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Populations of clays formed by alteration of subglacial hyaloclastites from Iceland
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd I N T E R N A T I O N A L SYMPOSIUM, July, 2-8, 1990, Alx en Provence, France.
261
P O P U L A T I O N S OF CLAYS F O R M E D BY A L T E R A T I O N OF SUBGLACIAL HYALOCLASTITES FROM ICELAND
CROVISIER J.L. and DAUX V. Centre de G6ochimie de la Surface (CNRS) 1, rue Blessig 67084 Strasbourg Cedex, France.
Subglacial hyaloclastites from Iceland with age ranging from 2,000 y to 2.2 My were sampled in Husafell and Hengill regions, located about 100 km NE and 40 km E of Reykjavik, respectively. No trace of hydrothermal activity could be observed in the sampling areas (SAEMUNDSSON, 1967; SAEMUNDSSON and NOLL, 1974; CROVISIER, 1989). The samples are essentially composed of angular, vesicular grains of pale yellow tholeiitic glass whose size ranges from about 10 ~tm to a few mm. The glass contains primary minerals such as olivine, plagioclase and clinopyroxene which do not appear to have been affected by the alteration processes. The glass granules are surrounded by an alteration crust, i.e. palagonite, and cemented with submicroscopic clayey material filling up the intergranular pore spaces. Zeolites are also sometimes present in the cement. None of the identified secondary products indicate hydrothermal conditions during alteration. Both palagonites and intergranular clayey cement are made up of more or less crystallized particles exhibiting a smectite morphology. The latter is generally better crystallized. Electron diffraction indicates that some of the particles with a smectite morphology are actually amorphous. Palagonite and intergranular clayey material were analyzed by flame and arc emission spectrometry after a carefull separation. It is shown that the chemical composition of palagonite is almost identical to that of the clayey material filling the intergranular spaces of the rock. The clayey material is made up of two single particles populations : the f'n'st one is Si, Mg and Ca-rich with a smectite structure while the second one is amorphous, Fe, Ti and Al-rich with a smectite
morphology (Fig. 1). Both types of particles also co-exist in palagonite crusts but it was not possible to check in ultrathin sections wether the Mg-rich particles or aggregates are also cristallized as they are in the intergranular material. The explaination for these two coexisting types of particules is difficult : they either correspond to different simultaneous but local equilibria or to successive equilibria, or they represent actual parageneses, i.e., both types of particles coexist at equilibrium with the same aqueous solution. As a first attempt at answering this question, the case of a mineral paragenesis has been examined. Using the ideal solid solution model proposed by FRITZ (1981), TARDY and FRITZ (1981) for clay minerals, it is shown that two populations of clay with the following composition : 1 2
(Si3.90Alo.lo) (Mg3.01Feo.oo)Olo (OH)2 Na0.00Ca0.04 (Si3.0oAll.00) (Mg0.09Fe2.00)O10 (OH)2 Na0.01Ca0.40
can coexist at equilibrium with each other during alteration of basaltic glass by meteoric waters. However the actual existence of the paragenesis has not been demonstrated since the chemical compositions of the various particles forming the mixture are not exactly known. Nevertheless the result of the calculation makes it clear that concepts of metastability or local equilibria are not necessary to explain why two particle populations with such a difference in chemical composition coexist in the hyaloclastite samples from Iceland. The chemical composition of the clayey fraction has been followed from 2000 y to 2.2 My. It has been shown that the ratio :
262
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en P r o v e n c e , France.
2nd
j
..../ ,/
~6o s,o2 °
2O
/// '
2~
,~
'
,
~;
~,
e83 e22
,
,
,
AMORPHOUSPARTICLES
/ 3o
/
._]~;0, <
Fe203
SiO2/MgO
J
W <
o21 n,"
//
,2
MgO
._J
,5
2o
A
1o
~'~
7 <
o15
/
023 024 cao //
/
/
/ = , 2= 4i 6L 8i 10 12
Figure 1 : Comparison between the chemical composition of crystallized and amorphous particles making up the intergranularclayeycementof the hyaloclastites. SiO2/MgO in the glass R= SiO2/MgO in the alteration products is close to unity for those of the Icelandic samples which contained zeolites, i.e., samples 86/10, 61 and 75 (see Fig. 2). These samples can be considered as the most evolved ones. The following facts one worth mentionning : 1- The youngest sample 86/83 which is 2000 y old, exclusively contains amorphous secondary products and plots the farthest away from the ideal straight line. It can be considered as the least evolved sample. 2- Samples 86/23, 24 and 26, about 90.000 y old, contain intergranular material displaying incipient cristallization, namely cristallized particles observed with the TEM, and fall not very far from the ideal
GLASS
A
Figure 2 : SiO2/MgO ratio in the glass compared to the same ratio in the intergranular material. line for which the SiO2/lVlgO ratio is similar in both alteration material and initial glass. 3- Sample 86/21, also 90.000 y old, contains only amorphous secondary material and its R ratio is lower than that of other contemporaneous samples containing crystallized secondary products. 4- Sample 86/22, again 90.000 y old, and containing amorphous secondary products, has the smallest specific area of that group of samples (CROVISIER, 1989). It fall near the youngest sample 86/83. Hence, the R ratio appears to be a good indicator of the reaction rate until zeolites start cristallizing. This ratio has been called T.R.Z. by CROVISIER (1989), i.e., "Taux de R6action jusqu' la formation de Z6olites" or reaction rate up to the formation of zeolites. The T.R.Z. were calculated for other Icelandic samples considered as resulting from subglacial eruptions. Samples containing
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd I N T E R N A T I O N A L SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France.
zeolites have been studied by FURNES (1984) and their T.R.Z. are the highest : 0.93. The T.R.Z. o f all the other samples either studied by FURNES (1984) or by JERCINOVIC and E W I N G (1987) are smaller than 0.56 except one sample from Mosfell which has a T.R.Z. o f 0.90; they do not contain zeolites. A m o n g t h r e e R a u d a f e l l samples studied by J E R C I N O V I C and E W I N G (1987) the highest T.R.Z. (0.56) is found in an ash-tuff which has the largest specific area w h e r e a s the o t h e r two contemporaneous samples having a T.R.Z. o f 0.10 correspond to a hyaloclastite and a pillow lava rim. T h e T.R.Z. c o n c e p t cannot be applied to submarine conditiong because it is based on the hypothesis that Si and M g are either lost to the solutions during alteration or are trapped in the secondary products. Any exogeneous contribution is by definition excluded. It is a clear evidence o f the strong limitations o f the T.R.Z. since most o f the natural systems are more complicated than the rather recent subglacial hyaloclastites from Iceland that we have studied. In other hyaloclastite occurences some chemical elements could be brought in the water/rock system for instance trough convection or advection.
263
REFERENCES
CROVISIER J.L. (1989) - Dissolution des verres basaltiques dans reau de mer et dans reau douce. Essai de mod61isation. Th~se de 1%lniv.Louis Pasteur, Strasbourg, 253 p. FRITZ B. (1981) - Etude thermodynamique et mod61isation des r6actions hydrothermales et diag6n6tiques. Sci. G6ol. M6m., Univ. Louis Pasteur, Strasbourg, 65, 197p. FURNES H. (1984) - Chemical changes during progressive subaerial palagonitization of a subglacial olivine tholeiite hyaloclastite : a microprobe study. Chem. Geol., 43, p. 271-285. JERCINOVIC M.J. and EWING R.C. (1987) - Basaltic glass from Iceland and the deep sea : Natural analogues to borosilicate nuclear waste-form glass. JSS Project Tech. Rep. 88-01, SKB Ed., Box 5864 S-102 48 Stockholm, 221p. SAEMLrNDSSON K. (1967) - Vulkanismus und Tektonik des Hengil-Gebietes in Sfidwest-Island. Acta Naturralia Islandica, Reykjavik, 2, 105p. SAEMUNDSSON K. and NOLL H. (1974) - K/Ar Ages of Rocks from Husafell, Western Iceland, and the Development of the Husafell Central Volcano. JOkull, 24, pp.40-58. TARDY Y. and FRITZ B. (1981) - An ideal solid solution model for calculating solubility of clay minerals. Clay Mineral, 16, pp.361-373.
261
P O P U L A T I O N S OF CLAYS F O R M E D BY A L T E R A T I O N OF SUBGLACIAL HYALOCLASTITES FROM ICELAND
CROVISIER J.L. and DAUX V. Centre de G6ochimie de la Surface (CNRS) 1, rue Blessig 67084 Strasbourg Cedex, France.
Subglacial hyaloclastites from Iceland with age ranging from 2,000 y to 2.2 My were sampled in Husafell and Hengill regions, located about 100 km NE and 40 km E of Reykjavik, respectively. No trace of hydrothermal activity could be observed in the sampling areas (SAEMUNDSSON, 1967; SAEMUNDSSON and NOLL, 1974; CROVISIER, 1989). The samples are essentially composed of angular, vesicular grains of pale yellow tholeiitic glass whose size ranges from about 10 ~tm to a few mm. The glass contains primary minerals such as olivine, plagioclase and clinopyroxene which do not appear to have been affected by the alteration processes. The glass granules are surrounded by an alteration crust, i.e. palagonite, and cemented with submicroscopic clayey material filling up the intergranular pore spaces. Zeolites are also sometimes present in the cement. None of the identified secondary products indicate hydrothermal conditions during alteration. Both palagonites and intergranular clayey cement are made up of more or less crystallized particles exhibiting a smectite morphology. The latter is generally better crystallized. Electron diffraction indicates that some of the particles with a smectite morphology are actually amorphous. Palagonite and intergranular clayey material were analyzed by flame and arc emission spectrometry after a carefull separation. It is shown that the chemical composition of palagonite is almost identical to that of the clayey material filling the intergranular spaces of the rock. The clayey material is made up of two single particles populations : the f'n'st one is Si, Mg and Ca-rich with a smectite structure while the second one is amorphous, Fe, Ti and Al-rich with a smectite
morphology (Fig. 1). Both types of particles also co-exist in palagonite crusts but it was not possible to check in ultrathin sections wether the Mg-rich particles or aggregates are also cristallized as they are in the intergranular material. The explaination for these two coexisting types of particules is difficult : they either correspond to different simultaneous but local equilibria or to successive equilibria, or they represent actual parageneses, i.e., both types of particles coexist at equilibrium with the same aqueous solution. As a first attempt at answering this question, the case of a mineral paragenesis has been examined. Using the ideal solid solution model proposed by FRITZ (1981), TARDY and FRITZ (1981) for clay minerals, it is shown that two populations of clay with the following composition : 1 2
(Si3.90Alo.lo) (Mg3.01Feo.oo)Olo (OH)2 Na0.00Ca0.04 (Si3.0oAll.00) (Mg0.09Fe2.00)O10 (OH)2 Na0.01Ca0.40
can coexist at equilibrium with each other during alteration of basaltic glass by meteoric waters. However the actual existence of the paragenesis has not been demonstrated since the chemical compositions of the various particles forming the mixture are not exactly known. Nevertheless the result of the calculation makes it clear that concepts of metastability or local equilibria are not necessary to explain why two particle populations with such a difference in chemical composition coexist in the hyaloclastite samples from Iceland. The chemical composition of the clayey fraction has been followed from 2000 y to 2.2 My. It has been shown that the ratio :
262
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en P r o v e n c e , France.
2nd
j
..../ ,/
~6o s,o2 °
2O
/// '
2~
,~
'
,
~;
~,
e83 e22
,
,
,
AMORPHOUSPARTICLES
/ 3o
/
._]~;0, <
Fe203
SiO2/MgO
J
W <
o21 n,"
//
,2
MgO
._J
,5
2o
A
1o
~'~
7 <
o15
/
023 024 cao //
/
/
/ = , 2= 4i 6L 8i 10 12
Figure 1 : Comparison between the chemical composition of crystallized and amorphous particles making up the intergranularclayeycementof the hyaloclastites. SiO2/MgO in the glass R= SiO2/MgO in the alteration products is close to unity for those of the Icelandic samples which contained zeolites, i.e., samples 86/10, 61 and 75 (see Fig. 2). These samples can be considered as the most evolved ones. The following facts one worth mentionning : 1- The youngest sample 86/83 which is 2000 y old, exclusively contains amorphous secondary products and plots the farthest away from the ideal straight line. It can be considered as the least evolved sample. 2- Samples 86/23, 24 and 26, about 90.000 y old, contain intergranular material displaying incipient cristallization, namely cristallized particles observed with the TEM, and fall not very far from the ideal
GLASS
A
Figure 2 : SiO2/MgO ratio in the glass compared to the same ratio in the intergranular material. line for which the SiO2/lVlgO ratio is similar in both alteration material and initial glass. 3- Sample 86/21, also 90.000 y old, contains only amorphous secondary material and its R ratio is lower than that of other contemporaneous samples containing crystallized secondary products. 4- Sample 86/22, again 90.000 y old, and containing amorphous secondary products, has the smallest specific area of that group of samples (CROVISIER, 1989). It fall near the youngest sample 86/83. Hence, the R ratio appears to be a good indicator of the reaction rate until zeolites start cristallizing. This ratio has been called T.R.Z. by CROVISIER (1989), i.e., "Taux de R6action jusqu' la formation de Z6olites" or reaction rate up to the formation of zeolites. The T.R.Z. were calculated for other Icelandic samples considered as resulting from subglacial eruptions. Samples containing
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd I N T E R N A T I O N A L SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France.
zeolites have been studied by FURNES (1984) and their T.R.Z. are the highest : 0.93. The T.R.Z. o f all the other samples either studied by FURNES (1984) or by JERCINOVIC and E W I N G (1987) are smaller than 0.56 except one sample from Mosfell which has a T.R.Z. o f 0.90; they do not contain zeolites. A m o n g t h r e e R a u d a f e l l samples studied by J E R C I N O V I C and E W I N G (1987) the highest T.R.Z. (0.56) is found in an ash-tuff which has the largest specific area w h e r e a s the o t h e r two contemporaneous samples having a T.R.Z. o f 0.10 correspond to a hyaloclastite and a pillow lava rim. T h e T.R.Z. c o n c e p t cannot be applied to submarine conditiong because it is based on the hypothesis that Si and M g are either lost to the solutions during alteration or are trapped in the secondary products. Any exogeneous contribution is by definition excluded. It is a clear evidence o f the strong limitations o f the T.R.Z. since most o f the natural systems are more complicated than the rather recent subglacial hyaloclastites from Iceland that we have studied. In other hyaloclastite occurences some chemical elements could be brought in the water/rock system for instance trough convection or advection.
263
REFERENCES
CROVISIER J.L. (1989) - Dissolution des verres basaltiques dans reau de mer et dans reau douce. Essai de mod61isation. Th~se de 1%lniv.Louis Pasteur, Strasbourg, 253 p. FRITZ B. (1981) - Etude thermodynamique et mod61isation des r6actions hydrothermales et diag6n6tiques. Sci. G6ol. M6m., Univ. Louis Pasteur, Strasbourg, 65, 197p. FURNES H. (1984) - Chemical changes during progressive subaerial palagonitization of a subglacial olivine tholeiite hyaloclastite : a microprobe study. Chem. Geol., 43, p. 271-285. JERCINOVIC M.J. and EWING R.C. (1987) - Basaltic glass from Iceland and the deep sea : Natural analogues to borosilicate nuclear waste-form glass. JSS Project Tech. Rep. 88-01, SKB Ed., Box 5864 S-102 48 Stockholm, 221p. SAEMLrNDSSON K. (1967) - Vulkanismus und Tektonik des Hengil-Gebietes in Sfidwest-Island. Acta Naturralia Islandica, Reykjavik, 2, 105p. SAEMUNDSSON K. and NOLL H. (1974) - K/Ar Ages of Rocks from Husafell, Western Iceland, and the Development of the Husafell Central Volcano. JOkull, 24, pp.40-58. TARDY Y. and FRITZ B. (1981) - An ideal solid solution model for calculating solubility of clay minerals. Clay Mineral, 16, pp.361-373.