REFERENCE MATERIALS FOR LA-ICP-MS
Several reference materials have been developed and distributed to several overseas LA-ICP-MS laboratories. These were investigated in collaboration with our partners in Europe (Fernando Corfu, Axel Gerdes, Micheal Widenebeck, Craig Storey, Daniela Rubato), South Africa (Ian Buick) and Canada (Sandra Kamo).
The BB zircon comes from a batch of Sri Lanka megacrysts (0.5–1.5 cm3) obtained by Prof Ricardo Scholz. Within‐grain rare earth element (REE) compositions for these zircons are largely homogeneous, albeit with some variation between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains, the variation in U content for all analysed crystals ranged from 227 to 368 μg g−1 and the average Th/U ratios were between 0.20 and 0.47. The Hf isotope composition (0.56–0.84 g/100 g Hf) is homogeneous within and between the grains – mean 176Hf/177Hf of 0.281674 ± 0.000018 (2s). The calculated alpha dose of 0.59 × 1018 g−1 for many BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the same crystal (analysed by ID‐TIMS in four different laboratories) gave consistent U‐Pb ages with excellent measurement reproducibility (0.1–0.4% RSD). Interlaboratory assessment (by LA‐ICP‐MS) from individual crystals returned results within uncertainty equivalent to the TIMS ages. Finally, we report on within‐ and between‐grain homogeneity of the oxygen isotope systematics of four BB crystals (13.16‰ VSMOW).
Most monazite reference materials (RMs) for in situ U‐Pb geochronology are rich in Th; however, many hydrothermal ore deposits contain monazite that is low in trace element contents, including Th, U and Pb. Because of potential problems with matrix effects and the lack of appropriate matrix‐matched RMs, such variations can bias the dating of hydrothermal deposits. We investigated a polycrystalline low‐U and low‐Th Diamantina monazite from the Espinhaço Range, SE Brazil. It has a U‐Pb ID‐TIMS weighted mean 207Pb*/235U ratio of 0.62913 ± 0.00079, 206Pb*/238U of 0.079861 ± 0.000088 and 207Pb*/206Pb* of 0.057130 ± 0.000031, yielding a weighted mean 206Pb*/238U date of 495.26 ± 0.54 Ma (95% c.l.). In situ dates acquired with different methods (LA‐(Q, SF, MC)‐ICP‐MS and SIMS) are within the uncertainty of the ID‐TIMS data. U‐Pb LA‐(Q, MC)‐ICP‐MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within < 1% deviation. The LA‐MC‐ICP‐MS analyses yielded homogeneous Sm‐Nd isotopic compositions (143Nd/144Nd = 0.511427 ± 23, 2s; 147Sm/144Nd = 0.1177 ± 13, 2s) and εNd(495 Ma) of −18.7 ± 0.5 (2s). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2s), confirming Diamantina’s relative homogeneity at test portion masses below 1 ng.
Monazite from the largest pegmatite bodies in the district (the Bananeira, Coqueiro and Paraíso pegmatites) are Ce-monazite, with negligible amounts of the huttonite and brabantite components. They are homogeneous in major and trace elements, which makes them potential candidates as compositional reference materials. U–Pb LA-ICP-MS and ID-TIMS analyses yielded identical ages within error. Although the ID-TIMS ages (507.7 ± 1.3 (207Pb⁎/
235U) and 513.6 ± 1.2 Ma (206Pb⁎/ 238U)) were reversely discordant, spot ages determined by LA-ICP-MS geochronology were concordant at ca 508 Ma. The Bananeiro monazite was assessed as a LA-ICPMS U–Pb primary reference material against other known reference materials (treated as unknowns). This approach successfully reproduced the previously published ages of the reference materials. MREE/HREE fractionation (ie, (La/Gd)N and (Gd/Lu)N values), Eu/Eu⁎ and the chondrite-normalized REE patterns suggest that the “Itambé” monazite aliquot is very similar to that from the Coqueiro pegmatite. This similarity is likewise apparent in their Sm–Nd isotope compositions. Moreover, the εNdi values of the “Itambé” monazite fragment
(εNdi = −4.2) and those from all the major pegmatites in the district, are distinct from other reference materials (eg, Managountry; εNdi = −22.3) as well as gem-quality monazite from c. 490–520 Ma pegmatites from the
Araçuaí Orogen, further to the south. The εNdi can provide a further distinction for tracing Brazillian gem-quality monazite reference materials.
Five megacrysts of xenotime (XN01, XN02, XN03, XN04, and XN05) as
potential reference materials (RMs) for U-Pb geochronology. These crystals belong to a 300-g xenotime assortment, collected from alluvial deposits in SE Brazil. Electron microprobe and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analyses show that the selected crystals are internally homogeneous for most rare earth elements, (REE, except some light REE) but are relatively heterogeneous for U and Th. The xenotime REE patterns are consistent with an origin from hydrothermal quartz veins that cut greenschist-facies metasediments and that locally contain other accessory phases such as rutile and monazite. High-precision U-Pb Isotope Dilution-Thermal Ionization Mass Spectrometry (ID-TIMS) analyses showed slight age heterogeneity for the XN01 crystal not observed in the XN02 sample. The two crystals have slightly different average 206Pb/238U ages of 513.4 ± 0.5 Ma (2 s) and 515.4 ± 0.2 Ma (2 s),
respectively. In situ U-Pb isotope data acquired via LA-(Q,SF,MC)-ICP-MS are within the uncertainty of the ID-TIMS data, showing homogeneity at the 1% precision of the laser ablation (and probably ion microprobe) technique. U-Pb LA-(MC, SF)-ICP-MS analyses, using XN01 as a primary RM, reproduced the ages of other established RMs within less than 1% deviation. Other Datas crystals (XN03-05) also display a reproducibility in Pb/U dates better than 1% on LA-ICP-MS, making them good candidates for further testing by ID-TIMS.
Kahn River and Bear Lake Titanite
The Khan River (Namibia) and Bear Lake (Canada) titanites are investigated as potential reference materials (RM) for LA-ICP-MS applications. The Bear Lake titanite is texturally and compositionally homogeneous. The Khan River titanite is texturally heterogeneous and characterised by variable trace element compositions and total rare earth element contents. However, both titanites have consistent U-Pb and Nd-isotope ratios. U-Pb isotope dilution-thermal ionisation mass spectrometry analyses yielded Pbc-uncorrected intercept ages of 516.3 ± 1.3 Ma (2s, n = 5, MSWD = 2.4) and 1067.81 ± 0.74 Ma (2s, n = 4, MSWD = 0.35) for Khan River and Bear Lake titanites, respectively. Multiple U-Pb LA-SF/MC-ICP-MS analyses gave consistent Pbc-uncorrected intercept ages for both, Khan River (517 ± 1/5 Ma, 2s, n = 262, MSWD = 1.5) and Bear Lake (1070 ± 1/11 Ma, 2s, n = 325, MSWD = 0.88). U-Pb SHRIMP analyses on the same material returned identical (within uncertainty) ages. Khan River and Bear Lake gave internally consistent solution MC-ICP-MS 143Nd/144Nd ratios of 0.511587 ± 0.000027 (2s, n = 2) and 0.512321 ± 0.000004 (2s, n = 2), respectively. The 143Nd/144Nd ratios via solution-MC-ICP-MS and LA-ICP-MS all agree within uncertainty and suggest that both titanites can be used as RMs for Nd-isotope analyses.
Monazite (Vermilion) from the ∼2.7 Ga Vermilion Granitic Complex (Minnesota, USA) was investigated as a potential reference material for U-Pb geochronology, using electron microprobe, high-precision ID-TIMS and high-spatial resolution techniques (LA-(Q/SF/MC)-ICP-MS and SHRIMP). Vermilion monazite is relatively homogeneous in BSE images. It is characterised as a monazite-(Ce) with uniform, relatively LREE-enriched chondrite-normalised REE patterns (Eu/Eu* = 0.074–0.085). ID-TIMS U-Pb analyses yielded concordant to slightly (−0.6% to 1.8%) discordant isotope data with weighted mean 207Pb*/206Pb* dates of 2668.6 ± 1.8 Ma (2σ, MSWD = 1.3, n = 3) and 2666.8 ± 1.8 Ma (2σ, MSWD = 4.0, n = 4). High-spatial resolution data from SHRIMP and LA-(Q/SF/MC)-ICP-MS agree with the ID-TIMS age. Using Vermilion monazite as primary reference material, LA-SF-ICP-MS analyses reproduced the age of several other reference monazites within −1.13% to +1.64% accuracy. Nd isotopes obtained via ID-TIMS and solution MC-ICP-MS returned consistent 143Nd/144Nd ratios of 0.510694 ± 0.000040 (2σ) and 0.510726 ± 0.000014 (2σ), respectively. The LA-MC-ICP-MS 143Nd/144Nd ratios of 0.51071 ± 0.00001 (2σ) and 0.51065 ± 0.00001 (2σ) are also in agreement within uncertainties. Results of Sm-Nd isotope analyses by LA-MC-ICP-MS from two laboratories yielded mean 147Sm/144Nd ratios of 0.0869 ± 0.0002 (2.5% RSD, 2σ, n = 58) and 0.0835 ± 0.0001 (2.2% RSD, 2σ, n = 50), reflecting small variations in individual fragments. The initial epsilon Nd (εNdi) varies from −0.8 ± 0.5 to 0.6 ± 0.4 (2σ). The reproducibility of our results shows that Vermilion monazite can be used as a primary reference material for U-Pb geochronology and is suitable as a secondary reference material for Sm-Nd isotopic tracing. The Archean age of Vermilion monazite, the oldest available as a reference material, will facilitate the investigation of Archean crustal evolution processes and broaden the prospect of studying ancient metamorphic events by combining matrix-matched U-Pb geochronology and Sm-Nd isotopic tracing.
This study has determined the trace element and the U-Pb, Sm-Nd and Sr-Sr isotope composition of the Sumé apatite (from NE Brazil) to assess its suitability as a primary/secondary reference material for LA-ICP-MS. Reproducibility tests demonstrate that one batch (~ 100 g) of the Sumé apatite (Sumé-570) is uniform in terms of Nd-Nd and U-Pb isotope compositions. Bulk isotope dilution TIMS/ICP-MS and LA-MC-ICP-MS analyses confirm that the apatite is well suited for use as quality control material for Nd isotopes and U-Pb geochronology. U-Pb ID-TIMS analyses yield weighted mean ratios of 0.09211 ± 0.00053 (2s; 206Pb*/238U) and 0.06120 ± 0.00063 (2s; 207Pb*/206Pb*) and a weighted mean 206Pb*/238U date of 568 ± 3 Ma (95% c.l.). U-Pb LA-(SF/MC)-ICP-MS runs using Sumé-570 as a primary RM and reproduces the dates of other established RMs within 1% deviation (except for Durango 2–4%). Major and trace element abundances show that Sumé-570 is a fluorapatite derived from a syenitic source. It also strongly shows LREE-enriched chondrite-normalised REE patterns with significant negative Eu anomalies, due to crystallisation of plagioclase in the residue.