LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) procedures for U-Th-Pb isotope analysis used at Multilab (UERJ, Brazil).

In this work we describe the LA-ICP-MC-MS procedures used for U-Th-Pb
isotope analysis at the MultiLab laboratory (UERJ). The Neptune is characterized by
being a high-resolution multi-collector for measurements of isotopic ratios, and its
configuration allows to simultaneously detect a wide range of nuclides, using nine
Faraday collectors and six ion counters, which can be combined in an appropriate
configuration for the isotopes of interest for the U-Pb method. During the analysis the
collectors geometry (with 2 Faradays and 5 ion count). The U-Th-Pb isotope analysis
includes 204 Pb, 206 Pb, 207 Pb, 208 Pb, 232 Th, and 238 U. Mercury represents a common
contaminant in He and Ar gas and 204 Hg interferes with 204 Pb mass counts. Mass 202 Hg,
mass 204 ( 204 Pb + 204 Hg) as well as 206 Pb, 207 Pb and 208 Pb were detected by the ion counters
while 238 U and 232 Th was measured with a Faraday cup.
Isotopic data are obtained using the static mode through 40 cycles of 1.054
seconds acquirement time with a gas inlet flow (Ar) of 15 L / min, auxiliary flow (Ar)
0.8 L / min, in MC-ICP-MS. The laser a gas inlet He flow (two input streams with
volumes of 0.800 l / m and 0.220 l / m, totaling 1.020 l / m). Repetition of the laser was
at 5-7 Hz, with 4-7 J / cm2 output power and 40-30 μm crater size. The acquisition
process consists of a sequence comprised of the analysis of one blank, which is the
measurement of the data performed of the passage of only Ar and He gases, being
measured as a background, followed by the measurement of a shot made on a reference
material zircon (GJ-1). If the data are compatible with the true value reported in the
literature ( 206 Pb/ 207 Pb = 0.06389 and 206 Pb/ 238 U = 0.09812), so the analysis of nine grains
of unknown age can be started.
The choice of the grain to be analyzed and the position where the crater will be
made is aided by the cathode-luminescence images and the image provided by the
equipment's camera. The laser is positioned at the chosen location and the beam is
manually activated, initiating the ablation process and data acquisition is initiated in 40
cycles. With the completion of the unknown analyzes, data of a second reference

material (91500) is acquired, ending the spreadsheet with the reading of a second GJ-1
and a blank.
The U-Pb results obtained by LA-ICP-MC-MS are treated in an offline
spreadsheet for blank and GJ-01 correction. Finally, the program ISOPLOT is used to
calculate ages and construct concordia diagrams and all values are referred to 2σ for all
uncertainties. Data on the GJ-1 zircon standard for a 30μm ablation spot yielded usually
432,000-114,000 cps of 206 Pb, 25,000-7,000 cps of 207 Pb, 6,500-4,200 cps of 208 Pb, 4,400-
4,200 cps of 202 Hg and 1,060-1,090 cps of 204 (Hg+Pb). For 232 Th and 238 U, measurements
on faraday cups the values are 0.78 mV and 6.06 mV, respectively, yielding an age of
610.8 ± 2.6 Ma. The GJ-01 reference material comprises a large amount of zircon
crystals approximately 1 cm in size from African pegmatites with reported
crystallization age of 608.5 ± 0.4Ma.