OQ

O

fel

a

g

Chemical Comparison of 

Glassy and Crystalline 

Volcanic Rocks

GEOLOGICAL SURVEY BULLETIN 1201-D

Prepared on behalf of the

>. C/./S. Atomic Energy Commission

Chemical Comparison of 

Glassy and Crystalline 

Volcanic Rocks

By P. W. LIPMAN

CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1201-D

Prepared on behalf of the

U.S. Atomic Energy Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON s 1965
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 15 cents (paper coyer)
CONTENTS

	Page 

Abstract..... ._________________.__________ Dl

Introduction_____________________-___-______-_____________-_-_____ 1

Alteration of volcanic glasses______________________________________ 2

Compositional changes in examples of primary crystallization._________ 4

Analytical data_____________________________________________ 4

Causes of the major compositional variations____________________ 10

Estimation of absolute compositional changes _____________________ 16

Compositional changes in examples of secondary devitrification________ 19

Implications for petrochemical studies_______________________ __ 21

References cited__________________________________________________ 23

GRAPHS

Page 

FIGURE 1. Compositional differences between glassy and crystalline

rocks______________________________________ D9

2-5. Alkali variations in 

2. Topopah Spring Member of the Paintbrush Tuff______ 12

3. Yucca Mountain and Tiva Canyon Members of the

Paintbrush Tuff ___ _ ________________ - 13

4. Timber Mountain Tuff____________________ 14

5. Grouse Canyon and Tub Spring Members of the

Indian Trail Formation________________________ 15

6. Relations between alkali compositions and shard size in samples of glassy Pearlette Ash Member of the Sappa Formation from three localities____________________________ 17

7. Compositional differences between glassy and crystalline 

rocks with glassy rocks recalculated to Al2 Oa content of 

crystalline rocks______-_-_-_-_-_-_____-_________---_- 18

8. Alkali variations in vitrophyres of Walcott Tuff, southcentral

Idaho._______.____._._______________ 22

TABLES

TABLES 1-5. Chemical analyses of Page

1. Glassy and crystalline rocks that have undergone

primary crystallization._______________________ D6

2. Rocks listed in table 1, recalculated.--.___________ 8

3. Ground water from the Rainier Mesa area, southern

Nevada, and other semiarid volcanic terranes____ 10

4. Glassy and crystalline rocks that have undergone

secondary devitrification_____________________ 20

5. Rocks listed in table 4, recalculated.______________ 20

CONTRIBUTIONS TO GENERAL GEOLOGY

CHEMICAL COMPARISON OF GLASSY AND CRYSTALLINE 

VOLCANIC ROCKS

By P. W. LIPMAN

ABSTRACT

Groundmass crystallization of volcanic rocks is considered in terms of primary 

crystallization, which occurs during original emplacement and cooling of the 

body, and secondary devitrification, which occurs subsequent to original cooling. 

Analyses of glassy and crystallized parts of 13 volcanic bodies, mostly rhyolitic 

lavas and ash-flow tuffs from southern Nevada and all examples of primary 

crystallization, show similar trends. The glasses show less oxidation of iron, 

higher water content (+105° C) from secondary hydratton, higher aluminum, and 

lower silicon and sodium contents. These last variations are thought to represent ground-water leaching of silicon and sodium constituents which are 

abundant in local ground-water analyses and resultant residual concentration 

of relatively insoluble aluminum. Leaching is most advanced in porous glassy 

tuffs having large surface areas, but is also significant in many dense vitrophyres 

from lavas and welded tuffs. Those parts of initially homogeneous ash-flow tuffs 

that have undergone primary crystallization seem to be relatively uniform in 

present composition and are probably closer compositional approximations of .the 

original magma than are the variably leached glassy margins of the ash-flow 

sheets. Secondary devitrification of volcanic rock is apparently accompanied by 

major alkali transfer in many cases.

INTRODUCTION

Consideration of the numerous volcanic rock analyses that have been 

made for U.S. Geological Survey investigations at the Nevada Test Site 

has revealed greater compositional scatter within genetically related 

suites of rocks than expected from magmatic processes. The present 

study, an investigation of chemical variations between glassy and 

crystalline parts of volcanic bodies, attempts to determine which parts 

of a volcanic body are most likely to approximate the original magma 

in composition. A previous investigation of zonal compositional variations in a welded ash-flow sheet (Lipman and Christiansen, 1964) 

showed no analytically detectable compositional variations among such 

differently crystallized zones as vapor phase, lithophysal, or densely 

devitrified as defined by Smith (1960, p. 151-153). The glassy parts 

showed an anomalous composition, however, and apparently have been

1)1
D2 CONTRIBUTIONS TO GENERAL GEOLOGY

leached by ground water to such an extent that they provide less reliable samples of primary magmatic compositions than do the crystallized parts. This tentative conclusion conflicted with the common 

assumption of petrologists that glasses "are the only rocks of which we 

can say with complete confidence that they correspond in composition 

with a liquid" (Bowen, 1928, p. 125), and indicated the need for a more 

detailed investigation. The following discussion is concerned mainly 

with chemical evidence but also draws on the author's experience in 

petrographic and field investigations of upper Tertiary rocks in southern Nevada, including more than 30 ash-flow tuff sheets and 20 rhyolitic lava flows.

Most of the data used in this report were gathered for purposes other 

than those of the present investigation; accordingly, interpretation 

presents problems not encountered when data are collected for a specific 

study. The data are considered from two approaches. First, glassy 

and crystalline paired analyses from individual localities are compared. Then, separate consideration is given to randomly collected 

glassy and crystalline rocks from those individual volcanic units for 

which abundant analytical data are available. Although each batch 

of data points to the same general conclusions, none permits completely 

unambiguous interpretation in itself; most conclusions are dependent 

on a multiple approach to the available data.

For permission to use unpublished chemical data I am grateful to 

many colleagues on the U.S. Geological Survey, particularly J. T. 

O'Connor, H. A. Powers, D. L. Hoover, and F. A. McKeown, who 

contributed glass-crystalline paired analyses for tables 1 and 2. I also 

wish to thank Irving Friedman, Frank Simons, K. E. Wilcox, and 

especially H. A. Powers for many stimulating suggestions.

ALTERATION OP VOLCANIC GLASSES

The types of alteration that may affect glassy parts of extrusive 

volcanic bodies are discussed in terms of four processes: primary 

crystallization, hydration, secondary devitrification, and leaching. 

The term "primary crystallization" is used in preference to "primary 

devitrification" because it is applicable both to ash-flow tuffs and to 

lava flows. In ash flows primary crystallization of glassy constituents 

is commonly ascribed to devitrification, as in the zone of devitrification 

of Smith (1960), but in lava flows crystallization can occur directly 

from a liquid without an intervening vitreous stage.

Primary crystallization occurs in the interiors of volcanic bodies 

at temperatures only slightly below melting during and immediately 

following emplacement. With dropping temperature the rate of 

crystallization decreases rapidly, and experimental determinations of
COMPARISON, GLASSY AND CRYSTALLINE VOLCANIC ROCKS D3

crystallization rates indicate that lavas cease crystallizing at a few 

hundred degrees below melting temperature (Irving Friedman, oral 

commun., 1963). Primary crystallization results in the zonation of 

volcanic bodies: the slowly cooled interior crystallizes and is surrounded by a quenched glassy envelope, as described for welded ashflow tuffs by R. L. Smith (1960). Similar crystallization zonations 

characterize unaltered rhyolitic lava flows (Christiansen and Lipman, 

1964).

Several recent studies have shown that most Tertiary volcanic 

glasses have undergone secondary hydration (Koss and Smith, 1955, 

p. 1086-1088; Boyd, 1961, p. 415). Such rocks contained only a few 

tenths of a percent magmatic water upon cooling; the remainder of 

the observed water, typically several percent H2O + , is secondary. 

The amount of hydration is dependent on time and surface area (Friedman and Smith, 1960); few Eecent glasses are hydrated, but only the 

least fractured Tertiary vitrophyres have escaped hydration. Obviously, chemical analyses of extensively hydrated volcanic glasses 

must be recalculated on a water-free basis for precise petrologic studies.

Secondary devitrification occurs after cooling of the volcanic body. 

As glasses are thermodynamically metastable below their melting temperatures (Morey, 1954, p. 29), devitrification may be expected whenever the kinetics of crystallization become significant for geologic 

time. The occasional preservation of glass as old as Precambrian 

(Simons, 1962, p. 882; Philpotts and Miller, 1963) indicates that 

glasses do not divitrify solely from aging, but the rarity of glasses 

older than Tertiary indicates that the quenched primary glassy borders 

of most older volcanic bodies have crystallized secondarily. Eecent 

studies by Marshall (1961) and Friedman and Smith (1964) emphasize 

the importance of water in catalyzing devitrification, and indicate that 

secondary devitrification of hydrated glasses occurs readily over a few 

million years at temperatures of no more than 200° C. Although the 

products of primary crystallization and secondary devitrification may 

in places be indistinguishable, products of secondary devitrification 

are suggested by structural relations between glassy and crystalline 

rocks that differ from the primary-crystallization zonations due to 

quenching. Crosscutting "devitrification dikes" that follow fractures 

(Simons, 1962; Terzaghi, 1948, p. 24) and certain glassy sills enveloped 

by crystalline material along joints and intrusive margins (Judd, 1893; 

Anderson and Kadley, 1915) are thought by the writer to represent 

secondary devitrification, although they are not necessarily so interpreted in the original reports. In the upper Tertiary volcanic rocks 

of southern Nevada studied by the author, such features are sparse 

and are confined to structurally complex rocks that have been sub-
D4 CONTRIBUTIONS TO GENERAL GEOLOGY

jected to hydrothermal alteration. In the following pages, rocks 

suspected of secondary devitrification on the basis of field relations will 

be shown to have undergone chemical changes that differ markedly 

from those of more obvious cases of primary crystallization.

The effect of leaching in aqueous solution, apparently not much considered by petrologists studying volcanic glasses, has been of considerable concern to the commercial glassware industry. Extensive cation 

leaching of artificial alkali silicate glasses has been demonstrated by 

much laboratory experimentation for instance, by Sen and Tooley 

(1955), and by study of historically ancient glass (Brill, 1963). 

Such glasses show compositional affinity to rhyolitic volcanic rocks, 

and a major purpose of this paper is to evaluate the effects of groundwater leaching on natural volcanic glasses over geologically significant 

spans of time.

COMPOSITIONAL CHANGES IN EXAMPLES OP PRIMARY 

CRYSTALLIZATION

ANALYTICAL DATA

Table 1 lists paired analyses of glassy and crystalline rocks from 13 

separate volcanic bodies including seven welded rhyolitic ash-flow 

tuffs, four rhyolitic lava flows, a rhyolitic dike, and a basaltic lava 

flow.1 In each the field relations indicate that the crystallized part of 

the body formed by primary crystallization. The glasses are from the 

outer parts of the bodies, and the crystallized equivalents were collected toward the interiors, a few feet across the glass-crystalline interface. All sample localities except Nos. 1,11, 12, and 13 have been 

examined by the author; relations at these localities have been described to the author by the original collectors D. L. Hoover, F. M. 

Byers, H. A. Powers, and F. A. McKeown, oral communications, 1963. 

All sample pairs except Nos. 10, 11, and 12 have been studied in thin 

section, and identical phenocryst ratios verify that they represent 

glassy and crystallized equivalents of single volcanic bodies. Sample 

5A is nonwelded glassy tuff; all other glassy samples are dense 

vitrophyres. Except for pair 11, both samples of any pair were 

analyzed simultaneously by the same chemist; therefore, few of the 

differences in the paired analyses should be ascribed to analytical 

error.

The glasses of sample pairs 11 (Pleistocene) and 12 (within historic 

times) show little secondary hydration; the glasses of all other sample 

pairs, which are Tertiary mostly Miocene and Pliocene are extensively hydrated. Accordingly, to facilitate comparisons, all analy-

^An additional glass-crystalline pair is given by Longwell (1963, table 2,, analyses 421, 

424), but it has not been used for the present study because the crystalline sample (No. 

424) is probably silicified. This sample shows more than 80 percent SiO2 when recalculated free of water.
COMPARISON, GLASSY AND CRYSTALLINE VOLCANIC ROCKS D5

ses have been recalculated to 100 percent on an anhydrous basis (table 

2). As the appreciable CO2 in several samples (especially 6B, 10B, 

13A, and 13B of table 1) clearly correlates with excessive CaO, all 

CO2 was calculated out as CaCO3 . Net changes in the weight-percent 

of oxides, obtained by subtracting the recalculated analyses of glasses 

(table 2, A columns) from the recalculated analyses of crystalline 

rocks (table 2, B columns), are shown graphically in figure 1. The 

changes in H2O + were obtained from table 1.

Despite considerable scatter, many changes are consistent. As 

already noted, H2O + is much lower in all crystalline samples except 

the two youngest lavas (pairs 11 and 12); their glasses have not had 

sufficient time to hydrate. The generally higher Fe2O3/FeO ratio in 

the crystalline rocks reflects a widespread conversion of magnetite 

crystallites in the generally black vitrophyres to hematite in the 

typically reddish crystallized samples. Total iron is nearly constant; 

the scatter is of small amplitude, shows no trend, and is probably 

within the expectable range of analytical error. MnO and TiO2 are 

minor constituents of all samples and show no detectable significant 

variations. Na2O is generally slightly to considerably higher in the 

crystalline samples, and K2O is generally slightly lower; interpretation of these apparently significant variations will be deferred 

until later in the paper.

MgO and CaO are nearly constant in most pairs; however, in a 

few crystalline samples especially IB, 4B, 5B, and 7B these constituents are both considerably lower. In thin section the glassy 

equivalents of these samples, all welded tuft's, showed minor fine dust 

at shard boundaries, and montmorillonitic clay was detected in samples 

4A and 5A by X-ray examination. Such minor argillic alteration 

probably explains the excess MgO and CaO in these glasses; it may 

also account for the slight excesses in samples 1A and 7A, even though 

no clay was revealed by X-ray studies because less than about 5 percent 

clay probably would not be detected. In addition to this minor argillic 

alteration of some samples, interpretation of the CaO variations is 

complicated by the presence of appreciable carbonate CO2 in the 

analyses in a few samples and by possible base exchange with ground 

waters enriched in calcium from nearby Paleozoic carbonate rocks.

SiO2 and A12O3 show sizable and meaningful variations; SiO2 is 

generally higher and A12O3 lower in the crystalline rocks. Although 

the determination of these two oxides by different analysts is known 

to be subject to sizable variation (Fairbairn and others, 1951; Stevens 

and others, 1960), relative differences within the pairs under consideration are likely to be significant because, with one exception, 

both samples of each pair were analyzed simultaneously by a single

1761-340 65-
TABLE 1. Paired chemical analyses, in weight percent, of glassy and crystalline roc

Symbols in column heads: 

A. Glassy rocks 

B. Crystalline rocks

[Analyses: 4A,B by Dorothy F. Powers; 7A,B by Christel L. Parker; 1A,B, 2A,B, 3A,B, 8A,B, 9A,B, and 13A.

Lowell Artis, and H. Smith]

8iOj ~ -~~

AljOs. . .... 

TiOs - -----

FeO-- - ...

MnO.- -- 

MgO.-- -

CaO 

"NTfloO

KzO 

H80+ - 

HiO--- 

Pz06 

COz. ..... ....

Total- --

Paired analyses 

1 

A

DLH-S2- 

19

68.8 

12.7 

.30 

2.3 

1.2 

.15 

.29 

.53 

2.6 

5.3 

4.5 

1.2 

.04 

<.05

100

B

DLH-62- 

23

71.8 

12.8 

.33 

3.9 

.04 

.16 

.04 

.19 

4.8 

4.9 

.63 

.22 

.07 

<.05

100

2 

A

62L-178- 

B

69.2 

15.0 

.32 

1.2 

.57 

.12 

.44 

1.0 

4.2 

5.0 

2.9 

.28 

.11 

<.05

100

B

62L-178- 

A

70.8 

14.8 

.34 

1.6 

.34 

.12 

.52 

1.0 

4.2 

5.0 

.92 

.40 

.10 

<.05

100

3 

A

63L-17-I

74.1 

12.5 

.09 

.64 

.29 

.06 

.22 

.53 

3.4 

4.5 

2.9 

.38 

.02 

<.05

100

B

63L-17- 

H

76.9 

12.2 

.10 

.84 

.13 

.07 

.29 

.40 

3.6 

4.8 

.51 

.22 

.02 

<.05

100

4 

A

AGE-2B

65.73 

15.46 

.40 

1.43 

.56 

.09 

.46 

1.39 

3.88 

5.70 

3.61 

.70 

.07 

.01

99.49

B

AGE-2A

69.02 

15.60 

.41 

1.93 

.18 

.08 

.25 

.94 

4.36 

5.93 

.31 

.20 

.07 

.01

99.29

6

} 
SiOj. ...-........ ...................

AhOs..-- --- ----------------

TiOs... ....................... ...........

FetOt. ...................................

FeO............ .......................

MnO

MgO. -.-. ........ . .. .....-. -

CaO _

NajO._ . ... 

KaO -.

HjO+.- 

HjO-.. ................................ ..

PaOs -

COj .

Total 

8-

A

63L-15-A

75.4

19 9

.18

.59

.34

.03

.16

.56

3.0

5.3

2.4

.18

.02

<.05

inn

B

63L-15-B

77.3

19 fi

.14

.73

.14

.02

.27

.46

3.0

5.0

.37

.02

<.05

100

9-

A

63L-14-A

75.7

11.7

.14

.51

<u

.04

.23

.42

2.8

5.3

3.0

.15

.03

<.05

100

B

63L-14-B

77.9

n

o

.14

.70

.16

.03

.25

.51

3.0

5.0

.49

.14

.02

<.05

100

10

A

MC-170-

B

74.68

12.20

.09

.41

.16

.07

.11

.38

3.42

4.86

3.13

.14

.01

.01

99.67

-

B

MC-170-

C

76.18

12.32

.09

.50

.13

.07

.20

.94

3.65

4.81

.16

.21

.02

.41

99.69

1. Khyolitic welded tuff, Grouse Canyon Member of 

Indian Trail Formation, Quartet Dome quadrangle, 

Nye County, Nev. Collected by D. L. Hoover. 

A. Vitrophyric welded tuff. 

B. Crystallized welded tuff from same locality 

as A.

2. Quartz latite welded tuff of Chocolate Mountain, 

Topopah Spring NW quadrangle, Nye County, 

Nev. Collected by P. W. Lipman. 

A. Vitrophyric welded tuff. 

B. Crystallized welded tuff, 50 ft from A.

3. Khyolitic welded tuff, Topopah Spring Member of 

Paintbrush Tuff, Topopah Spring SW quadrangle, 

Nye County, Nev. Collected by P. W. Lipman. 

A. Vitrophyric welded tuff. 

B. Crystallized welded tuff, 25 ft from A.

4. Quartz latite welded tuff, Topopah Spring Member 

of the Paintbrush Tuff, Topopah Spring NE 

quadrangle, Nye County, Nev. Collected by 

J. T. O'Connor.

A. Vitrophyric welded tuff. 

B. Crystallized welded tuff, from same locality 

as A.

EXPLANATION OF PAIRED SAMPLES

5. Rhyolitic welded tuff, Yucca Mountain Member 

Paintbrush Tuff, Topopah Spring NW quadrang

Nye County, Nev. (Lipman and Christians

1964, table 1.)

A. Glassy nonwelded basal ash-flow tuff. 

B. Crystallized welded tuff, 25 ft from A.

6. Rhyolitic welded tuff, Tiva Canyon Member 

Paintbrush Tuff, Bare Mountain quadrangle, N

County, Nev. (Cornwall, 1962, table 2, col. 5-

A. Vitrophyric welded tuff.

B. Crystallized welded tuff, 75 ft from A.

7. Rhyolitic lava flow of Fortymile Canyon, Topop

Spring NE quadrangle, Nye County, Nev. C

lected by J. T. O'Connor.

A. Vitrophyric rhyolitic lava. 

B. Crystallized rhyolitic lava from sa

locality as A.

8. Rhyolitic lava flow of Fortymile Canyon, Topop

Spring NW quadrangle, Nye County, Nev. C

lected by P. W. Lipman.

A. Vitrophyric rhyolitic lava.

B. Crystallized rhyolitic lava, 10 ft from A.
TABLE 2. Chemical analyses of rocks listed in table 

Symbols in boxheads:

A columns, glassy rocks recalculated without water or calcium 

B columns, crystalline rocks recalculated without water or cal

C columns, glassy rocks recalculated to AljOa content of crysta

[Values in parentheses are not included in total percen

SiOj _ . ...

AlsOs ... ....

TiOs ..

FeaOs- ...

FeO - ..

MnO_. ____ 

MgO . ____ 

CaO. .......... 

NazO.. ____ .

K80 .. ..._

P205 .. .........

Total....

SiOj... .... .....

AliOj... .. .-

TiOi. -.-

FeO 

MnO - ... ...

MgO. ..........

CaO.. ... ...

NazO.. .. ......

K20. ..........

PjOs.......-.-

Total... .

A

73.0

13.5

.32

2.4

1.3

(3.5)

.16

.31 

.56 

2.8

5.6

.04

99.99

1 

B

72.6

12.9

.33

3 Q

.04

(3.6)

.16

.04 

.19

4.8

4.9

.07

99.93

C

69.8

12.9

.32

2.3

1.2

/q o\

.16

.31

.54 

2.7

5.4

.04

95.67

A

71.2

15.4

.33

1.24

.59

(1.71)

.12

.45 

1.03 

4.3

5.2

.10

99.96

A

77.1

12.5

.18

60

.35

( 9&\

.03

.16

.57

3.1

5.4

.02

100.01

2 

B

71.6

15.0

.34

1.62

<u

(1.80)

.12

.53 

1.01 

4.3

5.0

.11

99.97

8 

B

77.5

12.6

.14

.73

.14

(.80)

.02

.27

.46

3.0

5.0

.02

99.88

C

f}Q A

15.0

.32

1 91

.57

(1. 57)

.12

.44 

1.00 

4.2

5.0

.10

97.36

C

77.7

12.6

.18

.60

.35

(.88)

.03

.16

.57

3.1

5.5

.02

100. 81

A

76 9

13.0

.09

.66

.30

f »Q\

.06

.23 

.55 

3.5

4.7

.02

100.01

A

77 Q

19 0

.14

.52

.35

f ft9A

ru

.23

.43

9 Q

t, 4

.03

99.94

3 

B

77.4

12.3

.10

QA

.13

(-89)

..07

.29 

.40 

3.6

4.8

.02

99.95

9 

B

78.2

19 n

.14

.70

.16

f 70^

.03

.25

.51

3.0

5.0

.02

100.01

C

79 ft

12.3

.09

.63

9ft

f Strt

,06

.22 

.52 

3.3

4.7

.02

94.92

C

77 9

12.0

.14

.52

.35

f R91

04

.23

.43

9 Q

5.4

.02

QQ Q3

A

69.10

1ft 93

At)

i f;n

KQ

a 

04.}

.09

.48 

1.46 

4.08

5.98

.07

100.00

A

77.48

12.66

09

.43

.17

(.56)

.07

.11

.38

3.55

5 04

.01

QQ QQ

4 

B

fiQ fi7

15.79

4.9

1 Qt.

1C

(i Q*tt a o8°

.25 

.96 

4.42

6.01

.07

100. 00

10 

B

77 4n

12.52

no

.51

.13

(.59)

.07

.20

.46

3.71

4.89

.02

100.00

C

67.23

15.79

.41

1.46

.57

a

CQ\

no

.47 

1.43 

3.97

5.82

.07

97.31

C

76.62

12.52

no

4.9

.17

(.56)

.07

.11

.38

3.51

4.98

.01

98.88

A

76.0

1Q A

.14

.85

.14

/ Q

(i io

.91 

.33 

3.0

5.1

.02

99.99

A

72.50

13.78

91

.59

2.33

/9 Q7

.09

.09

1.53

4.65

4.17

.06

100.00
COMPARISON, GLASSY AND CRYSTALLINE VOLCANIC ROCKS D9

PAIR NUMBER

11 3 10 

12 4 132 1/7 56\8/9

PAIR NUMBER

11 3 10

1.0

<+)

0^

(-)

1.0 

1.0

(+)

0^

(-)

UJ 

Q 1.0

X

o

IN WEIGHT PERCEN" 

plot,:

0 *CJl 

DIFFERENCE 7 ++ no en o<

lob *3 wic

i *i n i ii in i u.iu

(+) 

. " . '. . 0 *

 52.10 percent . Ol°* ( } 

0.10

(+)

1 1 1 1 II ! II II 1 u

(-)

 52.10 percent . * A1A 1 -°

* . . 1.0

L * (+) 

0*

1.0

: r ^ r-**- TiO, 2.0 

52.10 percent

( + ) 1.0

1 1 1 1 II 1 1 1 III 1 ( ) 

 52.10 peY.ceu.t , n

[ 1 11 1 1 II III 1

52.10 percent 

 i % *i

 

r i n i i 1 1 in i 

.52.10 percent A _

 

 

i i n i i ii in i 

.52.10 percent ..

 * * *

 

« * * 

52.10 percent *

. v % i e Total (as FeO) 

L 10 

( + ) 

0,

J_ 1 11 II 1 II III 1 ( _J 

1.0

52.10 percenj »*** 

1 1 1 , N

70 75 80 v ' 

SiO2 (AVERAGE OF PAIR), IN PERCENT 2-°

3.0

i u i i 1 1 in i

52.10 percent / ° t

52.10 percent

- 

. 

 9 

.1*1 i

MnO

MgO

CaO

NaQO

707580 

Si02 (AVERAGE OF PAIR), IN PERCENT

FIGURE 1. Compositional differences between glassy and crystalline rocks expressed as 

the percent variation of oxides. Each point represents the change in weight percent 

of oxide for one pair of analyses. Percent differences obtained from chemical analyses 

in table 2 by subtracting column A from column B.
D10 CONTRIBUTIONS TO GENERAL GEOLOGY

analyst. The consistently higher SiO2 and lower A12O3 in the 

crystalline samples indicates that the variation is meaningful; determinative error alone would have caused a more random scatter.

CAUSES OF THE MAJOR COMPOSITIONAL VARIATIONS

Although the evidence presented thus far does not indicate whether 

the variations in SiO2 , A12O3 , and Na2O are caused by modification 

of the glassy rocks, the crystalline rocks, or both, several lines of 

evidence suggest that the main variation is due to long-continued 

leaching by ground water. By this interpretation, silicon and sodium 

are the main constituents removed, and the nearly insoluble aluminum 

is relatively enriched in the residuum. The main evidence for groundwater leaching is (1) the composition of ground waters from volcanic 

terranes and (2) positive correlation between the cation deficiencies 

and surface areas of the glassy rocks.

Although reliable analyses of ground water originating entirely 

from volcanic rocks appear to be relatively scarce, many have recently 

been made of waters from the Rainier Mesa area of the Nevada Test 

Site (Clebsch and Barker, 1960). Rainier Mesa is underlain by 

glassy, zeolitic, and crystalline rhyolitic ash-flow and ash-fall tuffs, 

including several of the units sampled for this study (Gibbons and 

others, 1963). Six analyses of clear waters from this area (many of 

the reported analyses are cloudy or turbid due to suspended sediment) 

are listed in table 3, along with additional analyses from other semiarid volcanic terranes. Analysis 10 is also from southern Nevada, 

These analyses are in general rather consistent; all are very low in Al

TABLE 3. Chemical analyses, in parts per million, of ground water from the

Rainier Mesa area, southern Nevada, and other semiarid volcanic terranes

[Only major rock-forming elements listed]

SiOj. -

Fe

Mn__ 

Ca."IIII

Na......

Samples 1-6

1

68 

.4 

.00 

.00 

1.9 

13 

18 

2.8

2

67 

.5 

.40 

.00 

1.5 

9.6 

15 

2.8

3

47 

.1 

.20 

.00 

.00 

2.4 

32 

2.2

4

36 

.1 

.10 

.00 

.00 

.2 

24 

.6

5

52 

.0 

.06 

.00 

1.0 

16 

37 

7.8

6

54 

.2 

.00 

.00 

.00 

2.4 

26 

3.6

Average

54 

.2 

.13 

.00 

.8 

7.2 

25 

3.3

7

103

.1

1.1 

6.5

} 40

8

60

.01

7.5 

14 

f 41 

1 4.2

9

55 

.1 

.08 

.0 

1.4 

4.4 

11 

1.2

10

52 

.0 

.22 

.0 

1.0 

8.0 

62 

2.0

1. Seep in fracture, U12b tunnel, Rainier Mesa (Clebsch and Barker, I960, table 2, analysis 2589).

2. Seep hi fracture system, U12b tunnel, Rainier Mesa (Clebsch and Barker, 1960, table 2, analysis 2893)

3. Seep in fracture, U12e tunnel, Rainier Mesa (Clebsch and Barker, 1960, table 2, analysis 2913).

4. Seep in fracture, U12e tunnel, Rainier Mesa (Clebsch and Barker, 1960, table 2, analysis 3260).

5. Seep in fracture, U12e tunnel, Rainier Mesa (Clebsch and Barker, 1960, table 2, analysis 3354).

6. Seep in fracture, U12e tunnel, Rainier Mesa (Clebsch and Barker, 1960, table 2, analysis 3541).

7. Spring on Rio San Antonio, Sandoval County, N.Mex. Water-bearing formation: rhyolite (Hem, 

1959, table 6, analysis 2).

8. Basque Spring, Harney County, Oreg. Water-bearing formation: rhyolite and possibly basalt (Hem, 

1959, table 6, analysis 3).

9. Spring, west of Los Alamos, N.Mex. Flows from rhyolite of Tertiary age (White and others, 1963, 

table 1, analysis 2).

10. Spring, north of Beatty, Nev. Flows from rhyolite of Tertiary age (White and others, 1963, table 1, 

analysis 5).
COMPARISON, GLASSY AND CRYSTALLINE VOLCANIC ROCKS Dll

and high in SiO2, with the Na the next most abundant cation constituent. The analyses show that the main constituents taken into 

solution by ground water in these volcanic terranes are just those 

which are relatively the most deficient in the glasses of table 2. The 

fairly abundant calcium in the water analyses is probably due to 

widespread Paleozoic carbonate rocks near most of the sample 

localities.

Independent evidence of ground-water leaching can be inferred 

from relations between the composition and surface area of various 

glassy ash-flow tuff units. If the differences within the pairs of 

analyses in table 2 were due to modification during crystallization, 

then all glassy parts of any initially homogeneous volcanic body should 

show similar compositions, and there should be considerable scatter 

among crystallized samples. In contrast, if the variations were due 

to leaching of the glasses, then the crystalline samples from any single 

unit should be uniform, and the glassy samples should show considerable scatter correlative with variable surface area available for 

leaching. Variations due to leaching should be less extreme in dense 

vitrophyres (low porosity and surface area) than in nonwelded or 

partly welded tuffs (higher porosity and surface area).

The main problem with this approach is that most of the compositional variations indicated by table 2 are too small to recognize without 

the controls afforded by the paired samples. For example, the variations in SiO2 and A12O3 , which are among the largest in terms of 

weight-percent differences (fig. 1), are nevertheless relatively small 

compared to the total amount of these constituents in the rocks. 

These oxides, moreover, are the dominant constituents of rhyolitic 

rocks and have a reciprocal relationship to each other that makes 

recognition of even large variations difficult without some external 

standard. Fortunately, these difficulties do not seriously affect variations in Na2O. In table 2 the variations in Na2O average about 10 

percent and some are as high as 40 percent of total Na2O, a sufficiently 

large variation to be recognizable in less carefully controlled data. 

Also, as a relatively minor constituent, Na2O is not appreciably modified by the reciprocal effects of changes in SiO2 and A12O3 . Accordingly, Na2O variations provide a sound basis for examining some 

additional analyses from southern Nevada.

Considerable chemical data in addition to that already given in 

tables 1 and 2 is available for the Miocene(?) and Pliocene Paintbrush Tuff (Paul Orkild, written commun., 1964), an extensive 

ash-flow sequence in the vicinity of the Nevada Test Site. Figure 2 

shows Na 2O and K2O compositions of all available analyses of the 

Topopah Spring Member of the Paintbrush Tuff, and ash-flow cooling 

unit that is compositionally zoned from silicic rhyolite at its base to
D12 CONTRIBUTIONS TO GENERAL GEOLOGY

3.0

Densely welded glassy (vitrophyre)

O 

Partly welded glassy

68 70 72 74 

Si02, IN PERCENT

76 78

FIGURE 2. Alkali variations in Topopah Spring Member of Paintbrush Tuff. Generalized 

variation trends drawn by inspection through all analyses of densely welded rocks. 

Tielines connect multiple analyses from individual localities. Unpublished analyses 

provided by P. W. Lipman, J. T. O'Connor, and E. N. Hinrichs, U.S. Geological Survey.

quartz latite at its top. The crystallized tuffs, all of which are densely 

welded, plot on a linear variation trend and have very little scatter. 

Both alkalis increase systematically as SiO2 decreases. The vitrophyres, especially those of rhyolitic composition, plot fairly close to 

this trend, but for any given SiO2 content the vitrophyres are slightly 

lower in Na2O than are the crystallized welded tuffs. The glass that 

is lowest in Na2O (and highest in K2O) is only partly welded and 

accordingly has greater porosity and surface area than the other 

samples.

Na2O contents of available analyses of Yucca Mountain and Tiva 

Canyon Members of the Paintbrush Tuff, all rhyolitic ash-flow tuffs, 

show a pattern similar to that of the Topopah Spring Member (fig. 3).' 

The analyses of Yucca Mountain and Tiva Canyon Members show 

very limited SiO2 variation, and all devitrified samples have similar 

Na2O contents. The Na2O content is somewhat lower in a vitrophyre 

from the Tiva Canyon Member and decidedly lower in a nonwelded 

glassy basal ash-flow tuff from the Yucca Mountain Member. K2O 

tends to be higher in the glassy samples than in the crystalline ones.
COMPARISON, GLASSY AND CRYSTALLINE VOLCANIC ROCKS D13

5.0 r

Pair 5 of table 2 Pair 6 of table 2

4.0

3.0

5.0

4.0

EXPLANATION

X

Welded crystalline tuff,Tiva Canyon Member

Welded glassy tuff (vitrophyre), Tiva Canyon Member

Welded crystalline tuff, Yucca Mountain Member

O 

Nonwelded glassy tuff, Yucca Mountain Member

76.0 77.0

PERCENT

FIGURE 3. Alkali variations in Yucca Mountain and Tiva Canyon 

Members of Paintbrush Tuff. Tielines connect multiple analyses 

from individual localities. Analyses of Yucca Mountain Member from Lipman and Christiansen (19164) ; analyses of Tiva 

Canyon Member connected by tieline are from Cornwall (1962, 

table 2, col. 5-i6) ; the other analyses of Tiva Canyon Member are 

unpublished and were provided by J. T. O'Connor, U.S. Geological 

Survey.
D14 CONTRIBUTIONS TO GENERAL GEOLOGY

Many analyses are available for the Pliocene Timber Mountain Tuff, 

a somewhat more complex ash-flow sequence that includes several 

compositionally zoned cooling units. The considerable scatter of 

NaaO in figure 4 is probably due in part to this complexity and in 

part to the fact that many of the analyses were made primarily for 

purposes other than petrochemical study. Nevertheless, the analyses 

show that nonwelded porous glasses are strikingly lower in Na2O 

than are the dense vitrophyres and the approximately similar crystallized tuffs of comparable SiO2 content, a relation which suggests that 

sodium has been removed from the glasses in proportion to their surface 

area. The vitrophyres may average slightly lower in Na2O content 

than the crystallized welded tuffs, especially for less silicic compositions. The porous glasses are higher in K2O than are the vitrophyres 

and crystalline rocks.

Similar alkali variations between glassy and crystalline tuffs can be 

demonstrated for the Grouse Canyon and Tub Spring Members of

5.0

3.0

6.0

5.0

-4.0

EXPLANATION

Welded crystalline c

 

Welded glassy (vitrophyre)

Nonwelded glassy

68 70 72 74 

IN PERCENT

76 78

FIGURE 4u Alkali variations in Timber Mountain Tuff. Generalized variation trends 

drawn by inspection through all analyses of welded rocks. Unpublished analyses provided by W. J. Carr, F. M. Byers, Jr., E. N. Hinrichs, J. T. O'Connor, P. Orkild, and 

others, of the U.S. Geological Survey.
COMPARISON, GLASSY AND CRYSTALLINE VOLCANIC ROCKS D15

the Miocene and Pliocene ( ?) Indian Trail Formation (fig. 5), another 

major volcanic sequence in the Nevada Test Site area for which 

abundant analytical data are available. The scatter in overall NaaO 

values is so great that no trend can be seen; however, the devitrified 

samples make a well-defined trend, and most of the glassy samples

5.0

4.0

3.0

6.0

5.0 z

4.0 3'

3.0

68 70 72 74 

Si02 , IN PERCENT

76 78

EXPLANATION

Grouse Canyon Member Tub Spring Member 

x Densely welded crystalline + 

 Densely welded glassy (vitrophyre) + 

o Nonwelded and partly welded glassy -cj>-

FIGUBE 5. Alkali variations In Grouse Canyon and Tub Spring Members of Indian Trail 

Formation. Generalized variation trends drawn by inspection through analyses of 

densely welded crystalline rocks. Unpublished analyses provided by F. M. Byers, Jr., B. 

N. Hinrichs, K. D. Krushensky, K. A. Sargent, and others, of the U.S. Geological 

Survey.
D16 CONTRIBUTIONS TO GENERAL GEOLOGY

show significantly lower Na2O contents. Although there is considerable overlap, the nonwelded and partly welded glassy samples are 

generally lower in Na2O than are the less porous vitrophyres. Samples 

from the Grouse Canyon Member show an apparently unsystematic 

variation in K2O content, but the Tub Spring samples define an interesting series. The vitrophyres are slightly higher in K2O than are 

the devitrified samples, and the nonwelded and partly welded samples 

are higher than the vitrophyres. These K2O variations are similar 

to those in the Timber Mountain Tuff and, like the Na2O variations, 

appear to be related to surface areas of the glassy rocks.

Additional evidence of a correspondence between loss of Na2O and 

surface area of glassy rocks is available from an unpublished study by 

H. A. Powers, of the U.S. Geological Survey, on some samples of the 

glassy Pearlette Ash Member of the Sappa Formation of Pleistocene 

age. Powers separated shard tuffs from several localities into three 

size fractions and then had each fraction analyzed for alkalis. In 

each sample the finest fraction showed lower Na2O and higher K2O 

than the coarsest fraction (fig. 6); one intermediate fraction was 

anomalous. In conjunction with the previously presented data, these 

analyses suggest that sodium has been removed and potassium added 

to the tuffs in proportion to their surface area.

ESTIMATION OF ABSOLUTE COMPOSITIONAL CHANGES

Thus far in this study only the relative compositional differences 

between glassy and crystalline samples have been considered, but it is 

also desirable that the absolute compositional changes be estimated. 

Unfortunately, estimation of the absolute change requires several additional assumptions and, accordingly, is less precise than determination 

of the relative compositional changes. If the compositional differences 

between glasses and rocks that have crystallized by primary crystallization result mainly from ground-water leaching, the material removed 

from the glasses of table 2 can be approximated by assuming negligible 

leaching of silicon and sodium from the crystalline rocks and of aluminum from the glassy rocks. The very low aluminum in the groundwater analyses (table 3) suggests that this assumption is reasonable, 

at least with respect to aluminum.

Accordingly, each glass analysis in table 2 has been recalculated to 

the A12O3 content of its crystallized equivalent (table 2, C columns) 

and the differences between C and B columns of table 2 have been 

plotted graphically in figure 7. These differences are analogous to the 

relative compositional changes of figure 1, and should provide an approximation of the absolute compositional changes. The most pronounced difference in comparison with figure 1, the increased positive
COMPARISON, GLASSY AND CRYSTALLINE VOLCANIC ROCKS D17

6.0

5.0

EXPLANATION

Coarse

O 

Medium

A 

Fine

Very fine

Size of shards

2.0 3.0

Na,O, IN PERCENT

FIGCEK 6. Relations between alkali compositions and shard size in samples of the glassy 

Pearlette Ash Member of the Sappa Formation from three localities. Each sample 

collected from a single homogeneous bed and separated into three size fractions. 

Definition of size classification: Coarse shards, +150 mesh; medium shards, 150 

mesh to +200 mesh; fine shards. 200 mesh, sink in water in 2 minutes; very fine 

shards, 200 mesh, float in water for 2 minutes but sink in 2 hours. Unpublished 

analyses provided by H. A. Powers, U.S. Geological Survey.

values for SiO2, should approximate the amount of SiO2 actually 

leached from the glasses, but the large scatter of these values appears 

to reflect an inherent difficulty in the recalculation. Although analytical error in A12O3 determinations is probably not seriously large for 

the original data, as discussed previously, any determinative error 

would be multipled approximately five times in the recalculated SiO2 

values in column C of table 2 and is undoubtedly responsible for some 

of the SiO2 scatter in figure 7. In spite of such weaknesses, assumption of constant aluminum appears to be the best base from 

which to estimate the absolute compositional changes. Certainly, constant-volume calculations would be inaccurate because of volume
D18 CONTRIBUTIONS TO GENERAL GEOLOGY

changes during leaching and secondary hydration of the glasses. More 

precise determination of the absolute compositional changes will probably require experimental leaching of glassy and crystalline rocks 

under controlled laboratory conditions.

As an approximation, the values of figure 6 suggest that about 0.5 

percent Na2O and several percent SiO2 have been removed from many 

vitrophyres. Accordingly, the analyzed glasses have been leached of 

perhaps four times as much SiO2 as Na2O, while the analyses of ground

5.0

4.0

12

(-f)

2.0

1.0

1.0 L 

0.5

(-f)

0.5

PAIR NUMBERS

11 310 

4 132 1|7 56|8J9 0.5 12

1.0 L 

0.5

2.0-

( + ) 1.0

 Ti02

0.5

70 75 80 

Si02 (AVERAGE OF PAIR), IN PERCENT

PAIR NUMBERS 

11

4 132 1|7

310 

5618/9

MnO

 MgO

-CaO

 K20

65 70 75 80 

Si02 (AVERAGE OF PAIR), IN PERCENT

FIGOEB 7. Compositional differences between glassy and crystalline rocks with glassy 

rocks recalculated to A12O3 content of crystalline rocks. Each point represents the net 

change in weight percent of oxide for one pair of analyses. Percent differences obtained 

from chemical analyses in table 2 by subtracting column C from column B.
COMPARISON, GLASSY AND CRYSTALLINE VOLCANIC ROCKS D19

water average approximately twice as much SiO2 as Na2O (table 3). 

Because of the scatter in the present data, the significance of the variation between these ratios cannot be assured, but the data suggest that 

the ratio of SiO2/Na2O leached from glasses may be higher than that 

leached from crystalline rocks.

Consideration of K2O variations has been left until this point because the behavior of this oxide presents the most perplexing feature 

of the present data. Higher K2O in most glasses than in associated 

crystallized samples is indicated on both figure 1 and figure 7, although 

the recalculation to constant A12O3 reduces the amount somewhat. 

Generally similar behavior of K2O is clearly shown by many tuffs from 

the Nevada Test Site (figs. 3-5), although two units, the-Topopah 

Spring Member (fig. 2) and the Grouse Canyon Member (fig. 5), do 

not show a consistent trend. The variations are particularly striking 

for the Timber Mountain Tuff and the Tub Spring Member. Although any systematic variations between crystallized and densely 

welded glassy parts of the Timber Mountain Tuff are obscured by considerable scatter in the data, the porous nonwelded glasses are significantly higher in K2O than are welded rocks of comparable SiO2 

content, probably because of greater surface area. The Tub Spring 

Member (fig. 5) and the Pearlette Ash Member (fig. 6) also show a 

correlation between surface area (shard size) and increasing K2O in 

glassy tuffs. The reason for these K2O variations is not clear. Perhaps some excess K2O is related to the argillic alteration described previously that accounts for high MgO and CaO in a few glasses, especially 

sample 5A (table 2). However, sample 4A (table 2) contains clay in 

amounts detectable by X-ray, but is deficient in K2O. Most of the volcanic glasses containing high K2O also have low Na2O (figs. 3 through 

5), and some excess K2O is possibly due to base exchange, K2O substituting for some of the leached Na2O. Examples of base exchange 

of K2O (and CaO) in industrial glasses are cited by Stanworth (1950, 

p. 117,151).

COMPOSITIONAL CHANGES IN EXAMPLES OP 

SECONDARY DEVITRIFICATION

Compositional variations between glasses and crystallized rocks that 

have undergone secondary devitrification, although not the principal 

topic of this paper, are considered briefly because most previously described glass-crystalline pairs fall into this category, and because the 

typical compositional changes seem to differ from those observed for 

examples of primary crystallization.

Six analyzed pairs of crystalline and glassy rocks that probably exemplify secondary devitrification are listed in table 4; they have been 

recalculated free of water and calcium carbonate in table 5. According
D20 CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 4. Paired chemical analyses, in weight percent, of glassy and crystalline 

rocks that have undergone secondary devitrification

[A, glassy rocks; B, crystalline rocks]

SiOa-- - 

Ah03 . - 

TiOa - 

FeO ...

MnO _ . _ ..

MgO.........

CaO 

Na2O 

K2O_. ....... .

H2 0+ 

H2O-. .......

P2OS -

COS 

Total 

A

71.46

12.72

.21

.94

.31

.07

.22

.86 

3.40

4.72

3.67

.98

.03

.02

99.61

I

B

73.52

12.17

.19

1.08

.14

.05

.20

.74 

1.03

7.97

1.42

1.12

.02

.01

99.66

A

61.69

14.43

1.00

1.23

5.86

.30

2.81

4.97 

3.20

1.72

2.36

.25

.24

100.06

2

B

59.21

14.06

1.06

2.66

4.87

.24

3.71

5.95 

2.06

2.83

1.54

2.05

.20

100.44

A

72.37

11.64

1 49

1.08

.52

1.30 

4.15

3.98

J4.86

101. 32

}

B

75.31

13.62

2.31

00

.20

.97 

3.02

4.07

1.48

100.98

i

A

74.11

11.20

.11

.52

.28

.23

1.88 

3.17

1.04

f 5.14

I 2.20

.02

99.90

t

B

77.30

12.39

.12

.57

1Q

.10

.68 

3.14

4.36

.60

.31

.02

99.78

A

72.20

10.75

.06

.89

.80

.65

.45 

3.95

3.17

} 7.32

100.24

5

B

76.40

10.60

.06

1.40

.38 "Tw"

7.45

1.60

100.46

A

71.46

12.29

.17

.65

.45

.06

.32

1.20 

3.63

3.60

f 5.31

\ .58

.04

.04

99.80

5

B

75.94

12.40

.17

.97

.17

.06

.19

.50 

3.41

6.62

.28

.09

.04

.01

99.85

1. Rhyolitlc welded tuff, Klondyke quadrangle, Arizona. 

A. Vitrophyric tuff. (Simons, 1962, table 2, col. 1.) 

B. Same rock as A, devitrifled. (Simons, 1962, table 2, col. 2.)

2. Leidleite (dacite) sUl, Mull, Scotland.

A. Glassy part. (Anderson and Radley, 1915, p. 212, col. la.)

B. Stony (devitrifled) part, same locality as A. (Anderson and Radley, 1915, p. 212, col. Ib.)

3. Porphyry dike, Arran, Scotland. * 

A. Pitchstone. (Judd, 1893, p. 545, col. 3.) 

B. "Quartz-felsite" derived from A. (Judd, 1893, p. 545, col. 2.)

4. Extrusive rhyolite dome, Wenatchee, Wash. 

A. Perlite. (Coombs, 1952, table p. 202, col. 2.) 

B. Spherulitic breccia, presumably derived from A. (Coombs, 1952, table p. 202, col. 3.)

5. Rhyolitic lava flow, Colle de la Motte, Esterel, France. 

A. Black obsidian. (Terzaghi, 1948, table 1, col. 11.) 

B. Devitrifled obsidian a few hundred feet from A. (Terzaghi, 1948, table 1, col. 7.)

6. Rhyolitic lava flow, Bullfrog Hills, Nev.

A. Basal vitrophyre zone. (Cornwall, 1962, table 2, col. 8.) 

B. Felsitic zone, same flow as A. (Cornwall, 1962, col. 9.)

TABLE 5. Paired chemical analyses of rocks listed in table 4 recalculated without water or calcium carbonate

Symbols:

A, Glassy rocks 

B, Crystalline rocks

[Values in parentheses are not included in total percent]

SiOa -

AhOa 

TiO2 . _. ....

Fe2O3 _

FeO .......

FerotiiMnO... _ . _

MgO..........

CaO .... ...

Na2O. ........

K2O. .........

PjOs..........

Total...

A

75.29

13.41

.22

.99

.32

(1.21)

.07

.23

.88

3.58

4.97

.03

99.99

I

B

75.71

12.53

.20

1.11

.14

(1.14)

.05

.21

.75

1.06

8.21

.02

99.99

A

63.30

14.81

1.03

1.26

6.01

(7. 14)

.31

2.88

5.10

3.28

1.76

.25

99.99

2

B

61.14

14.52

1.09

2.74

5.03

(7.50)

.25

3.83

6.14

2.13

2.92

.21

100.00

A

75.02

12.07

1.47

1.12

f) AA\

RA

1.33

4.30

4.14

99.99

}

B

*rc an

13.69

2.32

.00

o ncN

.20

.97

3.03

4.09

99.99

t

A

80.06

12.10

.12

.56

.30

(

om

.25

2.03

3.42

1.12

.02

99.98

I

B

78.18

12.53

.12

.58

.19

(.71)

.10

.69

3.18

4.41

.02

100.00

,

A

77.70

11.57

.06

.96

.86

(1.72)

.70

.48

4.25

3.41

99.99

5

B

77.28

10.72

.06

1.42

.00

(1.28)

.38

2.60

7.54

100.00

(

A

76.14

13.10

.18

.69

.48

fl-JP

.06

.34

1.22

3.87

3.84

.04

99.96

>

B

76.31

12.46

.17

.97

.17 (1-Si>

.06

.19

.49

3.43

6.65

.04

99.94
COMPARISON, GLASSY AND CRYSTALLINE VOLCANIC ROCKS D21

to descriptions by the collecting geologists, the field relations of pairs 

]. to 5 are indicative of secondary devitrification, as defined at the beginning of this paper. The field relations of pair 6 from the Bullfrog 

Hills, Nevada, are not described, but it is tentatively included in the 

examples of secondary devitrification because it shows similar compositional variations. All the pairs are characterized chemically by 

higher K2O and lower Na2O in the crystallized samples than in the 

glassy samples, essentially the reverse of the relation obtained for exumples of primary crystallization in table 2. The K2O/Na2 0 ratios 

of several crystallized samples, especially IB and 5B, are much higher 

than normal siliceous igneous rocks, a relation that suggests potassium 

metasomatism during devitrification (Simons, 1962, p. 881-882; Terjsaghi, 1948, p. 21-26). The glass of pair 2 seems to be a normal dacite, 

but its crystallized counterpart shows a high K2O/Na2O ratio for rocks 

of its SiO2 content, and perhaps was also subjected to potassium metajsomatism. The compositional variations in pair 4 have been attributed 

to metasomatic addition of potassium during devitrification (Coombs, 

1952, p. 202-203). However, the K2O/Na2O ratio of the glass is 

atypically low, and the compositional changes in this pair possibly 

were due to modification of the glassy phase.

IMPLICATIONS FOR PETROCHEMICAL STUDIES

The data presented in this paper indicate the difficulty of determination of the composition of a volcanic magma. Most porous Tertiary 

glasses from the Nevada Test Site area have undergone secondary 

hydration and are so chemically modified by ground-water leaching 

;hat they are useless for petrochemical studies (figs. 2-5). The paired 

analyses-of tables 1 and 2 show the same trends for vitrophyres, alnhough in a less extreme fashion. This evident leaching of glassy 

rocks suggests that the most likely compositional approximations of 

original magma, at least for nonvolatile constituents, may be rocks 

that have undergone primary crystallization. This inference is supported by examples of consistent compositions among carefully sampled suites, such as the crystalline rocks of the Yucca Mountain and 

Tiva Canyon Members (fig. 3), and by linear variation trends such 

as that characterizing the crystalline rocks of the Topopah Spring 

Member (fig. 2). For some geologic purposes the compositional differences between vitrophyre and crystalline samples would be unimportant, but consideration of figures 2 through 5 indicates that variation 

trends utilizing only the crystallized samples would' differ considerably 

from trends using the glasses or mixed samples.

Perhaps obsidians that have not undergone secondary hydration 

may provide as close an approximation of original magma as samples
D22 CONTRIBUTIONS TO GENERAL GEOLOGY

of primary devitrification. Some interesting analyses of welded vitrophyres of the Walcott Tuff, provided by H. A. Powers, show a striking 

correlation between alkali changes and degree of secondary hydration 

(fig. 8). The least hydrated glasses appear to be least modified, suggesting that much of the modification in alkali composition takes place 

in conjunction with secondary hydration. Similar variations are 

shown by analyses of an obsidian core (Apache tear) having a low 

H2 0 content and its extensively hydrated glassy rind (Longwell, 1963, 

table 2, analyses 307 and 311). The hydrated rind contains 1.06 percent more K2O and 0.77 percent less Na2O (analyses recalculated on 

H2O-free basis); it also contains more A12O3 and less SiO2 than the 

obsidian variations characteristic of the leached glasses described previously. Probably only inconsequential leaching can take place without secondary hydration. Certainly the nonhydrated glassy basalt

5.5

5.0

3.5

3.0

2.5

1.0 2.0 3.0

TOTAL WATER, IN PERCENT

4.0 5.0

FIGURE 8. Alkali variations in vitrophyres of Walcott Tuff, south-central Idaho. Unpublished analyses provided by H. A. Powers, W. J. Carr, and D. E. Trimble, U.S.

Geological Survey.
COMPARISON, GLASSY AND CRYSTALLINE VOLCANIC ROCKS D23

and its crystalline equivalent (table 2, pair 12), erupted in 1949, are 

strikingly similar in composition. However, the only slightly hydrated glass of Pleistocene age (table 2, pair 11) shows in less extreme 

fashion the same trends of compositional modification that characterize 

the extensively hydrated pairs. Hydrated glasses cannot be used for 

precise petrochemical calculations without careful evaluation of their 

reliability.

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D24 CONTRIBUTIONS TO GENERAL GEOLOGY

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O