Petrology and geochemistry of South Mid-Atlantic Ridge(19°S) lava flows:Implications for magmatic processes and possible plume-ridge interactions

The South Mid-Atlantic Ridge(SMAR)19°S segment,approximately located along the line of Saint Helena volcanic chain(created by Saint Helena mantle plume),is an ideal place to investigate the issue whether the ridgehotpot interaction process affected the whole MAR.In this study,we present major and trace elemental compositions and Sr-Nd-Pb isotopic ratios of twenty fresh lava samples concentrated in a relatively small area in the SMAR 19°S segment.Major oxides compositions show that all samples are tholeiite.Low contents of compatible trace elements(e.g.,Ni=239-594 ppm and Cr=456-1010 ppm)and low Fe/Mn(54-67)and Ce/Yb(0.65-1.5)ratios of these lavas show that their parental magmas are partially melted by a spinel lherzolite mantle source.Using software PRIMELT3,this study obtained mantle potential temperatures(Tp)beneath the segment of1321-1348℃,which is lower relative to those ridges influenced by mantle plumes.The asthenospheric mantle beneath the SMAR 19°S segment starts melting at a depth of^63 km and ceases melting at^43 km with a final melting temperature of^1265℃.The extent of partial melting is up to 16%-17.6%with an average adiabatic decompression value of 2.6%/kbar.The correlations of major oxides(CaO/Al2 O3)and trace elements(Cr,Co,V)with MgO and Zr show that the parental magma experienced olivine and plagioclase fractional crystallization during its ascent to the surface.87Sr/86Sr(0.702398-0.702996),143 Nd/144 Nd(0.513017-0.513177)and 206Pb/204Pb(18.444-19.477)ratios of these lavas indicate the mantle source beneath the SMAR 19°S segment is composed of a three-component mixture of depleted MORB mantle,PREMA mantle,and HIMU mantle materials.The simple,binary mixing results among components from plume-free SMAR MORB,Saint Helena plume and Tristan plume show that asthenospheric mantle beneath the SMAR 19°S segment may be polluted by both Saint Helena and Tristan plume enriched materials.The abovementioned mantle potential temperatures,together with the low Saint Helena(<10%)and Tristan(<5%)components remaining in the asthenospheric mantle at present,show that the physically ridge-hotspot interactions at SMAR 19°S segment may have ceased.However,the trace element and SrNd-Pb isotopically binary mixing calculation results imply that these lavas tapped some enriched pockets left when Saint Helena and/or Tristan plume were once on the SMAR during earlier Atlantic rifted history.

Oceanic Plateau Formation Implied by Ontong Java Plateau, Kerguelen Plateau and Shatsky Rise

Oceanic plateaus are a significant type of large igneous provinces in the oceans,providing insights to regional tectonic events and mantle behavior.The three world's largest oceanic plateaus,the Ontong Java Plateau,the Kerguelen Plateau and the Shatsky Rise,are representatives in displaying extraordinary fluxes of magma from mantle to lithosphere.Detailed description incorporating transdisciplinary observations on marine geology,geophysics and geochemistry allow us to test the two lively-debated oceanic plateau formation hypotheses(mantle plume and plate boundary models).Predictions from either hypothesis merely obtain partial support.It is therefore unclear to differentiate one model from another one regarding the oceanic plateau formation.Careful comparisons of the three oceanic plateaus show many commonalities and even more differences in their formation and evolution.This diversity signifies one may not be typical of all.Notably,several key common features,i.e.,massive and rapid eruption and near-ridge formation setting,imply that the lithospheric volcanic emplacement of oceanic plateaus was controlled by seafloor spreading despite a mantle plume exists peripherally.If a coincidence of mantle plume and spreading ridge occurs,it may indicate a plume-ridge interaction.One possible mechanism is that spreading ridge is dragged by a plume and migrates to the location of the plume.Another possibility is that the asthenosphere is fed by a plume nearby and generates melting anomalies along the spreading ridge.

Implications of the melting depth and temperature of the Atlantic mid-ocean ridge basalts

Mid-ocean ridge basalts(MORBs) are characterized by large variations in trace element compositions and isotopic ratios, which are difficult to be interpreted solely by using magmatic process such as partial melting of a peridotitic mantle and subsequently fractional crystallization. Geochemical diversity of MORBs have been attributed to large-scale heterogeneity within the underlying mantle, and the heterogeneity might have been caused by addition of recycled crustal component, subcontinental lithosphere, metasomatized lithosphere and outer core contribution. In this study, we investigated the MORBs along the Mid-Atlantic Ridge(MAR) by estimating the temperature and pressure of partial melting, and comprehensively comparing trace element and isotope ratios. The data for MORBs from areas close to mantle plumes show large variations. Mantle plumes can affect mid-oceanic ridges 1 400 km away, but plume effects did not cover all of the ridge segments, and those segments without plume effects did not have any abnormalities in temperature, trace element or isotope ratios.We ascribed the above phenomena to result from the shapes of the plume flow, which we categorized as "pipelike channels" and "pancake-like channels". The "pancake-like channels" plumes affected the ambient mantle nondirectionally, but the range of the mantle affected by the "pipe-like channels" plumes were selective. Element ratios of MORBs reveal that the mantle source of the MORBs along the MAR is highly heterogeneous. We suggest that most of source heterogeneities of the MORBs may be due to the presence of subducted slab and delaminated lower crust in the source. In addition, the plume that carried materials from the core-mantle boundary may affect some of the segments.