1. peluruhan radioaktif

Upload: tito-hafiz

Post on 18-Jul-2015

351 views

Category:

Documents


8 download

TRANSCRIPT

7. GEOKIMIA ISOTOP

No Massa (P+N)No atom (P)

7. GEOKIMIA ISOTOP1912, Thomson mengukur perbandingan muatan thd massa (e/m) dr ion positif sinar kanal. Hasilnya dgn gas neon 91% atomnya memp massa normal 9% atomnya memp massa 22/20 lebih berat dr yg normal Jadi dlm atom yg mempunyai no atom sama dpt sedikit berbeda dlm massanya. ini disebut Isotop.

Metode Peluruhan RadioaktifNuclides Half-Life (year) 4,47 x 109 9 10

Decay Constant (year 1) 1,55 x 1010 10 11

Effectiv e Range (Year)10 7 109 9

Materials

238U 235U

206Pb+8 4He 207Pb+7 4He

Zircon, uraninite

0,71 x 101,39 x 10 4,88 x 10 1,06 x 10 1,31 x 10

9,72 x 104,99 x 10 1,42 x 10 6,54 x 10 4,72 x 10 1,21 x 10

10 7 1010 7 10 10 7 10 109

Zircon, uraniniteZircon, monazite Micas, igneous & met rocks Igneous rocks

208Pb+64He232Th 87Rb

9

87Sr 143Nd

10

11

9

147Sm 40K

11

12

40Ar

9

10

10 4 10 0 - 105

9

Micas, hornblende, sanidine, igneous and met rocks

14C

14N

5730 40

-1

Wood, charcoal, bone, shells

238U238 92

206Pb206 82

U

Pb

238U

= proton + netron proton

92U=

Neutrons = 238-92 = 146

Kegunaan Isotop di geologiIsotop strongtium utk mendeteksi magma pembentuk batuan. Magma yg asalnya dr mantel bgn atas mempunyai konsentrasi isotop 87Sr < dibanding magma kerak Dgn naiknya temp maka fraksinasi isotop stabil akan mengalami penurunan, dan pada bat sedimen fraksinasinya lebih besar dr pd bat beku Isotop carbon dipakai utk dating benda purbakala Isotop hidrogen dgn fraksinasi terbesar dipakai utk mendeteksi air (geohidrologi)

GEOKRONOLOGIPeluruhan radioaktif adalah jumlah atom yg terurai dlm waktu t selaras atau proporsional dgn jumlah atom yg ada N/ t = - N (1) N = Jumlah atom yg tidak berubah pada waktu t = konstante peluruhan, dinyatakan dlm jumlah atom yg meluruh per atom per detik Misal : = 0,01, berarti tiap-tiap detik 1/100 dari atom yg ada akan terurai

= menunjukkan N berkurangBila t = 0 t, Maka : N = N0 e - t N = N0 N (2)

Waktu Paruh (t)

0

1 2

3 4

0. P/D = 1:0P = parent D = daughter

(P0)

1:01:1 1:3 1:7

1. P/D = : 2. P/D = : 3. P/D = :

4. P/D = 1/16 : 15/16

1:15

Contoh soal Berdasarkan hasil analisa laboratorium, diketahui rasio antara dan 14 N suatu contoh fosil kayu = 1 : 31 Waktu paruh (t ) isotop 14 C = 5.730 tahun. Berapa umur fosil kayu tersebut?14

C

Soal Ujian Berdasarkan hasil analisa laboratorium, diketahui rasio antara 14 C dan 14 N suatu contoh fosil kayu = 1 : 13 Waktu paruh (t ) isotop 14 C = 5.730 tahun. Berapa umur fosil kayu tersebut?

Waktu Paruh (t)Waktu paruh (half time) adalah waktu yg diperlukan untuk terurainya setengah dari atom yg semula ada t=t, N = N0 e Maka : - t

N = N0 / 2

N0/2 = N0 e - t = e - t , t = ln 2

t = ln 2 /

(3)

Nt/No = (1/2) t/tNt = Jumlah zat yg ada N0 = Jumlah zat mula-mula

t = Waktu peluruhant = Waktu paruh

Contoh soalUnsur 222 Rn mempunyai waktu paruh 4 hari. Jika unsur ini mula-mula berjumlah 64 gram, berapa gram yang masih ada setelah 20 hari? Nt/64 = (1/2) 20/4 Nt = (1/2)5 x 64 Nt = 1/32 x 64 = 2 gram Unsur 137 Cs mempunayi waktu paruh 30 tahun. Berapa tahun yang diperlukan untuk menjadikan unsur tersebut tinggal 25% dari jumlah semula? 25/100 = (1/2) t/30 = (1/2) t/30 t/30 = 2 t = 60 tahun Unsur 80 Br mempunyai waktu paruh 18 menit. Jika setelah meluruh 1,2 jam unsur itu bersisa 5 gram. Berapa gram jumlah unsur tersebut mula-mula? 5/N0 = (1/2) 72 / 18 5/N0 = (1/2)4 N0 = 80 gram

Menentukan konstanta peluruhan ()R = Ro e -t 2 = e t t = ln2/ Ro = Ro e -t t = ln 2 = ln2/ t

t karbon = 5.730 tahun ln 2 = 0,693 = ln2/ t = 1,2 x 10-4

MENENTUKAN UMUR (T)Apabila N diganti P (parent) dan N0 dgn Po Po = Jumlah atom waktu mineral terbentuk P = Jumlah atom yg ada dlm suatu mineral D = Jumlah daughter atom = Po - P, krn terurainya 1 atom P menghasilkan 1 atom D Maka : Po P P =P+D = Po e - t = (P+D) e

- t

D

= P (e - t -1)= (P+D) e - t = e - t = P+D / P atau = Po / P = ln (1+ D/P)

(5)

P P/ P+D e t e t t

t = 1/ ln (D/P +1)t = 1/ ln (Po/P)

atau(6)

CARBON DATING (14C 14 N)Metoda yg berdasarkan peluruhan radioaktif memungkinkan penentuan umur batuan & benda yg mempunyai asal biologis Hal ini disebabkan peluruhan berlangsung dengan laju konstan dan tdk tergantung kondisi eksternal

Rasio antara jumlah nuklida induk dgn nuklida anak yg mantap / stabil akan menunjukkan umurnyaPada tanaman / binatang, proporsi 14 C dan 12 C mempunyai nilai tertentu. Namun setelah mati, fosil / mayatnya tdk menyerap 14 C (rasio karbon), bahkan 14 C meluruh menjadi 14 N dgn waktu paruh 5.730 tahun Umumnya metoda ini sampai 50.000 tahun

1. METODE RUBIDIUM STRONTIUMKomposisi Rb di alam = 27,8% 87Rb + 72,2 % 85RbKomposisi Sr di alam = 7,04% 87Sr + 9,87% 86Sr + 0,56% 84Sr + 82,53% 88Sr87Rb

87Sr

Persamaan (5) menjadi : 87Sr = 87Rb (e - t -1)87Sr

di ukur =87Sr semula (i) + 87Rb (e - t -1)

Karena perbandingan isotop lebih mudah utk diukur drpd harga absolut, maka persamaan 8 menjadi : (87Sr/86Sr) diukur = (87Sr/86Sr)semula + 87Rb (e-t -1) /86Sr

METODE RUBIDIUM STRONTIUMMencari T87Sr

87Rb

87Sr

/ 86Sr & 87Rb / 86Sr diukur : ditent dgn analisa kimia 87Sr / 86Sr i (semula) = ditent dgn menganalisa mineral dlm batuan yg sama yg tdk mengandung Rb, misal : apatit & Ca plagioklas Peluruhan 87Rb menyebabkan : Berkurangnya ratio 87Rb / 86Sr dan Bertambahnya ratio 87Sr / 86Sr

t = 1/ ln (D/P + 1), maka menjadi :87Sr _ 87 Sr 86Sr 86 Sr i +1 87 Rb 86 Sr

87Sr

di ukur = 87Sr semula + 87Rb (e87Rb

-t

-1)

t = 1/ ln

(e -t -1)= 87Sr di ukur - 87Sr semula

238U

206Pb- t

Dari persamaan D = P (e206Pb 206Pb

-1)- t

=

238U

(e

-1)

diukur = 206Pb semula (i) + 238U (e - t -1)diukur

(206Pb/204Pb)

= (206Pb/204Pb)semula (i) + 238U(e- t -1) /204Pb

t = 1/ ln (D/P + 1), maka menjadi : Pb _ 204 Pb206

t = 1/ ln

Pb 204 Pb i206

+1

U 204 Pb238

235UD

207Pb

= P (e - t -1) 207Pb = 235U (e - t -1)207Pb

diukur = 207Pb (i) + 235U (e - t -1)= (207Pb/204Pb)semula (i) + 235U(e-t -1) /204Pb

(207Pb/204Pb)

diukur

t = 1/ ln (D/P + 1), maka menjadi :207 204

t = 1/ ln

Pb _ Pb235

Pb 204 Pb i207

+1

U 204 Pb

232Th 208Pb

D

= P (e -t -1) 208Pb = 232Th (e -t -1)208Pb

diukur =

208Pb(i)

+

232Th

(e

-t

-1)

(208Pb/204Pb)

diukur

= (208Pb/204Pb)semula(i) + 232Th(e-t -1)/204Pb

t = 1/ ln

Pb _ 204 Pb208 232 204

Pb 204 Pb208

i +1

Th Pb

STABLE ISOTOPESPer Mil Isotopic Variation , of Heavier Isotop Element with Isotopic Standard Used Hidrogen Standard mean ocean water (SMOW) Isotop Measured2H

Minerals & Rocks -180 to +20

Natural Waters -410 to +50

/ 1H

Karbon Kalsit PDB (Pee Dee Belemnite)Oksigen SMOW Silikon Quartz vein, Mother Lode, California Sulfur Canyon Diablo meteorite troilite

13C 18O 30Si

// /

12C 16O 28Si

-35 to +5-2 to +36 -2,2 to +3,2 -50 to +15

34S

/

32S

-45 to + 60R = 18O / 16O atau 13C / 12C = (R sampel / R standard 1) . 1000

Hasil spektrometer massa:R = Rasio absolut misFaktor =18O/ 16O

atau

13C

/

12C

R sampel 1 . 1000 R standard

Terkadang masing-masing lab membuat standard sendiri, agar praktis = RA/ RB =18O

/ 16O kondisi A 18O / 16O kondisi B

3. Rasio Isotop

Diukur oleh Mass Spectrometry

Dinyatakan dlm (permil)Yang sering digunakan : 13 C/ 12 C Sampel yg mengandung S > menggunakan34S/32S

Oil dan bitumen 2 lebih ringan drpd kerogen.Apabila oil > kerogen, berarti korelasi negatif. Oil dan kerogen berasal dr ling evaporit = -20

Waxy oil = -30 Sebagian besar oil & kerogen = -24 28

Bissada et al, 1992

the n- paraffin distribution curve for the Persian Gulf oil is concave, the oil is inferred to be from a marine algal source.

Furthermore, because the n-paraffins are isotopically light ( 13 C values of -28 to -31), the source is inferred to be a shallow-water (500 m) water column above.

Bissada et al, 1992

Plot of C13 isotop values for oils in the South Sumatra basin, showing the subdivision of oils in to groups derived from terrigenous, lacustrine and mixed kerogens

Ginger & Fielding, 2005

Plot of carbon isotope data for pre-Ngimbang and Ngimbang sediments collected from some wells

NGIMBANG CLASTIC

NGIMBANG CLASTIC

NGIMBANG CLASTICPRE-NGIMBANG

Tabel Konsentrasi rata-rata Rb, K, Sr & Ca di batuan beku dan sedimen (Turekian and Wedepohl, 1961)

Tipe batuan Ultra basa Basaltic High Ca granitic Low Ca granitic Syenite Shale Sandstone Carbonate Deep sea carbonate Deep sea clay

Rb (ppm) 0,2 30 110 170 110 140 60 3 10 110

K (ppm) 40 8.300 25.200 42.000 48.000 26.600 10.700 2.700 2.900 25.000

Sr (ppm) 1 465 440 100 200 300 20 610 2.000 180

Ca (ppm) 25.000 76.000 25.300 5.100 18.000 22.100 39.100 302.300 312.400 29.000

Tabel Konsentrasi rata-rata U, Th, Pb di batuan beku, sedimen, & metamorf (Data U and Th from Rogers & Adams (1969), and Pb from Wedepohl (1974)

Tipe Batuan

U (ppm)

Th (ppm)

Pb (ppm)

ChondritesAchondrites Iron meteorites Troilite Ultramafic rocks Gabbro Basalt Andesite Nepheline syenit Granitic rocks Shale

0,010,07 0,008 0,009 0,014 0,84 0,43 2,4 8,2 4,8 3,2

0,040,36 0,01 ? 0,05 3,8 1,6 8 17 21,5 11,7

1,00,4 0,1 5,9 0,3 2,7 3,7 5,8 14,4 23,0 22,8

SandstoneCarbonate rocks Granitic gneiss Granulite

1,41,9 3,5 1,6

3,91,2 12,9 7,2

13,75,6 19,6 18,7

Carbon IsotopesCarbon isotopes on whole oil, saturate, aromatic and other fractions of the crude oil have been used to differentiate marine oils from terrestrial oils. But, Sofer (1984) showed that this was invalid, based on a statistical analysis of a wide range of oils of known source. However, Sofer demonstrated that marine oils could be distinguished from terrestrial oils based on a Canonical value (Cv) calculated from the saturate and aromatic fraction carbon isotope values (Table 3). In fact the Cv value differentiates algal sourced oils, marine or nonmarine (Cv< 0.47), from terrestrial (fluvio-deltaic) sourced oils (Cv> 0.47).

Robinson, 1987

Carbon IsotopesIsotopically light kerogen, l-amorphous, with 13 C values ranging between -26 to -35 ascribed to phytoplankton residing in environments overlying stratified, shallow (500 m) waters, within silled basins where the prevailing source of carbon in the photic zone is atmospheric C02 (with negligible influence from the deep organic C02 by virtue of its diffusion and dilution through a long and well-mixed water column).

Bissada et al, 1992

Carbon IsotopesIsotopic compositions of oils ascribed to the Miocene Monterey formation invert to kerogen isotopic composition of -20 to -22 , suggesting deep-water facies for the source rocks. In fact, the Monterey Fm. is believed to have been deposited in silled, deep-water (>1,000 m) basins with a common overlying water body open to the Pacific Ocean (Ingle, 1981). In contrast, 13 C values for oils ascribed to the Devonian/ Mississippian New Albany Shale of the Illinois basin invert to kerogen 13 C values of -28 to -30 . The New Albany is believed to have been deposited in a shallow (< 200 m), restricted epicontinental sea with a well developed anaerobic water column (Rich, 1951). One can therefore generalize that isotopically heavy oils ( 13 C values of -18 to 24), whether marine or lacustrine in origin, can be attributed to deep-water facies, and isotopically light oils ( 13 C values of -26 to -32, whether marine or lacustrine, can be attributed to shallow-water facies. Bissada et al, 1992

Contoh : Ro = Jika aktifitas suatu massa karbon dari tanaman / binatang yg masih hidup R = aktifitas massa karbon yg akan ditentukan umurnya

Menentukan Umur (t)R = Ro e et -t

R/Ro = e

-t

= Ro/R

t = ln Ro/R

t = 1/ ln Ro/R