Magmatic Di#erentiation Process Inferred from Plagioclase Zoning and Its Pattern

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1 .3 (,**.) /,.3,00,**- 3 3,**. 2,0 Magmatic Di#erentiation Process Inferred from Plagioclase Zoning and Its Pattern Akira TSUNE and Atsushi TORAMARU Relationships between whole rock composition of magma and plagioclase zoning were investigated on volcanic rocks from the Shirahama Group, Izu Peninsula, Japan, which have the various chemical compositions of tholeiitic series produced only by crystallization di#erentiation. The plagioclase zoning can be divided into the following three zoned regions, (+) oscillatory-zoned, (,) patchy-zoned, and (-) unzoned regions, based on Anderson(+32.) s classification. We described the flatness of zones and the spatial distribution of zoned regions from core to rim of the phenocrysts, and found the relations between the characteristics of plagioclase zoning and the whole rock compositions. As the whole rock SiO, increases, (+) the number of resorption zones, defined as zones with resorbed track, increases ; (,) the frequency of plagioclase with patchy-zoned region decreases ; (-) the frequency of plagioclase with the patchy-zoned region in central parts of crystals increases ; (.) the diversity in the zoning pattern becomes smaller. In basalts, various zoning patterns can be observed whereas in dacite, oscillatory zoned regions are dominant. Our observation can be explained by a simple model involving the homogenization processes of heterogeneity in a magma chamber, associating with the crystal growth or dissolution processes. According to the model and the observations of natural plagioclase zoning, we can give a constraint on the characteristics of homogenization process developed in an evolving magma chamber: parameters of the characteristics f, the volume ratio of relatively di#erentiated part in the magma to homogenize and g, chemical contrast of the magma. At the initial stage (basalt magma), the homogenization process with large f and large g is dominant. At the later stage (dacite magma), the homogenization process with large f or small g is dominant. Key words : plagioclase zoning, heterogeneous magma, homogenization process, the Shirahama group + Vance, +30, ; Wiebe, +302 ; Anderson, +32. ; Stamatelopoulou-Seymour et al., +33* ; Pearce and Kolisnik, +33* ; Singer et al., +33/ ;,+33/ ; Hattori and Sato, +330 ; Umino and Horio, +332 ; Kuritani, +332 ; Murphy et al.,,*** ; Stewart and Fowler,,**+ (Tsuchiyama, +32/), (Kawamoto, +33, ; Murphy et al.,,***). (Shore and Fowler, +330) (Singer et al., +33/) (L Heureux, +33-) 3,* ++3, Graduate School of Natural Sciences, Kanazawa University, Kakuma-machi, Kanazawa 3,* ++3,, Japan. : 2+, 2/2+ 0 +* + Present address: Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, 0 +* +, Hakozaki, Higashi-ku, Fukuokashi 2+, 2/2+, Japan. Corresponding author: Akira Tsune tsune@geo.kyushu-u.ac.jp

2 250 Anderson (+32.) Stamatelopoulou-Seymour et al. (+33*) SiO, wt. Anderson (+32.) SiO, wt. SiO, wt.,, + NDIC Nomarski Di#erential Interference Contrast microscopy ; ; Anderson +32. NDIC (Anderson, +32-). NDIC NDIC (Anderson, +32-). Ca Na Ca Na NDIC An + mol. (Anderson, +32-). NDIC NDIC NDIC mm SEM NDIC (Anderson, +32-) NDIC (Pearce and Clark, +323). X Rigaku -,1* /* kv,,* ma, - mm EPMA (JEOL JXA-22**R) +/ kv, +, na, - mm Nikon UFX-IIA, Nikon FX-/WA, Adobe Photoshop TM NIH Image,, (zoning) (zone) + + (ridge) (valley) (Anderson, +32.). NDIC (zoned regions) Anderson (+32.) (zoning pattern) Anderson

3 251 (+32.) : + (oscillatory zoned region),, (patchy zoned region or irregular zoned region), - (unzoned region). o, p, u u, o, p, o u-o-p-o Fig. + u-o-p-o Fig., a u-o-po NDIC (Figs., cand, d). (Figs., b,, eand, f). (Anderson, +32.), (Figs. + and, b).,* Fig. +. Schematic illustration showing zoning pattern of u-o-p-o and typical zones (flat, wavy, and resorption zones) of plagioclase. (Anderson, +32.) p p p (Fig., b). unknown Anderson (+32.) miscellaneous n.d. Pearce and Kolisnik, +33* p-p o-o (thickness) (flat zone) (wavy or curvy zone) (Fig. + ; Anderson, +32.). Pearce and Kolisnik(+33*), :An (+ +* An mol. ), (+ +* mm) + An (2-1 An mol. ), (- +** mm),, Pearce and Kolisnik(+33*), (resorption zone) Figs., c, d (Fig., d).,, (Mullins and Sekerka, +30.)

4 252 ῐ ΐῌῑ ῒ Fig.,. Photographs of plagioclase phenocrysts in the Shirahama group volcanic rocks. Photoes (a)-(e) are NDIC images, and (f), closs-polarized light image. (a) Zoning pattern of u-o-p-o (basalt -+.). (b) Patchy zoned region in, mm long phenocryst with zoning pattern of u-p-o (basalt -+.). The patchy zoned region can be recognized at the left crystal face, but not recognized at the right (arrow). (c) Flat zones in /** mm long phenocryst with zoning pattern of u-o (basalt -+.). (d) Resorption zones defined as eroded-like tracks at crystal corner (dacite -*+). The phenocryst is /** mm long, with o zoning pattern. (e) Honeycomb structure (andesite -+,ῌ-). The phenocryst is +./ mm long, with zoning pattern of p-o. (f) Dusty zoning (andesite -,0).

5 253 ; Kingery et al., , - (Anderson, +32.), (Tsuchiyama, +32/), (Kawamoto, +33,), (Lofgren, +31. ; +32*) (Fig., b; Anderson, +32.). (Fig., e) (Fig., f) (Kawamoto, +33,). + Lofgren, +31. (Tsuchiyama, +32/)., - (Tamura, +33.) Kano, +323, (Tamura, +33.). Tamura (+33/) + 3 Table + +** 3 Tamura (+33/) SiO, wt./* 00wt. (Table + ; Tamura, +33.). (Table +), -+. An 32 3, -,0, -*/A, -*/B, -+, - An , -**, -+, +, -*+ An 20./ (Fig. -). Table, (Table +) Table, SiO, wt. +* / (Tamura, +33/). SiO, wt. *.. mm, *.+ mm, *.*+ mm,

6 254 (Table,), +. cm, +** +** -,*** (Table,) - (o, p, u) *... mm u-p-o, o-p-o, p-o-p-o (Table -). Anderson (+32.) (Fig., b) +* /* + / mm +, (Fig., ). u-o-p-o u p o -, -,0 *.,, mm (Fig., f) (p-o) u-op-o + - +*,** mm +* 0* + / mm +*,* mm +*,* mm +, mm,, / mm + - -*/A *.. + mm (Table -), Fig. -. Histograms of An content (mol. ) of plagioclase phenocryst cores in the samples.

7 255 +, -* 0* + - mm +** +/* mm +* -* mm, mm / 2* +** mm, - mm, + / -*/B *./ *.2 mm *.- *./ mm o Table +. Whole-rock major element analyses and modal composition. Table,. Modal composition and area of plagioclase phenocrysts of the samples obtained by image processing.

8 256, +* mm +**,* mm +. u-o, p-o + - mm +*,* + -+, - *.+ - mm (Fig., e), +** mm (p-o) (o-p-o, p-o-p-o) (/*,** mm) + / mm,* +/* *.+, mm, *.+ *.- mm + mm (p-o) (o, u-o) (Table -). + +* mm +/* - -*. SiO, wt. SiO, wt. : + SiO, wt. (Fig.. a)., SiO, wt. (Fig.. b). SiO, wt. u (Fig.. c). - SiO, wt. (Table -).. SiO, wt. Table -. Variety of zoning patterns in basalt (-+.), andesite (-*/A) and dacite (-*+).

9 257 / / + SiO, wt. (Fig. /), SiO, wt. +, + + (self-mixing) boundary layer (Tait, +322 ; Kuritani, +332), +320 ; +321 (Jaupart and Tait, +33/) Fig... (a) Relation between the whole-rock SiO, contents (wt. ) and the number of resorption zones in oscillatory zoned regions of plagioclase phenocrysts. Representative number means the maximum number for phenocrysts. (b) Frequency of zoning pattern of o (open square) and those with p (solid square) in each sample plotted against the SiO, contents. (c) Frequency of zoning pattern with u for plagioclase phenocrysts in each sample plotted against the whole-rock SiO, contents. Fig. /. Schematic illustration showing the characteristics of plagioclase zoning in basalt, andesite and dacite.

10 258 (Couch et al.,,**+). + (McBirney et al., +32/). (Tamura, +33. ; +33/) An (Fig. -), (Table,) +, o, p (Mullins and Sekerka, +30.). Lofgren (Lofgren, +31. ; +32*). Tsuchiyama (+32/) p p (Anderson, +32.) o p o o o Tsuchiyama (+32/) simple dissolution o, Table., SiO, wt. u u o /, (a) A T A, C SA, C LA A B T B T A C LB C LA M T M, C LM a, (Fig. 0 a). Ca B B + A a B case+, A B b case, Table.. Relation between the driving forces and the formed zoned regions.

11 259 a C LM ; Fig. 0 a C - C / a T M ; Fig. 0 a C + C, C - C. T M A C LA C LA* a C LA C LB C LM Fig. 0 a C --C 0 C / M (C,) (C /) a M a a (C.) a a M (Fig. 0 a). ; T M C SA* C SA DC S C SA* C SA + : C LM T M C LA* DC L C LM C LA*, Fig. 0. The binary system of An-Ab and the representative composition of crystaland surrounding melt. The figures show sequence of events that the magmas of the local systems A and B produce the system M by mixing. (a) case +. (b) case,. DC L C LA C LB C LM DC S C LM C LB / - DC S DC L DC S DC L A f g A B

12 260 V A, V B f (V A V B) A V A g A B f g VA f V A V B g CLB C LA -. DC L f *./ V A V B DC S f V A DC S, DC L T M C LM C LA, T A, C LB, T B, f T M C SA* C LA* C LM T M T M ft A+ f T B C LM fc LA+ f C LB / 0 T M C SA* C LA* T M C SA* C LA* C SA* f T M C LA* g T M 1 2 T M T M f +, g + T M f + C SA* T M g + C LA* 3 +* DC S, DC L f C LA, C LB DC S C SA* C SA f ft A+ f T Bf T A f fg + C LA + f g + C LB f g + C LA ++ DC L C LM C LA* fc LA+ f C LBg T M fc LA+ f C LB g ft A+ f T B fc LA+ f C LB g fg + C LA + f g + C LB +, f, g (+31,) C SA* f T M + em e n e m C LA* g T Me n + em e n e m m m T DSAb T Tmelting Ab RT n n T DSAn T Tmelting An RT / +0 R DS Ab, DS An T melting Ab, T melting An Ab An (+31,) DS Ab +*.+* (cal K + mol + ), DS An +/.+* (cal K + mol + ), T melting Ab +-3+ (K), T melting An +2,- (K) f, g +31, DC S, DC L f ++ +, Fig. 1 a ++ +, f DC S, DC L C LA *., C LB *.2 g. DC L f DC S f p, o f, g Fig. 1 a DC S DC L DC L DC S p DC L DC S DC S, DC L, DC S p DC S

13 261 DC S DC c p DC c simple dissolution partial dissolution (Tsuchiyama, +32/) DC DC c p DC S DC c o DC S DC c DC c f f c (Fig. 1 a), f f c p f f c o o p g C LA *./ C LB *.1 Fig. 1 b g +.. g. Fig. 1 b g f DC S DC c p g p DC S f, g p DC S f A g A B o DC S f g /. A B b A B b case, case+ A B M b b (Fig. 0 b). case+ b C LM ; Fig. 0 b C - C / b, T M ; Fig. 0 b C + C, C - C. T M B C LB C LB* case+ C LA* b case+ DC L C LB T M C LB* DC LC LB C LB* +1 b case+ b Fig. 1. C as a function of f. (a) C LA *.,, C LB *.2. (b) C LA *./, C LB *.1. C LB C LA C LB C LM Fig. 0 b C --C 0 C / Ca DC L DC L case+ DC L :Fig. 0 b DC L C LB* C LA

14 262 DC L DC L (Fig. 0 b). b DC L case, p DC L DC L DC L f, C LA, C LB DC LC LB C LB* C LB g ft A+ f T B C LB g fg + C LA + f g + C LB +2 DC L case+ f, g Fig. 2 a Fig. 2 a C LA *./ C LB *.1 g C LA *., C LB *.2 g f DC L case+ DC L DC c ; f f c p p p DC L f A g Fig. 2. (a) C L as a function of f. (b) C as a function of g. A B case+ p case+ case, case+ case, f A A a p ; Tsuchiyama, +32/ B b o f A B b p ; Lofgren, +32* ; Anderson, +32. A a o g p g p o g A a b g A a b Fig. 2 b g DC p DC case+ case, f g p g f p g p case + case, f p p-o o SiO, wt.

15 263 (Fig.. a), p (Fig.. b) p p (Table.) / / SiO, wt. SiO, wt. (Fig. 3). SiO, wt. p g case, Fig. 3 case, f p Fig. 3. Schematic illustration showing summarized relationships between the whole rock composition and the characteristics of the homogenization process in a magma chamber. See text for details.

16 264 g case+ Fig. 3 case+ f p g case p (Fig. /), case+ case, Fig. 3 SiO, wt. SiO, wt. g p +320 (Singer et al., +33/). Anderson (+32.) o p / 0 An-Ab, Diopside DC p An An p DC S An (Kirkpatrick et al., +313) p + + Cashman and Marsh, +322 Anderson, +32. ; +33/ ; Umino and Horio, +332 Pearce et al., +321 SiO, wt. 0 A

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