Download Auxiliary Material for Constraints on the geological evolution of the

Auxiliary Material for
Constraints on the geological evolution of the Amerasia Basin, Arctic Ocean.
Wilfried Jokat
Michele Ickrath
John O’Connor
(Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany)
Geophysical Research Letters, 2013GL057275, 2013
Introduction
This data set contains the 40Ar/39Ar analytical data and isotopic age calculation for the rock sample from the
central Alpha Ridge. The 40Ar/39Ar age reported here (“ts01.pdf”) meets the following acceptability criteria and
thresholds widely used in deciding whether age plateaus are accepted as reliable (Fleck et al., 1977, Lanphere
and Dalrymple, 1978, Dalrymple et al., 1980; Pringle, 1993): (a) a well-defined high-temperature age spectrum
plateau is created by three or more concordant (within 2σ), contiguous steps representing at least 50% of the 39Ar
released, (b) a well-defined isochron exists for the plateau points, i.e., the mean squares of weighted deviates
(MSWD) are not greater than the cut-off value of 2.5, and (c) the 40Ar/36Ar intercepts found by regression
analysis are not significantly different from the atmospheric level of 295.5, i.e., the plateau and inverse
(isochron) ages are concordant. The 40Ar/39Ar age data reported in this study have been calculated using
ArArCALC v2.5. (Koppers 2002) and the ArArCALC data are provided here in and 4 different plots and 4
different tables: a summary of the incremental heating data is given in „ts02.pdf“, the age plateau diagram in
„fs01.pdf“, the K/Ca plateau diagram in „fs02.pdf“, the Normal Isochron Diagram in „fs03.pdf“, the Normal
Isochron Table in „ts03.pdf“, the Inverse Isochron Diagram in „fs04.pdf“, the Inverse Isochron table in
„ts04.pdf“ and the Relative Abundances table in „ts05.pdf“. The details of the ArArCALC tables and plots are
explained in full in the Auxiliary Material for Koppers et al. (2012) and adapted extracts are given below. A
TiO2-MnO-Na2O discrimination diagram for pyroxenes from Nisbet and Pearce (1977) is shown in „fs05.pdf“.
1. ts01.pdf Summary of 40Ar/39Ar age calculation for Alpha Ridge sample PS051-041-1CC
2. ts02.pdf Incremental Heating summary table containing the most important data to calculate the ages
and to build the age and K/Ca spectra plots. For each incremental heating step it lists the 36Ar (in V,
from the atmospheric component), 37Ar (in V, from Calcium), 38Ar (in V, from Chlorine), 39Ar (in V,
from Potassium), 40Ar (in V, radiogenic Argon), the calculated age (plus 2σ uncertainty), the percent
radiogenic 40Ar as recorded in that step, the percent 39Ar (from Potassium) in that step relative to the
total amount of 39Ar released in the experiment (a proxy for the step size), and the calculated K/Ca ratio
(plus 2σ uncertainty). In the first column the lab analysis number is listed and in the second column the
power (in Watts) for the CO2 laser used to heat up the sample material. The blue checkmarks indicate
the steps included in the calculations of the plateau age. The results section provides the weighted
means for the age plateau in terms of the 40Ar/39Ar ratio, its related age, and the K/Ca ratio. The mean
square of weighted deviations (MSWD) and its probability (in %) are given, as well as the total width
of the age plateau (in %) and the total number of steps included in the age plateau. The results give an
age plateau if the MSWD (dependent on n) is smaller than the shown 2σ confidence limit, but if the
MSWD is larger, the results are denoted as an error plateau. Finally, the total fusion age is reported,
computationally combining all steps (including the steps excluded in the age plateau calculation) into a
single fraction (summations are reported below the upper table) and then calculating the 40Ar/39Ar ratio
and its related age. The same is done for the K/Ca ratio. The uncertainties on the ages given (directly
next to the ages) are internal errors, including the error on the J-value. Analytical errors have these Jvalue uncertainties subtracted, whereas external errors include uncertainties on the total decay constant.
3. fs01.pdf Age Plateau plot showing the age spectra resulting from the incremental heating with the
black horizontal bar indicating the location of the age plateau. All errors shown are 2σ.
4. fs02.pdf K/Ca Plateau plot showing the K/Ca spectra resulting from the incremental heating with the
black horizontal bar indicating the location of the age plateau. All errors shown are 2σ.
5. fs03.pdf Normal Isochron plot showing the resulting isochron (pink line) with green squares indicating
the points included in the calculation. Blue squares are the excluded data points, while the red circle
denotes the location of the total fusion point, which should fall on the isochron if the rock or mineral
has remained a closed-system since its eruption and/or crystallization. The black line is the reference
line, starting with a 40Ar/36Ar intercept of 295.5 and assuming the plateau age to set its slope. All errors
shown are 2σ.
6. ts03.pdf Normal Isochron summary table listing the 39Ar(k)/36Ar(a) and 40Ar(a+r)/36Ar(a) ratios (plus
2σ uncertainties) used for the normal isochron age calculation, as well as the correlation coefficient
(r.i.) that is used in the same calculation and denotes the amount of correlation between the errors in
both ratios (with numbers approaching “1” indicating strongly correlated errors). In the first column the
lab analysis number is listed and in the second column the power (in Watts) for the CO2 laser used to
7.
8.
9.
10.
heat up the sample material. The blue checkmarks indicate the steps included in the calculations of the
isochron age. The results section lists the 40Ar(r)/39Ar(k) ratio and its related age as derived from the
slope of the isochron, as well as the intercept 40Ar(a)/36Ar(a) value, which provides an estimate of the
composition of the trapped (initial) argon in the geological material. The statistics section lists
computational information about the isochron calculations. The mean square of weighted deviations
(MSWD) and its probability (in %) are given. The results give an isochron if the MSWD (dependent on
n) is smaller than the shown 2σ confidence limit, but if the MSWD is larger, the results are denoted as
an error chron. The uncertainties on the ages given (directly next to the ages) are internal errors,
including the error on the J-value. Analytical errors have these J-value uncertainties subtracted, whereas
external errors include uncertainties on the total decay constant. By definition the data points selected
for the normal isochron are the same as those defining the age plateau, allowing for a direct comparison
between the plateau and isochron age calculations.
fs04.pdf Inverse Isochron plot showing the resulting isochron (pink line) with green squares indicating
the points included in the calculation. Blue squares are the excluded data points, while the red circle
denotes the location of the total fusion point, which should fall on the isochron if the rock or mineral
has remained a closed-system since its eruption and/or crystallization. The black line is the reference
line, starting with a (inverse) 40Ar/36Ar intercept of 295.5 on the 36Ar/40Ar axis and assuming the plateau
age to set the other intercept on the 39Ar/40Ar
ts04.pdf Inverse Isochron summary table listing the 39Ar(k)/40Ar(a+r) and 36Ar(a)/40Ar(a+r) ratios (plus
2σ uncertainties) used for the inverse isochron age calculation, as well as the correlation coefficient
(r.i.) that is used in the same calculation and denote the amount of correlation between the errors in both
ratios (with numbers approaching “0” indicating no correlated errors). In the first column the lab
analysis number is listed and in the second column the power (in Watts) for the CO 2 laser used to heat
up the sample material. The blue checkmarks indicate the steps included in the calculations of the
isochron age. The results section lists the 40Ar(r)/39Ar(k) ratio and its related age as derived from the
intercept of the isochron on the 39Ar(k)/40Ar(a+r) X-axis, as well as the (inverse) intercept
40
Ar(a)/36Ar(a) value on the Y-axis, which provides an estimate of the composition of the trapped
(initial) argon in the geological material. The statistics section lists computational information about the
isochron calculations, including the spreading factor (in %) of the data along the isochron (Jourdan et
al. 2009). The mean square of weighted deviations (MSWD) and its probability (in %) are given. The
results give an isochron if the MSWD (dependent on n) is smaller than the shown 2σ confidence limit,
but if the MSWD is larger, the results are denoted as an error chron. The uncertainties on the ages given
(directly next to the ages) are internal errors, including the error on the J-value. Analytical errors have
these J-value uncertainties subtracted, whereas external errors include uncertainties on the total decay
constant. By definition the data points selected for the inverse isochron are the same as those defining
the age plateau, allowing for a direct comparison between the plateau and isochron age calculations.
ts05.pdf Relative Abundances table containing the relative abundances of 36Ar, 37Ar, 38Ar, 39Ar and
40
Ar (plus their %1σ uncertainties) as measured in each incremental heating step. These relative
abundances (in V) are corrected for blanks, mass fractionation and radioactive decay (for 37Ar and
39Ar). The summations of each of these relative abundances are reported below the upper table and used
for the total fusion age and K/Ca ratio calculations. Furthermore, this table includes the calculated age
(plus 2σ uncertainty), the percent radiogenic 40Ar as recorded in that step, the percent 39Ar (from
Potassium) in that step relative to the total amount of 39Ar released in the experiment (a proxy for the
step size), and the calculated K/Ca ratio (plus 2σ uncertainty). In the first column the lab analysis
number is listed and in the second column the power (in Watts) for the CO2 laser used to heat up the
sample material. The blue checkmarks indicate the steps included in the calculations of the plateau and
isochron ages. The results section simply repeats the information from the age plateau, normal isochron
and inverse isochron, as described above. Finally, this table lists important metadata on the analysis
(IGSN number, preferred age, age classification, experiment type, extraction method, heating and
vacuum isolation time, instrument, lithology and lat-lon) and the constants used in the age calculations
(decay constants, production ratios, equation sets used).
fs05.pdf TiO2-MnO-Na2O discrimination diagramm for pyroxenes from Nisbet and Pearce (1977).
CESAR: Sample recovered from the Alpha Ridge (Jackson et al. 1985) during the Canadian CESAR
expedition. Abbreviations are:
A: Volcanic arc basalt (VAB) oceanic and continental,
B: Ocean floor basalt (OFB),
C: within plate alkali basalt (WPA),
D: all types,
E: Within plate tholeiites (WPT) + WPA + VAB, F: VAB + WPA.
References
Dalrymple, G.B., Lanphere, M.A. and Clague D.A. (1980). Conventional and 40Ar/39Ar ages of volcanic rocks
from Ojin (Site 430), Nintoku (Site 432), and Suiko (Site 433) Seamounts and the chronology of volcanic
propagation along the Hawaiian–Emperor chain, Init. Rep. Deep Sea Drill. Proj. 55, pp. 659–676.
Fleck, R.J., Sutter, J.F. and Elliot, D.H. (1977). Interpretation of discordant 40Ar/39Ar age spectra of Mesozoic
tholeiites from Antarctica, Geochim. Cosmochim. Acta 41, pp. 15–32.
Jackson H. R., Mudie P. J., and Blasco S. M. (1985) Initial geological report on CESAR - the Canadian
expedition to study the Alpha Ridge, Arctic Ocean, Geological Survey of Canada.
Nisbet E. G. and Pearce J. A. (1977) Clinopyroxene Composition in Mafic Lavas from Different Tectonic
Settings. Contrib. Mineral. Petrol. 63, 149-160.
Koppers, A. A. P. (2002), ArArCALC – software for 40Ar/39Ar age calculations, Computers Geosciences, 5,
605–619.
Koppers, A. A. P., T. Yamazaki, J. Geldmacher, J. S. Gee, N. Pressling, H. Hoshi, et al. (2012). Limited
latitudinal mantle plume motion for the Louisville hotspot, Nature Geoscience, 5, 911-917, doi:
10.1038/ngeo1638.
Lanphere, M.A. and Dalrymple, G.B. (1978). The use of 40Ar/39Ar data in evaluation of disturbed K–Ar systems,
Short Papers 4th Int. Conf. Geochronol. Cosmochronol. Isot. Geol. 78-701, pp. 241–243.
Pringle, M.S. (1993). The Mesozoic Pacific: geology, tectonics, and volcanism, Geophys. Monogr. 77, pp. 187–
215.