Download Investigation of plagioclase crystals from the ~2

SURF 2010 Project Description
Elissa Barris
Introduction
Development of the theory of geodynamo, the mechanism in the outer core responsible
for the Earth’s magnetic field, hinges upon our understanding of past changes in field strength,
termed paleointensity. In particular, the details of the relationship between the geodynamo and
the solid inner core still evade us. Some theoretical models predict that the crystallization of the
inner core may have stabilized the geodynamo and increased the intensity of the magnetic field,
effects which would appear in paleointensity records [1, 2]. Hence more extensive knowledge of
changes in paleointensity would lend great insight into the thermal evolution of the Earth.
Paleointensity is studied by measuring remanent magnetization in rocks containing
ferromagnetic minerals, which are originally magnetized by the field in which they form (during
the crystallization of lava). The conventional method of determining paleointensity involves
stepwise heatings of bulk rock specimens while measuring magnetization after each step [3].
During this process, the rock’s natural magnetization (NRM) is gradually replaced by a partial
thermal magnetization (pTRM) imparted in a known laboratory field. The paleointensity is then
determined by comparing the relative rates of NRM destruction and pTRM acquisition.
Paleointensity studies have given us a relatively accurate account of the recent field
history, primarily that of the last ~10 million years (Ma). However, conventional methods using
bulk rock specimens can be more problematic when dealing with older rocks, which become
increasingly likely to acquire secondary magnetizations. Over time it is also common for ironbearing clays to become incorporated into the crystal matrices of rocks, which upon heating can
form new magnetic minerals, resulting in a low field paleointensity bias [4]. We are particularly
challenged when measuring Precambrian paleointensities (older than ~550 Ma). As such very
little reliable data exist for this period, though it represents over 80% of Earth’s history, and
encompasses such events as the onset of inner core growth and the stabilization of the
geodynamo [5].
An alternative to treating bulk rock has recently been developed, using instead individual
silicate crystals from igneous rocks [6]. Certain such crystals can contain small ferromagnetic
inclusions that are capable of carrying stable remanent magnetizations. The inclusions are
protected from natural and heating-induced alteration by the silicate matrices, thereby
eliminating many of the problems associated with bulk rock measurement. The single crystal
approach is a promising tool for studying Precambrian paleointensities, and has already resulted
in a number of important results [7].
Proposed Research
In this project I plan to examine single plagioclase crystals extracted from rocks of the
Black Range dikes of Western Australia, formed ~2.7 billion years ago (Ga). My goal is to
determine whether these crystals could carry accurate records of Precambrian paleointensity.
This work will be part of an ongoing project in the Earth Magnetism Laboratory (EML) at
Michigan Tech, for which paleointensity has already been measured in bulk rock specimens of
the same origin. The previous data indicate that the ~2.7 Ga magnetic field may have been
significantly weaker than today’s, but being obtained from bulk rock samples these results may
be biased toward low paleointensities due to the presence of clays [8]. If the low field intensity is
verified by single crystal measurements, it would provide strong evidence of an inefficient
geodynamo at ~2.7 Ga. It could also be indicative of a period preceding the formation of the
inner core, which would have important implications for models of the thermal evolution of the
Earth.
However, before commencing paleointensity experiments on single silicate crystals, the
presence of ferromagnetic inclusions with proper magnetic properties must be confirmed.
Previous studies at the EML have already verified single-domain magnetic inclusions in two
other dike swarms of Western Australia similar to the Black Range swarm, making the
probability of finding suitable crystals from the latter source very high.
Procedure
My research plan consists of the following main stages (to be completed between July
rd
23 and August 27th).
1) Extraction of plagioclase crystals (10 days): Plagioclase crystals will be isolated from
eleven different dikes of the Black Range swarm using the separation technique described in [6].
The rocks will be ground using a mortar and pestle, and the powder will be repeatedly washed in
order to remove the majority of the groundmass (including clays). Once dry, plagioclase crystals
will be picked from the remaining mineral fraction with the aid of an optical microscope. The
selected crystals will be cleaned ultrasonically to remove any remaining clay before beginning
further analyses. The aim is to find 20 crystals from each dike.
2) Magnetic hysteresis measurements (15 days): Magnetic hysteresis parameters (saturation
remanence, Mrs, saturation magnetization, Ms, and coercivity, Hc) will be measured from the
extracted crystals using an Alternating Gradient Field Magnetometer (AGFM). Magnetic
hysteresis is the best indication of whether suitable ferromagnetic inclusions are present. From
the hysteresis loops I will calculate the squareness ratio (Mrs/Ms); ratios between 0.25 and 0.5
indicate nearly single-domain inclusions, which are desired for paleointensity measurements. I
will also use the AGFM to check crystals for magnetic anisotropy, which can result from a nonrandom orientation of magnetic inclusions. For this I will compare the magnetic hysteresis
parameters obtained from measuring each sample at multiple orientations with respect to the
applied field. Anisotropic samples will be excluded, since anisotropy may later bias pTRM
acquisition during heating.
3) Determination of unblocking temperatures (10 days): Several selected crystals will be
thermally demagnetized through a series of increasing heatings as described above. I will
measure the remaining NRM after each step using a 2G Superconducting Rock Magnetometer.
The resulting unblocking temperature spectra will be characteristic of the specific mineralogy of
the magnetic inclusions.
I will present the results of my project as a research report. If my analyses confirm the
suitability of Black Range plagioclase crystals for paleointensity studies, I, with other members
of EML, intend to continue working with them to determine the ~2.7 Ga paleointensity they
record. The EML has plans to develop a laser-based heating system that will enable more
accurate determinations of paleointensity from single crystal specimens, for which conventional
techniques, though acceptable for bulk rock specimens, are not always accurate. This will lead to
the acquisition of valuable data representing the Precambrian geomagnetic field, providing
important clues in regards to the functionality of the early geodynamo and the onset of
crystallization of the solid inner core.
References
1.
Hollerbach, R., and C. A. Jones (1993), Influence of the Earth’s inner core on
geomagnetic fluctuations and reversals, Nature, 365, 541–543.
2.
Costin, S. O., and S. L. Butler (2006), Modelling the effects of internal heating in the core
and lowermost mantle on the earth’s magnetic history, Physics of Earth and Planetary
Interiors, 157, 55–71.
3.
Coe, R. S. (1967), The determination of paleointensities of the Earth’s magnetic field
with emphasis on mechanisms which could cause non-ideal behaviour in Thelliers
method, Journal of Geomagnetism and Geoelectricity, 19,157–179.
4.
Tarduno, J. A. and A. V. Smirnov (2004), The paradox of low field values and the longterm history of the geodynamo, in Timescales of the Paleomagnetic Field (J.E.T.
Channell, D.V. Kent, W. Lowrie, and J.G. Meert, Editors), AGU Geophysical Monograph
Series 145, p. 75–84.
5.
Macouin, M., J. P. Valet, and J. Besse (2004), Long-term evolution of the geomagnetic
dipole moment, Physics of the Earth and Planetary Interiors, 147, 239–246.
6.
Tarduno, J. A., R. D. Cottrell, and A. V. Smirnov (2006), The paleomagnetism of single
silicate crystals: Recording geomagnetic field strength during mixed polarity intervals,
superchrons, and inner core growth, Reviews of Geophysics, 44, RG1002.
7.
Smirnov, A. V., J. A. Tarduno, and B. N. Pisakin (2003), Paleointensity of the early
geodynamo (2.45 Ga) as recorded in Karelia: A single crystal approach, Geology, 31,
415–418.
8.
Smirnov, A. V., and J. A. Tarduno (2005), Thermochemical remanent magnetization in
Precambrian rocks: Are we sure the geomagnetic field was weak?, Journal of
Geophysical Research, 110, B06103.