TY - JOUR
T1 - In situ measurements of plagioclase growth using SIMS depth profiles of 7Li/30Si
T2 - A means to acquire crystallization rates during short-duration decompression events
AU - Genareau, Kimberly
AU - Clarke, Amanda
PY - 2010/4
Y1 - 2010/4
N2 - Numerous petrologic studies have attempted to determine crystal growth rates in volcanic systems through several methods, including analyses of crystal size distributions in natural samples and de-compression experiments on hydrous magmas. Experiments have revealed that rim growth on existing crystals will occur under a wide range of decompression conditions and is favored under conditions of low to moderate undercoolings over microlite nucleation, which has typically been the focus of decompression-induced crystallization studies. For this study, samples of eruptive clasts were collected from a Vulcanian explosion that occurred following the July 12-13, 2003 dome collapse of Soufriére Hills volcano, Montserrat. Plagioclase phenocrysts were extracted and examined with secondary ion mass spectrometry (SIMS) depth profiling. Lithium inflection depths within the profiles, along with the observed time interval between the peak in the collapse decompression and the explosion that ejected the examined samples, were used to calculate the growth rates as a result of magma devolatilization, with an average of 8.3 × 10-8 mm/s. Anorthite content of the plagioclase rim growth indicates an av-erage decompression magnitude of 40 MPa, inducing an undercooling of ∼45 °C that favors crystal growth over microlite nucleation. However, variability in the final anorthite contents suggests that not all phenocrysts recorded an equilibrium composition reflecting accurate pressure conditions. In such events occurring over short timescales (<10 h), lithium is a more reliable indicator of decompression-induced growth than changing anorthite content due to lithium's rapid diffusivity.
AB - Numerous petrologic studies have attempted to determine crystal growth rates in volcanic systems through several methods, including analyses of crystal size distributions in natural samples and de-compression experiments on hydrous magmas. Experiments have revealed that rim growth on existing crystals will occur under a wide range of decompression conditions and is favored under conditions of low to moderate undercoolings over microlite nucleation, which has typically been the focus of decompression-induced crystallization studies. For this study, samples of eruptive clasts were collected from a Vulcanian explosion that occurred following the July 12-13, 2003 dome collapse of Soufriére Hills volcano, Montserrat. Plagioclase phenocrysts were extracted and examined with secondary ion mass spectrometry (SIMS) depth profiling. Lithium inflection depths within the profiles, along with the observed time interval between the peak in the collapse decompression and the explosion that ejected the examined samples, were used to calculate the growth rates as a result of magma devolatilization, with an average of 8.3 × 10-8 mm/s. Anorthite content of the plagioclase rim growth indicates an av-erage decompression magnitude of 40 MPa, inducing an undercooling of ∼45 °C that favors crystal growth over microlite nucleation. However, variability in the final anorthite contents suggests that not all phenocrysts recorded an equilibrium composition reflecting accurate pressure conditions. In such events occurring over short timescales (<10 h), lithium is a more reliable indicator of decompression-induced growth than changing anorthite content due to lithium's rapid diffusivity.
KW - Decompression
KW - Lithium
KW - Plagioclase
KW - SIMS
KW - Soufrière Hills
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U2 - 10.2138/am.2010.3292
DO - 10.2138/am.2010.3292
M3 - Article
AN - SCOPUS:77950902379
SN - 0003-004X
VL - 95
SP - 592
EP - 601
JO - American Mineralogist
JF - American Mineralogist
IS - 4
ER -