The Global Detrital Zircon Database: An Update Peter Voice Michał Kowalewski Ken Eriksson

Basic Detrital Zircon Methodology

Minimum Sample Size Constraints What is the minimum sample size necessary to have a sample that represents every “major” age fraction potentially present ? Several studies have attempted some variant of this What is the minimum sample size necessary to best estimate the age of the host sediment using detrital zircon geochronology? The goal of this study

Previous Sample Size Approaches Dodson et al. (1988) used: P = (1-f)n P = probability f = fraction of grains representing a subpopulation within the population n = sample size If f is set to 0.05 and P to 0.05, then n ≈ 60

Previous Sample Size Approaches Vermeesch (2004) used: Using this equation, Vermeesch (2004) shows that at least 117 grains need to be sampled in order to be 95% confident that any fraction of the population greater than 0.05% is not missed.

Our Approach The Previous sample size approaches are based on theoretical statistics; Our approach is based on empirical data from a very large database Defined a metric: the offset between the maximum age of the host sediment and the age of the youngest grain in the sample Calculated the offset for all samples in the GDZDb Averaged the offsets as a function of sample size for different filters

The Offset Host Sediment Time (Ma) Offset Time (Ma) Offset Youngest Detrital Zircon Age Minimum Host Sediment Age Maximum Host Sediment Age Host Sediment Ages Independent of Detrital Zircon Data

The Global Detrital Zircon Database Structure 1,248 studies compiled 5,529 Age Frequency Distributions 281,239 individually dated detrital zircons Samples from modern sediments, sedimentary rocks, and meta-sediments 78 variables (78 *281,239 = 21,936,642 cells) Variables include: host sediment lithology and metamorphic grade; minimum and maximum age constraints on host sediment; host sediment tectonic setting, etc.

Global Age Frequency distribution 1000 2000 3000 4000 n = 199,358 n = 118,682 n = 100,445 Geologic Age (Ma) Frequency of Dated Detrital Zircon Grains I. II. III. IV. V. Global Age Frequency distribution

Frequency of Dated Detrital Zircon Grains North America Europe Africa 1000 2000 3000 n = 43,160 n = 7,344 n = 7,153 n = 12,774 n = 5,094 n = 20,461 500 4000 n = 2,493 Geologic Age (Ma) Frequency of Dated Detrital Zircon Grains North America Europe Africa Antarctica South America Asia Australia

Sample Size Results

Full Database n = 4,885

- 200 100 300 400 500 600 20 40 60 80 120 140 Passive Margins Active Margins Rift Basins Binned n Average Offset between youngest grain age and Maximum estimate of host sediment age (Ma) Full Database

≤ ±5% discordant data n = 3,890

≤ ±5% discordant data Binned n - 100 200 300 400 500 600 20 40 60 80 120 140 Passive Margins Active Margins Rift Basins Binned n Average Offset between youngest grain age and Maximum estimate of host sediment age (Ma) ≤ ±5% discordant data

Modern River Samples n = 435

Modern River Samples Binned n 500 1000 1500 2000 2500 20 40 60 80 100 120 140 Passive Margins Active Margins Rift Basins Binned n Average Offset between youngest grain age and Maximum estimate of host sediment age (Ma) Modern River Samples

Sample Size Recommendations Recommended n Filter passive margins active margins rift basins Full Database 80 40 30 5% discordance filter 70 50 Phanerozoic Modern Sediment Average n = 51 grains

Conclusions Regardless of filter or Tectonic setting a conservative estimate of the sample size necessary to best estimate the host sediment age stabilize at approximately 50 grains minimum offset at n = 50 is 40-50 Ma Individual samples may be heavily influenced by source sediment age, local basin conditions, etc.