Major element data, 40Ar/39Ar step-heating and step-crushing data for anorthoclase megacrysts from the Newer Volcanic Province, south-eastern Australia

We provide the dataset associated with the research article “40Ar/39Ar ages of alkali feldspar xenocrysts constrain the timing of intraplate basaltic volcanism” Matchan et al. [1]. This dataset contains major element data for 15 large anorthoclase xenocrysts (‘megacrysts’) collected from six Pleistocene eruption centres (Mount Leura, Mount Shadwell, Mount Noorat, Mount Franklin, Lake Keilambete and The Anakies (East Cone)) in the basaltic Newer Volcanic Province of south-eastern Australia. It also contains multi-collector (Argus VI) 40Ar/39Ar step-heating for 13 of these anorthoclase megacrysts. 40Ar/39Ar vacuo step-crushing experiment data is also provided for three of these megacrysts.


a b s t r a c t
We provide the dataset associated with the research article " 40 Ar/ 39 Ar ages of alkali feldspar xenocrysts constrain the timing of intraplate basaltic volcanism" Matchan et al. [1]. This dataset contains major element data for 15 large anorthoclase xenocrysts ('megacrysts') collected from six Pleistocene eruption centres (Mount Leura, Mount Shadwell, Mount Noorat, Mount Franklin, Lake Keilambete and The Anakies (East Cone)) in the basaltic Newer Volcanic Province of south-eastern Australia. It also contains multi-collector (Argus VI) 40 Ar/ 39 Ar step-heating for 13 of these anorthoclase megacrysts. 40 Ar/ 39 Ar vacuo stepcrushing experiment data is also provided for three of these megacrysts.
& 2018 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Feldspar megacryst fragments free from obvious alteration and inclusions were mounted in resin blocks, polished and carbon-coated for electron microprobe analysis Hand-picked feldspar chips were cleaned ultrasonically in demineralised water, followed by acetone, prior to neutron irradiation and 40 Ar/ 39 Ar step-heating/step-crushing analyses.

Experimental features
Operating conditions used for electron microprobe analysis were: accelerating voltage of 15 kV, beam current of 10 nA, beam diameter of 10 μm; counting times of 20-40 s on peak positions and 10 to 40 s on two background positions located on either side of the peak position. A defocused electron beam was used to avoid loss of volatile species. For 40 Ar/ 39 Ar step-heating analyses, feldspar megacryst aliquots were stepheated in vacuo using a CO 2 laser with resultant gas cleaned prior to isotopic measurement on an Argus VI multi-collector mass spectrometer.
In vacuo, manual step-crushing experiments were conducted using Nupro s valves, with resultant gas cleaned prior to isotopic measurement on an Argus VI multi-collector mass spectrometer.

Data source location
Samples were analysed at the University of Melbourne, Parkville, Victoria, Australia.

Value of the data
The 40 Ar/ 39 Ar age data for feldspar megacrysts could be compared with basalt groundmass 40 Ar/ 39 Ar age data for the same eruption centres to evaluate time elapsed between feldspar crystallization and entrainment by basaltic melt.
Future 40 Ar/ 39 Ar dating and major element compositional studies on feldspar megacrysts (and potentially other K-bearing phases) from the Newer Volcanic province could provide insights into timeframes of megacryst formation by evaluating how many generations of megacrysts are represented at a single eruption centre.
The 40 Ar/ 39 Ar step-heating data presented here could be used in future studies evaluating mechanisms for argon isotopic disturbance (e.g. recoil, mass fractionation) in high-temperature feldspars. Table A1 contains electron microprobe data for sixteen feldspar megacrysts. Table A2 contains processed data from 40 Ar/ 39 Ar step-heating and step-crushing experiments on feldspar megacrysts.

Data
Data have been corrected for mass spectrometer backgrounds, discrimination, radioactive decay and interference corrections. Table A3 contains processed data from 40 Ar/ 39 Ar step-heating and stepcrushing experiments on feldspar megacrysts. Data have been corrected for mass spectrometer backgrounds, discrimination, and radioactive decay only. Table A4 contains processed data from 40 Ar/ 39 Ar fusion analyses of Alder Creek Rhyolite sanidine (double-grain aliquots). Data have been corrected for mass spectrometer backgrounds, discrimination, radioactive decay and interference corrections. Fig. A1 shows individual 40 Ar/ 39 Ar age spectra and inverse isochron diagrams for all analysed feldspar megacrysts. Hand-picked feldspar chips were cleaned ultrasonically in demineralised water, followed by acetone. Samples were then weighed and loaded into aluminium foil packets, placed in quartz tubes (UM#50, UM#51 and UM#70) along with the flux monitor Alder Creek Rhyolite sanidine (ACRs: 1.18144 7 0.00068 Ma [2]) and irradiated in the CLICIT facility at the Oregon State University TRIGA reactor (UM#50 -3 MWh; UM#51 -10 MWh; UM#70 -0.75 MWh).

Mineral chemistry
Electron microprobe analysis of feldspar fragments was undertaken using a Cameca SX-50 electron microprobe at the University of Melbourne. This instrument is equipped with four vertical wavelength dispersive spectrometers and operating conditions used were: accelerating voltage of 15 kV, beam current of 10 nA, beam diameter of 10 μm; counting times of 20-40 s on peak positions and 10-40 s on two background positions located on either side of the peak position; detection limits of r 400 ppm for all elements except Ba ( $ 800 ppm). Analyses were conducted using a defocused electron beam to avoid loss of volatile species (e.g. F, Cl and K). Elemental data, relative to natural and synthetic mineral and elemental standards, are reported in Table A1.

40
Ar/ 39 Ar analyses were undertaken in the Noble Gas Geochronology Laboratory at the University of Melbourne, using a multi-collector Thermo Fisher Scientific Argus VI mass spectrometer linked to a stainless steel gas extraction/purification line and a Photon Machines Fusions 10.6 CO 2 laser system [2,3]. 36 Ar was measured using a Compact Discrete Dynode (CDD) detector, with the remaining isotopes measured on Faraday detectors with low-noise amplifiers (1 Â 10 12 Ω resistors, with the exception of the more recent UM#70 analyses, where the 39 Ar collector was equipped with a 1 Â 10 13 Ω resistor).
Following neutron irradiation and cooling, separate aliquants of the samples were prepared for in vacuo step-crushing and detailed step-heating experiments. This approach, as opposed to two/three step-heating or direct fusion experiments, allowed for investigation of excess argon in the samples.
Step-crushing experiments were undertaken on several megacrysts to directly evaluate the isotopic composition of argon trapped in defects (e.g. fluid inclusions) in the following samples: AN1 (Mount Shadwell), AN2 (Mount Noorat), M51449 (Lake Keilambete), and M21210 (Mount Franklin). An aliquant size of either 60 mg or 120 mg was loaded across a series of four modified Nupro s valves ( $ 30 mg per valve) for in vacuo step-crushing experiments, following procedures described by Kendrick et al. (2006). Crushed samples were subsequently recovered for 40 Ar/ 39 Ar laser step-heating analysis.
Uncrushed and crushed sample aliquants (typically 70-100 mg) were loaded into custom-made copper sample holders, covered with a ZnS-glass cover slip, and placed into a stainless steel sample chamber. The sample chamber and extraction line were baked overnight at $ 120°C, and samples were outgassed at low laser-power using the 6 mm homogenised beam to remove any adsorbed atmospheric argon from grain surfaces. Air aliquots from an automated pipette system were analysed prior to sample analyses to monitor mass discrimination and detector bias. Samples were heated using the homogenized 6 mm beam. Taking into account the different low-temperature outgassing procedures, step-heating experiments (7-10 steps) were conducted over a heating interval of either 1.8-5.7 W (8-30% laser power; UM#50,51 samples), or 0.45-6.4 W (2-35% laser power; UM#70 samples). Gas introduced into the Argus VI mass spectrometer was equilibrated for 20 s, before peak centring on mass 40 (H1) and multi-collector analysis of the five argon isotopes. Peak signals were collected for a period of 300 s and regressed to the time of gas inlet. Line blanks, measured between blocks of 3-4 sample analyses, were typically 1-2 fA for 40 Ar, compared to typical sample signal sizes of 4100 fA. Line blanks were subtracted from succeeding sample results. Alder Creek Rhyolite (ACR) sanidine grains were analysed on the same system. Grains were either directly fused (35% laser power) or heated in two steps (14-18% and 35% laser power), with gas cleanup and measurement as for samples (data in Table A4). The weighted mean 40 Ar/ 39 Ar result was used to calculate the J-value, assuming an ACR sanidine age of 1.18144 7 0.00068 Ma (95% CI) [2].
Apparent ages (Table A2) were calculated based on: (i) a default atmospheric composition (298.56 [5]); and (ii) the corresponding inverse isochron determined ( 40 Ar/ 36 Ar) i value (Fig. A1). In the latter case, uncertainties in ( 40 Ar/ 36 Ar) i ratios were included in reported age uncertainties. Age spectra and inverse isochron diagrams (Fig. A1) were generated using ISOPLOT/Ex.3.75 [6]. Plateau ages are defined as including at least 50% of the 39 Ar, distributed over a minimum of three contiguous steps and with 40 Ar*/ 39 Ar ratios within agreement of the mean at the 95% confidence level (e.g. [7]). Plateau ages were calculated only in cases where ( 40 Ar/ 36 Ar) i values were within error of the atmospheric value and, by convention, do not include the uncertainty in the air ratio (298.56 7 0.62 (0.21%), 2σ [5]).

Transparency document. Supporting information
Transparency data associated with this article can be found in the online version at https://doi.org/ 10.1016/j.dib.2018.06.080.