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Since late 1994, open-vent eruptive activity and degassing at Popocatepetl volcano, Mexico, have released large masses of C[O.sub.2] and S[O.sub.2]. Tephra and lava produced by these eruptions show evidence for mixing of mafic and silicic magmas shortly before eruption. We present the first measurements of dissolved C[O.sub.2] in the mafic magma end member based on analyses of olivine-hosted melt inclusions that were trapped at pressures as high as ~400 MPa (~15 km depth) beneath the volcano. We combine our data with thermodynamic models to show that degassing of mafic magma at ~150-350 MPa pressure can explain the C[O.sub.2]/S[O.sub.2] mass ratios (1-8) of volcanic gases released from the volcano during 1995-1997. Our results demonstrate that mafic magma recharge was responsible for the high measured fluxes of C[O.sub.2] and S[O.sub.2] from 1995 to 1997. The total S[O.sub.2] emission of 9 Mt during this period requires intrusion and degassing of a minimum of 0.8 [km.sup.3] of mafic magma. Only ~0.3% of this new mafic magma has been erupted in the form of mixed (hybrid) lava and tephra. Our results suggest that the ongoing eruption of Popocatepetl is essentially an intrusive event. More generally, we suggest that intrusion and deep degassing may explain the high gas fluxes at some other open-vent volcanoes rather than convection of magma in the uppermost parts of subvolcanic conduits.

The Ontong Java Plateau in the western Pacific is anomalous compared to other oceanic large igneous provinces in that it appears to have never formed a large subaerial plateau. Paleoeruption depths (at 122 Ma) estimated from dissolved [H.sub.2]O and C[O.sub.2] in submarine basaltic glass pillow rims vary from ~1100 m below sea level (mbsl) on the central part of the plateau to 2200-3000 mbsl on the northeastern edge. Our results suggest maximum initial uplift for the plateau of 2500-3600 m above the surrounding seafloor and 1500 [+ or -] 400 m of postemplacement subsidence since 122 Ma. Our estimates of uplift and subsidence for the plateau are significantly less than predictions from thermal models of oceanic lithosphere, and thus our results are inconsistent with formation of the plateau by a high-temperature mantle plume. Two controversial possibilities to explain the anomalous uplift and subsidence are that the plateau (1) formed as a result of a giant bolide impact, or (2) formed from a mantle plume but has a lower crust of dense garnet granulite and/or eclogite; neither of these possibilities is fully consistent with all available geological, geophysical, and geochemical data. The origin of the largest magmatic event on Earth in the past 200 m.y. thus remains an enigma. Keywords: large igneous province, basalt, volatiles, mantle plume, meteorite impact.

Although there is a growing body of data on [H.sub.2]O in arc magmas, there is still considerable uncertainty about the relationship between [H.sub.2]O and various incompatible elements during enrichment of the mantle wedge by subduction processes. We report data for [H.sub.2]O, other volatiles (C[O.sub.2], S, Cl), and trace elements in olivine-hosted melt inclusions from high-Mg basalts in central Mexico that exhibit varying degrees of subduction-related enrichment. Most melt inclusions were trapped at low pressure, but rare inclusions (Mg# 65-78, olivine hosts [Fo.sub.85-90]) trapped at upper to middle crustal pressures (1-6 kbar) contain high C[O.sub.2] (250-2120 ppm). The high-pressure inclusions indicate magmatic [H.sub.2]O contents from 1.3 to 5.2 wt%. Enrichment of [H.sub.2]O relative to Nb correlates positively with K/Nb, Ba/Nb, and La/Nb, indicating a clear link between [H.sub.2]O and trace element enrichment of the mantle wedge. Our results show that fluxing of the wedge with an [H.sub.2]O-rich component from the subducted slab is important in formation of magmas that are enriched in large ion lithophile (LILE) and light rare earth (LREE) elements relative to high field strength elements (HFSE). In contrast, magmas with low LILEs and LREEs relative to HFSEs have relatively low [H.sub.2]O, and must have formed largely by decompression melting of unmodified mantle. Our data for volcanoes Keywords: basalt, magma, subduction zones, trace elements, volatiles, water.

The Ontong Java Plateau in the western Pacific is anomalous compared to other oceanic large igneous provinces in that it appears to have never formed a large subaerial plateau. Paleoeruption depths (at 122 Ma) estimated from dissolved H2O and CO2 in submarine basaltic glass pillow rims vary from ∼1100 m below sea level (mbsl) on the central part of the plateau to 2200–3000 mbsl on the northeastern edge. Our results suggest maximum initial uplift for the plateau of 2500–3600 m above the surrounding seafloor and 1500 ± 400 m of postemplacement subsidence since 122 Ma. Our estimates of uplift and subsidence for the plateau are significantly less than predictions from thermal models of oceanic lithosphere, and thus our results are inconsistent with formation of the plateau by a high-temperature mantle plume. Two controversial possibilities to explain the anomalous uplift and subsidence are that the plateau (1) formed as a result of a giant bolide impact, or (2) formed from a mantle plume but has a lower crust of dense garnet granulite and/or eclogite; neither of these possibilities is fully consistent with all available geological, geophysical, and geochemical data. The origin of the largest magmatic event on Earth in the past 200 m.y. thus remains an enigma. [ABSTRACT FROM AUTHOR]

Although there is a growing body of data on H[sub 2]O in arc magmas, there is still considerable uncertainty about the relationship between H[sub 2]O and various incompatible elements during enrichment of the mantle wedge by subduction processes. We report data for H[sub 2]O, other volatiles (CO[sub 2], S, Cl), and trace elements in olivine-hosted melt inclusions from highMg basalts in central Mexico that exhibit varying degrees of subduction-related enrichment. Most melt inclusions were trapped at low pressure, but rare inclusions (Mg# 6578, olivine hosts Fo[sub 85-90]) trapped at upper to middle crustal pressures (1-6 kbar) contain high CO[sub 2] (250-2120 ppm). The high-pressure inclusions indicate magmatic H[sub 2]O contents from 1.3 to 5.2 wt%. Enrichment of H[sub 2]O relative to Nb correlates positively with K/Nb, Ba/Nb, and La/Nb, indicating a clear link between H[sub 2]O and trace element enrichment of the mantle wedge. Our results show that fluxing of the wedge with an H[sub 2]O-rich component from the subducted slab is important in formation of magmas that are enriched in large ion lithophile (LILE) and light rare earth (LREE) elements relative to high field strength elements (HFSE). In contrast, magmas with low LILEs and LREEs relative to HFSEs have relatively low H[sub 2]O, and must have formed largely by decompression melting of unmodified mantle. Our data for volcanoes <50 km apart show evidence of significant variability in the composition of H[sub 2]O-rich subduction components that are added to the mantle wedge beneath central Mexico. [ABSTRACT FROM AUTHOR]

Since late 1994, open-vent eruptive activity and degassing at Popocatépetl volcano, Mexico, have released large masses of CO2 and SO2. Tephra and lava produced by these eruptions show evidence for mixing of mafic and silicic magmas shortly before eruption. We present the first measurements of dissolved CO2 in the mafic magma end member based on analyses of olivine-hosted melt inclusions that were trapped at pressures as high as ~400 MPa (~15 km depth) beneath the volcano. We combine our data with thermodynamic models to show that degassing of mafic magma at ~150-350 MPa pressure can explain the CO2/SO2 mass ratios (1-8) of volcanic gases released from the volcano during 1995-1997. Our results demonstrate that mafic magma recharge was responsible for the high measured fluxes of CO2 and SO2 from 1995 to 1997. The total SO2 emission of 9 Mt during this period requires intrusion and degassing of a minimum of 0.8 km3 of mafic magma. Only ~0.3% of this new mafic magma has been erupted in the form of mixed (hybrid) lava and tephra. Our results suggest that the ongoing eruption of Popocatépetl is essentially an intrusive event. More generally, we suggest that intrusion and deep degassing may explain the high gas fluxes at some other open-vent volcanoes rather than convection of magma in the uppermost parts of subvolcanic conduits. [ABSTRACT FROM AUTHOR]