Lecture IV Recap of Laschamp directions & relative ...gilder/P43/P43_2020_lect… · MAGNETIC...

Lecture IV Recap of Laschamp directions & relative

paleointensity

Magnetic anisotropy

P4.3 2020 Paleo- and Geomagnetism, S. Gilder

Stepwise alternating field (af) demagnetization from the A core isolates linear trajectories that decay toward the origin on “Zijdervelt” plots above ~30 mT.

We systematically fit line segments between the 30 and 60 mT steps and did not force the line to the origin.

“A”


Stepwise alternating field (af) demagnetization from the D core isolates linear trajectories that often do not decay toward the origin. We systematically fit line segments between the 30 and 70 mT steps and did not force the line to the origin.


Low field susceptibility (Xlf) reflects the contribution from all minerals (dia-, para- and ferrimagnetic). Because the susceptibility of magnetite is 1000x that

of other minerals, Xlf of magnetite often dominates the total contribution. SIRM reflects the concentration of ferrimagnetic minerals only.

Variations in magnetic parameters correlate well in the two cores. There might be a shift below 18.6 m between the two.


ARM vs. SIRM correlate better with each other than with bulk susceptibility (X), so we will not use X as a normalizer for relative paleointensity.


Proxies for high-coercivity magnetic minerals, such as hematite and

goethite, vs. soft magnetic minerals (magnetite) can be estimated by the S-

ratio and hard IRM (HIRM):

The S-ratio300mT (−IRM−300mT/SIRM) is scaled so that a value of 1 reflects

100% of magnetic remanence-bearing minerals with a coercivity lower than 300 mT, (magnetite or maghemite).

When the S‐ratio is close to zero or if it has negative values, contributions

from hematite and/or goethite will be significant.

The HIRM100mT is defined as 0.5 × (SIRM + IRM−100mT), where IRM−100mT represents the remanent magnetization obtained by first

saturating the sample in a high field (e.g., 1 or 1.5 T), and then applying a back‐field of −100 mT to reverse the

saturation isothermal remanent magnetization (SIRM) contributed by

magnetite or maghemite.

S-ratios and HIRM values are very coherent between the two cores. The consistently high S-

ratios indicate the cores contain mostly magnetite.


We imparted partial anhysteretic magnetizations in the 30-60 mT and 30-70 mT windows, then stepwise AF demagnetized the ARM.

All samples show very similar demagnetization spectra, which means the destruction of the NRM magnetization by AF fields should be comparable (uniform) for all

samples.


Here is a comparison of the normalized AF demagnetization between the NRM and the ARM data as a function of depth.

Note how consistent the ARM data are compared to the NRM. From this data we can calculate the relative paleointensity.

“A core”


Here is a comparison of the normalized AF demagnetization between the NRM and the ARM data as a function of depth.

Note how consistent the ARM data are compared to the NRM. From this data we can calculate the relative paleointensity.


Here are two ways to calculate the relative paleointensity plotted against inclination.

“A”


Relative Paleointensity Four ways to estimate the relative paleofield strength are quite similar--they are all based on the magnetization intensity of the NRM after AF demagnetization at 30 mT. The one using 51 mT gives a comparable, yet

somewhat different picture.


Relative paleointensity plotted against inclination.


Here is the combined data set of relative paleointensity plotted against inclination.


Here is the combined data set of relative paleointensity plotted against normalized declination and inclination. Note that pInt minima coincides where inclination = 0, but not declination, which never goes “antipodal”.


MAGNETIC ANISOTROPY Because magnetic grains have shape anisotropy or crystalline anisotropy,

their magnetizations vary as a function of the direction of the measurement. This is also true for an assemblage of grains in a rock.

Magnetic fabrics


The magnetization (M) of magnetically isotropic material is proportional to a weak external field (H):

M=kH where k is magnetic susceptibility.

In magnetically anisotropic material: M1 = k11H1, M2 = k22H2, M3 = k33H3

where k11 ≥ k22 ≥ k33 = k1 ≥ k2 ≥ k3 = kmax ≥ kint ≥ kmin

with k1/k3 = P (degree of anisotropy); k1/k2 = L (magnetic lineation) and k2/k3 = P (magnetic foliation)

Anisotropy of magnetic susceptibility measurements take 5-15 minutes/ sample.


In magnetically anisotropic material:

M1 = k11H1, M2 = k22H2, M3 = k33H3 where k11 ≥ k22 ≥ k33 = k1 ≥ k2 ≥ k3 = kmax ≥ kint ≥ kmin

with k1/k3 = P (degree of anisotropy); k1/k2 = L (magnetic lineation) and k2/k3 = F (magnetic foliation)

Other definitions : T = shape parameter = (2η2 - η1 - η3)/( η1 - η3) = 2·ln·F/ln·P - 1

T = 0 = triaxial; T > 0 to 1 = oblate; T < 0 to -1 = prolateη1 = ln k1; η2 = ln k2; η3 = ln k3

P’ = corrected degree of magnetic anisotropy = exp (2[(η1 - ηm)2 + (η2 - ηm)2 + (η3 - ηm)2])1/2

Mean susceptibility = km = (k1 + k2 + k3)/3


One can thus quantify the fabric

in rocks using magnetic methods.


A plot of lineation vs. foliation = Flinn diagram


AMS measurement schemes-scalar (AGICO) 15 position manual vs. 3 position “spinning” = (64

measurements / revolution)


Anisotropy of anhysteretic remanent magnetization (AARM) 12 (6x2) measurement scheme (SushiBar)


Magnetic susceptibility changes as a function of

composition and grain size.


Single domain grains yield

“inverse” fabrics because the

susceptibility measured along the long axis of

the grain ≈ 0.

AARM does not have this problem.


AMS of redeposition experiments under flowing conditions:

(1) k1 points in the direction of flow for low stream velocities

(2) k1 spreads out, with inclination to 0, as stream velocity increases.

One can thus deduce flow direction and velocity in sediments.


Evolution of a sedimentary fabric getting increasingly overprinted

by a tectonic fabric as one approaches the Pyrenees


A sedimentary fabric gets increasingly overprinted by a tectonic fabric approaching the Pyrenees


Here is another example from Japan. Note the use of P’ and T--

these are quite useful.


P

T

Superposition of two magnetic fabrics

Housen et al. (1993) P

T

k3

Two equal fabricsGrowth of a new fabric


Central Asia Fast uplift rates, very good exposures


Kuitun

Jinguohe Yaha

Jinguohe The Tianshan Sections


Throughout the Tianshan domain, Cenozoic sections coarsen upward-- going from red-yellow, clay-rich mudstone to gray conglomerate.

Does this coarsening sequence reflect changes in climate and/or uplift?

Does the stratigraphy correlate in time?

Xiy

u Fo

rmat

ion


Jinguohe : Northern flank of the Tianshan Mountains


Magnetostratigraphy allows one to precisely date the ages of

sediments by comparing against the global geomagnetic polarity time

scale.

Our magnetostratigraphic study of the 4571-m-thick Jingou River section (Xinjiang Province, China) identified 67 polarity chrons, which correlated with the reference geomagnetic polarity time scale between ~1 Ma and ~23.6 Ma.


The magnetostratigraphy allows us quantify sedimentation rates, which increase through time at Jinguohe.


Do increases in sedimentation rate occur smoothly or punctually?

At Jinguohe, we found that sedimentation rate correlates with magnetic susceptibility,

which is a proxy for magnetite concentration, with important changes seen at 10-11 Ma and

at 15 Ma.

Charreau et al. (2009)


Housen et al. (1993)

P

T k3

Superimposition of fabrics can explain k3 directions but not changes in P and T


Magnetite concentration increases from the base to

the top of the section, which reflects a change in

sediment source No change in relative grain

size


Anisotropy of anhysteretic remanent magnetization vs. anisotropy of magnetic susceptibility from the same section


ODP 1063 magnetic fabrics are typical of sedimentary rocks with k3

perpendicular to bedding and k1+k2 in the sedimentation plane.


There should be a relationship between magnetic field intensity and magnetic anisotropy. When the field is stronger, the torque to align the

magnetic minerals is greater. High fields should lead to increasingly prolate fabrics with k1 axes pointing toward the magnetic field direction.

There is a hint of a relationship in this in the ODP1063 cores.


Here is a plot of the virtual geomagnetic poles of the Laschamp event.


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