Nain geochemistry reviewed: anorthosite parent liquids and contamination problems

Conference · Tue Jan 01 00:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6317067

Morse, S A

Sr-An relations of PLAG and candidate natural LIQ (dikes, chills) agree with partition coefficients derived from Kiglapait in predicting parent LIQuids like rift basalts (Servilleta) and flood basalt (Keweenaw, Parana). All LIQ are richer in Sr than consistent with proposed lower crust or MORB. Many diverse parent LIQS had 275-675 ppm Sr, with mean 430 ppm Sr, 8 ppm Rb. Referred to 1.4 Gry, Archean crust (ARCH) at Lost Channel has I(Sr) = 0.717, R = Rb/Sr = 0.4. Minimum melt granites (GR) 2.3-1.3 Gyr old average Sr = 190, Rb = 160 ppm at I(Sr) = 0.753. By contrast, Nain feldspars have R from 0.001 to 0.01, implying liquids 0.003 - 0.02. Neither GR nor ARCH can contaminate normal mantle-derived magmas (I= 0.7028) to achieve the I(Sr) 0.703-0.708 observed, without adding 15-50 ppm Rb, not observed. Therefore, the I(Sr) signature is either from an old, enriched mantle source later depleted, or form assimilation of Rb-poor supracrustals (marble.) with huge I(Sr), ppm Sr values. The latter are not yet demonstrated to exist. Proximal contaminants should show up in local Sr-isotopic heterogeneity (as at Rhum). Local isotopic uniformity could arise for mold deeper-seated, multi-cycle or conceivably lower mantle source component, brought up in a rift setting and degassed with exit of LIL as per Galer and O'Nions (J. Petrol., submitted). Rb-rich GR of all ages show the regional persistence of undepleted crust available for melting. Either ANORTH plumbing was very restricted or new crustal slices of GR source were delivered to the hot spot on a regular basis to yield new GR melts.

• Research Organization: Univ. of Massachusetts, Amherst (USA)
• OSTI ID: 6317067
• Report Number(s): CONF-8510489-
• Journal Information: Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
15 GEOTHERMAL ENERGY
ANORTHOSITES
PETROGENESIS
PETROLOGY
GEOTHERMAL FLUIDS
ROCK-FLUID INTERACTIONS
CHEMICAL REACTIONS
FELDSPARS
GEOCHEMISTRY
ISOTOPE RATIO
MAGMA
MAGMATISM
METAMORPHISM
STRONTIUM COMPOUNDS
ALKALINE EARTH METAL COMPOUNDS
CHEMISTRY
FLUIDS
GABBROS
GEOLOGY
IGNEOUS ROCKS
MINERALS
PLUTONIC ROCKS
ROCKS
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