ESTUDIOS DE CASOS-CONTROLES PARA DETECTAR FACTORES DE RIESGO

The mineral phases in the Group II eruption deposits show a wide range of compositions with large core-rim variations, as well as large variations within cores and rims (Table 3.3; Appendix B). Complex zoning patterns and mineral textures are displayed by clinopyroxene and plagioclase phenocrysts, which are discussed in detail in Section 3.2.2.

3.2.1.1 Plagioclase

The abundance of plagioclase varies between units, from trace to 10 vol.% (Table 3.2). Unit N contains the highest abundance, while plagioclase phenocrysts in unit F are rare. Phenocrysts are typically euhedral to subhedral, up to 2mm in length, with a prismatic habit. Both fresh and altered, sieve textured, phenocrysts are present in all samples, commonly displaying complex zoning patterns. The Group II units have a high microphenocryst population relative to phenocrysts. Microphenocrysts (<0.3mm; Wilcox, 1954) are euhedral and prismatic, and have a fresh appearance.

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Glass SiO2 (wt.%) Plagioclase An (mol.%) Clinopyroxene Mg# Orthopyroxene Mg# Olivine Mg#

Unit Average Cores Rims Cores Rims Cores Rims Cores Rims

N 59.1 (n=55; 49.5-64.8) 57 (n=12; 53-64) 56 (n=25; 47-65) 0.81 (n=12; 0.74-0.89) 0.80 (n=21; 0.72-0.82) 0.87 (n=2; 0.87) 0.79 (n=13; 0.76-0.81) 0.80 (n=8; 0.77-0.80) 0.79 (n=10; 0.77-0.80) F 58.5 (n=5; 56.8-58.7) 52 (n=11; 42-62) 52 (n=10; 44-59) 0.83 (n=25; 0.70-0.90) 0.81 (n=22; 0.73-0.85) - - 0.87 (n=8; 0.86-0.88) 0.81 (n=9; 0.79-0.82) D 59.2 (n=41; 54.4-66.9) 54 (n=33; 34-78) 46 (n=22; 24-74) 0.82 (n=37; 0.72-0.88) 0.81 (n=25; 0.75-0.83) 0.86 (n=2; 0.86) 0.79 (n=2; 0.78-0.81) 0.86 (n=14; 0.85-0.87) 0.79 (n=13; 0.79-0.81)

Table 3.3 Mineral chemistry of scoria from the Group II eruption deposits.

For each mineral, the average composition is given with the number of analyses and the range of compositions in italics. Analyses were carried out on the Jeol 8234 electron microprobe at the University of Leeds. Anorthite content is calculated as Ca/(Ca+Na+K), and Mg# is calculated as Mg/(Mg+Fe). The full dataset is presented in Appendix B.

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Cores and rims display a large variation in anorthite content ranging from andesine (An30) to

bytownite (An78; Figure 3.3). One plagioclase crystal from unit D has a rim composition of

An24 (Table 3.3; Figure 3.3). The microphenocrysts have a narrower range in composition,

varying from An48 to An65.

Figure 3.3 Feldspar ternary classification diagram after Deer et al. (1992) for units N, F and D of the Group II eruption deposits.

Figure 3.4 shows the range of core and rim anorthite contents in plagioclase phenocrysts from the Group II eruption deposits. Units N and F show similar ranges in core and rim compositions (10-20% An), while unit D shows a much wider range of anorthite contents from An34 to An78 in

cores, and An24 to An74 in rims (Figure 3.4). These variations in plagioclase composition are

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Figure 3.4 Ranges of core and rim anorthite (An mol%) composition of plagioclase phenocrysts for the Group II eruption deposits.

Also shown are the ranges of plagioclase compositions for the Group I units.

3.2.1.2 Clinopyroxene

Clinopyroxene is present in all units varying in abundance from trace to 5 vol.% (Table 3.2). As observed in the Group I eruption deposits, individual phenocrysts are rare, more commonly occurring in pairs or in glomerocrysts. Crystals are typically ~0.5mm across, but can reach up to 1.5mm, and are euhedral to subhedral with rounded corners suggesting some disequilibrium. Zoning is common, and sieve textured growth zones are present in many of the phenocrysts. These features are discussed in detail in Section 3.2.2.3.

Cores and rims cluster in the diopside and augite fields on the clinopyroxene Ca-Mg-Fe classification diagram of Morimoto (1988; Figure 3.5) showing little variation between core and rim compositions.

Figure 3.5 Classification of clinopyroxenes in Group II scoria samples in the pyroxene quadrilateral after Morimoto (1988).

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The ranges of core and rim Mg# in clinopyroxene of the Group II deposits are shown in Figure 3.6. Clinopyroxenes from unit F show the widest range in Mg# with cores varying from Mg# 0.70 to 0.90 and rims ranging from Mg# 0.73 - 0.85. Core and rim compositions in units N and D typically display narrower ranges (Table 3.3). These variations in clinopyroxene Mg# are discussed in detail with respect to zoning patterns in Section 3.2.2.4.

Figure 3.6 Ranges of Mg# in cores and rims of clinopyroxenes from the Group II eruption deposits. Also shown are the range Group I clinopyroxene Mg#.

3.2.1.3 Orthopyroxene

Orthopyroxene crystals are rare in the Group II eruption deposits, with trace orthopyroxene present in units N and D, generally occurring together with clinopyroxene. Crystals are euhedral to subhedral, and show signs of disequilibrium with slightly embayed rims.

Figure 3.7 Ca-Mg-Fe classification diagram for orthopyroxenes after Morimoto (1988).

The rare orthopyroxene phenocrysts observed in the Group II eruption deposits, units N and D, lie in the enstatite field with clear differences in core and rim composition.

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Cores and rims plot as enstatite on the orthopyroxene classification diagram of Morimoto (1988; Figure 3.7); cores are more magnesian (Mg# 0.86-0.87) than rims (Mg# 0.76-0.81; Table 3.3; Figure 3.8).

Figure 3.8 Ranges of orthopyroxene core and rim Mg# for units N and D of Group II.

Orthopyroxene is not present in unit F. Core compositions do not show any variation, while rim compositions range from 0.78-0.81 and 0.78-0.81 for units N and D, respectively. Also shown are the ranges of Mg# in orthopyroxenes from the Group I eruption deposits.

3.2.1.4 Hornblende

Hornblende phenocrysts and microphenocrysts are present in all three of the Group II eruption deposits. Hornblende is the dominant mineral phase in unit D, which has 3-7 vol.%, while units N and F have only trace abundances of hornblende (Table 3.2). Phenocrysts in unit D are typically up to 2mm in length and are subhedral with rounded edges (Figure 3.9a). Reaction rims of pyroxene, plagioclase and Fe-Ti oxides are rare; however, holes in the centre are common (Figure 3.9) and many phenocrysts appear broken. Hornblende in units N and F occurs as microphenocrysts with an anhedral, broken appearance (Figure 3.9d).

The Group II hornblende phenocrysts and microphenocrysts are calcic-amphiboles (i.e. CaB ≥

1.50; see Section 2.2.1.4) and according to the classification of Leake et al. (1997), are magnesiohastingsite. Core and rim compositions show little variation; however, backscatter SEM images reveal zoning textures which are discussed in detail in Section 3.2.2.5 (Figure 3.9c).

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Figure 3.9 Backscatter SEM images of hornblende from Group II samples.

Images are from phenocrysts and microphenocrysts from samples VF01-05N and VF00-06O from unit D: (a) is a phenocryst with a large, vesiculated melt inclusion (M.I.); (b) is a phenocryst displaying a breakdown reaction rim of Fe-Ti oxides, pyroxene and plagioclase; (c) is a zoned hornblende with an embayed and partially broken rim; and (d) is an image of microphenocrysts from sample VF95-09G from unit N. All the phenocrysts contain holes.

Figure 3.10 Classification of calcic-amphiboles with (Na+K)A > 0.5 after Leake et al. (1997).

Core and rim phenocryst and microphenocryst compositions cluster in the magnesiohastingsite field.

(c)

(b)

(d)

(a)

M.I.

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3.2.1.5 Olivine

Olivine is present in all three units varying in abundance from trace to 3 vol.% (Table 3.2), occurring as rounded and embayed phenocrysts up to 1mm across and as microphenocrysts (Figure 3.11a). Olivine is also common in glomerocrysts with clinopyroxene and less commonly, orthopyroxene and phlogopite (Figure 3.11b).

Figure 3.11 Backscatter SEM images of olivine phenocrysts from sample VF00-06O from unit D. (a) is a rounded phenocryst with a hole in the centre; and (b) is an olivine (ol) and clinopyroxene (cpx) glomerocryst. Both olivines are zoned, with a more Fe-rich rim, as indicated by the light grey colour of these.

Core and rim Mg# of olivine phenocrysts from the Group II eruption deposits are typically more magnesian than olivines from the Group I units, with cores of up to Mg# 0.88 (Figure 3.12). The ranges of core and rim compositions are narrow; for example, unit D cores have Mg# 0.85- 0.87 and rims have Mg# 0.79-0.81 (Figure 3.12; Table 3.3).

Figure 3.12 Ranges of olivine core and rim Mg# in the Group II eruption deposits.

Also shown are the ranges of olivine Mg# in the Group I deposits. The cores of the Group II olivines in units F and D are highly magnesian with Mg# up to 0.88.

(a)

(b)

cpx

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3.2.1.6 Phlogopite

One of the distinguishing features between the Group I and Group II eruption deposits is the presence of phlogopite in the Group II tephra samples. Phlogopite varies in abundance from trace (in unit N) to 2 vol.% (in unit F; Table 3.2). Phlogopite occurs as microphenocrysts in units N and D, and phenocrysts up to 1mm in length in unit F. Phenocrysts are distinctive in thin section with a fibrous, sinuous texture (Figure 3.13), and commonly have a speckled appearance as a result of chloritisation along cleavage planes and crystal edges (Figure 3.13c). Phlogopite also occurs in glomerocrysts together with clinopyroxene, olivine and less commonly orthopyroxene (Figure 3.13d).

Figure 3.13 Backscatter SEM images of fibrous phlogopite from the Group II samples.

Phlogopite phenocrysts (a and b) from sample VF01-02Ps from unit F; (c) fibrous phlogopite displaying dark grey, speckled patches at the crystal edges and along the cleavage planes reflecting chloritisation from sample VF00-06P (unit F), and; (d) glomerocryst of phlogopite (phlog) and clinopyroxene (cpx) from sample VF02-01G (unit F).

(a)

(b)

(c)

(d)

phlog

cpx

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3.2.1.7 Fe-Ti oxides

Fe-Ti oxides including titanomagnetite and ilmenite are common in the groundmass in all the Group II eruption deposits, with abundances varying from ~1 to 3 vol.% (Table 3.2). Unit D contains the highest abundance of Fe-Ti oxides. Fe-Ti oxides are also common in glomerocrysts and inclusions in olivine and clinopyroxene. Rare breakdown rims of hornblende phenocrysts in unit D also contain Fe-Ti oxides.

3.2.1.8 Accessory Minerals

The only accessory mineral observed in the Group II tephra is apatite, which occurs as inclusions in hornblende phenocrysts and as microphenocrysts in the groundmass. Luhr and Carmichael (1982) reported spinel inclusions in olivine and in the groundmass of scoria samples, however, no spinel was observed during this study.

3.2.1.9 Groundmass

The groundmass typically comprises mafic to felsic glass with 49.5 to 66.9 wt.% SiO2 (Table

3.3) and commonly has a streaky appearance in units N and D. Units N and D show the widest range of glass SiO2 contents within the Group II deposits, with variations of up to 15 wt.%

(Figure 3.14). By contrast, the groundmass glass of unit F scoria shows very little variation in SiO2 content (Figure 3.14) and does not have a streaky appearance. These variations in glass

chemistry are also reflected in other elements such as TiO2 and MgO.

The groundmass is typically highly vesiculated and contains abundant microlite crystal phases dominated by plagioclase and clinopyroxene. These textures are discussed in Section 3.2.2.7.

Figure 3.14 Ranges of glass SiO2 content of the Group II scoria samples.

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