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Radiogenic argon and strontium diffusion parameters in biotite at low temperatures obtained from Alpine Fault uplift in New Zealand
Geochimica Acta, 1962. Vol. 20, pp. 67 to 80. Pergamon Press Ltd.
Printed in Northern Ireland
Radiogenic argon and strontium diffusion parameters in biotite at low temperatures obtained from Alpine Fault uplift in New Zealand* P. M. HVRLEY, H. HUGHES, W. H. PINSON, Jr. and H. W. FAIRBAIRX Massachusetts
Institute
of Technology,
Cambridge,
Massachusetts,
U.S.A.
(Received 9 May 1961) Abstract-Rapid uplift along the Alpine Fault zone in X_aw Zealand has exposed ancient mica schists which are estimated to have been at depths up to 9000 ft prior to t,he fault displacement, on the assumption that the smooth surface represented by the present, flat topprd, peak elevations is truly rcpresentaK-Ar age measurements on the tive of the mature Pliocene surface prior to uplift and glaciation. micas show losses of radiogenic argon that vary with depth of burial up to almost complete loss from biotite at a depth of 9000 ft. The mean value of the diffusion parameter (D/a*) calculated is 6 x 1O I6 set-i at an estimated temperature of 110°C. This value is much higher than expected but is compatible with effectively complete retention of argon at surface temperatures, and with measured diffusion losses Do/o” = 1 set-’ and E ~- 27 at high temperatures, if in the relationship D/u z = (Do/a’) exp (--E/R!!‘) Kcals/mole. A single Rb-Sr analysis on one of the low argon biot,ite samples showed an almost equal loss of radiogenic Sri”.
THE great uplift along the Alpine Fault zone of New Zealand has provided an opportunity to estimate the diffusion parameters of radiogenic argon and strontium in biotite in a temperature range much lower than is possible in the laboratory. The rise of rocks from a depth of about 3 km in a short period of time, and in a region of unusually high heat flow, has exposed biotite-bearing rocks in which the biotite shows varying argon loss as a function of depth, up to almost complete loss in the deepest cases. Pre-fault estimates of the depth of burial are based on our belief that the distribution of elevations of the mountainous region indicate with a high degree of assurance that the pre-uplift erosion surface passes through the tops of the present In other words, the mountain region is a deeply dissected peaks and ridges. erosion surface of fairly low relief, and was no higher than the top elevations indicate, prior to glaciation and post-fault erosion. The estimates of temperature are based to a lesser extent on calculations of above-normal heat flow resulting from the earlier rise of the geosynclinal prism over a period of at least 100 m.y. This requires a study of the geological history of prior to the fault displacement. the region, and a knowledge of rock types for thermal conductivity estimates. The determination of the diffusivities of radiogenic argon and strontium in common rock minerals at low temperatures is of much inherest, not only in geochronology but also in providing a new method of measuring depths of burial or time of uplift or erosion in the vertical motions of the earth’s crust. These diffusivities can not be determined in the laboratory because of the lengths of time involved, and other practical difficulties, so that they must be obtained from l
M.I.T. Age Studies h’o. 23. 67
favorable geological circumstances in which there is good knowledge of the It is therefore necessary to tsploit these o~~~)ortunities controlling parameters. where they are found t,o exist, et’cn if all the necessary data can not be known with certainty. By comparison of several such attempts it will be possible t,o approach final figures with assurance. l’hus if we are wrong in our interpretation that no significant post-fault depth of cover was rcmoved from above the surface passing through the present, heights-of-larval in the Alpine region, t,his work will not agree with subsequent test oases and will be abandoned.
CONTOURS IN
OF ALPINE
PEAK
ELEVATIONS
REGION,
SHOWING
SOUTH Q Il*f”TT
ISLAND,
‘P -4, YI1CI
Fig.
’ PRE-FAULT NEW
RELIEF
ZEPLAND
=?-Ys?
1.
Samples of the various metamorphic rock faoies acro68 the fault zone were kindly provided for age study by BRIAN MASOS of the American -Museum of Natural Hietory in New York. They were collected in the Southern Alps of the South Island of New Zealand during the mapping of this area by MASON, who is reporting elsewhere on his petrological stud& of the area. The position of the semples with respect to the bounderies of the metamorphic facies in the uplifted blook, shown approximately in Fig. 1, and also the eignificance of the age measurements in the geologiaal history of the region, is being discussed by MASOX in his report. We are concerned here only with the question of the diffusion loss of the radiogenia Ar and
Radiogenic
argon and strontium
Sr and its relation major fault uplift.
diffusion
to the ambient
parameters
temperature
in biotite
at low temperatures
under burial before
69
the recent
DATA 0~ SARIPLES The uplifted rocks of the central part of the island represent a succession of metamorphosed geosynclinal sediments and volcanics that have been isoclinallqfolded, and buried at great depth (GRISDLEY, HARRINGTO~ and WOOD, 1959). A comprehensive study of these rocks was made by TURNER (1938), who established the foundation of the metamorphic sequences across the fault zone. The succession of facies of rocks of graywacke composition from SE to NW is at present described as follows. Unmetamorphosed and argillites
greywackes
Chlorite Zone
Sheared greywackes Schistose greywackes Phyllonites, with chlorite, chlorite and muscovite
Biotite Zone
Phyllonites, Phyllonites, Phyllonit.es, Gneisses
or
with biotite with almandine with oligoclase
Fault Zone N\I* of Fault,
Sediments of various ages, and granite intrusions
These zones are indicated in Fig. 1, and are represented in the samples supplied by MASON. The locations of the samples are shown in Fig. 1. Their geologic associations and approximate elevations are listed below, going from SE to NW across the fault zone. All the samples were -40 + 200 mesh except B 3955, which was cut into thin flakes about 2 mm in diameter. M.I.T. Number R 3958
Argillite (whole rock) from zone of unmetamorphosed graywackes. Junction of the Hopkins and Dobson Rivers, Otago, New Zealand. This material has been analysed by H. B. WIIK, with the following results: SiO,, 60.73; TiO,, 0.76; Al,O,, 18.71: Fe,O,, 0.19; FeO, 5.05; MnO. 0.08; MgO, 2.62; CaO, 2.34; Na,O, l-60; K,O, 3.92; P,O,, 0.21: H,O+, 3.28; H,O-, O-02; CO,, O-03; C, trace; total, 99.55. A t’hin section and an X-ray powder photograph shows that the mineralogical composition is quartz, albite, muscovite and chlorite. It appears to have recrystallized completely from a clayey sediment at a very lou metamorphic grade. Thus the potassium is believed to be almost entirely in mica structures. Elevation 1750 ft.
70
M 3957
B 3953
B 3952
B 3956
B 3951
B 3955
B 3964
H3950
P. M. HURLEY,
H. HUGHES,W. H. PINSON,JR. and H. W. FAIRBAIRN
Muscovite concentrate. Looality, Haast-Makarora road, at Clarke Bluff The rook is coarsely (junction of Haast and Clarke Rivers, Westland). foliated quartz-albite-clinozoisite-chlorite-muscovite shist (Chlorite 4 subzone); it contains accessory amounts of pyrrhotite and apatite. Muscovite separated from crushed rock by Frantz Isodynamic Separator followed by centrifuging in methylene iodide-acetone mixture and 2.89). About 90 per cent pure. (fraction with density between '3.78 Elevation 200 ft. Biotite concentrate. Locality, Wilson Rock, Waiho Valley, near terminal The rock is a quartz-oligoclaseof Franz Josef Glacier, Westland. muscouite-biotite-almandine schist,, with accessory opaque material. About 95 per cent pure; t,he impurity almost entirely almandine. Elevation 700 ft. Locality, south bank of Wanganui River, just below Biotite concentrate. Hende Creek, Mount Bonar Survey District. The rock is a quartzoligoclase-muscovite-biotite schist, with accessory almandine, clinozoisite, tourmaline, and opaque material. About 95 per cent, pure. Elevation 500 ft. Biotite concentrate. Locality, Haast-Makarora road, at Halfway Bluff, 13-l miles from Haast Airfield. The rock is a quartz-oligoclasemuscovite-biotite gneiss, with accessory tourmaline, apatite, clinozoisite, and opaque material. More than 95 per cent pure. Elevation 100 ft. Biotite concentrate. Locality, east bank of Hokitika River, 1.4 miles, at 202” from Trig CW, Toaraha Survey District. The rock is a quartzoligoclase-museovite-biotite schist, with aocessory almandine, clinozoisite, tourmaline and opaque material. More than 95 per cent pure. Elevation 300 ft. Biotite. Locality, Moeraki River, Westland. Quartz-atbite-biotitemuscovite pegmatite. Sample cut from large plates of biotite (possibly contains a small amount of interleaved muscovite). Elevation 2000 ft. Biotite concentrate, from granite NW of fault. Locality, 1 mile north of l’aringa River, a few yards west of Main South Road. The rock is a granite consisting of quartz, microcline, plagioclase, biotite and hornblende, with accessory sphene, apatite, and zircon; some biotite flakes are partly replaced by chlorite. More than 95 per cent pure, t,he only impurity being a small amount of chlorite. Elevation 100 ft. Biotite concentrate, from granite NW of fault. Locality, cutting 0.4 miles south of Rotomanu railway station. The rook is a granite consisting of quartz, microcline, piagioclaae, biotite (with a IittIe replacement by chlorite), and museovite, with aocessory apatite. More than 95 per cent pure, chlorite being the impurity. Elevation 300 ft. K-Ar
RESULTS ANI) GEOLOGICAL INTERPRETATIONS
Argon 40 was determined by isotope dilution, using a spike gas containing 90 1”’ cent argon 38. Air contamination was measured by the relative abundance of
Radiogenic argon and strontium diffusion parameters in biotite at low temperatures
71
36. Samples were fused in an induction furnace at a temperature of 1200”(‘., with a small amount of NaOH flux. Fine material was prevented from decrel)itnt,iny out of the crucible by tamping it with Pyrex glass wool. Crucibles were of sinteretl alumina, inside a molybdenum casing, with a molybdenum lid. The furnace was a water-jacketed bell jar. Gas was purified by hot copper oxide and titanium sponge, and the purity of the final argon, particularly in respect to the content of hydrogen, argon
Table 1. Sample number
Mineral and metamorphic fit&es
._ R 3958 M
B B B B B B u
Unmetamorphosed graywacke zone 3957 i Muscovite. Chlorite schist zone 3953 j Biot,ite, with almandine and oligoclase. Schist 3952 1 Biotite, with oligoclase. Schist Biotite, with oligoclase. Gneiss 3956 3951 / Biotite, with oligoclase and almandine 3955 ’ Biotite plates in pegmatite 3954 I Biotite. Paringa river I Granite WV of fault 3950 / Biotite. Rotomanu granite NW of fault
Total Ar40/K40
KCj, _.
’ Argillite.
3.72 8.01
.0112 .0065
6.50 6.57 7.19
.OOOQ .0034 ~0010
6.63 7.81
+oll .0081
.OOOP - WOl
.OOl.i ~j .OOOl
6.12
was monit’ored by a cycloidal-focussing mass spectrometer. The isotopic composition of t,he spiked gas sample was analysed by a Reynolds-type mass spectrometer (REYXOLDS, 1954) and the cycloidal-focussing instrument, both directly coupled to the gas purification train. In analyses with air corrections less than 30 per cent the estimated precision Air corrections above this (reproducibility) was 3 per cent standard deviation. figure cause the precision error to rise, for although the error in 36138 measurement is fixed, it enters as the difference in subtracting the air argon 40. SystemaCc error of the mean is believed to be less than 1 per cent, based on the measurement) of the pure argon, and on int,crlaborator> argon content of air, on spectroscopically agreement in the analysis of a standard sample of biotite (B 3203). The potassium was measured by a Perkin-Elmer flame photometer using lithium internal standard. A single K analysis involves two solutions of separate portions Reproducibility of the sample. each with two separate photometric measurements. in potassium measurements is estimated to be 2 to 3 per cent standard deviation for a complete analysis, with an absolute error of the mean not exceeding 2 per cent. Decay constants for potassium 40 have been adopted as follows: 1,. = 0.585 x 10-10 yr-1; 1 -= 5.30 s IO-10 yr-l (ALDRICH and WETHERILI,, 1B.W). Analytical results are shown in Table 1. All of the samples in the biotite zone, coinciding with t.he greatest uplift close to the fault on the east, side, show ages that are remarkably young for metamorphic rocks and. except for R 39.55. agree with
72
P. 31. HI:RLEP,H.
HVGHES,
W. H. PINSON, JR. and H. w’. FAIRBAIRN
each other within experimental error. The apparent loss of argon can not he ascribed to hydrothermal alterations in fault zones as the biotite would not Iw developed under these condit.ions, and rekograde metamorphism is not prominent in these samples. As there is no intrusive activity of any consequence in the belt. the biotite in the oligoclase zone must have been developed in an environment 01 elevated temperature f5W-:i.Wf’) that was the result of burial at a depth that 1~;~s many t,imes the depth of the sa,mple region during t,he late Tertiary. \Ve mllst conclude therefore that the low a,rgon age values are the result of diffusion loss 01 argon at depths and temperatures much less than those in which the biotite \v:l.s formed. T&k
2. Estimated
geological
Event 1. Paringa R. granite is part of a basement that has bern stable since Pennsvlvanian times ‘. Sedimentation in trough tlruing Permian and Triixssic. Downgarping of geosyncline due to deep 3. Metamorphism burial. Probably ending with uplift in Jurassic times 4. Major uplift in Cretaceous times and erosion to low relief 5. Early Tertiary tag in uplift 6. Now uplift continuing in late Tertiary with erosion keeping pace 7. Rapid fault uplift of block bounded on NW by Alpine Fault
hisiory
11~1 in calculations
of thermal history of aam&
E vldence
..--x-“.“. -- .--.-Age of 286 m.y. on B 3954 indicates
-
.
inconsequential
uplift
since that t.ime
Triassic fossils. Jlinimum agrees w&h this,
age of argillite
It 3958 at, 133 m.y.
Metamorphic grnde reached oligoclaee facies in center of trough, equivalent to about 550°C
tow grade schist pebbles in Cretinous conglomerates. Cessation of argon diffusion loss in M 3957. Cessation of moat of uplift of Rotomanu granite, exposing it and giving pebbles of similar granites in the baaal Eocene coal measures partial sea transgression in Eocene, to complete traznrgresaion in Oligocene with Iimeetone deposition Burial and folding of Oligocene limestone with terrigenous sediments and development of smooth, low-relief erosion surface that is indicated prior to fault uplift Sot requiring more than 5 my. from argon dif%.rsion data, and probably not more than 2 m.y. Therefore in Plei&oeena and latest Pliocene. Pebbles of biotite schist faeiee fir& Been in Pleistocene moraines
Before studying the question of diffusion constants we may utilize the age data where possible to assist in the establishment of the geological history of the region, of interest in the estimates of temperature at depth. In particular B 3964, t,he Paringa River granite with an age value of 286 m.y., and R 3968, the unmetamorphosed argillite with an age value of 133 m.y., are helpful. Using the general geological eonulusions of GRINDLEY, HARRINQTON and WOOD, 1959, and MASON (personal communication), and making elight modifioations in accordance with these age measurements, we have decided on the sequence of events, shown in Table 2, to be used in the temperature caloulatione.
Radiogenic
argon and strontium
diffusion
parameters
in biotitc
at low teml)eraturcs
73
CALCULATION OF THE SECULAR EQUILIBRIUM RATIO [Ar40]/1K4n] IS TERMS 0~ THE DIFFI'sI~?; P_4RAwwEK. D/a2 The number of argon atoms generat,ed per cm3 per WC (n,,) is given by: no == Af[K40]
where [K4”] is the’K40 concentration in atoms/cm3; I is the K40 decay constant = 1.677 x 10-l'set-l; f = -585/5.30 = ratio of APO produced per K40 decay. For times much smaller than this is the case here where we the grain, i.e., the equilibrium Consider a spherical grain sphere of radius r (r < a) with
the half-life of K40 n, can be treated as a const,ant; calculate the average t’ime the argon atoms spend in concentration developed. of radius a and equate the argon produced within a the diffusion loss across the spherical boundary at 1’: noi7rr3
=
-D4rr2!!F
where n is the concentration
of argon atoms at 1‘: giving
on introducing the boundary in the grain is
condition
s
n = 0 at r == n. The total amount of argon
ndV=%
r2)4nr2 dr
(a”-
60 s
2)
4i7noa5 45
where V is the volume of the grain. [Ar40]
W4”l
D
Substituting *585 ___. 5.30
= 1.234
a2
V =ino-z
for no we have
1.677 x 10-l’ --.-~-i5_~~
x
x
lo-l9
a2 .~
a2 --
D
In the samples showing greatest argon loss (3951, 3952, 3953 and 3956) the to an age of 6 m.y., giving a average value of Ar40/K40 is *0003, corresponding minimum value of D/a2 equal to 4*-*) x lo-16. However, the fault movement must have taken place over part of this time, and after initial uplift the temperature in the region of the samples would have decreased and argon diffusion loss ceased. Therefore, if t, is the length of time in m.y. since diffusion loss ceased (i.e., the fault uplift was well initiated),
the equilibrium
value of Ar40/Kd0 is equal to .0003
and the average value of D/a2 pertaining
C
1 -
tf
61
to these samples 3951, 3952, 3953 and
--
_
, ,
0+046 o-0102
0.6092 owO2 0.00153
0~0006 0.0004
Ar40/K40
200 1760
100 500 700 300 2000
l%vation. feet
L-
/
j
8200 7500
QE&o 8060 8000
8000 8500
L
/
!
I
I
I
I
/
I (1) j _---_:_--_ I /
Tablo
9390 7770
9940 9570 9960 9080 10,350
j_
: :
1 j (11) ; .~*
Elevation of original surface, (feet)*
_--_~I___
* Two methods of estimating described in text.
M 3968
M-llWovite M 3957
3956 3962 3953 3961 3955
Biotite
Sample number
!
f .30 f .20 z .20 i ,25 f .35
2.60 f -20 1.80 * .20
2.70 2.60 2*75 2.60 2.20
O-1
+ * f f f
109 i 81 f
113 109 114 109 95
PJ
13 10
15 13 13 14 15
Pre-fault temperature _..__..
-I*-
j
!
--.--..-- -ll.--.---.-
Depth of burial,
3.
I /
I
___-
1.6 2.0 4.0 4.0 0.52
__._ -. .
Minimwn D/a2 ( x 10-lfi) set+
-_-“-_
-L
D/d
-
<;.os
q.17
3.2 4.0 8.0 8.0 0.59
( x lo--16) see-’
Probable Maximum
Hadiogenic
argon and strontium
diffusion
parameters
in biotitt
at low temperatures
7.i
3956 is D/a2
25
=
42
x
lo-
q1
-l,t).
In the case of biotite sample 3955, occurring in the same region, its age value of m.y. would result in a D/a2 relationship on the same basis, as follows: D/a2 = 0.80 x lo-
The other samples of biotite were west of the fault uplift. The two samples of muscovite-bearing rocks, 3958 and 3957, may also be considered. Their greater age values suggest that secular equilibrium between argon loss and gain had not been reached and that the values would have increased if they had remained a greater time at their pre-fault depth. Therefore, the D/a2 values are maxima only if calculated on the same basis. A summary of the values of D/a2 for these different samples is given in Table 3, calculated on the basis of t, = 0 and t, = 3. In the case of t, = 0, allowing zero time for exposure and cooling of the samples, the values of D/a2 are minima. In the case oft, = 3, or cooling of the biotite at 3 m.y. ago in the late Pliocene, the values of D/a2 are probably close to the maximum permissable. These limits are indicated on the points plotted in Fig. 2, at t’he limits of error in the vertical direction. ESTIMATED DEPTH OF BVRIAL The depth of burial of each sample, before uplift and erosion, is calculated by subtracting its present elevation from the estimated height of the upthrust mass immediately after the faulting. This height, it is suggested, can be obtained by interpolating across the ridges left by the recent glaciation. In Fig. 1 the distribution of ridge-top and peak elevations is used t’o provide a contour plan of the surface before uplift. The two-dimensional spatial correlation of these points into a smooth surface is our argument for the nonexistence of any significant thickness of eroded material above the surface. In particular, the uniform elevations along the extensive sets of ridges left by the glaciers would be difficult to explain if they were This geomorphological interrandomly left by erosion from higher elevations. pretation is the key to the conclusions in this investigation and the reader may judge for himself on this point by studying a topographic map of the area. The specific estimate of the original height at each sample location was made in alternative ways. In one, this original late Tertiary erosion surface, shown contoured in Fig. 1, was assumed to be symmetrical about the line of highest elevations parallel to the fault. Thus the original height at each sample location is the height, in Fig. 1, of its mirror image through this line. Another estimate was obtained by ignoring the low relief prior to faulting and finding the plane surface most nearly passing along all the ridges in the upthrust block. A least squares solution over a 75 mile radius showed that the strike of this plane is almost parallel to the fault, the dip to the SE being 1000 ft every 114 miles, By thus assuming that the original elevation of the upthrust block continued
1’. Y.
76
HUIALEY,
H. HUMIES,
W.
H. PINSON,JR. and H. W.
FAIRBAIRS
increasing right up to the fault and beyond the line of the present maximum elevat,ions, an upper limit to t,he depth of burial is obtained. The results are listed in Table 3, the error representing the difference between 2 km, whichever is the greater. these alternative estimates, or
THERMAL
GRADIENT
JUST PRIOR TO FAULT UPLIFT
The thermal gradient in the region may be regarded aa the sum of two terms, one constant and one transient. The “steady state” gradient is due to the almost
ctdspar. Ptwse I eldrpar, PhosaH
07 08
09
IO
It
12 13 14
13 16
17
18 19 2.0 2.1 2.2 23 moo T
24 2.5 25
2.7 2.8 29
30 31 32
33 34
'$(
time invariant loss of the earth’s internal heat. Taking this heat 108s86 (I.2 -& 0.15) x 1O-6 Cal/cm2 880, and taking the thermal conductivity of the material (granites and schists) to be 0.005 to 0.006 Cal/cm set “C (BIRCH, 1942), the resulting temperature gradient is (22 _:- 3-5) “(‘/km. The transient temperature gradient is a consequence of the uplift of the rook, metamorphosed at 5OO’Y!., to shallow depths prior to the final faulting. It may be oeicu18bed for a sudde?L upward dksplecement by expanding the resulting exees8 temperatures above the steady state values in a Fourier series, the amplitudes of the sucaessive terms of which decay exponentially with time, the higher the term
Ratliogenie
argon anti strontirlm
diffusion
parameters
in biotke
at low ietttpertlt,ures
77
the more rapid the decay as determined by the one-dimensional heat conduct,ion equat’ion. On approximating the actual uplift by a series of discrete displacements spaced in accord with the geological history given in Table 3 (and in particular assuming that uplift has continued since the Cretaceous at a de~eler~~tillg rate) the resulting transient temperature gradient was probably 14 & Y(‘/km near t,he surface immediately prior to the final faulting. Thus the most likely value for the total thermal gradient ah that time, is (36 &_ 4)‘Cjkm. With this gradient, and assuming a surface temperature of 15Y!., t.he temperature of each sample prior to fault uplift is obtained and listed in Tabie 3 along wit’h its standard error arising from the uncertainty in the &ermal gradient and the depth of burial. These limits together with those of D/a2 are shown graphically by the boxes in Fig. 2.
CONCLUSION
ON ARGON DIFFUSION PARAMETERS
The diffusion results obtained here are derived from measurements showing almost, complete loss of argon; one can not appeal to the loss of a small adsorbed fraction or of a minor mobile argon “phase” (in notation cf AMIRKHANOV, 1961) to account for the observations. While it is conceivable that diffusion has been facilitated by some aton+c rearrangement this is a detail unaffecting our main conclusion: that these biotites have lost essentially all their radiogenic argon at a temperature of about 100’32, or conceivably on cooling from 100°C. We now show that such diffusion losses are not incompatible with t.he effectively complete retention of argon at surface temperatures, or with the measured diffusion Rather, these additional data may be used to estimate rates at high temperatures. the temperature dependence of the diffusion rate. The diffusion loss of argon from biotite at temperatures found at the earth’s surface has appeared to be finite in some of the oldest cases, but there is some confusi,on resulting from the uncertain decay constant of Rb8’. For example, tests on biotite from the Superior Province in Western Ontario have shown age values slightly less than the age of the area estimated on the basis of lead and strontium These lower argon values are equivalent to at most a 15 per age determinations. cent lowering of age in 2600 n1.y.; even allowing for some strontium or lead diffusion also will not alter the general magnitude of this value. On substituting t,his data into the relation7 given by GOLES, FISH and ANDERS (1960, p. 180), the value of l>/a2 is found to be of the order 1O-2o set-I. This point is plotted in Figure 2, taking an average near surface te~nperature of 20”(1, and fixes t.he rni~linlunl rate for the reduction in diffusivity with deqreasing temperature. The values obtained in the laboratory at high temperatures by AMIRKHAMV et al. (1959) and HART (1960) are replotted in Fig, 2, and serve to determine an upper limit for the change in diffusivity wiih t,emperature. With the usual relation between diffusivity and temperature
where
f * is t.he apparent age (about 2300 m.y.), and t is the true age (about. 2600 m.y.).
P. M. HURLEY, H. HUGHES, W. H. PCNSON, JR.
78
and H. U’. FAIRBAER~V 4
and E = 27 kcalsfmole. Some uncertainty in the value of E arises from possible different values of the effective diffusion radius (a) among the different biotites but this seems unlikely to be comparable to the range of 1016spanned by the observations. A different value of E is possible if a phase change in biotite occurs between the different sets of observations. It must be recognised, however. that diffusion losses from biotites of igneous origin, or from other geologioal environments, may differ substantially from that determined by t,he above parameters beoause of such variations in the diffusion radius. set-l
ThFUSION
Loss
OF
&ADIOCENIC
Srs7
A thorough analysis of the Wb- -Sr age value in a single biotite sample. R 3Q;id, yielded the following : Rb, p.p.m.
Rba7, p.p.m.
Sr? p.p.m.
520.1
147.3
85.5
W/8$6 09710 * .OOl
A recent survey of the SFe’/SP ratio in ooeanio and oontinental basalts in this laboratory leads us to believe that the initial ratio in the biotite, whieh was formed from geosynolinal sediments, could not have been less than 0.707 even if the sediments had been oomposed entirely of volcanic materials. Therefore the initial S$7/Srs6 value in the biotite was between Q-707 and 0.710, and the radiogenic SP7 in the biotite is determined as being between the limits zero and 0.026 p.p.m. The age values equivalent to these limits are zero and 12 m.y. It is clear that the radiogenie S$’ also has been leaving the biotite. As before, considering secular equilibrium between loss and gain of daughter atoms, the total number of radiogenic SP atoms in a grain is
w&exeRbs’ is the number of parent atoms per cm 3, I’ is the volume of the grain, and Am,is the decay constant of Rb*’ which is approximately equal to 47 x lo-16 set--1. The measured ratio of daughter to parent is *SF -- --- < 0.026/147.3. Rb8’ Therefore D/a2 > 1.75 x lo-l6 se+. This minimum value of D/a2 for B 3952 resulted from a calculated environmental temperature of 109 f 13YJ. (Table 3.) As in the case of argon, the Rb-Sr age values on very ancient biotite suggest that the loss of *Sti’ is less than 10 per cent at earth surface temperatures over a time of 2600 “1.9. This sets an upper limit for D/a2 at 20°C of 1W20. * Itadiogcnio
Radiogenic argon and strontium diffusion parameters in biotite
8t
iow
temperatures
79
These values of D/a2 at 109°C and 20°C are close to those for argon. It is therefore concluded tentatively that similar values for E and D,/aa may be applied to *Sr*’ as for argon in biotite in this low temperature region. DISCUSSTON The observations recorded here constitute, we suggest, a close equivalent to a diffusion experiment of several million years duration in contrast to the few hours durat$ion common in laboratory diffusion measurements (GERLING and MOROZOVA, 1958; REYXOLDS, 1957; EVERNDEN, CURTIS, KISTLER and OBRADOVICH, 1960; AMIRKHAXOV, BRANDT, BARTNITSKII, GASANOV end GURVICH, 1959B; AMIRKHASOV, ef al., 1959a and 1961; FECHTIG, GENTNER and Z;~HRINGER, 1960; GEISS and HESS, 1958). The enormous extrapolation of such laboratory data to make them relevant to geological problems is thus avoided. There have been several cases in our own work and in that of others where biotite known to have been buried at substantially greater depths has retained its ancient K-Ar ages. We submit that it is too simple to declare that our depth or temperature estimate is faulty in this case, as these estimates are not insecurely b&sed. This is a highly mobile belt, relative to the other cases, and the micas occur in a metamorphic environment that has been subject to not only changing physical What is clearly observable here may be true in conditions but also to deformation. lesser extent in other mobile belts of metamorphic rocks. Whether the loss of radiogenic argon and strontium is due to diffusion or some other mechanism may not be settled easily, but the knowledge that it can occur in seemingly normal biotite at approximately 100°C is of interest and the purpose of this report. If this effect is commonly the case it will explain some of the observations that otherwise are difficult to account for in geochronology. For example, most investigators, including ourselves, have referred to the biotite K-Ar age values as commonly representing the time of la& metamorphism or heating. Instead, a continuous uplift of a region would yield a fairly uniform age value representing the t,ime since the last major erosion, without any need for an erogenic or metamorphic event at this time. Thus the difficulty of repeated deep folding of regions of crystalline rocks to account for observed age patterns could be explained simply by variations in time of uplift and depth of erosion. Geological history has been recorded in terms of episodes of sedimentation because of paleontological correlation. Owing to the difficulty of dating sediments the geochronologists have been making a plea for a swing toward erogenic and volcanic episodes as a frame of reference for the extension of earth history into the If the above conclusions are true, the ages commonly measured by Pre-Cambrian. K-Ar and Rb-Sr on biotite may actually reflect the time of major uplift and erosion, which coincides with that of sediment,ation, and not the initial period of metamorphism and igneous intrusion in the erogenic belt. For example, in New Zealand the biotite age values post-dated the time of metamorphism by more than 100 m.y. Thus the geochronologists may be actually dating episodes that are more nearly comparable to times of sedimentation than to metamorphism, and the frame of reference provided by each method of dating may be more nearly matching thsn expectred.
Ark,co,cZedgntents-TThe authors gratefully acknowledge the ussistancc of H. \\-. K;R\‘Is(;EK /it some of the analyses, and tho critica.1 review and interest of S. li. HART a-ho ~LISO IJ~o\‘I~II~~z translations of the Russian lit,eratuw. \Vc are indt~btod to 13~1~s ,\laso~ not onI>, fiw Jlib IW+ vision of the samlllt:s bnt also for I,ackgromltl information and litwuturc on the gvolop~~ of’ SI,,~ Zealand. The support of t,hr I)ivision of Researcli of the I7.S. Atomiv Energy t ‘otnr~~k~~~~! II~.-. made the investigation possible. REFI~REXCJ~S ALDRICH L. T. and ~VETHEKILL (:. VI'.(1958) C;eochronology by radioactive decay. dwt. flec.. Nucl. sci. 8, 257-2998. AMIRKHANOV K. I.,BRANDT S. B. and I~ARTNITSKII E. S. (1959a) Diffusion of radiogenw argon in feldspars. Uo,C1. &ad. Sci. lis,)‘iZ, 125 So. 6, Cieochem. Ser. AMIRKHANOV K. l., BHANDT, 8. B., B~RTWIT~KIIE. s., GASANOVs. -4. and C:I.HVI(‘H \‘. s. (1959b) On t)he mechanism of losses of radiogenic argon in micas. il~nrl. $5’~;. C;%S’R no. 3. 1’. 104. AWIRKHANOV K. I.,UHANDT S. H. and BARTNITSICIIE. S. (1961) HadiogenicargoninInitwrals and its migration. (;eochronology of Rock Systems, (.J. L. KTTLP,ed.), Ann. S. Y. dcrr
glauconite, microcline, sanidine, loucite and phlogopite. Allaer. .7. Sci., 268, 583-604. FECHTIGH., OENTNERW. and ZKKRINGEII.J. (1960) Argonbestiminungen an Kaliummineralien, VII. Diffusionsverlurte von Argon in Mineralien and ihre Auswirkung auf die Kaliurn-ArgonAltersbestinqnung-. Geochim. et Cotwnochim. Actu 19, 70-79. CEISS J. and HESS D. C. (1958) Astrophys. J. 12’7, p. 224. GERLING E:K. and MOROZOVAJ. M. (1958) The kinetics of argon ixolat,ion from microclincperthite. Geokhimiia 7, 175-781. GOLES C. Q., J~FIBHR. A. and ANDERS E. (1980) The record in -the meteorites, I: The formel environmenf of stone meteorites as deduced from Ka”-Ar40 ages. Geochim. et Cownochinc. Acta 19, !77-195.
GRINDLEY G. W., HARRINGTONH. J. and Woou 13. L. (1959) The geological map of Stl\r Zealand. N.Z. Geol. Survey Bull. 66, p. 111. HART S/R. (1960) Mineral uges and mdamorphisnl. Natrs. Inst. Tech. Ph. D. Thesis, L’19‘1,1>. REYNOLDSJ. H. (1954) A high-sensitivity mass spectrometer. Phys. Rev. 98, p. 283. EEYNOWS feldspar.
J. H. (1957) Comparative study of argon Geochim. et Co.wnockim. Acta 12, 177-184.
content
and argon
diffusion
in mica
and
.
TURNER %. J. (1958) Progressive regiona metamorphism in southern Sew Zealand. LXX\‘, No. 886, 160-174.
Geol. Mug.
Printed in Northern Ireland
Radiogenic argon and strontium diffusion parameters in biotite at low temperatures obtained from Alpine Fault uplift in New Zealand* P. M. HVRLEY, H. HUGHES, W. H. PINSON, Jr. and H. W. FAIRBAIRX Massachusetts
Institute
of Technology,
Cambridge,
Massachusetts,
U.S.A.
(Received 9 May 1961) Abstract-Rapid uplift along the Alpine Fault zone in X_aw Zealand has exposed ancient mica schists which are estimated to have been at depths up to 9000 ft prior to t,he fault displacement, on the assumption that the smooth surface represented by the present, flat topprd, peak elevations is truly rcpresentaK-Ar age measurements on the tive of the mature Pliocene surface prior to uplift and glaciation. micas show losses of radiogenic argon that vary with depth of burial up to almost complete loss from biotite at a depth of 9000 ft. The mean value of the diffusion parameter (D/a*) calculated is 6 x 1O I6 set-i at an estimated temperature of 110°C. This value is much higher than expected but is compatible with effectively complete retention of argon at surface temperatures, and with measured diffusion losses Do/o” = 1 set-’ and E ~- 27 at high temperatures, if in the relationship D/u z = (Do/a’) exp (--E/R!!‘) Kcals/mole. A single Rb-Sr analysis on one of the low argon biot,ite samples showed an almost equal loss of radiogenic Sri”.
THE great uplift along the Alpine Fault zone of New Zealand has provided an opportunity to estimate the diffusion parameters of radiogenic argon and strontium in biotite in a temperature range much lower than is possible in the laboratory. The rise of rocks from a depth of about 3 km in a short period of time, and in a region of unusually high heat flow, has exposed biotite-bearing rocks in which the biotite shows varying argon loss as a function of depth, up to almost complete loss in the deepest cases. Pre-fault estimates of the depth of burial are based on our belief that the distribution of elevations of the mountainous region indicate with a high degree of assurance that the pre-uplift erosion surface passes through the tops of the present In other words, the mountain region is a deeply dissected peaks and ridges. erosion surface of fairly low relief, and was no higher than the top elevations indicate, prior to glaciation and post-fault erosion. The estimates of temperature are based to a lesser extent on calculations of above-normal heat flow resulting from the earlier rise of the geosynclinal prism over a period of at least 100 m.y. This requires a study of the geological history of prior to the fault displacement. the region, and a knowledge of rock types for thermal conductivity estimates. The determination of the diffusivities of radiogenic argon and strontium in common rock minerals at low temperatures is of much inherest, not only in geochronology but also in providing a new method of measuring depths of burial or time of uplift or erosion in the vertical motions of the earth’s crust. These diffusivities can not be determined in the laboratory because of the lengths of time involved, and other practical difficulties, so that they must be obtained from l
M.I.T. Age Studies h’o. 23. 67
favorable geological circumstances in which there is good knowledge of the It is therefore necessary to tsploit these o~~~)ortunities controlling parameters. where they are found t,o exist, et’cn if all the necessary data can not be known with certainty. By comparison of several such attempts it will be possible t,o approach final figures with assurance. l’hus if we are wrong in our interpretation that no significant post-fault depth of cover was rcmoved from above the surface passing through the present, heights-of-larval in the Alpine region, t,his work will not agree with subsequent test oases and will be abandoned.
CONTOURS IN
OF ALPINE
PEAK
ELEVATIONS
REGION,
SHOWING
SOUTH Q Il*f”TT
ISLAND,
‘P -4, YI1CI
Fig.
’ PRE-FAULT NEW
RELIEF
ZEPLAND
=?-Ys?
1.
Samples of the various metamorphic rock faoies acro68 the fault zone were kindly provided for age study by BRIAN MASOS of the American -Museum of Natural Hietory in New York. They were collected in the Southern Alps of the South Island of New Zealand during the mapping of this area by MASON, who is reporting elsewhere on his petrological stud& of the area. The position of the semples with respect to the bounderies of the metamorphic facies in the uplifted blook, shown approximately in Fig. 1, and also the eignificance of the age measurements in the geologiaal history of the region, is being discussed by MASOX in his report. We are concerned here only with the question of the diffusion loss of the radiogenia Ar and
Radiogenic
argon and strontium
Sr and its relation major fault uplift.
diffusion
to the ambient
parameters
temperature
in biotite
at low temperatures
under burial before
69
the recent
DATA 0~ SARIPLES The uplifted rocks of the central part of the island represent a succession of metamorphosed geosynclinal sediments and volcanics that have been isoclinallqfolded, and buried at great depth (GRISDLEY, HARRINGTO~ and WOOD, 1959). A comprehensive study of these rocks was made by TURNER (1938), who established the foundation of the metamorphic sequences across the fault zone. The succession of facies of rocks of graywacke composition from SE to NW is at present described as follows. Unmetamorphosed and argillites
greywackes
Chlorite Zone
Sheared greywackes Schistose greywackes Phyllonites, with chlorite, chlorite and muscovite
Biotite Zone
Phyllonites, Phyllonites, Phyllonit.es, Gneisses
or
with biotite with almandine with oligoclase
Fault Zone N\I* of Fault,
Sediments of various ages, and granite intrusions
These zones are indicated in Fig. 1, and are represented in the samples supplied by MASON. The locations of the samples are shown in Fig. 1. Their geologic associations and approximate elevations are listed below, going from SE to NW across the fault zone. All the samples were -40 + 200 mesh except B 3955, which was cut into thin flakes about 2 mm in diameter. M.I.T. Number R 3958
Argillite (whole rock) from zone of unmetamorphosed graywackes. Junction of the Hopkins and Dobson Rivers, Otago, New Zealand. This material has been analysed by H. B. WIIK, with the following results: SiO,, 60.73; TiO,, 0.76; Al,O,, 18.71: Fe,O,, 0.19; FeO, 5.05; MnO. 0.08; MgO, 2.62; CaO, 2.34; Na,O, l-60; K,O, 3.92; P,O,, 0.21: H,O+, 3.28; H,O-, O-02; CO,, O-03; C, trace; total, 99.55. A t’hin section and an X-ray powder photograph shows that the mineralogical composition is quartz, albite, muscovite and chlorite. It appears to have recrystallized completely from a clayey sediment at a very lou metamorphic grade. Thus the potassium is believed to be almost entirely in mica structures. Elevation 1750 ft.
70
M 3957
B 3953
B 3952
B 3956
B 3951
B 3955
B 3964
H3950
P. M. HURLEY,
H. HUGHES,W. H. PINSON,JR. and H. W. FAIRBAIRN
Muscovite concentrate. Looality, Haast-Makarora road, at Clarke Bluff The rook is coarsely (junction of Haast and Clarke Rivers, Westland). foliated quartz-albite-clinozoisite-chlorite-muscovite shist (Chlorite 4 subzone); it contains accessory amounts of pyrrhotite and apatite. Muscovite separated from crushed rock by Frantz Isodynamic Separator followed by centrifuging in methylene iodide-acetone mixture and 2.89). About 90 per cent pure. (fraction with density between '3.78 Elevation 200 ft. Biotite concentrate. Locality, Wilson Rock, Waiho Valley, near terminal The rock is a quartz-oligoclaseof Franz Josef Glacier, Westland. muscouite-biotite-almandine schist,, with accessory opaque material. About 95 per cent pure; t,he impurity almost entirely almandine. Elevation 700 ft. Locality, south bank of Wanganui River, just below Biotite concentrate. Hende Creek, Mount Bonar Survey District. The rock is a quartzoligoclase-muscovite-biotite schist, with accessory almandine, clinozoisite, tourmaline, and opaque material. About 95 per cent, pure. Elevation 500 ft. Biotite concentrate. Locality, Haast-Makarora road, at Halfway Bluff, 13-l miles from Haast Airfield. The rock is a quartz-oligoclasemuscovite-biotite gneiss, with accessory tourmaline, apatite, clinozoisite, and opaque material. More than 95 per cent pure. Elevation 100 ft. Biotite concentrate. Locality, east bank of Hokitika River, 1.4 miles, at 202” from Trig CW, Toaraha Survey District. The rock is a quartzoligoclase-museovite-biotite schist, with aocessory almandine, clinozoisite, tourmaline and opaque material. More than 95 per cent pure. Elevation 300 ft. Biotite. Locality, Moeraki River, Westland. Quartz-atbite-biotitemuscovite pegmatite. Sample cut from large plates of biotite (possibly contains a small amount of interleaved muscovite). Elevation 2000 ft. Biotite concentrate, from granite NW of fault. Locality, 1 mile north of l’aringa River, a few yards west of Main South Road. The rock is a granite consisting of quartz, microcline, plagioclase, biotite and hornblende, with accessory sphene, apatite, and zircon; some biotite flakes are partly replaced by chlorite. More than 95 per cent pure, t,he only impurity being a small amount of chlorite. Elevation 100 ft. Biotite concentrate, from granite NW of fault. Locality, cutting 0.4 miles south of Rotomanu railway station. The rook is a granite consisting of quartz, microcline, piagioclaae, biotite (with a IittIe replacement by chlorite), and museovite, with aocessory apatite. More than 95 per cent pure, chlorite being the impurity. Elevation 300 ft. K-Ar
RESULTS ANI) GEOLOGICAL INTERPRETATIONS
Argon 40 was determined by isotope dilution, using a spike gas containing 90 1”’ cent argon 38. Air contamination was measured by the relative abundance of
Radiogenic argon and strontium diffusion parameters in biotite at low temperatures
71
36. Samples were fused in an induction furnace at a temperature of 1200”(‘., with a small amount of NaOH flux. Fine material was prevented from decrel)itnt,iny out of the crucible by tamping it with Pyrex glass wool. Crucibles were of sinteretl alumina, inside a molybdenum casing, with a molybdenum lid. The furnace was a water-jacketed bell jar. Gas was purified by hot copper oxide and titanium sponge, and the purity of the final argon, particularly in respect to the content of hydrogen, argon
Table 1. Sample number
Mineral and metamorphic fit&es
._ R 3958 M
B B B B B B u
Unmetamorphosed graywacke zone 3957 i Muscovite. Chlorite schist zone 3953 j Biot,ite, with almandine and oligoclase. Schist 3952 1 Biotite, with oligoclase. Schist Biotite, with oligoclase. Gneiss 3956 3951 / Biotite, with oligoclase and almandine 3955 ’ Biotite plates in pegmatite 3954 I Biotite. Paringa river I Granite WV of fault 3950 / Biotite. Rotomanu granite NW of fault
Total Ar40/K40
KCj, _.
’ Argillite.
3.72 8.01
.0112 .0065
6.50 6.57 7.19
.OOOQ .0034 ~0010
6.63 7.81
+oll .0081
.OOOP - WOl
.OOl.i ~j .OOOl
6.12
was monit’ored by a cycloidal-focussing mass spectrometer. The isotopic composition of t,he spiked gas sample was analysed by a Reynolds-type mass spectrometer (REYXOLDS, 1954) and the cycloidal-focussing instrument, both directly coupled to the gas purification train. In analyses with air corrections less than 30 per cent the estimated precision Air corrections above this (reproducibility) was 3 per cent standard deviation. figure cause the precision error to rise, for although the error in 36138 measurement is fixed, it enters as the difference in subtracting the air argon 40. SystemaCc error of the mean is believed to be less than 1 per cent, based on the measurement) of the pure argon, and on int,crlaborator> argon content of air, on spectroscopically agreement in the analysis of a standard sample of biotite (B 3203). The potassium was measured by a Perkin-Elmer flame photometer using lithium internal standard. A single K analysis involves two solutions of separate portions Reproducibility of the sample. each with two separate photometric measurements. in potassium measurements is estimated to be 2 to 3 per cent standard deviation for a complete analysis, with an absolute error of the mean not exceeding 2 per cent. Decay constants for potassium 40 have been adopted as follows: 1,. = 0.585 x 10-10 yr-1; 1 -= 5.30 s IO-10 yr-l (ALDRICH and WETHERILI,, 1B.W). Analytical results are shown in Table 1. All of the samples in the biotite zone, coinciding with t.he greatest uplift close to the fault on the east, side, show ages that are remarkably young for metamorphic rocks and. except for R 39.55. agree with
72
P. 31. HI:RLEP,H.
HVGHES,
W. H. PINSON, JR. and H. w’. FAIRBAIRN
each other within experimental error. The apparent loss of argon can not he ascribed to hydrothermal alterations in fault zones as the biotite would not Iw developed under these condit.ions, and rekograde metamorphism is not prominent in these samples. As there is no intrusive activity of any consequence in the belt. the biotite in the oligoclase zone must have been developed in an environment 01 elevated temperature f5W-:i.Wf’) that was the result of burial at a depth that 1~;~s many t,imes the depth of the sa,mple region during t,he late Tertiary. \Ve mllst conclude therefore that the low a,rgon age values are the result of diffusion loss 01 argon at depths and temperatures much less than those in which the biotite \v:l.s formed. T&k
2. Estimated
geological
Event 1. Paringa R. granite is part of a basement that has bern stable since Pennsvlvanian times ‘. Sedimentation in trough tlruing Permian and Triixssic. Downgarping of geosyncline due to deep 3. Metamorphism burial. Probably ending with uplift in Jurassic times 4. Major uplift in Cretaceous times and erosion to low relief 5. Early Tertiary tag in uplift 6. Now uplift continuing in late Tertiary with erosion keeping pace 7. Rapid fault uplift of block bounded on NW by Alpine Fault
hisiory
11~1 in calculations
of thermal history of aam&
E vldence
..--x-“.“. -- .--.-Age of 286 m.y. on B 3954 indicates
-
.
inconsequential
uplift
since that t.ime
Triassic fossils. Jlinimum agrees w&h this,
age of argillite
It 3958 at, 133 m.y.
Metamorphic grnde reached oligoclaee facies in center of trough, equivalent to about 550°C
tow grade schist pebbles in Cretinous conglomerates. Cessation of argon diffusion loss in M 3957. Cessation of moat of uplift of Rotomanu granite, exposing it and giving pebbles of similar granites in the baaal Eocene coal measures partial sea transgression in Eocene, to complete traznrgresaion in Oligocene with Iimeetone deposition Burial and folding of Oligocene limestone with terrigenous sediments and development of smooth, low-relief erosion surface that is indicated prior to fault uplift Sot requiring more than 5 my. from argon dif%.rsion data, and probably not more than 2 m.y. Therefore in Plei&oeena and latest Pliocene. Pebbles of biotite schist faeiee fir& Been in Pleistocene moraines
Before studying the question of diffusion constants we may utilize the age data where possible to assist in the establishment of the geological history of the region, of interest in the estimates of temperature at depth. In particular B 3964, t,he Paringa River granite with an age value of 286 m.y., and R 3968, the unmetamorphosed argillite with an age value of 133 m.y., are helpful. Using the general geological eonulusions of GRINDLEY, HARRINQTON and WOOD, 1959, and MASON (personal communication), and making elight modifioations in accordance with these age measurements, we have decided on the sequence of events, shown in Table 2, to be used in the temperature caloulatione.
Radiogenic
argon and strontium
diffusion
parameters
in biotitc
at low teml)eraturcs
73
CALCULATION OF THE SECULAR EQUILIBRIUM RATIO [Ar40]/1K4n] IS TERMS 0~ THE DIFFI'sI~?; P_4RAwwEK. D/a2 The number of argon atoms generat,ed per cm3 per WC (n,,) is given by: no == Af[K40]
where [K4”] is the’K40 concentration in atoms/cm3; I is the K40 decay constant = 1.677 x 10-l'set-l; f = -585/5.30 = ratio of APO produced per K40 decay. For times much smaller than this is the case here where we the grain, i.e., the equilibrium Consider a spherical grain sphere of radius r (r < a) with
the half-life of K40 n, can be treated as a const,ant; calculate the average t’ime the argon atoms spend in concentration developed. of radius a and equate the argon produced within a the diffusion loss across the spherical boundary at 1’: noi7rr3
=
-D4rr2!!F
where n is the concentration
of argon atoms at 1‘: giving
on introducing the boundary in the grain is
condition
s
n = 0 at r == n. The total amount of argon
ndV=%
r2)4nr2 dr
(a”-
60 s
2)
4i7noa5 45
where V is the volume of the grain. [Ar40]
W4”l
D
Substituting *585 ___. 5.30
= 1.234
a2
V =ino-z
for no we have
1.677 x 10-l’ --.-~-i5_~~
x
x
lo-l9
a2 .~
a2 --
D
In the samples showing greatest argon loss (3951, 3952, 3953 and 3956) the to an age of 6 m.y., giving a average value of Ar40/K40 is *0003, corresponding minimum value of D/a2 equal to 4*-*) x lo-16. However, the fault movement must have taken place over part of this time, and after initial uplift the temperature in the region of the samples would have decreased and argon diffusion loss ceased. Therefore, if t, is the length of time in m.y. since diffusion loss ceased (i.e., the fault uplift was well initiated),
the equilibrium
value of Ar40/Kd0 is equal to .0003
and the average value of D/a2 pertaining
C
1 -
tf
61
to these samples 3951, 3952, 3953 and
--
_
, ,
0+046 o-0102
0.6092 owO2 0.00153
0~0006 0.0004
Ar40/K40
200 1760
100 500 700 300 2000
l%vation. feet
L-
/
j
8200 7500
QE&o 8060 8000
8000 8500
L
/
!
I
I
I
I
/
I (1) j _---_:_--_ I /
Tablo
9390 7770
9940 9570 9960 9080 10,350
j_
: :
1 j (11) ; .~*
Elevation of original surface, (feet)*
_--_~I___
* Two methods of estimating described in text.
M 3968
M-llWovite M 3957
3956 3962 3953 3961 3955
Biotite
Sample number
!
f .30 f .20 z .20 i ,25 f .35
2.60 f -20 1.80 * .20
2.70 2.60 2*75 2.60 2.20
O-1
+ * f f f
109 i 81 f
113 109 114 109 95
PJ
13 10
15 13 13 14 15
Pre-fault temperature _..__..
-I*-
j
!
--.--..-- -ll.--.---.-
Depth of burial,
3.
I /
I
___-
1.6 2.0 4.0 4.0 0.52
__._ -. .
Minimwn D/a2 ( x 10-lfi) set+
-_-“-_
-L
D/d
-
<;.os
q.17
3.2 4.0 8.0 8.0 0.59
( x lo--16) see-’
Probable Maximum
Hadiogenic
argon and strontium
diffusion
parameters
in biotitt
at low temperatures
7.i
3956 is D/a2
25
=
42
x
lo-
q1
-l,t).
In the case of biotite sample 3955, occurring in the same region, its age value of m.y. would result in a D/a2 relationship on the same basis, as follows: D/a2 = 0.80 x lo-
The other samples of biotite were west of the fault uplift. The two samples of muscovite-bearing rocks, 3958 and 3957, may also be considered. Their greater age values suggest that secular equilibrium between argon loss and gain had not been reached and that the values would have increased if they had remained a greater time at their pre-fault depth. Therefore, the D/a2 values are maxima only if calculated on the same basis. A summary of the values of D/a2 for these different samples is given in Table 3, calculated on the basis of t, = 0 and t, = 3. In the case of t, = 0, allowing zero time for exposure and cooling of the samples, the values of D/a2 are minima. In the case oft, = 3, or cooling of the biotite at 3 m.y. ago in the late Pliocene, the values of D/a2 are probably close to the maximum permissable. These limits are indicated on the points plotted in Fig. 2, at t’he limits of error in the vertical direction. ESTIMATED DEPTH OF BVRIAL The depth of burial of each sample, before uplift and erosion, is calculated by subtracting its present elevation from the estimated height of the upthrust mass immediately after the faulting. This height, it is suggested, can be obtained by interpolating across the ridges left by the recent glaciation. In Fig. 1 the distribution of ridge-top and peak elevations is used t’o provide a contour plan of the surface before uplift. The two-dimensional spatial correlation of these points into a smooth surface is our argument for the nonexistence of any significant thickness of eroded material above the surface. In particular, the uniform elevations along the extensive sets of ridges left by the glaciers would be difficult to explain if they were This geomorphological interrandomly left by erosion from higher elevations. pretation is the key to the conclusions in this investigation and the reader may judge for himself on this point by studying a topographic map of the area. The specific estimate of the original height at each sample location was made in alternative ways. In one, this original late Tertiary erosion surface, shown contoured in Fig. 1, was assumed to be symmetrical about the line of highest elevations parallel to the fault. Thus the original height at each sample location is the height, in Fig. 1, of its mirror image through this line. Another estimate was obtained by ignoring the low relief prior to faulting and finding the plane surface most nearly passing along all the ridges in the upthrust block. A least squares solution over a 75 mile radius showed that the strike of this plane is almost parallel to the fault, the dip to the SE being 1000 ft every 114 miles, By thus assuming that the original elevation of the upthrust block continued
1’. Y.
76
HUIALEY,
H. HUMIES,
W.
H. PINSON,JR. and H. W.
FAIRBAIRS
increasing right up to the fault and beyond the line of the present maximum elevat,ions, an upper limit to t,he depth of burial is obtained. The results are listed in Table 3, the error representing the difference between 2 km, whichever is the greater. these alternative estimates, or
THERMAL
GRADIENT
JUST PRIOR TO FAULT UPLIFT
The thermal gradient in the region may be regarded aa the sum of two terms, one constant and one transient. The “steady state” gradient is due to the almost
ctdspar. Ptwse I eldrpar, PhosaH
07 08
09
IO
It
12 13 14
13 16
17
18 19 2.0 2.1 2.2 23 moo T
24 2.5 25
2.7 2.8 29
30 31 32
33 34
'$(
time invariant loss of the earth’s internal heat. Taking this heat 108s86 (I.2 -& 0.15) x 1O-6 Cal/cm2 880, and taking the thermal conductivity of the material (granites and schists) to be 0.005 to 0.006 Cal/cm set “C (BIRCH, 1942), the resulting temperature gradient is (22 _:- 3-5) “(‘/km. The transient temperature gradient is a consequence of the uplift of the rook, metamorphosed at 5OO’Y!., to shallow depths prior to the final faulting. It may be oeicu18bed for a sudde?L upward dksplecement by expanding the resulting exees8 temperatures above the steady state values in a Fourier series, the amplitudes of the sucaessive terms of which decay exponentially with time, the higher the term
Ratliogenie
argon anti strontirlm
diffusion
parameters
in biotke
at low ietttpertlt,ures
77
the more rapid the decay as determined by the one-dimensional heat conduct,ion equat’ion. On approximating the actual uplift by a series of discrete displacements spaced in accord with the geological history given in Table 3 (and in particular assuming that uplift has continued since the Cretaceous at a de~eler~~tillg rate) the resulting transient temperature gradient was probably 14 & Y(‘/km near t,he surface immediately prior to the final faulting. Thus the most likely value for the total thermal gradient ah that time, is (36 &_ 4)‘Cjkm. With this gradient, and assuming a surface temperature of 15Y!., t.he temperature of each sample prior to fault uplift is obtained and listed in Tabie 3 along wit’h its standard error arising from the uncertainty in the &ermal gradient and the depth of burial. These limits together with those of D/a2 are shown graphically by the boxes in Fig. 2.
CONCLUSION
ON ARGON DIFFUSION PARAMETERS
The diffusion results obtained here are derived from measurements showing almost, complete loss of argon; one can not appeal to the loss of a small adsorbed fraction or of a minor mobile argon “phase” (in notation cf AMIRKHANOV, 1961) to account for the observations. While it is conceivable that diffusion has been facilitated by some aton+c rearrangement this is a detail unaffecting our main conclusion: that these biotites have lost essentially all their radiogenic argon at a temperature of about 100’32, or conceivably on cooling from 100°C. We now show that such diffusion losses are not incompatible with t.he effectively complete retention of argon at surface temperatures, or with the measured diffusion Rather, these additional data may be used to estimate rates at high temperatures. the temperature dependence of the diffusion rate. The diffusion loss of argon from biotite at temperatures found at the earth’s surface has appeared to be finite in some of the oldest cases, but there is some confusi,on resulting from the uncertain decay constant of Rb8’. For example, tests on biotite from the Superior Province in Western Ontario have shown age values slightly less than the age of the area estimated on the basis of lead and strontium These lower argon values are equivalent to at most a 15 per age determinations. cent lowering of age in 2600 n1.y.; even allowing for some strontium or lead diffusion also will not alter the general magnitude of this value. On substituting t,his data into the relation7 given by GOLES, FISH and ANDERS (1960, p. 180), the value of l>/a2 is found to be of the order 1O-2o set-I. This point is plotted in Figure 2, taking an average near surface te~nperature of 20”(1, and fixes t.he rni~linlunl rate for the reduction in diffusivity with deqreasing temperature. The values obtained in the laboratory at high temperatures by AMIRKHAMV et al. (1959) and HART (1960) are replotted in Fig, 2, and serve to determine an upper limit for the change in diffusivity wiih t,emperature. With the usual relation between diffusivity and temperature
where
f * is t.he apparent age (about 2300 m.y.), and t is the true age (about. 2600 m.y.).
P. M. HURLEY, H. HUGHES, W. H. PCNSON, JR.
78
and H. U’. FAIRBAER~V 4
and E = 27 kcalsfmole. Some uncertainty in the value of E arises from possible different values of the effective diffusion radius (a) among the different biotites but this seems unlikely to be comparable to the range of 1016spanned by the observations. A different value of E is possible if a phase change in biotite occurs between the different sets of observations. It must be recognised, however. that diffusion losses from biotites of igneous origin, or from other geologioal environments, may differ substantially from that determined by t,he above parameters beoause of such variations in the diffusion radius. set-l
ThFUSION
Loss
OF
&ADIOCENIC
Srs7
A thorough analysis of the Wb- -Sr age value in a single biotite sample. R 3Q;id, yielded the following : Rb, p.p.m.
Rba7, p.p.m.
Sr? p.p.m.
520.1
147.3
85.5
W/8$6 09710 * .OOl
A recent survey of the SFe’/SP ratio in ooeanio and oontinental basalts in this laboratory leads us to believe that the initial ratio in the biotite, whieh was formed from geosynolinal sediments, could not have been less than 0.707 even if the sediments had been oomposed entirely of volcanic materials. Therefore the initial S$7/Srs6 value in the biotite was between Q-707 and 0.710, and the radiogenic SP7 in the biotite is determined as being between the limits zero and 0.026 p.p.m. The age values equivalent to these limits are zero and 12 m.y. It is clear that the radiogenie S$’ also has been leaving the biotite. As before, considering secular equilibrium between loss and gain of daughter atoms, the total number of radiogenic SP atoms in a grain is
w&exeRbs’ is the number of parent atoms per cm 3, I’ is the volume of the grain, and Am,is the decay constant of Rb*’ which is approximately equal to 47 x lo-16 set--1. The measured ratio of daughter to parent is *SF -- --- < 0.026/147.3. Rb8’ Therefore D/a2 > 1.75 x lo-l6 se+. This minimum value of D/a2 for B 3952 resulted from a calculated environmental temperature of 109 f 13YJ. (Table 3.) As in the case of argon, the Rb-Sr age values on very ancient biotite suggest that the loss of *Sti’ is less than 10 per cent at earth surface temperatures over a time of 2600 “1.9. This sets an upper limit for D/a2 at 20°C of 1W20. * Itadiogcnio
Radiogenic argon and strontium diffusion parameters in biotite
8t
iow
temperatures
79
These values of D/a2 at 109°C and 20°C are close to those for argon. It is therefore concluded tentatively that similar values for E and D,/aa may be applied to *Sr*’ as for argon in biotite in this low temperature region. DISCUSSTON The observations recorded here constitute, we suggest, a close equivalent to a diffusion experiment of several million years duration in contrast to the few hours durat$ion common in laboratory diffusion measurements (GERLING and MOROZOVA, 1958; REYXOLDS, 1957; EVERNDEN, CURTIS, KISTLER and OBRADOVICH, 1960; AMIRKHAXOV, BRANDT, BARTNITSKII, GASANOV end GURVICH, 1959B; AMIRKHASOV, ef al., 1959a and 1961; FECHTIG, GENTNER and Z;~HRINGER, 1960; GEISS and HESS, 1958). The enormous extrapolation of such laboratory data to make them relevant to geological problems is thus avoided. There have been several cases in our own work and in that of others where biotite known to have been buried at substantially greater depths has retained its ancient K-Ar ages. We submit that it is too simple to declare that our depth or temperature estimate is faulty in this case, as these estimates are not insecurely b&sed. This is a highly mobile belt, relative to the other cases, and the micas occur in a metamorphic environment that has been subject to not only changing physical What is clearly observable here may be true in conditions but also to deformation. lesser extent in other mobile belts of metamorphic rocks. Whether the loss of radiogenic argon and strontium is due to diffusion or some other mechanism may not be settled easily, but the knowledge that it can occur in seemingly normal biotite at approximately 100°C is of interest and the purpose of this report. If this effect is commonly the case it will explain some of the observations that otherwise are difficult to account for in geochronology. For example, most investigators, including ourselves, have referred to the biotite K-Ar age values as commonly representing the time of la& metamorphism or heating. Instead, a continuous uplift of a region would yield a fairly uniform age value representing the t,ime since the last major erosion, without any need for an erogenic or metamorphic event at this time. Thus the difficulty of repeated deep folding of regions of crystalline rocks to account for observed age patterns could be explained simply by variations in time of uplift and depth of erosion. Geological history has been recorded in terms of episodes of sedimentation because of paleontological correlation. Owing to the difficulty of dating sediments the geochronologists have been making a plea for a swing toward erogenic and volcanic episodes as a frame of reference for the extension of earth history into the If the above conclusions are true, the ages commonly measured by Pre-Cambrian. K-Ar and Rb-Sr on biotite may actually reflect the time of major uplift and erosion, which coincides with that of sediment,ation, and not the initial period of metamorphism and igneous intrusion in the erogenic belt. For example, in New Zealand the biotite age values post-dated the time of metamorphism by more than 100 m.y. Thus the geochronologists may be actually dating episodes that are more nearly comparable to times of sedimentation than to metamorphism, and the frame of reference provided by each method of dating may be more nearly matching thsn expectred.
Ark,co,cZedgntents-TThe authors gratefully acknowledge the ussistancc of H. \\-. K;R\‘Is(;EK /it some of the analyses, and tho critica.1 review and interest of S. li. HART a-ho ~LISO IJ~o\‘I~II~~z translations of the Russian lit,eratuw. \Vc are indt~btod to 13~1~s ,\laso~ not onI>, fiw Jlib IW+ vision of the samlllt:s bnt also for I,ackgromltl information and litwuturc on the gvolop~~ of’ SI,,~ Zealand. The support of t,hr I)ivision of Researcli of the I7.S. Atomiv Energy t ‘otnr~~k~~~~! II~.-. made the investigation possible. REFI~REXCJ~S ALDRICH L. T. and ~VETHEKILL (:. VI'.(1958) C;eochronology by radioactive decay. dwt. flec.. Nucl. sci. 8, 257-2998. AMIRKHANOV K. I.,BRANDT S. B. and I~ARTNITSKII E. S. (1959a) Diffusion of radiogenw argon in feldspars. Uo,C1. &ad. Sci. lis,)‘iZ, 125 So. 6, Cieochem. Ser. AMIRKHANOV K. l., BHANDT, 8. B., B~RTWIT~KIIE. s., GASANOVs. -4. and C:I.HVI(‘H \‘. s. (1959b) On t)he mechanism of losses of radiogenic argon in micas. il~nrl. $5’~;. C;%S’R no. 3. 1’. 104. AWIRKHANOV K. I.,UHANDT S. H. and BARTNITSICIIE. S. (1961) HadiogenicargoninInitwrals and its migration. (;eochronology of Rock Systems, (.J. L. KTTLP,ed.), Ann. S. Y. dcrr
glauconite, microcline, sanidine, loucite and phlogopite. Allaer. .7. Sci., 268, 583-604. FECHTIGH., OENTNERW. and ZKKRINGEII.J. (1960) Argonbestiminungen an Kaliummineralien, VII. Diffusionsverlurte von Argon in Mineralien and ihre Auswirkung auf die Kaliurn-ArgonAltersbestinqnung-. Geochim. et Cotwnochim. Actu 19, 70-79. CEISS J. and HESS D. C. (1958) Astrophys. J. 12’7, p. 224. GERLING E:K. and MOROZOVAJ. M. (1958) The kinetics of argon ixolat,ion from microclincperthite. Geokhimiia 7, 175-781. GOLES C. Q., J~FIBHR. A. and ANDERS E. (1980) The record in -the meteorites, I: The formel environmenf of stone meteorites as deduced from Ka”-Ar40 ages. Geochim. et Cownochinc. Acta 19, !77-195.
GRINDLEY G. W., HARRINGTONH. J. and Woou 13. L. (1959) The geological map of Stl\r Zealand. N.Z. Geol. Survey Bull. 66, p. 111. HART S/R. (1960) Mineral uges and mdamorphisnl. Natrs. Inst. Tech. Ph. D. Thesis, L’19‘1,1>. REYNOLDSJ. H. (1954) A high-sensitivity mass spectrometer. Phys. Rev. 98, p. 283. EEYNOWS feldspar.
J. H. (1957) Comparative study of argon Geochim. et Co.wnockim. Acta 12, 177-184.
content
and argon
diffusion
in mica
and
.
TURNER %. J. (1958) Progressive regiona metamorphism in southern Sew Zealand. LXX\‘, No. 886, 160-174.
Geol. Mug.