Geochemical Constrains on MORB Composition and Magma Sources at East Pacific Rise Between 1°S and 2°S

The East Pacific Rise(EPR)is a typical fast spreading ridge.To gain a better understanding of the magmatism under ridges,Mid Ocean Ridge Basalts(MORBs)with remarkably heterogeneous compositions are obtained from(EPR)1?–2?S and multielement geochemical and radioisotope analyses are conducted.Results show that these MORBs have wide variation ranges in trace element concentrations and isotopic ratios.Sample 07 has low concentrations of incompatible elements,and very low ^(87)Sr/^(86)Sr,and high ^(143)Nd/^(144)Nd from 0.70213 to 0.702289 and 0.513234 to 0.513289,respectively.However,other samples show enrichment in incompatible elements to varying degrees,and medium values of ^(87)Sr/^(86)Sr and ^(143)Nd/^(144)Nd from 0.702440 to 0.702680 and 0.513086to 0.513200,respectively.This study proposes that one depleted source and two enriched sources contribute to the formation of MORBs from EPR 1?–2?S.Samples 02 and 10 are formed by mixing between one enriched source and one depleted source,while sample 07 is crystallized from the depleted source with no mixing process involved.However,the formation of samples 06 and 11are different,and thus further research is required to determine genesis.

Spreading-rate dependence of hydroacoustic and teleseismic seismicity of ridge-transform systems:East Pacific Rise,Galapagos Ridge,and Mid-Atlantic Ridge

Seismicity in ocean ridge-transform systems reveals fundamental processes of mid-ocean ridges,while comparisons of seismicity in different oceans remain rare due to a lack of detection of small events.From 1996 to 2003,the Pacific Marine Environmental Laboratory of the National Oceanic and Atmospheric Administration(NOAA/PMEL)deployed several hydrophones in the eastern Pacific Ocean and the northern Atlantic Ocean.These hydrophones recorded earthquakes with small magnitudes,providing us with opportunities to study the seismic characteristics of ridge-transform systems at different spreading rates and make further comparisons of their differences.This study comparatively analyzed hydroacoustic and teleseismic data recorded on the fast-spreading East Pacific Rise(EPR,10°S to 12°N),intermediate-spreading Galapagos Ridge(103°W to 80°W),and slow-spreading Mid-Atlantic Ridge(MAR,15°N to 37°N).We present a systematic study of the spatial and temporal distribution of events,aftershock seismicity,and possible triggering mechanisms of aftershock sequences.Our analysis yields the following conclusions.(1)From the hydroacoustic data,the EPR transform faults had the highest average seismicity rate among the three regions.(2)Along-ridge event distributions show that a high number of earthquakes were concentrated on the EPR,while they became dispersed on the GR and fewer and more scattered on the MAR,reflecting that the different tectonic origins were closely correlated with the spreading rate.(3)Analysis from mainshock-aftershock sequences shows no significant differences in the aftershock decay rate among the three regions.(4)Multiple types of aftershock triggering models were inferred from Coulomb stress changes:strike-slip mainshocks triggered strike-slip aftershocks and normal faulting aftershocks,and normal faulting mainshocks triggered normal faulting aftershocks.Although these results are case studies,they may be applicable to other ocean ridge-transform systems in future investigations.Our results provide important new insights into the seismicity of global ocean ridge-transform systems.

Magma Dynamics of Axial Melt Lens at Fast-Spreading Mid-Ocean Ridges

Multichannel seismic studies performed at fastspreading mid-ocean ridges revealed the presence of a thin(tens to hundreds of meters high), narrow(< 1-2 km wide) axial melt lens(AML) in the mid-crust, which is underlain by crystal/melt mush that is in turn laterally surrounded by a transition zone of mostly solidified material. In order to shed light on the complexity of magmatic and metamorphic processes ongoing within and at the roof of axial melt lenses, we have focused on the petrological and geochemical record provided by fossilized AMLs. Of particular significance is Hole 1256D in the equatorial Pacific drilled by the International Ocean Discovery Program(IODP), where for the first time, the transition between sheeted dikes and gabbros in intact fast-spreading crust was penetrated, providing a drill core with a more or less continuous record of the upper part of an AML(Teagle et al., 2006;Koepke et al., 2008). This can be regarded as rosetta stone to answer longstanding questions on the complex magmatic evolution within an AML, as well as on metamorphic and anatectic processes ongoing at the roof of a dynamic AML, rising upward in the midcrust as a consequence of a replenishment event. The plutonic rocks drilled from Hole 1256D consist of quartz-bearing gabbros, diorites and tonalites, which might represent the upper part of a fossilized AML. The gabbros and diorites are consistent with modeled products of MORB fractional crystallization, composed of mixed melt and cumulate in varying ratios. Modeled trace elements support a model in which the tonalites originated from low-degree partial melting of the sheeted dikes overlying the AML, rather than extreme fractional crystallization(Erdmann et al., 2015;Zhang et al., 2017a). Therefore, the upper part of AML, largely composed of low density and high-viscosity felsic magmas, may serve as a barrier to eruptible MORB melts in the lower part of AML. Zoning of apatites from three different lithologies, tonalites, diorites, and gabbros, is common and shows a consistent evolution trend with depletion in Cl and REEs from core to rim. The cores are usually homogenous in composition and interpreted as magmatic origin, whereas zones with lower Cl and REEs are disseminated with heterogeneous concentrations, indicating exchanges with hydrothermal fluids. The high-Cl apatite core indicates assimilation of high-Cl brines at a magmatic stage, which is interpreted as immiscibility product from cycling seawater-derived fluids at a high temperature(Zhang et al., 2017b). The variation of F/Cl and Br/Cl ratios of bull rocks may reflect the mixing between MORB magmas and seawater-derived fluids, crystallization of apatite and amphibole, and/or extraction of magmatic fluids(Zhang et al., 2017c).