GENERATION OF RHYOLITIC MELT IN AN ARTIFICIAL MAGMA :
IMPLICATIONS FOR FRACTIONAL CRYSTALLIZATION PROCESSES IN
NATURAL MAGMAS
DUNBAR NW, RICIPUTI LR, JACOBS GK, NANEY MT, CHRISTIE W
JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH
v. 57(#3-4) pp. 157-166 1993
(Title at LANL MAIN.)
- Institutions:
- NMIMT,INST GEOSCI/SOCORRO//NM/87801
- OAK RIDGE NATL LAB,DEPT ENVIRONM SCI/OAK RIDGE//TN/37831
- OAK RIDGE NATL LAB,DEPT ANALYT CHEM/OAK RIDGE//TN/37831
- Abstract:
- A large (1.3x 10(7) g) artificial mafic melt with a bulk composition similar
to an unusually calcic basalt (17 wt.% CaO) was produced by in-situ heating of
soil, and subsequently cooled and crystallized. The final crystalline
assemblage consisted dominantly of acicular diopsidic to hedenbergitic
pyroxene and anorthitic feldspar, with a subordinate amount of potassic
feldspar. Electron microprobe analyses reveal the presence of a small amount
(approximately 10 vol.%) of rhyolitic glass (71% SiO2) within the crystalline
network. This glass is the residual material left after crystallization of
pyroxene and feldspar, analogous to rhyolitic melt which may be generated from
fractional crystallization of a basaltic magma. Ion microprobe imaging
indicates that the rhyolitic glass is generally present in isolated
triangular- to rectangular-shaped interstices left by crystallization of
acicular and tabular phases, and that the glass is enriched in trace elements
(such as Zr and Cs) which are incompatible within the crystalline phases. No
evidence of coalescence or migration of the rhyolitic melt is suggested by
glass morphology. Although these observations were made in an artificial
magmatic system, analogies to a natural system may be drawn. The isolated
nature and low abundance of the rhyolitic glass highlights the difficulty of
extracting and segregating evolved melts produced by high degrees of
crystallization of a primitive parent, particularly one characterized by
elongated, rather than equant, crystals. The distribution of the rhyotitic
glass supports the suggestion that extraction of significant amounts of
evolved melts from rocks may require repeated partial melting of crystallized
material in order to allow the evolved liquid to phasically separate from the
parent, and also suggests that the geometry of crystals may be an important
factor in melt segregation.
- Keywords:
- DIFFERENTIATION; CHAMBERS; ROCKS
- Categories:
- GEOLOGY
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