Composition and origin of a K2O-rich granite melt in the Mid-Atlantic Ridge, 13°34′ N: Evidence from the analysis of melt inclusions and minerals of the gabbro-plagiogranite association

Abstract

Participation of seawater in large scale convective circulation in the global system of mid ocean ridges is an important factor of heat and mass transfer resulting in cooling and hydrothermal alteration of the newly formed basic crust and precipitation of sulfide-poly metallic ores on the oceanic floor [1,2]. The deep zones of hydrothermal systems and the areas of hydro thermal-magmatic interaction are the least studied. It is suggested that the local partial melting of the basic crust under the influence of hydrothermal seawater derived fluids and the appearance of oceanic plagiog ranite (OPG), i.e., plutonic quartz-feldspar rocks with occasional pyroxene and hornblende, may be one of the extreme manifestations of such interaction [3,4]. However, there are other models of OPG origin: extreme differentiation of basaltic magma, phase sep aration of differentiated basaltic magma, magmatic assimilation, and partial melting of the hydrated crust [3]. Although the phenomenon of acid magmatism in the modern oceanic crust and paleoanalogs is of spe cial interest, the likely combinations of various petro genetic factors in the generation of OPG melts are poorly studied. In particular, the role of the basic crust composition is not clear.In this paper, we report the results of the study of an occasional biotite bearing gabbro-plagiogranite assemblage discovered in the Mid Atlantic Ridge (MAR), where the formation of the oceanic crust is significantly controlled by mantle magmas of the E MORB type enriched in incompatible elements. The composition of the granitoid melt was estimated by the study of melt inclusions in zircon, the bulk chemistry of OPG veinlets, and clinopyroxene/melt REE parti tioning.The gabbro-peridotite (strongly serpentinized) massif exposed in the footwall of the large offset low angle detachment fault in the range of 13°28′-13°35′ N on the western slope of the MAR was studied and sampled in three cruises of R/V Professor Logachev in 2007-2011 (Fig. 1). It is established that the structure of the massif is complicated by volcanic formations and a series of hydrothermal sulfide fields. In addition, samples of OPG veinlets in amphibolite and gabbro, as well as massive OPG rocks, were dredged at 8 sites ( Fig. 1) [5]. The analysis of fresh lavas with quench glasses from 21 sampling sites showed that within the massif the composition of magmas of volcanic struc tures varied from normal (Nb N /Zr N = 1.2; H 2 O (8) = 0.15 wt %; K 2 O = 0.04 wt %; 87 Sr/ 86 Sr = 0.7024) to sig nificantly enriched (Nb N /Zr N = 3.5; H 2 O (8) = 0.45 wt %; K 2 O = 0.7 wt %; 87 Sr/ 86 Sr = 0.7029) oceanic basalt.Biotite bearing OPGs were found at Sites 97 and 101, on the northern margin of the massif, at a dis tance of ~3 km from each other (Fig. 1). They occur as fine grained biotite-quartz-plagioclase veinlets in coarse graned gabbro. Gabbroids and hydrothermally altered mafic rocks prevailed among the dredged sam ples. Mafic rocks contain quartz-sulfide mineraliza tion. There are samples of massive biotite free OPG as well.S...

Oriented Magnetite Inclusions in Plagioclase: Implications for the Anisotropy of Magnetic Remanence

“…The felsic stringers are mostly <2 mm wide with some parts broadened up to 5 mm. They are fine-grained with variable mineral modes from plagioclase + quartz + biotite to local clinopyroxene-rich and zircon-rich plagiogranite micro-lithologies (Aranovich et al, 2015;Pertsev et al, 2021). Samples L-2612-41 and L-2612-49 are coarse-grained oxide gabbros consisting of plagioclase, clinopyroxene, and minor orthopyroxene.…”

Crystallographic and shape orientations of magnetite micro-inclusions in plagioclase

“…The Fe-Ti oxides are present as large interstitial grains of magnetite and as micron-and sub-micron-sized inclusions in plagioclase and pyroxene. Detailed descriptions of the mineralogy and petrography of the gabbros with complex petrogenetic history (Samples L32-101-1, L2612-41, L2612-47) can be found in Pertsev et al (2015), Aranovich et al (2015) and Ageeva et al (2017).…”

Halogens in Seawater, Marine Sediments and the Altered Oceanic Lithosphere

“…For example, Nehlig which is stronger than known from vent fluids (Fig 7), have been reported (Vanko et al, 1992) and brine inclusions with salinities of up to 50 wt. % salts appear to be a common minor component at all levels of the oceanic crust (Fig 13;Juteau et al, 2000;Kelley and Delaney, 1987;Lécuyer et al, 1999;Vanko, 1988;Aranovich et al, 2015).…”

“…The ultimate fate of brines within this model is unknown but brines might eventually be flushed out of the crust by advection of more typical low salinity fluids in the waning stages of hydrothermalism or they might be transported laterally and leak out of the ridge flanks away from the vents (Bischoff and Rosenbauer, 1989). In addition, a portion of these brines could be assimilated by magmas driving the hydrothermal system (Kendrick et al, 2013a;2015) or involved in flux melting and the genesis of plagiogranites (Aranovich et al, 2010(Aranovich et al, , 2015.…”

“…The brine assimilation data are significant because: i) some of the glasses affected have high concentrations of H2O and CO2, which demonstrate that assimilation must have occurred at depth in crustal magma chambers rather than on the seafloor (Coombs et al, 2004;Kendrick et al, 2013a;le Roux et al, 2006), and ii) the data provide evidence that seawater-derived brines, with Br/Cl and I/Cl distinct from the mantle, not only penetrate the lower crust but come into direct contact with, and are assimilated by crustal magmas (Fig 16). Mass balance calculations suggest the affected magmas from the NW part of the Lau Basin, the Galapagos, and Samoa assimilated up to 0.5 % of their total mass in brine which introduced 0-70 % of their total Cl and 0-30 % of their total H2O (Kendrick et al, 2013a;2015). Therefore, assimilation of brines generated in hydrothermal systems can alter the concentrations of halogens and relative abundance ratios (F/Cl, Br/Cl and I/Cl) of newly forming crust (e.g.…”