C-O-H ratios of silicate melt inclusions in basalts from the Galapagos Spreading Center near 95°W: A laser decrepitation mass spectrometry study

Geochrmica PI Cosmochimica Copyright 0 1989 Pergamon

0016.7037/89/$3.00

Acla Vol. 53. pp. 3 145-3 I54 Press Printed in U.S.A.

plc.

+ .CUl

C-O-H ratios of silicate melt inclusions in basalts from the Galapagos Spreading Center near 95”W: A laser decrepitation mass spectrometry study ROBERT N. YONOVER,’JOHN M. SINTON,’ MICHAELA. SOMMER,’and EVERETTK. GIBSON~ ‘Hawaii Institute of Geophysics, University of Hawaii at Manoa, Honolulu, HI 96822, U.S.A. 21243 Bay Area Blvd. #2512, Houston, TX 77058. U.S.A. ‘SN4, NASA/Johnson Space Center, Houston, TX 77058, U.S.A. (Received February 4, 1988; accepted in revisedform September 20. 1989)

Abstract-Volatile ratios (primarily of Hz0 and CO*) in individual silicate melt (glass) inclusions in minerals have been analyzed using laser volatilization and mass spectrometry. A Nd-glass laser was used to produce 50-micrometer diameter pits in silicate melt inclusions. Released volatiles were analyzed directly with a computer-controlled quadrupole mass spectrometer. The detection limits for COZ and Hz0 were on the order of 3 X lo-I4 and 3 X lo-l3 moles, respectively. The reproducibility for C02/H20 was better than f9%. The total range of volatile ratios from vitreous silicate glass inclusions contained in a suite of Galapagos lavas were: 0.0 18 to 1.193 for C02/H20: 0.002 to 0.758 for CO/H,O; 0 to 0.454 for CH4/H20; and 0 to 0.432 for Ar/H20. The mean C02/H20 from the propagating rift (0.245 ? 0.068) silicate glass inclusions is significantly lower than that of the actively failing rift (0.641 + 0.241); this difference probably reflects different degrees of degassing during contrasting magmatic histories for the two regions. Relatively undifferentiated failing rift magmas must have relatively short crustal residence times prior to eruption and, therefore, have not undergone significant degassing of COZ, as would appear to be the case for the more highly fractionated propagating rift magmas. The laser-mass spectrometric system described herein has the ability to act as a point-source probing device that can differentiate between the various volatile sites in minerals and rocks (as well as synthetic materials) on a micrometer scale. INTRODUCTION SILICATEMELTINCLUSIONSare mineral encased volumes of magma (now quenched silicate glass) captured during crystallization (ROEDDER, 1984; SOMMERand SCHRAMM,1983). Melt inclusions can sample the evolving melt as crystallization, effervescence, and assimilation proceed. Providing that no fractionation or alteration has occurred, the chemical composition of silicate melt inclusions can be representative of their host melt at the time of entrapment (ANDERSON, 1974). ROEDDER (1984) reviewed the uncertainties in our understanding of the genesis of silicate melt inclusions, due to leakage, host mineral crystallization, diffusional gradients, and uncertain origin. Despite the potential limitations, silicate melt inclusions have the ability to provide a window into the chemical interactions of magma chambers and the constraints imposed by physical changes during ascent, provided that accurate compositions of the inclusion can be obtained. Microprobe analysis of melt inclusions reveals both a similarity to host magmas and deviations suggestive of magma mixing (ANDERSONand WRIGHT, 1972; ANDERSON,1974; WATSON, 1976; SOMMER, 1977; DONALDSON and BROWN, 1977; HARRIS and ANDERSON, 1983; SOMMER and SCHRAMM, 1983). The volatile contents of magma have been inferred, most commonly, by analyzing glassy pillow rind separates using a variety of analytical methods. FINE and STOLPER(1985) used infrared (IR) spectrometry to determine the volatile content of a selected suite of glass separates. KILLINGLEY and MUENOW (1975), MUENOW et al. (1979) GARCIA et al. (1979), and BYERSet al. (1983, 1985, 1986) used mass spectrometry to analyze CO, CH4, Cl, F, and S, as well as CO1 3145

and H20. These data have been used to evaluate outgassing processes (ANDERSONand WRIGHT, 1972; MATHEZ, 1984) mantle source compositions, and magmatic oxygen fugacity (ARCULUS, 1985; BYERSet al., 1986; CHRISTIEet al., 1986). Determining the volatile content of silicate melt inclusions is made difficult by the low total volatile abundances and the nature of the confining silicate glass medium. Various analytical methods yield absolute concentrations of a few volatile species. Vacuum fusion combined with manometric measurements (SOMMER, 1977; SOMMERand SCHRAMM,1983; HARRIS, 198 1; HARRIS and ANDERSON, 1983) and mass spectrometry (DELANEYet al., 1978) have been used to analyze minerals containing silicate melt inclusions, but measurements of individual melt inclusions are rare in the literature. An exception is the work of HARRIS and ANDERSON (1983) who analyzed individual inclusions in olivine crystals by vacuum pyrolysis and capacitance manometry. The purpose of the present study was to analyze volatile ratios of individual silicate glass inclusions in samples from the Galapagos Spreading Center near 95”W (Fig. 1) and to apply this information to the petrology of an evolving oceanic propagating/failing rift system. In this region, a propagating rift is breaking westward through old lithosphere and preempting a doomed rift, eventually causing the rift to “fail” (cease spreading and eruptive activity) (HEYet al., 1986, 1989; Fig. 1). Lavas that were recovered in this region by DSRV ALVIN and rock dredges (R/V Atlantis ZZcruise #A 112-24) exhibit a wide variation in chemical composition from MgO-rich basalts through highly evolved ferrobasalts. SCHILLINGet al. ( 1982) and VERMAet al. ( 1983) documented gradients in La/

R. N. Yonover et al.

3346

the analysis of individual silicate melt inclusions possible. SOMMER et al. (1985) used laser decrepitation combined with capacitance manometry to determine COa and Ha0 concentrations in fluid inclusions in minerals. Their limit of detection was approximately 5 X 10-‘” moles for CO* and H,O, this translates to aqueous inclusions ap proximately 25 micrometers in diameter. The analysis of aqueous fluid inclusions requires decrepitation or fracturing of the inclusion wall by laser-induced energy, which volatilizes the contained fluid for manometric analysis. In contrast, focused laser heating must quanti~tiveiy release the dissukd volatiles within the silicate meh or glass inclusions (YONOVERet al., 1985) prior to either manometric or mass spectrometric analysis. In this paper, we report advances in the dete~ination of minute quantities (on the order of IO-” moles) of volatile species dissobed in individual silicate melt inclusions using focused laser volatilization and subsequent mass spectrometric analysis. The analytical system is shown schematically in Fig. 2. Doubly polished thick sections of the samples to be analyzed were contained within a high-vacuum stage assembly for microscopic viewing in the optical path of the laser. The volatiles released from the melt inclusions following laser heating were analyzed with a quadrupole mass spectrometer under dynamic high-vacuum conditions.

A COCOS PLATE

NAZCA PLATE

We used a Jar&Ash Laser Micropro~ 45-604 Mark III to heat and decrepitate the inclusions. A 30 nano~cond pulse of hgbt (wavelength 6943 A) is generated by means of a Q-switched laser (an eleo troptic shutter necessary to produce puised laser energy). The energy output is controllable throughout the range of 0.1 to 1.OJoules with an initial pulse peak above 0.4 MW for a duration of l-2 microseconds. The intense beam of light is focused by means of a trinocular metallurgical microscope onto a preselected area of the sample, producing pits about 50 micrometers in diameter (Fig. 3); focusing apertures along the laser path permit the mechanical adjustment of the beam diameter.

Vacuum stage assembly 0.m

m

ol”d* u9B DI.ml

FIG. 1.a) Location map of the Galapagos pro~~tin~failing rift system near 95”W (after HEY et al., 1989). b) Individual sample locales superimposed on the Seabeam bathymetry map with affiliated tectonic provinces (HEY et al., 1989). Solid dots represent ALVIN submersible dive sites. Asterisks represent Atlantis II (cruise #I 12-

24) rock dredge stations.

Sm and St and Nd isotope compositions for ridge lavas west of the Galapagos hotspot, located near 91’W (Fig. l), and proposed that a mantle source boundary exists between the propagating and failing rifts near 9S’W. A study of pillow rind glasses from the Galapagos Spreading Center at 95.5”W showed that failing rift lavas are more reduced than propagating rift lavas at similar extents of fractionation (BYERSet al., 1984; CHRISTIEet al., 1986). In this study we report ratios of C, 0, and H dissolved in silicate glass inclusions and pillow rind glasses, and assess these data in terms of mantle source compositions, oxygen fugacity, kinetic (diffusional) fractionation, and magmatic degassing. ANALYTICAL METHOD The recent advent of focused laser light to decrepitate preselected individual fluid inclusions (MIKHAYLOVA et al., 1973; TSUI and HOLLAND, 1979; ISHKOV and REYF, 1980, KOTRA and GIBSON, 1982) circumvents the problem of collectively analyzing multiple inclusions of varying ~rn~~t~ons and origins, consequently making

The vacuum stage consists of two 3.38 cm Cu-gasket& higb-vacuum stainless steel flanges with 1.91 cm silica-glass windows mounted on opposite sides of the flange. The flanges are attached to a mechanical stage in the optical path of the laser. After the sample is loaded, the stage can be evacuated first with a Hg diffusion pump to a pressure of about 1 X lo-’ torr, then with an ion pump to an ultimate pressure of about 1 X 10-s ton: This pressure, in combination with external heating (2OO”C, produced with heating tape that is applied throughout pump-down and analysis steps), is sufficient to obtain small residual volatile system backgrounds. The pressure of 1 X 10-r torr and the 200°C temperature prohibits significant “pumping” (adsorption and fractionation) of the gases released from the melt inclusions onto the walls of the vacuum stage during laser heating.

CAPACITANCE MANOMETER

VACUUM STAGE

4’

MASS

I OUAORUPOLE SPECTROMETER

FIG. 2. Schematic diagram of the system for laser voiatili~tion and mass spectrometric analysis used in this study.

Volatile content of basalt glass

3147

FIG. 3. SEM photomicrographs of typical laser pits in silicate melt inclusions contained in plagioclase following laser decrepitation. The bar scale in each photo provides the scale. Note the remelted texture at 18000X which alludes to the manner of volatile extraction. Mass spectrometry system Volatiles released from glass inclusions by laser heating were emitted directly into the source (distance about 10 cm) of a Finn&an 3200 Quadrupole Mass Spectrometer (Fig. 2). The high-vacuum system was constructed from 0.64 cm diameter stainless steel tubing assembled with Swagelock fittings. The valves were stainless steel Nupro high-vacuum bellows valves. A MKS Baratron capacitance manometer was built into the high-vacuum gas-handling system and used for calibration purposes in a manner similar to that of SOMMERet al. (I 985). The system is free of volatile organic materials and is outgassed by heating to temperatures of 200°C. Volatile collection “blanks” were determined prior and subsequent to every analysis. The quadrupole mass spectrometer was driven by a computer system permitting continuous mass spectral scanning to be monitored in real time and recorded for subsequent data reduction (typical scanning rate for mass/charge = 2 to 100 was 1 set). Baselines for individual masses were corrected and integrated digitally (Fig. 4). Owing to the different activation energies, fragmentation patterns, and adsorption characteristics, individual volatile species display unique decay profiles that were integrated quantitatively to provide relative ratios of volatile abundances (Fig. 4). The mass spectrometer was calibrated by introducing standard gases of known compositions. Standard relative sensitivity and fragmentation patterns were obtained. Backgrounds were subtracted

graphically prior to integration of the peaks. Hz (m/z 1, 2) was determined to be below the level of detection of the system based on fragmentation patterns for the major hydrogen-bearing species, namely Hz0 and C&. The measured fragmentation species for water are Hz0 (m/z 18), OH (m/z 17), 0 (m/z 16), HI (m/z 2), and H (m/z 1). Methane fragments were determined at CH4 (m/z 15), C (m/z 12), Hz (m/z 2) and H (m/z 1). Given the contributions of hydrogen from water and methane, all the determined free hydrogen was accounted for and assumed to have been derived from these species. We have, therefore, concluded that free hydrogen is not being released from the inclusions. If small amounts of free hydrogen were present during the extraction procedure, then it must have been combined with other hydrogen bearing species (e.g., water, methane, or other trace hydrocarbons) and will not significantly affect the locus of points and the fields generated on C-O-H plots which are dominated by the major components of water, methane, carbon dioxide, and carbon monoxide. Using the m/z values obtained from the mass spectrometer and the fragmentation corrections discussed above, water is determined at m/z 18 (HzO), carbon dioxide at m/z 44 (COz), and methane at m/z 15 (CH,). Carbon monoxide and nitrogen both have parent contributions at m/z 28 and must be determined from daughter peaks. Nitrogen m/z 28 is determined as m/z 14 (N), having determined the fragmentation ratio for (m/z 28 (Nz))/(m/z 14 (N)). Once nitrogen m/z 28 is determined, the remaining m/z 28 is derived from CO

R. N. Yonover et al.

3148 MALACHITE

CALIBRATION

Table 1. (Continued) ___rArLrnc ar~~------------_-----------------------~~~~~~~~~

0

7,

32767

,

-

,

,

, ,

MASS

I

,

,

,

,

,

44

,

,

,

,

,

,

,

,

,

,

,

,

1550-4A

l sw

15

0.486

0.016

0

0.051

61

8.24

1550-4B

L*SW

15

0.367

0.223

0.087

0.084

61

8.24

20

1550-4c

SWD

10

0.669

0.193

0.057

0.153

61

8.24

21

1550-4D

C+swD

15

0.740

0.184

0.034

0.163

61

8.24

22

1552-7A

CN?

30

0.826

0.437

0.314

0.167

62

8.43

23

1552-7B

20

20

0.468

0.269

0.295

0.144

62

8.43

24

1552-7C

C

25

0.585

0.322

0.315

0.239

62

8.43

25

1554-1A

2X

30

0.925

0.075

0.454

0

62

8.49

26

1554-2A

21pc

30

0.588

0.190

0.154

0.085

65

8.87

65

8.87 8.46

,

h

TIME

18 19

bed

27

20

0

60

40 ATOMIC

MASS

00

100

UNITS

(m/z 28) and a major m/z 28 fragment from CO*. Since the fragmentation ratios for CO* are known, they can be subtracted from m/z 28, yielding the true contribution of CO within the samples. The laser volatilization process was calibrated by analyzing the volatile-bearing mineral malachite (Cu2(COr)(OH)z). The analyzed C02/H20 ratio (Table 1) is 0.938 f 0.022, in close agreement with

1. Molar

ratios

volatilization ---MAUCHITE

and

analyzed

MALl

by

released

mass

by

laser

spectrometry.

0.992

0.129

0.119

28 A3-81

C+D

35

1.193

0.433

0.272

0.432

63

29 A3-B2

2D

30

0.723

0.181

0.091

0.363

63

8.46

30 A3-83

1p?

20'

0.938

0.758

0.573

0.265

63

8.46

I

30

0.512

0.257

0

0.162

na

na

lpdv?

40

0.505

0

0.197

0.017

65

9.53

Llp*sw

30

0.556

0.005

0.225

0

64

8.64

0.346

0.068

0.230

0

64

8.64

Ddvsw

20

0.605

0.097

0.210

0.115

62

8.73

36 AlO-1C

Ddvsw

20

0.402

0.169

0.270

0.073

62

8.73

37 AlO-1D

XC*="

35

0.293

0.073

0.141

0.058

62

8.73

38 AlO-1E

DBW

10

0.382

0.128

0.243

0.071

62

8.73

32 A4-1F 33 A5-18 34 AS-lGR-1

GR

35 AlO-lB

-_-DOOMED

R~~~_--_____________--____________________------__ 30

0.608

0.322

0.208

0.099

59

7.95

LC

25

1.001

0.214

0.042

0.135

59

7.95

1555-6C

DX

20

0.791

0.359

0.285

0.109

59

7.95

1555-7A

X*swC

35

0.108

0.002

0.229

0

59

7.94

43

1555-7c

P,LDX

40

0.307

0.103

0.399

0.090

59

7.94

44

1555-7D

XCL

25

0.250

0.138

0.348

0.081

59

7.94

59

7.94

39

1555-6A

40

1555-68

41 42

Xm,SC

45

1555-73

IC

46

1555-7GR-1

20

0.220

0.182

0.418

0.084

0.393

0.127

0.321

0.077

59

7.94

47 Al)-ALTC

sw

15

0.418

0.242

0.137

0.216

"a

"a

48 A13-XA

swc

20

0.556

0.648

0.187

0.102

60

8.12

0.729

0.593

0.226

0.098

60

8.12

GR

C*L~B~TION______________________________--____

lml?.l=sYEeb 1

of volatiles

GR

31 A4-XA

FIG. 4. Plot of representative ion count rates versus time for the malachite calibration runs in which the integrated peak areas of Hz0 compared to CO2 were found to be within a I:1 ratio (+7%). The associated mass spectrum of the volatiles released by laser decrepitation at time = 14 set is shown in the lower portion of the figure. Note the different nature of the volatile release patterns for Hz0 and COZ, emphasizing the importance of quantitative peak integration as opposed to absolute ion counts.

TABLE

1554-2GR-1

0.074

C02/H*O 0.963

xta1

2

MAL2

xta1

3

MAL3

xta1

---PRD**G*TING

49 Al)-XGR-1

GR

'Samples

from

AII/ALVIN

refer

ALVIN

0.932 0.920 RIFT----------------------------------_______

& alQ&zl= ky!xbaizS(

rn) CO2/H20

CO/H20

CH4/H20

4

1544-3A

p/LC*sw

20

0.220

0.028

0.018

0.153

50

6.80

5

1544-3Bl

p/*sw

25

0.201

0.041

0.010

0.169

50

6.80

6

1544-3B2

P/*sw

25

0.240

0.039

0.014

0.126

50

6.80

0.269

0.014

0.072

0.141

50

6.80

0.204

0.090

0.094

0.147

50

6.80

1544-3c

P/C1

8

1544-GR-2

GR

25

9

1540-1Bl

LD

35

0.283

0.247

0.344

0.150

44

5.99

IO

1540-182

dv

45

0.365

0.284

0.248

0.150

44

5.99

11

1540-183

Ldv

50

0.539

0.291

0.446

0.211

44

5.99

12

1545-48

Ddv

35

1.446

0.338

0.003

0.101

61

8.42

13

1545-A4A

x*sw

30

0.347

14

1545-A48

C+sw

25

0.183

15

1545-A4C

X*sw

35

0.137 0.257

0.091

0.290

0.324

0.081

0.040

16

1545-4GR-2

17

1546-48

GR l swc

15

Cruise

numbers,

A112-24: numbers

A3

numbers

1540-1555

through

Al3

lp,

phase;

are

from

dredges.

bpIelt inclusion two

separate

Clear;

D,

average

of

i"ClUsi.0"; host

(all

inclusions: *,

located

0.128

0.023

61

8.42

0.093

0.193

0.013

61

8.42

0.114

0.231

0.015

61

8.42

=based

0.018

61

8.42

separates

0.102

61

8.19

0.055

dive

Ar/Ii,oJ$&&& rock

7

to

on

smple

description

inclusions

dark: two m,

dv, spot

devitrifiad: analyses:

multi-phase

others X,

analyzed

hosted reddish

key: with GR-2,

I,

brow"

glassy

P,

olivine); color;

one

laser

isolated:

inclusion; by

one

L,

zap;

rind

swam

xtal,

with

elongate

plagioclase

SW,

2, c,

crystal

of

crystal:

+,

and:

within.

electron performed

microprobe at

the

analyses

Hawaii

of glassy

Institute

of

rind Geophysics.

3149

Volatile content of basalt glass the stoichiometric proportions ( 1:1) for the dissociation of malachite: Cuz(OH)&09 + 2CuO + CO* + Hz0 (sample numbers 1 through 3 in Table 1 and Fig. 4). Other standards included obsidian glass (volatile-poor) and silicate melt inclusions in quartz and olivine phenocrysts (volatile-rich) whose volatile contents had been previously determined by electron microprobe difference, manometry, and maST s~c~omet~cmeth~s (A~~E~~,~~~~,~MMER and SCHRAMM, 1983). The method employed in this study allows the dete~ination of volatile ratios, but not concentrations; the latter require determination of inclusion volumes prior to laser extraction and analysis. RESULTS Silicate glass inclusions in olivine and plagioclase from the Galapagos Spreading Center near 95.5”W were selected for analysis. Samples were recovered during the 1985 Atlantis IZ/DSRV AL YIN submersible/dred~ng cruise # 112-24 (Fig. 1). Devitrified and vitreous type melt inclusions are common in the Galapagos 95”W lavas examined (Fig. 5). The determined abundance ratios of COz/H20, CO/H20, CHg/HzO, and Ar/H20, and a description of the selected melt inclusions, host minerals, and glassy matrices that were analyzed, are given in Table 1. Sample numbers 4- 17 were from the propagating rift, 18-38 from the failing rift, and 39-49 from the doomed rift. A cluster of silicate glass inclusions from the Galapagos 95.5”W propagating rift (Fig. 5.a) were volatilized and ana-

lyzed separately (Table 1, nos. 4 through 6) to test the reproducibility of the system, assuming that all of the inclusions had similar volatile ratios. The inclusions were all vitreous silicate glass inclusions contained in the core of a plagioclase crystal from a quenched pillow rim of a propagating rift lava. The CO*/H20 ratio results agree within 9% (mean = 0.220 & 0.019). Allowing for minor variability between inclusion compositions, this value represents a maximum for the reproducibility of this technique for C02/H20. The reproducibility of the other volatile ratios is more variable, probably owing to the lower concentrations of these volatiles within the melt inclusions: 19% (0.036 t 0.007) for CO/HzO, 29% (0.014 5 0.004) for CH4/H20, and 15% (0.149 + 0.022) for Ar/HzO (Table I), Sample numbers 4-45 (Table 1) include laser analyses of vitreous silicate melt inclusions, devitrified silicate melt inclusions, and glassy pillow rinds (Table 1). In most cases, devitrified melt inclusions identified petrographically (Fig. 5b) have relatively higher COJHZO ratios (sample nos. 911, 30, 32, and 33; Fig. 8) than their vitreous counterparts (Fig. 5a), suggesting that Hz0 loss upon devitrification may be an important process, as previously noted by ANDERSON (1974) and SOMMER (1977). The total range of values for vitreous silicate melt inclusions from the Galapagos lavas is from 0.108 to 1.193 for COz/H,O: 0.002 to 0.758 for CO/ HzO; 0 to 0.454 for CH,/H,O; and 0 to 0.432 for Ar/HzO. The mean C02/Hz0 for only vitreous silicate melt inclusions from the various Galapagos 95”W tectonic regions are as follows: propagating rift = 0.245 i 0.068 (1 CT);failing rift = 0.641 + 0.241; doomed rift = 0.473 4 0.292 (Fig. 6). The propagating rift samples are restricted to generally low COz/ H,O and CO/H20 values, whereas the failing and doomed rifts have more variable and generally higher COJH20 and CO/H20 ratios. A crude positive correlation between COZ/ Hz0 and CO/H20 exists when all of the inclusion data are considered. CH4/H20 and Ar/HzO ratios do not correlate with one another or with any of the other volatile ratios, nor do they exhibit significant variation with respect to the specific tectonic provinces (e.g., propagating, failing, or doomed rift). ANALYTICAL

UNCERTAINTIES

Laser heating to volatilize glass

FiG. 5. Photomicro~phs of typical silicate melt inclusions in minerals from the Galapagos 95.5”W propa~tin~f~l~ng rift lavas. Top figure represents a group of presumed contemporaneous (?) vifreous silicate melt inclusions. Lower figure displays a silicate melt inclusion that is partially devitr@ed. The bar scale in both photomicrographs is 30 micrometers.

Whether or not the laser energy introduced into the sample medium outgasses the melt inclusion without fractionation poses one of the major uncertainties in this research. Previous work on the gas-release characteristics of various volatile materials by GIBSONet al. f 1974) has shown that CO, requires a higher temperature than H20 for gaseous extraction. Due to the focusing effects of the laser beam, a temperature gradient can be expected at the edge of the laser target. Laser analyses of previously volatilized melt inclusions produce negligible volatile ion counts, implying complete volatile extraction. We have also attempted to test the degree of volatile extraction by comparing laser-released volatile counts of melt inclusions with their respective volumes, assuming the inclusions were spherical. An observed positive correlation between volatile species count rates and melt inclusion volume suggests that the laser energy removes the dissolved volatiles in proportion to the inclusion size.

R. N. Yonover et al.

3150

5

PROPAGATING

RIFT

4

g: 3 2j

O 4

99.85% is silicate glass and other volatile components). Assuming that this value is representative of the glass inclusion volatile content, it translates into a spherical, basaltic glass inclusion of about 25 micrometers in diameter. The typical signal-to-noise ratio for individual inclusions is over an order of magnitude larger than the signal-to-noise ratio obtained by a “blank” laser analysis of the glass stage that supports the sample chip.

8 3 @z 2 g f

I

7

0

-

31

Speciution DOOMED RIFT

CO,/H,O

(MOLE RATIO)

FIG. 6. C02/HZ0 histogram for laser-extracted volatiles from the Galapagos 95”W prop~tin~failing rift system. Solid symbols rep resent v~~eo~s melt inclu~on~ open symbols refer to devitrified melt inclusions; striped symbols represent individu~ glassy rim point analyses.

Dlyerential rates of transmission of gas to mass spectrometer The decay profiles of Hz0 (mass 18) versus CO2 (mass 44) are quite diffe~nt (Fig. 4). The character of the II20 release suggests that Hz0 undergoes a slower transfer from the sample

to the mass spectrometer source and thus requires a longer period of time to reestablish a stable baseline (Fig. 4). This could in part reflect slow pumping of H20. The adsorption of water onto the newly enhanced sample surface area (see Fig. 3) or the extraction line itself (not likely at 2OO’C) could also contribute to the characteristically drawn-out Hz0 peak. We have com~n~ted for these differences by integrating the area under the peak rather than using the peak height alone (Fig. 4). The areas comprising the decay profiles for individual species were integrated from their initial peak pulse to their reestablished baseline point (defined graphically as the first point in which the slope of the successive scans becomes horizontal; Fig. 4). Relative to CO,, Hz0 typically requires approximately 7X more time to reestablish the initial baseline. Sensitivity We were able to determine the maximum sensitivity of the laser system using uqueous (- 100% Hz0 by volume) fluid inclusion standards. Photomicrographs of fluid inclusions with low homogenization temperatures and therefore small vapor bubbles were used to estimate inclusion volumes, assuming sphericity. The aqueous inclusions were analyzed in an identi~ manner to the glass in~lu~ons. Appro~mately 3 X lOPI3 moles of Hz0 can be detected (a maximum signalto-noise ratio of 10 to 1) by our method. Concentrations of other species are typically an order of magnitude lower than HzO, suggesting detection limits for CO* of -3 X lo-l4 moles. IR data on Galapagos propagating rift glassy pillow rind K12-40 by FINE and STOLPER( 1986) indicate that this sample contains approximately 0.15 wt% water (i.e., the remaining

The intense energy (0.4 MW) created by the pulsed laser induces a high temperature on the target area that could cause the volatile species to break down (e.g., COz into C and CO) in proportions greater than typical mass spectrometric cracking patterns. In addition, the analyzed volatile ratios in their species-specific form may not be related to high-temperature chemical equilibria. By plotting the volatile ion count data onto an elemental C-O-H ternary diagram (Fig. 7), we have circumvents any speeiation problems. In addition, our analysis of the malachite standard with results close to the stoichiometric COJH20 ratio of I:1 (within 7%) argues against an important role for species breakdowns. However, it must be noted that the bonding and release of Hz0 and CO* from malachite should occur in stoichiometric proportions and may not be directly comparable to that of complex silicate melts. The malachite analyses mainly test for the effects of pr~u~e~ndent calibration factors and rates of migration. DISCUSSION

A variety of mechanisms could be responsible for the result that propagating rift melt inclusions have significantly lower C02/HZ0 than those of the doomed and failing rifts. These mechanisms can be categorized according to their time of occurrence reIative to inclusion entmpment. Prior to mineral formation and inclusion entrapment, the volatile composition of the melt can vary as a result of shallow level degassing, crystal fractionation, and mantle source ‘(including f0,) composition. During the solidification of the inclusion, kinetic fractionation (diffusion) processes can alter C02/H20 compositions. Subsequent to inclusion formation, post-entrapment crystallization (or alteration) can also affect volatile ratios. Although the laser system permits the analysis of selected glass inclusions and therefore minimizes the effects of postentrapment and kinetic processes, any geologic interpretation of the C02/H20 ratios of glass inclusions requires careful consideration of all of the relevant factors that can alter the C0JH20 content. D@iision/kinetic effects Melt ~omp~itions in small inclusions may be modifi~ by kinetic effects such as chemical diffusion (DELANEY and KARSTEN, 1981; WATSON et al., 1982). WATSON~~ al. (1982) found that CO,

diffuses faster than Hz0 in basaltic magmas and suggested that diffusion-induced fractionation is plausible in rapidly quenched, extremely small inclusions (10 micrometers), as in the case of inclusion growth bands (swarm inclusions) commonly found in plagioclase. ANDERSON

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Volatile content of basalt glass

/

”

”

”

”

0

”

”

”

H2O

”

\

H

FIG. 7. a) C-O-H ternary plot of laser-released volatiles from vitreous silicate melt inclusions from the Galapagos Spreading Center with superimposed Mg#‘sof their respectiveglassyrinds (determined by electron microprobe analysis). Atomic compositions are based on absolute ion counts (corrected for mass spectral cracking patterns) and calculated in a manner similar to HOLLOWAY (1987). Theoretical fO2 isobars from thermodynamic calculations at 1 kb and 1175°C (HOLLOWAY, 1987) are shown withfOz indicated in log 10 units relative to the QFM buffer; i.e., QFM-1 is one order of magnitude lower than QFM. The tectonic provinces correspond to the following symbols: propagating rift = squares; failing rift = circles; doomed rift = triangles. b) C-O-H ternary plot exhibiting the variation between glassy rim analyses and melt inclusions from the same lavas (tie lines connect the data from individual lavas). (1974) and HARRIS and ANDERSON(1983) showed that inclusions larger than about 50 micrometers seem to be representative of unaltered melt with diffusion effects being undetectable or less than about 10% of the average amounts of slowly diffusing components like Al and Si. Although the inclusions analyzed in this study are smaller than 50 micrometers, their compositions may nevertheless be representative of unaltered melts in terms of volatile ratios. Preliminary ion probe determinations of relatively nonvolatile trace element concentrations (Ba, Cr, K, Sr, Ti, Zr, and others) of isolated melt inclusions suggest that dilfusional effects due to varying inclusion size and relative position within the inclusion are minimal (YONOVERand SHIMIZU,unpubl. data). Because most of the analyzed inclusions are larger than 10 micrometers, isolated from other inclusions (not occurring in dense bands or swarms), and located in large olivine crystals, we believe they represent equilibrium assemblages.

Inclusions that occur as swarms (noted as “SW” in Table 1) are typically found in plagioclase crystals. They may have been trapped at periods of accelerated growth along advancing crystal fronts and therefore may represent melts trapped along diffusional gradients near the crystal interface. In general, the swarm inclusions from this study are concentrated in the lower half of the C02/H20 ratio plot (Fig. 8) and have lower ratios than non-swarm inclusions from the same lava. These results apparently corroborate the work by WATSON et al. (1982), with rapid crystal growth trapping melt inclusions with lower COJHzO ratios than the actual melt values. Although the swarm inclusion data may not reflect true magmatic compositions, these data do not affect the overall observed differences in volatile ratios between the failing and propagating rift (see Fig. 8 and Table 1). Also, diffusional effects alone are not likely to produce systematic differences between these two provinces.

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R. N. Yonover et al.

Vit. PRm FRO DRr

3

SW Devit. Q 0 0 0 A A

Ek

w

$ 0

N

5, 0.5 8

MgO (wt.%)

FIG. 8. Molar C02/H20 versus MgO (wt%) for samples analyzed in this and other studies. Results of laser volatilization/mass spectrometry of silicate melt inclusions are shown as squares (propagating rift), circles (failing rift) and triangles (doomed rift), where vitreous = filled symbols;devitrified = open symbols; swarm inclusions = halffilled symbols, and glassypillow rims = asterisks.Error bars represent 2 (r values for data from this study. Other volatile data performed on glass separates are shown in the figure and include: bold line = Galauaaos 95”W PR/FR (BYERSet al.. 1984): stiz&e = IR suectrometry of oceanic ridges/Hawaii (FINE’~~~&oL;;R, 1986);“‘~” stipple = Hawaii samples, Loihi and Kilauea (BYERSet al., 1985; MUENOWet al., 1979; GARCK et al., 1989); oceanridges = Galapagos 85”W/95”W/EPR 2l”N (BYERSet al., 1983, 1984, 1986); Atlantic/ Pacificridges(DELANEYet al., 1978; FINEand STOLPER,1986).

Volatile evolution

In the absence of magma mixing effects, one might predict that inclusions in phenocrysts should record earlier, higher temperature melts than their corresponding glassy rims which represent quenching during submarine eruption. Major element microprobe data are equivocal in this regard, at least in part because post-entrapment crystallization can severely change glass inclusion compositions. Similarly, no systematic relation between glass inclusions and pillow rim glasses emerge from our limited data on corresponding pairs. Of the six glassy rim analyses performed, three were higher in CO*/ HzO, one was lower, and two were about the same as their corresponding melt inclusions (Fig. 7). Considering only nonswarm inclusions, two have higher COJHzO ratios and one has a lower ratio than that of their host glassy rim (Fig. 8). DICK and BRYAN (1978) and MATHEZ (1984) suggested that the oxygen fugacity of an ascending/cooling magma body should increase as a result of crystal fractionation of olivine or by the loss of hydrogen. We do not observe this relationship between inclusions and their host glassy rims in our limited data. D@erences between the propagating and failing rift

Degassing of COZ during magma storage, differential diffusion rates between CO* and H20, and graphite formation on the fractures created by the laser blast are processes that could decrease the C02/H20 ratios. Conversely, accidental analysis of COz-rich filled vesicles and/or alteration carbonate will increase the observed C02/H20 ratios. Many of these

processes are unlikely to produce systematic differences related to tectonic setting, and extraneous materials were intentionally avoided during analysis. Therefore, there appear to be two possible explanations for the contrasting C02/H20 between the propagating and failing rifts: 1) differences in source composition, and 2) differences in magmatic degassing histories. Source composition. The documented differences in COZ/ HZ0 between lavas from the propagating rift and failing rift coincide with differences in major, minor, and trace element composition (SCHILLINGet al., 1982; YONOVER,1989; YONOVERet al., unpubl. data). The low COJHzO from this study for propagating rift samples compared to failing rift samples (Fig. 6) also holds when comparing only samples with Mg#‘s greater than 60 (Fig. 8). SCHILLINGet al. (1982) and VERMA et al. (1983) documented gradients in trace element and isotope compositions for ridge lavas west of the Galapagos hotspot, located near 9l”W (Fig. 1) and proposed that a mantle source boundary exists between the propagating and failing rifts near 95”W. The propagating rift is slightly closer to the hotspot than the failing rift; hence, the low C02/H20 of propagating rift glasses (relative to those of the failing rift) might be attributed to a hotspot signature. SCHILLINGet al. (1983) proposed that many hotspots are also “wetspots” (H*O-rich), and hence low COdH20 values of propagating rift inclusions might signify an increasingly H,O-rich mantle source. Relative to the failing rift, the propagating rift field is displaced toward the field for Hawaii data in Fig. 8. However, based on a limited sample suite, BYERS et al. (1984) showed that Hz0 contents for lavas from the 95”W propagating and failing rifts are similar. Furthermore, the possibility of a significant difference in hotspot geochemical signature between Galapagos 95”W propagating and failing rift lavas is unlikely due to the large distance between the Galapagos hotspot and the study area (about 500 kilometers) relative to the slight difference in distance between the propagating and failing rifts. In addition, separate mantle sources are not required to account for the variations in (nonvolatile) trace element composition between the propagating and failing rifts (YONOVER et al., unpubl. data). Magmatic degassing. MATHEZ (1984), STOLPER and HOLLOWAY (1988), and EABY-DIXON et al. (1988) showed that most submarine lavas are oversaturated with respect to CO2 and suggested that CO2 content of submarine lavas varies inversely with their crustal residence time because of shallow level degassing effects. Therefore, magmas that are readily transported to the surface should have higher COZ contents and be more indicative of primary magmatic volatile conditions. Failing rift lavas, in contrast to the propagating rift lavas, are relatively unfmctionated and probably did not reside for long in shallow magma chambers or conduits prior to eruption (CHRISTIE and SINTON, 198 1; YONOVERet al., 1986; HEY et al., 1989). This may account for the higher C02/H20 volatile ratios for inclusions from the failing rift lavas compared to propagating rift lavas. The generally more differentiated propagating rift lavas, being derived from less thermally mature magma chambers or conduits, apparently had more opportunity to degas as they ascended and resided in the crust, resulting in lower COz/H20 ratios relative to the failing rift lavas. Although propagating rift magmas have ap-

3153

Volatile content of basalt glass patently undergone significant degassing corresponding to advanced crustal residence time, we do not observe a correlation between COZ/HZO and MgO that may be indicative of systematic CO, degassing with progressive fm~onation. However, due to relatively high vapor diffusion rates and the much larger density differences between vapor and melts than between crystals and melts, the time scale of degassing is much more rapid relative to crystal fractionation and therefore may occur with no noticeable change in MgO. Recent studies have shown that the oxidation state of midocean ridge basalts (MORB) is si~ifi~ntly more reduced (logfOl two orders of magnitude below QFM) than previously thought (CHRISTIE et al., 1986). Because the oxygen fugacity of magma may be affected by shallow level degassing processes (e.g., MATHEZ, 1984) this variability is difficult to interpret in terms of mantle or magma chamber processes. By recasting the glass inclusion volatile ion count data into elemental components (i.e., carbon, oxygen, and hydrogen), inferences regarding the oxidation state of the system can be made independently of any possible problems related to speciation (see above). In addition, volatiles trapped in the glass inclusions should provide a record of magmatic compositions during mineral formation, but prior to any degassing associated with the extrusive event. Theoretical thermodynamic fugacity values at 1 kb and 1170°C (after HOLLOWAY,1987) are superimposed on the C-O-H ternary diagram (Fig. 7a) to help ascertain the oxidation states of the extracted volatiles. Except for a few outlying values, extracted volatiles from the failing rift appear to be more reduced (fO2 - 2 to 4 orders of magnitude below QFM) than those from the propagating rift, which cluster at values approaching QFM (f0, - 0 to 1 log units difference), even when considering only propagating rift lavas that are relatively undifferentiated (i.e., Mg#‘s greater than 60; Fig. 7a). BYERSet al. (1984) determined that glasses from the propagating rift were higher in ferric/ferrous mtios than failing rift glasses at similar MgO values. CIIRISTIB et al. ( 1986) found the oxidation state of the failing rift lavas (glassy rinds) to be approximately 3 orders of magnitude below QFM compared to nearly 1 order of magnitude below QFM for the propagating rift lavas. In addition, Fe3+ compositions of spinels from the propagating rift are significantly higher than spinels hosted by failing rift lavas at similar Mg##‘s (YONOVER,1989; YONOVERet al., unpubl. data). Therefore, failing rift lavas have apparently been derived from a more reduced environment than the propa~ting rift lavas. It ap nears that the oxygen fugacity of the Galapagos lavas is more or less inversely related to C0JH20, regardless of whether this ratio is controlled by source differences or shallow level degassing processes. CONCLUSIONS 1) Glass inclusions in minerals provide a unique sample type for ascertaining the volatile composition of magmas prior to submarine extrusion. 2) Glass inclusions in Galapagos 95”W failing rift lavas are higher and more variable in COJH20 (0.641 + 0.241) relative to inclusions in propagating rift lavas (0.245 f 0.068). 3) Magmatic degassing of CO2 in ascending propagating magmas can account for their lower COZ/HZO relative to

failing rift lavas. The higher COz/H,O composition of inclusions from failing rift lavas correlates with limited crustal residence prior to eruption and therefore the inclusions are more representative of their parental magmatic values. 4) Failing rift lavas with high C02/Hz0 also have low Fe)+/ Fezi and therefore low f 4. 5) Although source differences between the propagating and failing rift cannot be precluded, variations in shallow magmatic histories may be sufficient to explain the contrasting COz/HzO ratios. ~cknowled~en~s-This research was funded by NASA Training Grant #NGT- 12-004-80 1 and NSF grants OCE 83- 16547 and GCE 87-01118. We are grateful to J. Eaby-Dixon, P. Michael, and W. Taylor for careful criticisms of various drafts of this paper which resulted in substantial imorovements. A. T. Anderson, M. Garcia, J. Mahoney, and D. Muenow also contributed to the revision of this manuscript. Harold Thompson of Harvard University was instrumental in sample preparation. L. Schramm operated the SEM at the NASA-J% facility. The samples from this study could not have been recovered without the expertise of the captain and crew of the Atlantis ZfIDSRV ALVZH, as well as the rest of the Galapagos propagating rift team. This is Hawaii Institute of Geophysics contribution #2198. Editorial handling: F. A. Frey

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O’HARAM. J. (1977) Geochemical evolution during fractional crystallisation of a periodically refilled magma chamber. Nature 266, 503-507. ROEDDERE. (1984) Fluid Inclusions; Reviews in Mineralogy 12. Mineralogical Society of America. SCHILLINGJ.-G., KINGSLEYR. H., and DEVINEJ. D. (1982) Galapagos Hot Spot-Spreading Center System 1. Spatial petrological and geochemical variations (83”W-101”W). J. Geophys. Res. 87, 5593-56 10. SCHILLINGJ.-G., ZAJACM., EVANSR., JOHNSTONT., WHITE W., DEVINEJ. D., and KINGSLEYR. ( 1983) Petroloaical and aeochemical variations along the Mid-Atlantickidge frim 29”N-to 73”N. Amer. J. Sci. 283, 510-586. SOMMERM. A. (1977) Volatiles HzO, COz, and CO in silicate melt inclusions in quartz phenocrysts from the rhyolitic Bandelier airfall and ashflow tuff, New Mexico. J. Geol. 85,423-432. SOMMERM. A. and SCHRAMML. S. (1983) An analysis of the water concentrations in silicate melt inclusions in quartz phenocrysts from the Bandelier Tuff, Jemez Mountains, New Mexico. Bull. Volcanol. 46, 299-320. SOMMERM. A., YONOVERR. N., BOURCIERW., and GIBSONE. K. (1985) Determination of Hz0 and CO2 concentrations in fluid inclusions in minerals using laser decrepitation and capacitance manometer analysis. Anal. Chem. 57, 449-453. STOLPERE. and HOLLOWAY J. R. (1988) Experimental determination of the solubility of carbon dioxide in molten basalt at low pressure. Earth Planet. Sci. Lett. 87, 397-408. TSUIT. F. and HOLLANDH. D. (1979) The analysis of fluid inclusions by laser microprobe. Econ. Geol. 74, 1647-1653. VERMAS. P., SCHILLINGJ.-G., and WAGGONERD. G. (1983) Neodymium isotopic evidence for Galapagos hotspot-spreading centre system evolution. Nature 306,654-657. WATSONE. B. (1976) Glass inclusions as samples of early magmatic liquid: determinative method and application to a South Atlantic basalt. J. Volcanol. Geotherm. Res. 1, 73-84. WATSONE. B., SNEERINGERM. A., and Ross A. (1982) Diffusion of dissolved carbonate in magmas: experimental results and applications. Earth Planet. Sci. Lett. 61. 346-358. Y~NOVERR. N. (1989) Petrological effects of rift failure at the Galapagos spreading center near 95S”W including analyses of glass inclusions by laser mass spectrometry and ion microprobe. Ph.D. dissertation, University of Hawaii, Honolulu. YONOVERR. N., SOMMERM. A., and GIBSONE. K. (1985) Laser decrepitation and mass spectrometric volatile analysis of silicate melt inclusions (abstr.) Eos 66, 1108. YONOVERR. N., SINTONJ. M., and CHRISTIED. M. (1986) ALVIN investigations of the petrological effects of rift failure, Galapagos Spreading Center near 95.5”W. (abstr.) Eos 67, 1185.