Mechanical Modeling of Dike Propagation: Methods, Recent Results and Links to Geophysics

Magma is transported in brittle rock by diking.Diking plays a key role in tectonic phenomena such as continental rifting and plate divergence at mid-ocean ridges.In spite of the importance of understanding how magma is

Origin of Peridotites and Chromitites in the Pozanti-kar Santi Ophiolite, Turkey

The Pozanti-Karsanti ophiolite(PKO)in Turkey’s eastern Tauride belt comprises mantle peridotites,ultramafic to mafic cumulates,isotropic gabbros,sheeted dikes and pillow lavas.The mantle peridotites are dominated by spinel harzburgites with minor dunites.The harzburgites and dunites have quite depleted mineral and whole-rock chemical composition,suggesting high degrees of partial melting.Their PGEs vary from Pd-depleted to distinct Pd-enriched patterns,implying the crystallization of interstitial sulphides from sulphur-saturated melts(e.g.MORB-like forearc basalt).U-shaped or spoon-shaped REE patterns indicate that the PKO peridotites may have also been metasomatized by the LREE-enriched fluids released from a subducting slab in a suprasubduction zone.Based on the mineral and whole-rock chemical compositions,the PKO peridotites show affinities to forearc peridotites.Chromitites occur both in the mantle peridotites and the mantle-crust transition zone horizon(MTZ).Chromitites from the two different horizons have different textures but similar mineral compositions,consistent with typical high-Cr chromitites.Chromitites hosted by mantle harzburgites generally have higher total platinum-group element(PGE)contents than those of the MTZ chromitites.However,both chromitites show similar chondritenormalized PGE patterns characterized by clear IPGEs,Rh-enrichments relative to Pt and Pd.Such PGE patterns indicate no or only minor crystallization of Pt-and Pd enriched sulphides during formation of chromitites from a sulphur-undersaturated melt(e.g.boninitic or island arc tholeiitic melt).Dunite enveloping chromitite lenses in the ho*s ting harzburgite resulted from melt-rock reaction.We have performed mineral separation work on samples of podiform chromitite hosted by harzburgites.So far,more than200 grains of microdiamond and more than 100 grains of moissanite(Si C)have been separated from podiform chromitites.These minerals have been identified by EDX and Laser Raman analyses.The diamonds and moissanite are accompanied by large amounts of rutile.Additionally,zircon,monazite and sulphides are also common phases within the heavy mineral separates.Both diamond and moissanite have been analyzed for carbon and nitrogen isotopic composition using the CARMECA 1280-HR large geometry Secondary Ion Mass Spectrometer at the Helmholtz Zentrum Potsdam.In total,61δ13CPDB results for diamond were acquired,exhibiting a range from-28.4‰to-18.8‰.31δ13CPDB results for Moissanite vary between-30.5‰to-27.2‰,with a mean value of-29.0‰.Diamond has relatively large variation in nitrogen isotopic composition with 40δ15NAIR results ranging from-19.1‰to 16.6‰.The discovery of diamond,moissanite and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new support for the genesis of ophiolitic peridotites and chromitites under high-pressure and ultra-high reducing conditions.Considering the unusual minerals,the high Mg#silicate inclusions,and the needle-shaped exsolutions in the PKO chromitites,the parental melts of these chromitites may have been mixed with deep asthenospheric basaltic melts that had assimilated materials of the descending slab when passing through the slab in a subduction zone environment.We suggest melt-rock reactions,magma mixing and assimilation may have triggered the oversaturation of chromites and the formation of PKO chromitites.

Secondary Ion Mass Spectrometry Analyses of Diamond and Moissanite in Ophiolite

The Cameca 1280-HR large geometry SIMS instrument is a highly versatile analytical tool which can support a broad range of geochemical applications.Research using the Potsdam 1280 instrument focuses primarily on isotope ratio determinations in geomaterials.Optimized measurement protocols have already been established forδ18O determinations in zircon,and we are also working towards routine oxygen isotope determinations for quartz,calcite,mica,apatite and titanite.The primary challenge in developing such measurement systems are the identification and characterization of suitable reference materials（RMs）,and this is made particularly challenging due to the matrix dependent ion yields of the SIMS ion source.Here we wish to report our progress towards establishing new analytical protocols for the determination ofδ13C in both diamond and moissanite.In the case of diamond,our facility possesses three natural RMs with which we are able to produce data with a typical analytical repeatability of;.15‰（1sd）.An inter-comparison of our three diamond RMs demonstrates an overall data quality of better than 0.5‰in terms of systematic offset between the various materials characterized using gas source mass spectrometry（Palot et al.,2012）.A single suchδ13C determination in diamond requires 80 s of data acquisition and involves a test portion mass of;00 pgof material.In-house diamond reference materials forδ15N calibration allow us to measure this isotopic system to a total analytical uncertainty of±1.6‰（1sd）at nitrogen concentrations reaching down to 250μg/g.Due to the relatively low abundance of nitrogen in diamonds,such isotope ratio determinations require around 9 minutes of data collection.With respect toδ13C determinations in moissanite,we use a kimberlitic Si C as calibrant（Mathez et al.,1995）,on which we achieve a repeatability of;.2‰（1sd）on a;50 pg test portion mass.Total data acquisition time for such measurements is 80 s.We are currently in the process of developing a second moissanite RM based on a synthetic,coarse-grained powder.We will also investigate this new material for itsδ30Si characteristics.

Mechanical Modeling of Dike Pathways in the Crust: Effect of Layering and Surface Loads

Magma is generated mostly in the Earth’s mantle by decompression melting and transported through the crust to reach the Earth’s surface.The main mechanism for magma transport is diking,but the pathways taken by