The major element composition of 39 sectioned spherules (Table DR1) from Victoria Land

Transcription

1 DR0000 Folco, 0 DATA REPOSITORY Bulk chemistry The major element composition of sectioned spherules (Table DR) from Victoria Land Transantarctic Mountains (TAM) was obtained by averaging multiple electron microprobe analyses (EMPA). EMPA were carried out using a CAMECA SX0 electron microprobe at the IGG CNR in Padova. Running conditions were kv accelerating voltage, 0 na beam current. Counting time was s for Na, K and Si, and 0 s for the other analyzed elements. A μmdiameter defocused beam was employed to reduce migration of volatile elements. The manifacturer-supplied PAP procedure was employed for raw data reduction. Synthetic, mineral and glass standards were used for instrumental calibration. The trace element composition of sectioned particles was determined by averaging multiple laser ablation - inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses performed at the CNR Istituto di Geoscienze e Georisorse in Pavia. The adopted instrument couples a Nd:YAG laser operating at nm with a quadrupole ICP-MS (Drc-e, Perkin Elmer). Analyses were carried out with a spot 0 μm in diameter and NIST SRM 0 and Si as external standards (see also Tiepolo et al., 00). 0 0 Ar- Ar dating Analytical methods. 0 Ar- Ar dating experiments were carried out on a selection of TAM spherules from three different sample sites (Frontier Mountain, Miller Butte and Timber Peak). Thirty-six of the freshest (i.e., devoid of weathering products) spherules in the 00 to 00 μm diameter range were chosen under the stereomicroscope and the scanning electron microscope. The aim of the experiments was twofold: i) to further check that the studied spherules belong to

2 DR0000 Folco, 0 0 one single microtektite population; ii) to date their formation event. Spherules were washed with methanol and rinsed many times in an ultrasonic bath always using distilled water. The sample was irradiated for hours in the core of the TRIGA reactor, Pavia University, with FCT biotite as flux monitor (see Laurenzi et al., 00 for more details). 0 Ar- Ar analyses were performed at the Ar Geochronology Laboratory, IGG, CNR, Pisa. Laser 0 Ar- Ar step-heating and total fusion analyses were performed with a Quanta System diode pumped Nd-YAG laser, operated in continuous mode at 0 nm. Evolved gas was purified using two AP-0 SAES getters operated at about 00 and 00 C. Argon was measured with a MAP (Mass Analyzer Products) -0 mass spectrometer, in static mode, using a Balzer SEV- multiplier as collector. Raw data were corrected for blanks, mass discrimination, and nuclear interferences through the ArArCALC software (Koppers, 00). Ages were calculated using the Isoplot.0 software (Ludwig, 00). Results. We first performed laser 0 Ar- Ar single-spherule total fusion (TF) dating on spherules (Table DR). These analyses provide apparent ages, calculated assuming an initial atmospheric 0 Ar/ Ar ratio, ranging from. to. Ma, with two distinctly older ages of and Ma. A preliminary age estimate for the TAM microtektites (based on spherules) published in abstract form (Folco et al., 00) proved to be erroneous because not corrected for the excess Ar subsequently revealed by the improved data set. The K/Ca ratio of the spherules, calculated from the K-derived Ar and the Ca-derived Ar, varies from 0. to, wider but consistent with electron microprobe data (0.0-0.). K/Ca ratios define an inverse correlation with apparent ages. The radiogenic 0 Ar yields (corresponding to the ratio between radiogenic 0 Ar and total 0 Ar) display narrow variations, with an average percentage value of. ±. This strengthens the argument that the studied spherules belong to a single population as hypothesized previously on the basis of geochemical data. In the 0 Ar/ Ar vs. Ar/ Ar isotope correlation diagram, data points define a rough linear correlation (MSWD = 0.) with an isochron age of. ±.

3 DR0000 Folco, 0 0 Ma and a trapped initial 0 Ar/ Ar = 0 ±. The difference between the apparent age from the TF experiments and the isochron age value is due to the automatic correction for the trapped component that the isochron calculation involves. In this paper we will use the normal isochron approach in the place of the more common inverse isochron approach in order to give a more immediate vision of the 0 Ar/ Ar ratio. We then performed stepwise heating (SH) 0 Ar- Ar analysis (Table DR). In order to obtain a sufficiently high signal to background ratio, spherules (0 to 00 μm in diameter) were used in a single multi-grain laser incremental heating experiment (Table DR). As previously noticed (Laurenzi et al., 00), grains tend to join together well before final fusion, with the result that the numbers and dimensions of the analysed objects change continuously during the step-heating experiment. The age spectrum (Fig. DRa) shows: i) high apparent ages for low temperature steps; ii) an apparent age plateau (.0 ±. Ma) that comprises four concordant incremental heating steps totalling % of the whole amount of Ar gas released. The integrated age,.00 ± 0. Ma, falls in the range of the total fusion apparent ages of the individual spherules. The K/Ca ratio (weighted mean = 0. ± 0.0; Fig. DRa) is consistent with the average obtained from the TF analyses and suggests that the K carrier is solely the glass, consistently with the observation of the total lack of weathering minerals and mineral inclusions in the analyzed spherules. Isochrons with a statistically acceptable MSWD (~ ) can be calculated using the plateau points only or by iteratively adding the other contiguous steps one at the time, provided that the mathematical solutions gives good statistics. A more rigorous approach involves the use of the Plateau-Isochron procedure to perform an isochron regression on sequential steps containing at least three steps and 0% of Ar release (see Ludwig, 00, for more details). In practice, the Plateau-Isochron age is the isochron age defined by the set of contiguous steps with the greatest amount of argon that defines a true isochron. The isochron

4 DR0000 Folco, 0 regression accepted out of 0 steps, including. % of the released Ar, with a result of 0. ± 0. Ma (MSWD = 0.; probability = 0., initial 0 Ar/ Ar = ± ) (Fig. DRb). Both TF and SH experiments give poorly determined age values (errors exceeding 00%) and initial 0 Ar/ Ar higher than the atmospheric ratio (.), evidencing the presence of 0 Ar not derived from in-situ decay of 0 K. Both experiments display a very narrow interval of variation of the 0 Ar/ Ar ratios: ± (standard deviation) for the TF, and ± for the SH. These values are slightly above, but within error, of the trapped components obtained from the isochron diagrams, evidencing that the amount of radiogenic 0 Ar formed by in-situ decay of 0 K was too small to be detected with such a high background. The apparent age variations displayed by the TF data might in part be due to the difference in the K content of the spherules. Despite the poorly resolved age values, however, the whole data set indicates that the Quaternary age of TAM microtektites is plausible. As the spherules are totally glassy, two processes might be responsible for the initial radiogenic trapped component: the decrepitation of fluid inclusions, as microbubbles were indeed observed under the stereomicroscope in some of the analysed spherules, and/or Ar dissolved into the glass, due to incomplete degassing of the melt formed by the impact. The last process might be further affected by the correlation between SiO content, that shows variability in the analysed spherules, and Ar solubility in melts (see Kelley, 00 and references therein). 0 REFERENCES CITED Folco, L., Rochette, P., Perchiazzi, N., D Orazio, M., Laurenzi, M. A., and Tiepolo, M., 00, Microtektites from the Transantarctic Mountains: Abstracts, Meteoritics and Planetary Science, 00, v. (Suppl.), p. A0.

5 DR0000 Folco, 0 0 Glass, B. P., Huber, H., and Koeberl, C., 00, Geochemistry of Cenozoic microtektites and clinopyroxene-bearing spherules: Geochimca et Cosmochimica Acta, v., p. -00, doi: 0.0/j.gca Glass, B.P., and Koeberl C., 00, Australasian microtektites and associated impact ejecta in the South China Sea and the Middle Pleistocene supereruption of Toba: Meteoritics and Planetary Science, v., p. 0-. Kelley, S., 00, Excess argon in K-Ar and Ar-Ar geochronology: Chemical geology, v., p. -. Koeberl, C., Bottomley, R., Glass, B. Y., and Storzer, D.,, Geochemistry and age of Ivory Coast tektites and microtektites: Geochimica et Cosmochimica Acta, v., p. -. Koppers, A.A.P., 00, ArArCALC software for 0 Ar/ Ar age calculations. Computer and Geosciences, v., p. 0-. Laurenzi, M. A., Balestrieri, M. L., Bigazzi, G., Hadler Neto, J. C., Junes, P. J., Norelli, P., Oddone, M., Osorio Araya, A. M., and Viramonte, J.G., 00, New constraints on ages of glasses proposed as reference materials for fission-track dating: Geostandards and Geoanalytical Research, v., p. 0-. Laurenzi, M. A., Bigazzi G., Balestrieri M. L., and Bouška, V., 00, 0 Ar/ Ar laser probe dating of the Central European tektite-producing impact event: Meteoritics and Planetary Science, v., p. -. Ludwig, K. R., 00, User s manual for Isoplot.00: Berkeley Geochronology Center, Special Publication No., p. 0. Tiepolo, M., Bottazzi, P., Palenzona, M., and Vannucci, R., 00, A laser probe coupled with ICP Double-focusing sector-field mass spectrometer for in situ analysis of geological

6 DR0000 Folco, samples and U-Pb dating of zircon: Canadian Mineralogist, v., p. -, doi: 0./gscanmin...

7 DR0000 Folco, Table DR. Bulk chemistry of TAM microtektites by means of EMP (major elements) and LA- ICP-MS (trace elements) analyses. Data for Australasian microtekties (normal type) available from the literature (Glass t al., 00) are shown for comparison. TAM (this work) Australasian (normal) (Glass et al., 00) Average Silica-poor Silica-rich Average Range #.-0 #.-0 Major elements (wt %) SiO. ±...0. ± TiO 0. ± ± Al O. ±.... ± FeO.0 ± ± MnO 0.0 ± MgO. ± ± CaO. ± ±.. -. Na O 0. ± ± K O 0. ± ± Total. ± 0... Trace elements (μg g - ) Li ± Sc ±... ±.. -. V ± Cr 0 ± ± 0-0 Co ±.0. ±. -. Rb ± ± - Sr 0 ± 00 ± 0 - Y ±..0 Zr 0 ± ± - 0 Nb 0 ±..0 Cs. ± ± Ba ± 0 ± - La ±. 0.. ± Ce 0 ± 0. ±. - Pr ± 0..0 Nd ±... ± Sm ±.0.. ± Eu. ± ± Gd.0 ± ± Tb.0 ± ± Dy. ±.0.. Ho. ± Er. ± Tm 0. ± ± Yb. ± ± Lu 0. ± ± Hf. ± ± Ta. ± ± Pb 0.0 ± Th ±.0..0 ±.. -. U 0. ± ± Average composition and s. d. (σ) calculated on the basis of the bulk composition of and individual particles for major and trace elements, respectively. Bulk composition of Si-poor and Si-rich representative particles #--0 and #.-0. All iron given as FeO.

8 DR0000 Folco, Table DR. Argon isotopic analytical data for TAM microtektites. Argon isotopes are in moles. J= ± Laser total fusion analyses #ID Ar(a) Ar(Ca) Ar(Cl) Ar(K) 0 Ar(r) Apparent age ± σ 0 Ar(r) (Ma) (%) K/Ca ± σ #.E-.E-.E-.E-.E-. ± ± 0.0 #.0E-.0E-.E-.E-.E-. ±.. 0. ± 0.0 #.0E-.0E-.0E-.E-.E-. ± ± 0.0 #.E-.E- b.d.l..0e-.e-.0 ± ± 0.0 #.E-.E-.E-.E-.E-. ± ± 0.00 #.E-.E-.0E-.E-.0E-. ± ± 0.0 #.E-.E-.E-.E-.E-. ± ± 0.0 #.E-.E-.E-.0E-.0E-. ± ± 0.0 #.E-.E-.E-.E-.E-. ± ± 0.0 #0.0E-.E-.0E-.E-.0E-. ±.. 0. ± 0.0 #.E-.E-.E-.E-.0E-. ±.. 0. ± 0.00 #.E-.E-.E-.E-.E-. ± ± 0. #.E-.E-.0E-.E-.E-.0 ± ± 0.0 #.E-.E-.0E-.0E-.E-. ± ± 0. #.E-.E-.E-.E-.E-. ± ± 0. #.0E-.E-.E-.E-.E-. ± ± 0.0 #.E-.E-.0E-.E-.E-. ± ± 0. #.0E-.E-.E-.0E-.E-. ± ± 0.0 #.E-.E-.E-.E-.E-. ± ± 0.0 #0.E-.E-.0E-.E-.E-. ± ± 0. #.E-.E-.E-.0E-.E-. ±.. 0. ± 0. #.E-.0E-.E-.E-.E-.0 ± ± 0.0 #.E-.0E-.E-.0E-.E-.0 ± ± 0.00 #.E-.E-.0E-.E-.E ±.. 0. ± 0.0 #.E-.E-.E-.E-.E-. ± ± 0. Laser step-heating experiment Laser Ar(K) Apparent Age ± σ 0 Ar(r) Intensity Ar(a) Ar(Ca) Ar(Cl) Ar(K) Ar(r) (%) (Ma) (%) (W) K/Ca ± σ..e-.0e-.0e-.e-.e-.. ± ± 0.0..E-.E-.E-.0E-.E-.0. ± ± 0.0..E-.E-.E-.0E-.E-.. ±.. 0. ± 0..0.E-.E-.E-.E-.E-.0. ± ±.0.0.E-.E-.E-.E-.0E ± ± 0..0.E-.E- b.d.l..e-.e-.. ±.. 0. ± 0..0.E-.E-.0E-.0E-.E-.. ±.. 0. ± 0..0.E-.E-.E-.E-.E-.. ±.. 0. ± 0..0.E-.E-.0E-.E-.0E-.0.0 ± ± 0.0 fuse.0e-.e-.0e-.e-.e-.. ± ± 0.0 Column headings are as follows: Ar (a) = atmospheric Ar; Ar(Ca) = Ca-derived Ar; Ar (Cl) = Cl-derived Ar; Ar (K) = K-derived Ar; 0 Ar(r) = radiogenic 0 Ar; 0 Ar(r) (%) = ratio (%) of radiogenic 0 Ar over total 0 Ar. The correction factors for reactor induced interfering reactions were: Ar/ Ar (Ca) = ± ; Ar/ Ar (Ca) = ± ; 0 Ar/ Ar (K) = 0.00 ±

9 DR0000 Folco, Laser total fusion analyses are divided into two sets by a blank row, and each set of data is listed in order of increasing ages. The set # to # was analysed before the failure of the mass spectrometer filament. The set # to # and the step heating experiment were performed five months later, after replacing the filament.

10 DR0000 Folco, 0 0 Fig. DR. Trace element compositional diagrams for TAM microtektites relative to literature data (Koeberl et al., ; Glass et al., 00) for Ivory Coast, Australasian, and North American microtektites. TAM microtektites show a geochemical affinity to Australasian microtektites. In the refractory element ratio diagram (A), TAM microtektites plot within the compositional field of Australasian and North American microtektites, whereas they distinctly overlap with Australasian microtektites at the high end of a common Zr/Rb versus Th/Cs volatilization trend (B).

11 DR0000 Folco, Fig. DR. Multi-grain (n=) 0 Ar- Ar step heating experiment of a Transantarctic Mountain microtektite sample. A: Apparent age spectrum with K/Ca ratio. B: Isochron plot; the nine data points used for age calculation are shaded.

12 DR0000 Folco, Fig. DR. Map showing the TAM microtektite find location in Victoria Land, Antarctica, relative to the Australasian tektite strewn field (dashed line; modified after Glass and Koeberl, 00) which is outlined by tektite finds on land and microtektites finds in cored deep-sea sediments. The square, circle and triangle are the locations of the hypothetical source crater of the Australasian tektites proposed by Glass and Pizzuto (), Prasad et al. () and Glass and Koeberl (00), respectively.