An unmetasomatized source for the Malaitan alnoeite (Solomon Islands): Petrogenesis involving zone refining, megacryst fractionation, and assimilation of oceanic lithosphere

Journal Article · Tue Aug 01 00:00:00 EDT 1989 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(89)90318-9· OSTI ID:7266115

Neal, C R ^[1]; Davidson, J P ^[2]

---

1. Univ. of Tennessee, Knoxville (USA)
2. Univ. of Michigan, Ann Arbor (USA)

The Malaitan alnoite contains a rich and varied megacryst suite of unprecedented compositional range. The authors have undertaken trace element and isotope modeling in order to formulate a petrogenetic scheme which links the host alnoeite to its entrained megacrysts. This requires that a proto-alnoeite magma is the product of zone refining initiated by diapiric upwelling (where the initial melt passes through 200 times its volume of mantle). Isotopic evidence indicates the source of the proto-alnoeite contains a time-integrated LREE-depleted signature. Impingement upon the rigid lithosphere halts or dramatically slows the upward progress of the mantle diapir. At this point, the magma cools and megacryst fractionation begins with augites crystallizing first, followed by subcalcic diopsides and finally phlogopites. Garnet probably crystallizes over the entire range of clinopyroxene fractionation. Estimated proportions of fractionating phases are 30% augite, 24.5% subcalcic diopside, 27% garnet, 12.9% phlogopite, 5% bronzite, 0.5% ilmenite, and 0.1% zircon. As this proto-alnoeite magma crystallizes, it assimilates a subducted component of seawater-altered basalt which underplates the Ontong Java Plateau. This is witnessed in the isotopic composition of the megacrysts and alnoeite.

• OSTI ID: 7266115
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Vol. 53:8; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
INCLUSIONS
ISOTOPE RATIO
MINERALOGY
VOLCANIC ROCKS
PETROCHEMISTRY
PETROGENESIS
CHEMICAL COMPOSITION
CRYSTAL STRUCTURE
CRYSTALLIZATION
DIOPSIDE
GARNETS
ILMENITE
INDONESIA
MAGMA
NEODYMIUM ISOTOPES
OXYGEN ISOTOPES
STRONTIUM ISOTOPES
ZIRCON
ALKALINE EARTH ISOTOPES
ASIA
CHALCOGENIDES
DEVELOPING COUNTRIES
IGNEOUS ROCKS
IRON COMPOUNDS
IRON OXIDES
ISLANDS
ISOTOPES
MINERALS
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
PYROXENES
ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
TITANIUM COMPOUNDS
TITANIUM OXIDES
TRANSITION ELEMENT COMPOUNDS
ZIRCONIUM COMPOUNDS
ZIRCONIUM SILICATES
580000* - Geosciences