l07 paleointensity ssrm · the experiments described up to this point use multiple heating cycles....

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Absolute Paleointensity Estimates• Theoretical Justification

– Almost all grounded in Neel theory of SD TRM and involve heating the sample. R d i i l i iti i th l b– Reproduce original remanence acquisition process in the lab.

– “Instantaneous”, discontinuous in time• Absolute Paleointensity Methods

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oto from

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Good Review Papers: Tauxe and Yamazaki, Paleointensities, in Kono (Ed.), Treatise on Geophysics, v. 5, Elsevier, 2007.Dunlop, D.J., Physical basis of the Thellier–Thellier and related paleointensity methods, Physics of the Earth and Planetary Interiors, 187, 118–138, 2011

Igneous materials Fired clay potsMafic dike Pallasite meteorite

Absolute Paleointensity Methods

Archaeomagnetism:Historical lava flows, pottery, baked clay, archaeological slag., p y, y, g g

Paleomagnetism:Basaltic glass, volcanic rocks, inclusions in silicates.

Extra‐terrestrial magnetism: Meteorites, lunar rocks

Igneous materials Fired clay potsMafic dike Pallasite meteorite

Paleointensity

NRM =F(B0, grain size, concentration, composition, “recording process”)NRM= {magnetic activity}B0

Theoretical Bias

NRM is assumed to be ~linear with applied field.

If the slope can be determined through laboratory proxy measurements (Mlab/Blab), then the NRM of a given specimen Mnrm can be mapped to an estimate of the ancient field Banc

Tauxe, 2008

If NRM was acquired by a mechanism difficult to reproduce in the laboratory,the normalization will be more difficult or even impossible.

CRM and DRM very difficult to exactly reproduce. TRM acquired over a timescale >>laboratory experiment (cooling of a pluton), anc

and lab may be different by as much as a factor of two.Multicomponent NRM (difficult to isolate primary remanence)

Néel Model for SD Thermoremanent Magnetization

0vM ( )TRM(T ) =M (T )tanh

vM ( )

s B eo r

B

T BkT

T B

Bias is always small, soTanh(b)b

nsity

0

0

vM ( )TRM(T ) M (T )

vM ( )TRM(T ) M (T )

s B eo r

B

s Bo r e

B

T BkT

T BkT

“constant” External field during cooling

at T

BeTR

M inte

Absolute paleointensity – Underlying assumptions at TB

NRM is a TRMpTRM Laws of Additivity, Independence, and Reciprocity serve as the foundation of most paleointensity methods. (SD grains)

TRM intensity is linearly proportional to the external field applied at TB (for small B)

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Partial Thermoremanent MagnetizationA rock’s total TRM can be viewed as portions acquired in distinct temperature intervals, or windows of blocking temperatures.

pTRM(T2,Tc,0)=0tanh(0)=0

pTRM(T0,T1,0)=0tanh(0)=0

T0 T1 T2 Tc

B0= 0 B00 B0= 0

Only those SD grains with T1 <TB <T2 will acquire a TRM

pTRM(T1,T2,B0)

Th lli (1938) d t t d 3 i t l l i TRM

tanh(0)=0tanh(0)=0

Thellier (1938) demonstrated 3 experimental laws governing pTRMs

Law of AdditivityLaw of IndependenceLaw of Reciprocity

Partial Thermoremanent Magnetization

PTRMs are only influenced by the magnetic field that is present during cooling through their respective TBinterval.

Law of Additivity

pTRM3

pTRM4

pTRM5

TRM

If you add up each of the PTRMs you should arrive at the total TRM.

Total TRM =pTRM1 + pTRM2 + pTRM3+…

Blocking temperature spectrum can be decomposed into non‐overlapping fractions

Butler, 1990.

pTRM1

pTRM2pTRM6

pTRM7pTRM8

overlapping fractions

Paleointensity

Underlying assumption of linearity...

For most natural assemblages of magnetic minerals, the assumption of linear acquisition of magnetization is validmagnetization is valid.

Especially for intensities within the scope of the modern field (max: ~65 µT)

This assumption should be tested whenever possible

Linearity holds true for equant particles in < 100 mT.Strongly elongate particles will behave in a more nonlinear fashion

Tauxe and Yamazaki , 2007

Predicted TRM expressed as a fraction of saturation for various particle sizes and distributions of magnetite

Underlying assumption of linearity...

SD particlesSDMD particles

op & Özdem

ir, 199

7

Range of Modern Field Values

Dunlo

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Paleointensity There are many caveats...

1. The proportionality “constant” may not be constant because NRM acquisition may not be a linear function of the applied field.

2. The specimen may have altered its ability to acquire remanence through natural weathering, chemical alteration, or lab heating.

3. If the original NRM is carried by MD or PSD grains, then the exact conditions of NRM acquisition will be difficult to reproduce because unblocking may occur at a different temperature than blocking.

4. If the sample is anisotropic in its remanence acquisition and the lab field is not parallel to the ancient field, then the proportionality constants can be quite different. ** We can in many cases correct for anisotropy

Paleointensity

There are caveats...

5. If the NRM was acquired by a mechanism difficult to reproduce in the lab ( CRM TCRM DRM ) h i ill b l i ibl i h(e.g., CRMs, TCRM, DRMs), then it will be almost impossible to estimate the proportionality constant.

6. If the NRM contains multiple components (e.g., IRMs, VRMs, etc.), then it may be difficult to isolate the primary remanence.

7. TRM acquisition varies with cooling rate. The laboratory cooling rate is almost always faster than the natural cooling rate, resulting in overestimate of paleofield. ** We can correct for this – to a point.

Absolute Paleointensity Methods

• Methods requiring heating– Königsberger‐Thellier‐Thellier (KTT) MethodKönigsberger Thellier Thellier (KTT) Method

• Stepwise heating – Shaw‐type Methods

• Single (or 2x) heating to T > Tbmax

– Continuous high‐temperature measurements– Multi‐specimen methods– Microwave MethodMicrowave Method

• Isothermal methods– ARM/IRM normalization methods (quasi‐absolute)– “FORC” (Preisach) paleointensities

KTT Method: samples are heated up to an intermediate temperature and allowed to cool in a known lab field:

I Fi ld (T B B )

Paleointensity: Stepwise Heating Experiments

In‐Field (T1, B=Blab)M1 = NRM + pTRM(T1,TRT)

Sample is reheated to the same temperature and allowed to cool in the same field in the opposite orientation:

In‐Field(T1,B=‐Blab)M2 = NRM ‐ pTRM(T1,TRT)

Tauxe, 2008

Redrawn from Königsberger (1938)Based on the pioneering work of Thellier and Thellier, 1959.

p ( 1 RT)

vector subtraction allows you to determine the pTRM gained at each step.

0.5 (M1+M2) = NRM0.5(M1‐M2)=pTRM

This relies heavily on the Law of Reciprocity.

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Thellier method: Coe protocol (Coe, 1967)

As magnetic shielding improved, a new technique was devised, “the Coe technique”. after Coe (1967). Currently the most widely used technique.


The first heating step is done in zero field, which allows you to see how much magnetization is unblocked.

Zero‐Field (T1, B=0): M1 = NRMIn‐Field (T1, B=Blab): M2 = NRM + pTRM(T1,Trt)

Again, the lab magnetization is calculated by vector subtraction. M ‐M = pTRM(T T )M2‐M1 = pTRM(T1,Trt)

This is a zero field in‐field step (ZI)

Alternatively you can do an in‐field zero field step (IZ) as described by Aitken (1988).


In‐Field (T1, B=Blab): M1=NRM +pTRM(T1,TRT)Zero‐Field (T1, B=0): M2=NRMM1 M2 TRM (T T )

Tauxe, 2008

In either case, the NRM decreases and the PTRM grows:

M1‐M2= pTRM (T1,TRT)

Paleointensity: Stepwise Heating Experiments Paleointensity: Stepwise Heating Experiments

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The PTRM Laws of Additivity, Independence, and Reciprocity serve as the foundation of most paleointensity methods.


If these laws didn’t apply, then this technique would not work.

Often times, researchers will plot this data directly against each other. This type of diagram is called an Arai plot (Nagata et al., 1963)


RM re

maining

Tauxe, 2008

NR

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Low temperature steps can be repeated to determine whether the magnetic mineralogy has


g gychanged during subsequent heating.

These tests (triangles) are called “pTRM checks”.

If the sample shows differences in pTRM acquisition at the same

NRM

remaining

Tauxe, 2008

temperature step, then the data is suspect and is often rejected.

Examples

Radiocarbon‐dated Hawaiian lave flow

Age: 3850±250 3720±250

Coe et al., 1978

Examples:Hawaiian Lava Flows (17,860±670 BP)Samples from the same flow

Coe et al., 1978

PaleointensityNone of these are SD size!PSD and MD grains may also cause

paleointensity experiments to fail.

Dunlop and Özdemir gave a suite of samples of known grain size a pTRM across a narrow temperature interval (350‐375˚C).

Then they thermally demagnetized the samples

Low‐T pTRM tailUnblocking below <TB> High‐T pTRM tail

Unblocking above <TB>

Tauxe, 2008

demagnetized the samples.

The Law of Reciprocity failed spectacularly!

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Paleointensity

ing

This sort of behavior, where the Law of Reciprocity fails results in Arai plots that look like this:

ctional rem

anen

ce re

main

Tauxe, 2008

Frac

This sagging away from the ideal SD line is an expression of pTRM tails.

Paleointensity

If a portion of the curve is used, then the result will be biased.

ng

This means that the proportionality constant has not been correctly estimated.

People often try to interpret portions of Arai plots. e.g.,

“I’ll only use the low temp steps because the high temp steps are influenced by mineral alteration”

tional rem

anen

ce re

mainin

Tauxe, 2008

“I’ll only use the high temp steps because the low temp steps are influenced by viscous remanence.”

Fract

Both of these interpretations could be wrong!

In order to identify imbalances between blocking and unblocking temperatures, Tauxe et. al. invented the “IZZI protocol”


IZZI protocol (Tauxe and Staudigel, 2004) + pTRM checks (Coe, 1978) + tail check (Riisager and Riisager, 2001)

This method alternates the ZI and IZ steps.

It embeds a pTRM check in each ZI step.

1. Measure NRM2. Zero‐Field T13. In‐Field T14. In‐Field T25. Zero‐Field T26. Zero‐Field T37. pTRM‐check T18. In‐Field T3

And it has an extra zero field step between ZI and IZ steps to look for pTRM tails.

Tauxe, 2008

39. Tail‐check: zero‐field T3

Paleointensity

Tauxe, 2008

Four heatings (the pTRM check and the pTRM tail check steps) at every other temperature stepDouble heatings at the intervening temperature steps

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PaleointensityThe “IZZI protocol” is extremely sensitive to pTRM tails, which appear as zig‐zag behavior in Arai plots.

Blue squares are pTRM tail checks.The fact that many are negative indicate that

Low temperature pTRM tail

High temperature pTRM tailThe fact that many are negative indicate that

the pTRM tail is a low temperature tail.

If the pTRM tail was a high temperature tail, then the difference in the NRM remaining would be positive.

If you see zig‐zag behavior in an Arai plot, be suspicious!

Tauxe, 2008

pTRM tail

Open (closed) symbols are the IZ (ZI) steps.p

IZ steps are typically farther from the ideal line than are the ZI steps

Zig‐zagging should raise warning flags about the reliability of data acquired by such non‐ideal specimens.

( ) p

Modifications for “non‐ideal” MD behaviorNote that pTRM checks, pTRM tail checks, IZZI protocol, etc. do not allow us to “correct” the data

Only allow us to identify non‐ideal behavior for rejection.

The experiments described up to this point use multiple heating cycles. The IZZI protocol sometimes heats a sample as many as 50 times!

Alternative paleointensity techniques aim to minimize thermochemical alteration

Success rate on lavas typically 5‐15%

Heating in vacuum or inert atmosphereChoose a method that minimizes time spent at high temperaturesMicrowave heatingMaterials resistant to alteration

Glass‐hosted magnetite, Silicate‐hosted magnetite

Paleointensity

Alternative paleointensity techniques aim to minimize the adverse effects of repeated heating:

Controlled atmospheres

By heating your samples in a vacuum or in an inert atmosphere, you can slow and impede mineralogic alteration.

Commonly used atmospheres are nitrogen, argon, and helium.

The original Thellier team tried controlling the heating atmosphere.

Generally, this usually helps improves the number of successful experiments.

Multi‐specimen Methods.

PaleointensityAlternative paleointensity techniques aim to minimize the adverse effects of repeated heating:

Collect many samples from a single unit and assume they represent a homogenous material.

Each specimen is heated only once, always at the same temperature and in a direction parallel to the NRM.

Various fields are used and all the results are stacked to yield a single paleofield estimate.

The paleofield estimate is the field strength that produces a magnetization equal to the original NRM intensity.


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Materials That Resist AlterationExsolved ferrimagnetic particles in silicate single crystals

plagioclase crystal

magnetic separate from a plagioclase crystal.

Tardun

o et al., 2006

Silicate crystals extractedfrom lava flows can yield excellent results while the lava flows themselves may be prone to alteration or other nonideal behavior.

Inclusions more SD like

Low‐T measurements indicate magnetiteThe trace amounts of magnetic

material silicates demand high‐sensitivity magnetometers.

Archaeological Slag as a Paleointensity Tool

“Slag” is the glassy residue left behind from metallurgical smelting and is common at archaeological sites throughout the Near East.

Materials That Resist Alteration

Backscattered SEM image of a magnetite dendrite in archaeological copper slag from Israel. Shaar, et al., EPSL, 2009.

Example of a successful paleointensity measurement that demonstrates that archaeological slag is an excellent recorder of the strength of the Earth’s magnetic field. (Shaar et al. EPSL, 2009)

10 m

Paleointensity ‐ The Microwave Method

Walton et al., 1994

Methods that minimize heating

Your kitchen microwave is tuned to the vibrational frequencies in water.

If we choose the right frequency, we can find the vibrational frequency of magnetite.

In theory this would allow us to heat

Dunlop

, 201

1

In theory, this would allow us to heat only the magnetic grains in the sample and avoid alteration of the rest of the mineral phases in the rock or archeological sample

A comparison of Thellier and microwave paleointensity determinations on sister specimens of a Hawaiian lava sample. The microwave determination gives the correct slope of –1 and utilizes the entire data set.

The Shaw MethodsShaw (1974)

1. NRM is progressively AF demagnetize to establish the coercivity spectrum of the specimen prior to heating.

Methods that minimize heating

2‐3. Specimen then given an anhysteretic remanence (ARM) in the a small bias field then AF demagnetized

4‐5. Sample given a total TRM in a known lab field, AF demagnetized to determine the coercivity spectrum of the lab TRM.

6. ARM is imparted and then AF demagnetized, to determine the coercivity distribution of the ARM afterdetermine the coercivity distribution of the ARM after heating.

Compare the pre‐ and post‐heating ARM coercivity spectra plots. If they are different, then the sample has experienced alteration and the estimate is suspect Plot NRM(AF) vs. TRM(AF)

Plot ARM(AF)0 vs. ARM(AF)1

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The Shaw MethodsModifications/variations for “non‐ideal” behavior

• The deviation from unity between ARM0 and ARM1 at each AF step is used to “correct” the TRM data for the effects of h l l i (K 19 8 l h & Sh 198 )thermal alteration (Kono, 1978; Rolph & Shaw, 1985).– Theoretical basis for this correction remains to be proven.

• A second TRM and a third ARM can be added as a check for the validity of the above ARM correction (Tsunakawa & Shaw, 1994).

• Low‐T (77K) demagnetizations are added to reduce contributions from MD grains. This can also be used in KTT‐type experiments.– Energy barriers to domain wall motion low at TV

• Can combine all of the above modifications in the LTD‐DHT Shaw method (e.g., Tsunakawa et al., 1997).

Modifications/variations for “non‐ideal” behavior

MTRM1* = MTRM1 x MARM1/MARM2

(From Tauxe and Yamazaki, 2007; data from Yamamoto et al., 2003)

KTT – vs ‐ Shaw

• KTT proso Gradual heating means

• Shaw proso Sample spends less o Gradual heating means

data from low‐T steps may provide useful paleointensity estimate, even if sample alters at higher temperatures.

• Cons

cumulative time at high temperature, which may reduce alteration.

o In a variation a “correction” for high‐temperature alteration is made using ARM data.

• ConsIf lt ti i t to Longer cumulative time

at elevated temperatures means sample may be more likely to alter

o If alteration is temperature‐dependent, heating immediately to T>Tc precludes use of low‐T blocking interval.

o Poor understanding of the applicability of the ARM correction.

Methods with NO heating

• IRM normalization – Provides relative to quasi‐absolute paleointensity

• “FORC” paleointensity– Models TRM acquisition from first principles, so theoretically provides an absolute paleointensity without heating the sample.

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Paleointensity IRM Normalization

Certain samples cannot be heated at all. (e.g. meteorites, lunar samples)

REM ratio as a function of magnetizing field for various minerals: magnetite, titanomagnetite, Fe5Ni, monoclinic pyrrhotite

and Roche

tte, 2004

If TRM acquisition is linear, then normalizing by a material’s SIRM may provide a way to get order of magnitude estimates of intensity

REM=NRM/SIRM

This only works (moderately well) for SD grains.

The problem with the TRM/SIRM approach is that d i t t li it f TRM d th t f

Gattacceca a

Martian meteorite ALH84001

., 2003

domain state, linearity of TRM, and the nature of the NRM cannot be assessed.

These results are difficult to interpret

REM~10‐3BMARS ~0.1BEARTH

Antretter e

t al

IRM Normalization

•Order of magnitude variation in remanence acquisition can arise purely from grain size variations.

•BUT sometimes that’s good enough to say something important about planetary evolution (for example).

•Lower uncertainty may be achieved for well‐characterized, homogonous samples that can be carefully calibrated.

TRM/SIRM

Tauxe, Lectures in Paleomagnetism, 2007

Extra Slides

Paleointensity ‐ Remanence Anisotropy

Samples may also fail paleointensity experiments due topaleointensity experiments due to remanence anisotropy.

You can identify this by comparing the direction of the pTRM to the orientation of the lab field.

If it is >5˚, then you should measure the sample’s anisotropy tensor.

Selkin et al., 2000

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Paleointensity ‐ Cooling Rates

Paleointensity experiments may also need to be corrected for differences in the cooling rate in the lab vs. that in naturein nature.

The longer a sample remains in the presence of a field at elevated temperatures, the more alignment will occur.

These are theoretical curves, but people have also collected empirical p p pdata on different cooling rates.

Tauxe, 2008

o During standard thermal heating, heat is transferred to magnetic minerals via conduction through the sample matrix.

Microwave heating (Walton et al., 1994)

o During microwave heating, magnetic component of the microwave electromagnetic field couples with spins in the magnetic grains, transmitting energy directly to the spins.

o The system is tuned so that the resonant frequency is optimized for exciting magnetic spin waves.

o Can use microwave heating in KTT or Shaw‐type experiment.

Tauxe, Lectures in Paleomagnetims, 2007

“FORC” (Preisach) Paleointensity

•Given a coercivity and interaction distribution as described by a FORC distribution, forward calculate the expected TRM acquired in a variety of fields.

•Assumes assemblage of randomly‐oriented, interacting, SD grains

•In theory, can account for cooling rate, and can eliminate undesirable MD & SP parts of the distribution.

•In practice, the method seems to perform slightly better than the REM method, but worse than KTT methods. But the technique has not yet been extensively tested.

Muxworthy & Heslop, 2011

Muxworthy & Heslop, 2011

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Cenozoic and Mesozoic “dipole low” is probably a common state of the geomagnetic field

Paleointensity (0‐200 Ma)

Tauxe, 2008

geomagnetic field

Anomalously high values occurring in the latter part of the Cretaceous and early Cenozoic and during the last few thousand years.

Summary of data in the MagIC database meeting minimum acceptance criteria for last 200 Ma. Blue dots are submarine basaltic glass data. Red diamonds are single crystal results. Triangles are all other data meeting the same consistency criteria

Paleointensity (0‐3500 Ma)


The Oldest Paleointensity

The geomagnetic field was ‘on’ by 3.2 Ga.

Geomagentic field strength 3.2 billion years ago recorded by single mineral crystals (Tarduno et al., Nature, 5 Apr 2007)

Archean Data (Tauxe and Yamazaki , 2007)

Archeomagnetism

Ben‐Yosef et al., 2010

l07 paleointensity ssrm · the experiments described up to this point use multiple heating cycles....

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