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Structural Geology Notes for October 11, 2016

by: Brandon Cook

Structural Geology Notes for October 11, 2016 Geology 355

Marketplace > University of South Carolina - Columbia > Geology (GEOL) > Geology 355 > Structural Geology Notes for October 11 2016
Brandon Cook


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About this Document

The notes covers Geochronology and Radioactive Dating
Structural Geology
Dr. J Knapp
Class Notes
geology notes
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This 5 page Class Notes was uploaded by Brandon Cook on Monday October 10, 2016. The Class Notes belongs to Geology 355 at University of South Carolina - Columbia taught by Dr. J Knapp in Fall 2016. Since its upload, it has received 12 views. For similar materials see Structural Geology in Geology (GEOL) at University of South Carolina - Columbia.


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Date Created: 10/10/16
Geology 355 Notes October 11, 2016 Geochronology PowerPoint on Appalachians will be on Blackboard Susie will be available from 2 to 4:30 Fall Break!!! Deformation Metamorphism and Time (chapter 13) Metamorphic Facies Predictable set of mineral comp. From P and T Aluminosilicate triple jct. Polymorphs but same exact composition. Al is highest in clay minerals rocks started as siltstone or claystone Garnet is one of the most important mineral: large Stability range rep. In many conditions. Look at comp. From P and Temp difference -Metamorphic Fabrics Pre-tectonic (pre-kinematic) Syn-tect. (syn-kinematic) Post-tect. (post-kinematic) I-Clicker Question If we find a shale (pelite) metamorphosed blueschist facies conditions, we should expect to find blue minerals such as glaucophane. True False (not formed in shales but in basalts) Petrologic Terminology Porphryoclast-relict large crystal in a metamorphic rock. Large crystal predated meta. Porphyroblast-large crystal grown during metamorphism. (Garnets) rotating during metamorphism. Poilkiloblast-overgrowth of a mineral around small inclusions of other minerals Schistosity- layering developed during metamorphism Foliation planar fabric strike and dip Measure (foliation of Appl. mtns NE strike and SE dip) Geochronology (time of earth study--study of time on earth) Tell time relative time A < B< C a before B Absolute time (talk about how to put geologic age and number to an event) Not until radiometric dating/ radioactivity, before then, the use of fossils Isotopic dating Radioactive elements (parents) decay to stable (daughters) non-radioactive elements The rate at which this decay occurs is constant and known If we know the rate of decay and the amount present of parent and daughter, calculate how long this reaction had been occurring. Compare to decay constant chemical mineral put in a certain point Atoms of elements w/ the same # of p and varying # of neutrons (n) Z = atomic number # of protons N= neutron number # of neutrons A= mass number= sum of protons and neutrons Is Z+N=A Examples: 235U, 238U, 87Sr, 86Sr, 14C, 12C ( C is known as carbon-dating) Types of Radioactive Decay (know this!) -Alpha Decay: loss of a 4He (2n, 2p) -e.g. 147Sm decays to 143Nd (dating materials extracted from Earth’s mantle) -Beta Decay: n to p -e.g. 87Rb decays to 87Sr (not electron in outer shell) shoot electron out of nucleus Electron capture: p to n - 40K decays to 40Ar (no change in atomic mass but changing the element) -important radioactive decay for Geology as K is in alot of different minerals Half Life The time half of the parent to decay to daughter (is constant) Half-lives for different isotopes have a wide range 1 half-life is ½ 2 half-lives is ¼, to ⅛ so on 5 half-lives is 1/32 U238 U235 has long half-lives decay to Lead U235 half-life is 4.5 by U235 half-life is .7 by (Zircons and Apatites) (Zircons crystalizes in magma tough don’t weather away- limited amount) K40 to Ar40 is 1.3 billion yrs half-life (electron capture) C14 to N14 5730 year half-life (used in dating biological, water from dissolved carbon dioxide, and artifacts <70,000 years) Rb87 to Sr87 half-life is 47 billion years Closure Temperature Above a certain temperature (Tc), elemental species will diffuse out of the mineral structure If parent and /or daughter atoms are lost, the radiometric clock will not be accurate. Low closure temp calculated since last above the last closure temperature--- key in metamorphicism function of T. We calculate the time since the mineral was last above Tc for the particular element on interest Rb-Sr System Decay to Sr from Very long half-live 48 to 49 billion years Usually measured several samples of the whole rock and plagioclase I-Clicker Question 2 The decay of 87Rb to 87Sr , involving the change …. Alpha Decar Beta Decay Gamma Decay Electron Capture U-Ph System Typically date individual U-bearing minerals: -Zircons Analyze different pop. of these mineral, which record different amounts o K-Ar System Modified in the 1970s to the Ar-Ar system to date moon rocks 40Ar 39Ar System Measure the ratio of these two isotopes in stepwise increments Analyze separate mineral fractions from same rock Hornblende Biotite Muscovite K-Feldspar Each mineral has a diff. closure temp which respect to Ar Fission Track dating When radioactive decay takes place, damage occurs to the crystal lattice of minerals With time, these fission tracks will heal or anneal By counting the # of tracks in a mineral, we can place quantitative estimates on the time at which mineral has been below the annealing temperature Different minerals heal at different temperatures and over different timescales I Clicker question #3 If the horizontal lines in the diagram at the right represent schistosity, the metamorphism that….. Pre-kinematic Syn-kinematic Post-kinematic All of the above


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