Sager, W.W., Sano, T., Geldmacher, J., and the Expedition 324 Scientists Proceedings of the Integrated Ocean Drilling Program, Volume 324 Massive basalt flows on the southern flank of Tamu Massif, Shatsky Rise: a reappraisal of ODP Site 1213 basement units1 A.A.P. Koppers,2 T. Sano, 3 J.H. Natland,3 M. Widdowson,3 R. Almeev,3 A.R. Greene,3 D.T. Murphy,3 A. Delacour,3 M. Miyoshi,3 K. Shimizu,3 S. Li,3 N. Hirano,3 J. Geldmacher,3 and the Expedition 324 Scientists3 Chapter contents Abstract Abstract . 1 Drilling during Ocean Drilling Program Leg 198 at Site 1213 re- covered three massive basalt units (8–15 m thick) from the south- Introduction . 1 ern flank of Tamu Massif at Shatsky Rise. Originally, these igneous Volcanology and igneous petrology of Site 1213. 2 units were interpreted to represent three diabase sills. During In- Interpretation and conclusions. 5 tegrated Ocean Drilling Program Expedition 324, this core was re- Acknowledgments. 6 described leading to the new conclusion that these diabase units References . 6 represent three submarine massive basalt flows. These massive Figures . 8 submarine flows were probably emplaced as inflated compound Table . 20 sheet flows during eruptions similar to those in large oceanic pla- teaus and continental flood basalts. Introduction The main objective of Integrated Ocean Drilling Program (IODP) Expedition 324 was to test competing mantle plume and plate tectonic models for ocean plateau formation at Shatsky Rise (Fig. F1). In these tests, determining the timing, duration, and source of volcanism at Shatsky Rise is of pivotal importance to under- stand the origin of this oceanic plateau. A short time span of vol- canism less than a few millions of years would imply a plume head–like eruption (e.g. Campbell, 2007; Courtillot et al., 2003), and a southwest to northeast age progression along Shatsky Rise would suggest a transition from plume head to plume tail. On the contrary, a longer duration of volcanism forming the Tamu, Ori, and Shirshov massifs at Shatsky Rise (Fig. F1) would indicate much lower rates of lava effusion that are more consistent with 1 Koppers, A.A.P., Sano, T., Natland, J.H., alternative ridge-controlled mechanisms and the effects of rapid Widdowson, M., Almeev, R., Greene, A.R., upwelling at a ridge-ridge-ridge triple junction. In the latter cases, Murphy, D.T., Delacour, A., Miyoshi, M., Shimizu, estimation of source temperatures and degree of partial melting K., Li, S., Hirano, N., Geldmacher, J., and the Expedition 324 Scientists, 2010. Massive basalt will prove important in showing the possibility of shallower man- flows on the southern flank of Tamu Massif, tle sources and testing associated extreme mantle fertility melting Shatsky Rise: a reappraisal of ODP Site 1213 models (e.g., Anderson, 1995; Foulger, 2007). basement units. In Sager, W.W., Sano, T., Geldmacher, J., and the Expedition 324 Scientists, As a large igneous province (LIP), Shatsky Rise currently is an Proc. IODP, 324: Tokyo (Integrated Ocean Drilling enigma in the Earth sciences because it has characteristics consis- Program Management International, Inc.). tent with both a deep mantle plume and that of a shallow mantle doi:10.2204/iodp.proc.324.109.2010 source controlled by plate tectonic processes. Its large volume and 2 College of Oceanic and Atmospheric Sciences, apparently rapid eruption of the entire volcanic pile are a possible Oregon State University, 104 COAS Administration Building, Corvallis OR 97331-5503, USA. consequence of a massive plume head impinging at the base of [email protected] the Pacific lithosphere at ~146 Ma (Sager and Han, 1993; Ma- 3 Expedition 324 Scientists’ addresses. honey et al., 2005). Yet, lava compositions with mid-ocean-ridge Proc. IODP | Volume 324 doi:10.2204/iodp.proc.324.109.2010 A.A.P. Koppers et al. Massive basalt flows on the southern flank of Tamu Massif basalt type signatures found in previously drilled In this report we briefly describe the major litholo- Shatsky Rise basalt units (Mahoney et al., 2005) may gies of the basement sequence and their petrogra- provide evidence pointing to a shallow mantle phy. On board, the core sections were digitally im- source. Importantly, Shatsky Rise represents only aged and measurements of point-source one of the very few places in the world oceans where susceptibility and optical reflectance were acquired we can distinguish between these two end member at 1 cm intervals. New lithologic units were assigned, models. Because M-series magnetic anomalies are and the basement rocks were described macroscopi- preserved in the Jurassic to Late Cretaceous ocean cally for volcanological structures, igneous petrol- crust surrounding Shatsky Rise, the buildup of this ogy, and alteration. Microscopic descriptions and large ocean plateau can be directly compared in digital photomicrographs were made from nine thin space and time with the formation of ancient seg- sections from Leg 198 available to the scientists on- ments of oceanic plate. Drilling of volcanic base- board. All of this new shipboard data was entered ment at five sites, from the oldest Tamu Massif (~146 into LIMS. To distinguish our new data and observa- Ma; Mahoney et al. 2005) to the younger Ori Massif tions from those obtained during Leg 198, Expedi- and Shirshov Massif (~140 Ma), provides us with crit- tion 324(198), Site U1213, and Hole U1213B are the ical samples that will help us distinguish between sample identifiers used in the databases according to the mantle plume and plate tectonic models. IODP policy. However, in this report the prefix “U” Ocean Drilling Program (ODP) Leg 198, which sailed will not be used. Detailed visual core description re- in 2001, was the first expedition to penetrate into ig- ports were automatically generated from the ship- neous basement of Shatsky Rise at Site 1213 on the board database entries (see “Site 1213 visual core southwest flank of Tamu Massif (Fig. F1). Drilling ex- descriptions” in “Core descriptions”). The major tracted 46.6 m of basaltic rock at a high recovery of lithologic features of each unit are summarized in 72.3% (Shipboard Scientific Party, 2002). This site is Figures F2, F3, and F4. important to the testing of the two models of forma- tion as it may contain older eruptive sequences than Physical volcanology lavas drilled during Expedition 324. Drilling at Site During Leg 198, the igneous basement unit (IV) of 1213 recovered three massive basalt units (8–15 m Hole 1213B was divided on the basis of lithologic thick) originally interpreted to represent three dia- patterns into three stratigraphic subunits (IVa–IVc) base sills (Shipboard Scientific Party, 2002). During separated by intercalated sediments and interpreted Expedition 324 and the first postcruise scientific as “diabase sills” (Shipboard Scientific Party, 2002). meeting in College Station, Texas (USA), the Expedi- In a similar fashion, we divided the Hole 1213B vol- tion Scientists, including igneous petrologists, volca- canic basement into nine lithologic units (1–9) based nologists, alteration specialists, and structural geolo- on the presence of sediments (including some dis- gists, redescribed all Site 1213 igneous basement placed dropstones fallen into the hole from the sedi- cores and processed all this information using the ment sections above) and variations in the basaltic new Laboratory Information Management System groundmass grain size (Table T1). We have combined (LIMS) database. The principal outcome of this reap- these lithologic units into three stratigraphic sub- praisal was the reinterpretation of these diabase units representing three individual cooling units as units as three submarine massive basalt flows, simi- thick as 15 m: Subunit IVa is composed of six litho- lar to those found in other LIPs and flood basalts. In logic units (1–6), whereas Subunits IVb and IVc cor- this study we provide macroscopic and petrographic respond to lithologic Units 7 and 9, respectively. descriptions, some limited shipboard measurements, Lithologic Unit 8 is a fragment of intercalated sedi- and a reappraisal of the basement units cored at Site ment between Units 7 and 9. All three cooling units 1213. are characterized by a finer grained top and base, in- dicating they are more likely massive submarine ba- saltic lava flows (see the “Methods” chapter) and Volcanology and igneous not sills. petrology of Site 1213 Hole 1213B is located ~200 km southwest of Site First massive basalt flow (stratigraphic Subunit U1347 on the eastern flank of Tamu Massif and 400 IVa; lithologic Units 1–6) km south of Site U1348 on the northeastern high Subunit IVa is a 14.3 m thick massive basalt flow. (Fig. F1). Volcanic rocks recovered at Site 1213 were This basement unit consists of typically nonvesicu- only sparsely sampled and studied (e.g., Mahoney et lar, aphyric to moderately phyric, cryptocrystalline al., 2005), yet this site obviously complements the to fine-grained basalts that are slightly to moderately drilling objectives of Expedition 324. altered. The top of the unit is bounded by overlying Proc. IODP | Volume 324 2 A.A.P. Koppers et al. Massive basalt flows on the southern flank of Tamu Massif sedimentary rocks with “baked” chert (indicated by eroporphyritic character to that of the overlying Sub- dark discoloration of the chert near contact with the units IVa and IVb. Both plagioclase and clinopyrox- igneous rocks) at interval 324(198)-1213B-28R-1, 17 ene phenocrysts are subhedral and as large as 10 cm. Although a glassy rim is not identified at the top mm. The top of this unit is defined by an overlying of the unit, a cryptocrystalline chill zone ~10 cm sedimentary rock unit (lithologic Unit 8) and is thick is present. The base of Unit IVa is defined by a marked by the presence of a chill zone ~20 cm thick.