AGE DETEMINATIONS OF MASSACHUSETTS GRANITES FROM RADI0(ImIC LEAD IN ZIRCON
by
GEORGE ROGER WEBBER
B.Sc., Queen's University, 1949 MoSc., Moester University, 1952
SUBMITTED IN PARTIAL FULFILLMENT ssp OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF -INDGREN PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHIOLOGY
J 1955
Signature of Author
DetrTment oft Geo rg nd GeophySloe 0aY 16s 1955 Certified by____________________
Thesis Supervisor
rman$ par nae Co dtee on Graduate Students
AGE DETEMINATIONS OF MASSACHUSETTS GRANITES
FROM RADIOGENIC LEAD IN ZIRCON
by
George Roger Webber
Submitted to the Department of Geology and
Geophysics on My 16, 1955, in partial fulfillment of the
requirements for the degree of Doctor of Philosophy. ABSTRACT
Radiogenic lead in zircon was used to determine the ages of fourteen granite samples from eastern
Massachusetts and two samples from Maine. Zircon from the granites was analysed for lead with an optical
spectro-graph and activity was measured with a proportional alpha
counter. A gamma ray scintillation spectrometer was used to obtain the ratio of uranium to thorium in the samples.
Zircon from each rock sample was split into three fractions according to slight differences in
magnetism wherever there was a sufficient amount. Age
determination results on the various fractions are given in the following table:
Ages in Millions of Years from Different Magnetic
Splits
of Zircon
Peabody Cae Ann Milford graniteIt ~ 1 Dedham granodiorite It OfMica diorite near Fitchburg
Northbridge granite gneiss
I i t
Chelmsford granite
Calais, Maine
LaOke irtv "
* appears anomalously high
Most Inter- Least Magnetic mediate Magnetic
495 504 448 559 42 460 - 415 438 58- 769 678 581 662 632 712 580 736 54140*--382
338
392
976
57 477 45576
-829 756 - -713 -0-Granite Quincy type graniteThe Calais granite from Maine which is believed to be Devonian is the only granite of the above group for which there is good field evidence of age., The age of
756 million years appears much too old for a Devonian rock.
Further investigation Is needed to find the cause of this discrepancy.
Both lead and activity values are higher in the
magnetic than in the nonmagnetic zircon from the same rock. This is probably due to a tendency for iron and uranium to
be concentrated in the zircon structure at the sam stage
of crystallization. The ages from the different fractions of zircon agree fairly well, with the exception of one sample of Northbridge granite gneiss which gives ages of
338, 392 and 976 million years.
Ages for the Quincy granite are the most
self-consistent. It is a distinctive rock type and contains
more zircon than the other granites. It would, therefore, be a good rock for more detailed age work.
Thesis Supervisor: Patrick K. Hurley
Title: Professor of Geology
Thesis Supervisor: William H. Dennen
Title: Assistant Professor
of Geology
ACKNOWLEDGMiMTS
I would like to thank my thesis advisors,
Professor L. U. Ahrens (thesis advisor until his
departure from M.I.T. in 1953), Professor W. H. Dennen
and Professor P. M. Hurley for their guidance in the course of the thesis investigation. Thanks also to
Professor H. W. Fairbairn, who prov,ded instruction
and assistance in separatory techniqaes.
George Shumway and Mark Smith, former graduate
students at M.I.T., did earlier work on the problem of analysing zircon for lead. Their notes were of great
assistance.
Dr. W. H. Pinson, Jr. contributed helpful discussion of analytical problems. Many others in the Department of Geology and Geophysics provided discussion on various phases of the work.
I wish to thank my wife, Joan, for typing the
thesis and assisting with some of the routine p.rocedures.
This work was sponsored by D.I.C. Project 6618,
Navy Contract N5ori-07830 and D.I.C. Project 6617, Navy Contract N5ori-07829.
TABLE OF CONTENTS
ABSTRACT...*****...*..**** .0.0..0 2
ACENOWLEDGMLB1 TS... 4
INTRODUCTION...
8
COLLECTION AND LOCATION OF SAMPLES... 9
GEOLOGICAL EVIDENCE FOR AGE... 13
1. Massachusetts Granites... . 13
(a) Relation to fossiliferous strata...
14
(b) Intrusive relationships between the granites. 15 (c) Indirect estimates of ag... 15
2. Maine Grnt...16
EXPERIMENTAL PROCEDURES AND RESULTS... 17
1. Separation of Zircon from the Rock... 17
2. Measurement of the Activity of the Zircon and Determination of the UfrhRatio... 19
3.
Spectrographic Lead Analyais... 21(a) Sadr ... 21 (b) Sample preparation... 23 (c) Arcing conditions... 24 (d) Calculation ofresults... 25 4. Computation of Ages... .... .... 30 DISCUSSION...o... . .g... .... 35
1. Consistency of Age Resuts... 35
3. Variation of Magnetism of Zircon and Its
Relation to Zircon Composition... 40
4. Relation of Zircon Age to the Age of Its Source Rock... 43
CONCLUSIONS AND RECOMMENDATIONS... 45
APPENDIX PRECAUTIONS AGAINST CONTAMINATION... 51
SEPARATION OF ZIRCON...**...*...*.** 53
ARCING TESTS... 56
PRECISION OF FILLING ELECTRODES...
60
INTERNAL STAND ARDS... 62
ANALYTICAL PROCEDURE FOR LEAD DETERMINATION... 65
CORRECTION FOR VARIATION OF INTENSITIES WITH TIME... 74
NONUNIFORMITY OF SLIT ILLUMINATION... 78
RECOMMENDATIONS... 81
BIBLIOGRAPHY FOR APPENDX... 83
TABLES
Table Page
1 Emerson's Classification of Massachusetts
Granites... 13
2 Activity Neasurements... 22
3 Analysis of Ceylon Zircons... 30
4 Results of Spectrographic Lead Determination.... 32
5 Age Determination Results...
33
FIGURES Page 1 Location of Samples... 12
2-7 Addition Curves for Determination of Lead Content of Zircon 1644... 28
8 Lead Working Curve... 29
9 Age DeterminationB...
34
10 Bismuth as an Internal Standard... 64
11 Indium as an Internal Standard... 64
12 Result of Weighing Electrodes... 64
13 Acetylene Tetrabromide Separation quipment... 72
14 Exterior of Grating Spectrograph.. ... 72
15 Grating - Showing Masking... 72
16 Hilger Microphotometer... 72
17 Sample Calibration Curves...
73
18 Variation of Intensities from Different Photographic Plates... 77
INTRODUCTION
The determination of the age of a number of
granites from eastern Massachusetts was undertaken as part
of a program of age determination in the Department of
Geology and Geophysics at Mtassachusetts Institute of
Technology.
Radiogenic lead in zircon was used to determine the ages. This method was originated by E. S. Larsen, Jr.,
of the U. S. Geological Survey, and his co-workers (Larsen,
Keevil and Harrison, 1952; Larsen, Waring and Berman, 1953). In accordance with this method, zircon from the granites was analysed for lead by means of an optical spectrograph by the writer and the activity was measured with an alpha counter by P. N. Hurley, Professor of Geology at Matssachusetts Institute of Technology. Assuming that the lead in zircon
is nearly all radiogenic, the age of the specimen is given
by the relationship T
=
A where T a age in millions ofyears, Pb
=
concentration of lead in parts per million, A=
activity in alphas per milligram-hour, C=
a constant whose value is dependent on the relative amount of activity due to uranium or thorium.COLLECTION AND LOCATION OF SPLES
Samples of granite, ranging in weight from 50 to
100 lbs., were collected from a number of quarries (described by Dale, 1923) in eastern Massachusetts. Their location is indicated in Figure 1. In general, the aim was to collect samples from at least two quarries of each quarry area
sampled. In two cases it was necessary to take samples from non-quarry locations. The samples were carefully chosen as representing average-looking, fresh-appearing granite (with the exception of sample R3107 which was obtained from a small outcrop and was considerably weathered).
In addition to these samples, five samples of
granite were used which had been collected by H. W. Fairbairn, Professor of Geology at assachusetts Institute of Technology.
Three of these were from eastern Massachusetts (locations
shown in Figure 1) and two were from Maine.
The types of granite,locations and references to descriptions of the quarries are given below 3
R3004 - Hornblende-augite granite from the Linehan quarry,
about three miles west-southwest of Peabody, Mass.
R3005 - Hornblende granite from the Flat Ledge quarry, about half a mile north-northwest of Rockport,
Mass. (Dale, 1923, p. 294).
R3006 - Hornblende granite from the Blood Ledge quarry,
about two miles west-northweat of Rockport, Mass. (Dale, 1923, p. 300).
R3011 - Mica diorite from the Leavitt quarry, two miles west of Leominster, Mss. (Dale, 1923, p. 353).
R3012 - Biotite granite from the Norerose quarry, about tc miles northeast of ilford, Aass. (Dale, 1923,
p. 348).
R3013 - Biotite granite gneiss from the Blanchard quarry,
about one and a half miles west-northwest of Uxbridge
station, Mass. (Dale, 1923, p. 352).
R3014 - Hornblende granite from t1he Curry quarry, about twc and a half miles east-sousheast of Wrentham station,
Mass. (Dale, 1923, p. 314).
R3105 - Granite from outcrop on Tigh Rock, 1.7 miles east of Wrentham, Mass.
R3106 - Biotite granite from th'v West quarry, one and three
quarter miles north-northeast of Milford, Mass. (Dale,
1923, p. 348).
R3107 - Granite gneiss from ati outcrop at the north end of Whitinaville, Mass.
R3108 - Muscovite-biotite grinite gneiss from the Fletcher quarry, one and one quarter miles northwest of West
R1982 - Coarse gray granodiorite from road out on Route 1
in Dedham, Mshs., north of Route 128 (collected by
H. W. Fairbairn).
R3051 - Granodiorite on Route 1, three miles north of
B. N. 188 which is located at the north end of
North Attleboro, Mafss, (collected by H. W. Fairbairn).
R3052 - Granodiorite on Route 1, 8.2 miles north of B. N. 188
which is at the north end of North Attleboro, Mass. (collected by H. W. Fairbairn).
R3063 - Granite on Hollingsworth and Whitney lumber road,
about six miles west of Lake Parlin on Route 201 in Maine (collected by H. W. Fairbairn).
R3078 - Granite on Route I, seven miles south of Calais, Maine (collected by H. W. Fairbairn).
QUATERNARY GRANITE.ETC Ct'vONIAN On GNANITECtTC. CAMSRIAN GNUSS AND GNUISS AND
(POST-PENN- CARGONIIIOuS (PME-PENN- (I0sIIEtROuS SCHIST SCH1ST
SYLVAN IAN) (5IDIMENTAN SY LVANIAN) STUATA) (PARTLY PRI-C.AM- (MAINLY
Pt-AND VOLCANIC) BRIAN.PARTLY CAMSRIAN)
PALE OZ IC) *K, CRETACCOUS AND MIOCENE STRATA
SENCATW QUATERNARY
LOCATION OF SAMPLES
(Base map and geology from LaForge, 1932)
GEOLOGICAL EVIDENCE FOR AGE
1. Massachusetts Granites
The geological classification of the Massachusetts granites, as given by B. K. Emerson in his report on the Geology of Massachusetts (Emerson, 1917), is shown below:
Sample Name of Granite Age
No. f(Erson)
(Eerson)-R3108 Ayer granite (Chelmsford) Late Carboniferous or
R3011 Fitchburg granite Quincy granite "3 "t Milford granite it I Dedhatm granodiorite "1 " Northbridge i" Pobt-Carboniferous Late Carboniferous or Post-Carboniferous Early Carboniferous i t Devonian? I" Devonian? i i I "1
granite gneiss Archean? "f I I R3004 R3005 R3006 R3012 R3106 R3014 R3105 R1982 R3051 R3052 R3013 R3107
In Figure 1 which shows the location of these
samples on a map by LaForge (1932), the above rock types
are divided into two age groups, Pre-Pennsylvanian and Post-Pennsylvanian.
(a) Relation to fossiliferous strata
Unfortunately there is a notable lack of fossils in sediments associated with Massachusetts granite.
Cambrian fossiliferous strata are found near Quincy (about 10 miles southeast of central Boston) and at Hoppin Hill
(about 30 miles southwest of central Boston)(see Figure 1). At Quincy, the Middle Cambrian Braintree slate is intruded
by Quincy granite, a distinctive rook similar
petrographi-cally to the Quincy granites at Peabody and Cape Ann, and correlated with them by field workers. From this evidence it is probable that the Cape Ann and Peabody rocks are post-Middle Cambrian. The relationship of the Dedham granodiorite to the Cambrian is more controversial. Rock which has been mapped as Dedham varies considerably petro-graphically, and it is not unlikely that very different intrusives are inoluded in the classification. At Hoppin Hill, near the Massachusetts-Rhode Island border, the
relationship between Lower Cambrian Hoppin slate and adjacent granite (classified as Dedham) has been a subject of
considerable controversy. Warren and Powers (1914) contended that the granite is younger than the sediments. For a
sediments and therefore Precambrian in age. Quincy granite in Rhode Island is overlain by basal strata of
the Carboniferous Narragansett Basin (3aerson, 1917, p. 188).
(b) Intrusive relationships between the granites
In areas where Quincy-type granite and Dedham-type granite occur, it has been reported that the Quincy is intruded into the Dedham (Emerson, 1917, pp. 187, 188). In Rhode Island, Quincy granite is intrusive into Milford granite (3nerson, 1917, p. 188). Emerson states that the Milford granite is intrusive into the Northbridge granite gneiss (Emerson, 1917, p. 155).
(c) Indirect estimates of age
Emerson assigned the Northbridge granite gneiss to the Precambrian but had no direct proof of this.
The. mica diorite (Dale's classification) from the Leavitt quarry is on the eastern border of an area mapped
by Enerson as Fitchburg granite. The Fitchburg pluton has
been tentatively assigned to Late Devonian by N. P. Billings (Billings, Rodgers and Thompson, 1952).
The Chelmsford granite is of controversial origin and is discussed in some detail by Currier and Jahns (1952).
Jahns assigns it to late Paleozoic.
The Quincy granite has been correlated with the White Mountain magma series of New Hampshire by some writers
(Williams and Billings, 1938) on the basis of its alkaline nature. The White ountain magma series has been dated as probably Mississippian by Billings (Billings, Rodgers and Thompson, 1952). Age determinations of the Conway granite of the White Mountain magma series made by Larsen, Gottfried, Waring and others, are tabulated by Faul (1954, p. 267).
They range from 201 to 255 million years (average - 235
million years).
2. Maine Granite
The granite at Calais, Maine, from which sample R3078 came, is very well dated from field evidence,
according to J. B. Thompson, Jr., Professor of Geology at
Harvard University (personal communication). The granite is overlain by, and contributes pebbles to, the fossiliferous Perry Formation (Upper Devonian). It is intrusive into the
fossiliferous Eastport Formation, which is late Silurian in
age. Thus the granite was probably intruded during the early Devonian. For a description of these formations see Bastin
and Williams (1914), and Smith and White (1905).
The granite from near Lake Parlin in Maine has been mapped as older than adjacent sediments which are classified as Silurian or Devonian (Hurley and Thompson,
EXPERIMENTAL PROCEDURES AND RESULTS
The experimental prlocedure for determination of the age of a rock from radiorenic lead in zircon may be divided into four steps:
1. Separation of zircon from the rock.
2. Measurement of the activity of the zircon and determination of the* U/RIh ratio.
3. Spectrographic lead analysis.
4* Computation of ages.
1. Separation of Zircon from the Rock
The granite collected in the field was first broken up on a steel plate with a sledge hammer. The size
o.' the fragments was further decreased with a rock splitter and then the sample was comminuted successively in a small
jaw crusher and disc grinder.i This ground product was then screened to obtain a size fraction of -60 and +200 mesh. FIfteen to thirty pounds of this size fraction was obtained from each sample.
A combination of magnetic separation by use of a
using acetylene tetrabromide, methylene iodide and clerici solution was then used to separate zircon, which is
comparatively nonmagnetic and heavy. The zircon was treated with hot nitric acid to remove pyrite in the concentrate.
Impurities were handpicked from the zircon.
H. W. Fairbairn had noted in previous work that the zircon concentrate could be separated into various
fractions according to its varying, weak magnetic properties, and that these fractions had widely different activities.
It was therefore considered desirable to see what effect this would have on age determinations. Accordingly, wherever a
sufficient amount of zircon was obtained, it was split into
three fractions according to its magnetic properties, by
means of the Frantz isodynamic separator. The various fractions are listed below. The separations were made by using a full field of 1.4 amperes and varying the inclination of the separator. The fraction that is magnetic at 50 is the
most magnetic, and the fraction that is nonmagnetic at 20 is the least magnetic.
R3004A - Magnetic at 50. B - Nonmagnetic at 50, magnetic at 20. C - Nonmagnetic at 20. R3005A - Magnetic at 54. B - Nonmagnetic at 50, magnetic at 20. C - Nonmagnetic at 20. R3006A - Magnetic at 20. B - Nonmagnetic at 20. R3011A - Nonmagnetic at 20.
R3012A - Magnetic at 20. B - Nonmagnetic at 20. R3013A - Magnetic at 50 B - Nonmagnetic at 54, magnetic at 24. C - Nonmagnetic at 20. R3014B - Nonmagnetic at 20. R3105A - Magnetic at 50 B - Nonmagnetic at
5
0, magnetic at 20 . C - Nonmagnetic at 20. R3106A - Magnetic at 50* B - Nomagnetic at 54, magnetic at 24. C - Nonmagnetic at 20. R3107A - Magnetic at 50. B - Nonmagnetic at 54, magnetic at 24 C - Nonmagnetic at 20. R3108A - Nonmagnetic at 20.Zircon from the granites collected by H. W. Fairbairn was separated by him, ground to -400 mesh size, and split into two fractions, magnetic and nonmagnetic at 20 on the Frantz isodynamio separator. Only the magnetic fraction was aval
-able at the time the lead analyses were made, so that the zircons R1982, R3051, R3052, R3063 and R3078 which were analysed were all magnetic at 20.
2. Measurement of the Activity of the Zircon and Determination of the Uf.h Ratio
iho has contributed the following information:
The equivalent uranium was determined by thick source alpha counting in a proportional counter. The instrument was a Nuclear Measurements Corporation low background proportional alpha counter with 2% geometry, continuously flushed with argon. The counter
character-istics were well known from several years of previous
operation. The alpha count from a standard source prepared by the National Bureau of Standards could be duplicated by the instrument to within a fraction of a per cent over a fairly broad plateau in collection voltage.
A solid source absorption factor for zircon was calculated by the method described by Nogami and Hurley
(1948), on the assumption that alpha particles would be
counted that emerged from the source with an air range exceeding 0.5 cm. This factor was calculated to be 0.493; that is, the number of alpha particles per sq. cm. per hour multiplied by 0.493 gives a value for the radium equivalent
in the zircon in units to 10-12 gmse/gm. It was found experimentally that this factor was 11% too low for this instrument, probably reflecting the fact that the
instrument's threshold was set so as to exclude ion
collections from alphas with a substantially greater
residual range than 0.5 cm. Standard zircon samples measured by other laboratories were found to be in close agreement with their established values when this corrected absorption factor was used.
A method of radiometric determination of uranium
and thorium developed by P, K. Hurley utilizing a gamma ray scintillation spectrometer was used to obtain the ratio of uranium and thorium in the samples, Thorium produces lead at only 1/3 the rate of uranium, so that the age ratios in the zircons are dominantly Pb2 0 6/j 2 3 8 ratios. This means
that the radium equivalent in units of 10-12 ps/Sm Multi-plied by a factor of 1.04 is numerically equal to the number
of alpha particles per milligram per hour within the zircon, which is sometimes referred to as the Activity Index, The
low proportion of thorium contribution in the age ratio also establishes the constant (C) to be used in the age formula, T = C , where T a age in millions of years, Pb =
concent-ration of lead in parts per million, and A
=
activity in alphas per milligram-hour. The average value for C was 2590.Table 2 shows the results of the activity measurements.
3.
Spectrographic Lead Analysis(a) standards
For age determination work it is necessary that the lead determinations be as accurate as possible as well as precise. Replicate chemical and spectrochemical results
from a single laboratory may be precise (i.e., reproducible) but differ from equally precise analyses of the same material from another laboratory (Fairbairn and others, 1951).
Table 2
Ativy
Measurents
aMl No.
R3004A R3004B R3004C R3005A R3005B R3005C R3006A R3006B R3011A R3012A R3012B R3013A R3013B R3013C R3014UB R3105A R3105B R3105C R3106A R3106B R3106C R3107A R3107B R3107C R3108A R1982 R3052 R3051 R3063 R3078 Activity in alphs_ er millM-hour)4115
190 185 1105 900 560395
280535
435
365
925760
330 525675
680
460 885 770 570 720605
375
830 237 630 500476
920standards used for comparison with the unknowns. If the
standards do not behave in the same manner as the unknowns, the results may be reproducible but not accurate. Ideally,
the standards for these particular analyses would be zircons similar to the unknowns and containing known amounts of
natural, radiogenic lead (i.e., not artificially added). The standards actually used in these analyses were made up by adding varying amounts of lead to a zircon which was available in considerable quantity (zircon R1644
from North Carolina beach sand). During the course of experimentation two batches of standards were made up. In
one, the lead was added as Pb02 , in the other, lead was
added in the form of a Pb-Ba glass (Bureau of Standards Standard Sample No. 89) containing 17.5% PbO.
(b) Samlie preparation
The zircon unknowns and standards were ground to a fine powder in an agate mortar, weighed, and mixed with NaCl so that the resultant mixture contained 80% zircon and 20% NaCI by weight. The sodium chloride was added to improve the arcing qualities of zircon.
The zircon mixture was packed tightly in the cavity of a 1/8 inch graphite electrode which held about
12 mg. of material (weighing electrodes and applying a weight factor showed no detectable improvement in precision).
(a) Aring
conditions
Grating spectrograph - Wadsworth mount, 21 foot instrument,
with a dispersion of 2.54
R/m.
External optics - short focus spherical lens focussed on slit. Slit height - 10 mm.
Slit width - .05 mm.
Step sector.
Wavelength region - 3850
R
to 4450 A. Current - 3 amp.Line voltage - 220 volts.
Lead line used - Pb 4057.820 A.
Electrodes - 1/8 inch National Carbon Co. special graphite
electrodes.
Anode - the lower electrode--contained the sample in a cavity 1/20 inch in diamter and 5/32 inch deep. Cathode - the upper electrode--sharpened with a penel1
sharpener.
Plates - Kodak 103-0. Arcing tima - 30 seconds.
Two electrodes of material were used for each spectrum (i.e.. two arcings were superimposed)* Seven spectra were obtained per plate.
Development - 4-1/2 minutes in Kodak D-19 at 200 C t 10. Fixing - 15 minutes in Kodak Acid Fixer.
(d) Calculation of results
The basic principle of quantitative spectrographic analysis is that I w K.C or log I = log K + log C, where I is the intensity of line emission, C is the concentration of the analysis element, and K is a constant.
Intensity may be calculated from the degree of
blackening of the line on the plate. This involves calibration
of plates to determine reaction of photographic emulsion to changes in light intensity. The method used here was a self-calibrating method (Ahrens, 1950, p. 136) including a back-ground correction. When this relationship is determined,
standard curves of I versus C may be constructed from measurement of the darkness of the lead lines produced by samples containing known concentrations of lead. The lead
content of unknowns may then be read directly from the curves. The zircon used as a standard base contained a
certain amount of lead before any was added. It was there-fore necessary to determine this concentration. To do this,
the addition method was used (Ahrens, 1950, p. 135). Standards
consisting of the zircon base (zircon R1644) with varying known concentrations of added lead were arced, and the resultant
intensity values were plotted versus concentration of added lead. A curve was fitted to these values. During the course of experimentation several sets of these addition plots were
made using slightly different procedures (Figures 2 to
7).
When these curves are extrapolated back to zero intensity, the distance of the intercept on the concentration axis from zero
per cent added lead represents the concentration of lead in the zircon before addition. The final value for lead
in zircon R1644 was calculated by averaging the results of several addition plots. The values obtained from the various plots were 47, 45, 46 and 43 parts per million
PbO (Figures 2, 3,
4,
6), the average value being 45 partsper million PbO. Figures 5 and 7 were not used in this calculation because they represent different plots of the data in Figure 6 as will be explained below.
Theoretically these curves should be straight lines,, but nonuniformity of illumination of the slit in
the optical set up used in the arcing procedure introduces curvature to the lines. One of the addition curves
(Figure 5) was recalculated to correct for this nonuniform illumination. This resulted in the curve shown in Figure 7,
and this corrected curve is essentially a straight line in
the lower concentration ranges. Curvature at the higher concentration range is possibly due to self-absorption
produced in the arc when atoms in the outer portions of the arc absorb the energy produced by lead atoms in the inner portion of the arc and thus decrease the total light emitted.
The standards used for comparison with the unknowns were the artificial Pb-Ba glass standards which were arced
during the same timn period as the unknowns. Each
photo-graphic plate contained three spectra (3 duplicate exposures)
of standards and four spectra of the unknowns. The standard with no added lead appears on all the plates, and standards
with added lead appear on various plates throughout the series. It was thought, before the samples were run, that an inter-plate shift in intensity values might be detected, and that the unknowns on each plate could then be determined relative to the standard values on the same plate. However, as it turned out, the reproducibility of the standards did not warrant this shift correction from plate to plate.
There was a general tendency, however, for later intensity values in the series to be lower than earlier values.
Plates used during the series had the same emulsion number but were from four different boxes in succession. In order to minimize the effect of the time shift, mean intensity values for zircon R160 were calculated for each box of plates. All intensity values of standards and unknowns
were then shifted the appropriate amount (dependent on the box of plates from which the values came) to convert them to the level of one box. The original and corrected values of the standards are shown in the addition plots in Figures 5 and 6 respectively. The corrected values are shown in the log-log plot of Figure 8. Thus Figure 8 is the curve actually used in calculation of the final results.
As a check on the accuracy of the lead determination,
three Ceylon zircons, which had been analysed for lead by C. L. Waring, of the U. S. Geological Survey, were analysed and excellent checks were obtained. The results are shown in Table 3.
LEAD WORKING CURVE
100 200
PARTS PER MILLION PbO
Figure 8 0 - - - -- -;-- - *---5 0 500 1000
Table 3
Analysis of Ceylon Zircons
Parts Per Million Pb
Sample No. Waring (U.S.GS.) Webber
R3028 80 79
R3036 115 113
R3053 88 81
The results of the lead analysis of the Massachusetts and Maine granites are shown in Table 4.
4. Computation of Ages
The computation of age by a lead method depends on the decay of parent atoms U238, U235 and Th232 to
produce daughter atoms Pb206 Pb207 and Pb208 respectively. Knowing the rates of deoay, we can determine the time at
which decay in a closed system began if we can measure the
amount of parent and daughter atoms at the present time. In the age determination method used here, a
measure of the present amount of parent is obtained from the activity measurements (since each parent emits alpha particles at a constant rate) and a measure of the amount
of daughter is obtained from the spectrographic lead measurements., The age is calculated from the formula
C.Pb
T m of
content in parts per million, A is the activity in alphas per milligram-hour, and C is a constant whose value is
dependent on the U/fh ratio (C = 2590 in this case).
Use of this method is based on the assumptions
that lead content at the time of formation of the zircon was low enough not to introduce much error and that the zircon is resistant to chemical alteration. Larsen has reported consistent results using the method. For a
e of the difficulties involved in lead age measure-ments, see Paul (1954, pp. 282-300).
The results of the age dalculation are given in Table 5 and shown graphically in Figure 9.
Table 4
Results of Spectrograhie Lead Determination Sample No. R3004A R3004B R3004C R3005A R3005B R3005C R3006A R3006B R3011A R3012A R3012B R3013A R3013B R3013C R3014B R3015A R3105B R3105C R3106A R3106B R3106C R3107A R3107B R3107C R3108A R1982 R3052 R3051 R3063 R3078 Results of Replicate Analyses in pm PbO (86, 83, 88, 77)? 88, 90 (43, 38, 39.5, 39), 40.5, 39 (33.5, 28, 36.6, 40), 34.6, 34*5 244, 300, 228 (150, 153, 164, 157), 158, 160 83, 130 (60.5, 60, 54, 83), 72, 79 (51.5, 52, 51), 50 (89, 86, 80), 86 159, 123, 135 117, 89, 102 (120, 135, 121, 118), 163, 120, 135 (118, 155, 127, 105), 120, 122 (130, 134, 138) (122, 133, 123), 135 200 163, 165 140, 150, 132 202, 222, 218 218, 235, 182 (150, 165, 133), 133, 171 130, 142, 140 (139, 120, 100, 121), 118 71 286 520, 554 124, 118, 119 103, 90, 108 137, 144 291, 290, 290
* Values bracketed together are from
Average Concentration in 2pm PbO 85.3 39.8 34.5 257 157 107 68.1 51.1 85.2 139 103 130 124 134 128 200 164 141 214 212 150 137 120 71 286 537 120 100 141 290
Table 5
Ae Determination Results
Ages in
Different Millions of Years fromMaxnetic Solits of Zircon
Peabody Cape Ann
Milford granite
Dedham granodiorite 11 ~ I
Mica diorite near Fitchburg
Northbridge granite gneiss
" I it Chelmsford granite Calais, Maine LaKe Pt1;i "f Sample No. R3004 R3005 R3006 R3012 R3106 R3014 R3105 R3052 R3051 R1982 R3011 R3013 R3107 R3108 R3078 R3063 Most Magnetic A* 495 A 559 A A5812 480 5440** A 338 A 457
756
713 Intermediate B* B A A B 504 20 415 76-9 662 B 580 392 Least MagneticC* 448
C 460 B 438 B 678 C 6 B 0 C736
A 382 C 976 C 455 A 829* Letters refer to sample numbers of different
' Appears anomalously high
magnetic splits of zircon Granite
Quincy type
AGE 9 IN 10 o 0 5 F,) -R 3078 MAINE R3063 R3107 NORTHBRIDGE R3013 R3052 R3051 R1982 DEDHAM R3105 R3014 R3106 MILFORD R3012 R3006 R3005 QUINCY R3004 YEARS 5 L I L ~ I I I I g .1
Li:
R3011 FITCHBURG R3108 AYER 11111 I I I 11111 I I I I I IU)
I I I I I I I I-I...'
L:~i
lI
I I I I I IDISCUSSION
1. Consistency of
Ag
ResultsAlthough the lead measurements and activities are different in the different magnetic fractions of
zircon from the same granite, the ages agree fairly well.
A notable exception is the value of 976 million years for sample R3013C as compared to 338 and 392 million years for
R3013A and R3013B respectively.
The ages of granites which have been mapped as
the same rock type are boxed in Figure 9. Where these specimens were close together geographically or are distinctive rock types (e.g., the Quincy granites) they
are boxed with a solid line. Where the relationship is less certain, a dotted line was used. It is evident that the results are generally self-consistent. Exceptions to this are found in the Dedham series. The age from zircon
R1982 is anomalously old. The two samples of Dedham granodiorite from near Wrentham (R3105 and R3014) give similar ages but samples R3051 and R3052 appear to be younger. Unfortunately only one split of zircon was available from each of R1982, R3051, and R3052.
The relative ages for the Fitchburg, Quincy (Cape Ann and Peabody). Milford, and Dedham (samples R3014 and R3105 near Wrentham) agree with the relative order given by Emerson (Table 1). However, the one sample of zircon from Ayer granite (R3108) gives an age
greater than the average of any of the other groups, whereas Emerson classed it as younger than the Quincy granites. The Northbridge granite gneiss, which he classed as Archean, gives values ranging from less than the Fitchburg sample to the sam age as the Quincy
granites, with one anomalously high value., Two samples of Dedham granodiorite (R3051 and R3052) give ages
similar to those of the Quincy granites.
The Maine granites give an age about the sam as the Milford granites and the Dedham granodiorite near Wrentham (samples R3014 and R3105).
2. AbsoluteAe
The only granite of those analysed which is well dated from geological field evidence is the granite from Calais, Maine. According to field evidence, this granite
is early Devonian in age. The value obtained from zircon
analysis is 756 million years. This is much older than our present geological time scale would estimate for Lower
Devonian (estimates are around 300 to 350 million years). So far only one split of zircon from the Calais granite has been analysed.
The fact that zircon ages for the Quincy
granites average about 460 million years, whereas similar
granites in New Hampshire have given an age of 201 to 255
million years and are believed to be Mississippian from geological field evidence, is suspiciously suggestive when viewed in conjunction with the Calais granite data. It is
possible that there is a systematic error in the results given here.
Excessively high age values could result from high lead values or low activity values. In the lead
determinations, precautions were taken to avoid systematic errors. A zircon base was used for the standards to avoid matrix differences; standards were arced throughout the period of analysis of the unknowns; analysis of three
samples of Ceylon zircon gave values which agreed very well with the results obtained by C. L. Waring. The only
apparent possibility of an error in the lead analysis is
that the zircon unknowns may have behaved very differently
from the standard zircon and the Ceylon zircons in the spectrographic procedure.
P. M. Burley reports that close agreement has
been obtained in interlaboratory checks of the activity of
standard zircon samples.
The possibility that lead contamination was
introduced in the laboratory is always present in
individual cases. Great care was taken to avoid this
possibility. The zircon from the Calais granite gave quite
a high lead result, so that the contamination needed to produce an error of the magnitude of a factor of two would
be quite large. None of the low concentration zircon
standards which were arced in large quantities showed
variations which would suggest such a degree of contamination.
Some of these low standards were subjected to all the
preparation treatment that the unknown zircon received and showed no detectable contamination. The analysis of
nonmagnetic zircon from the Calais granite is highly desirable to test the consistency of the high age value and to eliminate the possibility of a random contamination error.
Impurities present in the zircon as a result of incomplete separation of the zircon could cause random
errors. These impurities would have to be very high in lead. The possibility of this would be checked by analysing the nonmagnetic zircon fraction of the Calais zircon. If age results are consistent, it is not likely that impurities caused the age discrepancy.
In this method of age determination it is assumed
that the lead in the zircon structure is all radiogenic.
If there was a large concentration of nonradiogenic lead in the zircon, the ages obtained would be too great. In general, the consistent age results of different magnetic
fractions of zircorb which contain varying amounts of lead, suggest that the lead is radiogenic. In particular, the Calais granite zircon contains an apparent concentration of
as a whole generally show only about 15 to 20 parts per
million lead. It does not seem reasonable that zircon, generally considered an unfavorable host for nonradiogenic lead, should be so excessively enriched in it in comparison with granite as a whole.
Loss of uranium or thorium from zircon would result in high ages. This loss could be due either to treatment of zircon in the separation procedure or to natural leaching processes. It is generally considered that zircon, being a mineral resistant to chemical attack with a dense crystal structure, will not be very susceptible
to leaching of elements held within it. It is, however, possible that some of the zircons of odd composition are not so impervious. Composition of zircon is quite variable and may produce differences in susceptibility to leaching which would make zircon reliable for age determination work in some cases and not. reliable in other cases. More work on leaching susceptibility of zircon is needed. It might be thought that the nitric acid treatment of zircon crystal s
could remove some of the radioactive elements from the near surface area and thus lower the activities. However, grinding
of the zircon and rechecking activities, although it did
reveal differences, did not indicate large differences, nor were the values consistently raised or lowered. Independent values of activity obtained from the gamma ray seintillatlon spectrometer agreed with results from the alpha counter so
cause of lowered activities.
Larsen and his co-workers report consistent age determinations and have not reported any evidence of major discrepancies. On this account we must view with caution the alternative of ascribing differences of analytical ages from geological field age determinations to fundamental
differences between a zircon age and the age of intrusion o f
granites. This is, however, a possibility and may hold true
in certain cases.
3. Variation of Magnetism of Zircon and Its Relation to
Zircon Composition
Tables 2 and 4 show that both the lead measurements and the activities tend to be higher in the more magnetic
fractions of zircon. The following discussion examines
possible causes for this.
Spectrographic plates containing Massachusetts zircon show that iron and manganese are considerably more abundant in magnetic fractions than in nonmagnetic fractions of the zircon. This could be the result of magnetic
inclusions or of differences in the composition of the zircon itself. The fact that the magnetism and activity are approxi-mately correlated suggests that the magnetism is not the
result of the presence of foreign minerals with the zircon unless they are rich in uranium or thorium. According to P. M. Hurley, the activity of the Massachusetts zircons is almost entirely due to uranium. E. S. Larsen, Jr., and
rock-forming minerals contain low amounts of uranium compared to the amount of uranium in zircon. It does not, there-fore, seem likely that any of the common rock-forming minerals could be the cause of the magnetic properties
of the zircon.
Inclusions of certain rather rare minerals that can contain large amounts of uranium might cause variability in magnetic properties. These include uranium minerals
themselves and accessory minerals such as monazite and xenotime. A mineral like xenotime may be present as the
result of exsolution from the zircon structure. R. C. Shields,
a graduate student at Massachusetts Institute of Technology, in the course of an unpublished investigation of the yttrium content of zircon (1955), analysed 15 zircons (not the
Massachusetts zircons) and found that 13 of these contained
from 1 to 4.5% Y203. In two samples in which yttrium was
not detectable, there is doubt that they are zircon. A correlation between magnetic fractions and yttrium content was not apparent.
Differences in magnetism in the zircon may be due
to different compositions of zircons which have crystallized
at different times or under slightly different environmental
conditions. In connection with this possibility, the
following information on variation of zircon from different environments is pertinent.
E. S. Larsen, Jr., and his co-workers report that metamict zircon and nonmetamict zircon from the same rock
may differ in radioactivity by as much as tenfold, and that a similar difference in radioactivity imay be found from zone to zone in a single zircon crystal (Larsen et al., 1953). They also found that, in the California batholith, zircon from the more basic rock types has much lower activities than zircon from granites (Paul, 1954,
p. 84).
V. M. Goldschmidt (1954, p. 563) predicted that uranium and thorium would be concentrated in the latest
fractions (outermost zones) of zircon crystals from igneous rocks when they are formed in a simple single sequence of crystallization. Goldschmidt also pointed out the difference in resistance of different zircons to hydrothermal alteration. Altered varieties are rich (up to several per cent) in hafnium, yttrium earth metals, thorium, uranium, phosphorus, niobium,
beryllium, and water of hydration (Goldschmidt, 1954, p. 425). Goldschmidt ascribed destruction of the zircon structure to
the presence of radioactive elements, as have recent workers such as P. M. Hurley and H. W. Fairbairn (Hurley and Fairbairn,
1953), and H. D. Holland and his co-workers (Holland, Schulz and Bass, 1953).
Zircon from beach sand is the stable type that has
been able to resist the effects of weathering. Goldschmidt noted that it was the variety which is low in hafnium
(Goldschmidt, 1954, p. 425). The writer has noticed (from
the lead analysis plates) that the beach sand zircon from North Carolina (1644), which was used as a standard base in
the lead analysis, is notably lower in iron and manganese than zircon from the Massachusetts granites and has a higher concentration of titanium, vanadium and chromium. Titanium, vanadium and chromium generally are concentrated in minerals at earlier stages of crystallization than are the elements iron and manganese. This suggests that the magnetism (which is related to iron content) and the
activity (which affects the stability of the zircon) are
due to the tendency for uranium and iron to be concentrated in the zircon structure at the same comparatively late
stage of crystallization. Local variations in supply of these elements and fluctuations in environmental conditions would tend to complicate the situation and give rise to
reversals in the zoning of zircon crystals.
4. Relation of Zircon Ag to the Age of Its Source Rock
Assuming for the moment that a true age can be determined for zircon, the relationship of this age to the age of the rock (granitic and related rocks in this case)
from which the zircon was obtained must still be established. There are many uncertainties in this problem and the remarks
here will be confined to an outline of the possibilities.
Generally, the field geologist is interested in the time of intrusion in the case of intrusive rocks. This is the time that is commonly thought of in connection with age determinations. More precisely, age determinations are
particular minerals (in this case, zircon), The age of the zircon may represent the time of intrusion but there
are other possibilities.
There may be a considerable delay between the time that a magma starts to crystallize and the time at which it is emplaced. Thus the zircon age might be greater than the age of intrusion.
If a granitic rock was formed by alteration of a
sedimentary rock, it is possible that old zircon present in the sedimentary rock could be preserved and give an age a
great deal older than the time of formation of the granite. Another possibility is that a period of metamorphism could
introduce or reconstitute zircon crystals to give an age considerably younger than the age of emplacement of the host rock.
The proof of the meaning of a zircon age lies in comparison with other methods of age determination and in detailed investigation of zircon from different environments.
Results which appear to be anomalous should not be discarded. It is possible that they are reflections of the complicated possibilities of zircon formation. For instar e,
the anomalously high age value in sample R3013C as compared to R3013A and R3013B might be an indication of two ages of zircon.
The age results from different magnetic
fractions of zircon from the same granite agree fairly well, as do the ages of granites which have been tuapped as the same rock type. There are a few exception to this rule.
The relative ages of the rocks agree with
field evidence insofar as the Fitchburg granite appears to be younger than the Quincy granite, which in turn appears
to be younger than the Milford granite and two samples of Dedham granodiorite. The latter two groups appear to be about the same age. Results which are different from
conclusions arrived at from field evidence are: four out of five values for the Northbridge granite gneiss indicate that it is the same age as or younger than the Quincy
granites; two samples classed as Dedham granodiorite appear
to be about the same age as the Quincy granite; the one sample of Ayer granite (Chelmsford) gives an age older than
all the other rocks except for one of the Dedham rocks.
The value of 756 million years obtained for a granite from Calais, Maine, which is probably early Devonian
on the basis of good field evidence, suggests that the
absolute values obtained in this investigation may be too high. This is also suggested by the fact that zircon ages for Quincy granites average about 460 million years,
which is about twice the age obtained by other workers for similar rocks in New Hampshire. Further work is needed to determine the possibility of a systematic error. This
would involve: (1) the analysis of another zircon fraction
from the Calais granite to check the consistency of the high result, (2) interlaboratory checks on lead and activity
measurements.
. It would be highly desirable to improve precision
of lead measurement, and further work in this direction is recomnended.
On a longer range basis, it would be advisable to
determine ages on rocks from areas sampled by other workers.
The White Mountain magma series of New Hampshire would be excellent for this purpose. A direct comparison of White
Mountain rocks with Quincy granite in one laboratory should
be enlightening. The Quincy granite is a good rock for
further work because of its high content of zircon.
The co-variation of activity, lead content and
magnetism of zircon is probably related to a tendency for iron and uranium to be concentrated in the zircon structure at the same stage of crystallization. Composition of zircon varies considerably and may produce differences in
determination work in som cases and not reliable in other
cases. It is recommended that laboratory investigation be done on susceptibility of various zircons to leaching. Since the composition of zircon depends largely on its
environment during formation, it may be possible to establish
some correlation between rock type and reliability of zircon
ages from these rocks.
Zircon ages determined for intrusive rocks may
not always represent the age of intrusion of the rock. For this reason, apparently anomalous results should not be disregarded but should be rechecked, It is therefore recommended that more samples of Dedham granodiorite and Northbridge granite gneiss be obtained to check the
'BIBLIOGRAPHY
Ahrens, L. H. (1950) Spectrochemical Analysis,
Addison-Wesley Press, Inc., Cambridge, Mass.
Bastin, E. S., and Williams, H. S. (1914) U. S.
Geological Survey Geological Atlas, folio 192. Billings, Marland P., Rodgers John, and Thompson,
James B., Jr. (19525 Geology of the Appalachian Highlands of East-Central New York, Southern Vermont, and Southern New Hampshire; Guidebook
for Field Trips in New England, sponsored by the
Geological Society of America, pp. 23-33. Currier, L. W., and Jahns, R. H. (1952) Geology of the
"Chelmsford Granite" Area; Guidebook for Field Trips in New England, sponsored by the Geological
Society of Aerica, pp. 105-117.
Dale, T. Nelson (1923) The Commercial Granites of New England, U. S. Geological Survey Bulletin 738.
Dowse, A. M. (1950) New Evidence on the Cambrian Contact
at Hoppin Hill, North Attleboro, Mass., American Journal of Science, vol. 248, pp. 95-99.
Emrson, B. K. (1917) Geology of Massachusetts and Rhode
Island, U. S. Geological Survey Bulletin 597.
Fairbairn, H. W., Schlecht, W. G., Stevens, R. E.,
Dennen, W. H., Ahrens, L. H., and Chayes, F.
(1951) A Cooperative Investigation of Precision and Accuracy in Chemical, Spectrochemical and
Modal Analysis of Silicate Rocks, U. S. Geological Survey Bulletin 980.
Faul, Henry (1954) Nuclear Geology, John Wiley and Sons,
Inc., New York.
Goldschmidt, V. N. (1954) Geochemistry, Oxford University
Press, London.
Holland, H. D., Schuls, D. A., and Bass, N. N. (1953) The
Effect of Nuclear Radiation on the Structure of Zircon (abstract), Trans. Am. Geophys. Union, vol. 34, p. 342.
Hurley, P. M., and Fairbairn, H. W. (1953) Radiation
Damage in Zircons: A Possible Age Method Bull. Geol. Soc. Amer., vol. 64, pp. 659474. Hurley, P. M1., and Thompson, J. B. (1950) Airborne
Magnetometer and Geological Reconnaissance
Survey in Northwestern Maine, Bull. Geol. Soc.
Amer., vol. 61, pp. 835-842.
LaForge, L. (1932) Geology of the Boston Area, Mass.,
U. S. Geological Survey Bulletin 839.
Larsen, E. S., Jr., Keevil, N. B., and Harrison, H. C.
(1952) Method for Determining the Age of Igneous Rocks, Using the Accessory Minerals, Bull. Geol. Soc. Amer., vol. 63, pp. 1045-1052. Larsen, E. S., Jr., Waring, C. L., and Berman, J. (1953)
Zoned Zircon from Oklahoma, Am. Mineralogist,
vol.
38,
pp. 1118-1125.Nogami, H. H., and Hurley, P. M. (1948) The Absorption Factor in Counting Alpha Rays from Thick
Mineral Sources, -Trans., Amer. Geophysical Union, vol. 29, No. 3, pp. 335-340.
Smith, 0. 0., and White, D,. (1905) The Geology of the Perry Basin in Southeastern Maine, U. S. Geological Survey, Prof. Paper 35.
Warren, C. H., and Powers, Sidney (1914) Geology of the Diamond Hill-Cumberland District in Rhode Island,
Mass., Bull. Geol. Soc. Amer., vol. 25, p. 460.
Williams, Charles R., and Billings, Marland P. (1938) Petrology and Structure of the Franconia
Quadrangle, New Hamshire, Bull. Geol. Soc. Amer., Vol. 49, p. 1011-1 L.
PRECAUTIONS AGAINST CONTAMINATION
Care was taken at all stages of the procedures to avoid introduction of lead contamination. Rock
pebbles low in lead were passed through the crushing and
grinding apparatus and analysed spectrographically to
test for lead. No contamination was detected. No materials which were liable to contain any appreciable content of lead were permitted to be used in grinding and crushing apparatus. An agate mortar and pestle used for final grinding and a glass spatula were reserved
specifically for these purposes. They were cleaned with dilute nitric acid between samples. A small plastic
tamping device was used to avoid contact with metal in
the final stage of filling electrodes. The sodium chloride flux was tested on the spectrograph and showed no detectable
lead.
Clerici solution is known to carry appreciable
concentrations of lead, so it was necessary to test its effect on zircon. Crystals of zircon R1644D which had
previously had no contact with clerici solution were soaked in it for one hour and then rinsed very lightly with water
and acetone.
Some of this clerici treated zircon was mixed with NaCI (80% zircon, 20% NaCl) and packed in four electrodes. Four other electrodes were also prepared containing zircon Rl6"D which had never been treated
with clerici solution. These electrodes were arced (two
electrodes per exposure) on the same plate. No contamination effect was evident.
SEPARATION OF ZIRCON
After the granite had been crushed and a
screened fraction (-60 mesh +200 mesh) obtained, zircon
was separated from other minerals by a combination of magnetic and heavy liquid separation. In general the sequence of operations was as follows:
1. The Frantz isodynamic separator was used in vertical position with full field.
2. The nonmagnetic fraction from step one was split into two fractions, sink and float, using acetylene 'tetrabromide. This was done by means of apparatus devised
by H. W. Fairbairn and shown in Figure 13. It consists of two large stainless steel beakers supported one above
the other on an aluminum framework. The sandy material to
be separated was mixed with acetylene tetrabromide in a separate beaker and poured into the upper beaker. A
propeller-type stirrer kept the sandy material in suspension.
This mixture was allowed to drain gradually into the lower beaker, which had been filled with acetylene tetrabromide,
by way of a valve-controlled tube. The sink then collected
in the bottom of the lower beaker and the float was allowed to overflow, by way of a lip around the top of the beaker,
into a large fritted glass funnel. Acetylene tetrabromide was recovered in a large flask under the funnel. The float
was washed with acetone to recover more of the heavy
liquid. The sink was recovered from the bottom of the lower steel beaker and washed with acetone.
3. Sink from the acetylene tetrabromide was
passed through the Frantz isodynamic separator with an inclination of 150 and current of 0.4 amperes.
4. The nonmagnetic fraction was treated with
boiling nitric acid (removes apatite and pyrite principally).
5.
The acid-treated material was split into sink and float fractions using methylene iodide. Small separatory funnels were used at this stage.6. The sink was separated magnetically success-ively at settings of 0.7, 1.2 and 1.4 amperes--all at 150. This was done to recover different fractional concentratio ns of other heavy minerals.
7. The nonmagnetic fraction from step six was separated into two fractions, sink and float, in clerici
solution. The sink at this stage was nearly all zircon as shown by examination with a binocular microscope.
8. The aircon was separated into three fractions, where possible:
a. magnetic at 1.4 amperes, 5*
00M~f0
b. nonmagnetic at 1.4 amperes, 54 but magnetic at
1.4 amperes, 20
9. Impurities were removed by handpicking.
Variations of the above procedure, and repetition of some phases, were used to suit the individual samples.
ARCING TESTS
Electrodes
Tests of burning qualities of zircon in carbon and graphite electrodes were made. The most satisfactory burn was achieved with 1/8 inch graphite electrodes. There
is less wandering of the arc when carbon is used but zirconium sparked erratically in the early stages.
Line
The Pb 2833
f
and Pb 4057 R lines wereexamined
in both the grating and prism spectrographs (using bothquartz and glass optics). The Pb 4057 line showed the best
sensitivity and had least background in all cases. The
grating spectrograph produced a lower background, which made it appear preferable to the prism spectrograph. Other
considerations favoring use of the grating are that the prism instrument has given some focussing trouble from time to time and that the wide dispersion of the grating (about 2.54 R per mm.) lessens the chance of interference from neighboring lines.
Grain Size
A smoother burn is obtained and the alkali
phase of arcing is longer when zircon is finely ground.
Tim
It was found necessary to stop the arc before zirconium made its appearance as there is some interference with the lead line, and erratic values result. In early experiments an arcing time of 35 seconds was used, but when the unknowns were run, early flashing of zirconium necessitated shortening the arcing time to 30 seconds.
Flux
A sodium chloride flux was used throughout these
experiments (mixture 80% zircon, 20% sodium chloride). The presence of sodium chloride lowers the temperature of
the are and suppresses the less volatile elements (e.g., zirconium). It also suppresses the CN emission which would otherwise interfere with Pb 4057 9.
