Juha Köykkä - Oulu

Res Terrae, Ser. A 32, J. Köykkä, Sedimentology of the Mesoproterozoic Telemark basin-fills, South Norway: implications for
sedimentation processes, depositional environments and tectonic evolution
ing, crushing, heavy-liquid separation, hand-picking, and the Wilfley concentration
table), and they were photographed using an electron-microscope to identify suitable
microanalytical locations. The U-Pb dating and isotope analyses were performed using
a Nu Plasma HR multicollector ICPMS and a New Wave / Merchantek LUV-213 laser
microprobe at the University of Oslo, Department of Geosciences. LA-ICP-MS in-
volves the ejection of matter from a solid by a laser beam, ionization of the ablated
material by argon plasma and the measurement of isotopic ratios by a mass spectrome-
ter. The main components of the system include the following: (i) a vacuum system;
(ii) a source (ICP) from which ions created by the sample are focused and accelerated;
(iii) an analyzer in which ions are separated by their mass to charge ratios; and (iv) a
detector, which converts the ion beam to a number.
U-Pb geochronology is based on the natural decay of 238 U and 235 U to stable 206 Pb
and 207 Pb with half-lives of 4.468 Ga and 0.704 Ga. In this thesis, masses 204, 206,
207, and 238 were measured, and 235 U was calculated from 238 U by using a natural
238 U to 235 U ratio of 137.88. Mass 204 was used as a monitor for common 204 Pb. The
raw data were reduced to U and 206 Pb concentrations and U-Pb isotope ratios by a
sample-standard bracketing routine similar to that used by Jackson et al. (2004). The
zircon used for calibration included zircon standards GJ-01 (609 ± 1Ma; Jackson et al.,
2004) and 91500 (1065 ±1 Ma; Wiedenbeck et al., 1995) and an in-house Paleoprote-
rozoic standard A382 (1876 ±2 Ma; Patchett and Kouvo, 1986). The program Isoplot
Ex 3.0 (Ludwig, 2003) was used to plot the concordia and probability density dia-
grams. Zircon standard 91500 was also run as an unknown to verify the accuracy of
the method. The concordia age calculated from all of the standards run as unknowns
was 1054 ±3 Ma, which is slightly younger than the reported TIMS age. The laser set-
tings were ca. 32 % power, 40-µm spotsize and 10-Hz repetition rate. The data are pre-
sented in the 207 Pb/ 235 U vs. 206 Pb/ 238 U diagram, called the concordia diagram, in which
the ensemble of the points having the same age with the two isotope systems ( 238 U-
206 Pb and 235 U- 207 Pb) defines the concordia line. The 207 Pb/ 206 Pb age can be visualized
as the projection of the analytical point on the concordia line from the origin of the di-
agram (0, 0). An example of the concordia diagram is given in Figure 7.
176 Lu decays to 176 Hf with a half-life of ca. 3.73 ±0.05 to 3.69 ± 0.02 × 10 10 yr. In
this thesis, masses 172-179 were measured for the Lu-Hf isotope analysis of zircon.
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