Geologic Chronology, Zircon Analysis, and the Early History of Earth
Zircons as Geologic Clocks and Multiphase Dating
Zircon Resilience and Multiple Dating Events: * Zircon crystals are capable of growing multiple times throughout geologic history. * When geologists attempt to reconstruct historical plate movements and orogenic events, they look for internal structural confidence within a single mineral. * Zircons can yield multiple age dates from a single crystal. In specific observed samples, researchers have obtained five distinct age dates from a single zircon grain. * Each of these distinct age growth zones presumably represents a specific "autogenic event."
Physical Properties: * Zircons are remarkably small, typically measuring only a couple of millimeters across. * Despite their size, they act as "remarkable geologic clocks" that provide exhaustive data on chronological histories.
Uranium-Lead (U-Pb) Decay Schemes and Dating Mechanics
Complexity of Uranium Decay: * The Uranium decay scheme is described as very complex, involving numerous intermediate elements. * The process involves radioactive elements that continue to break down in a variety of ways through a complex decay chain. * The ultimate, stable end product of this radioactive decay is Lead ().
U-Pb Graphical Analysis (Concordia Diagrams): * Dating techniques utilize ratios of specific isotopes: relative to and relative to . * In a theoretical "perfect" system, the analysis results in a curve known as the "Concordia curve." * The Discordia Line and Lead Loss: * During the history of a mineral block, diffusion of minerals often occurs, leading to "lead loss." * This results in a straight line that intersects the Concordia curve at two points. * The upper intercept (upper age limit) represents the initial crystallization of the mineral. In the example provided, this occurred at (2,600 million years). * The lower intercept (lower age limit) indicates a subsequent event such as recrystallization, metamorphism, or maturation.
Case Study: Canyon Diablo Meteorite: * The Lead-Lead dating technique was applied to samples from the Canyon Diablo area. * The age of this meteorite is determined to be . * This is the specific meteorite responsible for the formation of Meteor Crater.
Terrain Correlation and Detrital Zircon Analysis
Application in Orogenic Events: * Detrital zircons are used extensively to correlate different terrains across present-day continents. * This is fundamental in studying the Appalachian-Caledonian mountain-building events. * Historical terrains studied include North America, Scandinavia, and the United Kingdom (specifically Wales). * The challenge lies in picking apart which "bits and pieces" of volcanic arcs and continents were caught up in collisions and determining how they fit together.
Kernel Density Plots: * These plots are used for averaging age date data to identify significant "spikes" or peaks. * Geologists take a large collection of zircons from a site, perform U-Pb dating, and plot the ages. * Each peak in the plot marks a distinct orogenic (mountain-building) event. * Matching Terrains: * By comparing these peaks across different geographical areas (e.g., Newfoundland, Wales, and Rockville), researchers can see if the terrains shared a common tectonic history. * If peaks align across multiple plots, it indicates the terrains were part of the same event. Some peaks may be unique to a specific area, while others correlate across broad regions.
Comparative Examples: * Mesozoic Great Valley (California) vs. Oslo Rift (Norway): * The Great Valley sample shows a "monotonous" age grouping, suggesting a steady source of eroded material. * The Oslo Rift shows at least three separate peaks, resulting from diverse materials eroded from the surrounding landscape. * Amazon River Study: * Samples were collected along the entire length of the Amazon River to identify the specific bedrock being eroded into the river system.
The Jack Hills Study and the Hadean Earth
Jack Hills, Western Australia: * This site is famous for providing a glimpse back into the "Hadean" era (named after Hades due to the traditional view of early Earth as a molten ball). * The rock units here include siliciclastic detrital rocks, specifically conglomerates. * Sedimentary layering is clearly visible in the Jack Hills units.
Research Methodology: * The process is described as extremely tedious. * Rocks are crushed, and individual zircon grains (often hundreds or thousands) are picked out under a microscope and secured to a surface, such as sticky tape. * Laboratories often use multiple facilities for analysis to ensure reproducibility. * Analyses include U-Pb dating, oxygen isotope measurement, and trace element analysis.
Findings and the "Cool Early Earth" Hypothesis: * Zircons in this study show age ranges extending back to . * One specific zircon from Jack Hills is dated at , the oldest measured mineral on Earth. * Oxygen Isotopes: * The zircons contain oxygen isotopes that are "above the supracrustal" level. * This suggests the presence of liquid water during the Hadean time. * This evidence contradicts the traditional idea of a "red hot" early Earth and supports a "cool early Earth" model where water was present much earlier than previously thought.
The "Oldest Rock" Distinction: * There is a distinction between the oldest rock and the oldest mineral. While some rocks in Canada are marketed as the "oldest rocks," the individual zircon grains within certain sedimentary rocks (like those in Jack Hills) can be significantly older than the rock containing them.
Questions & Discussion
Q: Where is the oldest rock on Earth? * A: Locations in Canada are often cited. There was some mention of a place named "Athabasca," though the speaker was uncertain if that was the exact correct name. It was noted that people have even sold pieces of these rocks on the internet.
Q: What is the schedule for Tuesday's exams and review? * A: * The professor will be in the room until approximately on Tuesday to allow for review, after which the room will be closed to organize samples. * The regular hand specimen test is scheduled for Tuesday at in the lab. * The professor clarified that there is no lecture on Tuesday; the time is dedicated to review and the specimen test. * The final exam is scheduled for on a separate date, as per the regular exam schedule.