Preliminary 40Ar/39Ar geochronology of igneous 

intrusions from Uvalde County, Texas: Defining a 

more precise eruption history for the southern 

Balcones Volcanic Province 

By Daniel P. Miggins, Charles D. Blome, and David V. Smith 

This report is preliminary and has not been edited or reviewed for conformity with U.S. 


Geological Survey editorial standards or with the North American Stratigraphic code. 


Any use of trade, product or firm names is for descriptive purposes only and does not 


imply endorsement by the U.S. Government. 


U.S. Geological Survey Open-File Report 2004-1031

U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY 
Table of Contents 

Introduction ........................................................................................................................................................ 1
 

Figure 1. Showing location of igneous intrusions in Uvalde County ................................................................ 1
 

Figure 2a Aeromagnetic map ............................................................................................................................. 2
 

Figure 2b Geologic map showing inferred igneous outcrops and


 subcrops set against a regional geologic map for the Uvlade intrusions .......................................... 2
 

Methodology ...................................................................................................................................................... 3
 

Results ............................................................................................................................................................... 4
 

Table 1. 40Ar/39Ar data for Uvalde rocks .......................................................................................................... 4
 

Figure 3 Photograph and age spectrum for sample 01-CB-001A ...................................................................... 5
 

Figure 4 Photograph and age spectrum for sample 01-CB-002 ........................................................................ 6
 

Figure 5 Photograph and age spectrum for sample 01-CB-005 ........................................................................ 7
 

Figure 6 Photograph and age spectrum for sample 01-CB-006 ........................................................................ 8
 

Figure 7 Photograph and age spectrum for sample 01-CB-007 ........................................................................ 9
 

Figure 8 Photograph and age spectrum for sample 01-CB-008 ........................................................................ 10
 

Figure 9 Photograph and age spectrum for sample 01-CB-009 ........................................................................ 11
 

Figure 10 Age spectrum for sample 01-CB-010 .............................................................................................. 12
 

Conclusions ....................................................................................................................................................... 12
 

Future work ....................................................................................................................................................... 13
 

Acknowledgments ............................................................................................................................................. 13
 

References cited ................................................................................................................................................ 13
 

Appendix A 40Ar/39Ar age spectrum, K/Ca, and %Radiogenic yields for each sample ................................... 15
 

Appendix B 40Ar/39Ar incremental heating data for each sample .................................................................... 27
 
Introduction 

Volcanic and subvolcanic intrusive rocks in south-central Texas have been known since the earliest 

geological surveys of Texas (Lonsdale, 1927 and Sayre,1936). In addition to their geologic setting, the rocks 

provide a unique economic resource for the state of Texas (road aggregate and natural gas). The first subsurface 

occurrences of these Cretaceous rocks were reported in the 1920Õs, and by the 1950Õs numerous gas fields were 

discovered in Zavala and Dimmit counties. 

The Uvalde igneous field consists of fine to coarse-grained ultramafic and hypabyssal rocks that occur 

as dikes, plugs, and shallow intrusions. Olivine, clinopyroxene, and feldspathoids constitute 25-60% of the 

exposed rocks. There are five rock types within the Uvalde igneous field: alkali basalt, melilite-olivine nephelinite, 

olivine nephelinite, nepheline basanite, and phonolite. The field is centered in Uvalde County and extends to the 

west into Kinney County (Figure 1) and to the east as far as Bexar County. Isolated dikes and plugs also occur 

240 km to the northeast in Hill County near Austin, Texas. 

Knippa Quarry 

TEXAS 

Austin 

Balcones 

Province 

Uvalde 

Sabinal 

0 15 

km 

29o30Õ 

90 

29o15Õ 

90 

Cline 

2 

1 

3 

4 

5 

6 

7 

8 

Cretaceous rocks 

Faults County line 

Kinney County Uvalde County

Tertiary sediments 

100o15Õ 100o

00Õ 99o

45Õ 99o

30Õ 

Melilite-olivine nephelinite Olivine Nephelinite

Alkali basalt Nepheline basanite 

Phonolite Unclassified intrusions 

1 01-CB-001A 

2 01-CB-002 

3 01-CB-005 

4 01-CB-006 

5 01-CB-007 

6 01-CB-008 

7 01-CB-009 

8 01-CB-010 

Figure 1. The Uvalde area of the Balcones magmatic province modified from Wittke and Lawrence,

 1993 (after Barnes, 1974, 1977). Note. Sample 01-CB-010 is off the map approximately 5

 km south of Sabinal. 

1 
The intrusive rocks in Uvalde County are economically important; formed natural gas traps by intense 

folding and faulting as they intruded surrounding Late Cretaceous sedimentary rocks (Figure 1). The igneous 

rocks also act as barriers to groundwater flow. The vesicles in the rocks are by filling with secondary minerals, 

thereby decreasing permeability of the igneous rocks. The Uvalde igneous field may also influence the major 

Edwards aquifer flow paths where there is a markedly change in direction just east of the Uvalde intrusions 

(Maclay, 1995). 

A high-resolution aeromagnetic survey flown in 2001 (Smith and others, 2002) detected over 200 shallow 

igneous intrusive bodies, where fewer than 30 have surface expression. The aeromagnetic map, shown in Figure 

2a, reveals numerous small bump-like magnetic anomalies in the southwest quadrant. These anomalies are set 

against broader regional magnetic highs (red) and lows (blue). Igneous rocks contain much more magnetite than

limestone, thus the contrast in magnetic susceptibilities causes distinct distortions in the earthÕs magnetic field 

at the surface. These anomalies effectively distinguish the intrusive rocks from the surrounding country rock. 

Using matched-filtering techniques on the aeromagnetic data, the shallow (0 Ð 500 m) anomalies were separated 

from those due to moderately deep plutons and crystalline basement. A map of inferred igneous outcrops and 

subcrops, set against a regional geologic map, is shown in Figure 2b. 

2500 0

METERS 

2500 5000 7500 A

28 0 14Õ40Ó 28 0 22Õ30Ó 28 0 30Õ20Ó 28 0 38Õ10Ó 

990 100000Õ23Ó 52Õ30Ó 990 990

44Õ40Ó 36Õ50Ó 990 990

29Õ00Ó 21Õ10Ó 990 990

13Õ20Ó 05Õ30Ó 980 98 49Õ50Ó 0

57Õ40Ó 

990 1000

00Õ23Ó 52Õ30Ó 990 990

44Õ40Ó 36Õ50Ó 990 99 21Õ10Ó 029Õ00Ó 990 99 05Õ30Ó 0

13Õ20Ó 980 980

57Õ40Ó 49Õ50Ó

28 0 14Õ40Ó 28 0 22Õ30Ó 28 0 30Õ20Ó 28 038Õ10Ó 

98041Õ30Ó 

98041Õ30Ó 

-N­ High Resolution Aeromagnetic Survey, 2001 

Medina and Uvlade Counties, Texas Total Magnetic Intensity, Reduced to Pole 

2500 0 2500 5000 7500 

METERS B
 Igneous Intrusive Rocks 

Outcrops and Subcrops Shown in Red 

Medina and Uvlade Counties, Texas -NFigure 2. Aeromagnetic map (2a) and map of inferred igneous outcrops and subcrops, set against a regional

 geologic map (2b). 

2 
The apparent random distribution of these igneous bodies, and the sparcity of associated dikes or dikelike structures, raises the question of their origins in time, as to whether they represent a single intrusive episode 

or several. This question can best be answered by isotopic, geochemical, and geochronological analysis. 

This report contains preliminary 40Ar/39Ar geochronology of various mineral separates, as well as groundmass 

concentrates, from a variety of rock types exposed within Uvalde County. Previous K/Ar age constraints reported 

by Baldwin and Adams (1971) indicate that igneous activity spanned a range in age from 90 to 60 Ma. Due to 

the inherent problems associated with the K-Ar dating technique, especially on basaltic rocks, the overall goal 

of this work is to define a more precise emplacement history of these igneous rocks. The 40Ar/39Ar ages obtained 

for the Uvalde rocks yield two distinct groups of ages, one at approximately 82-20 Ma and the other at 74-72 

Ma. 

Methodology 

Mineral separates were analyzed using the 40Ar/39Ar age spectrum technique, a variant of the conventional 

K/Ar method. A mineral separate of unknown age and a mineral standard of known age (Fish Canyon Tuff [FCT] 

sanidine with an age of 28.03 Ma) were irradiated at the USGS TRIGA reactor in Denver, Colorado, following 

techniques described by Dalrymple et al., (1981) to produce 39Ar from 39K by neutron bombardment. After 

irradiation, the 40Ar radiogenic/39Ar potassium ratios of the sample and standard were determined. Standard 

techniques were employed to produce 40Ar/39Ar spectra as described by Snee (1982), Snee et al., (1988), and 

Snee, (2002). 

The isotopic composition of argon was measured at the USGS in Denver, Colorado using a MAP 215 

series rare-gas mass spectrometer made by Mass Analyzer Products Limited. Abundances of five argon isotopes 

(40Ar, 39Ar, 38Ar, 37Ar, and 36Ar) were measured in each sample. Argon was released from the samples in 7 to 

11 temperature steps. Radiogenic 40Ar (40ArR) is the total 40Ar derived from natural radioactive decay of 40K 

after all corrections from non-decay-derived 40Ar, including atmospheric 40Ar and reactor-produced 40Ar, have 

been made. Potassium-derived 39Ar (39ArK) is the total 39Ar derived from the epithermal neutron-induced 39K(n, 

p) 39Ar after corrections for non-39K-derived 39Ar, including 42Ca-derived 39Ar, are made. F is the quantity 

resulting from the division of radiogenic 40Ar and K-derived 39Ar. Quantities for radiogenic 40Ar and K-derived 

39Ar are given in volts of signal measured on a Faraday detector by a digital voltmeter. These quantities can be 

converted to moles using the mass spectrometer sensitivity at time of measurement of 9.736x10-13 moles of argon 

per volt of signal. The measured 40Ar/39Ar ratio used for mass descrimination correction is 298.9. 

Samples were irradiated in two irradiation packages (DD78, DD79) for times ranging from 20 to 40 hours 

at 1 megawatt in the USGS TRIGA reactor in Denver. The J-value for each sample was determined from adjacent 

standards; errors in the calculated J-value were determined experimentally by calculating the reproducibility of 

multiple monitors. Corrections for the irradiation-produced, interfering isotopes of argon were made by measuring 

the production ratios for the interfering isotopes of argon produced in pure salts of K2SO4 and CaF2 irradiated 

simultaneously with the samples of this study. 

Corrections were made for additional interfering isotopes of argon produced from irradiation of chlorine 

using the method described by Roddick (1983). Measured quantities of 37Ar and 39Ar were corrected for radioactive 

decay, and the 39Ar/37Ar ratios were corrected for this decay, as well as for interfering argon isotopes. By 

multiplying the 39Ar/37Ar ratios by 0.5, the relative approximate K/Ca distribution of the samples may be obtained.

 Error estimates for apparent ages of individual temperature steps were assigned by using the equations of Dalrymple 

et al., (1981). However, the equations were modified to allow the option of choosing the larger of separately 

derived errors in the F-value-either a calculated error or an experimental error determined from the reproducibility 

of identical samples. Age plateaus were determined by comparing contiguous gas fractions using the critical value 

test of Dalrymple and Lanphere (1969), and McIntyre (1963), and the error was determined using the equations 

of Dalrymple et al., (1981). Isochron analysis (York, 1969) for those samples that did not plateau was used to 

assess if nonatmospheric argon components were trapped in any samples and in some cases to define an apparent 

age of the sample. 

Sample Preparation 

Mineral separates of potassium feldspar (K-feldspar) and plagioclase, as well as groundmass concentrates 

were used in this study. High-purity mineral separates (>99%) of K-feldspar, as well as basalt groundmass 

concentrates (>95-98% purity), were obtained using standard separation techniques. Mineral concentrates were 

selected by choosing the largest size-range free of composite grains and were purified using magnetic and heavyliquid techniques. Final mineral separates were hand picked under a binocular microscope to a purity of >99%, 

with particular attention being paid to excluding grains with abundant inclusions, adhering material, carbonate, 

or alteration. All mineral separates and groundmass concentrations were treated in a dilute bath of HCl (<10%) 

for 15-30 minutes to dissolve any primary and secondary calcite and zeolites. K-feldspar separates were treated 

in a dilute bath of HF (~15%) for approximately 5-15 minutes, depending on the purity of the crystals, to remove 

glass, zeolites and other material adhering to the crystals. 

3 
All groundmass concentrates range in size from 46 to 100 mesh (180-150 mm). Visible phenocrysts of 

olivine, pyroxene, and feldspathoids were removed from the concentrate using a magnetic separator. Both mineral 

and groundmass concentrates were washed in acetone, alcohol, and deionized water (2X) to dissolve any organic 

residue and remove dust. 

Results 

Eight samples were collected and 11 40Ar/39Ar ages were obtained from K-feldspar, plagioclase and 

groundmass concentrates. Table 1 lists the interpreted ages for each sample, latitude and longitude of the collection 

site, and their location relative to the town of Uvalde. Replicate analyses for samples 01-CB-002 and 01-CB-005 

were done to check sample reproducibility. Plagioclase and a groundmass concentrate were analyzed for sample 

01-CB-008 to test the reproducibility of both mineral phases from the same rock. All age spectra and the justification 

for interpreted ages are described below. 

A groundmass concentrate for sample 01-CB-001A (Figures 1 and 3) comes from an outcrop along the 

banks of the Frio River. The age spectrum for this sample is disturbed exhibiting much older apparent ages in 

the middle temperature-release steps. The age spectrum shows that the argon system of this sample has been 

severely affected over geologic time. No interpretable age can be deduced. 

Table 1. Summary of sample data along with location and ages for the intrusions from Uvalde County, Texas 

Sample # on Location Min/GC Latitude Longitude Type of date Age (Ma) Initial 

Fig 1 Apparent 40 Ar/36 Ar 

01-CB-001A 1 Intrusion along Frio River GC 29 o

14'10" 99 o40'58" NA 

01-CB-002 2 Knippa quarry - Run #1 

(82-100 mesh) 

GC 29 o

17'12" 99 o39'25" Total-gas age 

Isochron age (steps 1-7) 

80.9 ± 0.6 

76 ± 7 241 ± 39

 Run #2 GC Average age (steps 4-6) 80.48 ± 0.29 

(46-60 mesh) Isochron age (steps 1-8) 82.7 ± 2 287 ± 16 

01-CB-005 3 Intrusion on highway 90 

east of Uvalde - Run #1 

K-feldspar 29 o

15.338' 99 o43.842' Average age (steps 5-6) 

Isochron age (steps 2-6) 

72.41 ± 0.15 

71.5 ± 1.1 423 ± 57

 Run #2 K-feldspar Average age (steps 4-7) 

Isochron age (steps 1-7) 

72.60 ± 0.26 

74.1 ± 1.0 268 ± 8 

01-CB-006 4 Intrusion at Fort Inge 

south of Uvalde 

K-feldspar 29 o

10.841' 99 o45.936' Average age (steps 9-11) 

Isochron age (steps 1-11) 

73.39 ± 0.13 

74.6 ± 1.2 336 ± 226 

01-CB-007 5 Intrusion southest of GC 29 o

15.576' 99 o34.723' Maximum age (step 7) 79.2 ± 0.6 

Uvalde Isochron age (steps 1-9) 105 ± 27 287 ± 67 

01-CB-008 6 Intrusion north of Cline Plagioclase 29 o

17.813' 100 o04.884' Average age (steps 3-7) 

Isochron age (steps 2-8) 

80.92 ± 0.16 

80.36 ± 0.91 300 ± 33 

GC Minimum age (step 5) 82.79 ± 0.13 

Isochron age (steps 3-6, 8) 82.68 ± 0.24 252 ± 7 

01-CB-009 7 Intrusion on highway 90 GC 29 o

12.869' 99 o55.911' Weight average 78.9 ± 1.4 

west of Uvalde (steps 2-7) 

Isochron age (steps 1-8) 74.0 ± 7 188 ± 7 

01-CB-010 8 Intrusion south of Sabinal GC 29 o

11'38" 99 o25'12" Maximum age (Step # 3) 78.52 ± 0.12 

at Pilot Knob Hill Isochron age (steps 1-9) 80.0 ± 7 281 ± 23 

G.C.- Groundmass Concentrate 

MIN - Mineral 

NA - No Age 

4 
01-CB-001A 


Intrusion exposed along Frio River 

25 

75 

125 

Unrealistic age 

35 

45 

95 

55 

85 

65 

115 

105 

40Ar loss 

39Ar recoil 

Apparent Age (Ma) 

110 Ma 

0 20 40 60 80 100 

Percent 39ArK Released
 

Figure 3. Age spectrum and photograph for sample 01-CB-001A. 

5 
Sample 01-CB-002, a melilite-olivine nephelinite, was collected from Knippa Quarry. This quarry is 

currently being mined for road aggregate (Figures 1 and 4). Two fractions of groundmass concentrate were 

prepared. The first sample was sieved to a final mesh size of 82-100 mesh. The age spectrum for this finer-grained 

fraction shows the effects of 39Ar recoil. Assuming that the recoiled 39Ar was distributed within the sample and 

not lost from the sample, and assuming the sample had not lost radiogenic 40Ar over time, the total-gas age for 

this sample would be the best estimate of its emplacement age. The total-gas age is 80.9±0.6 Ma. An isochron 

for this sample gave an age of 76±7 Ma. The large error reflects the relatively poor fit of the data to a single 2-

component mixing line; thus the total-gas age is likely disturbed. The argon analysis for the 46-60 mesh size 

fraction yielded a less discordant age spectrum, but likely exhibits the effects of excess argon. The age given for 

this second sample is 80.48±0.29 Ma and an isochron age that is slightly older, although within error, at 82±2 

Ma. The smaller error of the isochron age provides more confidence in the geologic meaning of the age. 

01-CB-002 

Knippa quarry 

20 

40 

60 

80 

100 

39Ar recoil 

Unrealistic age 

95 Ma 

30 

50 

70 

90 

01-CB-002 

82-100 mesh 

Groundmass Concentrate 

01-CB-002 

46-60 mesh 

46-60 mesh 

82-100 mesh ± 0.6 Ma 

± 7 Ma 

± 0.29 Ma 

± 2 Ma 

01-CB-002 

Intrusion at Knippa quarry showing columar jointing 

Apparent Age (Ma) 

Total-gas age 80.9 

Isochron Age (steps 1-7) 76 

Average Age (steps 4-6) 80.48 

Isochron Age (steps 1-8) 82.7 

0 20 40 60 80 100 

Percent 39ArK Released 

Figure 4. Age spectrum and photographs of sample 01-CB-002 at Knippa Quarry. 

6 
Sample 01-CB-005 is from a phonolite that is poorly exposed along highway 90 west of Uvalde (Figures 

1 and 5). Two fractions of K-feldspar from sample 01-CB-005 were analyzed. Both age spectra are disturbed, 

and the disturbance may be a result of excess argon or 39Ar recoil. Both K-feldspar fractions yielded average ages 

of 72.41±0.15 Ma and 72.60±0.26 Ma with corresponding isochron ages of 71.5±1.1 Ma and 74.1±1.0 Ma, 

respectively. Another phonolite rock (01-CB-006) was collected at Fort Inge, south of Uvalde (Figures 1 and 6). 

The K-feldspar separated from this sample yielded an average age of 73.39±0.13 Ma. An isochron for this sample 

yielded an age of 74.6±1.2 Ma. This sample also contained excess argon in the lower temperature release steps, 

but not as significant a component as in sample 01-CB-005. 

01-CB-005 

Intrusion on highway 90 east of Uvalde 

20 

30 

40 

50 

60 

70 

80 

90 

100 

39Ar recoil? 

Run #2 

Run #1 ± 0.15 Ma 

± 1.1 Ma 

± 0.26 Ma 

± 1.0 Ma 

Run #2 

Run #1

Apparent Age (Ma) 

Two fractions of K-feldspar 

Average Age (steps 5-6) 72.41 

Isochron Age (steps 2-6) 71.5 

Average Age (steps 4-7) 72.60 

Isochron Age (steps 1-7) 74.1 

0 20 40 60 80 100 

Percent 39ArK Released 

Figure 5. Age spectrum and photograph of sample 01-CB-005 on highway 90 east of Uvalde. 

7 
01-CB-006
 

Fort Inge intrusion 

20 

30 

40 

50 

60 

70 

80 

90 

100 

Excess 

K-feldspar 

± 0.13 Ma 

± 1.2 Ma 

0 

Apparent Age (Ma) 

Argon 

Average Age (steps 9-11) 73.39 

Isochron Age (steps 1-11) 74.6 

20 40 60 80 100 

Percent 39ArK Released 

Figure 6. Age spectrum and photograph of sample 01-CB-006 south of Uvalde. 

8 
Sample 01-CB-007 was collected from a gentle sloping mound mostly covered by vegetation southeast
 

of Uvalde (Figures 1 and 7). A groundmass concentrate yielded a very discordant age spectrum. At heating step
 

#3, the age is 150 Ma (Figure 7). This high apparent age is likely the result of the incorporation of large amounts
 

of excess argon in the sample. However, step #7 is the lowest point in the saddle and gives an age of 79.17±0.56
 

Ma, the maximum possible age for this sample. The isochron for this sample gives an age of approximately 105
 

Ma, again an unrealistic age for this intrusion.
 

01-CB-007 

Intrusion southeast of Uvalde 

groundmass concentrate 

0 

50 

100 

150 

200 

250 

Unrealistic 

age 

150 Ma? 

Maximum possible age (step #7) 

79.2±0.6 Ma 

± 27 Ma 

25 

75 

125 

175 

225 

0 20 40 60 80 100 

Apparent Age (Ma) 

excess argon 

Isochron Age (steps 1-9) 105 

Percent 39ArK Released 

Figure 7. Age spectrum and photograph of sample 01-CB-007 southest of Uvalde. 

9 
A basalt (01-CB-008) from an area north of Cline (west of Uvalde) is the westernmost rock analyzed 

(Figures 1 and 8). For this sample a concentrate of plagioclase and a groundmass concentrate were prepared. An 

average age of 80.92±0.16 Ma for the plagioclase and 82.79±0.13 Ma for heating step #5 for the groundmass 

concentrate are given for these samples. Their associated isochron ages are 80.36±0.91 Ma and 82.68±0.24 Ma, 

respectively. Both the plagioclase and the groundmass concentrate showed significant 40Ar loss in the lower 

temperature heating steps. 

01-CB-008 

Intrusion north of Cline 

Plagioclase 

30 

40 

50 

60 

70 

80 

90 

100 

40Ar loss 

Groundmass 

Concentrate 

(GC) 

± 0.16 Ma 

± 0.91 Ma 

± 0.13 Ma 

± 0.24 Ma 

Plagioclase 

GC 

Apparent Age (Ma) 

Average Age (steps 3-7) 80.92 

Isochron Age (steps 2-8) 80.36 

Minimum Age (steps #5) 82.79 

Isochron Age (steps 3-6, 8) 82.68 

0 20 40 60 80 100 

Percent 39ArK Released 

Figure 8. Age spectrum and photograph of sample 01-CB-008 north of Cline. 

10 
Sample 01-CB-009 was collected off of highway 90 west of Uvalde southeast of sample 01-CB-008. 

Again, like most of the groundmass concentrates in this study, a groundmass concentrate for this sample yielded 

abundant 39Ar recoil (Figures 1 and 9). The age spectrum is very discordant, but a weight average of steps 2 to 

7 gives an age of 78.2±1.4 Ma. This weight average age is within error of the isochron age of 74.0±7 Ma. 

01-CB-009 

Intrusion along highway 90 west of Uvalde 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

± 1.4 Ma 

Groundmass 

Concentrate 

39Ar recoil 

± 7 Ma 

40Ar loss

Apparent Age (Ma) 

Weight avarage age 78.9 

Isochron Age (steps 1-8) 74 

0 20 40 60 80 100 

Percent 39ArK Released 

Figure 9. Age spectrum and photograph of sample 01-CB-09 on highway 90 west of Uvalde. 

11 
Sample 01-CB-010 was collected from Pilot Knob Hill, located south of Sabinal (Figures 1 and 10). A 

groundmass concentrate for this sample yielded an age spectrum that may reflect either excess argon or 39Ar 

recoil in the lower temperature steps. However, 64.9% of the gas was released in the 1400oC heating step and 

most likely represents the closest possible age for this intrusion at 78.52±0.12 Ma. The isochron age for this 

sample is 80±8 Ma. 

120 

100 

80 

60 

40 

20 

0 

Excess 

Unrealistic age 

± 0.12 Ma 

± 7 Ma GC 

Groundmass 

Concentrate 

Apparent Age (Ma)
 

argon 

110 Ma 

Maximum Age (step #3) 78.52 

Isochron Age (steps 1-9) 80.0 

0 20 40 60 80 100 

Percent 39ArK Released 

Figure 10. Age spectrum for sample 01-CB-010 on highway 90 west of Uvalde. 

Conclusions 

Step-heating on minerals and groundmass concentrates for these rocks all yielded discordant age spectra 

and as a result, determining emplacement ages is difficult at best. However, confidence in the interpretation of 

complex age spectra from groundmass concentrates and mineral separates has been documented for igneous rocks 

associated with the northern Rio Grande Rift (Miggins, 2002 and Miggins et al., 2002). Moreover, there is a high 

degree of confidence in the ages for the K-feldspar samples from 01-CB-005 and 01-CB-006, as well as for the 

plagioclase and groundmass concentrate from sample 01-CB-008. By comparing the other samples to samples 

01-CB-005, 01-CB-006, and 01-CB-008 with a high degree of confidence in their crystallization age, two distinct 

emplacement ages have emerged from the 40Ar/39Ar data. 

The age spectra indicate that there are two distinct phases of magmatic activity in the southwestern Balcones 

igneous zone. The first phase of intrusions was emplaced approximately 82-80 m.y. ago. The younger phase of 

intrusions, the phonolites, was emplaced approximately 74-72 m.y. ago. 

12 
Future Work 

Future studies for the Uvalde rocks will include the dating of nepheline. Alunite from hydrothermal 

mineralization associated with the intrusions will be analyzed to determine if the formation of alunite occurred 

during or after the emplacement of the intrusions. Also, samples will be collected from the southernmost, 

westernmost, and northernmost exposures to determine if there is a geographic spread in the ages of the intrusions 

based on geochronological, geochemical, and isotopic data. 

Acknowledgments 

The authors wish to thank Ross Yeoman and Libby Prueher for their assistance in analyzing the samples 

and Jason Faith for helping to collect samples for this study. Reviews by Mel Kuntz and Larry Snee greatly 

improved the clarity of this report. 

References Cited 

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of Texas, The Texas Journal of Science, v. 22, no. 2&3, pp 223-231. 

Barnes, V.E., 1974, San Antonio Sheet, Geologic Atlas of Texas: Bureau of Econonic Geology, University of 

Texas, scale 1: 250,000. 

Barnes, V.E., 1977, Del Rio Sheet, Geologic Atlas of Texas: Bureau of Econonic Geology, University of Texas, 

scale 1: 250,000. 

Dalrymple, G.B., and Lanphere, M.A., 1969, Potassium-argon dating: Principles, techniques, and applications 

to geochronology: W.H. Freeman and Company, San Francisco, 258 pp. 

Dalrymple, G.B., Alexander, E.C., Lanphere, M.A., and Kraker, G.P., 1981, Irradiation of samples for 40Ar/39Ar 

dating using the Geological Survey TRIGA reactor: U.S. Geological Survey Professional Paper 1176, 55 

p. 

Lonsdale, J.T., 1927, Igneous rocks of the Balcones fault region of Texas: University of Texas Bulletin 2744, 178 

p. 

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McIntyre, D.B., 1963, Precision and resolution in geochronometry: in Albritton, C.C., ed., The fabric of geology, 

Addison-Wesley, Reading, Massachusetts, p. 112-134. 

Miggins, D.P., 2002, Chronologic, geochemical, and isotopic framework of igneous rocks within the Raton Basin 

and adjacent Rio Grande Rift, south-central Colorado and northern New Mexico, Univ. of Colorado, 

Boulder, Colorado, 417p. 

Miggins, D.P., Thompson, R.A., Pillmore, C.L., Snee, L.W., and Stern, C.R., 2002, Extension and uplift of the 

northern Rio Grande Rift: Evidence from 40Ar/39Ar geochronology from the Sangre de Cristo Mountains, 

south-central Colorado and northern New Mexico, in Menzies, M.A., Klemperer, S.L., Ebinger, C.J., and 

Baker, J., eds., Volcanic Rifted Margins: Boulder Colorado, Geological Society of America Special Paper 

362, p. 47-64. 

Roddick, J.C., 1983, Generalized numerical error analysis with applications to geochronology and thermodynamics: 

Geochimica et Cosmochimica Acta, v. 51, p. 2129-2135. 

Sayre, A.N., 1936, Geology and groundwater resources of Uvalde and Medina Counties, Texas: U.S. Geological 

Survey Water-Supply Paper 678, 146 p. 

13 
Smith, D.V., Smith, B.D., Hill, P.L., 2002, Aeromagnetic survey of Medina and Uvalde Counties, Texas: OpenFile Report 02-49, U.S. Geological Survey. 

Snee, L.W.,1982, Emplacement and cooling of the Pioneer Batholith, south-western Montana: Columbus, Ohio, 

The Ohio State University Ph.D. dissertation, 320 p. 

Snee, L.W., 2002, Argon thermochronology of mineral depositsÑa review of analytical methods, formulations, 

and selected applications: U.S. Geological Survey Bulletin 2194, 44 p. 

Snee, L.W., Sutter, J.F., and Kelly, W.C., 1988, Thermochronology of economic mineral deposits: Dating the 

stages of mineralization at Panasqueira, Portugal, by high-precision 40Ar/39Ar age-spectrum techniques 

on muscovite: Economic Geology, v. 83, 335-354. 

Wittke, J.H., and Lawrence, L.E.,1993, OIB-like mantle source for continental alkaline rocks of the Balcones 

Province, Texas: trace-element and isotopic evidence, The Journal of Geology, v. 101, pp. 333-344. 

York, D., 1969, Least squares fitting of a straight line with correlated errors: Earth and Planetary Science Letters, 

v. 5, 320-344 p. 

14 
Appendix A: 40Ar/39Ar age spectrum, K/Ca and % Radiogenic yields

 for each sample. 

15 
01-CB-001A/#12/DD78
 

groundmass concentrate 

25 

75 

125 K/Ca %Rad 0.12 

0.10 

0.08 

0.06 

0.04 

0.02 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

35 

45 

95 

55 

85 

65 

115 

105 

Apparent Age (Ma) 

0 20 40 60 80 100 

Percent 39ArK Released 

16 
01-CB-002/#11/DD78
 

groundmass concentrate 

K/Ca %Rad 100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

20 

40 

60 

80 

100 

1.0 

0.8 

0.6 

1.2 

0.4 

0.2 

1.4 

30 

50 

70 

90 

Apparent Age (Ma) 

0 20 40 60 80 100

Percent 39ArK Released 

17 
0.4 

01-CB-002/#8/DD79
 

groundmass concentrate 

%Rad
 100
 

90
 

80
 

70
 

60
 

50
 

40
 

30
 

20
 

10
 

0
 

2.4

K/Ca
 2.0 

1.6 

1.2 

1.8 

100 

90 

80 

70 

60 

50 

40 

Apparent Age (Ma)
 

0 20 40 60 80 100

Percent 39ArK Released 

18 
01-CB-005/#1/DD79
 

K-Feldspar 

K/Ca %Rad 

20 

30 

40 

50 

60 

70 

80 

90 

100 

28 

24 

16 

12 

8 

20 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

Apparent Age (Ma) 

0 20 40 60 80 100

Percent 39ArK Released 

19 
01-CB-005/#3/DD78
 

K-Feldspar 

K/Ca %Rad 

20 

30 

40 

50 

60 

70 

80 

90 

100 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

40 

30 

20 

10 

Apparent Age (Ma) 

0 20 40 60 80 100

Percent 39ArK Released 

20 
01-CB-006/#2/DD79
 

K-feldspar 

Apparent Age (Ma)
 %Rad
 K/Ca
20
 

16
 

12
 

100 

90 

80 

70 

60 

50 

40 

30 

20 

0 20 40 60 80 100 

Percent 39ArK Released 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

8 

4 

21 
01-CB-007/#7/DD79
 

groundmass concentrate 

K/Ca %Rad 

0 

50 

100 

150 

200 

250 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

0.12 

0.10 

0.08 

0.06 

0.04 

0.02 

Apparent Age (Ma) 

0 20 40 60 80 100

Percent 39ArK Released 

22 
01-CB-008/#14/DD79


Groundmass Concentrate 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 K/Ca %Rad 100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

0.28 

0.24 

0.20 

0.16 

0.12 

0.08 

0.04 

Apparent Age (Ma) 

0 20 40 60 80 100

Percent 39ArK Released 

23 
01-CB-008/#4/DD79
 

Plagioclase 

K/Ca %Rad 100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

30 

40 

50 

60 

70 

80 

90 

100 

0.3 

0.2 

0.1 

Apparent Age (Ma) 

0 20 40 60 80 100

Percent 39ArK Released 

24 
01-CB-009/#109/DD78
 

Groundmass concentrate 

K/Ca %Rad 100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

1.2 

1.0 

0.8 

0.6 

0.4 

0.2 

Apparent Age (Ma) 

0 20 40 60 80 100

Percent 39ArK Released 

25 
01-CB-010/#6/DD79


Groundmass concentrate 

K/Ca %Rad 

0 

20 

40 

60 

80 

100 

120 

0.6 

0.4 

0.2 

0.0 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

Apparent Age (Ma) 

0 20 40 60 80 100

Percent 39ArK Released 

26
Appendix B: 40Ar/39Ar incremental heating data for each sample.
 

27 
Appendix B. 40Ar/39Ar incremental heating data for volcanic intrusions from Uvalde County, Texas. 

40ArR 

1 39 40ArR/

39ArK 

39 Ar2 %40ArR %39 Temp (o

C) ArK Ar/37 Ar Apparent Age3 

(Ma at ± 1 sigma)

 01-CB-001A, #12DD78, Del Rio River Intrusion

 Groundmass Concentrate; .2806g; J-value=.005085±0.1%(1 ) 

500 0.15794 0.01714 9.215 0.25 23.4 21.1 82.61 ± 0.56 

800 0.17112 0.1621 10.554 0.19 58 19.9 94.3 ± 1.5 

900 0.08623 0.00684 12.598 0.11 60 8.4 112.0 ± 3.3 

1000 0.16485 0.01311 12.571 0.09 63.9 16.1 111.78 ± 0.61 

1100 0.09578 0.00878 

1200 0.08942 0.01132 

1350 0.06467 0.00788 

Total gas age: 91.31±4.35 Ma

10.905 0.05 49.5 10.8 97 ± 5 

7.898 0.01 51.7 13.9 71 ± 5 

8.208 0 28.3 9.7 74 ± 19 

01-CB-002, #11DD78, Knippa Quarry Intrusion

 Groundmass Concentrate (82-100 mesh); .1722g; J-value=.005085±0.1%(1 ) 

500 0.29237 0.03617 8.083 

700 0.44599 0.04269 10.448 

800 0.39042 0.04007 9.744 

1000 1.14557 0.12415 9.227 

1100 0.34951 0.03874 9.021 

1200 0.24766 0.03292 7.523 

1350 0.06802 0.0112 6.075 

Total gas age: 80.89±0.56 Ma 

Isochron age (steps 1-7): 76±7 Ma; (40Ar/36Ar)i=241±39

0.23 40.7 11.1 72.66 ± 0.15 

0.75 64.8 13.1 93.38 ± 0.15 

2.62 72.5 12.3 87.24 ± 0.13 

1.49 81.3 38.1 82.75 ± 0.14 

0.34 75.6 11.9 80.91 ± 0.17 

0.02 71.6 10.1 67.72 ± 0.10 

0 31.8 3.4 55 ± 9 

01-CB-002, #8DD79, Knippa Quarry Intrusion

 Groundmass Concentrate (46-60 mesh); .2438g; J-value=.0105±0.1%(1 ) 

650 0.19579 0.04779 4.097 0.31 30.3 4.1 75.99 ± 0.97 

750 0.64791 0.14923 4.342 0.41 63.1 12.8 80.42 ± 0.12 

850 0.77916 0.15743 4.949 2.16 80.5 13.5 91.39 ± 0.47 

950 1.24042 0.28353 4.375 4.92 91.5 24.3 81.02 ± 0.12 

1050 0.81858 0.18935 4.323 3.44 88.8 16.2 80.09 ± 0.29 

1150 0.61907 0.14273 4.337 2.08 88.2 12.2 80.34 ± 0.45 

1250 0.42218 0.0917 4.604 0.52 79.6 7.9 85.16 ± 0.43 

1400 0.43965 0.10404 4.226 0.1 79.3 8.9 78.32 ± 0.17 

Total gas age: 81.99±0.29 Ma. 

Average age (steps 4-6) 80.48±0.29 Ma; Isochron age (steps 1-8): 82.7±2 Ma; (40Ar/36Ar)i=287±16

 01-CB-005, #3DD78, Fort Inge Intrusion

 K-feldspar; .0064g; J-value=.005095±0.1%(1 ) 

650 0.05111 0.00894 5.718 54.73 47.5 2.6 51.8 ± 3.6 

850 0.37597 0.04482 8.388 62.81 87.9 13 75.49 ± 0.27 

950 0.38245 0.04568 8.373 73.46 92.4 13.2 75.36 ± 0.70 

1050 0.33549 0.04092 8.199 67.33 93.5 11.8 73.83 ± 0.83 

1200 0.97607 0.12087 8.075 87.08 96.5 35 72.74 ± 0.11 

1350 0.50003 0.06251 7.999 59.03 94.2 18.1 72.07 ± 0.19 

1450 0.15383 0.02162 7.115 4.51 76.5 6.3 64.24 ± 0.11 

Total gas age: 72.38±0.41 Ma. 

Average age (steps 5-6) 72.41±0.15 Ma; Isochron age (steps 2-6): 71.5±1.1 Ma; (40Ar/36Ar)i=423±57 

28
 
Appendix B continued 

40ArR 

1 39 40ArR/

39ArK 

39 Ar2 %40ArR %39 Temp (o

C) ArK Ar/37 Ar Apparent Age3 

(Ma at ± 1 sigma)

 01-CB-005, #1DD79, Fort Inge Intrusion

 K-feldspar; .0102g; J-value=.010504±0.1%(1 ) 

650 0.07715 0.03323 2.322 21.24 12.6 2.5 43.5 ± 1.5 

850 0.23548 0.05843 4.03 14.25 71.5 4.5 74.80 ± 0.61 

950 0.2632 0.06408 4.107 26.65 90.9 4.9 76.2 ± 1.1 

1050 0.40735 0.10285 3.96 34.03 93.1 7.9 73.53 ± 0.41 

1200 1.63445 0.41104 3.976 56.07 95.9 31.5 73.82 ± 0.21 

1300 1.39105 0.36365 3.825 56.52 96 27.9 71.07 ± 0.21 

1450 1.04681 0.27021 3.874 49.91 95 20.7 71.96 ± 0.20 

Total gas age: 72.04±0.33 Ma. 

Average age (steps 4-7) 72.60±0.26 Ma; Isochron age (steps 1-7): 74.1±1.0 Ma; (40Ar/36Ar)i=268±8

 01-CB-006, #2DD79, Intrusion on Highway 90 west of Sabinal

 K-feldspar; .0596g; J-value=.010487±0.1%(1 ) 

650 0.16164 0.07702 2.099 14.9 47.3 1.1 39.3 ± 1.1 

750 0.80127 0.22406 3.576 13.62 90.4 3.3 66.42 ± 0.24 

850 1.52209 0.37451 4.064 6.54 96.1 5.5 75.30 ± 0.49 

950 2.56789 0.62748 4.092 15.17 97.9 9.1 75.81 ± 0.22 

1000 2.0881 0.50851 4.106 31.17 98.4 7.4 76.06 ± 0.12 

1050 2.41534 0.58959 4.097 32.48 98.6 8.6 75.88 ± 0.23 

1100 1.37299 0.33823 4.059 33.45 97.9 4.9 75.21 ± 0.12 

1150 1.92512 0.47828 4.025 34.76 98.1 7 74.59 ± 0.12 

1200 10.77533 2.76641 3.895 44.67 98.6 40.3 72.22 ± 0.14 

1300 1.79115 0.44904 3.989 35.05 97.4 6.5 73.93 ± 0.11 

1400 1.74096 0.43605 3.993 29.98 97.1 6.3 74.00 ± 0.14 

Total gas age: 73.30±0.19 Ma. 

Average age (steps 9-11) 73.39±0.13 Ma; Isochron age (steps 1-11): 74.6±1.2 Ma; (40Ar/36Ar)i=336±226

 01-CB-007, #7DD79, Intrusion southeast of Uvalde

 Groundmass Concentrate; .2471g; J-value=.010095±0.1%(1 ) 

650 0.01327 0.00712 1.863 0.14 8.4 4.5 33.61 ± 0.66 

750 0.12742 0.03477 3.665 0.16 36.3 21.8 65.54 ± 0.47 

850 0.04454 0.00492 9.052 0.25 44.2 3.1 158 ± 8 

950 0.12969 0.01695 7.651 0.2 46.9 10.6 134.2 ± 0.98 

1050 0.10868 0.01785 6.089 0.19 61.4 11.2 108 ± 1.8 

1150 0.07936 0.01345 5.899 0.06 58.4 8.4 104 ± 3 

1250 0.15684 0.03529 4.444 0.01 64.1 22.2 79.17 ± 0.56 

1350 0.12626 0.0221 5.713 0 44.2 13.9 101.2 ± 0.15 

1450 0.0919 0.00676 13.593 0.01 65.1 4.2 232 ± 1.00 

Total gas age: 97.73±1.07 Ma. 


Maximum possible age (steps 7) 79.2±0.6 Ma; Isochron age (steps 1-9): 105±27 Ma; (40Ar/36Ar)i=287±67 


29
 
Appendix B continued 

40ArR 

1 39 40ArR/

39ArK 

39 Ar2 %40ArR %39 Temp (o

C) ArK Ar/37 Ar Apparent Age3 

(Ma at ± 1 sigma)

 01-CB-008, #4DD79, Intrusion north of Cline

 Plagioclase; .250g; J-value=.010282±0.1%(1 ) 

650 1.07178 0.35171 3.047 0.58 55.8 10.2 55.66 ± 0.09 

800 3.01013 0.70078 4.295 0.4 92.1 20.4 77.97 ± 0.27 

900 3.5217 0.79144 4.45 0.26 96.6 23 80.70 ± 0.12 

1000 2.78118 0.61814 4.499 0.11 94.4 18 81.58 ± 0.12 

1100 1.4454 0.32189 1.49 0.16 91.9 9.4 81.42 ± 0.26 

1200 1.84801 0.4162 1.44 0.24 95.4 12.1 80.53 ± 0.12 

1300 0.92759 0.20933 4.431 0.05 92.1 6.1 80.38 ± 0.18 

1400 0.11485 0.02556 4.494 0.02 82.1 0.7 81.5 ± 1.4 

Total gas age: 77.79±0.17 Ma. 

Average age (steps 3-7) 80.92±0.16 Ma; Isochron age (steps 2-8): 80.36±0.91 Ma; (40Ar/36Ar)i=300±33

 01-CB-008, #14DD79, Intrusion north of Cline

 Groundmass concentrate; .1722g; J-value=.009845±0.1%(1 ) 

650 0.03491 0.02336 1.494 0.47 19.4 1 26.35 ± 0.62 

750 0.31121 0.08122 3.832 0.44 76.2 3.3 66.80 ± 0.41 

850 1.83919 0.39138 4.699 0.51 93.9 16.1 81.59 ± 0.15 

950 3.98219 0.83885 4.747 0.34 97.9 34.4 82.40 ± 0.13 

1050 1.51506 0.3176 4.77 0.18 96.5 13 82.79 ± 0.13 

1150 2.24887 0.4754 4.73 0.23 96.6 19.5 82.12 ± 0.13 

1250 1.16106 0.25182 4.611 0.16 96.7 10.3 80.08 ± 0.12 

1400 0.26521 0.05689 4.662 0.01 79.6 2.3 81.0 ± 1 

Total gas age: 80.95±0.17 Ma. 

Highest point in spectrum (step 5) 82.79±0.13 Ma; Isochron age (steps 3-6, and 8): 82.68±0.24 Ma; (40Ar/36Ar)i=252±7

 01-CB-009, #109DD78, Intrusion west of Uvalde on highway 90

 Groundmass concentrate; .2628g; J-value=.005045±0.1%(1 ) 

500 0.75588 0.13924 5.429 1.31 49.4 23.4 48.74 ± 0.08 

700 0.73653 0.08397 8.771 2.46 82.2 14.1 78.11 ± 0.12 

800 0.52703 0.05949 8.858 2.33 76.8 10 78.9 ± 1.4 

900 0.67564 0.08965 7.536 1.89 80.6 15.1 67.32 ± 0.10 

1000 0.92449 0.1165 7.935 1.84 83.4 19.6 70.81 ± 0.11 

1100 0.47518 0.06742 7.048 0.37 82.6 11.3 63.03 ± 0.28 

1200 0.1667 0.02789 5.977 0.01 74.7 4.7 53.6 ± 1.8 

1350 0.07134 0.01088 6.556 0 49.4 1.8 58.70 ± 4.92 

Total gas age: 65.08±0.41 Ma. 


Highest point in spectrum (step 3) 78.9±1.4 Ma; Isochron age (steps 1-8): 74±7 Ma; (40Ar/36Ar)i=188±7 


30
 
Appendix B continued 

Temp (o

C) 40ArR 

1 ArK 

40ArR/

39ArK 

39Ar/37Ar2 %40ArR %39Ar Apparent Age3 

(Ma at ± 1 sigma)

 01-CB-010, #6DD79, Intrusion south of Sabinal

 Groundmass concentrate; .2868g; J-value=.010382±0.1%(1 ) 

600 0.01196 0.00409 2.926 0.18 3.7 0.4 54 ± 18 

700 0.12753 0.0534 2.388 0.16 20.9 4.7 44.18 ± 0.77 

800 0.07376 0.02655 2.778 0.16 36.6 2.3 51.30 ± 0.54 

900 0.073 0.01202 6.071 0.2 29.9 1 110.3 ± 4.4 

1000 0.09333 0.0166 5.622 0.46 44.2 1.4 102.3 ± 3.1 

1100 0.37774 0.07614 4.961 0.83 73.1 6.6 90.60 ± 0.60 

1250 0.95685 0.21032 4.55 1.11 84.2 18.3 83.26 ± 0.16 

1400 3.19008 0.74446 4.285 0.02 89.1 64.9 78.52 ± 0.12 

1500 0.01163 0.00297 3.914 0.01 46.9 0.3 72 ± 10 

Total gas age: 78.57±0.40 Ma. 

Maximum age (Step #3), 78.52±0.12 Ma; Isochron age (steps 1-9): 80±7 Ma; (40Ar/36Ar)i=281±23 

39

Note. These are abbreviated footnotes for Appendix B. 

1. 	 Radiogenic 40Ar(40ArR) is the total 40Ar derived from natural radioactive decay of 40K after all corrections

 from non-derived 40Ar, including atmospheric 40Ar and reactor-produced 40Ar, have been made. K-derived 39Ar

 (39ArK) is the total 39Ar derived from the epithermal neutron-induced 39K(n,p)39Ar after corrections for non-39Arderived 39Ar, including 42Ca-derived 39Ar, are made. F is the quantity resulting from the division of radiogenic 40Ar

 and K-derived 39Ar. Quantities for radiogenic 40Ar and K-derived 39Ar are given in volts of signal measured on a 

Faraday detector by a digital voltmeter. These quantities can be converted to moles, using the mass spectrometer

 sensitivity at the time of measurement of 9.736x10-13 moles of argon per volt of signal. The measured 40Ar/36Ar

 ratio used for mass discrimination correction is 298.9. 

2. 	 By multiplying the 39Ar/37Ar ratios by 0.5, the relative approximate K/Ca distribution of the samples may be

 obtained. 

3. 	 Error estimates for apparent ages of individual temperature steps were assigned by using the equations of 

Dalrymple and others (1981); however, the equations were modified to allow the option of choosing the larger of 

separately derived errors in the F-value-either a calculated error or an experimental error determined from the

 reproducibility of identical samples. 

31