Stable Isotope Geochemistry III - PowerPoint Presentation

Slide1

Stable Isotope Geochemistry III

Lecture 32 Slide2

The Antarctic Ice Record

Much subsequent

paleoclimate

effort has focused on δD in ice cores from Antarctica and Greenland.The Vostok core from Antarctica went back 400 ka. Subsequent work shifted to the EPICA core which went back >800 ka.Complications in interpretation arise here too because of changes in δD of the oceans and changes in atmospheric circulation result in complex relationship between T and δD, but temperatures can be worked out.Overall, agreement between the marine and Antarctic records is excellent, but shows some differences between Antarctic and global climate change.Slide3

Greenland Ice Record

Ice records from Greenland are not as long, but provide finer details of the last glacial cycle.

Greenland is ‘ground zero’ of glaciation.

They reveal extremely variable climate in the last ice age -Dansgaard-Oeschager events - likely related to iceberg events documented in deep-sea cores.Slide4

Feedback Factors

Milankovitch variations provide only a weak climate signal that has been apparently greatly amplified in the Quaternary by feedback factors.

June insolation at 60˚N appears to be the key sensitivity.

Feedbacks include:AlbedoShift of CO2 from atmosphere to oceans with consequent change in greenhouse effectChanges in ocean circulation, particularly with delivery of heat to the North Atlantic (ground zero for continental ice sheets).The role of CO2 is well documented by CO2 concentrations in bubbles in Antarctic ice.

Figure 12.45Slide5

The Next Ice Age?

From

Marcott

et al. (2013) Science, 339: 1198Slide6

Soil Paleoclimate Proxies

Hydrogen and Oxygen isotopes in soil clays reflect (with fractionation), the isotopic composition of meteoric water.

This allows reconstruction of

paleoprecipitation patterns - Cretaceous precipitation in N. America in this figure.Slide7

Pedogenic Carbonate

δ

18

O in pedogenic carbonate also reflects composition of meteoric water (with fractionation).In Pakistan, δ18O in paleosol carbonates record the evolution of the monsoons.Slide8

Stable Isotopes in High Temperature GeochemistrySlide9

Where does the water come from?Slide10

Hydrothermal Systems

One of the first of many important contributions of stable isotope geochemistry to understanding hydrothermal systems was the demonstration by Harmon Craig (another student of Harold Urey) that water in these systems was meteoric, not

magmatic.

For each geothermal system, the δD of the “chloride” type geothermal waters is the same as the local precipitation and groundwater, but the δ18O is shifted to higher values. The shift in δ18O results from high-temperature reaction (≲300°C) of the local meteoric water with hot rock. Acidic, sulfur-rich waters from hydrothermal systems can have δD that is different from local meteoric water. This shift occurs when hydrogen isotopes are fractionated during boiling of geothermal waters. The steam mixes with cooler meteoric water, condenses, Slide11

Importance of Hydrothermal Systems

Hydrothermal systems are the source of many ore deposits, including base metals (Pb, Zn, Cu), gold, tin, and many others.

Hydrothermal activity is also important in the chemistry of the oceans, the oceanic crust, and the plate tectonic cycle.Slide12

Water-rock ratios

For a closed system:

from which we can derive:

For an open system in which water makes 1 pass through the rock we start withand derive:Point is that maximum change in δ18O will be associated with maximum W/R.Slide13

Example: Lane Co., Oregon

Low δ

18

O in rocks, reflecting water/rock ratios, forms a bullseye around main area of mineralization and economic gold deposit.Slide14

ODP Site 1256, Eastern Pacific

δ

18

O in Hydrothermal SystemsBecause of the temperature dependence of fractionation, the effect of water-rock interaction at low and high temperature can be quite different.As seawater is heated, it exchanges O with the surrounding rock. At temperatures in the range of 300-400° C,

the net water-rock fractionation is

smal

l

,

1 or 2‰. Thus isotopic exchange results in a decrease in the

δ

18

O

of the rock and an increase in the

δ

18

O

of the water

.

At low

-temperature

fractionations

are quite

large, ~20‰.

The result of these reactions is to increase the

δ

18

O

of the

shallow

oceanic crust and decrease the

δ

18

O

of seawater.

Thus

the effects of low temperature and high temperature reactions are

opposing. Slide15

Sulfur Isotopes

Many ores are sulfides and sulfur isotopes provide important clues to their genesis, including temperatures of deposition.

Overview of δ

34S:Mantle, bulk Earth value ~0 (same as meteorites)modern seawater is +20 (has varied over Earth’s history with δ13C).Sedimentary sulfide, generally the result of bacterial sulfide reduction, can have δ34S as low as -40.Slide16

Mississippi Valley Sulfide Deposits

Mississippi Valley type Pb-Zn deposits are sediment-hosted (often carbonate) sulfides deposited from low-T hydrothermal solutions.

Source of sulfide is generally formation brine or evaporite sulfate (of ultimate seawater origin) that is subsequently reduced.Slide17

Archean MIF Sulfide

Most studies report only

34

S/32S as δ34S, but sulfur has two other isotopes 33S and 36S.We expect δ33S, δ34S, and δ

36

S to all correlate strongly, and they almost always do (hence few bother to measure

33

S or

36

S).

When Farquhar measured δ

33

S and

δ

34

S in Archean sulfides, he found

mass independent fractionations.

∆

33

S is the permil deviation from the expected δ

33

S based on measured δ

34

S.

Experiments show that SO

2

photodissociated

by UV light can be mass-independently fractionated.

Interpretation: prior to 2.3 Ga, UV light was able to penetrate into the lower atmosphere and dissociate SO

2

. In the modern Earth, stratospheric ozone restricts UV penetration into the troposphere(sulfur rarely reaches the stratosphere, so little MIF fractionation).

This provides strong supporting evidence for the

Great Oxidation Event

(GOE) at 2.3 Ga.Slide18

Stable Isotopes in the Mantle and MagmasSlide19

Oxygen in the Mantle

δ

18

O in olivine in peridotites is fairly uniform at +5.2‰.Clinopyroxenes slightly heaver, ~+5.6‰.Fresh MORB are typically +5.7‰Some OIB and IAV show deviations from this.Bottom line: no more than tenths of per mil fractionations at high T.Igneous rocks with δ18O very different from ~5.6‰ show evidence of low-T surface processing.At high-T, δ18O isotopes can effectively be used as tracers like radiogenic isotopes.Slide20

Hydrogen in the Mantle

Mantle sample restricted in hydrous minerals in xenoliths and submarine erupted basalts.

Mean

δD in solid Earth is about -70‰.Some variation in the mantle, but hard to pin down, partly because of fractionation during degassing.Slide21

Carbon in the Mantle

MORB and submarine erupted OIB have δ

13

C of close to -6‰.Most diamonds have similar δ13C, with average around -5‰.Carbonatites have the same δ13C, indicating the carbonate is mantle-derived, not from sediments.A subclass of diamonds, those with an eclogitic paragenesis, have much lighter carbon, with peak around δ13C ≈ -25‰.This carbon was likely organic in origin and was anciently subducted into the mantle.Slide22

δ18

O in Crystallizing Magmas

Fractionations between silicates and silicate magmas are small, but they can be a bit larger when oxides like magnetite and rutile crystalize.

We imagine two paths: equilibrium and fractional, the latter more likely. For fractional crystallization:In both theory and observation, there will be not much more than 1 or 2‰ change in δ18O.Slide23

Fractional Crystallization-Assimilation

Magmas intruding the crust can melt and assimilate crust (because the magmas are hotter than the melting temperature)

Energy to melt comes largely from the ∆H of crystallization, hence crystallization and assimilation will be linked.

If R is the ratio of mass assimilated to mass crystallized, the isotope ratio will change as:where subscripts m, 0, and a refer to the isotopic composition of the magma, the original magma, and the assimilant,

ƒ

is fraction of liquid remaining and

∆

is crystal/liquid fractionation factor.

This can lead to much larger change in δ

18

O.

Note error in

equ

. 9.69 in bookSlide24

Boron Isotopes

Stable isotope geochemistry has been expanding beyond the traditional isotopes.

The large mass difference between

10B and 11B results in large fractionations.Fractionation is mainly between trigonal (e.g., BOH3) and tetrahedral (e.g., BOH4–) forms.

Both forms in seawater.

Mainly borate (BO

3

) in boron minerals like tourmaline; BOH

4

-

in clays, probably substitutes for tetrahedral Si in other silicates.

Mantle, chondrites, most basalts: δ

11

B ~ -5‰. Variable in crustal rocks and sediments. Island arc volcanics are heavier - evidence of a

fluid

or seawater component.

δ

11

B = +39‰ in seawater. Seawater is heavier than anything else.

Fractionation, mainly as a result of adsorption of light B on clays, drives seawater to extreme isotopic composition.Slide25

Boron in the Ocean & Carbonates

Boron

is present in seawater both as B(OH)

3, and B(OH)4-. The reaction between them is:B(OH)3 + H2O ⇋ B(OH)4- + H+The relative abundance of these two species

depends on pH

The

isotopic composition of these two species must

vary with pH

if the isotopic composition of seawater is constant.

From

mass balance we have:

δ

11

B

SW

=

δ

11

B

3

ƒ

+

δ

11

B

4

(

1

-

ƒ)

where

ƒ

is the fraction of B(OH)

3

If the isotopic compositions of the two species are related by a constant fractionation factor, ∆

3-4

,

then:

δ

11

B

SW

= δ

11

B

3

ƒ

+ δ

11

B

4

-

δ

11

B

4

ƒ

= δ

11

B

4

-

∆

3-

4

ƒ

Solving

for δ

11

B

4

, we have:

δ

11

B

4

= δ

11

B

SW

+ ∆

3-

4

ƒ

δ11B4 depends on ƒ, which depends on pH.Boron is incorporated into carbonate by surface adsorption of B(OH)4-. Thus the δ11B in carbonates tracks δ11B4, which in turn depends on pH, assuming δ11B in seawater is constant.What will pH of seawater depend on?

Note error in book.Slide26

Seawater pH and Atmospheric CO2

from δ

11

BPearson and Palmer (2000) measured δ11B in foraminifera from (ODP) cores and were able to reconstruct atmospheric CO2 through much of the Cenozoic.Surprisingly, atmospheric CO

2

has been < 400 ppm through the Neogene, a time of significant global cooling. Much higher CO

2

levels were found in the Paleogene.

This has largely been confirmed by another paleo-CO

2

proxy, δ

13

C in 37-C

diunsaturated

alkenones

(Section 12.8.2; Figure 12.43). Atmospheric CO

2

conc

(397 ppm) is now higher than it has been for 35 million years.