Ulrike Weis

In situ Sr isotopic analysis of low Sr silicates using LA-ICP-MS

We have developed a LA-ICP-MS technique for the in situ determination of 87Sr/86Sr in silicates having low Sr concentrations (30–400 µg g−1). Because of its high sensitivity we used a single collector sector-field ICP-MS, which was combined with a 193 nm Nd:YAG laser ablation system. Experiments were performed using the fast electrical scan mode (0.1 s per pass) of the mass spectrometer and a spot size of 50 µm. Various corrections were made for the determination of 87Sr/86Sr, such as dead time of the ion counting system, blank, isobaric interferences of Kr and Rb, mass discrimination for Sr and Rb. An external precision (1 SD) of the Sr isotope measurements of about 0.0002–0.0004 were achieved for samples having Sr of 100–400 µg g−1 and Rb/Sr < 0.1. Lower Sr concentrations (30–100 µg g−1) yielded higher SD values of up to about 0.0015. The LA-ICP-MS data for MPI-DING, USGS and NIST reference glasses agree within uncertainty limits with high-precision TIMS values. We have applied this technique for in situ isotopic analysis of 100–300 µm glassy melt inclusions from Hawaii and of small glass fragments of oceanic basalts.

In situ 230Th-232Th-234U-238U analysis of silicate glasses and carbonates using laser ablation single-collector sector-field ICP-MS

We present a novel method for the determination of Th and U isotopic ratios by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) applicable to samples with U concentrations as low as 0.4 µg g−1. While previous in situ U-series determinations have used multi-collector (MC) ICP-MS, we use a single-collector sector-field ICP-MS connected to a 213 nm Nd:YAG laser. Precision and accuracy were determined for different matrices: NIST SRM 612, MPI-DING (T1-G, and ATHO-G) and USGS (BHVO-2G, NKT-1G, and BCR-2G) reference glasses, 91500 zircon, and a travertine. We used a laser beam with a diameter of 110 µm and tracks up to 1800 µm long in order to obtain adequate precision on samples with the lowest abundances of 230Th and 234U. We applied corrections for background, tailing of the 232Th, 235U and 238U beams on 230Th and 234U, instrumental mass discrimination, and elemental sensitivity. Mass discrimination and relative elemental sensitivities for Th and U are similar for igneous rock glasses, zircon, and calcium carbonate under the experimental conditions used; synthetic, transparent NIST SRM 612, however, yields different values. Therefore, we used BCR-2G (Th/U = 3.507 ± 0.018 (1 SE)) for calibration of relative elemental sensitivities. The internal precision of three line scans obtained for 230Th/238U, 234U/238U, and 230Th/232Th ratios varies between 0.1 and 2.8% (relative standard error, 1 RSE), limited by counting statistics on the minor isotopes. For repeated measurements of BCR-2G and the travertine, we obtained activity ratios (±2SD) of (230Th/238U) = 1.000 ± 0.006 and (234U/238U) = 0.994 ± 0.014, and (230Th/238U) = 1.493 ± 0.032 and (234U/238U) = 1.369 ± 0.011, respectively, concordant within 2% of TIMS values. This consistency indicates that LA-ICP-SF-MS is a suitable method for U-series analyses. Regarding future studies, it also provides the advantage of combining U–Th isotope ratio determinations with additional multi-element and isotope analyses, respectively, during a single line scan.

Whole‐Ocean Changes in Silica and Ge/Si Ratios During the Last Deglacial Deduced From Long‐Lived Giant Glass Sponges

Silicon is a keystone nutrient in the ocean for understanding climate change because of the importance of Southern Ocean diatoms in taking up CO2 from the surface ocean-atmosphere system and sequestering carbon into the deep sea. Here we report on silicon isotopes and germanium-to-silicon ratios in giant glass spicules of deep-sea spongeMonorhaphis chuni over the past 17,000 years. In situ measurements of Si isotopes and Ge concentrations show systematic variations from rim to center of the cross sections. When calibrated against seawater concentrations using data frommodern spicule rims, sponge data indicate that dissolved silica concentrations in the deep Pacific were ~12% higher during the early deglacial. These deep Pacific Ocean data help to fill an important global gap in paleo-nutrient records. Either continental sources supplied more silica to the deglacial ocean and/or biogenic silica burial was lower, both of which may have affected atmospheric CO2.