ChapterChapter----3333 Analytical Techniques, Analytical ...shodhganga.inflibnet.ac.in/bitstream/10603/31859/9/09_chapter 3.pdf · Analytical Techniques, Analytical Techniques, Methodology

ChapterChapterChapterChapter----3333

Analytical Techniques, Analytical Techniques, Analytical Techniques, Analytical Techniques, Methodology and ResultsMethodology and ResultsMethodology and ResultsMethodology and Results

CHAPTERCHAPTERCHAPTERCHAPTER----3333 ANALYTICAL TECHNIQUES, METHODOLOGY AND RESULTS

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CHAPTERCHAPTERCHAPTERCHAPTER----3333

ANALYTICAL TECHNIQUES, METHODOLOGY AND RESULTS

In the previous chapter, sampling methods, details of samples, field

observation and petrography of selected samples have been described. This

chapter describes all the process and the methods as well as instruments which

were used for analyzing the samples. Sample preparation is very crucial part to

get a good and an accurate data with little error. Different methods of sample

preparation are required for different analysis. Sample preparation involves size

reduction, phase separation, splitting and homogenization of samples etc.

3.1. Preparation of Samples

Geochemical analysis involves accurate and precise determination of

major and trace elements. It can be obtained up to desirable limits by negating

the human error as much as possible. In laboratory, due care was taken to

deduce better interpretation while preparing the samples particularly to avoid all

possible sorts of cross contamination. A total of eighty samples from Shillong

plateau which belong to Kyllang Pluton, Moudoh Pluton and Basement Gneiss

around Kyllang and Moudoh have been selected for the geochemical analysis

based on the criteria of freshness or least altered rocks.

Samples were collected during the field work, at least 5 kg weight samples,

and representative of rocks type have been collected from each location. It should

be remembered that the sample size is important consideration and is a function of

the grain size in any rock. The coarser and more porphyritic grain, then sample

should be larger. The collected rock specimens were broken into two parts on the

outcrop itself (in the field) to avoid any contamination during breaking in the

laboratory. After having done the megascopic studies, one part is kept as

reference and the other part is used in making thin sections for petrographic


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studies and the remaining part was broken into thumb nail chips which are cleaned

with water to remove surface contamination. The cleaned samples are air dried

before grinding and chipping. After chipping, homogenizations of chips were done

several times to get representative but small quantity of chips for grinding.

Figure 3.1. Flow Chart for Geochemical Methodology


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3.1.1. Sample Cleaning and Chipping

1. The sample are firstly cleaned by de-ionized water and dried by using air

blower to remove all the impurities lying on the surface of the sample.

2. The cleaned and dried sample has been taken for chipping. The sample is

broken into small elongate or thumb nail size chips of about 2cms in

length and about 0.5 to 1cm thick.

3. Homogenization of the chips has been done by using coning and

quartering process to get a representative small quantity of chips for

grinding.

4. The homogenized chips were put in grinding machine (jaw crusher) to

reduce the size of the homogenized chips upto -60 mesh. Then the

samples were put in Tema Mill for powdering to reduce the grain size upto

-230 mesh.

5. Homogenization, coning and quartering have done several times at every

stage. Re-homogenization before weighing for analysis is also must.

3.1.2. Jaw Crusher Mechanism

The Jaw Crusher crushes chipped samples by compression without

rubbing. Hinged overhead and on the centerline of the crushing zone, the

swinging jaw meets the material firmly and squarely. There is no rubbing action

to reduce capacity, to generate fines or to cause excessive wear of jaw plates.

Crushing is done by compression between two surfaces, with the work

being done by one or both surfaces. Jaw crushers with the compression method

are very suitable for reducing the size of extremely hard and abrasive rock. In

this step the chipped samples are put into a Jaw Crusher.


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Figure 3.2. A small picture showing the Jaw Crusher Mechanism.

The Jaw crusher grinds the chips into a coarse powder. The coarse

powder is generally of the size of -60 to -80 meshes from the first run. This step

of grinding in Jaw Crusher is repeatedly run for a couple of times to get finer

powder. After these runs the powder is taken out and emptied into an air tight

poly-bag. The Jaw Crusher is cleaned inside out with organic solvents like

acetone or alcohol and also with air blower before putting the next sample to

avoid any sort of contamination while running the next sample.

3.1.3. Laboratory Disc Mill Mechanism (TEMA Mill)

The powdered sample coming from the Jaw crusher (previous step) is

then taken for grinding in a TEMA Mill. The Laboratory Disc Mill is quick, dust

free grinding machine which can reduce the grain size of the powder of minerals

and rock samples to -200 meshes for analytical purposes. By means of

predominantly horizontal vibrations, the material is ground by impact and friction,

usually in minutes and at the same time homogenized the sample is more

strongly homogenized. The grinding apparatus is called Disc mill which


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comprises eccentric rotation of the heavy metal disc in the sample- disc

chamber. The sample-disc chamber assembly comprises a heavy metal disc at

the center of the chamber encircled by a metal ring. The sample is placed in

between the metal disc and metal ring while running the machine. During the

operation the disc hits the wall of metal ring and the closed container thus it

grinds the material stuffed in between the disc and the circular metal ring within

the container.

Figure 3.3. A picture showing the TEMA mill apparatus.

The whole apparatus should be cleaned properly before and after running

every different sample to get rid of cross contamination of the samples.

3.2. Loss on Ignition (LOI)

Loss on ignition is carried out to give a general indication of the "volatile"

species in a sample. Loss on ignition refers to the weight loss experienced by a

sample when it is heated in a furnace at a specific temperature and for a specific

time period.


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The LOI is generally carried out by heating the sample in a furnace at a

temperature range of 850° - 950°C for one hour to 90mins. The weight loss

caused by heating reflects all the volatile species that are lost at the given

temperature and time. The volatile species include surface water moisture as

well as water of crystallization and organic carbon species if present. In addition,

carbonates decompose to oxides with the loss of carbon dioxide and sulphates

decompose (but usually only partially) to oxides with the loss of sulphur trioxide.

Fluorides may also be partially lost. As a result of the complex nature of these

reactions, the LOI value is best used as a general indicator of the amount of

volatile species present. It cannot be used to determine the presence of

individual species. Some base metals will also be lost in this process.

Procedure followed for calculating the Loss on Ignition:

1. Start the digital weighing machine and leave it alone for about an hour, so

that it equilibrates with the conditions and then set it to zero.

2. Weigh the silica crucible (W1) and note down the reading on the weighing

machine.

3. Now add 2gm of the powdered rock sample very carefully into the crucible

without taking it out of the weighing machine after step 2 and note down

the reading on the weighing machine (W2). W2 should be approximately

W1 + 2gms.

4. Repeat the same for all the samples. And store them in desiccators to

avoid any moisture entering the sample.

5. Now switch on the furnace and put the crucibles in it and set it at a

temperature of 850° for a time interval of 120mins. Remember the pattern

(serial number) in which the samples were kept because any sort of exterior

labelling on the crucible is going to evaporate at that temperature.

6. After the 120mins, switch off the furnace and leave the crucibles in it for

13 – 14 hours so that it reaches the room temperature.


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7. Keep a desecrator nearby and swiftly shift the crucibles from the furnace

to the desecrator.

8. Now weigh the crucible (W3).

9. Therefore the LOI (Loss on Ignition) is = W2 - W3.

10. Then convert it into percentage.

3.3. Sample Dissolution for Inductively Coupled Plasma-Mass Spectrometry

(ICP-MS)

Open Vessel Digestion

Open vessel digestion method is an acid digestion method of preparation

of solution. This solution is used for determination of rare earth elements and the

trace elements concentrations by ICPMS. In this procedure the silica vaporizes

when it is attacked by acids such as Hydrofluoric acid (HF), Nitric Acid (HNO3),

Perchloric acid (HClO4) and Hydrochloric acid (HCl). During the preparation

0.05gms sample was weighed on a high precision digital weighing machine. The

weighed 0.05gms of powdered sample (200mesh size) was put in a Teflon

beaker. 10ml of acid mixture (Acid Mixture is HF, HNO3 and HClO4 added in the

proportion of 7:3:1) was poured to the precisely measured 0.05 gms of samples

in the Teflon beaker. Then that was left for overnight. On the next day the Teflon

beakers was kept on the hot plate with the lids on for 15-20 min. After that the

lids were removed and allowed to dry the sample completely on the hot plate. 5

ml of the above mentioned acid mixture was added again to each beaker and

dried that again completely by putting the Teflon beaker on the hot plate. 1:1

acid mixture of water (Deionised) and Nitric acid was added to the dried sample

in the Teflon beaker. Then the Teflon beaker was put again with the lids on to

slightly warm the solution. After warming up solution the beakers were removed

from the hot plate and added 5ml of 1ppm rhodium solution. The beakers were

again put on the hot plate to warm gently. After warming up slightly the volume

of the solution was made up to 100 ml by adding deionized water in a 100ml


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flask. At last the final solution was filtered through WHATMAN paper before

storing them in air tight plastic bottles with a storage capacity of 60-70ml each.

Thus the solution was ready to use for determination of rare earth

elements and the trace element concentration by ICP-MS.

3.4. Zircon Separation Technique for Geochronology

Zircon separation was done for generation of SHRIMP U-Pb isotopic

dating as a part of geochronological studies. The zircon separation from

granitoids was carried out using the steps from the adopted from “Zircon

Separator, ZR-2001” system of Laboratory of Planetary sciences, Tokyo Institute

of Technology. The steps essentially consisted of (i) crushing approximately 10

kg of the sample to size of approximately <1/4 mm; (ii) washing the crushed

sample with deionized water to remove the ultra-fine particles to prevent the

muddiness of heavy liquid used in the later stage; (iii) separation of the magnetic

minerals from dried sample powder by hand held magnet; (iv) separation of

heavy minerals by using the heavy liquid (Bromoform); (v) the heavy minerals

collected from the previous step were thoroughly washed with the chloroform and

de-ionized water to remove any fine particles clinging to the heavy minerals; (vi)

Finally the zircon separation is carried out under a binocular stereoscope by

hand picking.

3.5. XRF: X-Ray Fluorescence

The XRF method is widely used to measure the elemental composition of

materials. Since this method is fast and non-destructive to the sample, it is the

method of choice of analytical technique which offers high levels of accuracy and

precision. X-Ray Fluorescence can provide rapid, accurate and precise chemical

analyses for a wide range of geological, environmental, processed and synthetic

materials. The technique is based on the interaction of X-rays with matter to yield

both quantitative and qualitative elemental information. All elements of the


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periodic table from Beryllium (Be) to Uranium (U) can be measured qualitatively

and quantitatively in pressed powders, solids, fused glass beads or liquids. It has

high grade of analytical flexibility and easy integration to any kind of process

automation.

Figure 3.4. XRF Instrument

XRF Principle and Methodology

As the XRF is a highly sofisticated machine, the principle used in XRF is

so advanced. When a primary x-ray from any source like x-ray tube or

radioactive source strikes a sample, the x-ray can either be absorbed by the

atom of the element present in the sample or scattered through the material. The

X-rays can be absorbed by an atom by transferring all of its energy to an

innermost electron. This phenomenon occurred in an atom is called

"photoelectric effect." If the primary x-ray had sufficient energy, electrons are


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either ejected or transferred from the inner shells to the outer shells, creating

vacancies in the inner shells. The atom having vacancies present in the inner

shells is an unstable condition for the atom. When the atom returns to its normal

condition, electrons from the outer shells are transferred to the inner shells filling

the created vacancies. When electron from outer shell of higher energy level are

transferred to the innel shells of lower energy level, characteristic x-ray whose

energy is the difference between the two binding energies of the corresponding

shells is produced. Each element produces characteristic x-rays at a unique set

of energies. Thus one can measure the elemental composition of a sample by

using the characteristic X-rays with unique set of energies. The process of

emissions of characteristic x-rays is called "X-ray Fluorescence," or XRF. In XRF

machin, the characterictic X-rays poduced by elements are counted in kcps (Kilo

Count per Second) by a flow counter attached to the WDX spectrometer.

Respective concentration is calculated by drawing a calibration curve using a set

of standards. Thus elemental concentration is measured from the characteristic

emission line intensity of fluorescent X-ray by using a suitable calibration

procedure.

Standard Used

JG 2 -Japanese Granite, DGH, AGV, JR-2, and JR-3 were used as

standard sample. All the major elements and a series of trace elements

(Appendix -II) can be detected and measured in one short session by using this

method.

Sample Preparation for XRF

An ideal sample is prepared so that it is:

1. Representative of the material / rock

2. Homogenous


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Pressed Powder Method: It is almost universally adopted sample preparation

technique for XRF geochemical studies. The pressed powder pellet method is

also one of the most convenient adopted methods for XRF geochemical studies.

Sample powder (-200 mesh), boric acid and small aluminium cups are the

primary requirement for this method. Firstly the sample powder is thoroughly

mixed. About 4gm of the sample is put with a background binder i.e. boric acid

(used in this case) in a small aluminum cup. This is prepared in such a way that a

thin layer of sample shoud be above the binder (Boric acid in this case). Then the

cup is pressed under about 2000kg/cm2 hydraulic press to form a pellet which

has a thin layer of the sample over the boric acid background binder (Note:

Background binders should not enhance the background substantially and also it

should not contain the element of interest). The thickness of the pellet obtained is

about 3.5mm, which is sufficiently thick for the strongest X-Ray energy analyzed.

3.6. Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is an advanced

analytical technique used for elemental determinations. ICP-MS can be used for

the estimation of elements with atomic mass ranges 7 to 250. A typical ICP-MS

will be able to detect in the region of nano-grams per liter to 10 or 100 milligrams

per liter. It has become a powerful technique for estimation of trace, REEs, PGEs

and even isotope ratios in a variety of geological materials.

ICP-MS has the capability to run or scan for several elements at a same

time. This capability is an adavantage over the atomic absorption spectroscopy

(AAS) which can only measure a single element at a given time. This advantage

makes this technique faster in sample processing.

Principle of ICP-MS: An ICP-MS is a combination of a high-temperature

Inductively Coupled Plasma (ICP) source and a mass spectrometer. The sample

introduced into the ICP torch through a nebulizer spray chamber system is


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completely desolvated and converted first the atoms of the elements present in

the sample into gaseous atoms and then ionized towards the end of the plasma

with the help of the ICP source. These ions are then brought to mass

spectrometer to separate and detect on the basis of their mass to charge ratio by

using the mass spectrometer. These filtered ions are counted with the help of a

detector. Finally the resulting information is given after processing in a computer.

Figure 3.5. ICP-MS instrument

Geological Application: The precise and accurate determinations of trace and

REEs in geological samples are important to understand the evolution of the

magma and to quantify the processes, which have taken place in the rock

history. ICP-MS has proved to be extremely sensitive for the accurate and

precise estimation of the trace elements including the REEs and PGEs.


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Sample Preparation for ICP-MS

Sample Dissolution: sample dissolution is a pre-requisite for geological

samples. Majority of difficulties arising from the determination of elements are not

associated with the reliability of instrumental techniques, but with the reliability of

sample preparation methods. Many dissolution procedures are available for

digestion of geological samples; one of them is given above (paragraph 3.3).

3.7. Analytical Details

1. The samples were analyzed in National Geophysical Research Institute,

Hyderabad for major oxides and some of the samples were again

repeated and checked in Delhi University. In National Geophysical

Research Institute, Hyderabad the major oxides were determined by using

X-ray Fluorescence Spectrometry (Philips PW 1400) instrument. JG-2

was used as standard during the analysis. In Delhi University, while

repeating and checking the analysis, Jg-2, DGH-9, GeoPt-8, GeoPt-18,

GeoPt-21, SARM-48, JG-2, JG-3, JR-2 and JR-3 were used as standard

samples.

2. The rare earth elements and the trace elements were determined from

Geochemistry Division, ICP-MS Lab, National Geophysical Research

Institute, Hyderabad-5000007 by using ICP-MS instrument (Perkin Elmer

Sciex, ELAN DRC II). The precision and accuracy of analyses, as

observed on JG-2 standard, were better than ± 7 % for all the trace

elements.

Some of the samples are repeated for major oxides and trace elements by

using the instrument, Pan Analytical (AXIOS) XRF-WDS at Department of

Geology, Delhi University for checking data quality and better accuracy.

Results are given in the Appendix-II


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3.8. U-Pb Isotope Analysis Details

Zircon was extracted from each sample by standard crushing, sieving, and

heavy liquid and magnetic separation. Zircons from each sample and the

standard Temora-2 (Black et. al., 2004) were handpicked and set in epoxy resin

in a standard SHRIMP mount. The epoxy was left to set for 24 hours. The

mounts were polished to expose the midsection of each zircon grain. The

polished zircons were examined with a microscope and photographed under

transmitted and reflected light Cathodoluminescence (CL) images of the zircons

were obtained with a Hitachi S2250-N SEM.

Zircons that were free of inclusions and cracks were identified and

analyzed with Sensitive High Resolution Ion Micro Probe (SHRIMP-I) at the

Research School of Earth Sciences, at the Australian National University.

SHRIMP-I was run using a mixture of Oxygen and Argon primary source gas and

the primary ion beam operated between 2-2.5 nA.

The uranium concentration standard used was the Sri Lankan zircon

megacryst (SL13) (U = 238 ppm). The data from each SHRIMP session was

reduced with the Microsoft Excel add-in, SQUID II written by Ken Ludwig. Count

rates were normalized to the secondary-beam-monitor to correct for total beam

fluctuations. Besides these the results of each SHRIMP spot were compared with

the morphology of the corresponding zircon. All ages are reported using the

207Pb corrected 206Pb/238U system owing to the errors associated with low yields

of 204Pb, 207Pb and 208Pb from zircons of this age. This method effectively

assumes each analysis is a mixture of radiogenic and common

Lead and this is unmixed from the measured 207Pb/206Pb ratio.

Because of the complexity of the age spectra of zircon, a protocol is kept

to deconvolve the complexity of multiple episodes of zircon growth (White et al.,

2011). Because of this protocol it can be omitted the compromised ages which

are associated with: (1) errors that are too large to provide a geochronological


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constraint, (2) high counts of common Pb, (3) high 207Pb/206Pb and (4) a matrix-

dependent sputtering effect correlated with high uranium/thorium concentrations

(White & Ireland 2012). Thus the non-omitted results, the zircon petrology (cores,

rims, morphology) and chemistry (U, Th, Th/U) are taken into account to identify

the age populations of the Zircons.

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ChapterChapter----3333 Analytical Techniques, Analytical ...shodhganga.inflibnet.ac.in/bitstream/10603/31859/9/09_chapter 3.pdf · Analytical Techniques, Analytical Techniques, Methodology