Macquarie Ridge Complex basalt 40Ar/39Ar geochronology data

Brief description

40Ar/39Ar geochronology data of basalt samples from the Macquarie Ridge Complex (MRC).

The MRC samples include basalts collected from Macquarie Island, the only exposed portion of the submarine Macquarie Ridge, by R. Merle and K. Evans during a field trip in November 2017, and dredge samples collected by the National Institute of Water and Atmospheric Research, New Zealand, during the TAN0803 voyage in 2008.

Analytical methods of the 40Ar/39Ar geochronology data:

Samples were crushed and minerals/groundmass were separated using a Frantz magnetic separator. Plagioclase, pyroxene, amphibole, sericite, and basaltic glass crystals and groundmass were separated from either the 125–212 μm or the 212–355 μm size fractions using a Frantz isodynamic magnetic separator. Minerals and groundmass were subsequently hand-picked grain-by-grain under a binocular stereomicroscope. Plagioclase and groundmass were further leached using diluted HF (2N) for 5 minutes and thoroughly rinsed in distilled water. Samples were loaded into several large wells of 1.9cm diameter and 0.3 cm depth aluminium discs. The discs were Cd-shielded to minimise undesirable nuclear interference re-actions and irradiated for 40 hours in the Oregon State University nuclear reactor (USA) in the central position. The samples were irradiated alongside FCs and GA1550 standards, for which ages of 28.294 ± 0.037 Ma and 99.738 ± 0.100 Ma were used, respectively.

The 40Ar/39Ar analyses were performed at the Western Australian Argon Isotope Facility at Curtin University. The samples were step-heated using a continuous 100 W PhotonMachine© CO2 (IR, 10.4 µm) laser fired on the crystals during 60 seconds. Each of the standard crystals was fused in a single step. The gas was purified in an extra low-volume stainless steel extraction line of 240cc and using one SAES AP10 and one GP50 getter. Ar isotopes were measured in static mode using a low volume (600 cc) ARGUS VI mass spectrometer from Thermofisher© set with a permanent resolution of ~200. Measurements were carried out in multi-collection mode using four faradays to measure mass 40 to 37 and a 0-background compact discrete dynode ion counter to measure mass 36. We measured the relative abundance of each mass simultaneously using 10 cycles of peak-hopping and 33 seconds of integration time for each mass. Detectors were calibrated to each other electronically and using air shot beam signals. The raw data were processed using the ArArCALC software.

The criteria for the determination of plateau are as follows: plateaus must include at least 70% of 39Ar released. The plateau should be distributed over a minimum of 3 consecutive steps agreeing at 95% confidence level and satisfying a probability of fit (P) of at least 0.05. Plateau ages are given at the 2σ level and are calculated using the mean of all the plateau steps, each weighted by the inverse variance of their individual analytical error. Uncertainties include analytical and J-value errors.