Crystal chemistry of clinopyroxenes from Linosa

Mineralogical Magazine, December 2002, Vol. 66(6), pp. 953–968

Crystal chemistry of clinopyroxenes from Linosa Volcano, Sicily Channel, Italy: implications for modelling the magmatic plumbing system L. BINDI1, F. TASSELLI1, F. OLMI2, A. PECCERILLO3 1 2 3

AND

S. MENCHETTI1,*

Dipartimento di Scienze della Terra, Universita` degli Studi di Firenze, Via La Pira, 4, I-50121, Firenze, Italy CNR ÿ Istituto di Geoscienze e Georisorse, Via La Pira 4, I-50121 Firenze, Italy Dipartimento di Scienze della Terra, Universita` degli Studi di Perugia, Piazza Universita`, I-06100, Perugia, Italy

ABSTR ACT

The Island of Linosa is a small part of the large submarine volcanic complex which is located at the SW edge of the Linosa Graben, Sicily Channel. The island was built up from 1.06Ô0.10 to 0.53Ô0.07 Ma, through three main stages of activity: Paleo-Linosa, Arena Bianca and Monte Bandiera. Major and trace element data show that the compositional variability of the three activity stages is limited, with most of the rocks showing basaltic to hawaiitic composition. Evolved benmoreites and trachytes are found as lithics in some pyroclastic units of Paleolinosa. The mafic rocks of the three stages show porphyritic texture, with phenocryst assemblages characterized by olivine, clinopyroxene and plagioclase. The volume ratio of olivine vs. clinopyroxene decreases from early to late stages of activity in mafic rocks with comparable major element composition. Clinopyroxene phenocrysts from mafic rocks of the three stages have poorly variable composition, clustering in the augite field. Phenocrysts from the first activity stage (Paleo-Linosa), show a slight increase in TiO2, Al2O3 and CaO, and a decrease of Fe2O3 (total) with the increasing SiO2 content of the host rocks. Crystals from the second and the third stage (Arena Bianca and Monte Bandiera) display a slightly more restricted range of FeOtot, frequently with very high MgO, Al2O3 and TiO2 contents. Crystal chemical investigation of clinopyroxenes from rocks of the three stages with comparable degrees of evolution, revealed significant variation of structural parameters, in particular VM1 and Vcell. These show a consistent decrease, passing from clinopyroxenes of the early stage to crystals extracted from the mafic lavas of stages 2 and 3. Given the similar compositional ranges of the host rocks, structural variations of clinopyroxenes are interpreted to reveal modifications of crystallization pressure, which increased, passing from Paleo-Linosa to the Arena Bianca and Monte Bandiera stages. Given this information, the observed crystal-chemical variations provide information on the depth of magma reservoirs and on the evolution of the plumbing system of Linosa volcano.

K EY WORDS : crystal chemistry, clinopyroxene, volcanic plumbing system, magma chamber, Linosa island,

Italy. Introduction SEVERAL crystal-chemical studies on C2/c clinopyroxenes (cpx) have pointed out characteristic differences between crystals formed in different magmatic environments (Dal Negro et al., 1982, 1986, 1989; Cellai et al., 1994; Salviulo et al.,

* E-mail: [email protected] DOI: 10.1180/0026461026660070

# 2002 The Mineralogical Society

2000). The influence of temperature, pressure and cooling history has also been studied in detail (Molin and Zanazzi, 1991; Malgarotto et al., 1993a,b; Nimis, 1995, 1999; Nimis and Ulmer, 1998; Nazzareni et al., 1998). These studies demonstrate that cation substitution in pyroxene is constrained by both compositional characteristics of parent magmas and physical conditions of crystallization. Therefore, crystal-chemical studies on clinopyroxenes may represent a

L. BINDI ETAL.

can be tested by crystal-chemical investigation of clinopyroxenes. In this paper, we report a crystal-chemical study of clinopyroxenes from magmatic rocks with comparable major element compositions from various eruptive stages of Linosa, with the aim of investigating the possible modifications of crystallization conditions as a step towards using cpx crystal chemistry in modelling the plumbing systems of volcanoes.

useful tool for petrological and volcanological investigations (e.g. Nazzareni et al., 2001). Experimental studies have shown that reliable information on crystallization pressure of clinopyroxene can be obtained when crystals extracted from compositionally similar rocks are investigated (Nimis, 1995, 1999; Nimis and Ulmer, 1998). Clinopyroxenes from mildly alkaline or tholeiitic magmas give more precise barometric indications than crystals extracted from calcalkaline rocks, because of the bias related to high water pressure and higher compositional variability of the latter magmas. The Linosa Island represents a key place where crystal-chemical investigations of clinopyroxenes can furnish important indications on pressure of crystallization. The island has been constructed by three main stages of activity, all dominated by eruption of rather homogeneous mafic lavas with a Na-alkaline affinity. However, some benmoreitic and trachytic rocks are found as lithics in the lowest exposed products, testifying to a decrease in the degree of melt evolution through time. These variations may be related to modification of the plumbing system of Linosa, a hypothesis that

Geological setting The island of Linosa (Fig. 1) is a small part of the large submarine volcanic complex which is located at the SW edge of the Linosa Graben, in the Sicily Channel (Lanti et al., 1988). K-Ar ages have shown that the island was built up from 1.06Ô0.10 to 0.53Ô0.07 Ma, through three main stages of activity: Paleo-Linosa, Arena Bianca and Monte Bandiera (Rossi et al., 1996). The compositional variability of rocks is limited, with most of the volcanic material showing basaltic to hawaiitic composition. The most evolved rocks are represented by abundant lithic fragments of

N Legend 14

Observed crater rims

15

17

29,30

Inferred crater rims 11

31 32

Volcanic edifices Faults

18

9,10 6,8 28

27

Tectonic cliff 1

26 20-25

Lava fields

Sicily Channel

Samples

FIG. 1. Geological sketch map of the Linosa Island (after Lanzafame et al., 1994).

954

CLINOPYROXENES FROM LINOSAVOLCANO

benmoreitic and trachytic composition, occurring within some pyroclastic units of Paleo-Linosa. This indicates that more advanced stages of magma evolution were reached during early activity at Linosa.

same minerals as found in phenocrysts and rare opaque oxides. In these rocks the olivine crystals are often altered to iddingsite. Proportions of olivine vs. clinopyroxene decrease from early to late erupted hawaiites. Benmoreites and trachytes occur as lithics in some pyroclastic units from Paleo-Linosa (Rossi et al., 1996). They exhibit a porphyritic seriate texture (P.I. = 15ÿ30%), with large and abundant plagioclase phenocrysts (An35ÿAn55) beside minor clinopyroxene and kaersutitic amphibole phenocrysts set in a microcrystalline groundmass made of plagioclase, clinopyroxene and opaques. In some samples cm-sized megacrysts of plagioclase and kaersutite are present.

Sampling and petrography Sampling was carried out with the aim of collecting all the main rock types cropping out on the island. The samples collected were analysed for major and some trace elements in order to provide a petrological basis for crystal chemistry studies. A detailed petrographic study of the Linosa rocks was reported by Rossi et al. (1996). Here a description of the samples collected in this study is given. Basalts exhibit a porphyritic seriate texture (porphyritic index, P.I. = 35ÿ45%) with olivine and clinopyroxene phenocrysts and lesser amounts of plagioclase (An40ÿAn70), set in a microcrystalline groundmass of the same phases plus opaque oxides. Olivine (Fo75 to Fo85) is the dominant phenocryst in the basalts of the first stage, whereas in late basalts clinopyroxene prevails. Hawaiitic rocks show a lower P.I. value (P.I. = 20ÿ30%) with respect to the basaltic rocks. They are characterized by phenocrysts of deep green clinopyroxene and olivine (Fo72ÿFo80) associated with variable amounts of plagioclase (An60ÿAn75). The groundmass is usually microcrystalline to hypocrystalline and contains the

Petrology and geochemistry Representative rocks samples from the three eruptive stages were analysed for major and trace elements. The Na2O, MgO and LOI were determined by wet chemical methods. Other major and trace elements were determined by X-ray fluorescence using a full correction method for matrix effects. A total alkali vs. silica (TAS) classification diagram of the analysed samples, normalized to 100% on a LOI-free basis, is given in Fig. 2. This shows that the rocks analysed range in composition from basalt to benmoreite. Rock compositions from various activity stages basically overlap, except for the lithics from early products that extend to the benmoreitic field. Variation of

FIG. 2. K2O+Na2O vs. SiO2 classification diagram (Le Bas et al., 1986) for the studied samples. Symbols refer to the rocks from different stages of activity: Paleo-Linosa (circles), Arena Bianca (squares), and Monte Bandiera (triangles). The dashed-line is the divide between the alkaline and subalkaline fields of Irvine and Baragar (1971).

955

L. BINDI ETAL.

major and some trace elements against MgO are shown in Fig. 3. There is a continuous increase in Na2O, K2O, Al2O3 and Zr, and a decrease in Cr with decreasing MgO. TiO2 and CaO increase in mafic samples and decrease sharply in the most evolved rocks; Fe2O3 (total) is virtually constant in mafic samples and decreases in more evolved rocks. Inter-element variation diagrams (Fig. 4)

show smooth positive trends of Nb, Ba and Rb vs. Zr and poorly defined negative correlations for Cr and Co. Lanthanum also shows a positive correlation with Zr, although a flattening of the trend is observed in the most evolved rocks. Strontium and P increase with Zr in the mafic rocks to decrease sharply in the most evolved samples.

FIG. 3. Variation of major and some trace elements against MgO. Symbols as in Fig. 2. Filled symbols refer to the rocks from which clinopyroxenes were extracted.

956


Covariation diagrams of major and trace elements clearly indicate that there is an overlap among compositions of mafic rocks, which define smooth linear trends on variation diagrams of most major and trace elements. This suggests that primary melts did not change their composition through time. Moreover, trends of major and trace elements against MgO and Zr indicate that

fractional crystallization played a major role during magma evolution. Increase in CaO, Sr and decrease in Cr and Co with decreasing MgO observed in the mafic rocks, suggest that early stages of fractionation were dominated by separation of olivine with some clinopyroxene. The decrease in CaO, Sr, TiO2, Fe2O3 and P2O5 in benmoreites suggests that plagioclase, Fe-Ti

FIG. 4. Inter-element variation diagrams. Symbols as in Fig. 3.

957

L. BINDI ETAL.

Samples LNS11 (hawaiitic rock from the second stage – Arena Bianca) and LNS31 (basalt rock from the third stage – Monte Bandiera) show two types of clinopyroxene crystals. The first type is idiomorphic, pale brown in colour, with sharp edges and does not show any evidence of chemical reaction with the surrounding matrix; the second type is green in colour, slightly cloudy because of submicroscopic inclusions, and exhibits rounded edges suggestive of partial resorption. Both types of clinopyroxene were investigated. Intensity data were collected by means of an automated CAD4 diffractometer with Mo-Ka radiation. The intensities of the reflections with y 435º were recorded using an o-scan technique and were corrected for absorption following the semi-empirical method of North et al. (1968). Only reflections with Fo > 4s(Fo) were employed for the structure refinement, which was performed

oxides and apatite were the main fractionating phases at this stage of evolution. Separation of apatite also accounts for the flattening of La vs. Zr trend in the most evolved compositions. Apatite hosts abundant LREE but contains only small amount of Zr; therefore, apatite fractionation determines a decrease in the degree of incompatibility for La, with flattening of La vs. Zr trend. Sample selection and analytical techniques For the X-ray single-crystal diffraction study and electron microprobe analysis, 20 clinopyroxene crystals were chosen from nine representative rock-types (Table 1), which cover the three stages of activity (i.e. Paleo-Linosa, Arena Bianca, and Monte Bandiera). Crystals were selected under binocular and petrographic microscope after rock chip crushing, rejecting those showing twinning, crystal defects or zoning.

TABLE 1. Major (wt.%) and trace (ppm) elements for the collected rocks.

Stage SiO2 TiO2 Al2O3 Fe2O3*{ MnO MgO CaO Na2O K2O P2O5 LOI V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Ba La Pb Ce Th

LNS9* LNS10 LNS20 LNS21*LNS22* LNS23 LNS25 LNS29 LNS30 LNS1* LNS6 1 1 1 1 1 1 1 1 1 2 2 45.07 1.53 14.87 11.01 0.19 16.17 7.02 2.85 0.92 0.38 0 178 549 70 424 50 78 15 19 370 19 157 32 246 20 1 17 2

46.70 2.07 16.97 11.17 0.20 9.36 9.11 2.92 1.10 0.41 0 260 272 50 155 49 91 19 23 485 26 182 39 301 24 4 35 3

45.57 2.20 16.10 11.81 0.20 11.05 8.26 3.32 1.13 0.35 0.01 188 287 54 225 57 90 21 24 532 26 225 42 298 30 6 45 7

50.48 2.21 16.67 9.65 0.15 3.75 9.76 3.82 1.17 0.47 1.87 222 315 29 46 36 95 21 27 470 34 213 43 290 29 0 40 1

56.29 1.53 17.62 7.55 0.20 2.13 4.14 5.79 2.93 0.48 1.34 49 13 8 4 12 103 25 41 444 37 418 82 730 59 23 112 16

51.24 2.01 17.01 10.17 0.17 2.85 8.70 4.21 1.45 0.36 1.84 195 125 29 45 46 95 22 21 456 27 212 40 349 26 2 54 5

958

47.38 2.36 16.55 10.95 0.20 8.56 8.27 3.76 1.40 0.57 0 222 281 39 163 48 85 20 28 665 26 254 51 391 40 5 61 6

57.56 1.16 17.22 7.38 0.19 1.20 3.98 6.11 3.13 0.27 1.80 47 14 8 4 16 97 22 44 342 40 456 83 959 57 12 116 10

48.68 2.25 17.36 10.41 0.19 7.16 7.44 4.26 1.66 0.59 0 195 151 38 109 38 87 20 35 633 26 294 54 429 40 9 77 7

49.62 2.26 16.84 11.24 0.20 6.09 8.33 3.81 1.22 0.39 0 204 167 32 63 58 94 21 22 427 32 229 43 301 29 6 44 5

48.56 2.09 17.04 11.59 0.19 7.02 9.15 3.35 0.77 0.24 0 208 231 42 79 76 97 21 16 417 29 167 29 212 22 2 22 3


using the program SHELXL-93 (Sheldrick, 1993), in the space group C2/c. The scattering curves for neutral atoms – Ca vs. Na (M2 site), Mg vs. Fe (M1 site), Si, O – were taken from the International Tables for X-ray Crystallography, volume IV (Ibers and Hamilton, 1974). In all the crystals studied, the difference Fourier ˚ map showed significant residual density at ~0.7 A from the M2 position, suggesting the presence of the M2’ site (Dal Negro et al., 1982; Rossi et al., 1987). Further least-squares refinements using the Fe2+ atomic scattering factor for M2’ were performed. The temperature factor of M2’ was fixed equal to the equivalent isotropic temperature factor of M2. The introduction of M2’ significantly improved the R index according to Hamilton’s (1965) test. Bond distances and angles are reported in Table 2.

Chemical compositions were then determined on the same polished crystals by means of a JEOL JXA 8600 electron microprobe operating at 15 kV and 10 nA, with variable counting times: 10 s for Na, 15 s for other major elements, and 40 s for Ti, Cr and Mn. Matrix correction was performed using the Bence and Albee (1968) program modified by Albee and Ray (1970). Replicate analyses on different spots (six for each crystal) showed that the crystals examined were homogeneous within analysis precision. The estimated analytical precision is: Ô0.02 for CaO, FeO and MgO, Ô0.03 for SiO2 and TiO2, Ô0.05 for Cr2O3, Ô0.06 for Na2O, Ô0.07 for MnO and Ô0.1 for Al2O3. Compositional data reported in Table 3 are the average of these analyses. Atomic proportions were estimated on the basis of 4 cations; Fe3+ was evaluated on both the charge-balance and the

TABLE 1 (contd.) LNS8* LNS11* LNS14 LNS15 LNS17 LNS18 LNS26 LNS27* LNS28 LNS31*LNS32* Stage SiO2 TiO2 Al2O3 Fe2O3*{ MnO MgO CaO Na2O K2O P2O5 LOI V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Ba La Pb Ce Th

2 48.86 2.09 16.64 11.62 0.19 7.11 9.15 3.31 0.77 0.26 0 213 235 40 78 81 98 21 16 411 26 162 31 215 19 2 18 5

2 48.28 2.02 16.72 11.81 0.19 7.63 9.29 3.15 0.66 0.24 0 212 254 45 89 73 97 19 14 404 22 146 27 193 10 1 25 4

2 49.24 2.22 17.04 11.06 0.20 6.14 8.73 3.83 1.13 0.41 0 190 160 36 54 65 93 20 20 427 27 207 38 290 30 4 44 3

2 45.32 2.66 16.88 11.79 0.21 7.66 10.01 3.63 1.39 0.44 0 240 197 42 114 65 94 24 34 671 25 247 57 425 38 4 64 6

3 44.97 2.21 15.33 11.94 0.21 11.87 8.83 3.02 1.26 0.38 0 201 455 52 275 68 96 20 29 530 21 228 49 376 37 1 53 5

3 45.94 1.99 15.73 10.86 0.20 12.03 8.24 3.16 1.28 0.47 0.10 169 398 49 248 55 82 18 27 548 27 228 50 380 38 1 55 3

* selected samples from which clinopyroxenes were chosen Fe2O3 as total iron

959

3 49.35 2.22 17.4 10.91 0.21 5.68 8.34 4.07 1.39 0.43 0 184 127 36 53 48 96 21 29 467 35 271 54 338 38 5 61 4

3 46.60 2.17 16.93 11.95 0.21 7.34 10.27 3.39 0.83 0.31 0 208 200 42 83 67 95 22 15 469 28 177 34 227 24 3 24 1

3 47.16 2.55 17.86 10.99 0.20 6.25 8.78 4.14 1.55 0.51 0 199 116 32 68 47 92 22 36 733 27 283 64 466 52 3 81 7

3 45.67 2.21 15.35 11.62 0.21 11.31 8.43 3.14 1.19 0.52 0.35 214 389 55 238 54 88 20 26 580 24 235 50 379 41 7 66 5

3 48.06 2.73 18.00 10.76 0.20 5.01 8.57 4.26 1.76 0.65 0 234 53 35 43 44 90 21 36 703 29 317 69 516 54 4 104 7

L. BINDI ETAL.

experimental M1ÿO mean bond distance (best fit between observed and calculated M1ÿO mean bond lengths). The distribution of Mg and Fe2+ between M1 and M2 sites follows the procedure suggested by Dal Negro et al. (1982), which is based on the comparison between electron microprobe analyses (EPMA) and X-ray structure refinement (X-ray). Population M2 + M2’ was obtained using the total amount of Ca, Na and Mn derived from microprobe analysis and adding Fe2+ and Mg partitioned, taking into account eÿ(M2 + M2’)X-ray derived from the occupancy refinement.

The M1 site was considered filled the residual Fe2+, Mg, Al, Fe3+, and by Cr and Ti from the analysis. Comparisons between M1ÿOobs and M1ÿOcalc and between eÿ M1(X-ray) and eÿ M1(EPMA), are given in Table 3. Structural and compositional results In the conventional classification diagram (Morimoto, 1989) the compositions of the studied clinopyroxenes plot in the diopsideaugite field (Fig. 5). In detail, clinopyroxenes of

TABLE 2. Relevant crystal data, bond distances and polyhedron parameters for the examined clinopyroxenes. LNS9-1 1

LNS9-2 1

9.776(2) 8.948(2) 5.257(1) 106.06(2) 441.48(2) 440.01 879 1.91 35

9.779(1) 8.963(2) 5.259(1) 105.83(2) 443.47(3) 440.55 913 1.91 35

9.775(1) 8.958(2) 5.257(1) 105.83(2) 442.87(5) 440.01 907 2.04 35

9.770(1) 8.948(2) 5.257(2) 106.02(2) 441.73(5) 439.90 925 1.71 35

9.768(1) 8.951(2) 5.259(2) 106.05(2) 441.89(6) 440.17 878 2.63 35

9.766(1) 8.935(1) 5.265(2) 106.04(2) 441.53(4) 440.23 795 3.09 35

9.762(1) 8.923(1) 5.271(1) 106.09(2) 441.15(4) 439.81 815 2.38 35

9.745(1) 8.916(1) 5.261(1) 106.03(2) 439.34(4) 438.83 931 1.77 35

9.749(1) 8.898(1) 5.273(2) 106.15(2) 439.36(4) 438.90 830 2.24 35

9.731(1) 8.897(1) 5.268(2) 106.09(2) 438.22(5) 437.82 844 1.93 35

M2 site M2ÿO2 2.313(1) M2ÿO1 2.360(2) M2ÿO3C1 2.580(2) M2ÿO3C2 2.734(1) Mean 2.497 VM2 25.69 D (M2) 0.309

2.322(1) 2.361(1) 2.584(1) 2.732(1) 2.500 25.79 0.309

2.324(2) 2.365(1) 2.579(1) 2.730(1) 2.499 25.79 0.307

2.318(1) 2.363(1) 2.566(1) 2.748(1) 2.499 25.79 0.332

2.317(1) 2.364(1) 2.576(2) 2.732(1) 2.497 25.71 0.313

2.324(1) 2.365(2) 2.570(2) 2.726(1) 2.496 25.70 0.306

2.325(1) 2.364(1) 2.568(2) 2.724(1) 2.495 25.68 0.305

2.304(1) 2.360(1) 2.565(1) 2.722(1) 2.488 25.45 0.312

2.327(1) 2.363(1) 2.557(2) 2.715(1) 2.490 25.54 0.299

2.307(1) 2.358(1) 2.560(2) 2.720(1) 2.487 25.41 0.312

M1 site M1ÿO2 2.055(1) M1ÿO1A1 2.070(1) M1ÿO1A2 2.140(1) Mean 2.088 VM1 12.06 VM1norm 11.95 s2y (oct) 16.28 l (oct) 1.0051

2.058(1) 2.085(1) 2.143(1) 2.095 12.18 11.97 15.99 1.0050

2.054(1) 2.084(1) 2.145(1) 2.094 12.16 11.95 16.21 1.0051

2.054(1) 2.072(1) 2.139(1) 2.088 12.05 11.92 16.49 1.0052

2.053(1) 2.072(2) 2.139(1) 2.088 12.05 11.93 16.8 1.0053

2.048(1) 2.069(2) 2.134(1) 2.084 11.97 11.88 17.40 1.0055

2.044(1) 2.071(1) 2.133(1) 2.083 11.95 11.85 17.35 1.0055

2.041(1) 2.056(1) 2.132(1) 2.076 11.84 11.80 17.54 1.0056

2.033(1) 2.058(2) 2.130(1) 2.074 11.79 11.76 18.64 1.0059

2.035(1) 2.054(1) 2.130(2) 2.073 11.79 11.76 17.99 1.0057

T site TÿO2 1.594(1) TÿO1 1.610(1) TÿO3A1 1.665(1) TÿO3A2 1.683(1) TÿOnbr 1.602 TÿObr 1.674 Mean 1.638 VT 2.238 s2 y (tet) 25.56 l (tet) 1.0058

1.593(1) 1.607(1) 1.667(1) 1.683(1) 1.602 1.674 1.637 2.235 24.13 1.0057

1.596(1) 1.605(2) 1.665(1) 1.680(1) 1.601 1.673 1.637 2.233 23.25 1.0055

1.593(1) 1.609(2) 1.644(1) 1.702(1) 1.600 1.672 1.637 2.233 25.26 1.0060

1.594(1) 1.608(2) 1.666(2) 1.683(1) 1.601 1.675 1.638 2.237 24.57 1.0058

1.596(1) 1.613(2) 1.666(1) 1.686(2) 1.604 1.676 1.640 2.247 24.69 1.0058

1.596(1) 1.612(1) 1.667(1) 1.686(2) 1.604 1.677 1.640 2.246 24.55 1.0058

1.598(1) 1.614(1) 1.665(1) 1.683(1) 1.605 1.673 1.640 2.245 24.03 1.0056

1.600(1) 1.617(2) 1.670(1) 1.685(1) 1.609 1.678 1.643 2.256 24.85 1.0059

1.598(1) 1.615(1) 1.666(1) 1.683(1) 1.607 1.675 1.641 2.249 24.73 1.0058

Stage a b c b Vcell Vnorm cell Nobs. refl. Robs ymax

LNS9-3 LNS21-1 LNS21-2 LNS22-1 LNS22-2 LNS1-1 1 1 1 1 1 2

960

LNS1-2 LNS11-1 2 2


the first activity stage (Paleo-Linosa), show a slight increase in TiO2, Al2O3 and CaO, and a decrease of Fe2O3 with the increasing silica content of the host rocks (Table 3). Crystals from the second and the third stages (Arena Bianca and Monte Bandiera) display a slightly more restricted range of FeOtot, frequently with

very high MgO, Al2O3 and TiO2 contents. The two types of clinopyroxenes recognized at the microscope in samples LNS11 and for LNS31 have distinct compositions (Table 3). The crystals labelled LNS11-2 and LNS31-2, which represent the rounded cloudy crystals, are significantly different from those coexisting in the same rock

TABLE 2 (cont.)

Stage

LNS11-2 LNS8-1 2 2

a b c b Vcell Vnorm cell N.obs. Robs ymax

9.768(1) 8.941(2) 5.260(2) 105.93(2) 441.74(5) 440.08 799 2.80 35

9.742(1) 8.911(1) 5.266(1) 106.24(2) 438.91(4) 438.64 900 1.92 35

9.746(1) 8.912(1) 5.266(1) 106.21(2) 439.20(5) 438.90 904 1.88 35

9.733(1) 8.882(2) 5.279(1) 106.24(2) 438.15(4) 438.03 908 3.40 35

9.773(1) 8.948(1) 5.260(1) 105.91(2) 442.36(5) 440.46 959 2.04 35

9.733(1) 8.881(1) 5.277(1) 106.21(2) 438.00(5) 438.00 867 2.13 35

9.745(1) 8.905(1) 5.268(2) 106.26(2) 438.87(4) 438.61 937 2.52 35

9.749(1) 8.910(1) 5.267(1) 106.27(2) 439.19(4) 438.98 917 1.67 35

9.755(1) 8.897(1) 5.279(1) 106.09(2) 440.22(5) 440.04 889 1.74 35

9.748(1) 8.882(1) 5.286(1) 106.10(2) 439.72(4) 439.53 911 1.55 35

M2 site M2ÿO2 2.324(1) M2ÿO1 2.363(1) M2ÿO3C1 2.574(2) M2ÿO3C2 2.730(1) Mean 2.498 25.73 VM2 D (M2) 0.310

2.316(1) 2.361(2) 2.564(1) 2.720(1) 2.490 25.52 0.306

2.316(1) 2.362(1) 2.563(1) 2.720(2) 2.490 25.51 0.306

2.333(1) 2.367(2) 2.551(2) 2.705(1) 2.489 25.52 0.288

2.328(1) 2.366(1) 2.574(1) 2.728(1) 2.499 25.79 0.305

2.333(1) 2.366(1) 2.551(2) 2.704(1) 2.488 25.48 0.287

2.316(1) 2.363(1) 2.560(2) 2.716(1) 2.489 25.49 0.303

2.314(1) 2.362(1) 2.563(2) 2.721(1) 2.490 25.52 0.308

2.337(1) 2.372(1) 2.555(1) 2.706(1) 2.493 25.63 0.284

2.342(1) 2.376(1) 2.546(1) 2.701(1) 2.491 25.61 0.28

M1 site M1ÿO2 2.052(1) M1ÿO1A1 2.075(1) M1ÿO1A2 2.137(1) Mean 2.088 12.05 VM1 norm VM1 11.93 s2 y (oct) 16.49 l (oct) 1.0052

2.024(1) 2.055(2) 2.144(1) 2.074 11.79 11.77 20.54 1.0068

2.037(1) 2.056(1) 2.130(1) 2.074 11.80 11.78 18.23 1.0058

2.022(1) 2.049(1) 2.122(1) 2.064 11.62 11.61 20.45 1.0065

2.053(1) 2.076(1) 2.138(1) 2.089 12.07 11.93 16.59 1.0052

2.021(1) 2.047(1) 2.121(1) 2.063 11.60 11.60 20.55 1.0066

2.034(1) 2.054(1) 2.129(1) 2.072 11.77 11.75 18.46 1.0059

2.037(1) 2.056(1) 2.131(1) 2.075 11.81 11.80 18.17 1.0058

2.026(1) 2.054(1) 2.128(1) 2.069 11.71 11.70 19.92 1.0064

2.019(1) 2.057(1) 2.133(1) 2.070 11.71 11.70 20.96 1.0068

T site TÿO2 1.593(2) TÿO1 1.607(1) TÿO3A1 1.666(1) TÿO3A2 1.684(1) TÿOnbr 1.600 TÿObr 1.675 Mean 1.637 2.235 VT s2 y (tet) 25.02 l (tet) 1.0059

1.598(1) 1.615(1) 1.666(1) 1.686(1) 1.604 1.676 1.642 2.252 24.56 1.0058

1.600(1) 1.616(1) 1.667(1) 1.685(1) 1.608 1.676 1.642 2.254 24.44 1.0057

1.605(1) 1.621(1) 1.668(1) 1.690(1) 1.613 1.679 1.646 2.271 24.36 1.0057

1.594(1) 1.608(1) 1.666(1) 1.685(1) 1.600 1.675 1.638 2.238 24.90 1.0059

1.606(1) 1.624(1) 1.669(1) 1.688(1) 1.615 1.679 1.647 2.273 24.67 1.0058

1.601(1) 1.617(1) 1.667(1) 1.685(1) 1.609 1.676 1.643 2.257 24.22 1.0057

1.599(1) 1.616(1) 1.666(1) 1.685(1) 1.608 1.676 1.642 2.252 24.25 1.0057

1.606(1) 1.623(1) 1.671(1) 1.689(1) 1.614 1.680 1.647 2.275 25.18 1.0059

1.604(1) 1.620(1) 1.667(1) 1.693(1) 1.612 1.680 1.646 2.270 25.49 1.0060

refl.

LNS8-2 LNS31-1 LNS31-2 LNS31-3 LNS27-1 LNS27-2 LNS32-1 LNS32-2 2 3 3 3 3 3 3 3

˚ and Standard deviation is reported in brackets; atoms nomenclature after Burnham et al. (1967). Distances and angles are in A norm ˚ 3); Vnorm ˚ 3) (see text); s2y (oct), = normalized volumes (A degrees, respectively. Legend: Vcell = volume of unit cell (A cell and VM1 octahedral angles variance (Robinson et al., 1971); l (oct), mean octahedral quadratic elongation (Robinson et al., 1971); ?M2 = M2ÿO3C2 ÿ [(M2ÿO3C1 + M2ÿO2 + M2ÿO1)/3], distortion parameter for the M2 site (Dal Negro et al., 1982); s2y (tet), tetrahedral angles variance (Robinson et al., 1971); l (tet), mean octahedral quadratic elongation (Robinson et al., 1971)

961

L. BINDI ETAL.

Di

Wo

He

Monte Bandiera stage LNS31 LNS27 LNS32 Arena Bianca stage LNS1 LNS11 LNS8 Pa leo-Linosa stage LNS9 LNS21 LNS22

Fs

En

FIG. 5. Quadrilateral component classification diagram of clinopyroxene after Morimoto (1989).

and from those crystallized from the rocks of the same volcanic stage. Their chemical composition closely resembles that of crystals from the basaltic rocks of the first stage (Fig. 5 and following). In the clinopyroxenes studied the M1 site is dominated by Mg (0.53ÿ0.79 a.p.f.u.) with minor amounts of Fe2+ (0.08ÿ0.41 a.p.f.u.) and variable contents of R 3+ [Fe 3+ +Ti 4+ +Al 3+ +Cr 3+ ] M1 (0.06ÿ0.18 a.p.f.u.). A negative correlation is found between the mean M1ÿO bond length and the R3+ cations. The entry of R3+ in M1 causes volume reduction, mainly due to the s h o r t e n i n g o f t h e M 1 ÿO 2 d i s t a n c e . Clinopyroxene of the first stage can be distin-

TABLE 3. Electron microprobe analyses and crystal chemical formulae of refined clinopyroxenes.

Stage

LNS9-1 1

LNS9-2 1

LNS9-3 LNS21-1 LNS21-2 LNS22-1 LNS22-2 LNS1-1 1 1 1 1 1 2

LNS1-2 LNS11-1 2 2

SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O

51.55 0.78 1.85 0.00 1.42 9.92 0.39 12.93 20.93 0.49

51.62 0.27 1.02 0.00 1.04 15.13 0.72 9.55 20.86 0.54

51.55 0.30 1.15 0.01 1.40 14.73 0.74 9.60 20.58 0.62

52.21 0.70 1.86 0.00 0.00 11.46 0.40 12.33 20.79 0.56

52.13 0.60 2.01 0.00 0.36 11.01 0.42 12.48 20.82 0.51

51.33 1.24 3.14 0.04 0.36 9.13 0.35 13.11 21.30 0.53

50.98 1.19 3.23 0.02 0.72 9.29 0.32 12.98 21.41 0.41

51.76 0.80 3.32 0.55 0.37 6.60 0.14 15.88 20.20 0.33

50.62 1.13 4.33 0.43 1.06 6.03 0.18 15.02 21.13 0.32

51.14 0.88 4.25 0.65 0.74 5.97 0.12 15.56 20.20 0.46

100.25

100.75

100.68

100.31

100.34

100.53

100.54

99.95

100.25

99.97

Sum T site Si Al

1.934 0.066

1.973 0.027

1.969 0.031

1.959 0.041

1.954 0.046

1.909 0.091

1.900 0.100

1.906 0.094

1.865 0.135

1.881 0.119

M1 site Mg Fe2+ Fe3+ Al Cr Ti

0.712 0.211 0.040 0.016 0.000 0.021

0.531 0.414 0.030 0.019 0.000 0.006

0.529 0.401 0.040 0.021 0.000 0.009

0.656 0.284 0.000 0.040 0.000 0.020

0.690 0.238 0.012 0.043 0.000 0.017

0.646 0.262 0.014 0.045 0.001 0.032

0.666 0.239 0.020 0.042 0.000 0.033

0.784 0.116 0.010 0.052 0.016 0.022

0.738 0.136 0.029 0.053 0.013 0.031

0.744 0.126 0.021 0.065 0.020 0.024

M2 site Ca Na Mn Fe2+ Mg

0.841 0.036 0.012 0.100 0.011

0.854 0.040 0.023 0.070 0.013

0.842 0.046 0.024 0.070 0.018

0.836 0.041 0.013 0.076 0.034

0.836 0.037 0.013 0.107 0.007

0.848 0.038 0.011 0.022 0.081

0.855 0.030 0.010 0.050 0.055

0.797 0.024 0.004 0.087 0.088

0.834 0.023 0.006 0.050 0.087

0.796 0.033 0.004 0.058 0.109

eÿM1EPMA 15.74 eÿM1X-ray 15.61 M1ÿOobs M1ÿOcalc Charge Mg#

18.30 18.12

18.29 18.13

16.22 16.07

15.45 15.74

16.24 16.30

16.00 16.29

14.23 13.97

14.52 14.75

13.28 13.61

2.088 2.085

2.095 2.095

2.094 2.094

2.088 2.086

2.088 2.083

2.084 2.082

2.083 2.082

2.076 2.075

2.074 2.074

2.073 2.073

11.996

11.994

12.002

11.998

12.006

11.995

11.998

12.004

11.999

12.002

0.699

0.529

0.537

0.657

0.669

0.719

0.714

0.811

0.816

0.823

962


guished from the others by their longer ˚ ). The M2 site (VM2 = distances (2.083ÿ2.095 A ˚ 3) is dominantly occupied by Ca 25.41ÿ25.79 A (0.80ÿ0.86 a.p.f.u.). Na is characteristically low (