Mantle enrichment by volatiles as the Nazca plate subducts beneath the Payenia backarc of the Southern Volcanic Zone, Argentina: A geochemical study of melt inclusions, minerals and tephra

Volatile (Cl, S, F, H2O and CO2), trace and major elements have been determined in olivine hosted
melt inclusions in tephra samples selected in the Payenia backarc volcanic province of the Southern
Volcanic Zone (SVZ), Argentina. The chemistry of the melt inclusions are compared to the major
element chemistry of olivine and Cr-spinel and to clinopyroxene and fluid inclusion barometry.
Based on the collected data, the importance of source enrichment, subduction zone metasomatism,
partial mantle melting, oxygen fugacity, volatile solubility and degassing is discussed for magmas
in Payenia.
The concentrations of Cl and maximum S in melt inclusions in Payenia increase from south to
north. This latitudinal variation is also observed for certain major elements concentrations such as
TiO2, K2O and SiO2 in the melt inclusions as well as for olivine Fo-contents (ranging from Fo90-80).
The systematic increase in chlorine is coupled to an increase Cl/K ratios and is related to an
increase mantle source enrichments by fluids derived from the subducting materials. Degassing of S
from the magmas is investigated and maximum S/K ratios correlate positively with Cl/K ratios in
melt inclusions.
Melt inclusions from the northern Payenia have trace element signatures resembling upper
continental crust, whereas melt inclusions from the southern Payenia have trace elements signatures
resembling ocean islands basalts. The transition in mantle enrichment from south to north in
Payenia is gradual and the trace element concentrations of Rb, Sr, Ba, La, Ce, Th and U in melt
inclusions increase northwards. A northward increase in mantle enrichment by fluids and UCC is
indicated by the increase in Th/La, Cl/Nb, Ba/Nd and Sr/Nb. These variations for selected trace
element ratios are coupled variations in oxygen fugacity determined by Cr-spinel inclusions in
olivine phenocrysts. We estimate that sample averages of Fe2+/Fetot (melt) ratios range from 0.88
(southern Payenia) to 0.79 in (Northern Payenia) and suggest that the northwards increase in of
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Fe2+/Fetot (melt) is the consequence of fluid and UCC addition to the mantle sources. The degree of
melting range from 1-10 % in Payenia, but no strong north to south variation is indicated.
Pressure estimates based on clinopyroxene phenocryst and fluid inclusions indicate
crystallization at 1 to 6 kbar for Northern Payenia magmas. Fluid inclusions pressures of 3 to 6 kbar
is indicated for central Payenia magmas. Maximum pressures inferred from clinopyroxene and fluid
inclusions correspond well to pressures inferred from maximum H2O and CO2 MIs from the
northern Payenia whereas maximum H2O and CO2 MIs from central Payenia may have experienced
degassing at depth. The variation in H2O/Th, H2O/Nb, Ba/Nb and Ba/Th in MIs indicate that fluids
were involved in mantle enrichment in Payenia. Northern Payenia magmas additionally indicate
melt derivation from a mantle source enriched by UCC, unlike magmas from the southern Payenia.
Moreover, the compositions of fluids involved in mantle enrichment in Payenia change from south
to north. Maximum H2O melt inclusions from Northern Payenia MIs indicate derivation from a
mantle source enriched by saline fluids (Cl/H2O = 0.02; 2 wt. % NaCl) whereas maximum H2O
melt inclusions from the southern Payenia indicate derivation from a mantle source enriched by less
saline fluids (Cl/H2O = 0.04; 6 wt. % NaCl). The fluids involved in the mantle enrichment in the
southern Payenia proposedly derive from altered oceanic crust (AOC) whereas the more saline
fluids in northern Payenia proposedly derive from serpentine dehydration at depth.