210Po, 210Pb, AND 226Ra IN SAMPLES FROM MERAPI VOLCANO, INDONESIA: IMPLICATIONS FOR TIME-SCALES OF DEGASSING

Three samples of volcanic rocks from Merapi volcano in Indonesia were analyzed for 210Po, 210Pb, and 226Ra activities using alpha and gamma spectrometry. Two samples were from a 2006 eruption. One was a darker sample that was a basalt; the other was a light-colored basaltic andesite. The third sample was from 1995 and was an andesite. The principal goals were to determine if both 2006 magmas were juvenile before eruption, and the time-scales of magma differentiation. The dark sample was found to have (210Pb/210Po) values that were in equilibrium suggesting it had solidified between 2 and 20 years before being erupted. The light sample had near-zero initial (210Pb/210Po) values, indicating that it had been completely degassed prior to eruption. Thus, the dark sample did not represent new magma at the time of eruption, whereas the light sample was liquid when it erupted. The (210Pb/226Ra) values for these andesites were 0.88-0.85 (see also Gauthier and Condomines, 1999, EPSL, v. 172). This contrasts with most arc andesites, which typically have near equilibrium (210Pb/226Ra) values. There are three possible explanations for the 210Pb deficits in these samples: crystal fractionation of plagioclase, fluxed melting, and degassing. Plagioclase fractionation is an unlikely cause of these deficits because both elements are incompatible and the amount of fractionation required to produce the Merapi basaltic andesites from mantle-derived parent magmas is small. Melting of the slab is also an unlikely cause of these deficits because the slab fluid would have to have (226Ra)>>(210Pb) and the period between melting, fractionation, and eruption would have to be less than a century, which seems unreasonable. Degassing of 222Rn for 4 to 5 years before eruption is thus the most likely cause of the 210Pb deficit, which implies that Merapi's high-K basaltic andesites degas for longer periods of time and perhaps rise from a more deeply-seated reservoir compared to lower-K basaltic andesites from other arc volcanoes, such as Arenal (Reagan et al., 2006, JVGR, v. 157).