Which Types of Rocks Are Not Good for Radiometric Dating, and Why Understanding Their Limitations is Crucial for Geochronology

Which Types of Rocks Are Not Good for Radiometric Dating?

It’s a question that often pops up when you're delving into the fascinating world of Earth's history: which types of rocks are not good for radiometric dating? For a geologist, or even an enthusiastic amateur, the answer is fundamental. In short, rocks that have undergone significant alteration, have a mixed origin of different-aged components, or haven't incorporated the right radioactive isotopes in the first place are generally poor candidates for reliable radiometric dating. Think of it like trying to date a photo album that’s been through a fire; the original information might be there, but it's likely obscured or completely destroyed.

I remember vividly a field trip early in my career, brimming with excitement to date a particularly intriguing volcanic ash deposit. The lab results, however, were a confusing mess. What should have been a straightforward measurement yielded dates that were wildly inconsistent, some impossibly old, others suspiciously young. It was a humbling lesson in the practical limitations of radiometric dating. Not all rocks are created equal when it comes to revealing their age through these isotopic clocks. The key, as I learned, lies in understanding the rock's genesis and its subsequent geological journey.

Radiometric dating, at its core, relies on the predictable decay of radioactive isotopes into stable daughter products over time. By measuring the ratio of parent to daughter isotopes in a mineral, scientists can calculate how long that mineral has been "closed" – meaning no parent or daughter isotopes have escaped or been added since its formation. However, this elegantly simple principle is profoundly dependent on certain conditions being met. When these conditions are not met, the resulting dates can be misleading, or entirely useless. This article will delve into the specifics of why certain rock types pose challenges and explore the nuances that geologists consider when selecting samples for this powerful analytical technique.

The Fundamental Principles of Radiometric Dating

Before we can truly appreciate which rocks are *not* good for radiometric dating, it's essential to have a grasp of how the process generally works. Radiometric dating is a cornerstone of geochronology, the science of dating geological materials and events. It leverages the fact that certain elements have naturally occurring radioactive isotopes. These isotopes are unstable and, over a fixed period known as a half-life, they decay into a different element or isotope (the daughter product) at a constant and predictable rate. This decay rate is unaffected by external factors like temperature, pressure, or chemical environment.

The most common isotopes used in radiometric dating include:

Potassium-40 (40K) decaying to Argon-40 (40Ar) Uranium-238 (238U) decaying to Lead-206 (206Pb) Uranium-235 (235U) decaying to Lead-207 (207Pb) Rubidium-87 (87Rb) decaying to Strontium-87 (87Sr) Samarium-147 (147Sm) decaying to Neodymium-143 (143Nd) Carbon-14 (14C) decaying to Nitrogen-14 (14N) - primarily used for much younger organic materials.

The magic of radiometric dating lies in the isochron method and the simple ratio calculation. In a closed system, the ratio of parent to daughter isotopes will increase predictably over time. For example, if a mineral forms with zero daughter product and a certain amount of parent isotope, after one half-life, half of the parent will have decayed into the daughter. After two half-lives, three-quarters of the original parent will have decayed. By measuring the current ratio of parent to daughter isotopes and knowing the half-life of the parent isotope, scientists can calculate the time elapsed since the mineral solidified and effectively "closed" its isotopic system.

A crucial aspect is ensuring that the mineral or rock sample analyzed constitutes a "closed system." This means that once the radioactive decay began, neither the parent isotopes nor the daughter isotopes were added to or removed from the system through processes like diffusion, weathering, or alteration. If this system is compromised, the calculated age will not reflect the true age of formation.

Sedimentary Rocks: The Primary Culprits for Undesirable Dating

When we ask which types of rocks are not good for radiometric dating, sedimentary rocks are almost always at the top of the list. This is due to their fundamental nature of formation. Sedimentary rocks are formed from the accumulation and cementation of pre-existing rock fragments (sediments), organic matter, or chemical precipitates. These fragments, or clasts, are often derived from older rocks that have been weathered and eroded from their original locations.

The Problem of Mixed Origins

Imagine a sandstone. It's made up of sand grains, which are essentially tiny fragments of other rocks or minerals, like quartz or feldspar. These sand grains might have been weathered from a granite that formed a billion years ago. When you collect a sample of this sandstone, the quartz grains themselves are, in a sense, "older" than the sandstone formation event. If you were to try and radiometrically date these individual quartz grains using a method like Uranium-Lead (U-Pb) dating (which is done on minerals that contain uranium, like zircon, often found within the source granite), you would be dating the formation of the original granite, not the formation of the sandstone. The sandstone itself, as a whole, formed much, much later when these grains were deposited, buried, and cemented together.

Similarly, a conglomerate, a rock composed of rounded pebbles and cobbles, is essentially a collection of much older rock fragments cemented together. Trying to date the conglomerate itself using methods that date the individual clasts would yield a jumble of ages, none of which represent the time the conglomerate was formed. The cement binding these clasts also has its own formation age, which could be different from the age of the clasts.

When Sedimentary Rocks Can Be Dated (Indirectly)

This doesn't mean sedimentary rocks are entirely uninformative in radiometric dating. While dating the clasts themselves is problematic for determining the *deposition age* of the sedimentary rock, there are indirect ways sedimentary rocks can be dated or help date their surroundings:

Dating Interbedded Igneous Layers: Often, sedimentary sequences are cut through by or interbedded with igneous intrusions (like dikes or sills) or volcanic ash layers. If a layer of volcanic ash (tuff) is deposited on top of a sedimentary layer, dating the ash layer (which is igneous and contains minerals like zircon) provides a maximum age for the sedimentary layer beneath it. Conversely, an igneous intrusion that cuts through a sedimentary sequence provides a minimum age for that sequence – the sedimentary rocks must have been there *before* the intrusion. Authigenic Minerals: Sometimes, minerals that form *during* the sedimentary process (authigenic minerals) can be dated. For instance, certain types of clay minerals or evaporite minerals (like halite or gypsum) can incorporate parent isotopes. If these minerals form a closed system at the time of deposition, they can potentially be dated. However, this is less common and often more challenging than dating igneous rocks. Detrital Zircon Dating: While dating individual clasts in a sedimentary rock is usually not done to find the depositional age, scientists can date populations of detrital zircons from sedimentary rocks. This technique, called detrital zircon geochronology, doesn't give a single age for the rock's formation but reveals the range of ages of the source rocks from which the sediments were derived. This can provide invaluable information about the provenance (origin) of the sediments and the geological history of the source terrains. Specific Examples of Sedimentary Rock Challenges Sandstone: Composed of sand grains (clasts) from older rocks. Dating individual sand grains will give ages of the source rocks, not the sandstone formation. Shale: Fine-grained, often composed of clay minerals and rock fragments. Clay minerals can be susceptible to alteration, and the source of the fine particles can be very diverse and ancient. Conglomerate: Composed of large, rounded pebbles and cobbles, which are themselves older rocks. Limestone: Primarily composed of calcium carbonate, often from biological sources (shells, coral). While the fossils within might give clues to relative age, directly dating the limestone using methods like U-Pb or K-Ar is generally not feasible as it doesn't typically contain suitable radioactive isotopes in a way that records the limestone's formation age.

In essence, sedimentary rocks record a history of mixing and transport, making them inherently complex targets for radiometric dating aimed at determining their depositional age. The ages obtained are usually those of the constituent grains' original formation, not the age of the sedimentary rock itself.

Metamorphic Rocks: The Tale of Remetamorphosis and Closed System Issues

Metamorphic rocks are another category that can present significant challenges for radiometric dating. These rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical reactions, without melting. While metamorphism can reset isotopic clocks, it can also complicate them, especially if the metamorphism is not pervasive or if multiple metamorphic events have occurred.

The "Resetting" of Isotopic Clocks

Metamorphism, particularly at high temperatures, can cause diffusion of isotopes and create new minerals. This process can effectively "reset" the radiometric clock within certain minerals. For example, in a granite that has undergone low-grade metamorphism, the potassium-feldspar crystals might begin to lose argon (40Ar) as 40K decays to 40Ar. The heat facilitates the diffusion of 40Ar out of the crystal lattice. If the metamorphism is intense enough and lasts long enough, it can drive off most of the accumulated 40Ar, effectively resetting the clock to zero. A subsequent K-Ar or Ar-Ar dating of these minerals would then record the age of the metamorphic event, not the original formation age of the granite.

Partial Resetting and Complex Histories

The challenge arises when metamorphism is not complete or uniform. A rock might experience a period of intense metamorphism that resets the clock in some minerals, followed by a period of cooling where other minerals continue to accumulate daughter products. This can lead to discordant ages – different minerals within the same rock yielding different dates. This is where techniques like isochron dating become vital, as they can help identify and resolve these complexities by analyzing multiple minerals or whole rock samples with varying initial ratios.

Furthermore, some metamorphic rocks are formed by the recrystallization of existing minerals without significant new mineral growth or significant isotopic loss. In such cases, the original isotopic ratios might be preserved, and radiometric dating could, in principle, reveal the age of the protolith (the original rock before metamorphism). However, distinguishing between the age of the protolith and the age of metamorphism requires careful mineral selection and analytical techniques.

Specific Challenges with Metamorphic Rocks Repeated Metamorphism: Rocks that have undergone multiple episodes of metamorphism at different temperatures and pressures will have complex isotopic histories. Dating might yield ages related to any of these events, making it difficult to pinpoint a specific formation age. Hydrothermal Alteration: The presence of hot, chemically active fluids during metamorphism can lead to the addition or removal of isotopes, compromising the closed-system assumption. Mineral Inclusions: If a mineral that contains radioactive isotopes (like zircon) is enclosed within another mineral that has a different formation or metamorphic history, dating the zircon will reveal its own age, which might not be related to the bulk rock's metamorphic event. Low-Temperature Metamorphism: In some cases, low-temperature metamorphism might not be sufficient to fully reset isotopic clocks, leading to mixtures of formation and metamorphic ages.

When dating metamorphic rocks, geologists often aim to date specific minerals that are known to crystallize at a particular temperature range during metamorphism (e.g., using U-Pb dating on zircons that crystallize during metamorphism, or K-Ar dating on micas that close at specific temperatures). The interpretation of metamorphic rock ages requires a deep understanding of the specific mineralogy, petrology, and the geological context of the rock's formation and subsequent alteration history.

Igneous Rocks: Not All Are Created Equal

While igneous rocks are generally considered the "gold standard" for radiometric dating because they form from molten material (magma or lava) and often contain excellent dating minerals like zircon, feldspar, and mica, there are still exceptions where certain types of igneous rocks can be problematic.

Fine-Grained Volcanic Rocks and Gas Bubbles

Very fine-grained volcanic rocks, such as some basalts or rhyolites, can be tricky, especially for Potassium-Argon (K-Ar) and Argon-Argon (Ar-Ar) dating. This is because:

Volatile Loss: In rapidly cooled volcanic rocks, gases can become trapped within the molten material. As the rock cools, these gases (including argon) can escape. If significant argon is lost from the rock before the isotopic clock can fully "close," the calculated age will be younger than the actual eruption age. Recoil Effects: During radioactive decay (e.g., 40K to 40Ar), the daughter atom can be ejected from the crystal lattice (recoil). In very fine-grained rocks with small crystal sizes, this recoil can lead to the loss of daughter products, especially if the rock is then subjected to later low-temperature alteration. Inclusions of Older Material: Sometimes, volcanic eruptions can entrain fragments of older rocks from the conduit walls. If these xenoliths contain datable minerals, dating them will yield the age of the xenolith, not the volcanic eruption.

For these reasons, when dating fine-grained volcanic rocks, geologists often prefer to date coarser-grained minerals within the rock or use whole-rock samples if they are demonstrably part of a closed system. Ar-Ar dating, which involves incremental heating of a sample and analysis of argon isotopes, can sometimes help to resolve issues related to excess argon or minor volatile loss.

Pegmatites: A Unique Case

Pegmatites are exceptionally coarse-grained igneous rocks, often found as dikes or sills. While their large crystals can be excellent for mineral separation, they can also be problematic for dating:

Complex Mineralogy: Pegmatites can have a very complex mineralogy with many different mineral phases crystallizing over a prolonged period. Dating different minerals might yield a range of ages, reflecting this complex crystallization history rather than a single formation event. Late-Stage Fluids and Alteration: The formation of pegmatites often involves late-stage, fluid-rich processes. These fluids can cause alteration of existing minerals, leading to isotopic diffusion and resetting of radiometric clocks. Inherited Components: In some cases, pegmatites can incorporate fragments of the surrounding country rock, which can complicate dating if not properly identified.

Despite these challenges, pegmatites are often rich in minerals like zircon, monazite, and tourmaline, which are excellent for U-Pb dating. Careful selection of minerals that are known to be resistant to alteration and that crystallized early in the pegmatite's history is crucial for obtaining reliable ages.

Xenoliths and Xenocrysts

As mentioned briefly, xenoliths (foreign rock fragments) and xenocrysts (foreign mineral crystals) can be incorporated into igneous melts. If these fragments are not distinguished from the igneous rock's own minerals, dating them will yield ages that do not represent the igneous intrusion or eruption event. For example, if a basaltic dike intrudes into ancient granite, and a xenolith of that granite is sampled along with the dike, dating the xenolith will yield the granite's age, not the dike's age.

Geologists must be diligent in field identification and laboratory examination to distinguish between genuine igneous components and foreign inclusions. This often involves examining textures, mineral associations, and chemical signatures.

The "No-Go" Zones: Rocks Lacking Suitable Isotopes or Systems

Beyond the issues of alteration and mixed origins, some rocks are simply not good candidates for radiometric dating because they do not contain the necessary radioactive isotopes or the minerals that typically incorporate them in a way that can be analyzed.

Carbonate Rocks (Mostly)

While limestone and other carbonate rocks (like dolomite) are common and important sedimentary rocks, they are generally poor for standard radiometric dating methods like K-Ar, U-Pb, or Rb-Sr. This is because:

Lack of Parent Isotopes: The primary mineral in limestone, calcite (CaCO3), does not contain significant amounts of the parent isotopes used in most long-lived radiometric dating systems. Open System Potential: Carbonate rocks are susceptible to chemical alteration and recrystallization in the presence of fluids, making it difficult to establish a closed system for any isotopes they might incorporate.

There are exceptions, however. Some carbonate rocks can be dated using Uranium-series disequilibrium dating (e.g., U-Th dating) if they incorporate uranium and thorium. This method is typically used for relatively young deposits (tens of thousands to a few hundred thousand years old), such as speleothems (cave formations) or some marine sediments. Furthermore, some specific types of carbonate rocks, particularly those formed in evaporative environments, might contain minerals suitable for dating.

Cherts and Evaporites

Chert, a hard, fine-grained sedimentary rock composed primarily of silica (SiO2), and evaporites, rocks formed from the evaporation of water (like halite/rock salt, gypsum), can also be challenging. While they can sometimes contain trace amounts of uranium or other isotopes, they are often formed in environments where isotopic mobility and alteration are high, making them difficult to interpret.

Very Young Rocks and Materials

For rocks that are geologically very young (e.g., less than tens of thousands of years old), long-lived radiometric clocks like U-Pb or K-Ar are not useful. This is because not enough time has passed for a measurable amount of daughter product to accumulate. For these materials, techniques like Carbon-14 dating (for organic matter) or thermochronology (which uses the closure temperatures of minerals to date cooling events) are employed.

Rocks with Uniform Isotopic Signatures

Even in igneous rocks, if the initial ratio of parent to daughter isotopes is very high, or if the system has been significantly disturbed in a way that the ratio is uniform across different minerals, standard isochron methods may not work effectively. This is rare but can occur with certain geological processes.

Factors to Consider When Selecting Rocks for Radiometric Dating

Given the complexities, how does a geologist decide which rocks are good candidates? It's a multi-faceted process that involves:

Understanding the Rock Type and Genesis: Is it igneous, sedimentary, or metamorphic? What was its original formation environment? This is the first and most critical step. Mineralogy: Does the rock contain minerals that are known to host the radioactive isotopes of interest and that form robust closed systems? (e.g., zircon, apatite, mica, feldspar). Evidence of Alteration: Has the rock undergone significant weathering, hydrothermal activity, or metamorphism that could have disturbed the isotopic system? Visual inspection in the field and detailed petrographic analysis in the lab are crucial. Crystal Size and Homogeneity: For some dating methods (like K-Ar on fine-grained volcanics), larger, more homogeneous crystals are preferred. Geological Context: How does the rock fit into the broader geological picture? Is it an intrusion cutting through other rocks? Is it an ash layer interbedded with sediments? This context helps interpret the dating results. Availability of Multiple Dating Methods: Ideally, a sample can be dated using more than one radiometric system. If different methods yield concordant ages, confidence in the result increases significantly. Analytical Techniques: The sophistication of the analytical technique employed can also make a difference. For example, Ar-Ar dating is often more robust than simple K-Ar dating for certain sample types.

The Importance of Understanding Limitations

My own early struggles with those seemingly uncooperative volcanic ash samples underscored a vital point: radiometric dating is a powerful tool, but it's not a magic wand. It requires careful selection of samples and rigorous interpretation of results. Misinterpreting data from unsuitable rock types can lead to fundamentally flawed conclusions about Earth's history.

Understanding which types of rocks are not good for radiometric dating is as important as knowing which ones are. It’s about avoiding wasted effort, ensuring scientific accuracy, and building a reliable timeline of geological events. This knowledge allows us to:

Avoid Misleading Ages: Prevents researchers from publishing inaccurate dates that could confuse subsequent studies. Focus Resources: Direct analytical efforts towards samples that are most likely to yield meaningful results. Develop Better Techniques: Drives innovation in developing and refining dating methods to overcome specific challenges in certain rock types. Build a Robust Geological Record: Ensures that the geological timeline we construct is based on the most reliable data possible.

The study of Earth's history is a grand detective story, and radiometric dating is one of our most potent investigative tools. By understanding its limitations and knowing which rock types are unreliable, we can wield this tool more effectively, uncovering the deep time narrative of our planet with greater accuracy and confidence.

Frequently Asked Questions About Rocks and Radiometric Dating

Q1: Can any sedimentary rock be dated using radiometric methods?

No, not all sedimentary rocks can be reliably dated using standard radiometric methods to determine their depositional age. As discussed, sedimentary rocks are typically composed of fragments of older rocks. If you try to date these individual fragments (clasts) using methods like Uranium-Lead (U-Pb), you will get the age of the source rock from which the fragment was derived, not the age of the sedimentary rock's formation. This is a crucial distinction. However, sedimentary rocks can be indirectly dated or provide clues about age by dating interbedded igneous layers (like volcanic ash beds or lava flows) or by dating authigenic (formed *in situ* during sedimentation) minerals if they are present and have remained a closed system. Detrital zircon dating of sedimentary rocks is also a powerful technique, but it reveals the age distribution of the source rocks, not the depositional age of the sedimentary rock itself.

Q2: Why are fine-grained volcanic rocks sometimes difficult to date?

Fine-grained volcanic rocks, such as some basalts or rhyolites, can present challenges for radiometric dating, particularly for Potassium-Argon (K-Ar) and Argon-Argon (Ar-Ar) methods. One major reason is the potential for volatile loss. Volcanic eruptions involve rapid cooling. If gases, including argon (the daughter product of potassium-40 decay), become trapped within the molten rock, they can escape as the rock solidifies or even afterward due to subsequent low-temperature events. This loss of argon before the isotopic clock fully "closes" means that the measured ratio of parent to daughter isotopes will be lower than it should be, leading to an calculated age that is younger than the actual eruption time. Additionally, in very fine-grained rocks with tiny crystals, the radioactive decay process can cause "recoil" where the daughter atom is ejected from the crystal lattice, leading to further loss of the daughter product and artificially younger ages. While Ar-Ar dating techniques can sometimes help to identify and correct for these issues, it's a significant consideration when choosing samples.

Q3: What makes a metamorphic rock problematic for radiometric dating?

Metamorphic rocks can be problematic for radiometric dating because they have undergone transformation by heat and pressure, which can significantly disturb their original isotopic systems. The key issue is the "resetting" of isotopic clocks. High temperatures associated with metamorphism can cause the diffusion of isotopes, particularly daughter products like argon, out of mineral crystals. If a metamorphic event is intense enough, it can effectively erase the age recorded by the parent isotopes and reset the clock to zero, dating the metamorphic event itself rather than the original formation of the rock (the protolith). The complication arises when metamorphism is not uniform or complete. A rock might experience partial resetting, where different minerals or different parts of the same mineral yield discordant ages. This means that dating different minerals within the same metamorphic rock might give a range of ages, reflecting various stages of isotopic closure or resetting during the metamorphic history. Furthermore, the presence of fluids during metamorphism can introduce or remove isotopes, further compromising the assumption of a closed system. Therefore, dating metamorphic rocks often requires careful selection of minerals that are known to crystallize or close at specific temperatures during the metamorphic process, and a thorough understanding of the rock's metamorphic history.

Q4: Can you use radiometric dating on any mineral?

No, you cannot use radiometric dating on just any mineral. Radiometric dating relies on the presence of specific radioactive isotopes that decay at predictable rates. Therefore, suitable minerals must contain these parent isotopes. For example:

Uranium-Lead (U-Pb) dating is most effective in minerals that incorporate uranium, such as zircon, monazite, titanite, and uraninite. Potassium-Argon (K-Ar) and Argon-Argon (Ar-Ar) dating require minerals containing potassium, like micas (biotite, muscovite), feldspars (orthoclase, sanidine), and hornblende. Rubidium-Strontium (Rb-Sr) dating is applied to minerals rich in rubidium and strontium, such as micas and feldspars. Samarium-Neodymium (Sm-Nd) dating is often used on whole rock samples or on minerals like garnets and clinopyroxenes. Carbon-14 dating is used for organic materials (wood, bone, shells) and relies on the presence of carbon.

Furthermore, the mineral must have formed in a way that it can act as a "closed system" for the isotopes in question, meaning no significant gain or loss of parent or daughter isotopes since its formation or since a resetting event. Minerals that are very fine-grained, highly altered, or easily susceptible to diffusion of isotopes are generally not good candidates for reliable dating.

Q5: What are "open systems" and "closed systems" in radiometric dating, and why are they important?

"Open systems" and "closed systems" are fundamental concepts in radiometric dating. A closed system is a geological sample (like a mineral or a rock) that, since its formation or a specific point in its history (like a metamorphic event), has neither gained nor lost any of the parent radioactive isotopes nor their daughter products. In a closed system, the ratio of parent to daughter isotopes will increase solely due to the predictable radioactive decay process, allowing for accurate age calculation. Think of it as a perfectly sealed container where the contents change only through internal processes.

An open system, conversely, is one where there has been a gain or loss of parent or daughter isotopes. This can happen through processes like diffusion (driven by heat), weathering, hydrothermal alteration, or partial melting. If a system is open, the measured ratio of parent to daughter isotopes will not accurately reflect the time since formation. For instance, if a mineral loses daughter products due to heat, the calculated age will be artificially young. If it gains parent or daughter isotopes from surrounding fluids, the calculated age could be artificially old or young, depending on the nature of the addition. The validity of any radiometric date hinges on the assumption that the analyzed sample behaved as a closed system during the time period being investigated. Geologists spend considerable effort selecting samples and using analytical techniques that help them assess whether a system was likely closed and to identify situations where it may have been open, which would render the resulting date unreliable.

Q6: Are there any igneous rocks that are generally not good for radiometric dating?

While igneous rocks are often ideal for radiometric dating, there are indeed some types that can be problematic. Very fine-grained volcanic rocks (like some basalts or rhyolites) can be difficult because they may have lost volatile elements, including the daughter product argon, during rapid cooling, leading to artificially young dates. Also, if volcanic eruptions entrain fragments of older rocks (xenoliths) or minerals (xenocrysts) from the surrounding crust, dating these inclusions will yield the age of the inclusions, not the volcanic event itself. Pegmatites, though often rich in datable minerals, can be challenging due to their complex crystallization histories and susceptibility to late-stage fluid alteration, which can lead to a range of ages rather than a single, definitive formation date. Ultimately, even within igneous rocks, the degree of alteration, the presence of inclusions, and the specific mineralogy play significant roles in their suitability for reliable radiometric dating.

Q7: What is the difference between dating a rock and dating an event?

Dating a rock often refers to determining the time when that rock solidified from a molten state (igneous rocks) or when it underwent a significant metamorphic event that reset its isotopic clock. For sedimentary rocks, dating the rock itself is generally not straightforward; instead, geologists date associated igneous events or authigenic minerals to infer the age of deposition. Dating an event is broader and can encompass the formation of igneous intrusions, volcanic eruptions, periods of metamorphism, or even the deposition of sedimentary layers. Radiometric dating techniques are applied to minerals within rocks, and the resulting age is interpreted in the context of the rock's formation or transformation. For example, dating zircons from a granite intrusion gives the age of the intrusion event. Dating micas in a metamorphic rock might give the age of the metamorphic event that caused the argon to be retained in the mica. Dating a volcanic ash layer interbedded with sedimentary rocks can provide a maximum age for the underlying sediments, thus dating the depositional event.