Thrivikramji on ISOTOPE GEOLOGY

7/31/2019 Thrivikramji on ISOTOPE GEOLOGY

1/42

STABLEISOTOPEGEOCHEMISTRY

DR. THRIVIKRAMJI. K. P.

UNIVERSITY OF KERALA (RTTD)


2/42

ISOTOPES: 2 TYPESA) STABLE

E.G.,: ISOTOPES OF OXYGEN,SULPHUR & CARBON

B0. UNSTABLE: URANIUM,

POTASSIUM, CARBON


3/42

Atom = a nucleus of protons +neutrons in a cloud of electrons.

Element defined by no. ofprotons in nucleus

No. of protons in nucleus fixed.But no. of neutrons variable


4/42

NO. OF PROTONS IN NUCLEUS FIXED;BUT NEUTRONS VARY.

FOR CARBON,

PROTONS=6NEUTRONS VARY AS 6, 7, OR 8.

HENCE,

12C= 6+6; 13C = 6+7 &14C=6+8


5/42

STUDY OF VARIATIONS IN

RATIOS OF ISOTOPES OF 2H/1H,13C/12C, 18O/16O, and 34S/32S.

RATIOS ESTIMATED IN LABWITH A MASS SPECTROMETER

BY CONVERTING IT TOGASEOUS STATE.


6/42

Isotopic variation or fractionation in

minerals, water, and gasesarise out of several common

chemical and physical processes,like cooling of hydrothermal fluids inmineral deposition, evaporation or

condensation of water, mixing of twoor more sources of fluid, ormetabolic activity of organisms.


7/42

Isotopic compositions of O, H, C& S reported as "delta" (d), is permilli, permil, or per mille)

(symbol, ).Enriched, +ve or depleted, -ve

relative to standard of knowncomposition.


8/42

RESULTS REPORTED AS:* higher vs. lower values

* heavier vs. lighter ( "heavier" isone with higher value)

* more/less positive vs. more/lessnegative (eg., -10 is more positive

than -20)

* enriched vs. depleted (eg., amaterial rich in 18O or 16O relative

to some other material).


9/42

(in ) = ((Rspl/Rstd) - 1))x1000where, R =heavy to light isotope in

sample or standard. For S,C & O,average terrestrial abundance ratio of h/l

isotope, ranges from 1:22 (S) to 1:500(O); 1:6410 (H). A +d says sample has

more heavy isotope vs. standard & adsays sample has less heavy isotope vs.standard. A d18O of +30 is that there

are 30 ppt or 3% more 18Oin sample vs.

standard.


10/42

Mass differences in elements of lowatomic numbers are good enough

for many physical, chemical, andbiological processes or reactions to

effect fractionation of various

isotopes. Equilibrium processesand kinetic isotope effects cause

isotope fractionation, by whichwaters and solutes earn uniqueratios of h/l isotopes indicating

source or causative processes.


11/42

In phase changes, ratio of h/lisotopes of molecules also change.

In equilibrium reactions, heavierisotope generally prefers species orcompounds @ higher energy state.34S prefers sulfate than sulfide so

sulfide depleted in 34S -not in

sulfate. During condensation-equilibrium reaction of water 18O& 2H enriches liquid phase against

16O 1H (lighter) to vapor phase

C i i f i S d d


12/42

Composition of isotope Standardsexpressed as 0 .

O & H isotopes > SMOW (StandardMean Ocean Water, Craig,1961) orVSMOW (Vienna-SMOW).

C standard > PDB (Pee Dee Belemnite)or VPDB (Vienna PDB).

O isotope in carbonates reported

relative to PDB or VPDB.S & N isotopes to CDT (for Caon

Diablo Troilite) and AIR (for

atmos heric air res ectivel .


13/42

Abundance (%) of Isotopes

1H=99.985; 2H=0.01512C=98.89; 13C=1.11

14N=99.63; 15N=0.3716O=99.759; 17O=0.037

18O=0.20432S=95.00; 33S=0.76;34S=4.22; 36S=0.014


14/42

18O = [(18O/16O) spl (18O/16O)std] / [(18O/16O) std)] x 1000

H d l i l A li i


15/42

Hydrological Applications:# Isotopically distinct finger prints of watersrecharged at different times or locations or flow-

paths# Unlike many chemical tracers, isotopes arerelatively conservative in reactions with catchmentmaterials,

e.g.,O & H isotopes in water;Meteoric waters hold distinct fingerprints untilmixing with waters of another composition;Or isotopes of dissolved species have reactions

with minerals or other fluids.# In a catchment solutes in waters derived from

atmospheric sources are commonly isotopicallydistinct from solutes derived from geologic andbiologic sources in the catchment


16/42

# Both biological cycling of solutes andwater/rock reactions

often change isotopic ratios of solutes inpredictable and recognizable directions;these interactions often can be reconstructed

from the isotopic compositions.# If water from an isotopically distinctive

source (e.g., rain with an unusual isotopic

composition) is found along a flowpath, itprovides proof for a hydrologic connection,despite any hydraulic measurements ormodels to the contrary.


17/42

Equilibrium reaction C16O2 +H2 18O 18 CO2 +

H216O

heavy = [(h/l spl.) (h/l std)] /[(h/l std)] x 1000 per mil


18/42


19/42


20/42


21/42


22/42


23/42


24/42


25/42


26/42


27/42


28/42


29/42


30/42


31/42


32/42


33/42


34/42


35/42


36/42


37/42


38/42


39/42


40/42


41/42


42/42

Thrivikramji on ISOTOPE GEOLOGY

Documents

Transcript of Thrivikramji on ISOTOPE GEOLOGY