CHEMICAL VARIATIONS IN PYROCLASTIC SERIES

Transcription

1 RI:NDlCONTl Socktd Iralla. ill Minno/ovlo It Pd, 31 (U: pp. U7-1tl Counlcadone pracnlata al Con 8DlP dl Cacn&rt ottobre CHEMICAL VARIATIONS IN PYROCLASTIC SERIES NARA CoIlADOSSI, MARINO MARTINI htilll!o di Minertlogi., Petrografi. e Gcochiica dell'universidl, vi. La Pia 4, Fiff'lW'. Ccntro di studio per J. Mineraiogi. e I. Gcochiia dei Scdic:nd (CN.R.) ABsnACT. - The S1udy of pyroclaslic depo5.its hu bcc:n ainly bucd upon their lithoqial, granuloctric and staligaphic charactcn. It seeed possible that cheial peculiarities have been produced in response to the physialcheical conditions which gave rise to Ihe diffeff'nt eruptive processes, and a sludy of pyrodastic series was undert.ken to check whether the presenl distribution of specific eleents can provide soe addition.1 inforation. Saples representing the different types of pyroclastic deposits of Minoan eruption of Santorini and Popcian eruption of Vcwvius have been investigated, as well as Iavas fro the sae.re-s; 5Odiu, pot.&ssiu, iron, agnesiu, fluorine.nd chlorine have been deterined. The distribution of Na, K, F and Cl, along with the values for I«chablc Na,nd D,.ppears of, ettlllin interest. Since the sojubility of volatiles in aga sc=is to depend ainly on their cooccntalioos in alkalies, high lcvels of F and a in pyrodutk products can be justified by the K.nd N. values observed at Vesuvius; for Santorini this can apply to puiccs only, while a different explanation is needed for saples of surge atrix, because their low contents in alkali etals cannot account for the observed nol rqligible levels in a. Keeping also in ind the locuion (If the volcano an inflow (If sea water into the syste appcan the osl likely aong the natural echaniss which an produce.n enrichenl of D in the agatic producu, in spite of the lack of Jufficicnt conditions for ill solubility in the elt phase. The explosive processes which produced the base surge deposits at Sanlorini see then 10 be ascribed to the above entioned inflow, while for Vesuvius the cheical evidences point OUt to siilar role of phrcalic w.t. RussuHTO studio dei <kpositi piroclaslici si soprallutto buato sui 1010 aattcri Iitoqid, granuloctrici e staligafici.. scbato possibije cbe si sillllo prodolle pecu. liarilil chiiche in risposta.lie condizioni chiicofisiche che hanno dato luoge.i ditferenli processi erutcivi, cd e SIaIO inlraprcso uno studio di serie piroclastiche per verificare se I'.ttuale distribuzione di deenti specifici pub foire infonnuioni Jupp1centari. Sono stati considcrati depositi piroclastici den. eruzione Minoica di Santorino e di quclla Popciana del Vesuvio, e bve delle stessc aree, deterinando sodio, potassic, ferro, apcsio, dolo e ftuoro; qgiore inleresse e appresentato dai.i.a distribuzionc di N., K, F c a nelle foc't'e totali, e di N. e a nei Iisciviati delle stessc foc't'e. Poid,e la solubilili dei voladti ncl aga scbra dipen<lere $OpranullO d.1 suo oontenulo in atali, aid livclli di FeCI possono csscre giustificati dai valoci di Na e K osstrvali al Vesuvio; per Santorino db pub valere solo per le pornid, cd una dh'c:fsll spicgazionc necessaria per i eapioni di surpc'. cbe hanno basso contcnuto in.icali e v.loci signifialivi di a. Tcncndo conlo.nd1c dell'ubicazione del vubno, un.pporlo di acqu. arina al sistca appaff' il piu plausibile fra i proccssi narurali che possono prcxlurre un arricchienco in Cl dei prodotri agrn.tid, nonostante non esistano le condizioni per una sua aggioff' solubilitll ncl fuso. I proccssi esplosivi che h.nno prodono qucslo tipo di deposilo S.nlorino scbrano quindi d&.nribuirc a11'apporto suddello, cntff' per it Vesuvio le CINIneristichc chiichc indicano piullosto un ruolo siile di licque fre-tiche. The explosive volcanic eruptions and their products have been largely investigated during the last ten years, and considerable results have been obtained which besides their scientific eaning have a saj iportance in assessing volcanic hazards in populated areas (LIRER et al, 1973; WALKER, 1974,1977; BARBERI and GASPARINI, 1976; BoND and SPARKS, 1976; BARBERI et ai., 1977; :BooTH et al., 1977; BAKER, 1979; BooTH, 1979; DELlBRIAS et ai., 1979; SHEItIDAN, 1979; SHERIDAN et al., 1981). The guickline of these researches has consisted in a careful reconstruction of past eruptive events by eans of the study of pyrodastic forations, carried out following ainly Iithological, granuloetric and stratigraphic criteria. The cheical coposition of the products involved in explosive activity is norally not taken into consideration in the frae of the above entioned inve5ti-

2 288 N. CORADOSSI, M. MARTINI SMHOR'N' N Q o,, Fig. I. Santorini. Locations of the studied saples fro gations, probably because in any instances secondary processes of alteration obliterated to a great extent the original characters. On the contrary, if the eplaceent setting allowed but a inor degree of secondary changes, it sees to us that soe additional inforation could arise fro the distribution of specific eleents. This condition can be fulfilled for pyroclastic foration of recent age, characterized by high depositional rates, considerable thickness and an early fored or conteporary ipereable cover. To carry on a pre1iinar essay of this kind of investigations, saples fro pyroclastic series of Popeian eruption of Ve suvius (79 A.D.) and Minoan eruption of Santorini (1470 b.c.) are here taken into consideration, with reference to their contents in soe ajor (Na, K, Fe, Mg) and volatile constituents (F and Cl). Experiental In figg. I and 2 are given the locations of the studied saples, described in table 1. The analyses of Na, K, Fe, Mg have been carried out by classical procedures using absorption spectrophotoetry; specific electrodes have been eployed in the deterination of F and Cl. The leachate solutions have been obtained by cold extraction on 1 g of saple with I. ',,, 0123Krn Oplontl Fig. 2. _ Locations of the studied saples fro Vesuvius area. 25 l of water, without stirring. The analytical results are shown in table 2. Di8CU!l8ion * Pornpel * The analytical data for the series here considered allow to verify that, besides the basic differences in cheical characters, at Vesuvius significant copositional variations can be found along the different deposits, while at Santorini a substantial siilarity is observed with inor exceptions. At Vesuvius the cheical coposition of cineritic and lithic coponents of flow deposits gradually approaches that of basal lavas of Soa; this fact has been considered as a cerrain degree of evidence in favour of a phreatic event which involved lavas of older age with respect to those produced in the first stages of Popeian eruption (MAR TINI, 1982). Following the sae logical path, the persistence of the ajor features in the

3 CHEMICAL VARIATIONS IN PYROCLASTIC SERIES 289 TO TO n n n n n n n... no pp.,,, 0 P,,,. '12.., No pp. 'n pp.,.,,ps no.,.,,, TABLE 1 Description 01 the studied saples., 0 0 uooo,'oo UO, uo,. o'oo M'o'oo M'o'. o,oo 'oou 'oh '0'0<' 'on.,1<.,, -, ()p'''''''..u.. _10. r., to.«b of,d, toll.<. of _ p' 0&.,.roe.u Hoo,.,0< <1 oo 00. '0<''''''0 fl f.ll. of,,. 0'. _ 1'10<'''''0 fl Loo. 010<'. 1'10<'''0 n.,.,.'0< 0 ow.00. Pf'oc flow foil f.'l _. 'u, <l. or b u '0u, '1<'.,. uo 0.'00,. lava b.<><'. '<><'''''0 -,,,,, f.u -,,,,, f. ''''' f. f. ''''' '.11 0.,.11 Pwol,,,'0<'''''0 flow Pwol. ''''0 fl.,oe, uo n.,.u. n., 'PS'U. 1 PT,., _, >t, _ 1 p, fy', o fl o _.' l 0<0 01 '_0.,11_, cheical picture should allow to infer for Santorini a true phreatoagatic episode, producing poorly differentiated aterials fro the sae aga chaber, If we extend the observation to the studied volatile constituents, however, a good unifority IS still present for fluorine distribution, but two strong peaks are shown by chlorine concentrations (fig. 3). The 'llbundance of volatile constituents In agas are norally liited by their solubility, which in turn sees to depend. on the alkali etals content (KOGARKO et al., 1968; KOGARKO, 1974). The substantial unifority of sodiu and TABLE 2 Analytical data 01 the studied saples 7, '3 7, 70 H.,,. u u n 7 rn H n T n n 7Ji.l P',v 0 pp,v. IV. lo',,, pr,v, '0 0,,.v,U n.,oo pr,. >t '.n,,..,.. ls,.. '.10 ' 0 '.,.,. ',0 '.J3,.,.11,, '.13,,. 00 l. 1, I. I.', I.tl '.12 1.,..n ','.,. I.' '. '.1,. '.11, '.0',,., '.1.,. ' 0.,,,,.,.,. 1.,. 1. '.11 S I.O!,..., '. S1, '.H 0.11 I U 0. l.ll D.n o',.,.,.,, I. 0. l.n,. '.10.. I.U IJ,., 'I 03 ' ' I..t. Il 11,.. o..ll.,.,.,,.i..,., S., '.'. '.., '.,.,. l. II,.,. '...,. '. 'll 00' 0<. 0.., 0. ṃ., 0,, _,I,,Ul,.,0 'HO 02.0, U. >SOS lioo.0 0' lslo 0. l'.,n.,. '00,,,, n '0,no - - '1, I.., loll >J1O ll,.s tt,,,,.,.,,., 0.,.,.,,., IU.,. III,,,. It loo< '''0 '. 13' 10. ',,., 00. HO,., 0 0 '1\0,s'. '0, '0'0 0..,.,,.,0,.o '0' 00 >0' - -.,,,,, 0, ṃ,,, ' '00,. Total iron is expressed as FeO'; Na. and Cl. represent the results of cold extraction. potassiu content tn the whole series of pyrodastic products at Santorini appears to infer that the variations observed for chlorine cannot depend on different conditions for its solubility in the aga. The results obtained by siple leaching show that significant quantities of extractable chlorine correspond to the «highs» in total concentration of this eleent. Sodiu IS another iportant constituent of the sae >t,.,,

4 290 N. CORADOSSI, M. MARTINI, 5 0 I,, 0,, 0 la 0,, 3,. Na.%o CI.%o Nal '*' K/O'lb Cl, F%. 2 SAII9LES,,,,,,,,,, ? \ so } l 83 t t Fig Chical variationin series 'ro Santorini.. surge atrix; puice; lava. Cl.,*, Na,o% K1O% 0 0' o, ] 4 5 6,. F, Cl,, o,,,, I I I ,.15l rrr \( (j r!\ j,. HO! ' I t \ t l Fig Cheical variations in the series fro Vesuvius a. - surge atrix or pyrodastic flow (size less than ); 0 pyroclastic flow (size between 050 and 0.71 ); puice. )

5 CHEMICAL VARIATIONS IN PYROCLASTIC SERIES 291 solutions, with a distribution siilar to that of chlorine, but its absolute quantities repnt only a sall fraction of the total sodiu content of the saples. For Vesuvius the situarion is quite dilierent, because in spite of the significant concentrations of chlorine in the rocks, very inor concentrations are observed in leachates (fig. 4). The extent of leachable chlorine ds not appear as depending on the pyroclastic nature of the rocks nor on the total content, and ust be accounted for by a different process aong the natural ones which substantially can be restricted to: 1) leaching of rocks by percolating waters, with accuulation in levels of slight penneability; 2) alteration by secondary fuaroles, with enrichent in volatiles; oreover, taking into account the sodiu chloride character of the solutions obtained by leaching, we can consider 3) pollution by seawater spray; 4) contribution of seawater to the eruptive echanis which deterined the eplaceent of the studied pyroclastics. The freshness of the saples, collected on the: front of quarries under exploitation, s to rule our any alteration by fuarolic activity, while odifications by percolating waters cannot be copletely excluded. The observed concentrations in leachates, however, result fro a very short and weak treatenr of saples with cold water, and we think that during the tie elapsed since their deposition even a reduced ratc of percolation through the studied forations would have ptoduced an alost copletc depletion in their leachable coponents; any extent of heavier alteration should have inrcoduced also odifications in the unifority of the cheical picture, expecially between levels of different granuloetry. Besides this, no significant concentrations in leachates are observed for saples collected at Athinios, which also should have been interested by a general process of secondary cheical changes. If we consider the possibility of seawater as providing the excess of chlorine, we think that any pollution by arine spray should have interested to a siilar extent all of the exposed forations; oreover, our investigation on deposits of the sae nature located on a cliff near the seaside in the island of Vulcano has deonstrated that a very inor quantity of sodiu chloride, if any, can be provided through a process of that kind. The contribution of seawater to eruptive ecl1anis has therefore, in our opinion, the: best probability aong the processes here considered, and the location of the volcanic syste of Santorini s to allow such an hypothesis (BoND and SPARKS, 1976). Significant concentrations of sodiu and chlorine in well defined levels in the studied series could be thus considered as witnesses of an inflow of seawater into the volcanic conduit, and should pertain to those deposits ore directly associated to phreatoagatic activity triggered by such contribution. As a atter of fact, the highest peaks in chlorine and sodiu occur in base surge deposits, which are thought to be produced by the above entioned type of eruption. Nothing siilar is observed at Vesuvius, for which an inflow of phreatic waters has been proposed (SHERIDAN et ai., 1981). At present, no further general explanation can be provided, keeping in ind the preliinary character of this research, but we think that the obtained results stress out that a deepening of the studies is needed in order to verify possibilities and liits of a cheical approach in the investigation on recent pyroclastic forations. AcJowledgertft. - We are gnlleful [0 P. MA I\ETTI and R. SAA'TAC.OC who provided the saples of Saorini and Vesuvius, t'cipectively.

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