Mineralogical Magazine, August 2011, Vol. 75(4), pp. 2485–2494
A secondary ion mass spectrometry (SIMS) re-evaluation of B and Li isotopic compositions of Cu-bearing elbaite from three global localities T. LUDWIG1,*, H. R. MARSCHALL2,3, P. A. E. POGGE VON STRANDMANN2, B. M. SHABAGA4, M. FAYEK4 F. C. HAWTHORNE4 1 2 3
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Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 234 36, 69120 Heidelberg, Germany Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada [Received 18 March 2011; Accepted 13 June 2011]
ABSTR ACT
Cu-bearing elbaite from Paraı´ba (Brazil) is a highly-prized gem tourmaline. Specimens of similar quality from localities in Mozambique and Nigeria are being sold, and reliable provenance tools are required to distinguish specimens from the original locality from ‘Paraı´ba-type’ tourmaline from Africa. Here we present Li and B isotope analyses of Cu-bearing elbaite from all three localities and demonstrate the suitability of these isotope systems as a provenance tool. Isotopic profiles across chemically zoned grains revealed homogenous B and Li isotopic compositions, demonstrating a strong advantage of their application as a provenance tool as opposed to major, minor or trace element signatures. Li and B isotopes of all investigated samples of Cu-bearing elbaites from the three localities are within the range of previously published granitic and pegmatitic tourmaline. Anomalous isotope compositions published previously for these samples are corrected by our results.
K EY WORDS : tourmaline, isotopes, Paraı´ba, lithium, boron, secondary ion mass spectrometry (SIMS), ion
probe, LA-ICP-MS Introduction TOURMALINE close to the elbaite endmember composition typically occurs in pegmatite dykes as comb-like layers or in miarolitic cavities and veins. The rarest and most expensive varieties of elbaite were discovered in the late 1980s in the Batalha pegmatite mine of the Borborema Province in the state of Paraı´ba, northeastern Brazil. These elbaites display very impressive colours of blue, blue-green, green and pink, with
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[email protected] DOI: 10.1180/minmag.2011.075.4.2485
# 2011 The Mineralogical Society
Cu2+ and Mn3+ as chromophores (Rossman et al., 1991). ‘Paraı´ba-type’ tourmaline is produced today from three different pegmatite districts in Brazil, Mozambique and Nigeria. Prices for gem tourmaline vary by several orders of magnitude depending not only on the quality, colour and clarity, but also on the provenance, and gemmologists are challenged to develop effective provenance tools. Elbaite major- and minorelement compositions are variable, overlap between the three localities and provide no definite provenance criteria. Trace element abundances (e.g. Ga, Bi, Pb) have been used successfully to distinguish ‘Paraı´ba-type’ tourmaline from Brazil and Nigeria, but the grains show
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significant zoning and some overlap between localities (Rossman, 2009; Peretti et al., 2010). Further provenance tools are required to distinguish between the three localities. Shabaga et al. (2010) studied elbaite samples from all three localities using secondary ion mass spectrometry (SIMS) to analyse B and Li isotopes. They suggested using these isotope systems as provenance indicators for ‘Paraı´ba-type’ tourmaline, as the isotopic signatures they found were different for the three localities. SIMS requires a minimum of sample material with sputtered craters significantly 106 ions/s) during analysis of the tourmalines. In the latter case, accuracy could be seriously affected by the deadtime of the detector system and its subsequent correction. Furthermore, SRM610 was shown to be affected by SIMS matrix effects for Li isotopes (Kasemann et al., 2005) and B isotopes (Rosner et al., 2008) compared to other glasses with Li and B as trace elements. Analytical setup Li and B isotope ratios were determined by SIMS using a modified Cameca ims 3f ion probe at the Institute of Earth Sciences, Heidelberg University. The common parameters for both Li 2487
and B isotope analyses were: 1 nA, 14.5 keV 16O primary ion beam, diameter typically 5 mm; 4.5 kV secondary acceleration voltage; 0S50 eV energy window; secondary ion detection by a single electron multiplier (Balzers SEV217) in counting mode (deadtime t = 16 ns); n = 50 cycles per analysis; each cycle was an A B A sequence (e.g. 6 Li 7Li 6Li) which minimizes the influence of slow changes in secondary ion intensity on the measured isotope ratio. The integration times per cycle were 3.5 s for 6Li, 1 s for 7Li, 3.3 s for 10B and 1.65 s for 11B. For Li isotopes, the mass resolution m/Dm was set to 970 at 10% intensity ratio (860 at 0.1%) and for B isotopes it was 1200 at 10% (1000 at 0.1%). Prior to each analysis, a mass calibration of the magnet was performed. The pre-sputtering time (which includes the time needed for mass calibration) was 250 285 s. The typical internal precision of both the Li and B isotope analyses was 0.5% (1s). The measured isotope ratios were corrected for the instrumental mass fractionation ainst which was determined using the reference materials (see Table 1). Because Li is a major element in elbaite (and a trace element in schorl or dravite), only elbaite #98144 was used to determine ainst for Li isotopes. For the correction of B isotope data the mean ainst of the three reference tourmalines was used. The standard deviation (0.9% (1s) for the first analytical session and 1.1% (1s) for the second) of the mean ainst indicates that no significant matrix effect exists for B isotope SIMS analyses of tourmalines. The accuracy of the d11B values of tourmalines obtained by the ion probe in Heidelberg has also been demonstrated in Marschall et al. (2006, 2009) by comparison with independent TIMS data. From these comparisons the accuracy of our d11B values is expected to be better than 2%. Up to now neither additional d7Li reference materials (tourmalines and in particular elbaites) nor independent d7Li analyses of the same samples (elbaites) using different methods exist; it is therefore impossible to make a comparable statement about the accuracy of the d7Li data. Using the ims3f in Heidelberg and synthetic basaltic glasses as reference material, Marks et al. (2008) reported deviations of 6% for an amphibole (arfvedsonite) and 3.9% for a pyroxene (aegirine) for their d7Li SIMS analyses. In the present work, unlike that of Marks et al. (2008), the reference material (elbaite) and the unknown samples (also elbaites) are well matched, and the accuracy of the d7Li data can be expected to be much better.
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Molecular interferences There are two significant molecular interferences that must be suppressed in B isotope analyses: these are 9BeH for 10B and 10BH for 11 B. The mass spectra at masses of 10 u and 11 u, shown in Fig. 1, were recorded on elbaite #98144. At a mass resolution of m/Dm = 1200 at 10% the 10 B peak and the 9BeH peak overlap, but at the centre of the 10B peak, the 9BeH peak is suppressed by a factor of >1000: the mass difference between 10B and 9BeH is ~0.007 u and with m/Dm = 1000 at 0.1% the half width of the 9 BeH peak at 0.1% is 10.02/2000 & 0.005 < 0.007 u. Furthermore, as Be is a trace element in tourmaline (typically B/Be >1000), the 9 BeH interference shown in Fig. 1a has a very low intensity compared to 10B; the 9BeH/10B ratio is ~2610 5 = 0.02%. Therefore, even at insufficient mass resolution, the 9BeH interference is irrelevant. The more important interference in B isotope analysis of tourmaline is 10 BH as both B and H are major components of tourmaline. Figure 1b shows a mass spectrum at mass 11 u. Again, the 10BH interference is suppressed by at least a factor of 1000; the 10 BH/ 1 1 B ratio is 2.4610 2 /2.2610 5 & 1.1610 3. Therefore, in a worst case scenario (10BH not suppressed at all during analysis of the reference material and fully suppressed during analysis of the unknown sample or vice versa), the
error would be only ~1%. The speculation by Shabaga et al. (2010) that the B isotope data in Marschall and Ludwig (2006) may be in error by >+20% due to 10BH interference is therefore not correct. We spare the reader the same lengthy considerations about the molecular interferences of the Li isotopes: at a mass resolution of m/ Dm = 860 at 0.1%, the interferences are sufficiently suppressed. Results A total of 10 samples of Cu-bearing elbaite were analysed. Six samples (Brazil 1 6) are elbaites from the Batalha mine in Paraı´ba, Brazil, two elbaites (Mozambique 1 2) are from the Mavuco mine in the Alto Ligonha district, Mozambique, and the remaining two (Nigeria 1 2) are from an undetermined pegmatite mine in Nigeria. The samples Brazil 1 4, Mozambique 1 2 and Nigeria 1 2 are the same samples analysed by Shabaga et al. (2010). Li and B isotopes were analysed in separate sessions. Since we did not perform two analyses on the same spot, no pairs of d7Li and d11B values at exactly the same location were obtained. Typically, the distance between the d7Li spots and the d11B spots was 50 mm. In Table 2 and Fig. 3, we therefore do not present single d7Li d11B value pairs, but the mean values for each sample. A table with the complete dataset
FIG. 1. Mass spectra (recorded with a mass resolution of 1200 at 10% and with elbaite #98144 as the sample) at masses 10 u and 11 u. Note that, although the peaks overlap, the contribution of the 9BeH peak to the centre of the 10 B peak is negligible (
