June 2007 LIP of the Month

Corresponds to event #21 in LIP record database.

The Manihiki Plateau: Implications for Early Cretaceous Large Igneous Province Formation in the Pacific

Stephanie Ingle
School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA
Email: ingle@hawaii.edu

John J. Mahoney
School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA

Hiroshi Sato
School of Business Administration, Senshu University, Kanagawa 214-8580, Japan

Millard F. Coffin
Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan

Jun-Ichi Kimura
Department of Geoscience, Shimane University, Matsue 690-8504, Japan

Naoto Hirano
Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan

Masao Nakanishi
Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan

Excerpted from: S Ingle, JJ Mahoney, H Sato, MF Coffin, J-I Kimura, N Hirano, & M Nakanishi, 2007: Depleted mantle wedge and sediment fingerprint in unusual basalts from the Manihiki Plateau, central Pacific Ocean. Geology 35(7) 595-598. Download pdf (597 Kb)

Introduction

Cretaceous oceanic plateau formation in the Pacific vastly exceeded that during Cenozoic time, perhaps reflecting a fundamentally different mode of mantle dynamics. Early Cretaceous (> ~100 Ma) Pacific plateaus include (see Figure 1 for locations):

• Hess Rise
• Magellan Rise
• Shatsky Rise
• Manihiki Plateau
• Ontong Java Plateau
• Hikurangi Plateau

Plateaus may form by impingement of a mantle plume head (Figure 2; e.g., Richards et al., 1989; Saunders et al., 1992). Several of the Pacific plateaus have been suggested to be products of a single “superplume” (e.g., Larson, 1991; Tatsumi et al., 1998), and recent papers have considered the possibility that three, Ontong Java, Manihiki, and Hikurangi, formed as a single enormous feature, despite their current separation by thousands of kilometers (Taylor et al., 2006; Worthington et al., 2006). If true, the area affected covered more than 1% of Earth’s surface.

Manihiki Plateau Overview

• Area of ~7.7 x 10⁵ km² (Coffin & Eldholm, 1994)
• Crustal thickness up to 25 km (Viso et al., 2005)
• Bisected by a series of deep troughs (Danger Islands Troughs; Winterer et al., 1974)
• Formed during Cretaceous Normal Superchron (125.0 – 83.5 Ma; Gradstein et al., 2004, time scale)
• A minimum age of Late Aptian based on biostratigraphy (Bukry, 1976)

Previous Basement (Igneous) Samplings (also see Figure 3)

• DSDP Site 317 on the ‘high plateau’ (24.5m of basement; see Figure 3; Jackson et al., 1976); tholeiitic basalts have some (isotopic) affinity to Ontong Java lavas (e.g., Mahoney & Spencer, 1991)
• Limited dredging in the Danger Islands Troughs; melt-depleted, high-Mg lavas (Clague et al., 1976) isotopically distinct from Ontong Java (Mahoney & Spencer, 1991) [herein referred to as SOTW samples]
• Limited dredging of seamounts in the eastern portion; evolved alkalic rocks (Beiersdorf et al., 1995)

Special Note: An intensive geophysical surveying / dredging expedition of the Manihiki Plateau is occurring at present (http://www.ifm-geomar.de/index.php?id=manihiki0&L=1).

Objectives of the Study

The relationship between Manihiki, Ontong Java, and Hikurangi Plateaus is not well understood. Ages are not well-known for Manihiki (or Hikurangi), and no modern-quality incompatible-element data were available for Manihiki rocks. To attempt to remedy this:

• Four dredge hauls carried out in the Danger Islands Troughs (August, 2003); three returned igneous rocks of tholeiitic or alkalic basaltic characteristics.
• Ar-Ar dating was attempted on 8 of these samples, with two successful attempts.
• New, high-quality incompatible trace element data was obtained for Site 317, pre-existing SOTW samples, and the newly acquired samples.
• Sr, Nd, and Pb isotopic compositions of new samples were acquired.

Results

• An Ar-Ar age of 117.9 ± 3.5 Ma (2σ) for a tholeiite from the Danger Islands Troughs (Figure 4)
• An Ar-Ar age of 99.5 ± 0.7 Ma (2σ)  for an alkalic lava from a seamount within the Troughs (Figure 4)
• Primitive-mantle normalized incompatible trace element patterns for Site 317 rocks are comparable to, but slightly lower than, those of the OJP Singgalo magma type rocks (Figure 5).
• Incompatible element patterns for the Danger Islands Troughs rocks are U-shaped and completely unlike those of Ontong Java or rocks from anywhere else we are aware of (Figure 5).
• Radiogenic isotopes from Site 317 are similar to Ontong Java’s Singgalo magma type, but those from the Troughs, although similar to Ontong Java’s Kwaimbaita magma type in Nd isotopes, are notably higher in ²⁰⁶Pb/²⁰⁴Pb isotopes (Figure 6).

Interpretation of Unusual Rocks from the Danger Islands Troughs

Incompatible element patterns of the Danger Islands Troughs basalts appear to be unique among oceanic plateau, island, ridge, and arc basalts, implying a source with an atypical history. Characteristics of the source (with possible explanations in parentheses) include (see Figure 7 for additional details):

1. Melting of a previously melt-depleted mantle source in the garnet-stability zone (e.g., mantle wedge beneath a subduction zone?)
2. Re-enrichment of this source by a contaminant elevated in the highly incompatible elements (Rb, Th, Nb, Ta) relative to the light rare earth elements (e.g., ocean island-derived volcaniclastic sediment with high Nb/La and Ta/La transported to the mantle wedge via the subducting slab?)
3. Shallow remelting of the mixed source to large extents (e.g., during initial decompression melting of a mantle plume head?)

Although the solution is non-unique (lower melting extents during step 1 may permit addition of less sediment in step 2 and less melting in step 3, or vice versa) the components and melting conditions used, provide the best fit to the available data.

Implications for the relationship between Manihiki, Hikurangi, and Ontong Java Plateaus

The Manihiki and Ontong Java Plateaus are currently separated by ~2500 km and the HP lies ~3000 km to the south of Manihiki. Nonetheless, a relationship between two or all of these plateaus has been proposed by several authors using various lines of evidence, including age, geophysical, and geochemical data. Our data add new evidence in favor of temporal and petrogenetic connections among the three plateaus; however, they also somewhat complicate this story.

The main volcanic events on Ontong Java, Manihiki, and Hikurangi Plateaus appear to have been contemporaneous at ~120 Ma, consistent with the proposed formation of the three plateaus as a single immense feature (e.g., Billen and Stock, 2000; Mahoney and Spencer, 1991; Mortimer and Parkinson, 1996; Hoernle et al., 2005; Taylor, 2006; Worthington et al., 2006). The overlap in incompatible element characteristics and isotopic compositions is significant, but far from complete. Danger Islands Troughs tholeiites have isotopic compositions that, when combined with their distinctive trace element characteristics, reveal a component in the Manihiki Plateau mantle source not yet sampled at the other plateaus. However, Manihiki’s lava sequences are deeply exposed along the Troughs’ walls: it may be that similar rocks are yet to be found deeper in the lava stacks at Ontong Java and Hikurangi.

References