Igneous Rock Samples Research Articles

Epithermal neutron activation, radiometric, correlation and principal component analysis applied to the distribution of major and trace elements in some igneous and metamorphic rocks from Romania

Six major (Na, Al, K, Ca, Ti, Fe) and 28 trace (Sc, Cr, V, Mn, Co, Zn, Cu, As, Br, Sr, Rb, Zr, Mo, Sn, Sb, Ba, Cs, La, Ce, Nd, Eu, Sm, Tb, Hf, Ta, W, Th and U) elements were determined by epithermal neutron activation analysis (ENAA) in nine Meridional Carpathian and Macin Mountains samples of igneous and metamorphic rocks. Correlation and principal factor analysis were used to interpret data while natural radionuclides radiometry shows a good correlation with ENAA results.

Precise determination of Sr isotope ratios in igneous rock samples and application to micro-analysis of plagioclase phenocrysts

A two-step chemical procedure to separate Sr from whole igneous rock sample has been developed for the analysis of Sr isotopes by thermal ionization mass spectrometry (TIMS). Firstly, a Sr-bearing fraction and a REE fraction are separated from the major elements of a decomposed sample solution using H+ form cation-exchange chromatography. Then the Sr fraction is purified in a second column using extraction chromatography and a Sr specific resin. For small amounts of sample (< 0.1 mg) obtained by micro-milling, only the second step of column separation is applied, directly separating the Sr fraction from the decomposed sample solution. The total procedural blank of these chemical separation procedures are very low, comparable to the lowest blank levels ever reported. The purified Sr is loaded on to a single Re-filament with a Ta ionization activator and measured using TIMS in static multi-collection mode. The mean value and reproducibility for standard 87Sr/86Sr in NIST SRM 987 with Sr loads of 100 ng and 5 ng are 0.710240 ± 0.000012(2σ, n = 12) and 0.710248 ± 0.000022 (2σ, n = 7), respectively. These values agree well with those from previous studies. Additionally, Sr isotope ratios measured in GSJ igneous rock reference materials JB-2, JA-1 and JR-1 are within error of values from previous studies, verifying the versatility of the technique reported in this study.

Multivariate analysis of remote laser-induced breakdown spectroscopy spectra using partial least squares, principal component analysis, and related techniques

Quantitative analysis with laser-induced breakdown spectroscopy traditionally employs calibration curves that are complicated by chemical matrix effects. These chemical matrix effects influence the laser-induced breakdown spectroscopy plasma and the ratio of elemental composition to elemental emission line intensity. Consequently, laser-induced breakdown spectroscopy calibration typically requires a priori knowledge of the unknown, in order for a series of calibration standards similar to the unknown to be employed. In this paper, three new Multivariate Analysis techniques are employed to analyze the laser-induced breakdown spectroscopy spectra of 18 disparate igneous and highly-metamorphosed rock samples. Partial Least Squares analysis is used to generate a calibration model from which unknown samples can be analyzed. Principal Components Analysis and Soft Independent Modeling of Class Analogy are employed to generate a model and predict the rock type of the samples. These Multivariate Analysis techniques appear to exploit the matrix effects associated with the chemistries of these 18 samples.

Magmatism associated with Gondwanaland rifting and Neo-Tethyan oceanic basin development: evidence from the Mamonia Complex, SW Cyprus

Volcanic and intrusive rocks of the Dhiarizos Group, occurring within the Mamonia Complex of SW Cyprus, comprise mainly tholeiitic basalts, ocean island tholeiites (OIT) and other alkalic varieties, with minor alkaline lamprophyre intrusive rocks. These rocks were highly tectonized and dismembered during the collision with the Troodos oceanic lithosphere. Trace element data for the most primitive mafic igneous rock samples suggest that the tholeiitic basalts are derived from a depleted mantle source whereas the OIT and the alkalic basalts originated from variable degrees of partial melting of a spinel peridotite mantle. Alkaline lamprophyres are products of a small degree of partial melting of a deep-seated garnet lherzolite. The bulk of the tholeiites and alkalic varieties are Late Triassic in age and are interpreted as having formed during Neo-Tethyan sea-floor spreading and associated seamount volcanism. Amphibole separates from a lamprophyre intrusion have 40 Ar– 39 Ar ages of 140.7 ± 0.4 Ma. In this paper, we report for the first time an Early Cretaceous alkaline lamprophyric magmatism, which is probably associated with a renewed episode of Gondwanaland rifting. These results show that the development of the Neo-Tethyan oceanic basin in the Eastern Mediterranean was largely controlled by a two-phase rifting–drifting process.

Tectonostratigraphic and geochronologic constraints on evolution of the northeast Paleotethys from the Songpan-Ganzi complex, central China

The Middle to Late Triassic deep-water deposits that form the Songpan-Ganzi complex (SGC) of central China comprise an estimated ~ 2.0 × 10 6 km 3 of detrital material that accumulated in the northeasternmost branch of the Paleotethys. A review of existing data demonstrates significant spatial and temporal variations in the stratigraphic and petrologic character of these turbidites. These variations are used to divide the complex into different depocenters: a northeastern depocenter (SGC-NE), a eastern–central depocenter (SGC-EC) and a northwestern depocenter (SGC-NW). Turbidite strata of the SGC-NE and SGC-EC zones of the Songpan-Ganzi complex are linked to the collision of the North China and South China blocks, whereas turbidite strata of the SGC-NW area are likely to be more closely affiliated with evolution of the Kunlun deformation belt. To test the validity of the Songpan-Ganzi stratigraphic framework and interpretations of its tectonostratigraphic evolution, sixty-eight U–Pb zircon ages were determined from five samples of felsic intrusive igneous rock, two samples from felsic plutonic rock of the adjacent Yidun arc complex, and one sample of volcanic rock interbedded with Middle Triassic turbidites of the SGC using the Sensitive High Resolution Ion Microprobe-Reverse Geometry (SHRIMP-RG). Together these data indicate primarily Late Triassic (~ 214–211 Ma) felsic magmatism in the SGC, with some indication of magmatic activity beginning as early as Middle Triassic (220 Ma). Zircon ages from the Yidun arc complex support Middle–Late Triassic magmatism from 225–215 Ma, prior to deformation of the SGC, suggesting deformation of the SGC was not related to subduction of the SGC substrate southwestward beneath the Yidun arc. Inherited Neoproterozoic (880–740 Ma) zircon ages found in two samples from the SGC-EC indicate either inheritance of zircon crystals from the surrounding SGC turbidite strata or possibly involvement of South China basement during crustal thickening and magma genesis.

Simultaneous Determination of Mass-dependent Isotopic Fractionation and Radiogenic Isotope Variation of Strontium in Geochemical Samples by Multiple Collector-ICP-Mass Spectrometry

We present a method to determine (88)Sr/(86)Sr and (87)Sr/(86)Sr simultaneously. The former variation reflects the mass-dependent isotopic fractionation through the physico-chemical processes, and the latter originates from beta(-)-decay of the parent nuclide (87)Rb as well as the mass-dependent isotopic fractionation. In order to determine the mass-dependent isotopic fractionation, the mass-discrimination effect on (88)Sr/(86)Sr was externally corrected by an exponential law using Zr. For the radiogenic growth of (87)Sr/(86)Sr, the mass-dependent isotopic fractionation effect on (87)Sr/(86)Sr was corrected by a conventional correction technique using the (88)Sr/(86)Sr ratio. The reproducibility of the (88)Sr/(86)Sr and (87)Sr/(86)Sr measurements for a high-purity Sr chemical reagent was 0.06 per thousand (2SD, n = 20) and 0.07 per thousand (2SD, n = 20), respectively. Strontium isotopic ratios ((88)Sr/(86)Sr and (87)Sr/(86)Sr) were measured on six geochemical reference materials (igneous rock: JB-1a and JA-2; carbonate mineral: JLs-1, JDo-1, JCp-1 and JCt-1) and one seawater sample. The resulting (87)Sr/(86)Sr ratios obtained here were consistent with previously published data within the analytical uncertainties. The resulting (88)Sr/(86)Sr ratios for igneous rock samples did not vary significantly within the samples, whereas the carbonate samples showed enrichments of the lighter Sr isotopes over the seawater sample. The (88)Sr/(86)Sr ratio of geochemical samples could reflect the physico-chemical processes for the sample formation. Also, a combined discussion of (88)Sr/(86)Sr and (87)Sr/(86)Sr of samples will render multi-dimensional information on geochemical processes.

Abundances of radioelements (K, U, Th) in weathered igneous rocks in Hong Kong

Gamma-ray spectrometric measurements and geochemical determinations of major and trace element contents using ICP-MS and XRF were conducted on 58 igneous rock samples from Hong Kong. Stripping analyses on the gamma-ray spectra have yielded estimates on the abundance of K, Th and U. Major element contents were used to compute the Parker weathering indices for the samples. Only K shows a systematic variation in concentration with an increasing degree of weathering. The increase in porosity and interconnectivity of microfractures are probably the cause for the observed nonlinear decline in the K content during the weathering process. The concentrations of U and Th in the samples do not show any systematic variations with the weathering index, reflecting the complex mechanisms of dissolution and deposition of the two radioelements in the weathering profile. A curvilinear relationship between eK and Wp has been derived from the measurement data, which possibly provides a quick means of characterizing the extent of alteration in the igneous rocks.

U-Pb Geochronological Constraints On The Evolution Of The ASPY Terrane, Cape Breton Island: Implications For Relationships Between ASPY And BRAS D'OR Terranes And Ganderia In The Canadian Appalachians

New U-Pb zircon ages from nine samples of igneous and sedimentary rocks in the Aspy terrane, Cape Breton Island, show that Neoproterozoic rocks form a major part of the terrane and confirm that the terrane was affected by a major Silurian-Devonian tectonothermal event. A rhyolitic crystal tuff, a leucotonalite pluton and a felsic sheet yield ages of 618.8 ± 0.6 Ma, 619.7 ± 0.9 Ma and 573.5 ± 2.7 Ma, respectively. A metasedimentary unit contains zircon ranging in age from ca. 546 to ca. 1520 Ma. A diorite has an age of 428.6 ± 1.9 Ma, and another diorite unit and a quartz porphyry also have probable Late Silurian ages. A third diorite yields an age of 373.0 ± 0.5 Ma, part of a widespread bimodal igneous event represented by both volcanic and plutonic rocks throughout the Aspy region. Geochronological results and field relationships indicate that the Neoproterozoic rocks are similar to those in the Bras d9Or terrane and form the basement to the (Ordovician-)Silurian rocks in the Aspy terrane, and that the Aspy terrane is probably correlative with rocks in the Hermitage flexure of southern Newfoundland where Silurian metasedimentary and metavolcanic rocks lie unconformably on similar Neoproterozoic rocks. Likely correlative rocks also occur in the Kingston terrane of southern New Brunswick. The results of this study support the idea that the Aspy-Kingston terrane is part of Ganderia and that Neoproterozoic rocks in the Bras d9Or terrane and correlative rocks in southwestern Newfoundland and elsewhere represent exposed basement to Ganderia in the Canadian Appalachians.

Estimation of radiation hazard indices from natural radioactivity of some rocks

Different samples of igneous and metamorphic rocks from Egypt and Germany have been considered to measure γ-ray activity concentrations due to naturally occurring, potentially hazardous radonuclides 226Ra, 232Th and 40K. The radiation hazard parameters including radiation equivalent activity, gamma-absorbed dose rate, and external and internal hazard indices have been estimated. The gamma-absorbed dose rates in air of rocks in Egypt range from 4.2 to 128.5 nGy·h −1 with a mean value of 55.3 nGy·h −1. For igneous and metamorphic rocks from Germany, the values of absorbed dose rates fluctuate from 5.1 to 148.6 nGy·h −1, with a mean value of 60.9 nGy·h −1. Generally, it is found that the radiation hazard indices in common igneous rocks are distinctly higher in acidic than in ultrabasic rocks. The results are discussed and compared with the corresponding published data.

Unraveling the magnetic carriers of igneous cores from the Atlantic, Pacific, and the southern Indian oceans with rock magnetic characterization

Previous rock magnetic investigations of oceanic basement samples have been mainly restricted by single drill holes or dredge collections. In this study, we present new and published data on the rock magnetic properties of igneous rock samples recovered during seven Ocean Drilling Program (ODP) legs in the Atlantic, Pacific, and the southern Indian oceans. The recovered igneous rocks from these legs are mainly basaltic flows (both submarine and subaerial), diabase sills, and serpentinized peridotites with approximate ages of 140,000 years, 10–12, 34, 69, 100–108, and 110–121 Ma, which offers an opportunity to investigate magnetic properties and mineral changes attending alteration of basement rocks over a wide range of ages from variable tectonic settings. Titanomagnetite and titanomaghemite are commonly present in igneous rock samples of the Ontong Java Plateau, the Kerguelen Plateau/Broken Ridge, the eastern Equatorial Pacific, the middle Atlantic ridge (Trans-Atlantic Geotraverse area), and the Newfoundland-Iberia rifted margins. For basaltic rock samples, three general groups (A, B, and C, respectively) can be divided in terms of rock magnetic properties. Samples from group A have a single phase of Ti-poor titanomagnetite with Curie temperatures ranging between 480 and 580 °C and exhibit a strong Verwey transition in the vicinity of 110 K. Basalts from this group are most likely good paleomagnetic recorders and probably have preserved original and stable magnetic remanences. Group B is mainly observed in pillow lavas and is characterized by a Curie temperature of 260–280 °C, which is typical of low-temperature oxidized titanomaghemite or titanium rich titanomagnetite. The low-temperature curves for group B do not show the Verwey transition. Group C has more than one Curie temperature, which suggests the presence of multiple magnetic phases. The thermomagnetic signature indicates the inversion of titanomaghemite to a strongly magnetized magnetite, as shown by the irreversible cooling curves. For the serpentinized peridotites recovered from both sides of the Newfoundland-Iberia rifted margins, results from low-temperature measurements show that (titano)magnetites are present in the dark-colored peridotites, with a strong Verwey transition in the vicinity of 110 K, and with field- and frequency-dependent susceptibility curves that resemble those of synthetic TM0. In contrast to the magnetic properties observed from the dark-colored peridotites, the low-temperature curves for the yellow-brown-colored peridotites did not show any Verwey transition. Thermomagnetic analysis also failed to show evidence for titanomagnetites. The remanent magnetization is carried by a thermally unstable mineral that breaks down at about 420 °C, probably maghemite. The magnetic signatures of the serpentinized peridotites recovered from both sides of the Newfoundland-Iberia rift appear not in conflict with the notion that conjugate margins will have generally similar crustal structure and evolution history. Our rock magnetic data indicate that the differences in the rock magnetic properties of basaltic rocks are mainly a function of mineralogy and alteration. There is no apparent coincidence between the age of the rocks and the degree of low-temperature alteration, as suggested by the presence of nearly unoxidized titanomagnetite in the Cretaceous aged basalts and by an examination of Curie temperature versus sample age that does not unambiguously show a positive relationship between the degree of low-temperature oxidation and crust ages.

Cretaceous paleomagnetic apparent polar wander path for the Pacific plate calculated from Deep Sea Drilling Project and Ocean Drilling Program basalt cores

The apparent polar wander path (APWP) of the Pacific plate still has many uncertainties owing to the fact that paleomagnetic data are difficult to obtain for oceanic plates. After more than three decades of coring by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) there are now a large number of reliably dated basalt cores recovered from the Pacific plate and this provides an opportunity to determine paleomagnetic poles based on igneous rock samples, considered by many scientists to be the most reliable data type. Cretaceous Pacific plate basalt core data were compiled, corrected using a standard technique, divided into groups based on age, and combined to calculate five mean paleomagnetic poles with ages of 80, 92, 112, 121, and 123 Ma, the latter two being for two different coeval regions. In all pole analyses, the lack of azimuthal orientation for cored samples leads to large uncertainties in pole locations along a nearly east–west direction. This difficulty was mitigated by using declination data from magnetic anomaly inversions of dated Pacific seamounts for azimuth constraint. The two nearly same-age poles were calculated because paleocolatitudes from Ontong Java Plateau (OJP) are discordant compared to those from other Pacific locations. I interpret the discordant OJP results to indicate that the plateau is on crust that had an early history as an independent plate. The other poles (80, 92, 112, and 123 Ma) fall on a northeast-trending line that suggests slow apparent polar wander during the Early and mid-Cretaceous, followed by rapid polar wander between 92 and 80 Ma. Comparison of the 123 Ma pole with previously published paleomagnetic data of Jurassic age implies southward apparent polar wander followed by a turnaround. Because the 123 Ma pole is the farthest from the geographic pole, it implies the turnaround happened near that time and that the Pacific plate has moved ∼40° northward since then. The 80 Ma pole stands ∼17° from the geographic pole, indicating that ∼60% of the northward drift occurred prior to that time and ∼40% afterwards.

Verification of ideal semi-logarithmic, lognormal or fractal crystal size distributions from 2D datasets

Crystal size distributions can follow any function, provided the total crystal content is less than 100%. However, many theories suggest that they approximate a number of ideal distribution models. The most widely adopted model for igneous rocks produces a straight line on a ‘classic’ CSD diagram of ln (population density) versus size. Lognormal by size distributions have been proposed for igneous and metamorphic rocks and materials that crystallise at low temperatures. Lognormal models are difficult to prove with simple linear frequency histograms or on a classic CSD diagram. Instead, it is better to use a normalized cumulative distribution function diagram, which gives a straight line for lognormal distributions. Similarly, fractal size distributions can best be verified simply by using a bi-logarithmic diagram. Statistical measures can be used to invalidate a null hypothesis, but with much caution for real data, especially those derived from two-dimensional measurements and with a limited range in sizes. These methods are applied to CSDs of plagioclase in three igneous rock samples to illustrate the pitfalls of model fitting. A simple nomenclature is proposed for ideal CSD shapes.

Natural radioactivity in phosphate rock, phosphogypsum and phosphate fertilizers in Brazil

Phosphate deposits are generally characterized by enhanced radionuclide concentrations compared to natural levels. The mining and processing of this phosphate ore redistribute radionuclides throughout the environment and introduce them into phosphoric acid and phosphogypsum. Phosphoric acid is the starting material for triple superphosphate (TSP), single superphosphate (SSP), monoammonium phosphate (MAP), diammonium phosphate (DAP), NPK fertilizers and di-calcium phosphate (DCP). Contents of natural radionuclides from thorium and uranium series, 226 Ra, 210 Pb and 228 Ra, were measured in Brazilian igneous phosphate rock, phosphoric acid, phosphogypsum and phosphate fertilizer samples, using high-resolution gamma-spectrometry. Neutron activation analysis was used for the determination of U and Th in the same samples. The fertilizers samples which are derived directly from phosphoric acid, MAP and DAP, presented activity concentrations around the detection limits of the counting system for 226 Ra (<5.0 Bq . kg –1 ), for 228 Ra (<3.0 Bq . kg –1 ) and for 210 Pb (<19 Bq . kg –1 ). As for U and Th, the concentrations found in MAP and DAP are more significant, up to 374 and 250 Bq . kg –1 , respectively. SSP, TSP and NPK, which are obtained by mixing phosphoric acid with different amounts of phosphate rock and NH3, presented higher concentrations of radionuclides, up to 871 Bq . kg –1 for 226 Ra, 283 Bq . kg –1 for 228 Ra, 1255 Bq . kg –1 for 210 Pb, 413 Bq . kg –1 for U and 538 Bq . kg –1 for Th.

Instrumental neutron activation analysis of carbonatites from Homa Mountain, Kenya

Twenty eight (major and trace) elements including eight rare earth elements (REEs) in African carbonatite samples were determined by instrumental neutron activation analysis. The geochemical behavior of trace elements was studied in relation to the order of carbonatite intrusion from C1 to C4 through C2 and C3 at Homa Mountain, Kenya. The enrichment of Mn, Fe, Sr, Ba, Th, U and REE is found in the sixteen carbonatites examined in this study. The general increase in the concentrations of Na, Sc, Mn, Sb, Ba, Th, U and REE occurs from C1 to C4 through C2 and C2c, but C3 carbonatite shows a different pattern. The C3 carbonatite is extraordinarily enriched in Mn, Fe and Ba and is highly enriched in Cr, As, Sb and Th. The chondrite-normalized REE distribution pattern of the C3 carbonatite is not rich in the light REE. Strong fractionation between light and heavy REEs is found in the carbonatites, and moderate fractionation in the two alkalic igneous rock samples. In order to evaluate partitioning of REEs into carbonate, oxide and other mineral fractions, a selective chemical leaching technique on carbonatites was applied and is discussed in this study.

Development of a New Method of Neutron Activation Analysis with Multiple Gamma-ray Detection -A High-sensitivity and Non-destructive Trace Element Analysis

A new trace-element quantification method has been developed by combining multiple gamma-ray detection and neutron activation analysis. This method is characterized by high sensitivity and simultaneous analysis for multi-elements. It has been successfully applied to the analyses of igneous rock samples, long-lived radionuclide, 129I, and geologic samples. Future perspectives for an innovative pulsed neutron source and a new detector system will be presented.

A High-sensitivity and Non-destructive Trace Element Analysis Based on Multiple Gamma-ray Detection

A new trace-element quantification method has been developed by combining multiple gamma-ray detection and neutron activation analysis. This method is characterized by high sensitivity and simultaneous analysis for multielements. The quantification accuracy amounts to 3–20% depending on statistics. It has been successfully applied to the analyses of igneous rock samples, long-lived radionuclide, 129I, and geologic samples. Future perspectives for an innovative pulsed neutron source and a new detector system will be presented.

Application of multiparameter coincidence spectrometry using a Ge detectors array to neutron activation analysis

The method of multiparameter coincidence spectrometry based on γ–γ coincidence is widely used for the nuclear structure studies, because of its high sensitivity to γ-rays. In this study, feasibility of the method of multiparameter coincidence spectrometry for analytical chemistry was examined. Two reference igneous rock samples (JP-1, JB-1a) issued by the Geological Survey of Japan were irradiated at a research reactor, and the γ-rays from the radioisotopes produced via neutron capture reactions were measured using an array of 12Ge detectors with BGO Compton suppressors, GEMINI. Simultaneously 24 elements were analyzed without chemical separation. The observed smallest component was Eu contained in JP-1 with abundance of 4ppb.

Accurate and precise lithium isotopic determinations of igneous rock samples using multi-collector inductively coupled plasma mass spectrometry

A second-generation multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) was applied to lithium isotopic measurements. The high sensitivity of the ICP-MS enabled high precision (±0.82‰, 2 σ) analyses with small amount of Li (∼45 ng Li). A single-step column separation protocol was established with which rapid purification of lithium from rock solutions can be carried out with reduced blank (<10 pg). The influence of potential sources of error for acquisition of lithium isotopic data introduced during the separation, such as matrix effects and isotopic fractionation due to incomplete recovery, were examined with an artificially mixed solution of a composition similar to that of basalt, which was doped with Li isotopic standard reagent. The examinations demonstrated that our protocol suffered from negligible isotopic fractionation. The Li isotopic ratios obtained by our method for seawater and standard rocks (JA-1, JB-2, and JB-3) agree well with those of previously reported data by Moriguti and Nakamura [1,2], which were determined using a four-step column separation method and thermal ionisation mass spectrometry (TIMS). Our separation protocol combined with a sensitive MC-ICP-MS will enable Li isotopic analyses on silicate rock with low Li contents, such as meteorite and peridotites with increased sample throughput.

Bending strength properties of untreated and impregnated igneous, sedimentary and metamorphic dimension stones of different thickness

Abstract A production method has been developed that makes it possible to produce dimension stone tiles only 4 mm thick without high amounts of waste material. The tiles are impregnated with a mixture of potassium-based water-glass, water, colloidal silica, and Berol 048 (non-ionic surfactant), using a repeated cycling between vacuum and atmospheric pressure. Mineralogy, fabric, and porosity affect the mechanical properties of rock used as dimension stone in the building industry. Tests for bending strength have been performed on tiles of eight different untreated and impregnated samples of igneous, sedimentary, and metamorphic rocks. Samples of three different thicknesses (4, 7 and 10 mm) were used in the tests. The untreated rock samples that had a fine-grained texture, low crack density, high mafic mineral content or a distinct ductile metamorphic texture showed the highest bending strength values, whereas those that had high amounts of carbonate minerals, and high crack or void density exhibited the lowest values. The stone types that gained the most from impregnation, in general, were those with high crack or void density.

238U/230Th disequilibrium measurement for volcanic standard rock samples using a multiple-collector ICPMS.

The radioactive disequilibrium measurements between 238U and 230Th with a Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) were applied to igneous standard rocks distributed by Geological Survey of Japan and Geological Survey of the United States. We modified sample dissolution and Th separation method, as well as Th isotope analysis by MC-ICPMS described by Nakai et al. (2001). High sensitivity of our ICP-MS enabled to analyze less than 10 ng Th. The abundances of U and Th were determined by an isotope dilution method, using a 235U depleted uranium reagent and 230Th prepared by milking of natural U as spikes. We can determine 238U/232Th abundance ratio with a precision of about 2%. This procedure was applied to (238U/230Th) radioactive disequilibrium measurements of igneous standard rock samples. Samples older than 350 Ka were found to attain secular equilibrium, indicating the accuracy of our analysis. On the other hand, young igneous standard rocks were found enriched in 238U relative to 230Th, (238U/230Th)activity ratio > 1, which is often observed for igneous rocks from subduction zones.