[sup 226]Ra-[sup 230]Th disequilibrium in axial and off-axis mid-ocean ridge basalts

Conference · Mon Mar 01 00:00:00 EST 1993 · Geochimica et Cosmochimica Acta; (United States)

OSTI ID:6430480

Volpe, A M; Goldstein, S J ^[1]

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1. Lawrence Livermore National Lab., CA (United States) Los Alamos National Lab., NM (United States)

The authors describe [sup 226]Ra-[sup 230]Th disequilibrium in mid-ocean ridge basalt (MORB) glasses from the Juan de Fuca, Gorda, and East Pacific ridges. These first mass spectrometric measurements of [sup 226]Ra in MORB glasses at sub-picogram abundance levels confirm the large excesses over [sup 230]Th determined by radon-emanation techniques and alpha spectrometry. All off-axis MORB glasses have [sup 226]Ra-[sup 230]Th and [sup 234]U-[sup 238]U in secular equilibrium. This suggests that magmatic processes, not secondary post-eruption alteration, generate [sup 238]U-series disequilibrium in these MORB. Least evolved, N-MORB from axial valleys have ([sup 226]Ra/[sup 230]Th) between 2.2-2.3. Differentiated and enriched E-type MORB have consistently low ([sup 226]Ra/[sup 230]Th) ratios compared with N-MORB from the same ridge sections. Ra-Th fractionation may be less pronounced, or magma residence-transit periods may be long for differentiated MORB. Also, E-MORB may be generated by different melt extraction volumes and rates. Estimated [sup 226]Ra-[sup 230]Th ages for N-MORB agree with location on and off ridge segments, and with Th-U model ages. These preliminary results show that [sup 226]Ra-[sup 230]Th disequilibrium could be used to quantify volcanic episodicity at ocean ridges. 39 refs., 6 figs., 4 tabs.

• OSTI ID: 6430480
• Report Number(s): CONF-911017-; CODEN: GCACAK
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 57:6; Conference: Annual meeting and exhibition of the Geological Society of America, San Diego, CA (United States), 21-23 Oct 1991; ISSN 0016-7037
• Country of Publication: United States
• Language: English

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54 ENVIRONMENTAL SCIENCES
37 INORGANIC
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RADIUM 226
EQUILIBRIUM
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THORIUM 230
AGE ESTIMATION
BASALT
GEOLOGIC MODELS
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URANIUM 234
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MAGNESIUM 28 DECAY RADIOISOTOPES
NEON 24 DECAY RADIOISOTOPES
NUCLEI
RADIOISOTOPES
RADIUM ISOTOPES
ROCKS
SPECTROSCOPY
SPONTANEOUS FISSION RADIOISOTOPES
THORIUM ISOTOPES
URANIUM ISOTOPES
VOLCANIC ROCKS
YEARS LIVING RADIOISOTOPES
540310* - Environment
Aquatic- Basic Studies- (1990-)
400102 - Chemical & Spectral Procedures