The brevity of carbonatite sources in the mantle: evidence from Hf isotopes

Abstract

Hf, Zr and Ti in carbonatites primarily reside in their non-carbonate fraction while the carbonate fraction dominates the Nd and Sr elemental budget of the whole rock. A detailed investigation of the Hf, Nd and Sr isotopic compositions shows frequent isotopic disequilibrium between the carbonate and non-carbonate fractions. We suggest that the trace element and isotopic composition of the carbonate fraction better represents that of the carbonatite magma, which in turn better reflects the composition of the carbonatitic source. Experimental partitioning data between carbonatite melt and peridotitic mineralogy suggest that the Lu/Hf ratio of the carbonatite source will be equal to or greater than the Lu/Hf ratio of the carbonatite. This, combined with the Hf isotope systematics of carbonatites, suggests that, if carbonatites are primary mantle melts, then their sources must be short-lived features in the mantle (maximum age of 10–30 Ma), otherwise they would develop extremely radiogenic Hf compositions. Alternatively, if carbonatites are products of extreme crystal fractionation or liquid immiscibility then the lack of radiogenic initial Hf isotope compositions also suggests that their sources do not have long-lived Hf depletions. We present a model in which the carbonatite source is created in the sublithospheric mantle by the crystallization of earlier carbonatitic melts from a mantle plume. This new source melts shortly after its formation by the excess heat provided by the approaching hotter center of the plume and/or the subsequent ascending silicate melts. This model explains the HIMU-EMI isotope characteristics of the East African carbonatites, their high LREE/HREE ratios as well as the rarity of carbonatites in the oceanic lithosphere.

Appendix

All trace element concentrations (except were noted by isotope dilution) were determined at NHMFL using a Finnigan MAT ELEMENT 1, high resolution ICP-MS, with a CD1E interface. For sample introduction we used a Cetac MCN 6000 desolvating nebulizer with a 50 µl Teflon nebulizer in self-aspiration mode and Teflon tubing. The enhanced instrument sensitivity with this configuration allows the introduction of low total dissolved solid content in the instrument (100 ppm), which minimizes matrix-related drift and signal instabilities. A mixture of nitrogen and argon was used as sample gas in order to minimize oxide interference, which in the case of carbonatites can be significant on the middle and heavy REE and Hf, due to the high LREE/HREE and Ba/HREE ratios and in the case of Hf, high Dy/Hf ratios in carbonatites. The oxide formation of these elements varies monotonically with the Th oxide formation (ThO⁺/Th⁺) which was kept at less than 0.06% by adjusting the Ar/N ratio in the sample gas, while keeping signal intensity at optimum conditions. At these levels of Th oxide formation, the other interfering oxides resulted in negligible corrections to the measured elements (e.g. ¹⁶²Dy¹⁶O/¹⁶²Dy <0.001%, resulting in negligible corrections at ¹⁷⁸Hf).

The sample powders were dissolved in HF/HNO₃ mixture for at least 48 h, dried and redissolved in 2% HNO₃ at 100 ppm total dissolved solid content. For external standard we used the reference material BHVO-1 (Kilauea basalt) with the concentrations reported by Eggins et al. (1997). Measurement sequences were less than 1 h long and consisted of procedural blank, standard, two unknowns and the standard. Indium (at 1 ppb concentration) was used as an internal spike in all samples to monitor and correct for signal drift during measurement. The concentrations for each element were calculated by combining linear interpolation between the bracketing standards and normalizing concentrations to the ¹¹⁵In intensity for each sample. Reproducibility was determined by repeated measurements of the COQ sample and it was 5% or better for all elements.

The Hf, Zr and Ti depletions (Table 2) are defined here as follows:

Hf/Hf*:

=(Hf_C/Hf_Chon)/(Nd_C/Nd_Chon*1/3+Sm_C/Sm_chon*2/3),

Zr/Zr*:

=(Zr_C/Zr_Chon)/(Nd_C/Nd_Chon*2/3+Sm_C/Sm_chon*1/3),

Ti/Ti*:

=(Ti_C/Ti_Chon)/(Eu_C/Eu_Chon*1/2+Gd_C/Gd_chon*1/2), where subscript C denotes concentration in the sample and chon chondrite concentration.

Nd, Sr and Pb chemical separations were carried out at NHMFL using previously established techniques (Hart and Brooks 1977; Manhès et al. 1978; Richard et al. 1976; Zindler et al. 1979). Nd, Sr and Pb isotope determinations were performed at NHMFL using a FINNIGAN MAT 262 RPQ mass spectrometer. Nd and Sr isotopic determinations were performed with a multi-collector dynamic routine. Pb isotope compositions were measured in the static mode using fully automated temperature control at 1,250 °C (see footnotes in Table 3 for further details).

The Hf chemical separation employed here is modified after the method of Salters (1994). The use of concentrated HF during dissolution coupled with the high Ca contents of carbonatites and the large amounts of required sample resulted in very low Hf yields due to the co-precipitation of Hf with insoluble CaF₂. To efficiently separate Hf from Ca, we employed a double-dissolution technique: the carbonate fraction was first dissolved and separated from the silicate fraction using either HCl:HNO₃ mixture or hot 2.5 N HCl and the remaining silicate fraction (always less than 20 mg) was dissolved in hot HF:HNO₃ mixture (3:1) and converted to 2.5 N HCl. This technique resulted in clear solutions with no obvious undissolved material for both the carbonate and silicate fractions, which were then combined into one solution containing the whole rock in 2.5 N HCl. If any residue was present in any of the two solutions, the sample was aborted and the dissolution was repeated. Hf, Zr, Ti (along with some Al, Fe, and Mg) was separated from the rest of the matrix using all-Teflon columns filled with cation resin in 4 ml 2.5 N HCl/0.1 N HF acid. At this acid normality and despite the presence of fluorides, Sr and the REE remain in the column and can be subsequently mobilized in the same fashion as in the typical cation exchange techniques. This technique allows the efficient separation of Hf from Ca (critical for the following separation steps that require HF) and the quantitative recovery of Sr and the REE for Sr and Nd isotope determination. This column chemistry was employed for the determination of the Hf, Sr and Nd isotopic compositions of the silicate fractions from a single dissolution. The Hf cut from the cation columns was further purified following the method of Salters (1994). To verify the reproducibility of this double-dissolution technique, we also processed sample BD 1584 with the original method of Salters (1994) (whole rock HF dissolution: this was the only sample we could process this way because of its higher Hf contents). The measured ¹⁷⁶Hf/¹⁷⁷Hf ratio was identical within error for both methods (Table 5: BD 1584 wr and wr duplicate, respectively). For the Hf isotope determination of the carbonate fractions (which required at least 3 g of sample), we used larger cation columns (14-ml cation resin) in order to increase the amount of Ca retained in the column. Hf blanks where less than 10 pg, and less than 20 pg when the large cation columns where used.

The Lu/Hf ratios of the carbonate fractions were determined by isotope dilution. A fraction of the dissolved carbonate was spiked with ¹⁷⁵Lu and ¹⁷⁹Hf, and dried. The Lu/Yb fraction was collected from the cation columns and Lu was further purified from Yb using columns packed with HDEHP-coated teflon powder and eluted with 4 N HCl. Lu was measured on the 262 MAT Finnigan using the double Re filament technique. The reproducibility of the Lu/Hf ratio was determined by repeated measurements of the BIR-1 basalt standard: Lu/Hf=0.06102±0.00033 (1 SD, n=3).

All Hf isotope compositions were determined at NHMFL with the Lamont ISOLAB (England et al. 1992) using the hot SIMS technique (Salters 1994). The external reproducibility of the instrument over a 2-year period was determined by repeated measurements of the standard JMC-475: ¹⁷⁶Hf/¹⁷⁷Hf=0.282200±31 (2 SD, n=44) which is in excellent agreement with the reported value before the instrument was relocated to NHMFL from LDEO (Salters 1994). Because of the variable amounts of Hf present in carbonatites, the JMC-475 reproducibility was determined by loading variable amounts of Hf (50–150 ng). If only the larger loads are considered (110–150 ng), the reproducibility improves but the average value remains essentially identical: ¹⁷⁶Hf/¹⁷⁷Hf=0.282199±24 (2 SD, n=13). ¹⁷⁶Hf/¹⁷⁷Hf ratios were fractionation corrected using ¹⁷⁷Hf/¹⁷⁸Hf=0.6816. All Hf isotope compositions are reported relative to the value of 0.282160 for the JMC standard.