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Chemical and Isotopic Composition of Lavas from the Northern Mariana Trough: Implications for Magmagenesis in Back-arc Basins

Gribble, Robert F. and Stern, Robert J. and Newman, Sally and Bloomer, Sherman H. and O'Hearn, Tim (1998) Chemical and Isotopic Composition of Lavas from the Northern Mariana Trough: Implications for Magmagenesis in Back-arc Basins. Journal of Petrology, 39 (1). pp. 125-154. ISSN 0022-3530. doi:10.1093/petroj/39.1.125.

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We report the results of a geochemical and isotopic study of mostly basaltic glasses recovered from 25 dredge stations along the northernmost 500 km of the Mariana Trough extension axis. The distribution of samples links regions of seafloor spreading to the south with regions farther north where a progression of rifting styles accompanies the earliest stages of back-arc basin extension. Petrographic, chemical and isotopic compositions of igneous rocks reflect the changing styles of extension, with typical back-arc basin basalts in the south which become increasingly similar to arc lavas to the north. Felsic lavas also appear along the extensional axis in the north. Glassy, sparsely phyric basalts characterize regions of seafloor spreading. Felsic lavas and porphyritic basalts occur in the northern, rifting portion. Geochemical and isotopic compositions distinguish between mature arc portions (Ce/Pb <10, Ba/La >20; ^(206)Pb/^(204)Pb >18.5, ^(87)Sr/^(86)Sr >0.7032, ^εNd< +8) and regions of back-arc spreading (Ce/Pb >10, Ba/La <20, ^(206)Pb/^(204)Pb <18.9, ^(87)Sr/^(86)Sr <0.7032, ^εNd> +7). Samples from along the extensional axis of the northern Mariana Trough show progressive changes in chemical and isotopic compositions, from back-arc basin basalts that formed by seafloor spreading northward through increasingly arc–like basalts, until lavas that are indistinguishable from arc lavas are encountered in the northernmost portion of the rift. Batch-melting models indicate that northernmost rift lavas reflect higher degrees of melting, with 13 ± 5% melting where seafloor spreading occurs, doubling to 28 ± 8% for the northernmost part of the rift axis. The greater degree of melting in the north reflects the greater amount of water added to the mantle source, reflecting the arc-like nature of the source region and melt generation style characteristic of the initial stages of back-arc basin formation. Our data indicate that F = 0.44W + 0.07, where F is the degree of mantle melting and W is the percent water in the mantle. ‘True’ back-arc basin basalts are generated by adiabatic decompression associated with mantle upwelling in mature extensional settings. Eruption of ‘true’ back-arc basin basalts accompanies seafloor spreading, which begins when the basin is 100–150 km wide. The arc is disrupted during early rift formation, because arc magmatism is captured by the extension axis, but the generation of arc melts by hydrous melting of the mantle wedge continues whether or not back-arc extension is occurring. Back-arc basin seafloor spreading requires development of an upwelling mantle flow regime, allowing melting by adiabatic decompression, similar to that responsible for mid-ocean ridge basalt (MORB). The arc begins to re-form once extension progresses nearly to the point of seafloor spreading.

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Newman, Sally0000-0003-0710-995X
Additional Information:© Oxford University Press 1998. RECEIVED JANUARY 30, 1997; REVISED TYPESCRIPT ACCEPTED AUGUST 4, 1997. We thank the Captain and the crew of the R.V. Thomas Washington. This research was supported by NSF grants to R. J. S. and S. H. B. Special thanks are due to Drs Jongman Lee and Mohamed Gamal Abdelsalam for analytical assistance. We thank also James F. Allan and two anonymous referees for their helpful comments and criticisms. This is UTD Geosciences Contribution 869.
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Subject Keywords:arc magmas; back-arc basin basalt; isotope geochemistry; Mariana Islands; trace elements
Issue or Number:1
Record Number:CaltechAUTHORS:20181016-153903256
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Official Citation:Robert F. Gribble, Robert J. Stern, Sally Newman, Sherman H. Bloomer, Tim O'Hearn; Chemical and Isotopic Composition of Lavas from the Northern Mariana Trough: Implications for Magmagenesis in Back-arc Basins, Journal of Petrology, Volume 39, Issue 1, 1 January 1998, Pages 125–154,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:90297
Deposited By: George Porter
Deposited On:22 Oct 2018 16:05
Last Modified:16 Nov 2021 03:31

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