Dynamics of melt generation beneath mid-ocean ridge axes: Theoretical analysis based on [sup 238]U-[sup 230]Th-[sup 226]Ra and [sup 235]U-[sup 231]Pa disequilibria

Journal Article · Thu Apr 01 00:00:00 EST 1993 · Geochimica et Cosmochimica Acta; (United States)

OSTI ID:6607845

Qin, Zhenwei ^[1]

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1. Univ. of Chicago, IL (United States)

Although slow melting favors the generation of basaltic melt from a mantle matrix with large radioactive disequilibrium between two actinide nuclides (McKenzie, 1985a), it results in long residence time in a magma chamber, during which the disequilibrium may be removed. An equilibrium melting model modified after McKenzie (1985a) is presented here which suggests that, for a given actinide parent-daughter pair, there exists a specific melting rate at which disequilibrium between these two nuclides reaches its maximum. This melting rate depends on the decay constant of the daughter nuclide concerned and the magma chamber volume scaled to that of its source. For a given scaled chamber size, large radioactive disequilibrium between two actinide nuclides in basalts will be observed if the melting rate is such that the residence time of the magma in the chamber is comparable to the mean life of the daughter nuclide. With a chamber size 1% in volume of the melting source, the melting rates at which maximum disequilibrium in basalts is obtained are 10[sup [minus]7], 2 [times] 10[sup [minus]7], and 3 [times] 10[sup [minus]6] y[sup [minus]1], respectively for [sup 238]U-[sup 230]Th, [sup 235]U-[sup 231]Pa, and [sup 230]Th-[sup 226]Ra. This implies that, while large disequilibrium between [sup 238]U-[sup 230]Th and between [sup 235]U-[sup 231]Pa may occur together, large [sup 230]Th-[sup 226]Ra disequilibrium will not coexist with large [sup 238]U-[sup 230]Th disequilibrium, consistent with some observations. The active mantle melting zone which supplies melt to a ridge axis is inferred to be only about 10 km thick and 50 km wide. The fraction of melt present in such a mantle source at any time is about 0.01 and 0.04, respectively, if melting rate is 10[sup [minus]7] and 10[sup [minus]6] y[sup [minus]1]. The corresponding residence time of the residual melt in the matrix is 10[sup 5] and 4 [times] 10[sup 4] y. 27 refs., 3 figs.

• OSTI ID: 6607845
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 57:7; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
58 GEOSCIENCES
ACTINIDE ISOTOPES
EQUILIBRIUM
ISOTOPE RATIO
MAGMA
MELTING
BASALT
DAUGHTER PRODUCTS
DECAY
DYNAMICS
PROTACTINIUM 231
RADIUM 226
SEA-FLOOR SPREADING
THORIUM 230
URANIUM 235
URANIUM 238
ACTINIDE ISO
ACTINIDE NUCLEI
ALKALINE EARTH ISOTOPES
ALPHA DECAY RADIOISOTOPES
CARBON 14 DECAY RADIOISOTOPES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
HEAVY ION DECAY RADIOISOTOPES
HEAVY NUCLEI
IGNEOUS ROCKS
INTERNAL CONVERSION RADIOISOTOPES
ISOMERIC TRANSITION ISOTOPES
ISOTOPES
MECHANICS
MINUTES LIVING RADIOISOTOPES
NEON 24 DECAY RADIOISOTOPES
NUCLEI
ODD-EVEN NUCLEI
PHASE TRANSFORMATIONS
PROTACTINIUM ISOTOPES
RADIOISOTOPES
RADIUM ISOTOPES
ROCKS
SPONTANEOUS FISSION RADIOISOTOPES
THORIUM ISOTOPES
URANIUM ISOTOPES
VOLCANIC ROCKS
YEARS LIVING RADIOISOTOPES
400100* - Analytical & Separations Chemistry
580000 - Geosciences