HIGH-RESOLUTION ANALYSIS OF TRACE-ELEMENTS IN CORALS BY LASER-ABLATION ICP-MS

A method has been developed using laser ablation inductively-coupled p lasma mass spectrometry (LA-ICP-MS) for rapid high resolution analysis of B, Mg, Sr, Ba, and U in corals. Corals represent a challenge for a microbeam technique due to their compositional and structural heterog eneity, their nonsilicate matrix, and their unusual range of trace ele ment compositions relative to available standards. The method employs an argon-fluoride excimer laser (lambda = 193 nm), masked to produce a beam 600 mu m wide by 20 mu m across to average ablation sampling ove r a range of structural features. Coral sections are scanned at a cons tant rate beneath the laser to produce a continuous sampling of the co ral surface. Sensitivity drift is controlled by careful preconditionin g of the ICP-MS to carbonate material, and standardisation is carried out by bracketing each traverse down the coral sample by analyses of a CaSiO3 glass synthesised from coral powder. The method demonstrates e xcellent reproducibility of both the shape and magnitude of coralline trace element profiles, with typical precisions of between 1.0 and 3.7 % based on analysis of the synthetic standard. Accuracy varies between 3.8% for B and 31% for U. Discrepancies are attributed to heterogenei ties in the: synthetic standard, and matrix differences between the si licate standard and carbonate sample. The method is demonstrated by an alysis of a coral collected from Australia's Great Barrier Reef near a weather station recording in-situ sea-surface-temperature (SST), The elements B, Mg, Sr, and U show seasonal compositional cycles, and tent ative calibrations against SST have been derived. Using independent IC P-MS solution estimates of the coral composition to correct for standa rdisation uncertainties, the following calibrations have been derived: B/Ca (mu mol/mol)= 1000 (+/-20) - 20.6 (+/-0.8) x SST Mg/Ca (mmol/mol )= 0.0 (+/-0.3) + 0.16 (+/-0.01) x SST Sr/Ca (mmol/mol)= 10.8 (+/-0.1) - 0.070 (+/-0.004)x SST U/Ca (mu mol/mol) = 2.24 (+/-0.07) - 0.046 (/-0.003) x SST1 These calibrations agree with literature within experi mental errors, except for ME which displays a 35% greater temperature dependence than reported previously. None of the elements in the coral appear to be sensitive to decreases in salinity associated with heavy rainfall in the summer of 1991/1992. Copyright (C) 1998 Elsevier Scie nce Ltd.