Comparative Study of Magmatism in East Pacific Rise Versus Nearby Seamounts:Constraints on Magma Supply and Thermal Structure Beneath Mid-ocean Ridge

Major elements of 2202 basalts from the East Pacific Rise （EPR） and 888 basalts from near- EPR seamounts are used to investigate their differences in magma crystallization pressures and mantle melting conditions. Crystallization pressure calculation from basalts with 5.0wt%〈MgO〈8.0wt % shows that magma crystallization pressures beneath near-EPR seamounts are positively and negatively correlated with Nas and Fes, respectively. However, these correlations are indistinct in axial lavas, which can be explained by chemical homogenization induced by extensive mixing processes. In each segment divided by major transforms and over-lapping spreading centers （OSCs）, near-EPR seamount lavas have higher magma crystallization pressures, higher Fes and lower Nas than the EPR lavas, which indicate cooler lithosphere, lower degrees and shallower melting depths beneath near-EPR seamounts than the EPR. The correlations between magma crystallization pressures and melting conditions beneath near-EPR seamounts imply that the source thermal state controls the melting degree and melt flux, and then melting process controls the shallow lithosphere temperature and magma crystallization depth （pressure）. The cooler mantle sources beneath near-EPR seamounts produce a lower degree of melting and a less robust magma supply, which results in a deep thermal equilibrium level and high magma crystallization pressure. The magma crystallization pressure decreases significantly as spreading rate of the EPR increases from ~80 mm/year in the north （16~N） to ~160 mm/year in the south （19~S）, while this trend is unobvious in near-EPR seamounts. This suggests that the magma supply controlled by spreading rate dominates the ridge crust temperature and magma crystallization depth, while the near-EPR seamount magma supply is not dominated by the axial spreading rate. Because most seamounts form and gain most of their volume within a narrow zone of 5-15 km from ridge axis, they provide good constraint on magma supply and thermal structure beneath the EPR. High magma crystallization pressures in seamounts indicate dramatic temperature decrease from the EPR. The crystallization pressures of seamount lavas are well correlated with mantle melting parameters but in a blurry relationship with axial spreading rate. Despite the adjacency of the EPR and nearby seamounts, the thermal structure beneath the near-EPR seamounts are controlled by their own magma supply and conductive cooling, chemically and thermally unaffected by magmatism beneath the ridge axis.

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.

Characteristics of silicon and oxygen isotopic compositions of basalts near East Pacific Rise 13°N

In this study, 13 groups of silicon and oxygen isotopes and major elements of the basalts near the East Pacific Rise 13°N are used to study the fractionation of silicon and oxygen isotopes. Among these data, δ30Si values of basalts vary from -0.4%o to 0.2%o with a mean value of δ30Si of （-0.18±0.22）%o. The δ180 values range from 4.1%o to 6.4%o with a mean δ180 value of （＋5.35±0.73） %0. Since the δ30Si values increase in the series of basalt-basaltic andesite- andesite, and δ180 values display a positive correlation with the SiO2 content, we propose that the fractionation of silicon and oxygen isotopes is influenced by the SiO2 content in igneous rocks. Compared with the igneous rocks from Manus Basin with clinopyroxene as their dominant mineral phase, MORBs in this study containing olivine and plagioclase as primary minerals have lower δ180 and δ30Si values, indicating that the fractionation of silicon and oxygen isotopes is also affected by different Si-O bridges in silicate minerals. Furthermore, our samples from the EPR are defined as E-MORB based on K/Ti ratios. Probably, the difference in δ30Si and δ30O between our samples and a normal MORB are cause by the enriched components in E-MORBs.

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.

Genesis of anhydrite in hydrothermally altered basalt from the East Pacific Rise near 13°N

This study reports the occurrence of anhydrite in hydrothermally altered pillow basalt （12°50.55＇N, 103°57.62＇W, water depth 2 480 m）, which may have been produced in the basalt during seawater-basalt interaction in the laboratory. The existence of anhydrite in the altered basalt indicates extensive high- temperature hydrothermal alteration at the surface of seafloor pillow basalt. Microprobe analysis shows significant chemical zoning in the hydrothermally altered pillow basalt, in which Ca, Si and A1 contents de- crease and P, Fe, Mn, Cr and S contents increase from fresh basalt to altered basalt. The negative correlation between Rb-Sr and Li-Sr, and negative correlation between Li-Ca and Rb-Ca in the high-temperature vent fuids show that these fluids underwent anhydrite precipitation before fluid jetting due to mixing with sea- water in the sub-seafloor. Based on these observations, we show that not all Ca in the anhydrite comes from basalt in the reaction zone, and that the basalts on the seafloor or in the upflow zone may also provide Ca for anhydrite.

Sr Isotopes and REEs Geochemistry of Anhydrites from L Vent Black Smoker Chimney, East Pacific Rise 9oN–10oN

High resolution sampling, for Sr isotope and REE analyses, was carried out along a transaction of L vent chimney collected from East Pacific Rise 9oN–10oN. Sr isotopes show these anhydrites are precipitated from a mixture between hydrothermal fluid and seawater. The calculated relative proportion of seawater and hydrothermal fluid shows that the mixing is heterogeneous on the transection of the L vent chimney. Anhydrites from the chimney show uniform chondrite-normalized REE pattern with enrichment of LREE and positive Eu anomaly. While normalized to the REE of end-member hydrothermal fluid, anhydrites also show uniform REE pattern but with negative Eu anomaly and enrichment of HREE. Combining previous studies on REEs of hydrothermal fluids from different hydrothermal systems and the hydrothermal fluid data from this region, we suggested that REE-anion complexing, rather than crystallography controlling, is the main factor that controls the REE partition behavior in the anhydrite during its precipitation from the mixture of hydrothermal fluid and seawater.

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).