Geochemistry and volatile content of magmas feeding explosive eruptions at Telica volcano (Nicaragua)

Telica volcano, in north-west Nicaragua, is a young stratovolcano of intermediate magma composition producingfrequent Vulcanian to phreatic explosive eruptions. The Telica stratigraphic record also includes examples of(pre)historic sub-Plinian activity. To refine our knowledge of this very active volcano, weanalyzedmajor elementcomposition and volatile content of melt inclusions fromsomestratigraphically significant Telica tephra deposits.These include: (1) the Scoria Telica Superior (STS) deposit (2000 to 200 years Before Present; Volcanic ExplosiveIndex, VEI, of 2–3) and (2) pyroclasts from the post-1970s eruptive cycle (1982; 2011). Based on measurementswith nanoscale secondary ion mass spectrometry, olivine-hosted (forsterite [Fo] N 80) glass inclusions fall into 2distinct clusters: a group of H2O-rich (1.8–5.2 wt%) inclusions, similar to those of nearby Cerro Negro volcano,and a second group of CO2-rich (360–1700 μg/g CO2) inclusions (Nejapa, Granada). Model calculations showthat CO2 dominates the equilibrium magmatic vapor phase in the majority of the primitive inclusions (XCO2 N0.62–0.95). CO2, sulfur (generally b2000 μg/g) and H2O are lost to the vapor phase during deep decompression(P N 400 MPa) and early crystallization of magmas. Chlorine exhibits a wide concentration range (400–2300μg/g) in primitive olivine-entrapped melts (likely suggesting variable source heterogeneity) and is typicallyenriched in the most differentiated melts (1000–3000 μg/g). Primitive, volatile-rich olivine-hosted melt inclusions(entrapment pressures, 5–15 km depth) are exclusively found in the largest-scale Telica eruptions (exemplifiedby STS in our study). These eruptions are thus tentatively explained as due to injection of deep CO2-richmafic magma into the shallow crustal plumbing system. More recent (post-1970), milder (VEI 1–2) eruptions,instead, do only exhibit evidence for low-pressure (P b 50–60 MPa), volatile-poor (H2O b 0.3–1.7 wt%; CO2b 23–308 μg/g) magmatic conditions. These are manifested as andesitic magmas, recording multiple magmamixing events, in pyroxene inclusions.Wepropose that post-1970s eruptions are possibly related to the high viscosityof resident magma in shallow plumbing system (b2.4 km), due to crystallization and degassing.