Geochemical evolution within the Tonga-Kermadec-Lau Arc-Back-arc systems: The role of varying mantle wedge composition in space and time

Ewart A., Collerson K.D., Regelous M., Wendt J.I. and Niu Y. (1998) Geochemical evolution within the Tonga-Kermadec-Lau Arc-Back-arc systems: The role of varying mantle wedge composition in space and time. Journal of Petrology, 39 3: 331-368. doi:10.1093/petroj/39.3.331


Author Ewart A.
Collerson K.D.
Regelous M.
Wendt J.I.
Niu Y.
Title Geochemical evolution within the Tonga-Kermadec-Lau Arc-Back-arc systems: The role of varying mantle wedge composition in space and time
Journal name Journal of Petrology   Check publisher's open access policy
ISSN 0022-3530
Publication date 1998-01-01
Sub-type Article (original research)
DOI 10.1093/petroj/39.3.331
Open Access Status Not yet assessed
Volume 39
Issue 3
Start page 331
End page 368
Total pages 38
Publisher Oxford University Press
Language eng
Subject 1906 Geochemistry and Petrology
1908 Geophysics
Abstract New trace element and Sr, Nd, and Pb isotope data for lavas from the active Tonga-Kermadec arc in the southwest Pacific, the volcano of Niua fo'ou in the back-arc Lau Basin, and Pacific Ocean sediments from DSDP Sites 204 and 275, and ODP Site 596, are integrated with existing geochemical data for lavas from the Lau Basin, Samoa, the Louisville Ridge Seamount Chain (LR-SMC) and the extinct Lau Ridge arc, giving new insights into the petrogenesis of lavas in an active arc - back-arc system. Geochemical variations in Tonga-Kermadec arc lavas are the result of (1) differences in the amount and composition of the material being subducted along the arc, and (2) pre-existing heterogeneities in the upper mantle. Differences in the material being subducted beneath the arc have an important influence on the chemistry of the arc lavas. At the Kermadec Trench, ∼1 km thick layer of sediment is being subducted beneath the arc, compared with ∼200 m at the Tonga Trench. This results in the high Th/U and more radiogenic Pb isotope compositions of Kermadec lavas compared with Tonga lavas. The latter have Pb isotope compositions intermediate between those of Pacific sediments and Pacific mid-ocean ridge basalt (MORB), suggesting that much of the Pb in these lavas is derived from subducting Pacific Ocean crust. This is supported by the Pb isotope signatures of the subducting LR-SMC, which are also observed in lavas from the northern Tongan islands of Tafahi and Niuatoputapu. High field strength element (HFSE) and heavy rare earth element (HREE) concentrations are generally lower in Tongan lavas (particularly those from northern Tongan islands) than in Kermadec lavas. The Tonga Ridge basement, the proto-Tonga arc lavas (ODP Site 839) and the older Lau Ridge arc lavas are generally less depleted than the modern arc lavas. In the back-arc region, upper-mantle depletion as inferred from HFSE and HREE contents of the lavas broadly increases eastwards across the Lau Basin, whereas the subduction signature and volatile (CO and F) contents increase eastwards towards the modern arc. These observations suggest thai depletion is due to melt extraction during back-arc extension and vokanism, together with a long 'residence time' of mantle material within the mantle wedge. The upper mantle beneath the northernmost end of the Tonga arc and Lau Basin contains an ocean-island basalt (OIB) component derived from the Samoa plume to the north. This is reflected in high concentrations of Nb relative to other HFSE in lavas from Niua fo'ou, and Tafahi and Niuatoputapu islands at the northern end of the Tonga arc. Pb isotopes also suggest an LR-SMC contribution into Tafahi and Niuataputapu. Trace element and isotope modelling is used to investigate the combined effects of varying mantle source depletion and subduction on the geochemistry of the arc lavas. The results suggest that the arc lava geochemistry can be explained largely by the balance between a relatively constant subduction input of Pb, Th, U, Cs, Ba, Sr, Rb, K and Sc [corresponding to 0.001-0.005 weight fraction of the Stolper & Newman (1994, Earth and Planetary Science Letters, 121, 293-325] 'HO-rich component' composition), into the overlying, but variably depleted mantle wedge.
Keyword Isotopes
Mantle plumes
Petrogenesis
Tonga-Kermadec arc lavas
Trace elements
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Unknown

Document type: Journal Article
Sub-type: Article (original research)
Collection: Scopus Import - Archived
 
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