Radiogenic argon in glauconites as a function of mineral recrystallization

Natural glauconitic grains were subjected to hydrothermal treatment at a pressure of 2 kbars and temperatures of 200–414°C for the period of one month.The measurement of radiogenic argon before and after treatment shows that the mica-type glauconite mineral contains 92–96% of the argon initially present up to temperatures as high as 320°C.After recrystallization of the mixed-layered glauconitic minerals the new phases still contain significant amounts of argon in the structure. The mixed-layered glauconitic minerals are much more sensitive to temperature treatment at elevated pressure than glauconitic-type mica, they, however, still contain up to 84% of the initial argon of the untreated material at temperatures as high as 320°C, i.e., just below the thermal stability of glauconite mica. It is evident that low-temperature metamorphism will affect the apparent radiogenic age of glauconitic grains to an extent which depends upon their composition and the temperature of metamorphism.     

The Ninole Basalt — Implications for the structural evolution of Mauna Loa volcano, Hawaii

Lava flows of the Ninole Basalt, the oldest rocks exposed on the south side of the island of Hawaii, provide age and compositional constraints on the evolution of Mauna Loa volcano and the southeastward age progression of Hawaiian volcanism. Although the tholeiitic Ninole Basalt differs from historic lavas of Mauna Loa volcano in most major-element contents (e.g., variably lower K, Na, Si; higher Al, Fe, Ti, Ca), REE and other relatively immobile minor elements are similar to historic and prehistoric Mauna Loa lavas, and the present major-element differences are mainly due to incipient weathering in the tropical environment. New K-Ar whole-rock ages, from relatively fresh roadcut samples, suggest that the age of the Ninole Basalt is approximately 0.1–0.2 Ma, although resolution is poor because of low contents of K and radiogenic Ar. Originally considered the remnants of a separate volcano, the Ninole Hills are here interpreted as faulted remnants of the old south flank of Mauna Loa. Deep canyons in the Ninole Hills, eroded after massive landslide failure of flanks of the southwest rift zone, have been preserved from burial by younger lava due to westward migration of the rift zone. Landslide-induced depressurization of the southwest rift zone may also have induced phreatomagmatic eruptions that could have deposited widespread Basaltic ash that overlies the Ninole Basalt. Subaerial presence of the Ninole Basalt documents that the southern part of Hawaii Island had grown to much of its present size above sea level by 0.1–0.2 Ma, and places significant limits on subsequent enlargement of the south flank of Mauna Loa.     

The Ninole Basalt — Implications for the structural evolution of Mauna Loa volcano,Hawaii

Lava flows of the Ninole Basalt, the oldest rocks exposed on the south side of the island of Hawaii, provide age and compositional constraints on the evolution of Mauna Loa volcano and the southeastward age progression of Hawaiian volcanism. Although the tholeiitic Ninole Basalt differs from historic lavas of Mauna Loa volcano in most major-element contents (e.g., variably lower K, Na, Si; higher Al, Fe, Ti, Ca), REE and other relatively immobile minor elements are similar to historic and prehistoric Mauna Loa lavas, and the present major-element differences are mainly due to incipient weathering in the tropical environment. New K-Ar whole-rock ages, from relatively fresh roadcut samples, suggest that the age of the Ninole Basalt is approximately 0.1–0.2 Ma, although resolution is poor because of low contents of K and radiogenic Ar. Originally considered the remnants of a separate volcano, the Ninole Hills are here interpreted as faulted remnants of the old south flank of Mauna Loa. Deep canyons in the Ninole Hills, eroded after massive landslide failure of flanks of the southwest rift zone, have been preserved from burial by younger lava due to westward migration of the rift zone. Landslide-induced depressurization of the southwest rift zone may also have induced phreatomagmatic eruptions that could have deposited widespread Basaltic ash that overlies the Ninole Basalt. Subaerial presence of the Ninole Basalt documents that the southern part of Hawaii Island had grown to much of its present size above sea level by 0.1–0.2 Ma, and places significant limits on subsequent enlargement of the south flank of Mauna Loa.     

Potassium argon dating of gamma-irradiated minerals

A method for measuring potassium-argon ages making use of the reaction³⁹K(λ, n)³⁸K^β+→³⁸Ar to indirectly determine potassium is discussed. In principle, it is closely analogous to the⁴⁰Ar/³⁹Ar dating method and should possess all of the attributes of that technique. It is demonstrated that precise dating of mica samples with ages between 15 my and 1000 my can be carried out, and a discussion of potentially interfering reactions suggests that it may be possible to extend the method to the problem of induced argon isotopic dating of calcium-rich minerals.     

Molecular and carbon isotopic compositions of gas inclusions of deep carbonate rocks in the Tarim Basin

Gaseous components of gas inclusions in deep carbonate rocks (>5700 m) from the Tacan 1 well were analyzed by online mass spectrometry by means of either the stepwise heating technique or vacuum electromagnetism crushing. The carbon isotopic compositions of gases released by vacuum electromagnetism crushing were also measured. Although the molecular compositions of gas inclusions show differences between the two methods, the overall characteristics are that gas inclusions mainly contain CO2, whilst hydrocarbon gases, such as CH4, C2H6 and C3H8, are less abundant. The content of CO is higher in the stepwise heating experiment than that in the method of vacuum electromagnetism crushing, and there are only minor amounts of N2, H2 and O2 in gas inclusions. Methane δ13C values of gas inclusions in Lower Ordovician and Upper Cambrian rocks (from 5713.7 to 6422 m; -52‰-63‰) are similar to those of bacterial methane, but their chemical compositions do not exhibit the dry character in comparison with biogenic gases. These characteristics of deep gas inclusions may be related to the migration fractionation. Some deep natural gases with light carbon isotopic characteristics in the Tazhong Uplift may have a similar origin. The δ13C1 values of gas inclusions in Lower Cambrian rocks (7117-7124 m) are heavier (-39‰), consistent with highly mature natural gases. Carbon isotopic compositions of CO2 in the gas inclusions of deep carbonate rocks are similar (from -4‰ to -13‰) to those of deep natural gases, indicating predominantly an inorganic origin.     

Electric conductance of the consolidated crust and graphitization

The influence of inclusions of fluids, ore minerals, and graphite on the electric conductance of rocks of the consolidated crust is considered by using geoelectric investigations in the Ukraine as an example. The abundance of graphite and fluid inclusions is shown, and the anomalies associated with them are estimated.     

I-Xe dating of Allende inclusions: Antiquity and fine structure

Two inclusions from the Allende meteorite exhibit multiple I-Xe isochrons corresponding to an age difference of 3.7 ± 1.5 m.y. for their constituent minerals. Consequently, I-Xe dating can establish, independently of thermodynamic models, a partial sequence of formation for minerals that incorporated radioactive ¹²⁹I. Additionally, the results give an estimate of the cooling rate for the solar nebula when validity of thermodynamic models is assumed.     

Isotopic fractionation of a loosely held atmospheric argon component in the Picture Gorge Basalts

Seven lava flows from the Picture Gorge “type section”, central Oregon, U.S.A., have been dated at 15.2 ± 0.6 m.y. by the K-Ar method. The preparation of the samples for the argon extraction work strongly affects the ages obtained, due to the presence of very large quantities of atmospheric argon held “loosely” within most of the rocks. Use of single large pieces of rock results in incomplete removal of this gas, argon isotopic fractionation and grossly incorrect ages. When powdered samples are used, the increase in surface area per unit mass helps in the removal of the “loosely” held atmospheric argon. The Picture Gorge Basalts contain traces of clays of the montmorillonite group; on hydration these clays swell up and apparently shut off minute cracks in the rocks, thereby trapping substantial amounts of atmospheric argon. It is recommended that when K-Ar dating of whole-rock samples showing traces of alteration has to be attempted, the specimens be crushed to approximately 10–30 mesh size prior to the argon extraction work.     

Petrogenesis and tectonic setting of bimodal volcanism in the Sakoli Mobile Belt, Central Indian shield

The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO₂ shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial ¹⁴³Nd/¹⁴⁴Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNd_t (t = 2000 Ma) varies between ?0.19 and +2.22 for the basic volcanic rock and between ?2.85 and ?4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.     

Origin of a magnetic lineation on Kyushu Island, Japan

Abstract Several linear magnetic anomalies over continental crust have been identified in and around the Japanese Islands. The anomalies are probably related to island arc tectonic structures, but identifying specific sources has been difficult. Several deep holes were drilled in and around Aso caldera, where a linear anomaly occurs along an active fault. One drillhole located on the linear anomaly encountered a zone of highly magnetized and altered basement rocks at least 100 m thick at a depth of ∼1000 m. The other hole was located away from the anomaly and did not encounter any high-magnetic zones. Rocks from the zone have exceptionally strong remanent magnetization (several tens of A/m) sub-parallel to the present field. AF demagnetization experiments indicated that the magnetization is hard and stable. Magnetic modeling indicates that the linear anomaly is caused mainly by this layer. Microscopic examination of core samples shows that the highly magnetized zone includes secondary magnetic minerals and abundant hydrothermal alterations. Temperatures determined by fluid inclusions and down-hole temperatures show that the temperature of the highly magnetized zone was elevated in the past relative to surrounding rocks. The high temperature could destroy primary magnetic minerals and replace them with secondary magnetic minerals. Thus, the past hydrothermal system may have enhanced thermo-chemical remanent magnetization. The results can produce a model indicating that there was a past hydrothermal system related to the tectonic structure.     

Evidence of microstructures and fluid inclusions for the origin of polycrystalline quartz ribbons in high-grade metamorphic rocks in Daqingshan region

Polycrystalline quartz ribbons in high-grade metamorphic rocks from the Daqingshan region, are typi- cal microfabrics of, and provide information for, deep crust deformation and metamorphism. The quartz ribbons have straight boundaries and extend stably along gneissosity. They truncate other mineral grains in the rocks and may contain inclusions of such minerals that are lens-shaped and oriented. They frequently end into branching termination. Analysis fluid inclusions in polycrystalline quartz rib- bons reveal that the complex types of fluid inclusions are inhomogeneously distributed. They are ob- viously different from inclusions captured at granulite facies, in both fluid compositions and T-P esti- mations. Based on microfabric and fluid inclusion analysis, the polycrystalline quartz ribbons are suggested to be formed by SO2-rich fluids filling micro-fractures that are parallel to early gneissosity. The SO2 composition is derived from the deformed host rocks. The fluid phase has significant effects on the rheological characteristics, fracturing of rocks, and formation of quartz ribbons.     

Evidence of microstructures and fluid inclusions for the origin of polycrystalline quartz ribbons in high-grade metamorphic rocks in Daqingshan resion

Polycrystalline quartz ribbons in high-grade metamorphic rocks from the Daqingshan region, are typical microfabrics of, and provide information for, deep crust deformation and metamorphism. The quartz ribbons have straight boundaries and extend stably along gneissosity. They truncate other mineral grains in the rocks and may contain inclusions of such minerals that are lens-shaped and oriented. They frequently end into branching termination. Analysis fluid inclusions in polycrystalline quartz ribbons reveal that the complex types of fluid inclusions are inhomogeneously distributed. They are obviously different from inclusions captured at granulite facies, in both fluid compositions and T-P estimations. Based on microfabric and fluid inclusion analysis, the polycrystalline quartz ribbons are suggested to be formed by SO2-rich fluids filling micro-fractures that are parallel to early gneissosity. The SO2 composition is derived from the deformed host rocks. The fluid phase has significant effects on the rheological characteristics, fracturing of rocks, and formation of quartz ribbons.     

Emerald dating through Ar/Ar step-heating and laser spot analysis of syngenetic phlogopite

Emerald, occurring in K-metasomatic rocks developed at the contact of the Carnaíba leucogranite with serpentinite (Bahia State, Brazil), has been dated using an original ⁴⁰Ar/³⁹Ar procedure. It combines step heating and spot fusion experiments on two types of phlogopite crystals: (1) bulk samples and individual grains extracted from the enclosing K-metasomatic host rocks; and (2) syngenetic solid inclusions precipitated along growing zones of the emerald host crystals. The second procedure uses in situ laser probe experiments on rock sections. In spite of the huge amounts of excess ⁴⁰Ar detected in adjacent emerald, we could measure reliable ages of 1951 ± 8 Ma and 1934 ± 8 Ma for the Trecho Velho and Braulia occurrences, respectively. Spot fusion data had higher discrepancy than the step heating data, but minute crystals of phlogopite included in emeralds bearing excess argon do not reveal excess argon. A muscovite belonging to the same granite hydrothermal complex gave a plateau age of 1976 ± 8 Ma, which may correspond to a higher closure temperature of the KAr system during the cooling of the whole pluton and associated hydrothermal halo.

These accurate measurements lead to the following conclusions: (1) direct emerald dating is possible; (2) in spite of a polyphase history during the Transamazonian orogenesis (2 Ga), combined step heating and spot fusion experiments give a better precision for granite-related emerald mineralization than the scattered ages obtained by Rb-Sr and K-Ar methods; (3) the late-Transamazonian tectonothermal retrograde event which probably caused the dispersion of previous Rb-Sr and K-Ar data is not revealed by our procedure; (4) the emerald mineralization and K-metamorphism appear to be linked with the thermal history of the leucogranite; (5) in addition to its use in polyphase crustal domains, accurate ⁴⁰Ar/³⁹Ar dating is of major interest in the field of metallogenic models, even, for instance, for mineralizations characterized by disturbed isotopic systems, which record effects as excess argon.     

Fission-track dating of glass inclusions in volcanic quartz

We have extended fission-track dating techniques to volcanic pyroclastic rocks, using quartz-bearing uranium-rich glass inclusions. We use both the glass itself and the surrounding quartz as track detectors, the latter having a much higher resetting temperature. We can thus derive the age of the last low-temperature thermal event, as well as that of the most recent deposition of quartz on the walls of the inclusions. This method has been applied to a Sardinian sample and to a Moroccan Precambrian sample.     

Trace elements in alkali basalts and their inclusions in the Dariganga area,Mongolia

Alkali basalts of Dariganga contain inclusions of dunite, harzburgite, peridotite, and eclogite-like rock. The dunites and peridotites contain few trace elements, but these are somewhat more abundant in the eclogite-like rock, and the alkali basalts are rather rich in them. The trace elements show a more or less continuous change in abundance that can be related by recognized crystal-chemical laws to the major elements of the minerals that are present in the various rock types. Hypotheses for the origin of basaltic magma are discussed in relation to the trace-element distribution in the Dariganga rocks, but it is concluded that the available evidence does not permit any definite decision as to which is correct.     

Intergranular trace elements in mantle xenoliths from Russian Far East: Example for mantle metasomatism by hydrous melt

Based on both major and trace element chemistry, the occurrence of the intergranular component in mantle-derived xenoliths from far eastern Russia has been constrained. Whole-rock trace element measurements of one xenolith show apparent negative anomalies in Ce, Th, and high field strength elements on normalized trace element patterns. The trace element pattern of the whole rock differs from those of constituent minerals, indicating that the anomalies in the whole rock are attributable to the presence of an intergranular component. That assumption was confirmed using in situ analysis of trace elements in the intergranular substance and melt inclusion using laser ablation inductively coupled plasma–mass spectrometry. Both the intergranular component and the melt inclusions have identical trace element patterns, which mean that these materials are a cognate metasomatizing agent. The anomalies are regarded as mantle metasomatism related to an aqueous fluid. Hydrous minerals were observed on the wall of the melt inclusions using micro-Raman spectroscopy, indicating that the melt inclusions contained a large amount of water. Thus, this study reveals a trace element composition of a hydrous metasomatizing agent in the mantle.     

Excess argon, crustal fluids and apparent isochrons from crushing K-feldspar

Using a combination ofin-vacuo crushing and stepped heating, the⁴⁰Ar³⁹Ar technique has been applied to two K-feldspar-bearing sedimentary rocks from the Proterozoic of North China, with a view to studying ancient fluids trapped in these rocks and their chronology. Correlations between natural Ar isotopes and those produced by neutron irradiation permitted a clear distinction to be made between different components of natural argon. Crushing released Cl-correlated excess⁴⁰Ar (and palaeoatmospheric³⁶Ar) trapped in moderately saline fluid inclusions in quartz and possibly the K-feldspar. The very high⁴⁰Ar/Cl ratios of these fluids (2.7 × 10⁻⁴ and1.7 × 10⁻⁴) is interpreted as indicating a metamorphic source for the⁴⁰Ar. More surprisingly, crushing also released, from the K-feldspar, a K-correlated component with a very well defined⁴⁰Ar/K ratio, which for both samples corresponded formally to an apparent age comparable to or slightly less than the stratigraphic age. In contrast, stepped heating yielded⁴⁰Ar/K ratios corresponding to significantly younger ages which are interpreted as (low-grade) metamorphic ages. The explanation of the K-correlated component released on crushing and the significance (if any) of the distinct⁴⁰Ar/K ratio is obscure, although several effects involving microporosity and a combination of³⁹Ar recoil and diffusion are suggested. There is a contribution of this component to the stepped heating release pattern which may have implications for attempts to use K-feldspar as a⁴⁰Ar³⁹Ar thermochronometer.     

The Xe-Xe spectrum technique,a new dating method

A new method to date uranium-bearing minerals exclusively by means of a mass spectrometric determination of Xe and/or Kr isotopic ratios has been developed and experimentally tested. It is based on the compositional differences between Xe produced by spontaneous fission of²³⁸U in nature and Xe from²³⁵U fission induced by thermal neutrons in a nuclear reactor. Xe is extracted in 5–10 release fractions at successively higher temperatures. This relates the radiogenic Xe contained in various structural elements of different retention characteristics to the respective U concentrations and allows to account for natural Xe losses, which are quite common. A monazite monitor mineral of known age is included in each irradiation and its Xe isotopes are measured in the same way.The samples analyzed include uranium oxides, REE phosphates, tantalum niobates, zircon and others.The results and comparisons with reported ages obtained by other means demonstrate the applicability of the new dating method for minerals with U contents up to a few percent. In age spectra the low-temperature release fractions of some samples indicate radiogenic Xe losses, while at high temperatures age plateaux can be obtained.     

On possible application of nepheline for alkaline rock dating

Suitability of unmetamorphosed nepheline for alkaline rock dating is examined. It is shown that only a considerable amount (more than 10%) of secondary minerals such as cancrinite, wischnewite, and sometime spreustein (¹) can falsily the results of age determination from nepheline. The possible presence of excess argon in secondary minerals, though rather infrequent, should also be taken into account.     

Episodic mesozoic volcanism in Namibia and Brazil: A K—Ar Isochron study bearing on the opening of the south atlantic

An attempt is made to find a more objective and precise basis for the correlation of volcanics from southwestern Africa and South America than is possible by frequency diagrams of individual K—Ar ages. This leads to a critical appraisal of conventionally calculated K—Ar ages with the conclusion thata priori assumption regarding the isotopic composition of non-radiogenic argon and, hence, the standard atmospheric correction, are no longer tenable.K—Ar isotoopic data on Mesozoic basalts and dolerites from Namibia and Brazil are presented in terms of an isochron model. Plots for cogenetic rocks are unacceptably scattered on a “radiogenic”⁴⁰Ar vs. K diagram, but show a high degree of collinearity on⁴⁰Ar/³⁶Ar diagrams0K/³⁶Ar diagrams. Using the latter plots, a number of isochrons are generated which indicate that Mesozoic volcanism in these regions occured as several discrete episodes of fairly short duration. Effusion of the extensive Serra Geral basalts of Brazil and the Kaoko basalts of Namibia is shown to have occured simultaneeously at 121 m.y.B.P. Basalts from a series of boreholes along the central Parana Basin, as well as a group of dykes from Sao Paulo, yield isochrons of 128 m.y., which coincides with the postulated onset of separation of Africa and South America based on marine magnetic anomalies. Linear dyke swarms along the Namibian seaboard, interpreted as an expression of the earliest rift phase, have an isochron age of 134 m.y. Sills and dykes, mainly from southern Namibia, with isochron ages of 183 m.y. are considered to be the westernmost manifestation of Stormberg volcanism, not necessarily related to rifting. Most of the igneous suites examined have initial⁴⁰Ar/³⁶Ar ratios significantly different from the modern atmospheric value.