Heterogeneous Earth’s mantle drilled at an embryonic ocean

Abstract

Mantle processes control plate tectonics and exert an influence on biogeochemical cycles. However, the proportion of mantle sampled in-situ is minimal, as it is buried beneath igneous crust and sediments. Here we report the lithological characteristics of two mantle sections from an embryonic ocean drilled by the International Ocean Discovery Program (IODP) in the Tyrrhenian Sea. Contrary to the mantle drilled at Mid Ocean Ridges (MORs) and hyperextended passive margins, our findings reveal exceptionally heterogeneous and fertile mantle lithologies, ranging from fertile lherzolites to depleted harzburgites and dunites, interlayered with pyroxenites. Plagioclase- and clinopyroxene-rich layers, hydrous potassic magmatic veins, and mafic intrusions indicate substantial mantle refertilization and delayed inception of magmatic crust. We propose that magma-poor rifts do not require a chemically depleted mantle, too refractory to melt. Deep lithospheric processes such as mantle refertilization and prolonged lithospheric thinning delayed melt focusing and the formation of a steady-state spreading center.

Introduction

Since the discovery of large portions of the upper mantle exhumed at slow and ultraslow spreading MORs, scientific ocean exploration programs have aspired to collect in-situ sections of the underlying mantle . Currently, drilled sections of the oceanic mantle are limited to three locations along the central Mid-Atlantic Ridge - near the Kane Fracture Zone at 23°N (Ocean Drilling Program, ODP Leg 153), at the 15°20' N Fracture Zone (ODP Leg 209), and recently in the Atlantic Massif oceanic core complex at 30°N (IODP Expedition 399). The mid-oceanic mantle section in the Pacific Ocean has only been drilled in the Hess Deep, a tectonic window along the East Pacific Rise, during ODP Leg 147. The abyssal mantle peridotites recovered at these sites are extensively serpentinized and geochemically depleted due to partial melting of the ascending asthenosphere. Clinopyroxene-poor peridotites, such as harzburgites and dunites, are the most commonly drilled mantle lithologies at these sites and rarely consist of plagioclase-rich varieties, formed in response to grain-scale percolation and crystallization of MOR basaltic (MORB) magmas. In addition, a significant proportion of these depleted mantle sections consist of MORB-type magmatic intrusions of variable thickness.

In addition to MORs, mantle peridotites in the ocean have also been recovered in the distal portion of a passive continental margin, a geological setting that formed during the last phase of continent rifting, which is well distinct from that of a mature ocean ridge. In the late 20th century, the discovery of these exhumed mantle rocks by drilling during ODP Legs 103, 149, 173, and 210 at the conjugate magma-poor Iberia-Newfoundland rifted margins in the Atlantic Ocean, led to the formulation of a new model for the formation of Continent – Ocean Transition (COT). Prior to this, the COT was interpreted as a simple juxtaposition of stretched continental crust and new magmatic oceanic crust resulting from rapid lithospheric breakup and leading to a magmatically robust mid-ocean ridge system. Drilling offshore at the hyper-extended Atlantic-COT challenged this axiom, showing that continental breakup can be followed by exhumation of a chemically depleted sub-continental mantle with minimal or absent magma production, until the uprising asthenosphere produced enough melts and focused melt-extraction to generate a magmatic Penrose-type oceanic crust. Over the past decades, COT has been thereby classified into two end-member models of magma-rich and magma-poor types, characterized by contrasting structural architectures. Despite a global consensus that the magma-poor nature of hyper-extended COTs is caused by a combination of low extension rates and a depleted nature of the lower lithospheric mantle, the debate is still ongoing due to a lack of direct geologic sampling to test geophysical observations and geodynamic models of these contrasting end-members. The IODP Expeditions 367-368 drilled the intermediate South China Sea (SCS) continental margin and recovered unexpectedly thick MORB-type basaltic sections close to the thinned continental crust, revealing that abundant magmatism was produced during the breakup of a thinned continental lithosphere. The mechanical behavior of the lithosphere, which is also dependent on its inherited structures, exerts a key influence on the rifting evolution. Hence, understanding the COT formation requires access to deep lower crustal and mantle rocks to define the link between deep tectono-magmatic processes and the development of a rifted margin.
Exhumed mantle in the central Tyrrhenian Sea embryonic ocean

Continuing the legacy of in-situ mantle drilling in COT, the penultimate IODP Expedition (402) on the R/V JOIDES Resolution was conducted to drill into the oceanic basement of the Vavilov Basin, representing an embryonic oceanic basin located in the central part of the Tyrrhenian Sea . Different from the Iberia-Newfoundland rifted margins, the COT exposed in the area never developed a steady-state ocean ridge, and shares geological features with both magmatic and amagmatic COTs. The Tyrrhenian Sea was formed as a result of the northwestward subduction of the Ionian oceanic lithosphere beneath the Calabrian Arc and slab roll-back. This process stretched a thinned continental lithosphere, which may have included remnants of the Jurassic Alpine Tethys, to create an active back-arc basin. Lithospheric thinning began in the southern part of the Tyrrhenian during the Middle Miocene. With the rate of extension increasing progressively, lithospheric thinning moved southward until it breaks at latitude 41°N, leading to the opening of the Vavilov basin. Seismic data and previous drilling expeditions concur that the central portion of the Vavilov basin is formed by exhumed mantle peridotites covered by a thin layer of basalts and sediments. Although the basin opened at a full spreading rate ranging between 40 and 60 mm/yr, which is higher than ultraslow and slow-spreading ridges where mantle exhumation has been largely reported, the absence of a continuous magmatic crust suggests regional igneous starvation. The main objectives of IODP Expedition 402 were to ascertain whether the mantle in this nascent ocean differs from the depleted mantle observed at MORs and at Atlantic-type hyper-extended COTs and to constrain the effect of mantle composition in the transition from continental rifting to ocean formation.

Results

The post-Messinian (< 5 Ma) basement of the Vavilov basin was the focus of scientific drilling during the 1980s and 1990s when DSDP Site 373 and ODP Site 655 recovered exclusively < 150 m thick MORB-type tholeiitic basalts along linear ridges trending N10°E. Only ODP Site 651 (Leg 107) recovered a 40 m thick mantle section, below 136 m of basalts and basalt breccias, intercalated by a dolerite-albitic intrusion. The drilled peridotites were extensively serpentinized, and the original mantle fabric showed porphyroclastic textures with relicts of orthopyroxene and spinel in a serpentinized matrix. In contrast, IODP Expedition 402 recovered two continuous sections of moderately to highly serpentinized mantle peridotites (Sites U1614 and U1616) directly beneath a ~ 250 m thick sedimentary cover, with no intervening magmatic crust . The contact between brecciated serpentinized peridotites and dolomitic sediments is exceptionally preserved and lack a thick zone of cataclasites or tectonic gouges as observed at the top of exhumed mantle in other COT. Ophicarbonates and tectonic breccias were nonetheless recovered at different stratigraphic levels throughout both Expedition 402 sites . The 160 m thick mantle section sampled at Site U1614 is composed of approximately equal proportions of spinel-bearing lherzolite (24%) and harzburgite (26%), with minor dunite (10%), pyroxenites (4%) and mafic intrusions (3%) and Supplementary. Plagioclase-bearing lherzolites and harzburgites constitute the rest of the mantle section (27%). The 94 m thick mantle section cored at Site U1616 also recovered heterogeneous lithologies with spinel-bearing harzburgites and lherzolites making up to 50% and 19% of the recovered fraction, respectively . Mafic intrusions in both U1614 and U1616 mantle sections are relatively abundant and dispersed throughout the section . The different lithologies appear randomly distributed throughout the two sites. However, depleted harzburgites and dunites are prevalent at shallower depths at Site U1614 (< 320 mbsf). The results of Expedition 402 are in good agreement with the refractory nature of the peridotites previously sampled at ODP Site 651, which were restricted to a depth of 30 m below the sedimentary cover. The thicker mantle sections drilled at Sites U1614 and U1616 now suggests that refractory peridotites (26% at Site U1614 and 50% at Site U1616) are not prevailing lithologies in the Tyrrhenian COT mantle, which is exceptionally heterogeneous and on average mineralogically fertile. The relative proportions of fertile lithologies in the Tyrrhenian mantle are indeed exceptionally higher than mantle peridotites available from back-arc and fore-arc oceanic basins, with a lithological heterogeneity covering the entire spectrum of MOR mantle peridotites . Notably, the clinopyroxene modal proportions of Hole U1416C peridotites exceed by far that of Hole U1601C that drilled a 1268 m long section of depleted mantle at the Mid-Atlantic Ridge.


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