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The knowledge gathered from these studies
led to the preparation and the publication of the
second article included in this PhD thesis. This
paper had a very important role in allowing the
author to achieve the necessary abilities to publish
scientific results to the scientific community.
These results are presented in chapter 4.
After having a good idea of the subject of study
through the above-mentioned techniques, the
best samples were taken to perform radiogenic
isotope geochronology on zircon crystals. At first,
it was necessary to separate the zircon grains with
several mineral separation techniques. These
separations were run by the PhD candidate with
the initial supervision and guidance of his PhD
supervisors and laboratory technicians. These
separation techniqueswere appliedwithgreat care
to minimize possible laboratory contamination.
Samples were cleaned and dried before being
crushed in a jaw crusher and afterwards in
a tungsten disc mill. The light fraction was
removed by floatability using a Wilfley table, and
a Franz model magnetic separator was used, to
remove those minerals susceptible to a magnetic
field induced by an electric current up to 1.7 A.
Minerals with a density below 3325 kg·m
−3
were
removed using CH
2
I
2
(diiodomethane) heavy
liquid, using pertinent lab equipment due to the
toxicity and the difficulty of working with this
liquid. These separations were performed at the
ComplutenseUniversity ofMadrid.The following
techniques were performed at the J.W. Goethe
Universität of Frankfurt am Main. Zircon hand
picking of all types of zircons was carried out
under a binocular microscope before mounting
them, depending on their size, in epoxy resin.
These mounts were polished to approximately
half of the zircon crystal thicknesses. Then
the mounts were introduced into a JSM 6490
scanning electron microscope (SEM) to perform
cathodoluminiscence (CL) and back-scattered
electron (BSE) images to study the internal zoning
of the zircon grains in order to choose the best
areas for isotope analysis. Isotopic measurements
were taken for U–Pb and Lu–Hf isotopes. Both
types of measurements were performed with the
laser ablation technique. The laser used in both
cases was a RESOlution M–50, with 193 nm
wavelength ArF excimer (COMpexPro) laser,
which provided a maximum space resolution
of 23
μ
m laser spot diameters. The zircon was
ablated within a low volume cell in a He stream,
which was mixed directly after the ablation cell
with N
2
and Ar, before being introduced into an
Ar plasma attached to the mass spectrometers.
This plasma device ionized the ablated zircon
so that the spectrometers could measure the
different masses. In the case of U–Pb isotope
measurements the mass spectrometer (MS) used
was ThermoFinnigan Element 2 sector field MS
and for Lu–Hf isotopes the spectrometer used
was a ThermoFinnigan Neptune multicollector
MS. During the entire PhD project the total
amount of U–Pb analyses was around 2600 and
the amount of Lu–Hf analyses was around 1400
(in both cases excluding standard analyses). The
analytical sequences where sites of ablation and
other variables were defined, were programed
and performed by the PhD candidate with the
supervisors guidance.
Other isotopic experiments applied to
the studied rocks were the Nd whole-rock
determinations. Sm–Nd geochemistry allowed a
theoretical approach to provenance, model ages
and a “track of the juvenility” of the different
lithologies investigated at a hand-specimen
scale. These determinations were performed
at the geochronology laboratory (CAI) of the
Complutense University of Madrid, and involved
several laboratory skill developments. The
technique used required clean lab conditions
for sample dissolution with ultra-pure acids and
conventional ion-exchange chromatography
procedures. Rare earth element concentrates
were loaded on rhenium filaments onto the
sample load system of the thermal ionization
mass spectrometer (TIMS-Phoenix HCT040®). A
number of 36 Sm–Nd whole-rock determinations
were taken during the PhD thesis.
Data processing of the isotopic determinations
constituted an important part of the PhD project
and careful attentionwas takenon the calculations
involved, such as data reduction, interference
corrections, decay equation application, standard
deviation corrections, error determinations and
so on.
All this work, involving isotope geochemistry,
led to the publication of two articles which are
presented in chapters 5 and 6. These chapters
deal with the main lithology of the Banded Gneiss
2. OBJECTIVES AND METHODOLOGY
6