Kailasa Pandarinath

Statistical evaluation of tectonomagmatic discrimination diagrams for granitic rocks and proposal of new discriminant-function-based multi-dimensional diagrams for acid rocks

The four tectonic discrimination diagrams of Pearce et al. [Journal of Petrology, v. 25, p. 956–983] for granitic rocks were first evaluated using the literature cited by these authors as well as from our new database. The first diagram (Y−Nb) cannot discriminate volcanic-arc and collision settings. Both Y−Nb and Yb−Ta diagrams have an overlapping field for within-plate and ocean-ridge granitoids. The remaining two diagrams (Y + Nb−Rb and Yb + Ta−Rb) use a mobile element (Rb) in their y-axis. Although these diagrams successfully discriminate volcanic-arc and within-plate granites, they perform less well for collision tectonics. Besides, felsic or acid rocks are scarce in ocean-ridge settings, which limits the usefulness of these diagrams for this geological environment. Therefore, using an extensive database, we proposed a set of five new discriminant-function-based multi-dimensional diagrams for acid magmas from four tectonic settings (island arc, continental arc, continental rift, and collision). The very similar tectonic settings of island and continental arcs are discriminated for the first time. These diagrams are based on correct statistical treatment of compositional data, because they use natural logarithm transformation of major-element ratios and linear discriminant analysis (LDA). The use of discordant outlier-free samples prior to LDA improved the success rates by about 3–5%. Success rates of these diagrams as inferred from a testing set were between 76% and 88% for island arc, 60% and 92% for continental arc, and 72% and 84% for both continental rift and collision settings. Finally, application of these new diagrams to case studies not compiled in our initial database used for constructing these diagrams provided the following results: a collision setting for the Himalayas at about 30 Ma; an island arc setting for Quaternary acid rocks from geothermal boreholes in El Salvador; an island- or continental-arc setting for northern Italy at 35–52 Ma; a continental-arc setting for the Italy–Austria border at about 30 Ma; either a rift or a collision setting for northern Nigeria at about 164 Ma; a collision setting for central Nigeria at about 144 Ma and for the Cretaceous Masirah ophiolites of Oman; and an island arc setting for the Cretaceous Semail ophiolites of Oman. In spite of the relative mobility of major elements, these applications suggest utility of the new discrimination diagrams for all four tectonic settings.

Clay minerals in SW Indian continental shelf sediment cores as indicators of provenance and palaeomonsoonal conditions: a statistical approach

Two sediment cores from the inner continental shelf of SW India, SK‐148/13 (4.66 m long; from 50 m water depth) and SK‐148/14 (5.37 m long; from 22 m water depth), were studied for provenance and palaeomonsoonal implications. Sediment layers at different depth intervals in these cores were estimated for clay minerals and organic matter content. Five surface sediment samples from the nearby Kali River estuary of the adjacent hinterland were also analysed for clay mineral contents. In both cores and Kali River sediments, smectite is the dominant clay followed by illite, and kaolinite or chlorite. Based on the similarities of the abundances of clay minerals and their relative wt‐% in the inner shelf sediment cores and in surface sediments of the estuary as demonstrated by significance tests, Al‐rich illite, and negligible contribution of clay from the deep‐sea and aeolian sources, it can be inferred that the sediments of the two inner continental shelf cores were derived from the adjacent hinterland. Application of statistical discordancy and significance tests on the down‐core variations in the crystallinity index (CI) of illite and organic matter content in the sediment cores indicates intense monsoonal (high rainfall) conditions at the adjacent hinterland during about 4300–6200 and 9300–10,400 years BP. These inferences are comparable to those from other well‐established palaeomonsoonal indicators such as sedimentation rates and mineral magnetic properties. The CI of illite, an easily determinable and climatically sensitive parameter, can thus be a reliable palaemonsoonal indicator for inner shelf sediment cores. The statistical methodology used in this work highlights the advantages of a quantitative interpretation of the data instead of the conventional qualitative visual examination.

Solute geothermometry of springs and wells of the Los Azufres and Las Tres Vírgenes geothermal fields, Mexico

Subsurface reservoir temperatures of two important Mexican geothermal systems (Los Azufres and Las Tres Vírgenes) were estimated by applying all available solute geothermometers for 88 and 56 chemical data measurements of the spring waters and fluids of the deep geothermal wells, respectively. Most of the chemical data for spring water of these two geothermal fields are for HCO3 water, followed by SO4 and Cl types. For the Los Azufres geothermal field (LAGF), the reservoir temperatures estimated by Na-K geothermometers for springs of HCO3 and SO4 waters, and by Na-Li and Li-Mg geothermometers for Cl water, are close to the average bottom-hole temperature (BHT) of the geothermal wells. However, all reservoir temperatures for spring waters from the Las Tres Vírgenes geothermal field (LTVGF) estimated by all solute geothermometers indicated significantly large differences (low temperatures) compared to the BHT. Evaluation of inferred reservoir temperatures for spring waters of the LAGF and LTVGF suggests that not all springs nor all solute geothermometers provide reliable estimation of the reservoir temperatures. Even though chemical equilibrium probably was not achieved in the water–rock system, Na-K geothermometers for HCO3 water (peripheral water mainly of meteoric origin with little geothermal component) and SO4 water (geothermal steam heated) and Na-Li and Li-Mg geothermometers for Cl-rich spring water (fully mature geothermal water) of the LAGF indicated reservoir temperatures close to the BHT. However, in comparison with the geothermometry of spring water of the LAGF and LTVGF, fluid measurements from geothermal wells of these two fields indicated reservoir temperatures in close agreement with their respective BHTs. For the best use of the solute geothermometry for spring water, it is advisable to: (1) chemically classify the springs based on water types; (2) identify and eliminate the discordant outlier observations by considering each water type as a separate sampled population; (3) apply all available solute geothermometers employing a suitable computer program such as SolGeo instead of using some specific, arbitrarily chosen geothermometers; and (4) evaluate the temperatures obtained for each solute geothermometer by considering the subsurface lithology, hydrological conditions, and BHTs or static formation temperatures whenever available.

Evaluation of the odd-even effect in limits of detection for electron microprobe analysis of natural minerals.

Limit of detection (LOD), being a fundamental quality parameter for analytical techniques, has been recently investigated and a systematic behavior has been observed for most odd-even element pairs for many techniques. However, to the best of our knowledge very few LOD data are available in published literature for electron microprobe analysis; these consist of three papers, two being on rare-earth elements and the third covering a large number of elements of atomic number between 21 and 92. These data confirm the systematic behavior of LODs for many odd-even pairs. To initiate to full this gap, we determined LODs for several major rock-forming chemical elements from Na to Fe with atomic numbers between 11 and 26, during the microprobe analysis of common minerals (olivine, plagioclase, pyroxene, amphibole, quartz, and opaques) in volcanic rocks. The odd-even effect of nuclear stability seems to be present in LOD data for most odd-even pairs investigated. Nevertheless, the experimental strategy concerning the reference materials, calibration procedure, and blank measurements, should be substantially modified to better evaluate the systematic behavior of LOD values in microprobe analysis.

X-Ray Diffraction Analysis of Hydrothermal Minerals from the Los Azufres Geothermal System, Mexico

Los Azufres is an active geothermal field located in the middle of the Mexican Volcanic Belt (MVB), a vast Miocene-Recent, E-W-oriented volcanic province spanning central Mexico. Three wells—Az-5, Az-28, and Az-31—from this geothermal field were selected to investigate the distribution, alteration sequence, and thermal stability of hydrothermal minerals. Maximum in situ measured temperatures are 280°C at 1493 m, 265°C at 1700 m, and 288°C at 1300 m depths in Az-5, Az-28, and Az-31, respectively. The host rocks in these wells are dominantly andesite followed by dacite, rhyolite, and basalt. Rock cuttings from different depths were analyzed for clay and non-clay minerals by X-ray Diffraction (XRD) methods. Hydrothermal quartz, calcite, and pyrite, as well as other alteration mineral phases (e.g., chabazite and chlorite) that are difficult to identify by traditional petrography were identified and their abundances semi-quantitatively estimated by XRD. We show that these mineral data present a better perception of distribution trends of hydrothermal minerals in geothermal wells than the qualitative mineral identifications generally used for this purpose. Homogenization temperatures measured in fluid inclusions of hydrothermal minerals, in situ measured temperatures in the wells, and K+/H+ vs. Mg2+/(H+)2 activity diagrams for the chemical characteristics of the present geothermal fluids were used to define the thermal regime and the resultant stability conditions of the clay minerals. Smectite, illite, and chlorite are present in the <2 mu;m size fraction. Gradual variations in relative abundances of clay minerals range from smectite dominant at shallow well depths to a combination of smectite, illite, and chlorite at intermediate depths, and to illite and chlorite in the deepest levels. Excellent crystallinity and lack of mixed-layered clay minerals support a model involving a discontinuous change from smectite to chlorite and/or illite, rather than that involving continuous mixed-layering of smectite-illite and/or smectite-chlorite. Mineralogical and fluid inclusion data suggest that mineral distribution trends documented for the Los Azufres geothermal system reflect the prevailing thermal regime. The mineral parageneses of the Los Azufres geothermal field are broadly comparable with those reported in other geothermal systems of the world.

Solute and gas geothermometry of geothermal wells: a geochemometrics study for evaluating the effectiveness of geothermometers to predict deep reservoir temperatures

Deep reservoir temperatures of 10 important geothermal systems of the world were estimated by applying 13 solute (Na/K) and 21 gas geothermometers. The predicted temperatures were comprehensively evaluated and compared with measured bottom-hole temperatures using geochemometric techniques. The present study reveals (1) high prediction performances in most of the Na/K geothermometers for the majority of the geothermal fields with liquid-dominated reservoirs, whereas low prediction performances were indicated for the geothermal fields with vapour-dominated and high-temperature reservoirs; (2) the gas geothermometers, in comparison to Na/K, are more successful in predicting the subsurface temperatures in high-temperature geothermal systems; (3) the geothermal systems for which Na/K geothermometers have indicated a high prediction performance, the gas geothermometers have specified a low prediction performances, and vice versa; (4) both Na/K and gas geothermometers, generally, overestimated the reservoir temperatures for the majority of the low-enthalpy geothermal fields and underestimated for the majority of the high-enthalpy geothermal fields; (5) the reservoir temperature predictions of gas geothermometers have more scatter than those temperatures inferred from Na/K geothermometers; and (6) in general, Na/K geothermometers seem to be a more successful geochemical tool in predicting reliable reservoir temperatures than gas geothermometers.

Dating of Sediment Layers and Sediment Accumulation Studies along the Western Continental Margin of India: A Review

Modern surface sediments and organic matter/peat in sediment cores along the western continental margin of India have been dated by excess 210Pb and conventional or accelerator mass spectrometry 14C techniques. We have established an extensive database of these radiometric dates and related parameters, and have calculated sedimentation rates. Modern sedimentation rates are higher (1.8-19.0 mm/yr) on the continental margin of northwestern India and decline (0.01-2.6 mm/yr) toward the southwest. This trend is attributed to differences in load-carrying capacity of the rivers and lithology of the adjacent hinterland. Sedimentation rates decrease away from the coast, except for higher sedimentation rates at few places on the mid-slope compared to inner slope regions. The data reveal lateral as well as temporal variations in accumulation rates, which are explained in terms of environment of deposition and paleomonsoon conditions. The onset of intensive monsoonal conditions during the Holocene is reflected in very high sedimentation rates on the inner shelf. Peat and carbonized wood layers in the inner shelf cores are of Early Holocene age and indicate marine transgression. Age inversions in the cores of continental shelf and slope regions are attributed to either sediment slumping or reworking and redeposition of foraminifera shells. This study provides additional information to the worldwide database for continental margin sedimentation, and is useful for understanding Late Quaternary sedimentation records, sea level changes, the Holocene marine transgression event, the role of Last Glacial Maxima (LGM), and climatic changes in the input of terrigenous sediments to continental margins.

Magnetic susceptibility of volcanic rocks in geothermal areas: application potential in geothermal exploration studies for identification of rocks and zones of hydrothermal alteration

Magnetic susceptibility and petrographic studies of drilled rock cuttings from two geothermal wells (Az-26 and Az-49) of the important electricity-generating geothermal system, Los Azufres, Mexico, were carried out to determine the relation between the magnetic susceptibility of rocks, the concentration of magnetic minerals and hydrothermal alteration. For this purpose, low-frequency magnetic susceptibility (χlf) was measured and compared its distribution trends with those of magnetic and Fe–Mg silicate minerals, and with the extent of hydrothermal alteration in rocks of the two geothermal wells. The study indicates a decrease in χlf values with depth in the two geothermal wells corresponding with: (1) an increase in the reservoir temperature and hydrothermal alteration; and (2) a decrease in the concentrations of Fe–Mg silicates and opaque minerals. The data suggest that ferromagnesian minerals and opaque minerals like ilmenite are the main contributors to the χlf of rocks. The decrease in χlf, ilmenite, and Fe–Mg mineral contents with an increase in the hydrothermal alteration degree, pyrite and haematite contents suggests the hydrothermal alteration of ilmenite and Fe–Mg minerals (characteristic of high χlf values) to pyrite, haematite and other opaque minerals (with low χlf values). The interaction of hydrothermal fluids with rocks results in the hydrothermal alteration of primary minerals. In a geothermal area, an anomaly of low magnetic susceptibility values of rocks in a homogenous litho unit characterized by high magnetic susceptibility may suggest hydrothermal alteration. Magnetic susceptibility can be a useful parameter, during the initial stages of geothermal exploration, in identifying hydrothermally altered rocks and zones of hydrothermal alteration both at the surface and from drilled wells in geothermal systems.

Tectonomagmatic Origin of Igneous Rocks from the Western Mexican Volcanic Belt

The origin of magmas in the western part of the Mexican Volcanic Belt (W-MVB) was constrained from tectonomagmatic discrimination diagrams under the assumption that the magmas originated from different tectonic settings are distinguishable from differences in their chemical compositions. For the W-MVB, the diagrams for basic rocks generally indicate a continental rift setting whereas those for intermediate and acid rocks show either an arc or a transitional arc to rift setting but with relatively low total percent probability values. Besides these diagrams, the conventional significance tests (Fisher F, Student t, and ANOVA) were also applied to understand the complex petrogenetic and tectonic processes in the W-MVB.