Abstract
We present new olivine-hosted melt inclusion volatile (H2O, CO2, S, Cl, F) and major element data from five historic eruptions of Nyamuragira volcano (1912, 1938, 1948, 1986, 2006). Host-olivine Mg#'s range from 71 to 84, with the exception of the 1912 sample (Mg# = 90). Inclusion compositions extend from alkali basalts to basanite-tephrites. Our results indicate inclusion entrapment over depths ranging from 3 to 5 km, which agree with independent estimates of magma storage depths (3–7 km) based on geophysical methods. Melt compositions derived from the 1986 and 2006 Nyamuragira tephra samples best represent pre-eruptive volatile compositions because these samples contain naturally glassy inclusions that underwent less post-entrapment modification than crystallized inclusions. Volatile concentrations of the 1986 and 2006 samples are as follows: H2O ranged from 0.6 to 1.4 wt %, CO2 from 350 to 1900 ppm, S from 1300 to 2400 ppm, Cl from 720 to 990 ppm, and F from 1500 to 2200 ppm. Based on FeOT and S data, we suggest that Nyamuragira magmas have higher fO2 (>NNO) than MORB. We estimate the total amount of sulfur dioxide (SO2) released from the 1986 (0.04 Mt) and 2006 (0.06 Mt) Nyamuragira eruptions using the petrologic method, whereby S contents in melt inclusions are scaled to erupted lava volumes. These amounts are significantly less than satellite-based SO2 emissions for the same eruptions (1986 = ∼1 Mt; 2006 = ∼2 Mt). Potential explanations for this observation are: (1) accumulation of a vapor phase within the magmatic system that is only released during eruptions, and/or (2) syn-eruptive gas release from unerupted magma.
Highlights
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Our results indicate inclusion entrapment over depths ranging from 3 to 5 km, which agree with independent estimates of magma storage depths (3-7 km) based on geophysical methods
Based on FeOT and S data, we suggest that Nyamuragira magmas have higher f02 (>NNO) than MORB
Summary
[2] Volatiles in magma play a key role in the timing, magnitude, and style of volcanic eruptions [e.g., Parfitt and Wilson, 1994; Sparks, 1997; Sutton et al, 2001; Edmonds and Gerlach, 2007; Shinohara, 2008], and can provide valuable insight into the depth and source of magma, whether the conduit is open or sealed and, potentially, the rate at which magma is erupting [e.g., Greenland et a/. , 1985; Sutton et a/., 2001]. We present new volatile (H20, C02, S, Cl, F) and major element data from olivinehosted melt inclusions from six Nyamuragira lava and tephra samples from five historic eruptions (1912, 1938, 1939, 1986, and 2006). These results are compared to so emissions derived from TOMS and OMI data. [16] Major and volatile (S, Cl, and F) element compositions of the melt inclusions, tephra glasses, and olivine host crystals were determined using the Cameca SX- 100 electron microprobe at the University of Oregon and the JEOL-JXA-733 electron microprobe at the Massachusetts Institute of Technology (MIT) (Table 2). On the other hand, resulted in an 1146% change between the original and corrected values
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