GRENVILLE SKARN TITANITE: POTENTIAL REFERENCE MATERIAL FOR SIMS U-Th-Pb ANALYSIS

Abstract

We have investigated the homogeneity, chemical composition, structure, degree of radiation damage, and post-formation evolution of titanite crystals from skarns of the Grenville Province of the Canadian Shield using SHRIMP, TIMS, Raman and PL spectroscopy, EBSD, and EPMA-WDS. These results are used to assess the potential of the titanite as Reference Material (RM) for micro-analytical U-Th-Pb age dating. The SHRIMP data show that these megacrysts (5-31g) have concordant U-Pb isotope systematics, 60 to 500 ppm U, 120 to 1200 ppm Th , 206 Pb/ 204 Pb between 500 and 2500, ages of ~1 Ga, and excellent homogeneity at the scale of the analytical volume of the ion probe. The ID-TIMS titanite data for OLT1, OLT2 and TCB show that these crystals are essentially concordant. Data for OLT1 and OLT2 show slight scatter (i.e., in excess of that expected from the uncertainty in an individual analysis). For OLT1, one of seven analyses shows Pb loss or, possibly, a younger period of growth. Crystals OLT1 and OLT2 have respective TIMS concordia ages of 1014.8 ± 2.0 Ma (2s, n = 6, MSWD = 1.8) and 998.0 ± 4.5 Ma (2s, n = 3, MSWD = 3.3) for domains that have not lost Pb. The TIMS analyses of TCB are tightly clustered and give a concordia age of 1018.1 ± 1.7 Ma (2s, n = 4, MSWD = 0.92). Raman and PL spectra show a low to moderate degree of accumulated radiation-induced damage in the Grenville Skarn Titanite crystals and uniform internal distributions of this damage. The EDSB contrast images indicate little or no crystallographic misorientation. The EMPA-WDS data show that the outer 50-100 mm of the OLT1 and TCB crystals are enriched in Al and F, and depleted in Fe and Nb, when compared with the interior. In spite of the variation in composition and degree of radiation damage amongst samples, there are no identifiable matrix effects in our SHRIMP data. Some Grenville skarn titanite (GST) crystals have potential as RM for micro-analytical U-Th-Pb age dating. Crystal TCB has excellent homogeneity of U-Th-Pb isotopic composition. Crystals OLT1 and OLT2 have minor TIMS age heterogeneity. However, this heterogeneity is smaller than that of the Khan titanite, our current in-house titanite standard. Careful selection of analysis areas during SIMS, and of chips for TIMS analysis, allows high-quality isotopic data to be obtained from these large crystals of titanite.

In situ U-Pb dating of titanite by LA-ICPMS

“…After (Figure 2(d)). All of these ages are identical to the reference value of 1014 Ma [33,34], within errors.…”

“…It is obvious that the matrix effect is slighting for same minerals with different compositions during LA-ICPMS U-Pb age determination. For example, using BLR-1 titantie as the external standard [32,34], the obtained U-Pb ages of OLT-1 titanite are consistent with its reference value for both single spot and line raster scan analyses, although BLR-1 and OLT-1 titanites have distinct chemical components.…”

“…This sample is dark brown titanite grain collected from pegmatite skarns of the Grenville Province of the Canadian Shield. Kennedy et al [33,34] obtained a concordia age of 1014.7±3.8 Ma (n=3, MSWD=3.4) from three titanite grains using solution analyses, while the other grain gave an older age due to the influence of surface common lead. Comprehensive analyses of SIMS, Raman, EBSD, EPMA-WDS and U-Th-He dating conclude OLT-1 titanite has potential as international reference material, with U-Pb heterogeneity smaller than that of the Khan titanite [35].…”

Photoluminescence of synthetic titanite-group pigments: A rare quenching effect

“…In the case of titanite samples, this interpretation is supported strongly by the fact that groups of REE 3+ emission bands show fine-structures that are typical of REEs in titanite (compare spectra published by Gaft et al, 2003;Kennedy et al, 2010). In contrast, if REE-bearing phases other than titanite were present (i.e.…”

“…A similar, Ti 4+ -related broad-band emission was observed from synthetic, Ti-activated malayaite (CaSn 0.97 Ti 0.03 SiO 5 ; Abe et al, 2010). The emission of natural titanite, in contrast, may in some cases be most complex, comprising numerous narrow and broad bands assigned to rare-earth elements (REE, including Sm 3+ , Eu 3+ , Pr 3+ , Nd 3+ , Tm 3+ , Er 3+ ) and transition metals (especially Cr 3+ ; Gaft et al, 2003;Kennedy et al, 2010).…”