Magma‐Mush Interactions in the Lower Oceanic Crust: Insights From Atlantis Bank Layered Series (Southwest Indian Ridge)

Abstract

The main igneous processes responsible for the oceanic crust accretion, which is responsible for the formation of ∼70% of the Earth's surface, have been primarily ascribed to equilibrium and fractional crystallization

Brown Amphibole as Tracer of Tectono-Magmatic Evolution of the Atlantis Bank Oceanic Core Complex (IODP Hole U1473A)

“…Associated clinopyroxene and plagioclase have compositions comparable with the data compilation of olivine gabbros from Hole U1473A from other studies (Sanfilippo et al, 2020;Zhang et al, 2020;Boulanger et al, 2021;Ferrando et al, 2021a), showing typical core-rim zonation. Clinopyroxene displays subparallel REE pattern (La N /Sm N = 0.08-0.21) compared to those of pargasites at lower concentration levels and exhibits a core-to-rim increase in Y-REE abundances, associated with steepening of Ti, Eu and Sr negative anomalies and flattening of the Zr-Hf anomaly (Fig.…”

“…In coarse-grained intervals of olivine gabbros, subophitic clinopyroxene commonly embay elongated and partially corroded plagioclase crystals, attesting that chemical disequilibrium triggered partial dissolution of plagioclase and crystallization of clinopyroxene . Mineral chemical compositions reveal that magma emplacement, melt extraction and accumulation processes are accompanied by melt reactive percolation (see also Zhang et al, 2020;Boulanger et al, 2021;Ferrando et al, 2021a). Clinopyroxene crystals, mostly coarseand medium-grained, display core-to-rim selective enrichments in highly incompatible trace elements relative to less incompatible trace elements (i.e., Ce to Y; Zr to Nd), similar to trace element zoning patterns documented in clinopyroxenes from gabbroic lithologies recovered in the adjacent Hole 735B (Gao et al, 2007;Lissenberg and MacLeod et al, 2016;Boulanger et al, 2020;Zhang et al, 2021).…”

“…The most widespread 'patchy' grain size variations have been interpreted as the result of compaction-aided melt extraction from the coarse-grained intervals and concomitant melt accumulation in discrete domains where finegrained gabbros rapidly formed (Ferrando et al, 2021a,b). At about 150 and 600 mbsf, the grain-size heterogeneities define decimeter-scale igneous layering that was interpreted as results of interfingered intrusive events of distinct magmas (Boulanger et al, 2021;Pieterek et al, 2022).…”

“…At ~57°E along the Southwest Indian Ridge (SWIR), a long-lived detachment fault has exhumed a >1500 m-thick section of lower oceanic crust, now forming the domal structure of the Atlantis Bank Oceanic Core Complex. The Atlantis Bank plutonic section formed through protracted accretion of intermittent intrusions (Dick et al, 2019a;Boulanger et al, 2020) and subsequent upward melt migration (Dick et al, 2000;Natland and Dick, 2001;Sanfilippo et al, 2020;Zhang et al, 2020;Boulanger et al, 2021;Ferrando et al, 2021a,b) in a complex dynamic environment during ongoing seafloor extension. Continuous exhumation and cooling of the lower crustal section led to progressive deformation mechanisms under decreasing temperature conditions (Miranda & John, 2010;Gardner et al, 2020;Taufner et al, 2021).…”

Plastic Deformation of Plagioclase in Oceanic Gabbro Accreted at a Slow‐Spreading Ridge (Hole U1473A, Atlantis Bank, Southwest Indian Ridge)

“…Fine-grained granular domains (CPF index 0, Figure 7a) display CPO with a [100](010) fabric, interpreted as resulting from magmatic flow (e.g., Benn & Allard, 1989;Ji et al, 2014;Satsukawa et al, 2013). Although coarse-grained subophitic domains shown herein display no clear CPO (CPF index 0; e.g., Figures 6a and 7a), it is documented in coarse-grained olivine gabbros from Hole U1473A by Boulanger et al (2021). The onset of plastic deformation in sampled domains (CPF indices 1 to 5) also results in the development of a [100](010) CPO (Figures 6b-6e and 7b-7f) with a common "host-controlled" relation between porphyroclasts and recrystallized grains (Figures 6b and 6c).…”