Age of the Earth – Geology – Radiometric Dating
Table of Contents
Radiometric Dating Helium in Zircon Crystals Overview
Radioactive helium (helium-4) is produced as uranium-238 decays through a series of alpha emissions in its decay chain to lead-206. Each alpha particle eventually becomes a helium atom, which may remain trapped within mineral crystals such as zircon or diffuse outward over time. Because helium is a very small and mobile atom, its retention or loss within zircon crystals depends on factors such as temperature, crystal structure, and diffusion rates.
Measuring the amount of helium retained relative to the amount produced provides a potential method for estimating how long the decay and diffusion processes have been occurring. Researchers have argued that the observed retention of helium in certain zircon samples appears higher than expected under conventional geological timescales, suggesting that helium diffusion rates and retention histories deserve careful investigation. These measurements form the basis for ongoing scientific discussion regarding helium diffusion, thermal history, and radiometric interpretations.
- Zircon (ZrSiO₄)- incorporates uranium – they reject lead initially – they are ideal for U-Pb dating – they produce helium as a byproduct
- Decay chain – U-238 → … → Pb-206 [Along the way, 8 alpha decays occur | Each alpha particle = helium-4 nucleus | After capturing electrons → helium atom.]
History of Helium Retention in Zircon Crystals
Zircon (ZrSiO₄) is a naturally occurring crystalline mineral that commonly incorporates uranium atoms into its lattice during formation while strongly excluding lead. Because uranium undergoes radioactive decay to lead through a well-known decay chain, zircon crystals serve as one of the most important minerals for radiometric dating.
During the uranium-238 decay chain to lead-206, eight alpha particles are emitted. Each alpha particle becomes a helium-4 atom after capturing electrons. As a result, zircon crystals continuously generate helium internally as long as radioactive decay occurs.
Robert Gentry’s Helium Retention Measurements
In the 1970s and early 1980s, geophysicist Robert V. Gentry measured helium concentrations in zircon crystals recovered from Precambrian granitic rock formations in New Mexico. These zircons had previously been dated using uranium-lead radiometric dating methods, yielding ages on the order of approximately 1.5 billion years. Gentry measured the amount of radiogenic helium retained within individual zircon crystals and compared this to the total amount of helium expected to have been produced based on the measured uranium content and decay rates.
Gentry found that some zircon crystals retained measurable fractions of their internally generated helium, with retention values reported as high as approximately 58% of the total helium expected from radioactive decay. These zircon crystals were microscopic in size, typically ranging from approximately 50 to 75 micrometers in length. Because helium is a small and highly mobile atom, its presence within crystal lattices depends strongly on diffusion processes controlled by temperature, crystal structure, and time.
Helium Diffusion as a Time-Dependent Process
The movement of helium atoms through zircon crystals is governed by diffusion physics, described by Fick’s laws of diffusion. The diffusion rate is quantified by the diffusion coefficient D, which depends exponentially on temperature according to the Arrhenius equation: [D=D0e−Ea/RT]
where:
- D = diffusion coefficient
- D0 = diffusion constant
- Ea = activation energy
- R = gas constant
- T = temperature in Kelvin
The characteristic diffusion timescale for helium to escape from a spherical crystal of radius a is approximately: [t≈Da2]
This equation shows that helium retention depends directly on:
- crystal size
- temperature
- and diffusion coefficient
These quantities can be measured experimentally.
RATE Project Diffusion Measurements
In the late 1990s and early 2000s, researchers associated with the RATE (Radioisotopes and the Age of the Earth) project conducted laboratory measurements of helium diffusion rates in zircon crystals recovered from the same geological formation studied by Gentry. These experiments measured helium diffusion coefficients at various temperatures and extrapolated the results to in-situ rock temperatures.
The experimental results were plotted as diffusivity versus temperature, allowing comparison between measured diffusion rates and diffusion rates predicted under different assumed timescales. The diffusion coefficient determines how rapidly helium escapes from zircon crystals over time. Higher diffusivity corresponds to faster helium loss, while lower diffusivity corresponds to greater retention. Measured diffusion values were compared to diffusion rates required to retain observed helium concentrations over assumed geological timescales.
Structure of the Diffusion Data Plot
The diffusion plot presented in the RATE study contains several key elements:
- Horizontal axis: temperature of the zircon crystals (°C)
- Vertical axis: helium diffusivity (cm²/s)
- Blue points: measured experimental diffusion rates
- Error bars: experimental measurement uncertainty (two-sigma)
- Model predictions: diffusion rates required to produce observed helium retention under assumed timescales
The vertical axis spans many orders of magnitude, reflecting the strong exponential dependence of diffusion rate on temperature. The measured diffusion rates provide an experimentally determined relationship between helium diffusion and temperature in zircon crystals.
The diffusion rates required for the 1.5-billion-year model are extremely low [Green graph], because helium would need to remain trapped in the zircon crystals for a vastly longer period of time without escaping. In contrast, the diffusion rates measured experimentally were much higher [blue graph], corresponding to helium loss over thousands of years rather than billions, which is why the measured data aligned with the ~6,000-year diffusion timescale under their model assumptions.
Based on their measured helium diffusion rates, crystal sizes, and estimated in-situ temperatures, the RATE research team calculated diffusion timescales on the order of thousands of years rather than hundreds of millions or billions of years. Their analysis concluded that the observed helium retention in zircon crystals was consistent with an apparent diffusion age of approximately 6,000 years under their model assumptions, which they noted corresponds closely with the biblical timescale.
Rebutting the Critics
The sequence of measurements, predictions, and experimental verification places the focus on explaining how the observed helium diffusion data aligns with the measured diffusion coefficients and retention levels. Any alternative interpretation must account for both the measured helium concentrations and the experimentally determined diffusion rates within zircon crystals.
Critics must address how zircons could contain helium retention levels consistent with measured diffusion coefficients if the crystals had experienced diffusion over hundreds of millions or billions of years at their present temperatures.
1. Critics State: “ICR relied on questionable Q/Q₀ (helium retention) values from Gentry et al. (1982a).”
Response:
ICR independently measured helium retention values in zircon crystals from the same geological formation and reported results consistent with Gentry’s original findings (Humphreys et al., Creation Research Society Quarterly, 2004). The retention fractions used in the analysis were based on measured helium concentrations, uranium content, and experimentally determined diffusion coefficients. These values were not assumed but derived directly from physical measurements.
2. Critics State: “ICR underestimated the Q₀ values (maximum possible radiogenic helium concentration).”
Response:
Critic Roger Henke suggested that Q₀ values could be higher, estimating approximately 41 ncc STP/µg compared to ICR’s estimate of approximately 15 ncc STP/µg.
However, Q₀ represents the theoretical maximum helium production based on uranium decay, and the critical parameter in diffusion analysis is the retention ratio Q/Q₀. Even if Q₀ were increased by a factor of two or more, the resulting retention fractions would still differ by orders of magnitude from those required to sustain diffusion over hundreds of millions or billions of years.
Additionally, independent measurements of uranium and lead concentrations in the zircon crystals provided direct constraints on total radiogenic helium production, supporting the retention ratios used in the diffusion calculations.
3. Critics State: “ICR selectively interpreted helium concentrations from specific samples.”
Response:
The diffusion analysis included multiple zircon samples spanning a range of temperatures and helium retention levels. Sample 5, which had very low retention at higher temperature, and Sample 6, which had higher retention at lower temperature, both fit the experimentally determined diffusion relationship.
These data points follow the expected temperature-dependent diffusion trend described by the Arrhenius equation. The agreement between multiple independent samples supports the validity of the measured diffusion coefficients and retention calculations. Furthermore, the overall diffusion trend is determined by the full dataset, not by any single sample.
4. Critics State: “The helium originated from external sources rather than radioactive decay within the zircons.”
Response:
If helium had entered the zircon crystals from external sources, diffusion physics predicts that helium concentration would be higher in the surrounding minerals and lower inside the zircons. However, measurements show the opposite relationship: helium concentration is significantly higher inside the zircon crystals than in the surrounding biotite.
According to Fick’s Law of diffusion, this concentration gradient indicates that helium is diffusing outward from the zircon crystals, consistent with internal radiogenic production. Additionally, elevated temperatures associated with magmatic activity would accelerate helium diffusion and result in significant helium loss, not retention. The measured retention levels, therefore, reflect diffusion behavior governed by the crystal’s thermal history and diffusion coefficients.
5. Critics State: “ICR’s diffusion models rely on unrealistic assumptions regarding temperature history and diffusion behavior.”
Response:
Helium diffusion rates depend strongly on temperature, crystal size, and experimentally measured diffusion coefficients. These parameters were determined through laboratory measurements and geological temperature estimates.
Even allowing for reasonable variations in thermal history, diffusion physics requires specific combinations of temperature and time to produce the observed helium retention levels. The experimentally measured diffusion coefficients provide a direct constraint on the diffusion timescale for helium escape from zircon crystals. The diffusion calculations are based on established physical laws governing atomic diffusion in crystalline solids and experimentally measured material properties.
Helium in Earth’s Atmosphere: Mass Balance and Measured Accumulation (Young Earth Resource)
Helium-4 is continuously supplied to Earth’s atmosphere through radioactive decay of uranium and thorium within the crust and mantle. This helium migrates through rocks and enters the atmosphere through diffusion, tectonic activity, and volcanic degassing. At the same time, helium atoms are continuously lost to space through thermal escape and magnetospheric polar wind processes. Because helium is chemically inert and does not react or bind permanently in the atmosphere, its total atmospheric inventory is governed by a simple mass-balance relation:dH/dt =P−L
where:
- H = total atmospheric helium (tons)
- P = helium input into the atmosphere (tons/year)
- L = helium escape to space (tons/year)
If P>L, then helium accumulates in the atmosphere over time.
Measured Atmospheric Helium Inventory
The total mass of Earth’s atmosphere is approximately: [5.1480×1018 kg]
Measured atmospheric helium concentration is: [5.24 ppm by volume]
From these measured values, the total atmospheric helium mass is: [H≈3.73×109 metric tons]
This value is derived directly from atmospheric sampling and mass measurements. [Data source: NOAA atmospheric composition measurements and global atmospheric mass determinations.]
Measured Helium Escape Rate (from the Atmosphere)
Modern satellite and ionospheric measurements estimate global helium escape at approximately: [50 grams per second.]This converts to: [L≈1,578 metric tons per year]
This escape occurs primarily through polar wind ion outflow along Earth’s magnetic field lines and thermal escape in the upper atmosphere. [Data source: Catling & Zahnle (2009), atmospheric escape measurements based on satellite ion flux observations and magnetospheric modeling.]
Measured Helium Supply Rate
Radiogenic helium production and degassing estimates from crust and mantle sources yield supply rates on the order of: [P≈103 tons per year]
These values are derived from measured uranium and thorium concentrations in crustal rocks, known nuclear decay constants, measured crustal helium flux values, and mantle degassing measurements at mid-ocean ridges.
Net Atmospheric Helium Accumulation
The net accumulation rate is: [N=P−L]
Using representative measured values: [N≈200 tons per year]
This implies that atmospheric helium is accumulating over time. The cumulative accumulation is described by: [H(t)=H0+Nt]
The chart above illustrates:
- The measured yearly escape rate (constant blue line)
- The cumulative atmospheric helium accumulation over time (orange line)
Even a modest net accumulation rate produces significant atmospheric helium increases over geological timescales.
Atmospheric Helium Replacement Timescale
Dividing the total atmospheric helium inventory by the net accumulation rate gives the characteristic accumulation timescale: [T=NH ] [T=2003.73×109 ]
T≈18.6 million years
Conclusion:
Based on measured present-day helium supply and escape rates, the atmospheric helium inventory corresponds to a replacement timescale of approximately 18 million years. This indicates that atmospheric helium is actively accumulating and that its present abundance reflects an ongoing balance between radiogenic production, crustal degassing, and atmospheric escape governed by measurable physical processes.
Measurement Methods and Data Sources
Atmospheric helium content – Direct atmospheric gas sampling, Mass spectrometry of air samples, NOAA global atmospheric composition datasets
Helium escape rate – Satellite ion detectors measuring polar wind helium ion flux, Magnetospheric plasma measurements, Upper atmosphere escape modeling calibrated by satellite data
Primary references – O’Nions & Oxburgh (1988), Jambon et al. (1986), Stacey & Davis (2008), Catling & Zahnle (2009), Scientific American, NASA Polar Wind and Magnetosphere ion escape measurements
Helium supply rate – O’Nions & Oxburgh (1988), crustal helium flux measurements, Jambon et al. (1986), mantle helium degassing flux, Stacey & Davis (2008), uranium/thorium radiogenic production models, Uranium and thorium concentration measurements in crustal rocks, Radiogenic decay rate calculations, Direct crustal helium flux measurements, Mid-ocean ridge mantle degassing measurements
References
Age of the Earth – Young Earth Evidence – Radiometric Dating
Institute for Creation Research – Age of the Earth pdf
Vardiman, L., A. A. Snelling, and E. F. Chaffin, editors., Radioisotopes and the Age of the Earth: Results of a Young-Earth Creationist Initiative, Institute for Creation Research, El Cajon, California, and the Creation Research Society, St. Joseph, Missouri, expected publication date, on or before November 2005.
DeYoung, Don, Thousands not Billions, Master Books, Green Forest, Arkansas, expected publication date, on or before November 2005.
(ICC 2003) Humphreys, D. R., S. A. Austin, J. R. Baumgardner, and A. A. Snelling, Helium diffusion rates support accelerated nuclear decay, 2003a, in Proceedings of the Fifth International Conference on Creationism, edited by R. L. Ivey, Jr., Creation Science Fellowship, Pittsburgh, Pennsylvania, pp. 175-195, 2003.
(CRSQ 2004) Humphreys, D. R., S. A. Austin, J. R. Baumgardner, and A. A. Snelling, Helium diffusion age of 6,000 years supports accelerated nuclear decay, Creation Research Society Quarterly, 41(1), 1-16, 2004.
Gentry, R. V., Glish, G. J., and McBay, E. H., Differential helium retention in zircons: implications for nuclear waste management, Geophysical Research Letters, 9(10), 1129-1130, 1982a.
Humphreys, D.R., Accelerated nuclear decay: a viable hypothesis?, in Radioisotopes and the Age of the Earth: A Young-Earth Creationist Research Initiative, edited by L. Vardiman, A. A. Snelling, and E. F. Chaffin, Chapter 7, pp. 333-379, Institute for Creation Research and the Creation Research Society, San Diego, CA, 2000.
Gentry, R. V., T. J. Sworski, H. S. McKown, D. H. Smith, R. E. Eby, and W. H. Christie, Differential lead retention in zircons: implications for nuclear waste containment, Science, 216, 296-298, 1982
For example, if the average length of zircons in that sample (number 2002) were larger than the average length in the other samples (about 60 microns), then the percentage of alpha particles retained would be higher. That would make Q0 higher than the value of 15 ncc/µg we used for the other samples, thus dropping the retention from 80.7 % to a smaller value. This affects Henke’s reasoning in item 8.
Stacey, F. D., Physics of the Earth, John Wiley and Sons, New York, p. 245, Table 9.3, 1969. The table says the average amount of uranium in basaltic crust is 0.8 ppm by weight. Assuming that at most an equal amount of uranium has already decayed to lead (the thorium, having a much greater half-life, would not have decayed nearly as much), and that all the helium produced thereby has remained in the basaltic magma, gives an average helium concentration of less than 80 nmol/g in such magmas.
Humphreys, D. R., Beyond Neptune: Voyager II supports creation, ICR Impact, No. 203, May 1990
Helium Evidence for a Young World Remains Crystal Clear, and Helium Evidence for A Young World Overcomes Pressure, by D. Russell Humphreys, Ph.D.