–20% expandable layers), and from g = 1 to g = 2 (<20% expandable layers). This pattern of conversion differs from the behavior of mica/smectites during burial diagenesis which undergo a continuous change in ordering type, and from the behavior of rectorite which displays a constant expandability and ordering (45–55%) over a wide range of conditions. Differences between these minerals were also found in the relationships between expandability and total layer charge, and between expandability and number of non-exchangeable interlayer cations. In mica/smectites from the Shinzan area, chemical changes in the interlayers and tetrahedral and octahedral sites are consistent with a reaction in which K-enrichment and K-fixation in the interlayers are controlled by an increase in negative layer charge. This conversion occurred in response to a steep geothermal gradient and migrating hydrothermal solutions.

Zeolites in Sedimentary Rocks, with Reference to the Depositional Environments and Zonal Distribution

Abstract: SUMMARY The authors have studied alterations of Cenozoic and Mesozoic pyroclastic rocks of Japan, which contain several kinds of zeolites in abundance. This paper summarizes zeolites in sedimentary rocks, with reference to the depositional environments and zonal distribution, by a survey of the literature in addition to the authors’ data. The zonal distribution of zeolites is recognized in buried sedimentary rocks as follows: The zeolites in syngenetic or early diagenetic origin depend strongly upon a specific sedimentary environment. Phillipsite occurs largely in pelagic sediments of the younger geologic age. Analcime is found in saline-lake and terrestrial sediments in a warm, rather arid region, frequently associated with phillipsite, chabazite and natrolite. The zeolites are not influenced by the sedimentary environments but depend upon the depth of burial, i.e., increasing temperature and pressure. Most of clinop- tilolite, mordenite and erionite, forming at a relatively shallow depth, occur only as an alteration product of acidic to intermediate volcanic glass and cement of the post- Jurassic pyroclastic rocks. Laumontite, forming at a greater depth, on the other hand, is widely distributed in the pre-Pliocene various sedimentary rocks.

Talc-bearing serpentinite and the creeping section of the San Andreas fault

Abstract: The section of the San Andreas fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr(-1) (ref. 1), and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust. Serpentinized ultramafic rocks have been associated with creeping faults in central and northern California, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength, and these minerals also have the potential for unstable slip under some conditions. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault, and its inherently stable sliding behaviour is consistent with fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the fault zone.

Low strength of deep San Andreas fault gouge from SAFOD core

Abstract: Laboratory measurements of the strength of core samples from a drill hole located northwest of Parkfield, California, near the southern end of a creeping zone of the San Andreas fault, demonstrate that the fault is profoundly weak at this location and depth. This is because of the presence of the smectite clay mineral saponite — one of the weakest phyllosilicates known. The finding suggests that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals, rather than by high fluid pressure or other proposed mechanisms. This study reports on laboratory-strength measurements of fault core materials from a drill hole located northwest of Parkfield, California, near the southern end of a creeping zone of the San Andreas fault. It is found that the fault is profoundly weak at this location and depth, owing to the presence of the smectite clay mineral saponite—one of the weakest phyllosilicates known. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms. The San Andreas fault accommodates 28–34 mm yr−1 of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7 km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault2,3. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms1. The combination of these measurements of fault core strength with borehole observations1,4,5 yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust.

An experimentally derived kinetic model for smectite-to-illite conversion and its use as a geothermometer

Abstract: The smectite-to-illite conversion during shale diagenesis has recently been used to constrain the estimate of a basin's thermal history. We have systematically investigated the kinetics for the con- version of a Na-saturated montmorillonite (SWy-1) to a mixed-layer smectite/iUite as a function of KC1 concentration (from 0.1 to 3 moles/liter) over a temperature range of 250* to 325"C at 500 bars in cold- seal pressure vessels using gold capsules. The results show that the conversion rate can be described by a simple empirical rate equation: - dS/dt = A. exp(- Ea/RT)- (K § S 2 where S = fraction of smectite layers in the I/S, t = time in seconds, A = frequency factor = 8.08 x 10 -4 sec -~ , exp = exponential function, Ea = activation Energy = 28 kcal/mole, R = gas constant, 1.987 cal/ deg-mole, T = temperature (degree Kelvin), (K § ) = K + concentration in molarity (M) in the fluid. The results also show that Ca 2 + in solutions barely affects the illitization rate, whereas Mg 2+ significantly retards the rate. The retardation, however, is not as severe as previously reported. Na + ion can significantly retard the rate only if the concentration is high. We found that by assuming a range 0.0026-0.0052 moles/liter (100-200 ppm) of K +, concentrations similar to the value typically reported in oil field brines, the present kinetic model can reasonably predict the extent of the smectite-to-illite conversion for a number of basins from various depths and age. This narrow range of potassium concentrations, therefore, is used to model the smectite-to-illite conversion in shale when the actual chemical information of pore fluid is not available. The kinetic equation has been tested using field data from a large variety of geologic settings worldwide (i.e., the Gulf of Mexico, Vienna Basin, Salton Trough Geothermal Area, East Taiwan Basin, Huasna Basin, etc). The results show that the equation reasonably predicts the extent of the reaction within our knowledge of the variables involved, such as burial history, thermal gradients, and potassium concen- tration.

Hydrothermal Minerals and Precious Metals in the Broadlands-Ohaaki Geothermal System: Implications for Understanding Low-Sulfidation Epithermal Environments

Abstract: The Broadlands-Ohaaki geothermal system is a boiling hydrothermal system hosted by a sequence of Quaternary felsic volcanic rocks and Mesozoic metasediments. More than 50 wells have been drilled (400 to >2,600 m deep) to assess the geothermal potential for the production of electricity. Fluids and precipitates sampled from wells, along with descriptions of the alteration minerals in more than 500 drill cores, provide a three-dimensional picture of the distribution of fluid types and secondary minerals. Interpretation of these features and the distribution of gold and silver highlight the relationship between alteration and mineralization in an active, low-sulfidation epithermal environment. Quartz, illite, K feldspar (adularia), albite, chlorite, calcite, and pyrite are the main hydrothermal minerals that occur in the deep central upflow zone at >250°C and> 600 m depth. These minerals form through recrystallization of the volcanic host rocks and incorporation of H 2 O, CO 2 , and H 2 S in the presence of a deeply derived chloride water containing ~1,000 mg/kg Cl and ~26,400 mg/kg CO 2 . At the same time, and on the periphery of the upflow zone, illite, smectite, calcite, and siderite form through hydrolitic alteration in the presence of CO 2 -rich steam-heated waters that contain 2 . Upward and outward from the deep central upflow zone, mineral patterns reflect the shift from rock-dominated to fluid-dominated alteration and the prevailing influence of boiling, mixing, and cooling on fluid-mineral equilibria. Accordingly, the abundance of quartz and K feldspar increase toward the upflow zone, whereas clay abundance increases toward the margin of the upflow zone (with smectite dominating at 200°C); the abundance of chlorite, pyrite, and calcite varies here, but albite is absent. Geothermal production wells with high fluid fluxes are the main sites of precious-metal mineralization. The deep chloride water (with or without minor amounts of vapor) enters the well at depths >500 m and undergoes a pressure drop that causes boiling. As a result, precious metals precipitate and accumulate as scales on back-pressure plates or as detritus in surface weir boxes; these deposits contain 1,000 mg/kg Au, 10,000 mg/kg Ag and ~10 to ~1,000 mg/kg As and Sb, each. Within production wells, platy calcite deposits as a scale at the site of first boiling near the fluid feed point, while crustiform-colloform-banded amorphous silica deposits in surface pipe work. By contrast, the hydrothermally altered host rocks contain low concentrations of gold, ranging from <0.01 to 1.0 mg/kg Au (68 analyses), and these correlate positively with arsenic (<100 to ~5,000 mg/kg) and antimony ( Reaction path modeling using SOLVEQ and CHILLER shows that calcite, K feldspar, gold, and amorphous silica deposit in sequence from a chloride water that cools along an adiabatic boiling path (300° to 100°C), analogous to fluid flow in a production well. By contrast, calcite, quartz, K mica, and pyrite deposit from a chloride water that cools due to mixing with CO 2 -rich steam-heated waters; dilution prevents precipitation of precious metals. Thus field observations and reaction path modeling demonstrate that boiling is the main process influencing the deposition of precious metals. The results of this study show how peripheral hydrolytic alteration by CO 2 -rich steam-heated waters relate to propylitic and potassic alteration by chloride waters in the epithermal environment of a hydrothermal system. Both the distribution of alteration mineral assemblages associated with the different water types and the broad-scale distribution of temperature-sensitive smectite and illite reflect the location of the upflow zone. On a local scale, the occurrence of platy calcite, crustiform-colloform silica, and K feldspar in veins indicates the existence of boiling conditions conducive to precious-metal deposition.