Journal
RATIONALE: Laser Ablation coupled to Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MCICPMS)<br />
is a powerful tool for the high-precision measurement of the isotopic ratios of many elements in geological<br />
samples, with the isotope ratio (11B/10B) of boron being used as an indicator of the pH of oceanic waters. Most geological<br />
samples or standards are polished and ablation occurs on flat surfaces. However, the shape and the irregularities of<br />
marine biocarbonates (e.g., corals, foraminifera) can make precise isotopic measurements of boron difficult. Even after<br />
polishing, the porosity properties and the presence of holes or micro-fractures affect the signal and the isotopic ratio<br />
when ablating the material, especially in raster mode.<br />
METHODS: The effect of porosity and of the crater itself on the 11B signal and the isotopic ratio acquired by LA-MCICPMS<br />
in both raster and spot mode was studied. Characterization of the craters was then performed with an optical<br />
profilometer to determine their shapes and depths. Surface state effects were examined by analyzing the isotopic<br />
fractionation of boron in silicate (NIST-SRM 612 and 610 standards) and in carbonate (corals).<br />
RESULTS: Surface irregularities led to a considerable loss of signal when the crater depth exceeded 20 μm. The stability<br />
and precision were degraded when ablation occurred in a deep cavity. The effect of laser focusing and of blank correction<br />
was also highlighted and our observations indicate that the accuracy of the boron isotopic ratio does not depend on the<br />
shape of the surface. After validation of the analytical protocol for boron isotopes, a raster application on a Porites coral,<br />
which grew for 18 months in an aquarium after field sampling, was carried out.<br />
CONCLUSIONS: This original LA-MC-ICPMS study revealed a well-marked boron isotope ratio temporal variability,<br />
probably related to growth rate and density changes, irrespective of the pH of the surrounding seawater.
