In situ Sr isotopic analysis by laser ablation

Abstract

We demonstrate that the Sr isotopic compositions of geological materials can be measured in situ through laser ablation using an ICP multiple-collector double focusing magnetic sector mass spectrometer (MC-ICPMS). This provides rapid, texturally sensitive, high precision (0.004%) Sr isotopic analysis without the need for chemical preparation of samples. The system used employs a Q-switched Nd-YAG infrared laser. The laser-ablated material is carried by Ar flow to an ICP torch. The resulting ions are extracted from the plasma and focused with d.c. quadrupole lenses and an electrostatic analyzer prior to entering a magnetic sector analyzer and collection in an array of Faraday collectors. A feldspar megacryst, with independently known 87 Sr 86 Sr of 0.703117 ± 13, and a modern gastropod shell, with independently known 87 Sr 86 Sr of 0.709170 ± 10, were used evaluate the method. Both have ∼ 2000 ppm Sr. Laser power was adjusted to give an average 88Sr of 2 × 10 −11 amps during analysis to replicate the beam size during typical thermal ionization mass spectrometry (TIMS). Within-run precisions in 87 Sr 86 Sr of < ± 4 × 10 −5 compare favorably with the precision from TIMS. Thirteen analyses of the Cameroon feldspar megacryst yielded a weighted average 87 Sr 86 Sr of 0.703106 ± 22, in perfect agreement with the TIMS measurement. Similarly, 22 analyses of the modern gastropod shell gave a weighted average of 0.709182 ± 22, identical to the known value. No memory effect was observed when switching from the Cameroon feldspar sample (run for several hours) to the modern shell material. The power of this technique is illustrated with a preliminary study of systematic differences in 87 Sr 86 Sr of ∼ 0.0004 between two feldspar crystals of a Long Valley basalt. One feldspar has identical 87 Sr 86 Sr to the host basalt, confirming the lack of any matrix effect with this method.