Journal of Volcanology and Geothermal Research, 58 ( 1993 ) 345-358 345 Elsevier Science Publ ishers B.V., Ams te rdam
Be isotope and B-Be investigations of the historic eruptions of Mr. Vesuvius
Julie Morr is a'l, Je f f Ryan a'b and W.P. Leeman c "Department of Terrestrial Magnetism, Carnegte lnstitution of Washington, 5241 Broad Branch Rd. N. Hi,
Washington, DC. 20015, USA bDepartment of Geology, UniversiO' of South Florida, 4204 E. Fowler Ave, Tampa, FL 33620, USA
CKeith-Wiess Geological Laboratortes, Rice Universio', Houston, TX 77251-1892, USA
(Received March 27, 1992; revised version accepted September 7, 1992 )
ABSTRACT
('osmogenic and radioactive ~°Be, stable 9Be and B concentrations have been determined for four alkaline lavas from the recent cycle of historical activity at Mr. Vesuvius, in the Central Campania Province (Italy). The goal of this study was to use the Be isotope and Be-B systematics of these lavas in a manner analogous to that used in regions unequivocally related to active subduction, in hopes of being able to document a subduction origin for the Vesuvian lavas. Four lavas measured to date have low ~°Be concentrations as well as low B/Be ratios. While the low ~°Be concentrations could reflect subduction and incorporation of old sediments and /o r only contributions from the basaltic part of the subducting plate, the combination of low ~°Be and low B/Be ratios must be interpreted as the absence of a subduction signature in these lavas. Unfortunately, the absence of a subduction signature in the lavas cannot be used unequivocally to argue against recent subduction in the Central Campania region. Subduction of an unusually young, hot slab which has lost its B through prograde metamorphic reactions at shallow levels could explain the absence of a subduction signature, as could extensive crustal contamination. In addition, recent studies in Java show that alkaline lavas in this region of active subduction never show a subduction signature, even when erupting in close proximity to calc-alkaline and tholeiitic lavas which do: b~ analogy, the Vesuvian lavas studied may have been generated in a part of the sub-arc mantle which either does not expe- rience or does not preserve the chemical signature of subduction modification. The present data set does not allow us to make definitive statements about the role of subduction in the origin of the Central Campania lavas.
Introduction
The relative roles of pre-existing mantle het- erogeneity, subduction modification of the sub- arc mantle and crustal contamination in gen- erating the trace element and isotopic charac- teristics of alkaline lavas erupting at conver- gent margins remain unclear. Even more difficult is the problem of assessing the parts played by these different processes in the for- mation of lavas erupting in regions where the tectonic setting is equivocal. The K-alkaline
lavas of Central Italy, recently described as be- longing to two separate geochemical and geo- dynamical provinces (Serri, 1990), represent just such a problem, erupting as they do through continental crust in a region that may or may not have experienced recent subduc- tion. Many geophysical studies, discussed be- low, document the complexities of the recent tectonic evolution of the Tyrrhenian Sea, cast- ing doubts upon a simple subduction setting but not completely precluding a subduction origin for the recent lavas. Numerous geo-
~Present address: D e p a r t m e n t & E a r t h and Planetary Sci- ences. Washington University, St. Louis, MO 63130, USA.
chemical studies, also discussed below, point to some characteristics which are typical of subduction related lavas, others which seem to reflect crustal contamination, and others yet which may be interpreted in several different ways.
This paper reports the result of an explora- tory study conducted to determine whether or not Be isotope and Be-B systematics of lavas from the historical cycle of activity at Vesuvius could be used to establish a subduction rela- tionship for these alkaline lavas of the Central Campania Province. In regions of active plate convergence and subduction, calc-alkaline and tholeiitic lavas almost always show B/Be ra- tios significantly elevated above values for normal mantle, and often show elevated l°Be/ Be ratios. These are characteristics of arc lavas which may be uniquely ascribed to contribu- tions from the subducting plate. If l°Be enrich- ments were to be observed at Vesuvius, they would represent strong arguments for a sub- duction origin for these lavas. In addition, l°Be enrichments at Vesuvius would represent the effects of recent subduction; the short l°Be half- life of 1.5 Ma (Yiou and Raisbeck, 1972) means that any subduction signature imposed on the Central Campania mantle prior to about 10 Ma would have decayed to unmeasureable levels in the interim. Given that the conver- gence rates at this margin were likely to have been slow, and that subduction may have ceased several million years ago, B determina- tions were added to the study, as its concentra- tions are not affected by decay over time.
In this paper we first briefly summarize the geophysical and geochemical evidence that leads to consideration of a possible subduction origin for lavas from Vesuvius. We then dis- cuss in more detail the behavior of the Be iso- tope system and the nature of observed Be-B systematics at active convergent margins. Sub- sequently, we present the results of this pilot study, and discuss the possible reasons for the absence of a subduction signature in the Be-B systematics of the recent Vesuvian lavas.
Are the Vesuvius lavas subduction-related?
Geophysical evidence
A unique and consensus model for the struc- ture and tectonics of the Tyrrhenian Sea re- gion has proved elusive, reflecting the com- plexity of the region, the diversity of opinion, and the relatively limited data. Studies within the last decade include models wherein the Central Campania volcanoes are considered to be an extension of the Aeolian arc, models where they are considered to be distinct from the Aeolian volcanoes but still subduction re- lated, and models where none of the Italian volcanism is thought to be directly related to subduction.
The location of Mr. Vesuvius is shown in Figure 1. Vesuvius lies south of a proposed Campanian lithospheric discontinuity, within the Central Campania Province of Serri (1990). Interpretation of physiographic, geo- physical and volcanological data led Gasparini et al. (1982) to suggest that the Italian arc could be considered the Mediterranean equiv- alent of the Pacific arcs, albeit in a different structural context. In this model, opening of the
A . " ' ~
~"~" ~ ~ Vesuvius " \ Sardinia \ ~ I ~ -L.,
.! Tyrhhenlan Sea / / / ~ ~ " ~ .... ' " " <:>
Aeolian Islands ^AIV~' I I / I ,
Fig. 1. Location map showing position of Mt. Vesuvius relative to sites of active volcanism in Aeolian Islands and other sites of recent K-alkaline volcanism in the north- west Roman and Central Campania Provinces. Contours indicate depth to the Wadati-Benioff Zone, from Ander- son and Jackson (1987), based on best-quality earth- quake relocations.
Be ISOTOPE A N D B-Be I N V E S T I G A T I O N S OF THE HISTORIC E R U P T I O N S { )F Nlt VESUVII JS t47
Tyrrhenian Sea was considered the analog of a Pacific marginal basin. Contours of the Wad- ati-BenioffZone, based on relocated hypocen- ters, were drawn so that Vesuvius lay some 300 km above a NW-dipping slab that penetrates the mantle to 500 km or more. Vesuvius was thought to be located above the northernmost edge of a slab that was continuous with that be- neath the Aeolian arc to the southwest. These authors suggest that significant amounts of continental lithosphere may have been in- volved in the subduction process. They recon- cile the evidence for deep subduction with the expected behavior of continental crust during collision by suggesting that the lower crust and lithospheric mantle of the African plate are subducted while deformation in the upper crest is accomodated at shallower levels. Estimated N-S convergence rates between Africa and Eu- rope are on the order of 1.4 cm yr -1 to 7:',.5 cm yr -~ (McKenzie, 1972; LePichon et al., 1973; Scandone et al., 1974).
More recently, Anderson and Jackson ( 1987 ) published a re-appraisal of the seismic data, assigning each earthquake relocation a quality rating based on criteria developed for relocation reliability in South America, where station coverage was also poor. Based on best quality relocations, they also interpret the pat- tern of seismicity beneath the Aeolian arc as reflecting subduction of the African plate to some 600 km beneath Europe. In their vie, w, however, the seismicity beneath Naples is iso- lated from that of the Aeolian arc to the south- west; they propose that Vesuvius is underlain by a separate, more shallowly dipping portion of a slab, rather than accepting the Gasparini et al. (1982) model wherein seismicity is con- tinuous between the two regions. Anderson and Jackson (1987) regard the nature of the crust subducted beneath the Aeolian arc as open to question, but interpret the deep seismicity as evidence that the subducted crust was primar- ily oceanic in character. They also cite the ab- sence of seismogenic shallow thrusting SE of Messina-Calabria as evidence that conver-
gence at the surface has now ceased. With re- gard to convergence rates, they say only that rates in the past must have been faster than present convergence ( ~ 1 cm yr -1 ) between Africa and Europe.
A few studies discuss the Calabrian arc in terms of E-W subduction (e.g., Thompson, 1977). Estimates of convergence rates are on the order of 5-6 cm yr- ~ ) ( Patacca and Scan- done, 1989).
Other studies question the role of recent subduction in the formation of the historical Aeolian arc, much less the Central Campania segment. In this interpretation, the pattern of seismicity is explained as the result of mantle convection, in which upwelling asthenosphere, centered on the Tyrrhenian Sea, encounters older, colder continental lithospheric mantle beneath the Italian margin (Locardi and Ni- colich, 1991 ). 'This follows on from models in which the volcanism is thought to result from asthenospheric uprise associated with recent rifting in the Tyrrhenian ( Horvath et al., 1981: Gillot, 1987). In many models, older subduc- tion is thought to have imposed a geochemical signature on the mantle, which subsequently melts during later rifting, producing lavas with a subdudction signature but without a direct relationship to plate convergence.
While covering only some highlights, this summary emphasizes the difficulties in either assuming or disproving a subduction origin for the Aeolian arc and, especially, for the Central Campania lavas. For the purposes of this pa- per, the question will be left open, with the controversy kept in mind. If the Vesuvian la- vas are related to recent subduction, however, it is likely that convergence rates were slow. Subducted continental crust may or may not have been involved.
Geochemical evidence
The unique geochemical characteristics of subduction related lavaa have been well docu- mented by a number of authors working in
348 J. MORRIS ET AL.
volcanic arcs around the world. The chemistry of the Aeolian, Northwest Roman and Central Campania lavas has been investigated in light of this body of knowledge. Depending on the suites studied and the techniques used, au- thors have variably attributed the geochemical signatures of these lavas primarily to crustal contamination, to subduction modification of a mantle source, to subduction modification of heterogeneous mantle sources or to various combinations of the above (Taylor and Turi, 1976; Turi and Taylor, 1976; Hawkesworth and Vollmer, 1979; Peccerillo, 1985, 1990; Rogers et al., 1985; Ferrara et al., 1986; Barker, 1987; Ellam et al., 1989; Serri, 1990; Beccaluva et al., 1991; Crisci et al., 1991; Esperanca et al., 1992).
In the remainder of this paper, we will focus on the Central Campania lavas in general and the historical lavas from Vesuvius in particu- lar. While unique in many aspects, the lavas of the Central Campania Province also show some chemical characteristics typical of sub- duction related lavas.
Central Campania lavas range in age from Pliocene to historical, and cover a chemical spectrum from high-K calc-alkaline through shoshonitic to leucite and leucite-basanite se- ries. Calc-alkaline lavas of ~ 2 Ma were re- covered from the base of a borehole northwest of Campi Flegrei. Lavas younger than 100 ka from Ischia, Procida and the Campi Flegrei are shoshonitic in character, with leucitic xeno- crysts found in shoshonitic volcaniclastic se- quences (Beccaluva et al., 1984, 1991). At Somma-Vesuvius, all recovered lavas have been of the leucitite and leucite-basanite series (Beccaluva et al., 1991 ). These lavas are sim- ilar in major-element chemistry to the high-K alkaline lavas of Indonesia (Varne and Foden, 1986; Stolz et al., 1988; Edwards et al., 1991 ), and to other, rare occurrences of potassic al- kaline volcanism, generally observed in re- gions of unusual (often tensional) tectonic setting, as in Mexico (Luhr et al., 1989 ).
The trace-element characteristics of the
Central Campania lavas lend themselves to a subduction model for their origin. Although all incompatible element abundances are high rel- ative to MORB and most calc-alkaline arc ba- salts and andesites, Central Campania lavas show a small depletion of Ti, Nb (and Ta, where data is available), and Zr relative to the other incompatible elements when plotted on an extended rare-earth-element diagram (Thompson, 1977; Hawkesworth and Voll- mer, 1979; Beccaluva et al., 1982; Rogers et al.. 1985; Civetta et al., 1991). Depletion of the high field strenght elements is one of the most diagnostic aspects of the chemical signature for subduction related lavas (e.g., Wood et al.. 1979; Pearce, 1983 ); this same subduction sig- nature in the high field strength elements is ob- served in the Somma-Vesuvius lavas (Haw- kesworth and Vollmer 1979; Rogers et al., 1985; Belkin et al., 1993-this volume).
Interpretation of the isotopic characteristics of lavas from this region is more equivocal. 875r/a6Sr values for Central Campania lavas are in the range 0.705-0.710, with values for Somma-Vesuvius being less than 0.708 (Haw- kesworth and Vollmer, 1979; Cortini and Hermes, 1981; Civetta et al., 1991; Caprarelli et al., 1993 - this volume). 143Nd/la4Nd val- ues for Roccamonfina (western Campania ) are in the range of 0.5122-0.5163; Somma-Vesu- vius lavas plot at the high end of the range (Hawkesworth and Vollmer, 1979; Civetta et al., 1991; Caprarelli et al., 1993-this volume). For comparison, older lavas from the eastern volcanoes of Pietre Nere and Vulture have more depleted values, with ratios between 0.5127 and 0.5129 (Hawkesworth and Voll- mer, 1979). Different authors have assigned different weights to the relative roles of en- riched mantle, subduction modification of the mantle, and crustal contamination in generat- ing these isotopic characteristics. Given the presence of calcareous crustal inclusions in the Somma-Vesuvius lavas (e.g., Savelli, 1967) unanimous agreement on the meaning of the Sr isotopes may remain elusive (see for exam-
Be ISOTOPE AND B-Be INVESTIGATIONS OF THE HISTORIC ERUPTIONS OF ML VESUVIUS ~49
pie Civetta et al., 1991; Caprarelli et al., 1993- this volume). The Nd isotopic compositions for Vesuvius are in the range observed for many continental volcanic arcs (Gill, 1981), where authors argue about the extent to which these values reflect mantle vs. subduction vs. crustal contributions.
In this context of contradictory and incon- clusive evidence for and against a subduction origin for the Vesuvius lavas, we undertook an exploratory Be-B study. In a number of vol- canic arcs around the world, elevated l°Be/Be and B/Be ratios have been interpreted as in- dicators of recent sediment subduction (Brown et al., 1982; Tera et al., 1986; Morris et al., 1990).
Be isotope and Be-B systematics
Several aspects of the Be isotope system make it especially suitable as a tracer for studying sediment subduction and recycling at active convergent margins, as shown in Figure 2. l°Be is cosmogenic, made in the atmosphere by cosmic ray induced spallation reactions on oxygen and nitrogen (Lal and Peters, 1967). The l°Be thus formed is quickly removed from the atmosphere via precipitation (Raisbeck et al., 1981 ). The l°Be that falls on the sea sur- face is removed to the sediment column through adsorption on settling sediment parti- cles, resulting in high concentrations (Brown, 1984; Bourles et al., 1989; Anderson et al., 1990), averaging 5000X 106 a toms/gram (hereafter ag-1 ). As l°Be is radioactive with a 1.5 Ma half-life (Yiou and Raisbeck, 1972), concentrations decrease with depth and age in a subducting sediment column, as shown in Figure 2. Because the source of l°Be is atmos- pheric and as the nuclide is short-lived, l°Be concentrations in the mantle are extremely low, giving rise to mantle-derived mid-ocean ridge and ocean island basalts with essentially un- measureable l°Be concentrations, less than 1 × 106 ag -1 (Brown et al., 1982; Tera et al., 1986). The high concentration of l°Be in sur-
face sediments means that a small amount of sediment mixed with the mantle can generate lavas with measureable l°Be concentrations. In fact, concentrations in the surface sediments are high enough that, when integrated over the entire sediment column, the average sediment often has high enough 1°Be concentrations ( 100 to 500X 106 ag-~ ) to produce measurea- ble signals. Stable 9Be concentrations in sedi- ments are typically 0.8-3 ppm (Bourles et al., 1989: S. Zheng, pers. commun., 1992 ), and are not significantly enriched above values ob- served for MORB and OIB, 0.5-8 ppm (Ryan and Langmuir, 1988). Consequently, high I°Be/Be ratios reflect primarily the addition of sediment-derived l°Be to mantle-derived ~Be, where normalization to stable Be eliminates effects of partial melting and crystal fractionation.
As shown in Figure 3, lavas from the Ku- riles, Aleutians, Central America, Southern Chile, and Bismarck arcs have high l°Be/Be ratios, as do those from S. Hokkaido (Brown et al., 1982; Tera et al., 1986, 1989; Morris and Tera, 1989). In contrast, lavas from Kam- chatka, E. Alaska, the Cascades, Mexico, Costa Rica, Sunda, Halmahera, the Marianas and Ja- pan (except Hokkaido) have low concentra- tions of l°Be, indistinguishable at the level of analytical uncertainty from values measured in MORB and OIB.
Previous studies (Tera et al., 1986; Mon- aghan et al., 1988; Morris and Tera, 1989 ) have concluded that high l°Be concentrations in the young ( < 200 years old) arc lavas studied to date do not reflect surface contamination, but rather require sediment incorporation. As most of the upper crust is so old that it contains no ~°Be, the general consequence of crustal con- tamination would be to lower ~°Be concentra- tions in contaminated lavas. While it is con- ceivable that occasional assimilation of young sediments during magma ascent could gener- ate a high l°Be lava, the simple and consistent pattern of ~°Be-B systematics observed in arcs around the world preclude a general model of
3 5 0 J. MORRIS ET AL.
Cosmic Ray ~ ,
O,N ~ B e - t O H s l f - l l f e : 1 . 5 m i l l i o n years
11 5000 ~ - - 0 m
50 ] ' " ] - ' 1 0 0 i10 M s )
- 2 0 0
o o c I- oo . ~ . ~ 400 l 0 - 2 , I
lOBe in units of 10 6 a/g _ .. N . ~ = ' ~ +
Fig. 2. t°Be cycle at convergent margins, showing low measured t°Be concentrations of MORB and OIB, high values for young pelagic sediments, and range of values observed for volcanic arcs (Brown et al., 1982; Brown, 1984; Tera et al., 1986; Bourles et al., 1989; Morris and Tera, 1989; Anderson et al.. 1990). Also indicated schematically is distribution of 1°Be in a pelagic sediment core, reflecting decrease in t°Be concentrations with depth as result of 1.5 Ma half-life. Core lithology taken from DSDP 495 outboard of Guatemala. HP= hemipelagic, P= pelagic, C= carbonate. For 1°Be to show up in convergent margin magmas, four conditions must apply: ( 1 ) 1°Be concentration in subducted sediments must be high enough that a few percent sediment admixed with the lava will produce a measureable signal; (2) t°Be bearing sediments must be subducted rather than accreted; ( 3 ) subducted ~°Be (and B) must be transferred from the slab to the arc source region and thence to the magmas; and (4) the total cycle for sediments from the surface through the subduction zone and back to the surface in lavas must occur in less than 10 Ms. At Vesuvius, convergence rates are likely to be slow in this collisional environment (McKenzie, 1972; Le Pichon et al, 1973; Scandone et al., 1974; Anderson and Jackson. 1987 ); t°Be decay during long subduction times may preclude the use of t°Be as a tracer here.
~°Be incorporat ion through near-surface sedi- ment assimilation (Morris et at., 1990).
As shown in Figure 2, the lavas with high l°Be concentrations require the subduction and recycling of some part of the upppermost sed- iment column, within the last 10 Ma. Given the dilution of high l°Be surface sediments with low t°Be older, deeper sediments, and the di- lution of any sediment signature by mixing with the mantle, 10 Ma is in fact an upper limit, and sediment recycling within the last 3-5 Ma may be required to generate a measureable robe signal in the lavas. Regardless of any preferred dilution a n d / o r mixing model, it is possible to state unequivocally that a subduct ion that is older than 10 Ma, a n d / o r recycling of sedi-
merits that are older than ~ 10 Ma at the t ime of magma generation and eruption will leave no detectable ~°Be enrichment in the lavas. In regions where young sediments are being ac- creted (e.g., Java, Costa Rica) or where con- vergence rates are slow and J°Be will decay during subduction (e.g., Cascades, Lesser An- tilles, E. Alaska) no l°Be is expected in the la- vas, and none is observed.
B is analogous to l°Be in that it makes an outstanding tracer of sediment recycling at convergent margins (Leeman, 1987; Ryan and Langmuir, 1993 ). High B concentrations in sea water (Upps t roem, 1974; Spivack, 1986 ) and strongly adsorptive behavior for B during sedi- mentat ion result in high concentrations in ma-
Be ISOTOPE AND B-Be INVESTIGATIONS OF THE HISTORIC ERL PTIONS OF Mt. VESUVIUS ~ 5 1
WbSl P&L.IFIC AROS Z Kamchatka III Kur,le
HoKkal~O c & S Japan
Mar,aras III ~latmamera f.t) SJ'<~ O B,swar¢~
ASl DAQIFIQ ARCS I:~ AJeuuans
Cascades Ue~ co
,1 M,dd'e Amer,ca (~ Cosla R,ca
~FS[J~IIJS
{No Sediment Component)
II
NO Evidence Ior Sedimentary Component
0 2 0 4 0 6 0 8 0 1 0 0
MAXIMUM 10Be/9Be
Fig. 3. ~°Be/Be of Vesuvius lavas, compared with the I°Be/ Be ratios measured in arcs around the world. The absence of ~°Be enrichment indicates only that young (i.e. < 10 Ma) sedimentary materials were not involved in the gen- esis of the Vesuvius lavas.
rine sediments, on the order of ~ 100 ppm (Harder, 1970; Dean and Parduhn, 1983). This is in contrast to concentrations of mantle derived lavas. For example, MORB generally contain less than 3 ppm B (Ryan and Lang- muir, 1993). Granulite-facies metamorphism results in a lower continental crust with low B concentrations and low B/Be ratios (Leeman et al., 1992 ); thus lower crustal contaminat ion of arc lavas will not impose B enrichments. Upper continental crust has higher and more variable concentrations (Moran et al., 1992), probably reflecting at least in part localized distribution of tourmaline, and could conceiv- ably result in occasional lavas with unusually high B concentrations. Again, however, con- sideration of coupled mBe-B systematics pre- cludes extensive contaminat ion of arc lavas with high-B upper crust as a general mecha- nism for producing B enrichment. Coupled J°Be/Be-B/Be relationships suggest that the residence time of B in the sub-arc mantle is less than 3-5 Ma, making B enrichment also an in- dicator of recent subduction.
Unlike l°Be, B is strongly enriched in the al- tered fraction of the subducting basaltic crust as well as in the sediments. Derivation of a subduction component largely or entirely from the altered basaltic crust could produce lavas
with high B/Be and low l°Be/Be ratios, as could involvement of old sediments. Figure 4 shows the range of B/Be values measured for a number of different arcs and arc segments. For calc-alkaline and tholeiitic lavas, only those from the Cascades show no B enrichment, per- haps reflecting the consequences on the B cycle of subduction of a young, hot plate. (Leeman et al., 1990; Moran et al., 1992 ).
Combination of these two ratios leads to strong general conclusions about subduction recycling, which are summarized elsewhere (Morris et al., 1990). One reason for the con- sistent systematics observed in the Be-B ap- proach, however, is that crustal contamination has little effect, as the 1°Be and B concentra- tions of the vast majority of upper plate mate- rials are too low to significantly affect the mass balance for these tracers. Essentially, the sub- duction component contributes l°Be and B, and all other possible constituents of the lavas simply act as undistinguished diluents. In fact, to the extent that upper crustal materials have
i - Z WORLD ARCS
~ : JAVA C ~, & Thol
I I I Atkalme
' ~ EAST FLORES ["Jr CA & ThoI
&lkal,q~
~,~ CASCADES
rr I1~ VESUVIUS
I All M O R B & OIB' hncl H I M U & DUPAL)
NO s lab c o m p o n e n t at aq
0 1 0 0 2 0 0
M A X I M U M B/Be
Fig. 4. B/Be ratios of Vesuvius lavas compared with those reported in arcs around the world. B data from Leeman (1987), Leeman et al. (1990), Ryan and Langmuir (1993), Ryan el al. (1992), Morris et al. (1990), Ed- wards et al. ( 1991, 1993) and this paper. Absence of B in the Cascades attributed to extreme B loss from slab at shallow levels due to unusually hot thermal structure of subducting Juan da Fuca plate (Leeman et al., 1990 ). Ab- sence of B enrichment in alkaline lavas of Indonesia sug- gests that they are derived from local pockets within the sub-arc mantle that escaped slab-derived metasomatism (Edwards et al., 1991, 1993 ). The alkaline lavas of Vesu- vius may show similar effects.
352 J. MORRIS ET AL.
low B/Be and 1°Be/Be ratios, the effect of crustal contamination is simply to shift the high ratios that a lava might have acquired in the mantle towards lower values, without causing significant scatter about a mixing line formed by mantle and subduction end-members. Thus, more crustal contamination of an ascending lava will have the same effect on the I°Be-B systematics as less sediment incorporation in the arc source region.
Detailed models for subduction recycling are easy to construct but difficult to defend (Mor- ris et al., 1990; Lao et al., 1992 ) at our current state of knowledge about the complete l°Be and B cycles. Quantitative modelling requires data about the distribution of ~°Be, Be and B in the sediment column supplied to the trench; such work is only now in progress (Zheng and Mor- ris, unpub, data). Be and B may behave differ- ently during the prograde metamorphism of subducting plates, with B concentrations de- creasing significantly with increasing T, while Be concentrations remain nearly constant (Leeman et al., 1990; Bebout et al., 1993; Moran et al., 1992 ). In consequence, B/Be ra- tios of subducted sediments and altered basal- tic crust may be significantly lower than their surface equivalents and significantly lower than those used in Morris et al.,. (1990). The effect of fluid moderated processes operating on the slab in the vicinity of magma generation is un- known but is expected to increase B/Be ratios, based on observations at lower temperatures and pressures (Seyfried et al., 1984; Tatsumi and Isoyama, 1988 ). The high B/Be ratios ob- served in many arcs (up to 150) suggest that just such enhancement of the B/Be ratio in slab-derived fluids does occur.
In summary, arc lavas are often character- ized by high l°Be/Be ratios and are almost al- ways associated with elevated B contents and B/Be ratios. Where these features are ob- served in the volcanics, subduction modifica- tion of the source region of the lavas within the last 10Ma, and possibly within the last 3-5 Ma can be established.
Results
The samples analyzed are from the 1760, 1855, 1872 and 1944 eruptions at Vesuvius. They are of the small-scale mainly effusive type of eruption discussed by Santacroce (1983), with eruptive volumes typically 0.01-0.03 km 3 They form a group spanning the leucite te- phrite and phonolite tephrite regions of the TAS diagram. Lavas are strongly porphyritic with principal phenocrysts of leucite, clinopy- roxene and calcic plagioclase. Olivine, biotite, Fe-Ti oxides and apatite are present occasion- ally. Major- and trace-element compositions for these and other lavas are given in Belkin et al., this volume. All samples analyzed have less than 48.5% SiO2. Minor within-flow variation in chemical composition has been noted for some of the Vesuvian lavas, attributed largely to variations in the proportions of the differ- ent phenocrysts (Villemant et al., 1993-this volume).
Results from this study are shown in Table I. Rock chips of ~ 2-3mm size were leached, and leachates from the oldest and youngest sample were analyzed for l°Be. Powders were prepared from the leached chips by grinding in a WC shatterbox; 5-g aliquots were measured for 1°Be, and l-g aliquots were fused for B and 9Be concentration determination. Total chem- istry and shatterbox blanks are reported in Ta- ble 1. Contamination of the shatterbox had ap- parently occurred prior to sample preparation and standard cleaning procedures were appar- ently not sufficient to remove all contamina- tion. Analytical techniques are discussed fur- ther in Tera et al. (1986), Morris and Tera (1989), and Ryan and Langmuir (1993). Un- certainties on the measurements are given as footnotes to the table.
B and stable Be concentrations of the lavas are at the high end of the reported range for low-silica rocks (Leeman, 1987: Ryan and Langmuir, 1988, 1993), reflecting the alka- line, incompatible element enriched nature of these samples Three of the 4 lavas reported in
Be ISOTOPE AND B-Be INVESTIGATIONS OF THE HISTORIC ERUPTIONS OF Mt. VESUVIUS 353
TABLE1
Be isotope and B data for Vesuvius Lavas
Sample ID 1°Be, 10 6 a / g 1 B, ppm 2 Be, p p m 2 ( l°Be/Be) × 10-~,3 B/Be ~
1At less than 1 million atoms/gram, external reproducibility is + 40% ( 1 or). At > 1 million, reproducibility is +_ 15% 1 c~. 2Uncertainties in B, Be concentrations at these levels are ± 5%, uncertainty in the ratio is + 7% 3Reported as atom ratio. Uncertainties are + 40% at low ~°Be concentrations, +_ 16% at high concentrations. 4Total ~°Be blank resulting from chemical processing. 5 roBe of quartz processed in shatterbox; 1944 sample first processed.
Table 1 have l°Be concentrations that overlap with those reported for MORB and OIB (Brown et al., 1982; Tera et al., 1986). The 1944 sample has higher concentrations of l°Be than the other samples and does contain de- tectable l°Be, although the l°Be/Be ratio is low and within the range reported for MORB and OIB. As the 1944 sample was the first pro- cessed, its high concentration may reflect the inadvertant contaminat ion of the shatterbox. The leaching process, designed to remove any l°Be due to surface alteration or contamina- tion imposed prior to powdering, removed negligible amounts of l°Be from these samples. That the leaches contain little 1°Be also indi- cates that little or no removal of l°Be that was intrinsic to the rock occurred. Taken together, these results indicate that the Vesuvius lavas are similar to a number of arc lavas that show very low to essentially unmeasureable amounts of l°Be, as indicated in Figure 3.
Given that, in subduction models, the slab beneath Vesuvius is quite deep and that con- vergence rates are expected to have been slow, the l°Be in subducted sediments may well have decayed prior to any incorporation of sedi- ments in the arc source region. The uppermost sediments in an oceanic sediment column have a t°Be/gBe atom ratio ~ 5000× 10 -11 (equiv-
alent to 5000X 106 ag- I lOBe and 1.5 ppm 9 B e )
at the trench, with values decreasing by half for every 1.5-Ma increment in increasing age with depth. If a depth of 300 km for the slab be- neath Vesuvius and a convergence rate of 2 cm yr- l is adopted, the highest ratio at the top of the subducted sediment column (after cor- rection for 1°Be decay during transport) would be ~ 1 × 10-l l, relative to our detection limit of ~ 1 × 10-11. In this case, no amount of sed- iment incorporation would yield a measurea- ble l°Be signal in the lavas. If the geometry were similar, but the convergence rate were 5 cm yr- l, the highest value at the top of the sed- iment column would be on the order of 150xl0-11; the average value of the entire sediment column would depend on the amount of older sediments available to dilute the high l°Be young sediments, but would be signifi- cantly less than the highest value. Assuming an average value for the sediments of 100 × 10- i l
(unrealistically high to illustrate a limiting case), and assuming 2% sediment incorpora- tion, the lavas would a mBe/9Be of ~ 2 × 10- i l, virtually indistinguishable from OIB and MORB. Given the effect of l°Be decay during long subduction times on the Be isotope sys- tematics and the likliehood of slow subduction in this region, it is difficult to use the Be iso-
354 J. MORRIS ET AL.
tope systematics to argue for or against a role for recent subduction in the genesis of the Ve- suvius lavas.
For these reasons, we turn to the B/Be ratio. Earlier work showed consistent enrichment of B in arc lavas (Leeman, 1987 ) and that the B/ Be ratio may be used as a proxy for the ~°Be/ Be ratio (Morris et al., 1990). The B/Be ratio is, of course, not subject to decay through time. As shown in Figure 4, none of the samples have B/Be ratios above values measured in all MORB and OIB. It is tempting to use these re- suits to conclude that there is no role for recent subduction in the genesis of these lavas. It is important to realize, however, that the Vesu- vian lavas are like all lavas from the Cascades, and also like the alkaline lavas from Indonesia, which show no l°Be or B enrichment and yet are erupting in regions clearly affected by re- cent subduction.
Discussion
Consideration of the general systematics of the Be and B approach, together with recent case studies in the Cascades (Leeman et al., 1990) and Indonesia (Edwards et al., 1991, 1993 ), lead to several different possible expla- nations for the absence of 1°Be and B enrich- ment even in clearly subduction related lavas. Any of these possibilities may explain the ab- sence of a subduction signature in the Vesu- vian lavas. Given that long subduction times alone could eliminate any possible 1°Be signal, the following discussion will focus primarily on use of the B/Be ratio.
( 1 ) Involvement of significant amounts of low-l°Be/Be and low-B/Be crust via assimila- tion processes could, in theory, explain the ab- sence of l°Be and B enrichment in the Vesu- vian lavas. In general, however, the 1°Be/Be ratio in a lava will not be rapidly lowered by crustal contamination, as the crust has Be con- centrations similar to those of the lavas; cal-
culations suggest that in excess of 80% crustal contamination is necessary to lower a l°Be/9Be ratio of 8 to a value of 1 × 10-11 (Morris and Tera, 1989). That B concentrations in crustal reservoirs are generally within a factor of 2-3 of those seen in most arc basalts mean that the B/Be systematics are not especially sensitive to crustal contamination either. The high B concentrations of the Vesuvius lavas relative to those of calc-alkaline and tholeiitic arc basalts make these particular lavas even less suscepti- ble to significant reduction of the B/Be ratio through crustal contamination. The nearly constant B/Be and l°Be/Be ratios of Vesuvius are seen in lavas where S7Sr/S6Sr varies from 0.7072 to 0.7079 (Cortini and Hermes, 1981; Caprarelli, this volume). The absence of any significant variation in the Be isotopes or B/ Be ratios precludes any detailed evaluation, but indicates that the data are not consistent with any simple model of B dilution through exten- sive crustal contamination.
(2) The absence of l°Be in lavas of the Cas- cades may be explained by l°Be decay during long subduction time (Morris et al., 1990) while the absence of B enrichment is probably due to B loss from the subducting plate (Lee- man et al., 1990). The oceanic plate subduct- ing beneath the Cascades is ~ 10 Ma old, is thermally blanketed by thick sediments of the Cascadia margin, and is subducting slowly, all of which will lead to an anomalously hot ther- mal structure (Abbott and Lyle, 1984; Kom- inz and Bond, 1986). As discussed earlier and elsewhere (Leeman et al., 1990; Moran et al., 1992; Bebout et al., 1993) such conditions are favorable to loss of B from the plate at shallow depths, where any expelled B would presum- ably be stored within the accretionary prism or the crustal section of the upper plate. Moran et al. ( 1992 ) have taken this argument a step fur- ther and proposed that the observed high B/ Be ratios of most arc lavas require that the temperature of the subducting plate at the point where the arc source region is modified by sub-
Be ISOTOPE AND B-Be INVESTIGATIONS OF THE HISTORIC ERUPTIONS OF Mt. VESUVIUS 3 5 5
duction be less than ~ 650°C. Ryan and Lang- muir (1993) show that, in general, B/Be ra- tios of arc lavas decrease as the estimated temperature of the slab increases. While it is difficult to know the thermal structure of any slab that may have been subducted beneath the Italian region, the agreement that convergence rates were likely to be slow suggests that the slab may have been anomalously hot. If true, this fact alone could account for the absence of a subduction signature in the Vesuvius lavas.
( 3 ) As seen in Figure 4, the potassic alkaline lavas of Java and Flores never show B enrich- ment over Be, despite their intimate associa- tion spatially and temporally with calc-alka- line and tholeiitic lavas that always have elevated B/Be ratios (Edwards et al., 1991, 1993 ). As these authors note, this observation reinforces many interesting questions that have been asked about the role of alkaline lavas :in convergent margins (e.g., Box and Flower, 1989, and accompanying papers): what is their relationship to the subducting plate and the far more abundant non-alkaline rock types that typify this tectonic setting? What does the ab- sence of a subduction signature in alkaline suites imply about the processes and plumbing of metasomatism in the mantle wedge, magma generation, and eruption? While more work is clearly necessary to address these questions, analogy with the Indonesian results would sug- gest that the alkaline Vesuvian lavas could simply be derived from localized pockets within the mantle that have not been affected by recent episodes of subduction modifica- tion. If true, the absence of a subduction s~g- nature in the alkaline lavas does not allow us to conclude that the entire sub-arc mantle has not undergone subduction modification within the last 10 Ma. The calc-alkaline lavas re- covered from drill core north o fCampi Flegrei are too old, and potentially too altered, tbr study, but analysis of Central Campania shoshonitic lavas and Aeolian calc-alkaline, shoshonitic and alkaline lavas of the last 25 ka may allow this ambiguity to be resolved.
Conclusions
Measurements of cosmogenic and radioac- tive l°Be, stable 9Be and B concentrations in four alkaline lavas from the historical cycle at Mt. Vesuvius were undertaken in hopes that these tracers might show a chemical imprint resulting from subduction within the last 3-10 Ma, with implications for tectonic reconstruc- tions of the Mediterranean region. No lava measured shows significant elevation of l°Be/ Be or B/Be ratios similar to those observed in many lavas from regions of active arc volca- nism. While the presence of L°Be and B enrich- ment in the lavas would have built a strong case for recent subduction, the absence of such en- r ichment does not constitute a strong argu- ment against recent subduction. Subduction modification of the mantle any time prior to 3-10 Ma B.P. may have introduced B, which may have migrated out of the source area, due to its strong affinity for hydrous fluids. Alter- natively, the postulated slow convergence rates for this margin would allow any subducted L°Be in pelagic sediments to decay to negligible lev- els prior to magma generation; recent studies have shown that high slab temperatures asso- ciated with slow convergence rates can pro- duce arc lavas, as in the Cascades, with neither B nor ~°Be enrichment. Alkaline lavas in In- donesia never show a subduction signature, even though closely associated calc-alkaline and tholeiitic lavas always do, raising the pos- sibility that the alkaline lavas analyzed in this study simply do not record or preserve the chemical imprint of any possible subduction modification. Either of these two explana- tions, together with a less likely model of 1°Be and B dilution through extensive crustal con- tamination, could explain the absence ofa L~'Be- B subduction signature in the Vesuvian lavas. Given these disappointingly negative results, the Be-B system cannot be used simply to fur- ther asses a role for subduction processes in the generation of Vesuvius lavas.
356 J, MORRIS ET AL.
Acknowledgements
We are very grateful to Drs. Benedetto De Vivo and Raffaello Trigila for involving us in this project, for their careful sampling of Ve- suvius lavas to meet our specific needs, and for their interest, encouragement and support as the study progressed. We are also grateful to Drs. Gino Crisci, Sonia Esperanca, Dan Bar- ker and Giancarlo Serri for discussions about Aeolian and Central Campania magmatism and tectonic setting. Dr. Caroline Edwards work on Indonesia, and discussions thereof, contributed significantly to this ms, as did the comments of an anonomous reviewer and Giancarlo Serri. As always, the advice and ex- pertise of Drs. Fouad Tera, Roy Middleton and Jeff Klein is invaluable in carrying out the an- alytical work required in this project. This work was supported in part by the National Science Foundation.
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