from three Quaternary basalts

Transcription

1 4 Ar/ 39 Ar Geochronology results from three Quaternary basalts y Richard P. sser MAY 1, 23 Prepared for r. Roy reckenridge Idaho Geological Survey University of Idaho PO ox Moscow, I NW MXIO GORONOLOGIAL RSAR LAORATORY (NMGRL) O - IRTORS R. MATTW T. IZLR R. WILLIAM. MINTOS LAORATORY TNIIANS LISA PTRS RIAR P. SSR Internal Report #: NMGRL-IR-287

2 Introduction Three basaltic rocks were submitted to the New Mexico Geochronology Research Laboratory for 4 Ar/ 3 9 Ar dating by r. Roy reckenridge of the Idaho Geological Survey. 4 Ar/ 39 Ar Analytical Methods and Results Groundmass concentrate was prepared from the three basalt samples (R-2-A, R-2-MA and R-2-WR). The groundmass concentrate samples were analyzed by the furnace incremental heating age spectrum 4 Ar/ 3 9 Ar method. Abbreviated analytical methods for the furnace sample is given in Table 1. The analytical data for the three groundmass concentrates is given in Table 2. igures 1-3 show the age spectrum and inverse isochron yielded by each of the groundmass concentrates. A summary plot of the ages yielded in this study is shown in igure 4. etails of the overall operation of the New Mexico Geochronology Research Laboratory are provided in the Appendix. Groundmass concentrate samples R-2-WR and R-2-MA yield age spectra that are very similar in appearance to one another. The age spectra for R-2-WR (igure 1) and R-2-MA (igure 2) are predominantly flat for the majority of the 3 9 Ar K released. owever, both spectra exhibit a minor increase in age at the temperature steps and then another increase in age at the 12 temperature steps. The radiogenic yields and K/a ratios for these two samples are very similar as well, ranging from % to about 12% radiogenic yield and from.5 to.5 for the K/a values. or both samples, radiogenic yields are generally highest for the intermediate temperature steps (~ ). K/a ratio decreases with increasing temperature. A weighted mean or plateau age assigned to the flattest portion of each spectrum (steps through G for both samples) yields apparent ages of 1.17±.21 Ma (45.4% of the cumulative 3 9 Ar K released; MSW=3.3) for R-2-WR and 1.45±.16 Ma (49.8% of the cumulative 3 9 Ar K released; MSW=1.53). In each case, the MSW values were greater than 1, so that the uncertainties were multiplied by the square root of the MSW according to the procedures set forth by Mahon (1996). The inverse isochron results for R-2-WR (age=1.6±.19 Ma; 4 Ar/ 3 6 Ar=296±4; MSW=3.8) and R-2-MA (age=1.25±.36 Ma; 4 Ar/ 3 6 Ar=298±4; MSW=3.3) are analytically indistinguishable from their respective plateau weighted mean ages and also yield trapped 4 Ar/ 3 6 Ar compositions equivalent to modern day atmosphere (295.5). The age spectrum for R-2-A (igure 3) is more precise than the spectra for R-2-WR and R-2-MA and does not exhibit the same increase in apparent age at the temperature steps. owever, there is an increase in age at the highest temperature steps. The radiogenic yields are not as high as those for the previous two samples, but are more consistent at about 9%. The K/a ratios vary from.9 to.69 and also trend to lower values as the heating temperatures increase. The

3 weighted mean of steps through G yields an apparent age of.515±.63 Ma (89.3% of the cumulative 3 9 Ar K released; MSW=1.29). The inverse isochron yields an apparent age of.45±.8 Ma with a 4 Ar/ 3 6 Ar ratio of 298±4 and a MSW of 2. iscussion All three of the R age spectra exhibit small increases in apparent ages at the highest temperatures of gas release. Specifically, the final 1 to 2 heating steps of each R age spectrum yield apparent ages slightly older than those ages comprising the weighted mean or flattest portion of the age spectrum. In addition, the R-2-WR and R-2-MA age spectra exhibit an increase in age at intermediate temperature steps. The anomalously old ages at the highest temperatures are likely caused by excess argon. The increase in age at the 75 temperature steps for R-2-WR and R- 2-MA may be caused by excess argon. xcess argon is non-atmospheric 4 Ar within a sample that is derived by a process other than the in situ radioactive decay of 4 K (Mcougall and arrison, 1999). Most commonly, excess argon refers to trapped 4 Ar/ 3 6 Ar compositions greater than (the present day 4 Ar/ 3 6 Ar composition). In the case of the R groundmass concentrates, small amounts of excess argon may have been incorporated into high temperature mafic mineral phases (e.g. pyroxene and/or olivine; the cover letter that accompanied the samples indicates an ophitic texture and consist of 1 mm augite grains enclosing.1 to.2 mm labradorite laths ) at elevated argon partial pressures (i.e. at depth or in a magma chamber). In some cases, an inverse isochron can be employed to test for trapped 4 Ar/ 3 6 Ar compositions greater than owever, the inverse isochrons for the three R groundmass concentrate samples yield trapped 4 Ar/ 3 6 Ar compositions statistically indistinguishable from An alternative explanation to excess argon for the increase in age at the 75 temperature steps (R-2-WR and R-2-MA) is alteration and argon loss causing anomalously young ages for the 7 and, to a lesser extent, the 75 temperature steps. The low radiogenic yields (<<12%) for R-2-WR and R-2-MA (and R-2-A) are indicative of alteration and/or hydration of groundmass phases, particularly glass. ydration and/or alteration can increase the quantity of atmospheric argon within a sample, partially or completely overwhelming the radiogenic argon. ydration and/or alteration may also cause potassium bearing phases to lose any in situ produced 4 Ar* relative to the parent 4 K, thereby lowering the 4 Ar/ 3 9 Ar ratio and apparent age. The 7 heating steps may have lost comparatively more 4 Ar*, thus decreasing their apparent ages. Generally, additional sample preparation and/or analytical manipulation are not effective for improving the quality of data from low radiogenic yield basalts. There does not appear to be any argon loss for the low temperature heating steps for the R-2-A groundmass concentrate, despite its low radiogenic yields. 2

4 Given the apparent combination of argon loss and excess argon for the R-2-WR and R- 2MA groundmass concentrate samples, the weighted mean of the flattest portion of each age spectra yields the most accurate age of each sample. or R-2-WR, the preferred age is the weighted mean of steps through G at 1.17±.21 Ma. or R-2-MA, the preferred age is the weighted mean of steps through G at 1.45±.16 Ma. ecause sample R-2-A yields a spectrum that is less disturbed than the other two samples (i.e. no low temperature argon loss), its preferred age of.515±.63 Ma is more precise. igure 4 shows a summary of the 4 Ar/ 3 9 Ar ages produced by this study plotted against the geomagnetic polarity time scale (GPTS) of Izett and Obradovich (1994). According to personal communications with r. Roy reckenridge, the stratigraphic relationships among the three R samples are unknown. owever, r. reckenridge does state that R-2-WR and R-2-MA have opposite magnetic polarities. ased on the 4 Ar/ 3 9 Ar apparent ages, it appears that R-2-MA should have yielded a reversed magnetic polarity as its age and error are entirely contained within the Matuyama reversed Subchron. In order to have normal polarity, R-2-WR must have been erupted during the Jaramillo Normal Subchron. The 4 Ar/ 3 9 Ar apparent age of R-2-A indicates that the sample was erupted during the runhes Normal hron. 3

5 References ited ande, S.., and Kent,.V., A New Geomagnetic Polarity Time Scale for the Late retaceous and enozoic. Journal of Geophysical Research, 97, 13,917-13,951. eino, A., and Potts, R., 199. Single-rystal 4 Ar/ 3 9 Ar dating of the Olorgesailie ormation, Southern Kenya Rift, J. Geophys. Res., 95, leck, R.J., Sutter, J.., and lliot,.., Interpretation of discordant 4 Ar/ 3 9 Ar age-spectra of Mesozoic tholeiites from Antarctica, Geochim. osmochim. Acta, 41, arrison, T.M., 1981, iffusion of 4 Ar in hornblende: ontributions to Mineralogy & Petrology, v. 78, p Izett, G.A., and Obradovich, J.., Ar/ 3 9 Ar age constraints for the Jaramillo Normal Subchron and the Matuyama-runhes geomagnetic boundary: Journal of Geophysical Research, v. 99, p Mahon, K.I., The New York regression: Application of an improved statistical method to geochemistry, International Geology Review, 38, Samson, S.., and, Alexander,.., Jr., alibration of the interlaboratory 4 Ar/ 3 9 Ar dating standard, Mmhb-1, hem. Geol., 66, Steiger, R.., and Jäger,., Subcommission on geochronology: onvention on the use of decay constants in geo- and cosmochronology. arth and Planet. Sci. Lett., 36, Taylor, J.R., An Introduction to rror Analysis: The Study of Uncertainties in Physical Measurements,. Univ. Sci. ooks, Mill Valley, alif., 27 p. Mcougall, I., and T. M. arrison, 1988, Geochronology and thermochronology by the 4 Ar/ 3 9 Ar method: Oxford Monographs on Geology and Geophysics, v. 9, p York,., Least squares fitting of a straight line with correlated errors, arth and Planet. Sci. Lett., 5,

6 Table 1. 4Ar/39Ar analytical methods used for the groundmass concentrate samples. Sample preparation and irradiation: Geological samples provided by r. Roy reckenridge. Groundmass concentrates were prepared using standard separation techniques (crushing, sieving, franzing and hand-picking). Samples were packaged and irradiated in machined Al discs for 1 hour in the -3 position, Texas A&M University Research Reactor. Neutron flux monitor ish anyon Tuff sanidine (-1). Assigned age = Ma (eino and Potts, 199) relative to Mmhb-1 at 52.4 Ma (Samson and Alexander, 1987). Instrumentation: Mass Analyzer Products mass spectrometer on line with automated all-metal extraction system. Samples step-heated in Mo double-vacuum resistance furnace. eating duration 7 minutes. Reactive gases removed by reaction with 3 SAS GP-5 getters, 2 operated at ~45 and 1 at 2, together with a W filiment operated at ~2. Analytical parameters: lectron multiplier sensitivity averaged 2.2x1-16 moles/pa. Total system blank and background for the furnace averaged 352, 16.5, 4., 4.4, 15.4 x 1-18 moles at masses 4, 39, 38, 37, and 36, respectively for temperatures <13. J-factors determined to a precision of ±.1% by O 2 laser-fusion of 4 single crystals from each of 3 radial positions around the irradiation tray. orrection factors for interfering nuclear reactions were determined using K-glass and a 2 and are as follows: ( 4 Ar/ 39 Ar) K =.2±.3; ( 36 Ar/ 37 Ar) a =.28±.1; and ( 39 Ar/ 37 Ar) a =.89±.3. Age calculations: Weighted mean age calculated by weighting each age analysis by the inverse of the variance. Weighted mean error calculated using the method of (Taylor, 1982). Total gas ages and errors calculated by weighting individual steps by the fraction of 39 Ar released. Isochron ages, 4 Ar/ 36 Ar i and MSW values calculated from regression results obtained by the methods of York (1969). ecay constants and isotopic abundances after Steiger and Jäger (1977). All final errors reported at ±2σ, unless otherwise noted.

7 Table 2. 4 Ar/ 39 Ar analytical data. I Power 4 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar 39 Ar K K/a 4 Ar* 39 Ar Age ±1σ (Watts) (x 1-3 ) (x 1-15 mol) (%) (%) (Ma) (Ma) R-2-WR, mg groundmass concentrate, J=.119±.1%, =1.623±.131, NM-153, Lab#= A G I Integrated age ± 2σ n= K2O=.5 % Plateau ± 2s steps -G n=4 MSW= Isochron±2σ n=6 MSW=3.8 4 Ar/ 36 Ar=296± R-2-MA, mg groundmass concentrate, J=.1111±.1%, =1.623±.131, NM-153, Lab#= A G I Integrated age ± 2σ n= K2O=.5 % Plateau ± 2s steps -G n=4 MSW= Isochron±2σ n=7 MSW=3.3 4 Ar/ 36 Ar=298± R-2-A, mg groundmass concentrate, J=.1117±.1%, =1.623±.131, NM-153, Lab#= A G I Integrated age ± 2σ n= K2O=.9 % Plateau ± 2σ steps -G n=6 MSW= Isochron±2σ n=7 MSW=2 4 Ar/ 36 Ar=298± Notes: Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interferring reactions. Ages calculated ralative to -1 ish anyon Tuff sanidine interlaboratory standard at Ma. rrors quoted for individual analyses include analytical error only, without interferring reaction or J uncertainties. Integrated age calculated by recombining isotopic measurements of all steps. Integrated age error calculated by recombining errors of isotopic measurements of all steps. Plateau age is inverse-variance-weighted mean of selected steps. Plateau age error is inverse-variance-weighted mean error (Taylor, 1982) times root MSW where MSW>1. Plateau and integrated ages incorporate uncertainties in interferring reaction corrections and J factors. ecay constants and isotopic abundances after Steiger and Jaeger (1977). # symbol preceding sample I denotes analyses excluded from plateau age calculations. iscrimination = ±.131 orrection factors: ( 39 Ar/ 37 Ar) a =.72 ± 2e-5 ( 36 Ar/ 37 Ar) a =.28 ± 5e-6 ( 38 Ar/ 39 Ar) K =.177 ( 4 Ar/ 39 Ar) K =.2 ±.3

8 L# 53247: R-2-WR, mg groundmass concentrate Apparent Age (Ma) % Radiogenic ±.21 Ma* (MSW = 3.3) G 175 I K/a -4 Integrated Age = 1.13 ±.65 Ma umulative 39 Ar K Released A G.35.3 A See Inset for I G more detail I 39 Ar/ 4 Ar Isochron age = 1.6 ±.19 Ma 4 Ar/ 36 Ar Intercept = 296 ± 4 MSW = 3.8, n = Ar/ 4 Ar igure 1. 4 Ar/ 39 Ar age spectrum and inverse isochron for the R-2-WR groundmass concentrate. The preferred age of this sample is the weighted mean of steps through G (1.17 ±.21 Ma). All errors are two-sigma.

9 L# 5325: R-2-MA, mg groundmass concentrate Apparent Age (Ma) % Radiogenic ±.16 Ma* (MSW = 1.53) G 175 I K/a -2-4 Integrated Age = 1.78 ±.97 Ma umulative 39 Ar K Released 39 Ar/ 4 Ar AI G See Inset for more detail Isochron age = 1.25 ±.36 Ma 4 Ar/ 36 Ar Intercept = 298 ± 4 MSW = 3.3, n = A I G Ar/ 4 Ar igure 2. 4 Ar/ 39 Ar age spectrum and inverse isochron for the R-2-MA groundmass concentrate. The preferred age of this sample is the weighted mean of steps through G (1.45 ±.16 Ma). All errors are two-sigma.

10 L# 5325: R-2-A, mg groundmass concentrate Apparent Age (Ma) % Radiogenic ±.63 Ma* (MSW = 1.29) G I K/a -4 Integrated Age =.62 ±.39 Ma umulative 39 Ar K Released 39 Ar/ 4 Ar A I G See Inset for more detail Isochron age =.45 ±.8 Ma 4 Ar/ 36 Ar Intercept = 298 ± 4 MSW = 2, n = 7 39 Ar/ 4 Ar.34 A I G igure 3. 4 Ar/ 39 Ar age spectrum and inverse isochron for the R-2-A groundmass concentrate. The preferred age of this sample is the weighted mean of steps through G (.515 ±.63 Ma). All errors are two-sigma.

11 runhes hron Matuyama Reversed hron Jaramillo Normal Subchron obb Mtn Normal Subchron Matuyama Reversed hron Olduvai Normal Subchron R-2-WR 1.17 ±.21 Ma 1.6 ±.19 Ma R-2-MA 1.45 ±.16 Ma 1.25 ±.36 Ma R-2-A.515 ±.63 Ma.45 ±.8 Ma Spectrum weighted mean age Inverse isochron age Apparent Age (Ma) igure 4. Summary plot of the 4 Ar/ 39 Ar ages yielded by this study. The age of R-2-WR indicates that this sample yield a normal or reversed polarity. Sample R-2-MA should yield a reversed polarity, while R-2-A should yield a normal polarity. All errors are reported at two-sigma.