Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
TRANSITION FROM M A G M A T I C TO HYDROTHERMAL CONDITIONS IN M A G M A CHAMBERS: EVIDENCE FROM MINERALOGY, CHEMISTRY AND FLUID INCLUSIONS I N ALKALI-SYENITE NODULES OF THE "BRECCIA MUSEO" PYROCLASTIC FLOW (CAMPI FLEGREI, NAPLES, ITALY)
T A R Z I A M.1, D E V I V O B .1. B E L K I N H. E.2, L I M A A.1, F E D E L E L.1
1 Dip. Geofisica e Vulcanologia, Univ. of Napoli Federico II, Via Mezzocannone 8, 80134, Napoli, Italy.
2U . S . Geological Survey, 9 5 3 National Center, Reston, VA 20192, USA.
E-mail: bdevivo@unina.it
Object of the present study are feldspathoid-bearing syenite nodules which belong to the lithic horizon (Breccia Unit) of the Breccia M u s e o formation. T h e Breccia Museo, is a volcanic breccia which crops out in the S W sector of the Phlegrean Fields, an active volcanic field located immediately west of Naples, Italy. The Breccia Museo has been interpreted by several authors as result of the explosive activities of different monogenetic vents (Perrotta, Scarpati, 1994) or the proximal facies of the Campanian Ignimbrite, a widespread pyroclastic flow deposit which occurred about 35.000 ybp (Rosi et al., 1983). Perrotta and Scarpati (1994), on the basis of the distinctive textural, structural and lithological characteristic, divide the deposit in four stratigraphic units grouped in two overlapped depositional units named, respectively, Lower Depositional Unit ( L D U ) and Upper Depositional Unit (UDU); the two depositional units are divided by a co-ignimbrite ash-fall deposit. Both the L D U and the U D U show facies variations interpreted by authors as resulting from the interaction between the flow and the paleomorphology.
The nodules are composed primarily of potassium feldspars (up to 80% of the constituents), subordinate plagioclase, scapolite (a S and Cl-rich member of the cancrinite-group) amphibole, pyroxene, biotite, magnetite, titanite and apatite. T h e homogeneous composition of feldspars from core to rim suggests a substantial equilibrium with the host liquid. Most of the observed fluid inclusions were hosted by potassium feldspar and, to a lesser extent, by scapolite.
A preliminary fluid inclusion study was carried out in thin sections and on small fragments of xenoliths, which were examined and analyzed by S E M - E D S . E D S analysis was carried out on daughter minerals, whose identification was based on E D S spectra and, where possible, on crystal habit. Room-temperature observations allowed to identify various types of fluid inclusion: (1) one-phase liquid-rich; (2) two-phase aqueous, liquid + vapor, liquid-rich inclusions (most of these inclusions were found in healed fractures or cleavages of the host minerals); (3) three-phase and multiphase aqueous inclusions containing one or more daughter minerals. The study of daughter minerals in fluid inclusions revealed the presence of many different phases: Chlorides-. Na (halite) and K (sylvite) chlorides were found in many opened inclusions. T h e y were often associated and occupied more than half of the volume of inclusion. Fe- and Mn-chlorides often occurred as irregular masses or coated the wall of the inclusions; Sulfides: minerals showing Fe, Cu and S peaks were identified as pyrite, pyrrhotite and chalcopyrite, based on peaks ratio and morphology. Noteworthy is the occurrence of sphalerite and argentite/acanthite;
Sulfates: although S E M - E D S analysis does not reveal the degree of hydration, a Ca and S phase was identified as anhydrite ( C a S 04) f r o m morphology; Carbonates: in some inclusions a mineral showing exclusively a Ca peak was observed. W e consider this mineral to be calcite in agreement with crystal morphology. Other phases, identified as carbonates, were dolomite, cerussite and a Zn daughter mineral tentatively identified as smithsonite ( Z n C 03) .
S E M - E D S and electron microprobe analysis of the nodules showed a high abundance of U, Zr Nb, Th and R E E elements in late stage minerals such as apatite, Zr bearing minerals (i.e. zircon baddeleyite and zirkelite), pyrochlore-group minerals, thorite and phosphate (monazite group); zircon and baddeleyite have U 02 concentrations of up to 9 and 4.5 wt.%, respectively and Th enriched zircons ( T h 02 up to 50 wt.%) were also observed. Baddeleyite shows also enrichment in N b205 (up to 7 wt.%, among the highest concentrations reported from terrestrial baddeleyite). Zirconolite is relatively enriched in N b and Th (up to 14 and 15 wt.%, respectively) and, to a lesser extent, in R E E and Y (up to 4 wt.% and 3 wt.%, respectively). Pyrochlore, which shows a large compositional variation, is the main mineral concentrating N b in the nodules (up to 37 wt.%) and has a relative abundance of Z r 02 (up to 16 wt.%), T h 02 (up to 8 wt.%) and U 02 (9 to 31 wt.%). Phosphates are enriched in T h 02 (up to 21 wt.%) and R E E (mainly C e203 and L a203 up to 13 wt.% and 12 wt.%, respectively) and often show zonation. Other minerals observed were sulfides, carbonates and tungstates.
Many of the accessory phases occur with typically hydrothermal textures (i.e. replacement, vugs, in-filling). Furthermore, the nodules show high abundance of small miarolitic cavities. On the basis of mineralogy and fluid inclusion evidences, these cavities have been interpreted by the authors to be magmatic in origin and to represent the probable site of vapor bubbles exsolving f r o m late-stage magmatic fluid separation. Complex daughter crystal assemblages found in multiphase fluid inclusions of the nodules of the Breccia M u s e o give evidence of high solute entrapment. The high salinity of the trapped fluids is testified by high abundance of chlorides, sulfides and, to a lesser extent sulfates and carbonates. Such evidences suggest that fluids trapped represent a hypersaline/sulfur rich with minor C 02 content fluids exsolved by a crystallizing magma.
Microprobe data and fluid inclusion observations of the nodules show the partition of fluorine, chlorine and sulfur in the syenite. Fluorine is incorporated in biotite (about 5 wt. %) and, subordinately amphibole; chlorine and sulfur are incorporated in scapolite, cancrinite-group minerals and trapped in the form of magmatic brine. The high abundance of halite and sylvite 209
Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
f o u n d in the fluid i n c l u s i o n s s u g g e s t a t e m p e r a t u r e of f o r m a t i o n in the r a n g e of 4 0 0 ° - 6 0 0 ° C . S u c h a r a n g e of t e m p e r a t u r e c a n b e c o n s i d e r e d as a l o w e r limit t e m p e r a t u r e f o r an e a r l y stage h y d r o t h e r m a l c o n d i t i o n . W e a re c u r r e n t l y c a r r y i n g o n m i c r o t h e r m o m e t r i c e x p e r i m e n t s to c o n f i r m this h y p o t h e s i s .
T e x t u r e , m i n e r a l o g y a n d c h e m i s t r y of the alkali-syenite of the B r e c c i a M u s e o s t r o n g l y s u g g e s t that t h e s e n o d u l e s r e c o r d e d the transition from m a g m a t i c to h y d r o t h e r m a l c o n d i t i o n s in the local m a g m a c h a m b e r . T h i s t r a n s i t i o n w a s sited in a n u p p e r part of the m a g m a c h a m b e r . A c c o r d i n g to M e l l u s o et al., ( 1 9 9 5 ) , the c o m p o s i t i o n of t h e e r u p t e d p r o d u c t s o f the B r e c c i a M u s e o D e p o s i t is c o m p a t i b l e with the p r o g r e s s i v e d o w n w a r d t a p p i n g of a s h a l l o w m a g m a c h a m b e r t h e r m a l l y a n d c h e m i c a l l y z o n e d . A f t e r an e a r l y e r u p t i v e p h a s e d u r i n g w h i c h the u p p e r , m o s t d i f f e r e n t i a t e d level of t h e m a g m a c h a m b e r w a s t a p p e d , the s u d d e n c o l l a p s e of the roof of the m a g m a c h a m b e r triggered the s u b s e q u e n t e r u p t i o n of t h e r e m a i n i n g m a g m a ( r a n g i n g in c o m p o s i t i o n f r o m t r a c h y t e to t r a c h i p h o n o l i t e ) , w h i c h f o r m e d the B r e c c i a U n i t a n d the U p p e r P u m i c e F l o w U n i t ( P e r r o t t a , S c a r p a t i , 1 9 9 4 ; S c a n d o n e , 1990).
H i g h t e m p e r a t u r e a n d h y p e r s a l i n e i m m i s c i b l e f l u i d s play a k e y - r o l e in o r e - m e t a l t r a n s p o r t p r o c e s s e s . M a n y s t u d i e s o n m a g m a t i c s y s t e m s ( R o e d d e r , 1984) report the p r e s e n c e of high-salinity fluid i n c l u s i o n s (i.e. t h e p r e s e n c e of a C l - r i c h , h i g h - d e n s i t y a q u e o u s p h a s e ) related to a m a j o r o r e deposit (i.e. C u and M o p o r p h y r y d e p o s i t s ) . T h e s e i n c l u s i o n s h a v e e q u i v a l e n t h o m o g e n i z a t i o n t e m p e r a t u r e s T h > 5 0 0 ° C and salinity > 4 0 % N a C l e q u i v a l e n t . S i m i l a r o c c u r r e n c e s a r e a l s o r e p o r t e d f r o m Italian s u b v o l c a n i c s y s t e m s ( D e V i v o et al., 1995; L o w e n s t e r n , 1994).
V a r i o u s e x p e r i m e n t a l w o r k s h a v e p o i n t e d o u t that m a n y o r e m e t a l s tend to p a r t i t i o n into a h y p e r s a l i n e c h l o r i d e - b e a r i n g b r i n e e x s o l v e d f r o m a silicate melt ( C a n d e l a , Piccoli, 1995). T r a c e and R E e l e m e n t s c a n a l s o b e e f f i c i e n t l y e x t r a c t e d f r o m m a g m a f r o m Cl-rich f l u i d s ( H a a s , S h o c k , 1994; K r a v c h u k , K e p p l e r , 1994). O n the o t h e r h a n d , the r o l e of S in t r a n s p o r t m e c h a n i s m s is o f t e n u n d e r e s t i m a t e d , e v e n if sulfate-rich f l u i d s are r e p o r t e d in p e r a l k a l i n e , silicic m a g m a t i c i n t r u s i o n s a n d in o r t h o m a g m a t i c fluids related to p o r p h y r y - c o p p e r d e p o s i t s ( H a n s t e e n , B u r k e , 1990). T h e o c c u r r e n c e of s u l f i d e s ( p y r i t e , pyrrhotite, c a l c h o p y r i t e ) a l o n g with c h l o r i d e s and s u l f a t e s a n d Fe- M n o x i d e s s u g g e s t s that the r e d o x state of the m a g m a t i c solutions s h i f t e d t o w a r d s relatively o x i d i z e d c o n d i t i o n s . C o n c e r n i n g the partition of R E E into S - r i c h f l u i d , W o o d ( 1 9 9 0 ) states that s u l f a t e - R E E c o m p l e x e s c a n p r e d o m i n a t e o v e r a q u e o u s species in a b s e n c e of o t h e r ligands. F u r t h e r m o r e , t h e t e m p e r a t u r e strongly a f f e c t s the R E E - c o m p l e x stability constant: i n c r e a s i n g t e m p e r a t u r e s p r o d u c e a n i n c r e a s e in c o n s t a n t v a l u e s f o r f l u o r i d e , sulfate and c h l o r i d e (in this order). T h e r e f o r e c h l o r i d e and sulfates, w h i c h w e r e p r e s e n t in the m a g m a t i c s y s t e m o f the B r e c c i a M u s e o D e p o s i t , c o u l d h a v e p l a y e d an i m p o r t a n t role in the R E E selective e n r i c h m e n t in the a c c e s s o r y m i n e r a l s o f t h e nodules.
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