Reprinted from

G E O D E W

Geoderma 86 (1998) 99-122

Weathering and soil forming processes under semi-arid conditions in two Mexican volcanic ash

.- soils

Didier Dubroeucq a, * , Daniel Geissert b, Paul Quantin a

QRSTQM, 32 avenue H. Varagnat, 93143 Bondy Cedex, France Instituto de Ecologia A.C., apartado postal 63, 91000 Xalapa, Veracruz, Mexico b

Received 14 May 1996; accepted 5 March 1998

GEODERMA A N INTERNATIONAL JOURNAL OF SOIL SCIENCE

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A. Ruellan, Montpellier M. Schnitzer, Ottawa, Ont. U. Schwertmann, Freising N. Senesi, Bari S. Shoji, Sendai G. Sposito, Berkeley, Calif. K. Stahr, Stuttgart A. Stein, Wageningen G. Stoops, Ghent L. Stroosnijder, Wageningen G.C. Topp, Ottawa, Ont. N. van Breemen, Wageningen E. van Ranst, Ghent W.H. van Riemsdijk, Wageningen A. van Wambeke, Ithaca, N.Y. Gy. Varallyay, Budapest M.J. Vepraskas, Raleigh, N.C. H. Vereecken, Jülich G.J. Wall, Guelph A.W. Warrick, Tucson, Ark. D.H. Yaalon, Jerusalem

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GEODERMJ ELSEVIER Geoderma 86 (1998) 99-122

Weathering and soil forming processes under serni-arid conditions in two Mexican volcanic ash

soils

Didier Dubroeucq ai * , Daniel Geissert b, Paul Quantin I a

a ORSTOM, 32 avenue H. Varagnat, 93143 Bondy Cedex, France Instituto de Ecologia A.C., apartado postal 63, 91000 Xalapa, Veracruz, Mexico

Received 14 May 1996; accepted 5 March 1998

Abstract

Weathering and neoformation of allophane, imogolite and halloysite in volcanic ash soils have been studied extensively in humid climates but little data are available on these soils in arid and semi-arid conditions. Weathering and soil formation under semi-arid conditions on acid pyroclas- tics have been investigated in two profiles, both at the micro-site scale and on bulk samples. X R microdiffractions and SEM-EDX microanalysis were performed on soil thin sections. Chemical, particle-size, XRD and mineral identification analysis were performed on conventional soil samples. Results show the interplay of various sub-processes: (a) intense division of the coarse minerals into small fragments 50-200 p m in size; (b) diagenesis of noncrystalline products in the zone of contact with the parent minerals; (c) transformation of noncrystalline minerals into halloysite in the compact soil microstructures and preservation of the noncrystalline minerals in the topsoil; (d) desiccation and condensation of the crystalline and noncrystalline minerals into microaggregates in the topsoil. The results of these interacting weathering processes are silty-loam soils with no cohesion and high susceptibility to wind erosion. Differences appearing between different analytical methods at different sampling scales need special precautions in explaining the results. O 1998 Elsevier Science B.V. All rights reserved.

Keywords: weathering; soil formation; volcanic ash; Andosols; semi-arid environment

1. Introduction

Volcanic ash is a unique parent material for soils. It is composed of non-welded glass particles &d fine-sized minerals which are more susceptible to

Fonds Documentaire ORSTOM * Corresponding author. E-mail: dubroeucq@bon@my.fr 4 0 EX : 5

0016-7061/98/$ - see front matter O 1998 Elsevier Science B.V. All rights reserved. PII: SOO 16-706 1 (98)00033-O

1 O0 D. Ditbroeucq et nl./Geodemza 86 (1998) 99-122

weathering than minerals found in crystalline and sedimentary rocks. The coloured glass of basaltic andesite composition, richer in cations than non- coloured glass of rhyolitic composition, is the most rapidly altered (Kobayashi et al., 1976; Shoji et al., 1993b). The fine particle size and the predominance of glass favour preferential formation of noncrystalline products, the most common includes allophane, imogolite, hisingerite and ferrihydrite, and a specific type of clay mineral, halloysite. Two causes are given to this process. The rapid weathering releases over-saturated soil solutions which precipitate into noncrys- talline hygroscopic phases (Shoji et al., 1993a). The second cause is the lack of layered minerals such as mica and chlorite in the parent ash that may serve as template for further crystallization. This probably impedes the formation of secondary layer silicates (Dahlgren, 1994) and could explain that, in Andisols, silicates are mostly in tubular and spherical structures rather than layered structures.

The weathering environment's influence, which regulates Al and Si activities in the soil solution, is determinant on the formation of either noncrystalline components or crystallized clay minerals, or both (Ugolini and Dahlgren, 1991). The effect of climatic conditions on mineral formation in Andisols has been largely studied in different regions of the world and particularly in Japan (Aomine and Wada, 1962; Saigusa et al., 1978; Shoji et al., 1993b), in New Zealand (Parfitt and Wilson, 1985; Parfitt and Kimble, 19891, in Italy (Quantin et al., 1985), on the islands of Martinique (Quantin et al., 1991) and Hawaii (Chadwick et al., 1994). These soils were studied under precipitations > 1000 mm, where andic characteristics have developed in situ and cannot be disputed. In these conditions the age of the soil and the presence or absence of a marked dry season are the most determinant factors on the mineral evolution of the Andisols. With the ageing of the soil and with climatic changes from cold plus' wet to warmer plus dryer climates, three colloidal assemblages tend to prevail in soils deriving from volcanic ashes (Shoji et al., 1993b; Mizota and Van Reeuwijk, 1989; Dahlgren, 1994): (a) Al-humus complexes together with hydroxy-Al polymers and/or hydroxy-Al interlayered 2: 1 layer silicates, (b) allophane and imogolite and/or ferrihydrite, (c) halloysite.

In comparison with Andisols from humid regions, very little information is available on volcanic soils of semi-arid climates. Carbonate accumulation, formation of 2:l layered clay minerals and surficial imports of Ca, Mg and quartz from aeolian dust and seaspray are active processes on volcanic ash soils from Lanzarote, Canary Islands (Fernandez Caldas et al., 1981; Jahn et al., 1987; Jahn and Stahr, 1994). In Kenya, siliceous iron oxides such as hisingerite are the predominant amorphous component formed in volcanic ash soils under ustic moisture regime and isohyperthermic conditions (Wakatsuki and Wiele- maker, 1985). In the semi-arid interandine plateaux of Central America, the weathering products from volcanic ashes are quite different from those in humid climates and soil formation in this environment is not yet entirely understood.

D. Dubroeihcq et al./Geodenna 86 (1998) 99-122 101

Most of these soils are compact in the middle part of the profile and, in the upper horizons, they crumble into very fine particles consisting of halloysite and smectite clay minerals mixed with glass fragments (Colmet-Daage et al., 1969). In the Mexican Altiplano, these soils are open to wind erosion and constitute the most important source of the aeolian dust which obscures the sky during the dry season.

We have studied the weathering processes and the pedogenic products of two different ash-flow deposits under semi-arid conditions in the Mexican Altiplano. One is of rhyolitic composition and the other of dacitic composition. The aims of this study are: (1) to establish the morphology, chemistry and mineralogy of the weathering products, (2) to establish their genesis, and (3) to propose explanations for the very uncohesive and fine structure of these soils.

2. Materials and methods

2.1. Location and general irzfomation

The study area belongs geologically to the complex andesitic strato-volcano Cofre de Perote (4250 m) in the eastern Transmexican Volcanic Belt (Fig. 1). The climate of the western leeside of the mountain range is cold temperate (midslope, above 3000 m) to temperate (footslope at 2600 m), with an average annual temperature between 11°C and 14°C (isomesic soil temperature regime).

Enlarged area

Fig. 1. Location map. L

102 D. Dubroeucq et al./ Geoderrna 86 (1998) 99-122

From the mountain side to the altiplano, the total annual rainfall decreases from 800 to 400 mm, and the dry period increases from 4 to 8 months (ustic to aridic moisture regime). Parent soil materials are generally andesitic or dacitic ashes of late Pleistocene age, with underlying lavas or ash-and-blocks flows of the same mineral composition, or thick rhyodacitic pumice deposits of middle Pleistocene age.

2.2. Field sites and soils

Two volcanic ash soils were selected as representative soil types of the region. Their major macromorphological characteristics are given in Table 1; their physical and chemical properties are presented in Tables 2 and 3.

Profile A is SW from Perote, Veracruz State (19’28’30”N, 97’15’20”W) at an altitude of 2500 m and on a 5% slope. Annual precipitation is 400 mm, with 8 months of dry season, and the evaporation is 1640 mm. Land-use is maize associated with pine reforestation. The volcanic substratum is composed of a 1-2 m thick pumice fall deposit overlain by less than 1 m of fine ash. These materials are discordant upon a rhyolitic ash-and-pumice flow deposit, about 10 m thick, unconsolidated, and which shows a subhorizontal banded pattern of slightly undulated lamellae, each 2 to 5 cm thick, that tends to disappear deep down (Fig. 2). According to Ferriz and Mahood (1984), the ash-and-pumice flow corresponds to the Xaltipan ignimbrite dated 460,000 yr BP, and the pumice fall to the Toba Faby formation dated 240,000 yr BP, both originating

Table 1 Major macromorphological characteristics of profiles A and B Horizon Depth (cm) Colour Texture Structure Coarse NaF test HCl test

Profile A

AB 60 B 110 CRb 170 Bkxb 220 BCb 310 Cb 400

AP 20 Gravel

10YR3/6 sand loose W.

IOYR5/3 sandy loam w. subang. bl. - 10YR5/4 sandy loam w. ang. bl. - 10YR6/2 loamy sand loose abundant - 10YR6/6 loamy sand massive 7.5YR5/6 sand loose common - 5YR5/4 sand nd. many -

Profile B Stones 40 7.5YR5/4 sandy loam loose few m. AP

B 1 O0 7.5YR5/6 sandy loam massive few m. Bb 150 10YR5/4 loam m. subang. bl. few W.

BCb 230 Cb 300

- - -

7.5YR5/4 silt loam w. ang. bl. common - - nd. silt loam m. subang. bl. many - -

subang. bl. = subangular blocky; ang. bl. = angular blocky; w. = weak, m. = moderate.

D. Dubroeucq et al./ Geoderrna 86 (1998) 99-122

Table 2 Chemical and physical characteristics of profiles A and B

103

Profile A Profile B

Depth (cm) Depth (cm)

20 60 110 170 220 40 100 150 230

Horizons Horizons

Ap AB B CRb Bkxb Ap B Bb BCb

Solid density 2.4 2.5 2.4 2.3 2.5 2.5 2.6 2.6 2.7 Bulk density 1.03 1.12 0.82 - 1.33 1.14 1.54 1.95 2.61 Moisturecontent 1.3 2.9 3.3 1.3 5.1 3.9 5.4 4.2 3.7 pH, HZO 6.6 6.5 7.3 8.3 8.2 6.3 6.3 6.1 5.6 pH, KC1 6.0 5.9 6.6 7.6 7.5 5.5 5.5 5.2 9.1 PH, 9.3 8.8 9.3 9.9 9.5 10.4 10.3 9.2 7.6 % allophane 0.04 0.03 0.20 - - 3.45 2.42 0.31 0.24 % Org. matter 1.2 0.7 0.6 0.1 0.2 4.0 0.7 0.2 0.1 % C 0.7 0.4 0.3 0.08 0.1 2.3 0.4 0.1 0.07

- - 1.4 0.3 0.2 0.1 % N Available P t t t t t t t t t CECcmol(+)/kg 4.6 6.0 8.2 3.6 9.4 10.8 8.0 7.4 7.6 Na cmol(-t)/kg 0.24 0.46 0.46 - - 0.28 0.36 0.24 0.50 Kcmol(+)/kg 0.49 0.78 0.45 - - 0.64 0.92 1.02 0.60 Cacmol(+)/kg 3.54 3.54 2.08 - - 6.66 4.47 4.47 1.59 Mgcmol(+)/kg 0.84 1.15 0.29 - - 0.47 1.03 1.03 ' 0.90 Extr.Hcmol(+)/kg 0.14 0.28 0.14 0.28 0.28 0.28 0.14 0.14 0.14

- - -

Extr. Al cmol( +)/kg - - - - - - - - -

from the Los Humeros caldera, 20 km to the NW of the site. This profile may be allocated to the Soil Taxonomy class Vitrandic Ustorthent (Soil Survey Staff, 1996).

Profile B is near Los Altos, district of Perote, Veracruz State (19"27'40"N, 97"ll'lO"W) at the altitude of 3100 m and on a 20% slope. The annual precipitation is 700 mm, with six months of dry season and the evaporation is 1300 mm. Land-use is potato crop in a partly cleared pine forest. The volcanic substratum is a 5-6 m thick rhyodacitic ash-and-blocks flow originated from the Cofre de Perote volcano. The ash has a sandy-loam texture and is brown coloured. The blocks are generally 0.5-1 m across, some are fragmented with radiating shearing cracks and others subangular, probably parts of the collapsed walls of the volcano (Fig. 2). Since the ash-and-blocks flow overlaps the Toba Faby formation downslope, it is younger than 240,000 yr BP, but postdates the last major explosive event of the Cofre de Perote volcano dated 38,800 yr BP in this study (sample OBDY 1011, ORSTOM, Bondy, France). This profile may be allocated in Soil Taxonomy as Typic Ustivitrand over Andic Eutropept (Soil Survey Staff, 1996).

104 D. Diibroeiicq et al. / Geodernia 86 (1998) 99-122

Table 3 Particle size distribution in profiles A and B Horizon Depth MD So 0-2 2-20 20-50 50-200 200-2000

(cm) ( p m > P m Pm Pm Pm P m Profile A

AB B Lapilli Bkxb Lamella Matrix Lamella Matrix

Profile B

B Bb BCb

AP

AP

10 49 2.10 40 37 2.81 80 45 3.48

130 100 9.38 190 62 6.63 240 97 7.89 240 182 3.36 300 185 2.45 300 250 2

20 48 3.4 60 41 6.24

120 11 7.34 190 12.8 6.9

13 15 14.5 5

17.5 23.5 11 18.5 3.2

10.5 17 22.5 22

24.5 27 27.2 39 24.1 15 15 2.8 4.2

24.5 28.5 35 36

28.5 18 11.7 27.8 9.3 26 4.5 2.8 6.5 25.9 2.1 26.2

14.2 25.3 1.3 30.6 2.1 31.7

13 35.5 5.5 37.5 8 19 7.5 17

16 18.5 23 48.7 26 30.2 34.5 46.8 58.8

16.5 28.5 15.5 17.5

MD is the medium size of the particles. So is the Trask' sorting index, equal to 1 for a perfect sorting and to higher values for nonsorted particle size distributions.

2.3. Laboratory analyses

The samples of fine-earth (30 g of < 2 mm soil fraction) were previously treated with H,O, and NaOAc buffer solution (pH 5) to remove organic matter and mechanically agitated for 8 h with 2 cc Na-hexametaphosphate N/loOO. The particle-size analysis was carried out in two steps: (a) conventional sieving for the 50 to 2000 p m fractions, (b) using a Sedigraph 5000 in automatic procedure for clay and silt fractions.

Chemical analyses were carried out on the fine-earth from each horizon. pHHZ0, pH,,, pH,, were determined potentiometrically, CEC by ammonium acetate saturation and removal by NaC1, Na and K by flame photometry, Ca and Mg by atomic absorption spectrometry. Exchangeable H+ and Al3+ were determined titrimetrically and available P by the Bray 1 method. Allophane content was calculated from the oxalate extractable Si, estimating the overall composition as (SiO,, Alzo,, 2.5H20) with SiO,/A1,0, = 1.

Heavy and light minerals of the sand fraction (> 50 pm) were separated by bromoform (density 2.89 at 20°C) and observed on glass slides with an optical microscope.

Undisturbed samples from each horizon were resin impregnated to obtain soil thin sections. Micromorphological observations were performed with a standard petrographic microscope, using the terminology of Bullock et al. (1985) and

D. Dubroeucq et al./ Geodenna 86 (1998) 99-122 105

AP AB

B

CRb

B k b

BCb

Cb PROFILE B

AP

B

Bb

BCb

Cb

PROFILE A Fig. 2. Profiles A and B, with horizon designations from Soil Taxonomy and showing sampling sites of soil thin sections.

Mackenzie and Guilford (1988). The soil thin sections were carbon-coated and examined with a scanning electron microscope (S.E.M.) Cambridge Stereoscan 200 equipped with an energy dispersive X-ray analyser (EDXRA) Link System AN10000 calibrated with a Co sample.

Conventional X-ray analyses were carried out on a portion of fine-earth, using a Siemens diffractometer with Cu anticathode and applying several sample treatments: (a) non-oriented clay fraction ( < 2 ,um), (b) heating to 110°C and 480°C, (c) Glycerol-treated clay sample, (d) fully oriented clay sample by a drop of clay suspension on a platelet and subsequent air drying.

X-ray microdiffractions were performed on soil thin sections using a Siemens microdiffractometer with Cu anticathode, linear localisation detector and 0.15 mm2 collimator producing irradiated areas of about 0.5 m2. X-ray intensity and timing were identical for each sample. In order to cut down the small angles spectrum baseline variations, a blank with a glass slide was made and the final spectra are the difference between the samples and the blank.

In the case of crystalline material, XR microdiffraction offers a good defini- tion of the diffraction peaks, in conformity with the conventional band intensi- ties of the mineral (Rassineux et al., 1987). In the case of paracrystalline minerals, clear reflections are absent, and diagrams are not interpretable. When both species are mixed, the relative peak height comparison is no longer valid.

106 D. Dubroeucq et al./Geodermn 86 (1998) 99-122

One can detect the absence or presence of noncrystalline material by the flat or bulging baseline between 0.5 and 0.3 nm, but not the abundance.

3. Results

3.1. Profile A

3.1.1. The coatings on the glass shards ia the C horizons In the rhyolitic ash flow, at a depth of 330 cm, more than 70% of the mineral

components were glass shards, the rest being pumice lapilli, small plagioclase crystals and dacite lava clasts. A majority of the glass shards were larger than 500 pm, had a smooth surface and were partially coated with a discontinuous, pale grey coloured, isotropic, fine-textured material. Under S.E.M. observation, traces of incipient weathering were evident on their surface, such as dissolution pits, alveoles and fine indents. The pale grey material is spongy and rich in very small mineral fragments (Plate 1). The spongy structure has been formed by strong dehydration, presumably in the vacuum chamber during the S.E.M. observation, suggesting that the pale grey material was initially a hydrated gel. Semi-quantitative EDXRA analyses show that the coatings are mainly siliceous and not very different in their composition from that of the rhyolitic glass, with a Si/Al molar ratio of 6.1 (Table 4).

Plate 1. SEM image of the parent glassy ash in soil A. The glass shards show discontinuous coatings rich in small mineral fragments.

D. Dubroeucq et al./Geoderina 86 (1998) 99-122 107

d Y

-2 s 8

8 2

U LI

111

7 x

O

1

+I O

.3 v)

Y s

8

U

rcI E O d u

%I3 E-.w

3

2

108 D. Dubroeiicq et al./ Geodernta 86 (1998) 99-122

a b

323 1

O O 5mm

Fig. 3. Microstructure of the parent rhyolitic ash of profile A. (a) In the glassy matrix, the glass shards are bridged by colorless isotropic coatings. (b) In the lamellae, the corroded glass shards are embedded in a brown halloysitic groundmass.

At a depth of 250 cm and between the lamellae, the glass shards, rarely bigger than 500 ,um, show a rough etched surface and are bridged and welded together by fine-textured coatings. These are colourless, isotropic, non-laminated and limpid in PPL, indicating a neoformed amorphous material (Fig. 3a). Semi-quantitative EDXRA analyses show a higher content in Fe and Al in these colourless coatings than in the former pale grey coatings and a Si/A1 molar ratio of 1.5 (Table 4). X-ray diffractograms of the clay fraction from the same glassy material (Fig. 4, samples 3231 and 3362) show a 1.0 nm halloysite plus plagioclases (andesine, labrador) and traces of calcite. The bulge of the baseline between 0.5 and 0.3 nm is attributed to noncrystalline products. After heating to 480°C a small 1.0 nm peak remained, pointing to traces of illite clay mineral (Fig. 5, sample 3362). No peaks were detected in the X-ray microdiffraction diagrams performed directly on the colourless coatings, except for two bulges which are attributed to noncrystalline material (Fig. 6, sample 3241).

3.1.2. The coatings on the pumice lapilli Unweathered pumice lapilli are subangular with a rough surface. Their

structure varies from being fibrous to vesicular. Weathered lapilli were gradually round and smooth and coated with an isotropic, pale yellow, fine material which partly infills the voids and the vesicles in the pumice. Needle-shaped calcite crystals were commonly found on the surface of the coatings, attesting to a secondary crystal growth from evaporation of Ca-saturated solutions in the macrovoids (Fig. 7a). The pale yellow coatings are isotropic, laminated and translucent. Observed under S.E.M., the surface of the coating is very spongy with a botryoidal surface and internal microvoids, suggesting an initial hydrated state of gel which, subsequently, was dehydrated in the field, and probably also in the vacuum chamber of the microscope. Similar structures of amorphous

D. Dubroeucq et al. / Geodernia 81 ‘I 998) 9 -122 109

3500

3000

2500

2000 P z 3 O

1500

1 O00

500

O

Profile A XRD-Graph

O 10 20 30 40 50 2 THETA

Fig. 4. X-ray diffractograms of clay fractions from profile A. Near the soil surface the small peaks at 0.74 and 1.0 nm refer to poorly crystalline and dehydrated halloysite and the bulge of the baseline indicates noncrystalline products.

alumino-silicate coatings have been described in Andisols of humid climates (Jongmans et al., 1995). The chemical composition given by the EDXRA corresponds to a silica gel with a low content of Ca, Fe and AI and an Si/& molar ratio of 20.5 (Table 4). X-ray diffractograms display small reflection peaks of plagioclases, calcite and cristobalite, but no clay was detected. The broad bulge of the baseline centred on 0.4 nm indicates abundant noncrystalline products.

3.1.3. The fine groundmass in the B horizons Between depths of 300 to 220 cm, the lamellae showed a dense microstruc-

ture compared with the loose bridged grain structure of the surrounding glassy

110

2500

u) 2000 I- z 3

8 1500

1000

500

O

O

D. Dubroeiicq et al./ Geodernin 86 (1998) 99-122

Profile A, sample 3362 XRD-Graph

I I I I

10 30 2 THETA

20 40 50

Fig. 5. X-ray diffractograms of the clay fraction from the glassy material between the lamellae in soil A. Diagrams show weak peaks of halloysite and a bulge of the baseline indicating noncrystalline products. The 1.0 nm peak remaining after heating refer to small quantities of 2:l clay minerals.

material. The glass shards were 0.4 to 0.6 mm in size and embedded in a brown-coloured, isotropic, fine groundmass (Fig. 3b). This material is richer in Si and Fe than the colourless coatings, and its composition is close to that of 2:l clay minerals, with Si/Al molar ratio = 2 (Table 4). As illuviation features and laminated structures are absent in the groundmass, the clay minerals are supposed to be formed in situ from noncrystalline products. X-ray diffrac- tograms of the < 2 ,um fraction from a lamellae showed clear peaks at 1.0 and

Profile A XRmicroD Graph I l

I I I l I O 10 20 30 40 50

2 THETA Fig. 6. X-ray microdiffractograms from selected areas of fine material between the glass shards, in the ash and in a lamella of the parent material from profile A. Contrary to the diffractograms from clay fraction samples, clear peaks of clay are not detected in the ash and in the lamella.

D. Dubroeucq et al./ Geoderrna 86 (1998) 99-122

a b

111

- - O 0.25” O lmm C

- O Imm

glass and plagioclase fragments brown fine groundmass nodules

dense calcitic infilling O p a l e yellow coating =voids pumice

Fig. 7. Microstructure of profile A. (a) Pale yellow amorphous coatings on the corroded pumice lapilli and secondary growth of acicular calcite crystals. (b) Compact microstructure of the buried B horizon with infillings of fine calcite crystals in root channels. (c) The continuity of the massive structure of the €3 horizon is broken by irregular vughs connected to fine planar voids developed generally around the coarser grains.

0.445 nm related to 1.0 nm-halloysite, small peaks of plagioclases and a bulge in the baseline corresponding to noncrystalline products (Fig. 4, samples 3232 and 3361). Diagrams of X-ray microdiffraction on the brown fine groundmass displayed only two bulges with a small peak at 0.445 nm (Fig. 6, sample 3231) which was interpreted as poorly crystallized halloysite mixed with noncrystalline products.

Between depths of 220 to 170 cm, the same dense microstructure as in the lamellae, was observed in the whole horizon. Abundant, corroded glass frag- ments, 0.1 to 0.3 mm in size, were embedded in an isotropic, brown-coloured, fine groundmass. XR diffractograms of the < 2 p m fraction displayed clear peaks of 1.0 nm-halloysite and a shoulder at 0.74 nm which was interpreted as a partly dehydrated halloysite (Fig. 4, samples 335 and 335s). Dense infillings of fine calcite crystals were observed in elongated voids and channels of about 0.4 to 1 mm in diameter (Fig. 7b). Several of them showed a radiating orientation of the crystals pseudomorphosing plant roots. Iron oxide nodules and coarse

112 D. Dirbroeucq et al. / Geodenna 86 (1998) 99-122

pumice lapilli were also observed. This material has been interpreted as the B horizon of a buried soil, whose maximum age corresponds to the overlying pumice deposit, i.e., 24,000 yr BP (Ferriz and Mahood, 1984).

3.1.4. The fine groundmass in the topsoil Between 110 cm and 60 cm, the soil texture is loamy sand with only 14 to

15% clay. This material corresponds to an ash-fall composed of abundant coarse grains of pumice lapilli, glass, plagioclase and dacite microliths. The ground- mass consists of fine particles of plagioclase, pyroxene and glass and of an undifferentiated pale brown fine material. An average of five microanalyses from the pale brown groundmass indicates an Si-rich product, with Fe and Na as predominant cations and Si/A1 molar ratio = 5.6 (Table 4). Compared with the B horizon, which has a Si/A1 molar ratio = 2, the weathering of the upper part of the soil is not so advanced, and the B horizon has been probably buried under more recent deposits. X-ray diffractograms of the clay fraction indicate a partly dehydrated halloysite with clear peaks at 1.0 and 0.74 nm and a little of plagioclase. The smooth bulge of the baseline between 0.5 and 0.3 nm was interpreted as having small quantities of noncrystalline products. At 30 cm below the surface (Fig. 7, sample 3321, the X-ray diffractograms show a more accentuated bulge of the baseline and a weak peak at 0.74 nm, indicating only small quantities of dehydrated halloysite together with noncrystalline minerals, presumably allophane.

The mineral evolution of the fine groundmass is accompanied by an evolution of the microstructure. Compact structures were observed within depths of 110 to 60 cm. Above 60 cm, loose structures were observed to be developing gradually (Fig. 7c). They are mainly vughy and crumb microstructures in which the fine material is dominantly consisting of dehydrated halloysite and noncrystalline products .

3.2. Profile B

3.2.1. The coatings and infillings in the weathered rock boulders Both the ash and the blocks of the pyroclastic flow are of dacitic composition.

The blocks are composed of 30 to 40% fenocrysts of plagioclase (essentially oligoclase, labrador and andesine), K-feldspar, pyroxene and amphibole grains; 20% smaller crystals, 0.5 to 1 mm in size, mainly of plagioclase and pyroxene; 40 to 50% of microlitic matrix, essentially plagioclases and glass.

In the weathered blocks, a yellow, isotropic and limpid fine mass infills the voids of the microlitic matrix (Fig. 8a). Since no laminated structure was noticed, the yellow fine mass is presumed to be formed in situ and is considered as the product of an initial stage of weathering, resulting from the co-precipita- tion of Al and Si released from the weathering of overlying and in situ primary minerals (Jongmans et al., 1995).

D. Dubroeucq et al./ Geodernia 86 (1998) 99-122

a b

113

34501 34401

34402

O b Imm -

0.5 mm

C d

- - O Imm O imm

yellow coatings and infillings feldspar Glass shards and nodules !

811 pyroxene microlitic matrix brown groundmass 0 Plagioclse

biologic microaggregates 0 voids

Fig. 8. Microstructure of profile B. (a) Isotropic yellow infillings in the microlitic matrix of the dacite blocks. (b) Centrifugal alteration of a feldspar phenocryst and yellow isotropic weathering products. (c) Thick yellow coatings and brown groundmass in a sample from the bottom of the B horizon. (d) In the topsoil are two types of microaggregates, one consisting of clusters of mineral particles and the other consisting of rounded bodies containing organic matter.

Between depths of 150 to 230 cm, yellow coatings of the same characteristics were found around and inside the weathered feldspar and plagioclase fenocrysts, and more particularly, along the cleavage planes and within the cavities of the crystals (Fig. 8b). Close to the parent minerals (Fig. 8b, sample 34401), the yellow coatings are optically isotropic and limpid, and the EDXRA analyses gave the following composition: SiO, = 59.7%, Alzo, = 24.6%, Fe,O, = 0.8%, Na,O = 5.7%, Ca0 = 7.4%, K,O = 0.9%; Si/A1 mol = 2.05. In the external zone of the minerals (Fig. 8b, sample 34402), the fine mass has a denser yellow colour and its chemical composition shows an increase in Fe, a decrease in bases and Si/Al molar ratio = 1.2 (Table 4). This latter composition is close to that of halloysite. Therefore, various grades of differentiation of the yellow coatings may coexist near the same crystal, from siliceous products rich in bases, to

114 D. Dubroeucq et al./ Geodermn 86 (1998) 99-122

aluminous products depleted in bases. X-ray microdiffractions on a yellow coating in a rock fragment (Fig. 8a) shows clear peaks of plagioclase, a bulge of the baseline at the small angles indicating abundant amorphous products and a hump at 0.445 nm indicating traces of poorly crystallized 1:l clay minerals (Fig. 9, sample 34501). X-ray microdiffractions of yellow coatings inside and around a weathered plagioclase (Fig. 8b) show plagioclase, sharp and high peak of cristobalite at 0.403 nm and small peaks of halloysite at 0.74 and 0.445 nm

Profile B XRmicroD Graph 4000

3000

u) t- z 3

8 2000

i ooa

t I l I I

O 'IO 20 30 40 50 2 THETA

Fig. 9. X-ray microdiffractograms of selected areas in thin sections from profile B. Diagrams show contrasted differences between the yellow and the brown fine materials: sharp peaks of cristobalite and bulges of the baseline for the yellow fine material (samples 34501, 34402, 3401, 34321), clear peaks of clay, plagioclase and hornblende for the brown groundmass (samples 34322, 34301). In the topsoil (sample 34101) the peaks of clay are not discernible and the baseline bulges are accentuated, indicating noncrystalline products.

D. Dubroeucq et al./ Geodeniia 86 (1998) 99-122 115

(Fig. 9, samples 34402 and 34401). X-ray microdiffractions of thick yellow coatings on the surface of a dacite block at a depth of 150 cm (Fig. Sc), indicate a mixture of allophane-like products with halloysite and cristobalite (Fig. 9, sample 34321).

3.2.2. The yellow coatings in the B horizon Between depths of 150 to 250 cm, the yellow coatings were very abundant,

isotropic, and 20 to 600 p m thick. They were covering the surface of the dacite blocks and pebbles, the walls of the planar voids and the coarse minerals and infilled the porosity, resulting in a relatively dense microstructure. Both struc- tureless and microlaminated features were observed in the same samples, indicating that neoformed and translocated coatings were coexisting in this part of the profile. At a depth of 150 cm (Fig. Sc), semi-quantitative EDXRA analyses show that a thick yellow coating consists predominantly of Si, Al and Fe with Si/A1 molar ratio = 1.3 and with an excess of Fe and Si in comparison with halloysite (Table 4). In accordance with the X-ray microdiffraction dia- grams of this sample (Fig. 9, sample 34321), the yellow coating composition is interpreted as a mixture of halloysite with minor amounts of allophane and substantial contents of Fe-oxides and silica minerals such as cristobalite. Above a depth of 120 cm, the yellow coatings were fragmented into rounded or amiboidal clay features and incorporated within the groundmass. Faunal activity is thought to be the cause of fragmentation.

3.2.3. The brown fine groundniass in the B horizon At a depth of 150 cm, only small areas of brown-coloured fine groundmass

were discernible among the yellow coatings (Fig. Sc). Higher in the profile, they were more abundant. The brown fine groundmass is reddish-brown in PPL, with a speckled b-fabric in XPL and a preferred orientation around the coarse grains. Its chemical composition from EDXRA analyses is dominantly of Si and Al with a substantial Fe content and %/A1 molar ratio = 4.1. At a depth of 120 cm in the profile, the brown fine material was denser, embedding the coarse minerals and forming a channel microstructure. X-ray diffractograms of the < 2 p m fraction have given evidence of a partly dehydrated halloysite by a clear peak at 1.0 nm and a shoulder at 0.72 nm, and of plagioclase (Fig. 10, sample 343). X-ray microdiffractions have shown a small peak of halloysite at 0.74 nm and peaks of plagioclase at 0.403, 0.322 and 0.252 nm, without evidence of noncrystalline products (Fig. 9, sample 34322). Semi-quantitative EDXRA analyses of the brown groundmass at a depth of 100 cm have indicated a silica-rich calcoalkaline product with substantial Fe content and Si/A1 molar ratio = 1.85 (Table 4). The X-ray microdiffractions on the same sample have shown halloysite by a small peak at 0.74 nm, hornblende at 0.843 nm and 0.270 nm, and plagioclase at 0.318 nm and 0.322 nm (Fig. 9, sample 34301). These results confirm that, in the B horizons, the brown coloured fine material is

116

4000

3000

2000

u) I- z 3 O o

1 O00

O

D. Dubroeiicq et al. / Geoderma 86 (1998) 99-122

Profile B XRD Graph

M In O

x 3

I I I l O 10 20 30 40 50

2 THETA Fig. 10. X-ray diffractograms of clay fractions from profile B. In the upper part of the soil (samples 342 and 341), small peaks at 0.74 nm and a bulge in the baseline between 0.5 and 0.3 nm indicate dehydrated and poorly crystallized halloysite clay with noncrystalline products.

mainly composed of 1.0 nm-halloysite accompanied by fine particles of plagio- clase and hornblende.

3.2.4. The fine groundmass in the topsoil At a depth of 100 cm, areas of microaggregated structures were observed to

be developing at the expense of dense microstructures, and above 80 cm, the microaggregation occupied the whole soil material. Each microaggregate is irregular-to-subrounded, 0.05 to 0.15 mm across, composed of coarse grains embedded in a brown fine material. The brown groundmass composition, from semi-quantitative EDXRA analyses, corresponds to a silica-rich product, with higher amounts of Fe and bases than in the groundmass from the underlying horizons and with Si/Al mol = 1.7. X-ray diffractograms of the < 2 p m soil

D. Dubroeucq et al./Geodertna 86 (1998) 99-122 117

fraction at a depth of 60 cm show a decrease of the 1.0 nm peak of halloysite and a bulge of the baseline between 0.6 nm and 0.3 nm, indicating allophane-like products, mixed with a significant amount of fine plagioclase grains (Fig. 10, sample 342). Near the soil surface, changes in the mineral composition of the clay fraction were even more pronounced. At a depth of 20 cm, X-ray diffractograms showed a wide hump of the diffraction line centred on 0.44 nm, indicating allophane-like products, and low peaks at 0.445 nm and 0.74 nm, indicating small quantity of dehydrated halloysite together with some plagio- clase grains (Fig. 10, sample 341). X-ray microdiffraction diagrams have shown the same results: halloysite peaks are not discernible and two bulges of the baseline indicate significant contents of noncrystalline products (Fig. 9, sample 34101).

Near the soil surface, other aggregates probably of faunal origin were observed. They were roundish, 0.03 to 0.08 mm across and dark-brown-coloured due to the presence of organic matter (Fig. 8d).

3.2.5. The division of the coar-se rninerals To a depth of 300 cm and above, plagioclase and feldspar fenocrysts were

intensely weathered. Alteration begins from the centre and extends to the periphery of the minerals by way of radiating and irregular anastomosed fingers of amorphous material. Upon ongoing weathering, increasingly small crystal clumps of 0.5 to 1 mm in size were isolated (Fig. 8b). In the middle part of the profile, between depths of 100 to 150 cm, micromorphological observations have shown that all the grains were fissured and smaller than 0.6 mm, with an average size of 0.2 mm. From the surface to a depth of 80 cm, plagioclase and feldspar grains were of about 0.5 mm and slightly weathered, attesting to the presence of another material, presumably an ashfall.

From the bottom to the middle part of the profile, the pyroxene grains became progressively smaller, round-shaped and deeply fissured, the amphibole grains were divided into small angular fragments along the cleavage planes. In the middle part of the profile, the pyroxene grains were impregnated with thick Fe-oxide coatings such as iron boxwork crystal pseudomorphs (Nhon, 1991) and gradually transformed into nodules. Between depths of 40 to 100 cm, small Fe-oxide nodules, relics of pyroxenes from the underlying material were ob- served together with sharp pyroxene fragments from the fresh volcanic deposits on the topsoil.

4. Interpretation and discussion

4. I. The first stages of weathering

The first stages of weathering in these soils are not very different from those described in wetter climates (Jongmans et al., 1995; Quantin et al., 1991).

118 D. Dubroeucq et al. / Geoderma 86 (I 998) 99-122

Yellow isotropic products were observed as coatings on glass shards, fenocrysts and rock fragments and also as infillings in the microlitic matrix. At the microsite scale, these coatings appeared mainly as noncrystalline products in X-ray microdiffractions. But the chemical extractions have revealed only low allophane contents (Table 2), and the X-ray diffractions on the < 2 ,um fraction have evidenced more halloysite clay than noncrystalline minerals. This has probably two causes. The first is the fine-sized particle separation, which has excluded the coarse minerals and the rock fragments in which features of initial weathering were abundant. The second could be a significant content of these products in amorphous siliceous iron oxides or proto-ferrihydrite (Parfitt and Childs, 19831, which are more easily extractable in DCB than in acid oxalate, and were found to be formed in ustic or drier moisture regimes rather than udic moisture regime (Wakatsuki and Wielemaker, 1985).

4.2. The origin of the fine groundnznss of the soils

In profile A, the colourless and noncrystalline coatings appeared as direct products of the weathering of the rhyolitic glass shards. In profile B, micromor- phological observations have shown that the yellow coatings develop from the coarse minerals in a centrifugal way, retaining the outline of the mineral

Table 5 Estimation of the mineralogical composition of the sand sized grains in profiles A and B, in percent of the total number of grains

Profile A Profile B

Horizon Horizon

AB CRb BCb Ap B Bb BCb

Depth (cm) Depth (cm)

40 140 240 20 70 130 180

Light mineral Quartz 1 .o 0.8 0.5 - Feldspar 33.6 23.0 7.9 57.9

Dacitic glass - - -

- - - - Biotite Rhyolitic glass 55.1 13.8 91.6 26.2

4.4 Rhyolitic pumice 10.3 62.4 - 11.5

- - - 76.9 85.5 79.7 - 0.3 15.1 1.0 -

-

7.2 13.0 20.2 0.8 - -

Heavy mineral Augite 28.6 10.3 25.7 21.4 24.7 15.6 10.2 Hypersthene 37.0 50.1 42.7 38.2 39.9 24.5 31.0 Green hornblende 21.7 34.5 17.5 25.3 14.4 18.2 2.3 Brown hornblende 6.6 2.0 3.3 6.1 6.6 22.4 21.4 Opaque 6.0 3.2 8.9 8.9 14.4 19.3 35.0

D. Dubroeucq et al./ Geodenna 86 (1998) 99-122 119

(Nahon, 1991). This yellow, optically amorphous fine mass is thus the product of an in situ weathering.

The genesis of the brown fine groundmass depends on the evolution of the yellow coatings. In the soil lamellae of profile A, a replacement of the colourless isotropic coatings by a brown fine groundmass has been observed, correlative with the formation of 1 .O- nm halloysite and interstratified hal- loysite-smectite clay minerals and with increasing Fe contents. In profile B, a gradual fragmentation of the yellow coatings with progressive admixtures of brown fine groundmass have also been observed. These two materials have different compositions. The yellow fine groundmass is composed of 1.0-nm halloysite accompanied by Fe oxides and cristobalite. The brown fine ground- mass is composed of 1.0-nm halloysite with small particles of hornblende and plagioclase. This difference reveals certainly different origins. The yellow coatings are a weathering product of the dacitic ash-and-blocks flow, whereas the brown groundmass results presumably from the weathering of the rhyo- dacitic ash of the topsoil which contains more rhyodacitic glass, green hom- blende and quartz (Table 5).

4.3. The existence of allophane in the topsoil

Both profiles have small amounts of allophane in the topsoil. This mineral is the product of the initial weathering of fresh minerals. Since the ultimate explosive event of the volcano is 38,800 yr BP, another newer source of wind-transported minerals is probable.

4.4. The sandy-loanz texture of the soils

Micromorphological observations showed that, as weathering proceeds, feldspar and hornblende fenocrysts were progressively divided into small grains, pyroxenes were transformed into fine nodules and glass shards were etched and considerably reduced in size. This evolution could explain the vertical particle size distribution of the profiles which shows a decrease of the 200-2000 p m fraction and an increase of the 0-2 p m and 50-200 p m fractions between unweathered and weathered parent materials. This general evolution is modified in the topsoil where an increase of the 50-200 p m and 20-50 p m fractions, and a medium size of the particles of about 50 pm is noticed (Table 3). The decrease of the 0-2 p m fractions and increase of the 20-200 p m fractions in the surface horizon could be interpreted as an input of fine sand from aerial dust. But this does not correspond to the particle size of deposited dust registered in West Africa (Drees et al., 1993). This could be also interpreted as the deflation of the 0-20 p m fraction and the relative increase of the remaining 20-200 p m fraction, more in accordance with the local observations of wind erosion effects on cultivated soils.

120 D. Dubroeiicq et al. / Geoderma 86 (1998) 99-122

4.5. The microaggregate structure of the topsoil

Another observation was the progressive dehydration of halloysite and con- comitant development of microaggregates near the soil surface. These microag- gregates are different from biological features. The presence of dehydrated halloysite clay together with allophane is an undisputed indicator of climatic changes towards dryer conditions. Desiccation may cause dramatic changes in volcanic soils and, in particular, the irreversible dehydration of crystallized and noncrystalline clay minerals (Shoji et al., 1993b). Since biological activity was not reported as the factor of microaggregation, desiccation, probably consecutive to cultivation, is thought to be the main cause of the loose structure of these soils in the upper horizons.

4.6. Susceptibility to wind erosion

Wind erodibility depends not only on low contents of clay and organic matter (Kemper et al., 1987) but also on the quantity of erodible particles (Chepil, 1950, 19511, the size of which has been experimentally established at 840 p m for aggregates (Gillette, 1978) and 85-95 pm for simple grains (CoudBGaus- sen, 1994). The surfaces of the two soils studied, being predominantly composed of microaggregates and grains of lesser values, respectively, and about 500 p m and 50 pm, are therefore very susceptible to be wind eroded.

5. Conclusion

The two studied soils are formed of superimposed volcanic materials of different composition and ages, spanning from recent surficial aeolian deposits, subsurface layers at most of 38,800 yr old and underlying materials undoubtedly much older. In the zone of contact with the primary minerals, the initial products of weathering are amorphous coatings in which silica predominates. In the outer zone of the minerals, the noncrystalline products are transformed into halloysite clay, accompanied by a notable depletion in bases, an enrichment in.Fe and a progressive division of the coarse minerals into small grains 50-200 p m in size. The result is the formation of a groundmass embedding small-sized grains. Under semi-arid conditions the small grains are preserved even in the subsurface horizons and contribute to form sandy-loam textured soils.

In the topsoil, the fine mineral grains are slightly weathered. This fine material is presumably a surficial import of fresh volcanic ash from aerial dust. The weathering products are Si-rich allophane and dehydrated halloysite. These clay minerals are assembled in a loose microaggregate structure interpreted as a result of their desiccation.

D. Dubroeucq et al./ Geodernta 86 (1998) 99-122 121

All these sub-processes of weathering lead to the formation of soils without any cohesion and very exposed to wind erosion.

Acknowledgements

This research was undertaken in ORSTOM laboratories, Bondy, France. It was supported in part by 'the Mexican National Council of Science and Technology (CONACYT) and by ORSTOM. We are grateful to A. Bouleau for assistance in scanning electron microscopy, to G. Millot for help in X-ray diffraction analysis, and to M. Delaune for carrying out heavy mineral determi- nation and automatic particle-size analysis. Special thanks are extended to N. Portilla (Instituto de Ecologia, Mexico) for helpful technical assistance in soil analysis.

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I

Volcanic Ash Soils Genesis, Properties and Utilization

by S. Shoj

Developments in Soil Science Volume 21

Jolcanic eruptions are pnerally viewed as. agents of destruction, ret they provide the )arent materials from which some of the most xoductive soils in the world are formed. The iigh productivity results rom a combination of inique physical,, :hemical and n ineralog ¡cal xoperties. The mportance and iniqueness of volcanic ish soils are ?xemplified by the ecent establishment of he Andisol soil order in ;oil Taxonomy. This mok provides the first :omprehensive tynthesis of all aspects )f volcanic ash soils in I single volume. It :ontains in-depth :overage of important opics including erminolog y, norphology, genesis, :lassification, nineralogy, chemistry, ,hysical properties, )roductivity and tilization. A wealth of ata (37 tables, 81 gures, and Appendix) iainly from the Tohoku hiversity Andisol Data

M. Nanzyo and R.A. Dahlgren

Base is used to illustrate major concepts. Twelve color plates provide a valuable visual-aid and complement the text description of the world-wide distribution for volcanic ash soils. This volume will serve as a valuable reference for soil scientists, plant scientists, ecologists and geochemists interested in biogeochemical processes occurring in soils derived form volcanic ejecta.

Short Contents: 1. Terminology, Concepts and Geographic Distribution of Volcanic Ash Soils. 2. Morphology of Volcanic Ash Soils. 3. Genesis of Volcanic Ash Soils. 4. Classification of Volcanic Ash Soils.

ELSEVIER SCIENCES

5. Mineralogical characteristics of Volcanic Ash Soils. 6. Chemical Characteristics of Volcanic Ash Soils. 7. Physical Characteristics of Volcanic Ash Soils. 8. Productivity and Utilization of Volcanic Ash Soils. Appendices. References Index. Subject Index.

1993 312 pages Dfl. 290.00 (US $165.75) ISBN 0-444-89799-2

Elsevier Science B.V. P.O. Box 1930 1 O00 BX Amsterdam The Netherlands

P.O. Box 945 Madison Square Station New York NY 1 O1 60-0757

The Dutch Guilder (Dfl.) prices quoted apply worldwide. US $prices quoted may be subject to exchange rate fluctuations. Customers in the European Community should add the appropriate VAT rate applicable in their country to the price.