•Garnet schists from the South Beishan Orogen experienced high-grade metamorphism at ca. 900Ma.•~900Ma metamorphism is coeval with extensive continental arc formation in the SBOB and CTA.•The SBOB ...and CTA occupied in the periphery of the Rodinia during the final assembly stage.
An early extensive Neoproterozoic (ca. 900Ma) continental magmatic arc system covering hundreds of kilometers has been reported to occur in the South Beishan Orogenic Belt (SBOB) and the Central Tianshan (CTA) in the southern Central Asian Orogenic Belt (CAOB). However, evidence for coeval high-grade metamorphism and thus the formation of an accretionary orogen in the framework of Rodinia is ambiguous or absent. This study provides new petrological, geochemical and geochronological data for garnet-bearing schists (quartz+garnet+biotite+plagioclase±muscovite) from the SBOB in order to constrain its Neoproterozoic metamorphic history. The metamorphic zircon rims are either unzoned or display sector zoning in CL-images and reveal REE patterns with flat HREE patterns and negative Eu anomalies, which are interpreted to be in chemical equilibrium with garnet and plagioclase. The zircon U-Pb dating yields concordant U-Pb ages of 900±3Ma, 897±2Ma and 898±4Ma for the metamorphic zircon rims. The inherited detrital zircon cores of one sample display a concordant U-Pb age of 1397±5Ma that is consistent with the timing of formation for the extensive Mesoproterozoic continental arc in the SBOB and CTA. Based on phase equilibrium geothermobarometry and average P-T thermobarometric calculations, minimum amphibolite-facies P-T conditions are estimated to be >600°C at pressure >0.6GPa, which is thought to have been overprinted by subsequent Paleozoic metamorphism. However, the Ti-in-zircon thermometer still reveals temperatures of up to 840°C using the composition of metamorphic zircon rims, suggesting former ca. 900Ma granulite-facies peak metamorphic temperatures. The combined petrological and geochronological evidence in conjunction with the continental affinity of the regional metamorphic rocks suggests that the SBOB and the eastern CTA experienced an early Neoproterozoic accretionary orogenesis during the final assembly stage of Rodinia.
We report new estimates of abundances of rarely analyzed elements (As, B, Be, Bi, Cd, Ge, In, Mo, Sb, Sn, Te, Tl, W) in the upper continental crust based on precise ICP-MS analyses of ...well-characterized upper crustal samples (shales, pelites, loess, graywackes, granitoids and their composites) from Australia, China, Europe, New Zealand and North American. Obtaining a better understanding of the upper crustal abundance and associated uncertainties of these elements is important in placing better constraints on bulk crust composition and, from that, whole Earth models of element cycling and crust generation. We also present revised abundance estimates of some more commonly analyzed trace elements (Li, Cr, Ni, and Tm) that vary by >
20% compared to previous estimates. The new estimates are mainly based on significant (
r
2
>
0.6) inter-element correlations observed in clastic sediments and sedimentary rocks, which yield upper continental crust elemental ratios that are used in conjunction with well-determined abundances for certain key elements to place constraints on the concentrations of the rarely analyzed elements. Using the well-established upper crustal abundances of La (31 ppm), Th (10.5 ppm), Al
2O
3 (15.40%), K
2O (2.80%) and Fe
2O
3 (5.92%), these ratios lead to revised upper crustal abundances of B
=
47 ppm, Bi
=
0.23 ppm, Cr
=
73 ppm, Li
=
41 ppm, Ni
=
34 ppm, Sb
=
0.075, Te
=
0.027 ppm, Tl
=
0.53 ppm and W
=
1.4 ppm. No significant correlations exist between Mo and Cd and other elements in the clastic sediments and sedimentary rocks, probably due to their enrichment in organic carbon. We thus calculate abundances of these elements by assuming the upper continental crust consists of 65% granitoid rocks plus 35% clastic sedimentary rocks. The validity of this approach is supported by the similarity of SiO
2, Al
2O
3, La and Th abundances calculated in this way with their upper crustal abundances given in Rudnick and Gao Rudnick, R., Gao, S., 2003. Composition of the continental crust. In: Rudnick, R.L. (Ed.), The Crust. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, vol. 3. Elsevier–Pergamon, Oxford, pp. 1–64.. The upper crustal abundances thus obtained are Mo
=
0.6 ppm and Cd
=
0.06 ppm. Our data also suggest a ∼
20% increase of the Tm, Yb and Lu abundances reported in Rudnick and Gao Rudnick, R., Gao, S., 2003. Composition of the continental crust. In: Rudnick, R.L. (Ed.), The Crust. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, vol. 3. Elsevier–Pergamon, Oxford, pp. 1–64..
The ChahJam–Biarjmand complex (CJBC), flanked by the Alborz Mountains in the north and the Lut–Tabas block to the south, is part of Central Iranian block, where the oldest continental crust of Iran ...is found. This complex contains granitic to tonalitic orthogneissic rocks (old plutons) and associated metasediments, amphibolites and paragneisses. Metamorphosed granitic and granodioritic dikes intrude orthogneisses as well as metasediments and are abundant close to the plutons (orthogneissic rocks). Based on the results of bulk rock trace and rare earth elements, the orthogneissic rocks are inferred to have crystallized from subduction-related melts. Amphibolites also have subduction-related signatures and are inferred to have formed both as metamorphosed volcanoclastic sediments and as attenuated basic dikes. The presence of para-amphibolites associated with paragneisses and metasediments (mica schists) could represent a sedimentary basin filled with magmatic arc erosional products. U–Pb zircon dating of the ChahJam–Biarjmand rocks yielded 238U/206Pb crystallization ages of ca. 550 to 530Ma (Ediacaran–early Cambrian). Sr–Nd isotope systematics on whole rocks (εNd(t)=−2.2 to −5.5) and zircon Hf isotope results indicate that CJBC Cadomian granitic rocks contain older, possible Mesoproterozoic, continental crust in their source. The ChahJam–Biarjmand granitic–tonalitic gneissic rocks are coeval with other similar-aged metagranites and gneisses within Iranian basement exposed in Central Iran, the Sanandaj–Sirjan Zone and the Alborz Mountains, as well as in the Tauride–Anatolide platform in western Anatolia and in NW Turkey. All these dispersed Cadomian basement rocks are interpreted to show fragments of Neoproterozoic–early Cambrian continental arcs bordering the northern active margin of Gondwana.
Display omitted
•Granitic gneisses in ChahJam–Biarjmand complex (Iran) were intruded ca. 550–530Ma.•Nd and zircon Hf isotopes suggest re-melting of Mesoproterozoic continental crust.•The rocks are related to Cadomian magmatism along the active margin of Gondwana.
A novel “wave” signal-smoothing and mercury-removing device has been developed for laser ablation quadrupole and multiple collector ICPMS analysis. With the wave stabilizer that has been developed, ...the signal stability was improved by a factor of 6.6–10 and no oscillation of the signal intensity was observed at a repetition rate of 1 Hz. Another advantage of the wave stabilizer is that the signal decay time is similar to that without the signal-smoothing device (increased by only 1–2 s for a signal decay of approximately 4 orders of magnitude). Most of the normalized elemental signals (relative to those without the stabilizer) lie within the range of 0.95–1.0 with the wave stabilizer. Thus, the wave stabilizer device does not significantly affect the aerosol transport efficiency. These findings indicate that this device is well-suited for routine optimization of ICPMS, as well as low repetition rate laser ablation analysis, which provides smaller elemental fractionation and better spatial resolution. With the wave signal-smoothing and mercury-removing device, the mercury gas background is reduced by 1 order of magnitude. More importantly, the 202Hg signal intensity produced in the sulfide standard MASS-1 by laser ablation is reduced from 256 to 0.7 mV by the use of the wave signal-smoothing and mercury-removing device. This result suggests that the mercury is almost completely removed from the sample aerosol particles produced by laser ablation with the operation of the wave mercury-removing device. The wave mercury-removing device that we have designed is very important for Pb isotope ratio and accessory mineral U–Pb dating analysis, where removal of the mercury from the background gas and sample aerosol particles is highly desired. The wave signal-smoothing and mercury-removing device was applied successfully to the determination of the 206Pb/204Pb isotope ratio in samples with low Pb content and/or high Hg content.
Here we describe an internal standard-independent calibration strategy for LA-ICP-MS analysis of anhydrous minerals and glasses. Based on the normalization of the sum of all metal oxides to 100 wt.%, ...the
ablation yield correction factor (AYCF) was used to correct the matrix-dependent absolute amount of materials ablated during each run.
A
Y
C
F
=
100
∑
j
=
1
N
(
c
p
s
sam
j
×
l
j
)
,
l
j
=
C
rm
j
/
c
p
s
rm
j
, where cps
sam
j
and cps
rm
j
are net count rates of analyte element
j of the sample and reference material for calibration,
C
rm
j
is concentration of element
j in the reference material,
N is the number of elements that can be determined by LA-ICP-MS. When multiple reference materials were used for calibration,
l value can be calculated with regression statistics according to the used reference materials.
Applying an AYCF and using the USGS reference glasses BCR-2G, BHVO-2G and BIR-1G as reference materials for external calibration, analyses of MPI-DING reference glasses generally agree with recommended values within 5% for major elements (relative standard deviation (RSD)
=
0.3–3.9% except for P
2O
5,
n
=
11), and 5–10% for trace elements. Analyses of anhydrous silicate minerals (clinopyroxene, orthopyroxene, olivine, plagioclase and garnet) and spinel generally agree with the results of electron microprobe analysis within 0.2–7% for SiO
2, Fe
2O
3, MgO and CaO. RSD are generally <
5% for elements with concentrations >
0.1 wt.%. The results indicate that, by applying an AYCF and using USGS reference glasses as multiple reference materials for calibration, elements of these anhydrous minerals can be precisely analyzed
in situ by LA-ICP-MS without applying internal standardization. The different element fractionations between the NIST glasses and those glasses with natural compositions indicate that NIST SRM 610 is a less than ideal reference material for external calibration of analyses of natural silicates.
LA-ICP-MS and LA-MC-ICP-MS have been the techniques of choice for achieving accurate and precise element content and isotopic ratio, the state-of-the-art technique combines the advantages of low ...detection limits with high spatial resolution, however, the analysis accuracy and precision are restricted by many factors, such as sensitivity drift, elemental/isotopic fractionation, matrix effects, interferences and the lack of sufficiently matrix-matched reference materials. Thus, rigorous and suitable calibration and correction methods are needed to obtain quantitative data. This review systematically summarized and evaluated the interference correction, quantitative calculation and sensitivity correction strategies in order to provide the analysts with suitable calibration and correction strategies according to the sample types and the analyzed elements. The functions and features of data reduction software ICPMSDataCal were also outlined, which can provide real-time and on-line data reduction of element content and isotopic ratios analyzed by LA-ICP-MS and LA-MC-ICP-MS.
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