Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
RHÖNITE IN A L K A L I BASALTS: STUDIES OF SILICATE M E L T INCLUSIONS IN O L I V I N E PHENOCRYSTS
S H A R Y G I N . V.V.'. K Ó T H A Y , K.2, P E T Ő , M.2, T Ö R Ö K , K.2, T I M I N A, T.Ju.3, V A P N I K , Y.4, K U Z M I N , D . V . ' , S Z A B Ó , C.2
' Institute of Mineralogy and Petrography, Koptyuga pr. 3, 630090 Novosibirsk, Russia.
2Lithosphere Fluid Research Lab, Department of Petrology and Geochemistry, Eötvös University, P á z m á n y Péter sétány 1/C, H - l 117 Budapest, Hungary.
3 Department of Geology and Geophysics, Novosibirsk State University, Pirogova st. 2, 6 3 0 0 9 0 Novosibirsk, Russia.
4D e p a r t m e n t of Geological and Environmental Sciences, Ben-Gurion University of the Negev, P.O. B o x 653, 84105 Beer- Sheva, Israel.
E-mail: sharygin@uiggm.nsc.ru Introduction
Rhönite, Ca2(Mg,Fe2 +)4Fe", +Ti[Al3SÍ302o], is an u n c o m m o n phase in alkali basalts. It scarcely occurs as phenocrystal or groundmass mineral in silica-undersaturated basalts (Cameron et al., 1970; Magonthier, Velde, 1976; Olsson, 1983; K u n z m a n n , 1999; Seghedi et al., 1995; Prestvik et al., 1999). In some basalts and their deep-seated xenoliths rhönite is observed in reactionary rims around amphibole (Kyle and Price, 1975; Gushchin et al., 1991). However, recent studies of silicate melt inclusions showed that rhönite is a c o m m o n daughter phase of silicate melt inclusions in olivine phenocrysts of most alkali and subalkali basalts (Ananiev, Okrugin, 1991; Kuz'min et al., 1999; Golovin et al., 2000; Kóthay et al., 2001), whereas it may be absent as phenocrystal or groundmass mineral in these rocks. This work is a present-day compilation of all data we collected concerning to rhönite f r o m silicate melt inclusions.
Rhönite from inclusions
T o study crystallization conditions of rhönite in alkali basalts, we used silicate melt inclusions hosted by olivine phenocrysts from different localities (Table 1). It should be noted that rhönite-bearing inclusions occur relatively rarely and are mainly confined to the central zones of host olivine, whereas silicate melt inclusions from the outer zones are generally free in rhönite.
Besides rhönite, silicate melt inclusions usually contain Ti-rich augite, apatite, Al-spinel, sulfide blebs, amphibole, Ti- magnetite, ilmenite as daughter phases, and Cr-spinel and sometimes Cr-diopside as trapped crystals. A halo of small inclusions around large silicate melt inclusions with rhönite is typical of some olivines (Fig. 1) that might indicate high pressure for trapping of inclusions and their subsequent leakage. According to petrography of inclusions, rhönite is o n e of the earliest daughter phases. It crystallized after trapped Cr-spinel and daughter Al-spinel and before Ti-Al-rich clinopyroxenes, apatite and ilmenite. Thermometric data for rhönite-bearing inclusions are available only for s o m e occurrences. According to these data, during heating apatite disappeared at 1060-1100°C, ilmenite - at 1030-1130°C, Ti-rich augite - in the range of 1130-1210°C, rhönite - at 1180-1245°C, and Al-spinel - at 1270°C (the Pécskő basalts). Homogenization temperatures of inclusions are in the range of 1250-1355°C, and possible pressure of their trapping estimated on coexisting C 02 inclusions is higher than 2 kb.
Oxygen fugacity estimated on the olivine-Cr-spinel pair (the Hegyestű basalts) is higher in 1-2 order than the Q F M buffer.
Table 1. Rhônite occurrences in olivine-hosted silicate melt inclusions from some alkali basalts of the world.
Locality Rock Phase composition
of inclusions
Tmelting
for Cpx
Tmelting
for Rhö
T1 honn °c P, kb
estim. Reference Kamchatka, Russia high-Al basalt Gl+Opx+Cpx+
Al-sp±Rhö±Amph+g
1250-1350 2-3 Ananiev, Okrugin, 1991 Tsagan-Khurtei ridge, Transbaikalia,
Russia
trachybasalt Gl+Cpx+Rhö±Mgnt
±Al-sp+Ap+Sulf+g
1280-1320 0.8-1 Kuz'min et al., 1999
Lurbun volcano, Udokan Volcanic Field, Transbaikalia, Russia
olivine nephelinite
Gl+Cpx+Rhö+
Ap+Sulf+g
>1250 >3
Tergesh and Bele pipes, Khakasia, Russia
basanite Gl+Cpx+Rhö±Ilm
±Mgnt+Sulf+g
1130-1190 >1200 1280-1310 >3 Golovin et al.. 2000
Makhtesh Ramon, Southern Israel basanite Gl+Cpx+Rhö+Sulf+g 1140-1180 1180-1230 1310-1355 >3 Hegyestű, Kabhegy, Badascsony,
Haláp volcanoes, Bakony-Balaton Highland Volcanic Field, Hungary
basanite Gl+Cpx+Rhö+
Al-sp+Ilm+Sulf+g
1190-1210 1220-1245 1270-1310 >3 Kóthay etal., 2001
Pécskő, Eresztvény, Magyarbánya, Terbelény, Nógrád-Gömör Volcanic Field, Hungary-Slovakia
hawaiite basanite
Gl+Cpx+Al-sp±
Rhö±Amph+Sulf+g
1165-1235 1300-1350 2-3
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Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
Figure 1. Rhonite as daughter phase of silicate melt inclusions in olivine phenocrysts from alkali basalts around the world.
D i s c u s s i o n
T h e d a t a o b t a i n e d s h o w that r h o n i t e in o l i v i n e - h o s t e d silicate melt i n c l u s i o n s c ry s ta ll iz e d in a n a r r o w t e m p e r a t u r e r a n g e ( 1 1 8 0 - 1 2 4 5 ° C ) and at p r e s s u r e less than 3 - 5 k b b a s e d on C 02 m i c r o t h e r m o m e t r y . T h e p e t r o g r a p h y of s t u d i e d i n c l u s i o n s indicates the f o l l o w i n g s e q u e n c e to c r y s t a l l i z e minerals: ( C r - s p i n e l , s o m e t i m e s C r - d i o p s i d e ) - > Al-spinel - > r h o n i t e - > T i - A l - c l i n o p y r o x e n e - T i - m a g n e t i t e (ilmenite). A c c o r d i n g to B o n a c c o r s i et al. ( 1 9 9 0 ) , the r h o n i t e structure r e p r e s e n t s an alternation of " p y r o x e n e " and " s p i n e l " slabs. In this s e n s e , r h o n i t e is n o t o n l y a transitional m e m b e r b e t w e e n Al-spinel a n d T i - A l - a u g i t e in a crystallization s e q u e n c e but also an i n t e r m e d i a t e m e m b e r of a p o l y s o m a t i c series h a v i n g spinel and p y r o x e n e as the e n d - m e m b e r s . O u r d a t a s o m e w h a t c o n t r a d i c t with c o n c l u s i o n s of K u n z m a n n ( 1 9 9 9 ) w h o s u g g e s t s that stability of r h o n i t e in alkali s i l i c a - u n d e r s a t u r a t e d b a s a l t s is restricted to p r e s s u r e s less than 0 . 6 k b , and t e m p e r a t u r e s f r o m 8 4 0 to 1200°C w i t h o u t a n y limitations o n o x y g e n f u g a c i t y . A p p a r e n t l y , s o m e c o n t r a d i c t i o n s m a y b e related to the c h e m i s t r y of rhonite b e c a u s e this m i n e r a l is h i g h l y variable in the c o n t e n t s of F e2 +, Fe3 +, and other c o m p o n e n t s d u e to i s o m o r p h i c substitutions. In a d d i t i o n , the c h e m i c a l a n d p h y s i c a l c o n d i t i o n s of solidification in oli vine-hosted melt i n c l u s i o n s and in the r o c k g r o u n d m a s s m i g h t b e d i f f e r e n t .
T h i s w o r k is s u p p o r t e d b y the R u s s i a n F o u n d a t i o n for B a s i c R e s e a r c h (grant no. 0 2 - 0 5 - 6 4 6 2 0 ) and the H u n g a r i a n N a t i o n a l S c i e n t i f i c F o u n d a t i o n ( O T K A ) : grants T 0 3 4 9 2 2 to T o r o k , K. and T 0 3 0 8 4 6 to S z a b o , C s .
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