geochronology – some basics...KE796Bt-4a 1300 14.28 2.01699 2.02881 0.0004 4233 14.25 297.2...
Transcript of geochronology – some basics...KE796Bt-4a 1300 14.28 2.01699 2.02881 0.0004 4233 14.25 297.2...
geochronology ndash some basics
Isotope geochemistry and geochronology M Tichomirowa
radioactive und radiogene isotopes changes of isotope abundances through decaybull spontanous decaybull induced decay (triggered by neutrones protones and other particles)
decay of atomic nuclei instable nuclei decay to stable nucleiemission of radiation (bdquoradioactivityldquo)bull α decay loss of helium-nucleusbull β- decay a neutron transfers into a proton + electron loss of an electronbull β+ decay a proton transfers into a neutron + electron loss of a positron
(positively charged electron)bull γ decay release of electromagnetic emissionbull electron capture one electron (mostly from the K-orbit ndash bdquocaptureldquo) is captured by the
nucleus There reaction with a proton results in a neutron Daughter elements often are inexcited state Transfer by γ radiation (eg through relocation of an electron from the L-orbitto the K-orbit) in a stable nucleuscan be used as γ emittors
Geochronologie - Einfuumlhrung
M Tichomirowa
Rutherford amp Soddy rule of the radioactive decay
-dNdt = λ middot Nλndash decay constant (probability that one
atom decays in a certain time)N ndash number of radioactive atomst - time
N = N0 e ndashλt
e ndash integration constant (2718)
rate of decay depends on the concentration of themother-isotope and slows down
Every decay sytem has a special decay constant= measure of the decay rate
t frac12 = ln 2λ = 06931λ
half-live time where half of the initial atoms (N0) decayed
Isotope geochemistry and Geochronology
stable and instable atomsbull stabile isotopes t frac12 gt 1016 abull instable (radioactive) isotopes t frac12 lt 1016 ain nature ndash continous transition
geochronology uses decay systems withhalf-lives between 109 and 1011 a
geochronology ndash calculation of time t
N = N0 e ndashλt
N0 = N + DD = N (e λt ndash 1)
rarr determination of the concentration of themother isototope
rarr determination of the concentration of thedaughter isototope
Determination of concentrations bybdquoisotope dilutionldquo (ID)(error le 1 for Rb-Sr le 01 for other
isotope systems eg Sm-Nd)
analyticsbull chemical digestionbull separation of elements (mother and
daughter on ion exchange columns)bull measurement of isotope ratios
(mother and daughter) TIMS SIMSSHRIMPLA-ICP-MS
mass spectrometer
TIMS ndash thermal ionization mass spectrometer ndash MS for solids (eg salts)
ICP-MSbull solutionbull laser
ion microprobebull SHRIMP (sensative high resolution ion microprobe)bull SIMS (secondary ion mass spectrometer)
Isotope geochemistry and Geochronology M Tichomirowa
Rb-Sr - method
87Rb rarr 87Sr + β- (λ = 142 middot 10-11 a-1)Nebel et al 2011 λ = 1393 middot 10-11 a-1
M Tichomirowa
alkali metal element alkali earth elementatom radius 148 Aring 113 Aring
substitutes K substitutes Ca
Rb Sr
Rb-Sr - method
Isotope geochemistry and Geochronology
M Tichomirowa
D = N middot (e λt ndash 1)
87Sr = 87Rb middot (eλt ndash 1) if 87Sri = 0
87Sr = 87Sri + 87Rb middot (eλt ndash 1)
( )87Sr 87Sr 87Rbmdash = mdash + mdash middot (eλt ndash 1)86Sr 86Sr 86Sri
t = ln + 11 87Sr86Sr ndash (87Sr86Sr)iλ 87Rb86Sr[ ]
Y = b + m middot X
Rb-Sr - method
Isotope geochemistry and Geochronology
M Tichomirowa
(87Sr86Sr)i
eλt ndash 1
Rb-Sr - method
M Tichomirowa
isochron diagramrequires several samples
pre-requisition for datingwith an isochron systembull closed systembull identical initial isotope ratios
(87Sr86Sri)
rarr dating of events(processes) that have homogenized87Sr86Sr ratios
Rb-Sr - method
M Tichomirowa
Dating of
1) magmaticrocks
2) metamorphicrocks
Rb-Sr - method
time
M Tichomirowa
Isochron diagram ndash resetting the system (bdquothe clockldquo)Rb-Sr - method
M Tichomirowa
closure or blocking temperature
definition
discussion- Implications for dating (processes ages)
Closure temperatures (oC) for frequent rock forming mineralsfor Rb-Sr- and Ar-Ar-dating methods
minerales Rb-Sr-system Ar-system
amphibolemuscovitebiotiteK-feldsparplagioclase
550-650a
450-550b c k
250-350b c d k
625-800e
505-800e j k
500-700c f h
325-425c d k l
280-375c g h k
150-200c i
175h
a Brabander and Giletti (1995) b Jaumlger et al (1967) c Burbank and Anderson (2001)d Del Moro et al (1982) e Giletti (1991) f Harrison (1981) g Harrison et al (1985)h Berger and York (1981) i Harrison and McDougall (1982) j Baxter and DePaolo (2000)k Baxter et al (2002) l Harrison et al (2009)
Rb-Sr - method
Isotope geochemistry and Geochronology
M Tichomirowa
mineral or whole rock isochronsRb-Sr - method
Isotopengeochemie und Geochronologie M Tichomirowa
2) Sedimentary rocks
- on authigenic minerals(eg clay minerals)
problemsa) alterationoverprint
(rarr opening of the system)b) detrital minerals
eg micararr reconstruction of provenance
(rocks areas direction ofsediment transportcomplicated by mixedprovenance sources)
Rb-Sr - method
Isotope geochemistry and Geochronology M Tichomirowa
Rb-Sr - method
M Tichomirowa
RbSr - Methode
Calculation of an isochron ndash error discussiondefine a line throughlinear regression
error of the age calculationdepends onbull error xbull error ybull diversity of x-values
- data points are weightedaccording to their deviationfrom the line for y-valuesand for x-valuesΣ(ac2ab2) = minimum
- data points more distant to theline (isochron) have a lower weight
MSWD ndash mean standard weighteddeviation
rarr should be low (ideally lt 1 -3)
ISOPLOT (shareware)Excel AddIn ISOPLOT-R
M Tichomirowa
Rb-Sr - method
scheme of an bdquoerrorchronldquo
Isotopengeochemie und Geochronologie M Tichomirowa
mixing lines mixing of two components
- with reliable age information- no reliable age
Rb-Sr - method
M Tichomirowa
Sr isotope geochemistry
initial 87Sr86Sr ratio of the earliest Earth
derived from basaltic achondrites(formation of silicate melts ndashsimilar to that of planets)
BABI ndash Basaltic AchondriteBest Initial
= 069896(=87Sr86Sri of the Earth)
LUNI ndash 87Sr86Sri of the moon= 069892age 45 Ga (meteorite age)basalts from lunar maria39 ndash 31 Mathrough meteorite bombardement
Rb-Sr - method
Isotope geochemistry and Geochronology M Tichomirowa
schematic developmentof 87Sr86Sr ratios in thesolar nebula
Rb-Sr - method
Isotopengeochemie und Geochronologie M Tichomirowa
Differentiation of crustal and mantle rocks
upper mantle87Sr86Sri = 0704 plusmn 0002
crust87Sr86Sri gtgt 0704
rarreg source of granitesassimilation of crustalmaterial regionaldifferences
rarr provenance of sedimentsstratigraphic variations ina sediment basin
rarr recent ocean87Sr86Sr = 070906
Rb-Sr - method
M Tichomirowa
Tim e (Ga)012345
CanadaFennoscandiaGreenland
07060
07050
07040
07030
07020
07010
07000
06990
06980
bulk E ar thR bS r = 003
D ev elopm ent l ine for theC anadian and F ennos c andiandepleted m antle R bS r ~ 002
FOZO
BABI
a
87 86Sr Sri
Tichomirowa M Grosche G Goumltze J Belyatski BV Savva EV Keller J Todt W (2006) The mineral isotope composition of two Precambrian carbonatite complexes from the Kola Alkaline Province ndash alteration versus primary magmatic signatures Lithos 91 229-249
Rb-Sr - method
Isotope geochemistry and Geochronology
Isotope geochemistry and Geochronology M Tichomirowa
basalts
Rb-Sr - method
M Tichomirowa
Ocean
Sr input
a) Input from riversthrough rockweathering(carbonatesilicate)
b) bdquomantleldquo-Sr in mid oceanic ridges
less importantc) Input from groundwater
(eg Sr from carbonates)
Rb-Sr - method
Isotope geochemistry and Geochronology
Isotopengeochemie und Geochronologie M Tichomirowa
Rb-Sr - method
Isotope geochemistry and Geochronology M Tichomirowa
application
1 metamorphic rocks
gneisses from the Erzgebirgeareal isochrons from bulk rock
Krentz (1985)
Rb-Sr - method
M Tichomirowa
Krentz (1985)
Rb-Sr - method
Isotope geochemistry and Geochronology
Caledonian metamorphism
M Tichomirowa
Tichomirowa et al (2001)
Rb-Sr - method
Isotope geochemistry and Geochronology
( )87Sr 87Sr 87Rbmdash = mdash + mdash middot (eλt ndash 1)86Sr 86Sr 86Sri
M Tichomirowa
2 magmatic rocks
Variscan granites from the Erzgebirge
Gerstenberger (1987)
Rb-Sr - method
Isotope geochemistry and Geochronology
P Schielinski (2018) master thesis
biotites
biotites
apatitefeldspar
initial Sr isotope ratio mantle or
crustal source I- or S-type
Why do biotites have different RbSr ratios
Tichomirowa et al (2019)
Sample- numberRb [ppm] Sr [ppm] 87Sr86Sr 87Sr86Sr
plusmn 2σ(mean) 87Rb 86Sr 87Rb86Sr
calculated measured [nmolg] [nmolg]KE796Bt-3a 1384 1403 211211 209348 00566 4507 1388 3247KE796Bt-3b 1316 1404 204859 215657 00030 4286 1397 3068KE796Bt-3a+b 1384 1403 208035 215657 00298 4507 1393 3237
KE796Bt-4a 1300 1428 201699 202881 00004 4233 1425 2972KE796Bt-4b 1311 1499 200104 201077 4270 1498 2850KE796Bt-4a+b 1305 202881 4252 1461 2909
KE796Bt-7a 1301 1341 211755 212323 00001 4236 1327 3192 -011KE796Bt-7b 1306 1350 211658 212111 00002 4252 1335 3185 011KE796Bt-7a+b 1303 1346 211706 212217 00001 4244 1331 3189
KE796Bt-8a 1318 1397 206175 206338 00085 4293 1388 3093 -016KE796Bt-8b 1320 1401 206386 00052 4298 1394 3083 016KE796Bt-8a+b 1319 1399 206175 206362 00069 4296 1391 3088
KE796Bt-9a 1216 1432 194552 194849 00002 3959 1438 2753 -030KE796Bt-9b 1226 1453 194216 194613- 3994 1459 2737 029KE796Bt-9a+b 1221 1443 194384 194731 00002 3977 1449 2745
P Schielinski (2018) master thesis
87Rb86Sr err () 1σ 87Sr86Sr err () 1σ
1 3237 10 215657 022 2981 10 202881 023 2981 12 202659 024 3188 05 212217 025 1903 05 071549 026 2939 18 072038 027 3088 10 206362 028 2745 10 194731 028 2867 05 197439 029 2825 05 196085 02
10 00599 25 070951 0211 3019 05 072067 0212 1470 05 134909 0213 1322 05 128630 02
1) What sample number (Nr 1-13) correspond to what minerals feldspar biotite apatite
P Schielinski (2018) master thesis