Isocon Method Research Articles

Shear strain and volume change associated with sigmoidal vein arrays in the Boda Claystone

The Boda Claystone Formation, a potential host rock for high-level nuclear waste in Hungary, contains numerous sigmoidal vein arrays with angular vein terminations. These veins are excellent strain markers and can be used in the interpretation of palaeo-environments. Sigmoidal vein arrays were investigated in the BAF-2 well, which penetrates the formation to depths above 900 m with nearly 100% core recovery. The veins have antitaxial growth morphologies and are filled by calcite, anhydrite and barite–celestine phases. To estimate the volume change in shear zones, we used geometric, mass-balance and density-change calculations. These methods exhibit different volume loss rates. The geometric approach indicates a large amount of volume loss, whereas the isocon method and density-change approach suggest that volume loss in the shear zones was less than 10%. We propose that the volume loss in the shear zone occurred because of tectonic compaction. Vein shapes indicate formation under compressional stress fields. Based on their intersections with other vein generations, the sigmoidal en échelon vein generation is determined to be one of the youngest structures in the well.

Aluminous gneiss derived by weathering of basaltic source rocks in the Neoarchean Storø Supracrustal Belt, southern West Greenland

The origin of amphibolite-facies aluminous gneiss from the gold-hosting Neoarchean Storø Supracrustal Belt in the Nuuk region of southern West Greenland is investigated in this study. An improved understanding of the formation of such aluminous gneiss has implications for genetic models (epithermal vs. orogenic style) for a local gold occurrence, which is hosted by sheeted quartz-veins within amphibolite in the hanging wall adjacent to the aluminous gneiss on the island of Storø. The aluminous gneiss mainly consists of garnet, plagioclase, sillimanite, quartz and biotite, which suggest a pelitic protolith. However, it has previously been postulated that the aluminous gneiss represents a hydrothermal alteration product, formed by leaching of a mafic precursor that was subsequently transformed to the current mineral assemblage during later regional metamorphism. In support of this hypothesis are decimeter-scale relicts of amphibolite, found within the decameter-wide aluminous gneiss, that share similar ratios of commonly fluid immobile elements, such as Al, Ti, Zr, Hf, Nb and Lu.Metasedimentary rocks sensu stricto are also present within the Storø Supracrustal Belt. These mostly comprise biotite schist with a similar mineralogy as the aluminous gneiss, and are located adjacent to the latter in the footwall. The aluminous gneiss has high bulk-rock Al2O3 contents and low SiO2 relative to the biotite schist, which is less aluminous, has higher SiO2, and is more potassic than the aluminous gneiss. The immobile element ratios of the biotite schist are significantly different from those of the amphibolite and the aluminous gneiss. Additionally, the biotite schist yields distinct detrital zircon age populations, whereas only metamorphic zircon has been found in the aluminous gneiss and amphibolite.In the present study, the isocon method was applied to provide mass balance constraints on the alteration of basalt to the protolith of the aluminous gneiss prior to regional metamorphism of the entire Storø supracrustal sequence. Accepting minor fractionation among otherwise immobile elements, an error of at least ±20% is estimated for the isocon mass-balance model. The results indicate that all major elements, except for K2O were leached from the basaltic precursor, resulting in a net mass loss ranging from −20 to −40wt.%. However, despite the similar trace element patterns and ratios of the aluminous gneiss and the adjacent amphibolite, in situ alteration or weathering of the basaltic precursor rock is not possible given the small, but persistent, fractionation among fluid immobile elements, due to the unusual accumulation of Cr, U, Ni and Th, as well as the observed modal layering within the aluminous gneiss. Instead, this points to physical transport during sedimentary reworking of a mafic protolith and potentially the addition of redox-sensitive elements such as U and Cr from the water column, and therefore suggest that this aluminous gneiss simply represents a metasediment with a mafic provenance.The implication of this study for gold exploration within the Archean supracrustal belts of the SW Greenland is that aluminous gneiss is unlikely to represent an indicator of acidic hydrothermal alteration as previously postulated. Instead, aluminous gneisses within these supracrustal belts are likely of sedimentary origin and may provide a venue to further understand the exogenous environments of the Archean Earth, and thus further geochemical studies of such rocks are recommended in order to place constraints on the composition of the hydrosphere at that time.

Microstructural Control on Perlite Expansibility and Geochemical Balance with a Novel Application of Isocon Analysis: An Example from Milos Island Perlite (Greece)

Representative perlite bulk rock samples from two areas of Milos Island, Greece were collected and the expansion properties of their industrial product were investigated. Coarse crude perlite from Tsigrado exhibits better expansibility, which is assigned to the presence of coarser crystallites in its bulk parent rock. During thermal treatment, the finer crystallites of the coarse crude perlite from Trachilas are entrapped in the groundmass and lead to overheating, which inhibits expansion and eventually results in shrinkage. Geochemical modification of the expanded perlites relative to their crude precursors were investigated, using the isocon method. Volatilisation of crystalline water is the main factor controlling mass reduction of the expanded perlites. Other elements, during the adequate expansion of the Tsigrado perlite, can be classified into three categories. The elements that participate preferentially in crystals decrease in the expanded material at amounts higher than the total mass loss of the rock, due to their escape controlled mainly by the removal of the crystalline phases. The elements equally participating in crystals and the groundmass show losses equivalent to the total mass loss of the rocks, as they escaped in the crystalline phases and airborne particles from the groundmass during thermal treatment. Decrease of highly incompatible elements, which mostly participate in the groundmass, in the expanded products is less than the total mass loss, as they escaped mainly in the airborne particles. The inadequate expansion and burst of the Trachilas perlite did not allow for a similar categorisation, due to random and unpredictable escape of the elements. We propose the application of this method to an artificial system to predict unexpandable mineral phases in bulk perlite, as well as elements that are most likely to participate in the amorphous perlite phase, which cannot be determined from a regular industrial production line. This graphical method may also predict environmental pollution of the atmosphere from the release of volatile compounds and airborne particles during thermal treatment of perlite or other processes of mineral treatment.

The multi-attribute anomaly structure model: An exploration tool for the Zhaojikou epithermal Pb-Zn deposit, China

Traditionally, a geochemical exploration index is based mainly on elemental concentration, and focusses on positive anomalies rather than negative anomalies. This results in two problems: firstly, it ignores depleted elements, negative anomalies, and immobile elements, and secondly, elemental concentration does not directly reveal the degree of elemental mobility. To discover information on element mobility and utilize negative anomalies, the Isocon method was employed to calculate mass-transfer, and study anomalous elemental characteristics in the Zhaojikou epithermal Pb-Zn deposit, SE China. Subsequently, the concept of a multi-attribute anomaly structure is proposed, which refers to enrichment and depletion zones of multiple elements co-existing in an ore deposit. This includes major elements, mineralizing agents, and ore-forming and associated elements.Results show that there are system mass-transfer (μ), Na2O, and S anomalies, correlations between S-FeO-Pb+Zn, and Pb, Zn, Cu, Ag, As, and Sb anomalies in the multi-attribute anomaly structure model of the Zhaojikou Pb-Zn ore deposit. The presence of the μ anomaly indicates there was at least one mass-exchange geological event, with the extent of the anomaly representing the impact area of the event(s). The extent of the negative Na2O anomaly represents the fluid-rock interaction zones. The positive S anomaly suggests the presence of sulphides. The correlations between S-FeO-Pb+Zn reveal there are numerous other economic metal sulphides (i.e., excluding noneconomic Fe-rich sulphides like pyrite). Ore-forming and associated elemental anomalies indicate the potential presence of polymetallic sulphides. μ>0.3, ΔCNa2OO-A<−7kg, and μNa2O<−50% can act as evaluation indices for ore prospecting. There are two anomalous centres in the Zhaojikou Pb-Zn ore deposit. The shallower and larger one corresponds solely to the presence of the known orebodies. However, the deeper and smaller one is not currently known, and suggests ores may be present at depth. In summary, the Zhaojikou district has great potential for additional ore deposits at depth. We suggest directing further work deeper, mainly along NE–SW Dongzhi Fault.

How fluids infiltrate through fractures and change metamorphic rocks – a case study from northern Spitsbergen [abstract

Metasomatism is a process leading to changes in the chemical composition of a rock or its portion. It involves introduction or removal of chemical components due to the interaction of the rock with aqueous fluid (Zharikov et al. 2007). Here we present a case study of metasomatic changes in metapelites from northern Spitsbergen. There is a visible metasomatic zonation in the studied samples. The most intense changes are observed close to the contact with fractures, whereas the zones farther away from them are less altered or not affected by fluids at all. Thin sections were prepared from all of the aforementioned zones. The mineral chemistry was obtained using a JEOL SuperProbe 8230 electron microprobe at Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology. Mineral assemblage varies from Grt-Bt-Qtz-Pl in zones that were not affected by fluids to Grt-Chl-Bt-Qtz-Pl in altered portions of the rock. Garnet forms subhedral crystals with abundant quartz inclusions. Its composition is changing from Alm0.56Sps0.11Prp0.05Grs0.28 in the core to Alm0.64Sps0.03Prp0.09Grs0.24 in the rim. It shows bell-shaped Sps profile and opposite Alm trend, indicating one-step progressive garnet growth. Plagioclase composition is characterized by Ab74.16An24.39Or1.44, K-feldspar also occurs along with Fe-chlorite. Point counting technique was used to estimate volume percentage of minerals. For each thin section one thousand points were counted. Plagioclase, quartz and biotite are the main phases, building more than 95% of the rocks. Amount of garnet oscillates around 2% and chlorite varies from 2.38% to 0%. Point counting technique was compared with image analysis, which was performed using the Matlab software. The image analysis revealed that the amount of biotite is changing in different zones from 35.42% to 37.38%, whereas chlorite from 1.54% to 0%, respectively. The redistribution of elements between biotite and chlorite was investigated by volume and mass-balance using Gresens (1967) approach. During chloritization of biotite amount of H2O in the system must have increased by ca. 8% in order to form 2.4% chlorite. Potassium released during this reaction was bonded to secondary K-feldspar, which is apparent on BSE images, and it is closely associated with biotite and chlorite. This study reveals that metasomatic zones significantly differ in mineralogical composition and fluid induced alterations are more pervasive closer to the fractures. It has also been concluded that replacement of biotite by chlorite required aqueous fluids, with inflow and outflow of metallic ions and formation of K-feldspar as a product of biotite breakdown reaction. Further investigations will involve X-ray mapping using the XMap Tools software in order to obtain more in depth information on volume of minerals and their chemistry. Pressure and temperature conditions of metamorphism will be determined using garnet-biotite geothermometry and thermodynamic modelling. Isocon method will be used to track changes in whole rock chemistry due to interaction with fluid (Grant 1986, 2005). Acknowledgements:The study was financed from the statutory research project No 11.11.140.319 and 11.11.140.613, AGH University of Science and Technology.

Mesoarchaean aluminous rocks at Storø, southern West Greenland: New age data and evidence of premetamorphic seafloor weathering of basalts

Metamorphosed Meso- to Neoarchaean supracrustal rocks in the central part of the island of Storø (Nuuk region, southern West Greenland), show field- and geochemical evidence of premetamorphic chemical alteration. This alteration changed basaltic precursors into aluminous lithologies, and following amphibolite grade metamorphism and penetrative ductile deformation, these garnet–biotite schists now resemble adjacent metapelitic schists of sedimentary origin. Mass balance calculations (isocon method), suggests that most major elements (Si, Fe, Mg, Ca, Na and P) were leached during alteration. The calculated overall net mass changes are between −18% and −45%, consistent with breakdown of olivine, pyroxenes, plagioclase and apatite in the basaltic precursor rocks. Major and trace elements such as, K, Cs, Rb, Ba, Pb, Zn, La and Ce were added during this alteration process, whereas high field strength elements (Ti, Al, Zr, Hf and Nb) remained essentially immobile and were thus residually enriched. Interestingly, Th which is generally assumed to be immobile in fluids, was also added during this process. These chemical changes reflect interaction between a basaltic protolith and hydrous fluids that established a new equilibrium and thus a different mineral assemblage. It is proposed that the premetamorphic alteration at Storø was due to low-temperature interaction between seawater and oceanic crust, and thus essentially represents in situ submarine seafloor weathering. This interpretation is consistent with the mass balances reported from well-documented examples in younger settings.New U–Pb zircon geochronology from the arc-related mafic sequences at Storø shows that they comprise at least two distinct age groups: an older anorthosite complex dated at 3051.3±2.6Ma and a younger supracrustal sequence with age brackets between 2840 and 2710Ma. The allochthonous nature of these two mafic igneous to sedimentary stacks is consistent with accretionary processes in island arc complexes and a compressional Archaean tectonic setting.

System closure during the combustion metamorphic “Mottled Zone” event, Israel

The Hatrurim Formation (or the “Mottled Zone”) is a unique rock complex in Israel and Jordan, which is composed mainly of high-temperature–low-pressure metamorphic minerals, typical of the sanidinite and pyroxene-hornfels facies. These rocks are the metamorphic product of combustion of the bituminous (“oil shales”) Maastrichtian to Paleocene chalk and marls of the Ghareb and Taqiye Formations.Temperatures of 500–700°C were common during the combustion metamorphic process producing the high-grade gehlenite–larnite, calcite–spurrite and anorthite–diopside fels (“dense-olive” rocks). In order to test the degree of system chemical closure the graphical isocon method was applied. We compared the chemical composition of the metamorphic rocks and their protolith equivalents — Ghareb Formation oil shales and Taqiye Formation rocks, on an outcrop scale. With respect to a set of 8 major, minor as well as 21 trace elements, the chemical composition remained similar during metamorphism and yielded almost no element mobility. The main losses were those of organic matter, which was oxidized during combustion, dehydration, upon heating and decarbonation of carbonates. Combustion resulted in an average mass loss of 30% and some volume loss (derived from the mass loss as well as the formation of denser mineral phases). These suggest that the combustion process was essentially isochemical on a large scale. On the other hand, the common presence of veins in some of the rocks implies that the Hatrurim metamorphism was isochemical on a large scale, whereas on a smaller scale open system fractionation characterized some of the elements, which are enriched in the veins.

Origin of omphacitites from the Nishisonogi metamorphic rocks, western Kyushu, Japan: comparison with jadeitites

Two types of omphacitites are distinguished in ultramafic melanges that are intercalated with epidote–blueschist facies schists of the Nishisonogi metamorphic rocks in western Kyushu, Japan. One is an omphacitite layer in a metabasite block, and the other is an omphacitite lens in an albitite block. These omphacitites are principally composed of omphacite with minor amounts of winchite/magnesiohornblende, epidote, albite and titanite. Although the petrographic features of both types are similar, the omphacitite layer is depleted in light rare earth elements (LREE; La, Ce and Nd) compared with the omphacitite lens. In the case of jadeitites, those interpreted as being formed by direct precipitation from aqueous fluids are LREE-depleted, while those interpreted as being formed by metasomatic replacement from protoliths are LREE-enriched. Thus, by analogy, the omphacitite layer may have directly precipitated from aqueous fluids, and the omphacitite lens may be metasomatic in origin. The omphacitite layer has amphibole-rich selvages, which are likely formed by the reaction between the omphacitite-forming fluid and the host metabasite. Mass-balance analysis using the isocon method indicates that the formation of the selvages involves additions of large ion lithophile elements (LILE; Rb, K and Ba), Na and Si to the host metabasite. These components were likely introduced via fluid infiltration. Although the source of the omphacitite-forming fluid is not clear, the LILE-rich composition suggests participation of fluids derived from metapelitic rocks.

Environmental suitability of ceramic raw materials: a geochemical approach to volatile emissions and leaching potentials

Growing environmental concern is promoting the necessity of additional ceramic tests. The use of unknown materials with potential contamination conditions requires further studies to demonstrate that the piece encapsulates the pollutant and that during its firing it does not produce the emission of harmful volatile elements. The objective of this work was to perform both tests in a ceramic paste made with slip-casting wastes and electro-plating residues. The leaching methods for determining the encapsulation of potential harmful elements were performed following the TCLP, EP-Tox norm. Having precise chemical analysis of both crude and fired brick, the problem of emissions losses during the firing can be solved by the gain/loss techniques used in geological studies. In particular, the Isocon method that permits a quick visualization of the lost elements is useful. Once the volatile elements were determined, their amount was calculated considering a constant element and the chemical concentrations normalized by the loss of ignition, or using the crude/fired brick ratio densities. The leaching tests indicate that the ceramic brick does not produce harmful leachates according to Argentinean specifications. The leachates of B and Ca are high. Ca does not seem to be a problem while the B content is beyond the permitted limits. During the firing, volatilized elements are Ag, Br, Cl, F, Hg, S, Se, and H2O. The loss of Ag, Br, and Se are negligible. For F and Cl the potential emission rate is very low (100 mg/kg) while in the case of S, the 2,600 mg/kg rate is high. However, extrapolated emission rates at the chimney assuming an air-to-brick ratio of 2.5 N m3/kg, are approximately 1,040 mg/N m3 for S and 40 mg/N m3 for F and Cl, falling inside environmentally acceptable values. The geochemical procedures proved to be useful tools to assess the element mobilization during firing of ceramic wares and in the analyzed case, the results indicate that the paste is environmentally acceptable.

Applying the box plot to the recognition of footwall alteration zones related to VMS deposits in a high-grade metamorphic terrain, South Africa, a lithogeochemical exploration application

Alteration zones (more commonly foot wall alteration zones) are related to volcanic-hosted massive sulfide (VMS) deposits and represent unique features that may be targeted during exploration. Of these, the chloritic foot wall alteration pipe is the most extensive and characteristic of VMS deposits. This feature is geochemically identified by a strong relative enrichment in aluminium and magnesium and a coupled depletion in calcium and sodium, giving rise to chloritic rocks in the primary environment of formation. During high grade regional metamorphism such chloritic precursor rock types are replaced by an unusual mineral paragenesis, typically containing magnesium rich cordierite, phlogopite, orthoamphiboles or orthopyroxenes and aluminium rich minerals such as sillimanite and corundum. This suggests that the unusual geochemical features of the alteration zone, retained during the deformation and metamorphism, should be recognisable in lithogeochemical exploration. The massive sulfide deposit in the eastern part of the metamorphic Namaqua Province, South Africa, at Areachap, Kantienpan and the defunct Prieska Cu–Zn Mine are hosted by a Mid-Proterozoic volcano sedimentary succession known as the Areachap Group. These deposits were affected by a complex deformation and metamorphic history and represent examples of upper amphibolite to granulite grade metamorphosed VMS deposits. The application of the known lithogeochemical methods is especially complicated where the geology is not well understood, due to the poor rock exposure of complexly deformed and metamorphosed areas, such as in the eastern part of the Namaqua Province. The box plot presents a more readily applicable lithogeochemical method to characterize and identify the alteration process, but it was designed for relatively un-metamorphosed environments. It is demonstrated here that the box plot may also be applied to high-grade metamorphic terrains and that the mineral phases used in defining the boxplot in low grade metamorphic environments may be replaced by their equivalents in high grade metamorphic terrains. The compositional trends of the metamorphic minerals themselves may be used in defining the boxplot for high grade metamorphic terrains. These include the transition of: annite to phlogopite; grossular to almandine or pyrope, augite to enstatite or clinoenstatite, and hornblende to gedrite or cummingtonite. Close to the ore zone, the relative Mg content of pyroxene, cordierite and biotite are higher than further away from this zone. It could be demonstrated that the changes in the mineral compositions are gradational when comparing unaffected rocks with progressively more altered wall rocks. Conclusions based on an application of the isocon method demonstrate that primary footwall alteration zones in the Areachap Group's VMS deposits are characterized by elemental depletion of Na 2O, CaO, Sr, Ni, V and La and enrichment of MgO, Fe 2O 3(total), S, Zn, SiO 2, Co and F. It is shown that the whole rock compositions of rocks that were independently identified as the metamorphic equivalents of altered rocks, using the isocon method, plot in the correct place in the box plot for high grade regionally metamorphosed terrains. This establishes the box plot as an effective and practical tool for lithogeochemical exploration for VMS deposits in complexly deformed high grade metamorphosed terrains.

A NORMALIZATION SOLUTION TO MASS TRANSFER ILLUSTRATION OF MULTIPLE PROGRESSIVELY ALTERED SAMPLES USING THE ISOCON DIAGRAM

This study complements the isocon method of Grant (1986) for describing mass transfer during metasomatic alteration by introducing a normalization solution to illustrate sequential mass transfers among multiple progressively altered geologic samples. The principle of the normalization solution involves adjusting all the isocons (defined by unaltered and altered sample pairs) to a single unified isocon without modifying the relative concentration changes of the corresponding components. When the normalization solution is applied to a sequence of progressively altered samples, an immobile component is defined or assumed, and the measured concentrations of all components in all altered samples are normalized by multiplying the corresponding normalizing factors; these factors are calculated using the concentration data for the immobile component. The normalized compositions of the altered samples are then plotted against the original measured composition of the unaltered sample in a single normalized isocon diagram that illustrates all the mass changes for a series of progressively altered samples. This study applies the above approach to a set of progressively altered hydrothermal quartz-feldspar-porphyries at the Millenbach Cu-Zn mine (Riverin and Hodgson, 1980). The graphically displayed result indicates that the normalized isocon diagram can illustrate not only the mass changes of any component between any two altered zones, but also mass transfer trends of all components in the entire alteration zone. The normalization solution can be applied to any group of progressively altered geologic samples.

Mineralogy, Lithogeochemistry and Elemental Mass Balance of the Hydrothermal Alteration Associated with the Gold‐rich Batu Hijau Porphyry Copper Deposit, Sumbawa Island, Indonesia

AbstractThis paper discusses the mineralogy, whole‐rock geochemistry and elemental mass balance of the hydrothermal alteration zones within the Batu Hijau porphyry copper‐gold deposit, Sumbawa Island, Indonesia. The hydrothermal alteration and mineralisation developed in four stages, namely (i) the early stage consisting of a central copper‐gold‐bearing biotite (potassic), proximal actinolite (inner propylitic) and the distal chlorite‐epidote (outer propylitic) zones; (ii) the transitional stage represented by the chlorite‐sericite (intermediate argillic) zone; (iii) the late stages distinguished into the sericite‐paragonite (argillic) and pyrophyllite‐andalusite (advanced argillic) zones; and (iv) the very late stage typified by the illite‐sericite zone. In general, major elements (particularly Ca, Mg, Na and K) and some minor and rare earth elements decrease from the least altered rocks towards the late alteration zones as a consequence of the breakdown of Ca‐bearing hornblende, biotite and plagioclase. Chemical discrimination by means of millicationic R1‐R2 diagram indicates that R1 [4Si − 11(Na + K) − 2(Fe + Ti)] increases while R2[6Ca + 2Mg + Al] decreases with increasing alteration intensity, from least‐altered, through early, transitional, to late alteration zones. Rare earth elements‐chondrite (C1) normalised patterns also exhibit the depletion of the elements through the subsequent alteration zones. These results are consistent with the elemental mass balance calculation using the isocon method which shows that the degree of mass and volume depletion systematically increases during alteration. A decrease of the elements as well as mass and volume from early, through transitional to late alteration stages may imply a general decrease of the element activities in hydrothermal fluids during the formation of the alteration zones.

A geochemical estimation of fluid flux and permeability for a fault zone in Mugi mélange, the Cretaceous Shimanto Belt, SW Japan

The fluid flux and permeability along seismogenic subduction interfaces were estimated roughly based on geological evidences. A decrease in silica from the host rocks to the deformed rocks due to rock–fluid interaction within the fault–fluid system was qualitatively examined. The area under study was the northernmost part of the Mugi mélange in the Cretaceous Shimanto Belt, Shikoku, SW Japan. A fault zone with pseudotachylytes is located at the area as the roof thrust of the Mugi mélange. The pressure–temperature conditions of the fault zone correspond to the depth around the onset of the seismogenic zone along the subduction interface. On the basis of deformation texture, rocks are classified into three types—mélange, fault breccia, and ultracataclasite. The bulk chemical analysis of the abovementioned rocks was conducted by employing the X-ray fluorescence (XRF) analysis method. The average mass loss from the mélange host rocks to the fault rocks was estimated by the isocon method. In particular, a decrease in silica was observed clearly. Based on the decrease in silica and certain other geological constraints such as the pressure–temperature conditions of fluid, length of the fault, and the duration between deposition and exhumation, the amount of fluid reacting with rocks and the fluid flux were examined roughly. The estimated fluid volume was approximately thousand times the rock volume, and the fluid flux was about 10 −8–10 −6 m/s. Based on the fluid flux and some constraints in temperature pressure conditions of fluid and pressure gradient, the permeabilities of fault zone rocks were estimated, which was approximately of the order of about 10 −17–10 −16 m 2 and about 10 −16–10 −14 m 2 in the fault breccia and ultracataclasite, respectively. While the estimated value of the permeability of the fault breccia is similar to that obtained in laboratory experiments, the estimated value of the permeability of the ultracataclasite is considerably larger than that obtained from laboratory experiments. This probably indicates that the permeability of the ultracataclasite is not constant as observed in the laboratory experiments but can evolve during seismic cycle.

Mass transfers and volume changes related to pressure solution–fracturing interactions in carbonate rocks: Example of the Oligocene deformation of the Coniacian chalk from the Omey area (Paris Basin, France)

The present contribution documents the interactions between pressure solution and fracturing in carbonate rocks by studying the mass and volume changes caused by the Oligocene deformation of the Coniacian chalk from the Omey area. The isocon method allowed to establish that the decreases in interstitial fluid pressure resulting from the development of one normal fault and related tension gashes within a 11.4 m wide zone induced mass and volume changes within a 30.2 ± 3.5 m wide zone. In the hangingwall as well as in the footwall, the deformed zones adjacent to the fault plane exhibit mass gains (up to 58%) while the outermost deformed zones show mass losses (up to 36%). The pressure solution–fracturing interactions caused a mass redistribution from the most porous zones (outermost deformed zones) to the least porous zones (deformed zones adjacent to the fault plane) linked to differences in stress–strain energy of grain aggregates. The mass transfers strongly controlled the volume changes. Chemical compaction occurred only within the outermost deformed zones where mass losses took place in response to the reduction in solid–solid contacts. The zones affected by chemical compaction show equal mass and volume losses.

Evidence for Biological and Hydrological Controls on the Development of a Paleoproterozoic Paleoweathering Profile in the Baraboo Range, Wisconsin, U.S.A.

Abstract We examined a complete late Paleoproterozoic paleoweathering profile beneath the Baraboo Quartzite from south-central Wisconsin, USA, in order to interpret biological and hydrological controls on ancient terrestrial weathering processes. Detailed thin-section investigations were used to identify characteristic pedogenic features associated with saprolitization and formation of an overlying regolith, and the isocon method (a geochemical mass-balance technique) was employed to characterize elemental gains and losses. The age of the sub-Baraboo profile is well constrained by the youngest detrital zircon ages previously reported from the basal Baraboo Quartzite (1712 Ma), and the 1749 Ma crystallization age of the underlying Baxter Hollow granite and rhyolite parent materials. Micromorphologic evidence of paleoweathering comprises clay illuviation features (indicative of clay movement), sepic-plasmic fabrics (formed by repeated wetting and drying cycles), redoximorphic features (redox-related depletions and concentrations of Fe indicative of variable drainage and redox conditions), and dissolution and alteration of feldspars and mafic minerals (now sericite and Fe-oxides). Geochemical evidence for weathering includes depth-dependent whole-rock losses of Na, Ca, Mg, Si, Ba, Sr, Fe, and Mn, and relative enrichment of several immobile elements; there is also evidence for post-weathering K metasomatism (previously dated as 1456 Ma) that includes 30% net additions of K and Rb. Geochemical patterns for the Baraboo profile resemble those of the previously studied and older Pronto (2.475–2.44 Ga) and Ville Marie (2.38–2.215 Ga) paleoweathering profiles, but with important differences related to Baraboo profile development beneath a regolith cover and under conditions of variable hydrology during weathering. In spite of its age and greenschist-facies metamorphism, the Baraboo paleoweathering profile is remarkably similar to some modern weathering profiles formed on granite.

Arbitrary scaling in ISOCON method of geochemical mass balance: An evaluation of the graphical approach

Rock alteration processes can be studied through geochemical Mass Balance (MB) estimation in several ways. Due to the ease of its adaptability, especially its graphical approach, the ISOCON method of mass balance is extensively used for such studies. However the technique suffers from a serious limitation, in particular with reference to the graphical treatment involving arbitrary scaling, which is explained in this paper with the help of illustrative examples. The reference frame for MB obtained by best fit regression is biased to the element scaled upward that plot away from the origin. The scaling factors actually act as weighting factors which bias the slope of the ISOCON line because it is controlled by data points having higher numerical value in the plot. Since scaling of concentration data are unavoidable in ISOCON analysis, the results of MB may potentially be misleading. The slope of the best fit ISOCON line may vary within the range of slopes defined by individual conserved elements. In case, no scaling is done, the ISOCON shall weight in proportion to the abundance of the immobile elements. The co-linearity of the immobile elements with origin is also subjective in nature with the elements near origin apparently seem to satisfy a wide range ISOCON slopes than those that are away from origin. When equal weight is assigned to all elements, regardless of their concentration level, an average of the slopes of the immobile elements (i.e., concentration ratios in altered to unaltered) may provide better approximation for reference frame without recourse to graphical plot and ISOCON solution. The existing weighted least-squares or equal weight least-squares ISOCON method is found to be more appropriate in case of higher uncertainty in the protolith compositions. In any case, a proper identification of the conserved species is an essential prerequisite to successful implementation of mass balance computation minimizing the inherent errors of the ISOCON technique.

Chemical mass balance in a reaction zone between serpentinite and metapelites in the Nishisonogi metamorphic rocks, Kyushu, Japan: Implications for devolatilization

Abstract Reaction zones of 0.5–10.0 m thick are commonly observed between serpentinite and pelitic schist in the Nishisonogi metamorphic rocks, Kyushu, Japan. Each reaction zone consists of almost monomineralic or bimineralic layers of talc + carbonates, actinolite (or carbonates + quartz), chlorite, muscovite and albite from serpentinite to pelitic schist. Magnesite + quartz veins extend into the serpentinite from the talc + carbonates layer, while dolomite veins extend into the pelitic schist from the muscovite layer. These veins are filled by subhedral minerals with oriented growth features. Primary fluid inclusions yield the same homogenization temperatures (145–150°C) both in the reaction zone and in the veins, suggesting their simultaneous formation. Mass‐balance calculations using the isocon method indicate that SiO2, MgO, H2O and K2O are depleted in the reaction zone relative to the protoliths. These components were probably extracted from the reaction zone as fluids during the formation of the reaction zone.

Mass transfer and reaction paths in alteration zones around carbonate veins in the Nishisonogi Metamorphic Rocks, southwest Japan

This paper describes local mass transfer and fluid-rock interaction in epidote-blueschist subfacies basic schist around retrograde dolomite + calcite veins in the Nishisonogi Metamorphic Rocks, southwest Japan. The veins have alteration zones on both sides. The mineralogical and chemical changes during alteration were documented along two sample traverses normal to the veins. Approaching the veins, the peak metamorphic assemblage (winchite + epidote + chlorite + calcite + albite + quartz) has been modified by progressive breakdown of amphibole, epidote, and calcite to illite and dolomite. Mass-balance calculations using the isocon method reveal that CO 2 and K were added to the basic schists, and Ca was removed during alteration. The mass balance does not hold for these components between the vein + alteration zone and protolith as a whole. These findings suggest that the vein formation and metasomatism represent a process driven by fluid flow and long-distance element transport. Possible reaction paths between the vein-fluids and basic schists were estimated from the progressive change of mineral assemblages in the alteration zones. The phase relations on ionic activity diagrams indicate an increase in log f CO 2 and decreases in log( a Ca 2+ /σ Ca 2+ a 2 H + ) and log( a Mg 2+ /σ Mg 2+ a 2 H + ) with progress of the alteration. Although a change in log( a K + /σ K + a 2 H + ) is poorly constrained on the activity diagrams, the mass balance suggests a concentration gradient of K across the alteration zones. Hence, the reaction paths and mass balance calculations suggest that a combination of the concentration gradients of CO 2 , Ca, Mg, and K produced by the vein-fluids drove the alteration process.

Miocene silcretes in argillaceous playa deposits, Madrid Basin, Spain: petrological and geochemical features

Four regressive sequences are present in the opaline rocks and related deposits of the Miocene Intermediate Unit of the Madrid Basin. The sequences consist of silty mudstones and argillaceous opals, separated by transitional facies. The silty mudstone consists mainly of dioctahedral smectites, whereas the argillaceous opal is principally opal‐CT and variable amounts of sepiolite. In the transitional facies, lamina of dioctahedral smectite co‐exist with neoformed opal‐CT and sepiolite. Petrological and geochemical features (major, trace and REE elements) indicate that the opaline levels and the transitional facies are related and are a consequence of silcrete formation in an argillaceous playa deposit. The isocon method was used to calculate changes in element concentration associated with silcrete formation. The geochemical data suggest silicification in an arid environment. The silcrete profile occurs four times, possibly as a result of highstand–lowstand fluctuations of the lake level caused by climatic changes. Structures and cements in the silcretes indicate that, although silicification may have commenced at the top of a groundwater table, it continued in the unsaturated zone above the water table.

A least-squares approach to mass transport calculations using the isocon method

A statistically rigorous approach based on the isocon method (Grant, 1986) to evaluate mass transport is presented. Chemical analyses of multiple samples of unaltered (parent) and altered rock are used to calculate the average oxide or element concentration and its standard deviation for each rock population. In a typical application, the uncertainties in element concentration associated with the lack of homogeneity in each population outweigh the analytical uncertainty. An efficient algorithm is presented to select the immobile elements by identifying the maximum number of elements that are, within their uncertainties, compatible with the same isocon. The actual isocon is constructed by forcing a line through the origin and optimizing its slope using a weighted least-squares procedure for the selected immobile elements. Thus, individual uncertainties for each immobile element are taken into account. As an illustration, the procedure is applied to a published set of data on potassic and sericitic alterations of quartz monzonite of the Bingham porphyry copper system. The surprising result indicates that the apparent increase of potassic alteration phases is a consequence of acid leaching of calcium and sodium. No mass transport of potassium is indicated. Silica, water, and sulfur were added during alteration. The best fit isocon requires a small mass increase during each alteration process which, however, is within the uncertainty obtained for the slope of the isocon. A software package is available from the authors in the form of a FORTRAN 77 source code.