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Titolo:
Insights into mid-ocean ridge basalt petrogenesis: U-series disequilibria from the Siqueiros Transform, Lamont Seamounts, and East Pacific Rise
Autore:
Lundstrom, CC; Sampson, DE; Perfit, MR; Gill, J; Williams, Q;
Indirizzi:
Univ Calif Santa Cruz, Earth Sci Board, Santa Cruz, CA 95064 USA Univ Calif Santa Cruz Santa Cruz CA USA 95064 d, Santa Cruz, CA 95064 USA Univ Florida, Dept Geol, Gainesville, FL 32611 USA Univ Florida Gainesville FL USA 32611 ept Geol, Gainesville, FL 32611 USA
Titolo Testata:
JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH
fascicolo: B6, volume: 104, anno: 1999,
pagine: 13035 - 13048
SICI:
0148-0227(19990610)104:B6<13035:IIMRBP>2.0.ZU;2-2
Fonte:
ISI
Lingua:
ENG
Soggetto:
BENEATH MIDOCEAN RIDGES; U-238 TH-230 DISEQUILIBRIUM; UPPER MANTLE; ISOTOPIC SYSTEMATICS; CRUSTAL THICKNESS; UPWELLING MANTLE; LAMONT SEAMOUNT; AXIS SEAMOUNTS; SPREADING RATE; MAGMA GENESIS;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
59
Recensione:
Indirizzi per estratti:
Indirizzo: Lundstrom, CC Brown Univ, Dept Geol Sci, Providence, RI 02912 USA Brown Univ Providence RI USA 02912 Providence, RI 02912 USA
Citazione:
C.C. Lundstrom et al., "Insights into mid-ocean ridge basalt petrogenesis: U-series disequilibria from the Siqueiros Transform, Lamont Seamounts, and East Pacific Rise", J GEO R-SOL, 104(B6), 1999, pp. 13035-13048

Abstract

Parent-daughter disequilibria between (Th-230)/(U-238), (Pa-231)/(U-235) and (Ra-226)/(Th-230) (parentheses refer to activities) have been measured by thermal ionization mass spectrometry and inductively coupled plasma-mass spectrometry in basalts from three tectonomagmatic settings of the East Pacific Rise (EPR) at 8 degrees 20'-10 degrees N. Mid-ocean ridge basalts (MORB) from the Siqueiros Transform, the Lament Seamounts; and the EPR ridge crest span a large compositional range from primitive, high-MgO basalts with strong incompatible element depletions (DMORB) to typical normal MORE (NMORB) to rare incompatible element enriched basalts (EMORB) derived from a more enriched source isotopically. Concentrations of U vary from <13 ppb in DMORB to >400 ppb in EMORB while Th/U ranges from 2 in DMORB up to 3 in EMORB. The young-looking high-MgO basalts have (Ra-226)/(Th-230) that ranges from 3.2 to 4.2, while EMORB appear old being near secular equilibrium. Initial (Pa-231)/(U-235) are very high (>2.5) in all of the Siqueiros basalts. Three basalts from the Lament Seamounts have low incompatible element concentrations and low Th/U and are in secular equilibrium for (Ra-226)/(Th-230) while the sample located closest to the ridge axis has significant Ra-226 and Pa-231 excesses and minor Th-230 excess. DMORB lack Th-230 excess, have high excesses of Ra-226 and Pa-231, and resemble experimentally determined melts of peridotite at 1 GPa, implying derivation from relatively shallow level melting of spinel Iherzolite at low residual porosity. Disequilibria for all three parent-daughter pairs are consistent with typical axial NMORB resulting from mixing of melts derived from heterogeneous sources, specifically 90-95% DMORB with 5-10% EMORB. The observation that all samples, regardless of tectonomagmatic setting, lie on the same mixing trend suggests thatmelting beneath seamounts and transforms is similar to melting beneath theridge axis. Variations in Th-230 excess over short spatial scales imply that garnet-bearing mafic veins create all of the Th-230 excess observed in typical NMORB.

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Documento generato il 02/04/20 alle ore 00:03:45