U-Pb Saturn Main Features

Main Features

Saturn’s main control panel presents a fairly efficient, self-intuitive configuration, with a broad range of operations easily accessed from the Main Menu. Calculations of U and Th concentrations, U-Pb and Pb-Pb isotope ratios and apparent spot dates are displayed in two separate display tables (hereafter described as the Reference Material (RM) Table and Sample (S) Table). The RM Table (Figure 1a) is designed to show and develop calculations (e.g., instrumental drift corrections) on selected primary reference material analyses. The S Table (Figure 1a) displays corrected isotope ratios and spot dates for samples and secondary reference materials. It also gives different options for common Pb corrections (²⁰⁴Pb, ²⁰⁷Pb, ²⁰⁸Pb based corrections), which can be applied to the entire dataset on display, or individually (for example, only on particular spot analyses that recorded high common Pb, or, in the case of ²⁰⁸Pb-based corrections, only those analyses characterized by low Th/U). These tables are linked to the main panel and are automatically updated after any change related to Pb_c, drift and U-Pb offset corrections.

The individual analyses are loaded into the two sample lists, linked to the main RM and S Tables (Figure 1a). Here the user chooses (1) the primary reference material analyses (to be loaded in the RM table) and (2) the sample plus secondary reference material analyses (to be loaded in the S Table). Once the RM data points are loaded, the software automatically runs calculations for drift and fractionation, and displays the results. The RM Table displays the average of drift-corrected Pb/Pb and U/Pb ratios (+uncertainties) and dates (+uncertainties). It also displays the calculated offset between the averaged data and the certified values. The certified values for all reference materials available can be loaded in a separate window, which can be easily called from the main panel.

_{Figure 1a: Saturn’s main screen control, showing the Sample list (left), Menu header (top left), the Reference Material (RM) Table (showing data for the 602 Ma GJ-1 zircon, Jackson et al. 2004, Horstwood et al. 2016) and the Sample (S) Table (showing data for the 560 Ma BB zircon, Lana et al. 2017, Santos et al. 2017; and the 337 Ma Plešovice zircon, Sláma et al. 2008).}

Before sorting out the spot analyses into the RM and S Tables, the user may add the certified values for primary reference materials (Figure 1b). This information is accessed from the main Menu (in Reference Materials). Access and modifications to the main list of reference values can be easily selected from the Main Menu.

_{Figure 1b: Window for adding reference materials (e.g., Blue Berry/BB – Lana et al. 2017, Santos et al. 2017; GJ-1 – Jackson et al. 2004, Horstwood et al. 2016; Plešovice – Sláma et al. 2008; 91500 – Wiedenbeck et al. 1995, 2004; SRM-NIST 614 – Hollocher and Ruiz 1995, Jochum et al. 2011; TANZANIA – in-house reference material; BCR-2 and BHVO-2 – Jochum et al. 2016). New references can be added to the dropdown list by simply typing in a new reference material (and certified values) and clicking Add}.

The visualization and interaction of the raw and corrected time-resolved signals of individual analyses is done in a separate window (the Signal Window), which plots the background and signal intensities, and the calculated ratios against time (Figure 2a). Here, the user has the option of visualizing and slicing the background and sample signals. The graphic interfaces are linked to the S and RM Tables, and the average signal is recalculated and displayed instantaneously after any small change in the signal slices.

_{Figure 2a. Signal Window displaying all measured masses in time-resolved mode. Note the background slice (vertical red lines) and signal slice (vertical green lines). The window is interactive and linked to the S and RM tables and the Plot Window (Figure 2b).}

A second graphic window (the Plot Window) displays options for U-Pb Wetherhill and Tera-Wasserburg concordia plots and Pb-Pb isochrons (Figure 2b). Key features such as linear regression, anchoring and age calculations are easily accessed from the Plot Window. The window reads and plots the analyses from the RM and S Tables. This feature is particularly useful when treating complex samples, which record multiple (igneous, metamorphic and inherited) ages. The Tera-Wasserburg diagram is particularly useful for analyzing the effects of common Pb (Pb_c) in Pb_c-uncorrected analyses and identifying offsets between non-matrix matched normalization (see below).

_{Figure 2b. Plot Window displaying U-Pb data points for carbonate sample on a Terra-Wasserburg diagram. In this window, the user can create Terra-Wasserburg, Wetherill or Isochron plots. The window is interactive and linked to the main table and plot screen (see text for more details). The upper and lower intercept ages should be taken as an estimated ages. The age calculations in the Plot Window do not take into account the uncertainties of the spots and do not calculate uncertainty for the intercept ages.}

File Structure

Saturn reads any extension (.cvs, .txt, .exp) of data files from a wide range of mass spectrometers, including multi-collector. As ICP-MS output data files vary in extension and format (heading, spacing and content), the software identifies and uses specific import data modules that convert them into a Saturn generic format. A primary requirement is that individual spot analyses should be exported in separate output files; a feature that is easily set in most mass spectrometers. Also, the isotope mass measurements (e.g., ²⁰²Hg, ²⁰⁴Hg+Pb, ²⁰⁴Pb) should be displayed in separate rows of the raw data file. Once loaded, the software instantaneously reads the file and recognizes the masses. For each of the measured masses, it creates a databank with stored information from the background and sample signal. It repeats this routine for up to 1000 loaded data points in less than a minute.

The file structure is fundamental for fast data reduction of large datasets. Individual files normally contain hundreds of lines reporting mass measures in counts per second (CPS); each line repressing a swept through all masses in milliseconds. In the case of large datasets, it is recommended to set up the spectrometer to generate the smaller individual files (e.g., longer mass measurements with longer swept times, or shorter laser ablation periods). The individual files should have a maximum of 300 lines of mass measurements. Longer files can potentially slow down data processing.

Data processing

The raw data from individual analyses are processed in time-resolved mode and, here, any fraction of the entire period of one analysis is called a time-slice. The software automatically separates the time-resolved signal into two time-slices: (1) background and (2) signal (Figure 2a). The background signal region relates to the baseline detection at a particular mass, and the signal is the additional counts from the ablated sample material. Thus the main data input requirement is that each raw data file should include background and signal information for all measured masses.

Like Glitter and Iolite (Jackson et al. 2004, Paton et al. 2011, McLean et al. 2016), Saturn has a feature that recognizes the rise of the time-resolved (238U or 206Pb) signal of individual analyses and automatically separates background (He + Ar gas) signal from sample signal. The width of the windows can be easily modified by the user in two different ways: It can be modified for the whole dataset including the reference materials in the Settings Window (signal parameters; Figure 3), or changed individually via the graphic interface that plots the signal against time (Plot Window; Figure 2a).

The background-subtracted signals of all of the analysed masses are directly converted to time-resolved U-Pb/Pb-Pb ratios. The individual masses and Pb-Pb ratios are then averaged and transformed into background subtracted signals. The calculations are done internally and the user has the option to select averaged ratios (each segment of the signal is rationed and then the ratios are averaged) or averaged masses (signal of the individual masses are averaged and then rationed) (Figure 2a).

There are options for removing outliers (10% at 1 sigma level) from both signal and background of the masses and from the U/Pb and Pb/Pb ratios (Figure 3).