Melt extraction and mantle source at a Southwest Indian Ridge Dragon Bone amagmatic segment on the Marion Rise

Abstract

This paper works on the trace and major element compositions of spatially associated basalts and peridotites from the Dragon Bone amagmatic ridge segment at the eastern flank of the Marion Platform on the ultraslow spreading Southwest Indian Ridge. The rare earth element compositions of basalts do not match the pre-alteration Dragon Bone peridotite compositions, but can be modelled by about 5 to 10% non-modal batch equilibrium melting from a DMM source. The Dragon Bone peridotites are clinopyroxene-poor harzburgite with average spinel Cr# ~27.7. The spinel Cr# indicates a moderate degree of melting. However, CaO and Al 2 O 3 of the peridotites are lower than other abyssal peridotites at the same Mg# and extent of melting. This requires a pyroxene-poor initial mantle source composition compared to either hypothetical primitive upper mantle or depleted MORB mantle sources. We suggest a hydrous melting of the initial Dragon Bone mantle source, as wet melting depletes pyroxene faster than dry. According to the rare earth element patterns, the Dragon Bone peridotites are divided into two groups. Heavy REE in Group 1 are extremely fractionated from middle REE, which can be modelled by ~7% fractional melting in the garnet stability field and another ~12.5 to 13.5% in the spinel stability field from depleted and primitive upper mantle sources, respectively. Heavy REE in Group 2 are slightly fractionated from middle REE, which can be modelled by ~15 to 20% fractional melting in the spinel stability field from a depleted mantle source. Both Groups show similar melting degree to other abyssal peridotites. If all the melt extraction occurred at the middle oceanic ridge where the peridotites were dredged, a normal ~6km thick oceanic crust is expected at the Dragon Bone segment. However, the Dragon Bone peridotites are exposed in an amagmatic ridge segment where only scattered pillow basalts lie on a partially serpentinized mantle pavement. Thus their depletion requires an earlier melting occurred at other place. Considering the hydrous melting of the initial Dragon Bone mantle source, we suggest the earlier melting event occurred in an arc terrain, prior to or during the closure of the Mozambique Ocean in the Neproterozoic, and the subsequent assembly of Gondwana. Then, the Al 2 O 3

Jurassic zircons from the Southwest Indian Ridge

“…The analogous geochronology and isotopic signatures between the ancient diorite and the Karoo magmatic rocks 27 29 , imply that the diorite may be correlated with the Jurassic magmatic rocks that emplaced prior to the breakup of southern Gondwana and the opening of the SW Indian Ocean. The rock may represent the relicts onto the margin of the southern Gondwana during the rifting of Africa and Antarctica, possibly a mixture of juvenile and recycled crust in a continental magmatic arc 30 .…”

“…There is substantial evidence, based on the geochemistry of the peridotites from the Dragon Bone spreading segment, immediately north of that of the dredged diorite, that the present mantle source of the SW Indian Ridge there represents such old arc-mantle wedge mantle drawn up between the plates along the ridge 30 . The Gallieni FZ nucleated at the margin of a bathymetric plateau lying to the north flanking the eastern side of the Madagascar Plateau.…”

Petrogenesis of ODP Hole 735B (Leg 176) Oceanic Plagiogranite: Partial Melting of Gabbros or Advanced Extent of Fractional Crystallization?

“…The partial melt composition (at degree of melting F = 1%, 5%, 10%) of modal equilibrium melting is calculated using experimentally determined mineral modes of Pl 0.69 + Cpx 0.18 + Opx 0.09 +Am 0.04 (Brophy, 2009;Koepke et al, 2004) from average Ocean Drilling Program Leg 176 gabbros (Niu & O'Hara, 2003; detailed calculation parameters are given in supporting information Table S6). Southwest Indian Ridge (SWIR) mid-ocean ridge basalt (MORB) data are from Gao et al (2016). The partition coefficients used to calculate melt compositions in equilibrium with amphibole (Amp) in the felsic veins and clinopyroxene (Cpx) in the gabbros are given in Table S6. 10.1029/2019GC008320 Geochemistry, Geophysics, Geosystems Toplis & Carroll, 1995;Regelous et al, 1999;Berndt et al, 2005;Koepke et al, 2018;[2] in Table 1).…”

He–Ar–S Isotopic Compositions of Polymetallic Sulphides from Hydrothermal Vent Fields along the Ultraslow-Spreading Southwest Indian Ridge and Their Geological Implications

“…The Southwest Indian Ridge (SWIR), a divergent tectonic boundary separating the African and Antarctic plates, is among the world's slowest spreading ridges with an average rate of only 14-15 mm/yr [53,54]. It extends~7700 km from the Bouvet to the Rodriguez Triple Junction (RTJ), and is characterised by asymmetric (oblique/orthogonal) spreading, numerous large non-transform offsets, intermittent volcanism, thin crust, and extensive outcrops of serpentinised peridotite or gabbro exposed by extensional detachment faulting [55][56][57][58]. Distinct variations have been observed in axial topography, segmentation patterns, crustal thickness, mantle composition, and melt supply along the strike, from west to east [24,[59][60][61][62][63][64][65][66].…”