petrology and rock magnetism of the gabbro of troodos ophiolite abstract in order to determine the nature of remanence carriers of the layered gabbro of the troodos ophiolite cyprus we report optical and electron microscopic observations together with rock magnetic and paleomagnetic experiments above all the study aims to understand and clarify the time of magnetic acquisition relative to the brittle deformation of the oceanic crust manifested by the ridge transform intersection solea graben and the arakapas transform petrographic examination of pyroxene grains revealed isolated magnetite inclusions ranging in size from single domain sd to multi domain md and in addition md pyrrhotite inclusions residing in veins and cracks thermal demagnetization and thermomagnetic procedures indicate two components low and high temperature we argue that the low temperature component the sd magnetite inclusions exsolved in pyroxene are the dominant carriers of magnetic remanence in troodos gabbro the initial formation of these inclusions occurred via exsolution reaction at temperatures between 520 and 850 c above the curie temperature of pure magnetite during the solidification of the magma therefore acquisition of remanent magnetization of the troodos gabbro took place during the earliest stages of crustal accretion before any brittle deformation associated with the spreading ridge and the transform fault occurred

preferred orientation of ferromagnetic phases in rock forming minerals insights from magnetic anisotropy of single crystals in the early days of paleomagnetism david strangway was interested in understanding why igneous rocks are faithful recorders of the earth s magnetic field he recognized that ferromagnetic s l

salvaging primary remanence from hydrothermally altered oceanic gabbros in the oman ophiolite a selective destructive demagnetization approach abstract widespread hydrothermal alteration and formation of secondary magnetite have been problems for paleomagnetic work on gabbros in the oman ophiolite mechanical removal of hydrothermally altered ferromagnesian minerals from gabbro and gabbronorite in the wadi rajmi area revealed a cryptic remanence which could not be detected by stepwise demagnetization of bulk rock core samples after the mechanical removal samples consist of plagioclase and clinopyroxene these samples exhibit remanence directions of southeast declination and shallow inclination this direction is consistent with previously reported paleomagnetic directions at crystallization of the oman ophiolite in contrast bulk rock core samples yielded north declination resembling the younger remanence directions associated with the obduction of the ophiolite microscopic observation and paleomagnetic directional comparison concluded that the cryptic remanence is a primary magnetization carried by exsolved magnetite in plagioclase and clinopyroxene our results suggest that previous paleomagnetic data from whole rock gabbros in the oman ophiolite as well as tectonically active ocean floor should be taken with care

interstellar hydroxyl in meteoritic chondrules implications for the origin of water in the inner solar system abstract ion probe determinations of water concentrations and d h ratios have been performed on different phases pyroxene olivine and mesostasis of chondrules from the bishunpur and semarkona ll3 chondrites mean water concentrations average 1290 ppm in olivine 500 to 2100 ppm 2400 ppm in pyroxene 400 to 9800 ppm and up to 3950 ppm in the mesostasis 1000 to 16500 ppm in one chondrule an iron oxide phase exhibits a mean water concentration of 1 70 wt 1 21 to 2 15 wt hydrogen isotopic measurements reveal the presence of two water sources with high and low d h ratios pyroxenes range from 74 to 479 10 6 with a mean value of 186 10 6 n 54 the mesostasis from 124 to 203 10 6 with a mean value of 152 10 6 n 8 olivines from 63 to 209 10 6 with a mean value of 130 10 6 n 13 the iron oxide phase from 157 to 360 10 6 with a mean value of 219 10 6 n 7 hydroxyls in pyroxenes have been identified as submicroscopic poorly crystallized amphibole lamellae through high resolution transmission electron microscopy considering the broad range of d h ratios in pyroxene chondrules alteration in a unique meteoritic parent body seems to be excluded an early hydroxylation of the pyroxene occurring during chondrule formation due to the presence of water bearing minerals among chondrule precursors seems the only possible interpretation of the data accordingly the distribution of d h ratios in chondrules reflects that of their precursors such a distribution is quite similar to that found in matrix minerals as recorded by water concentration and d h ratio the mesostasis has been subjected to hydrothermal alteration after the incorporation of the chondrules into the meteoritic parent body this alteration took place with water having a mean d h ratio reflecting the mixing of the d depleted and d rich sources the d rich component is interpreted as a preserved interstellar source while the low d h ratio reflects a water component that underwent isotopic exchange with protosolar molecular hydrogen

hardened subtidal stromatolites bahamas hardened high relief stromatolites have been discovered along the margins of some bahamian platforms they occur in high energy tidal oolitic sand environments in waters ranging in depth from about 1 to 5 meters physical stress produced by actively migrating bed forms of oolitic sand appears to exclude grazing gastropods and subsequent community successions permitting stromatolite growth

depositional bias and environmental change important factors in sequence stratigraphy abstract differences among depositional systems here called depositional bias strongly influence sequence patterns siliciclastics and shallow water carbonates for instance shed most of their sediment during opposite phases of a sea level cycle lowstand shedding and highstand shedding respectively furthermore the two systems generate their own system specific relief on the sea floor disperse their sediment load along different avenues and differ in the way they are deactivated reefs and carbonate platforms can be drowned whereas siliciclastic deposition can be shut off and renewed at any depth as a consequence of these differences pronounced unconformities drowning unconformities develop where carbonate platforms are terminated and buried by siliciclastics the siliciclastic to carbonate transition tends to be more gradual drowned platforms and drowning unconformities appeared world wide in great abundance in the miocene cretaceous valanginian turonian jurassic toarcian and devonian frasnian famennian examples of drowning unconformities interpreted as sequence boundaries include those of the early cretaceous platforms off new jersey and off morocco the mid cretaceous unconformity in the gulf of mexico and miocene unconformities on top of reefs in the far east the eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms either the rates of rise are an order of magnitude lower than the growth potential of platforms or cycle amplitudes are too small or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone the critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems thus drowning unconformities demonstrate the importance of environmental change as a control on sequence development other examples of dominantly environment controlled sequences include slope deposits shaped by shifting currents e g florida and blake plateau and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland e g gulf of mexico environmental change must be considered as a third independent factor that competes with eustatic and tectonically driven regional changes of sea level for control of sequences the definition of sequences and sequence boundaries should be broad enough to include the possibility of non sea level controls

the 3100 bp eruption of the soufriere of guadeloupe a transmission electron microscopy study of the cryptodome andesite fragments of unaltered andesite found at all levels in the deposits of the catastrophic flank failure directed blast eruption of the soufriere of guadeloupe in 3100 bp are thought to be remnants of the cryptodome they were observed in analytical transmission electron microscopy for clues to the evolution of the intrusion prior to the eruption several features that could potentially be used as temperature markers were identified among which the angle between microexsolutions of magnetite in augite phenocrysts was used to find an upper boundary of the temperature of the intrusion before the eruption 600 700 c calculation of the time a dyke or sphere shaped intrusion may have taken to cool from the emplacement temperature down to the temperature of exsolution of the magnetite leads to an estimate of the time between emplacement of the intrusion and the eruption which could not have been less than a few tens of years it therefore seems probable that the emplacement of the magmatic intrusion was not the immediate cause of the flank destabilization and catastrophic eruption of the soufriere in 3100 bp

carbonates and relative changes in sea level abstract in the geologic record some of the most accurate gauges of changes in sea level are the sediment type geometry and diagenesis of carbonate shelves and platforms this is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics world wide changes in relative sea level the sum of eustatic sea level changes sedimentation and crustal movements have occurred repeatedly and cyclicly through geologic time producing characteristic responses in carbonates 1 1 relative rises in sea level usually caused by the cumulative effect of tectonic subsidence and eustatic rise may result in the following 1 1 a drowned carbonate reefs or platforms here carbonate growth potential is exceeded by relative sea level rise and is characterized by shallow water sediments overlain by hardgrounds and or deep water sediments some of which may be condensed sequences 1 2 b platforms where only the fast growing rim and patches of the interior are able to match sea level rise while the remainder of the platform is drowned temporarily 1 3 c platforms which keep up and maintain a flat top at sea level and contain shallow water sediments whose thickness at the least matches the height of the sea level rise if terrigenous supply is limited prograding sheets of shelf carbonate occur frequently capped by supratidal evaporites with prograding shelf margin carbonate clinoforms and turbidites if terrigenous supply is high the shelf carbonates encroach on deltaics 2 2 relative drops in sea level caused by crustal uplift or by subsidence being outpaced by a eustatic drop in sea level cause karst and soil development over shelves and platforms deposition of deep water evaporites in adjacent semi enclosed basins and in open marine basins the deposition of deltaic and aeolian clastics that bypassed the shelf falls are accompanied by platform wide fresh water diagenesis during relative sea level rises marine diagenesis is common in the subtidal portions of the shoaling upward carbonates and fresh water diagenesis and dolomitization and sulphate deposition is common in their intertidal and supratidal portions the stratigraphic significance of these reponses to relative sea level change is that many are tied to eustatic events and so are predictable within a basin of deposition

crystallographic and shape orientations of magnetite micro inclusions in plagioclase plagioclase hosted oriented magnetite micro inclusions are a frequently observed phenomenon in magmatic and metamorphic rocks understanding the orientation relationships between these inclusions and the plagioclase host is highly relevant for interpreting paleomagnetic measurements the systematics of the shape and crystallographic orientation relationships between needle and lath shaped magnetite micro inclusions and their plagioclase host from oceanic gabbro were investigated using optical microscopy including universal stage measurements scanning electron microscopy and crystal orientation analysis by electron backscatter diffraction the magnetite inclusions show preferred shape orientations following six well defined directions and with specific crystallographic orientation relationships to the plagioclase host these relationships are rationalized based on angular and dimensional similarities between the crystal structures of magnetite and plagioclase which favor the parallel alignment of oxygen layers with similar lattice spacing in both phases the parallel alignment of oxygen layers in plagioclase and magnetite can be traced back to the oriented nucleation of magnetite which occurs by the accommodation of feo6 octahedra in six membered rings of sio4 and alo4 tetrahedra of the plagioclase structure the orientation systematics of the magnetite micro inclusions is related to four orientation variants for placing the feo6 octahedra into the plagioclase structure

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

rapidity of freshwater calcite cementation implications for carbonate diagenesis and sequence stratigraphy abstract in less than 10 years subaerially exposed holocene oolitic sand on eleuthera island bahamas was completely case hardened by freshwater vadose calcite cementation this phenomenon demonstrates the rapidity of freshwater carbonate diagenesis and explains in part why carbonate platforms preferentially shed unconsolidated sediment during highstands and not during lowstands when they are subaerially exposed it further proves contrary to what the exxon conceptual sequence stratigraphic models imply that carbonate deposits do in fact respond differently to subaerial exposure than siliciclastic deposits case hardening of unconsolidated carbonate sediment in response to subaerial exposure and early near surface freshwater diagenesis helps explain why shallow water platform sediments are preferentially shed into adjacent basins during highstands of sea level and not during lowstands during lowstands rapid subaerial lithification of carbonate sediments retards their reworking by wind or water once lithified these deposits are further stabilized by vegetation and karstification inhibiting their transport as unconsolidated sediment into adjacent deeper water basins observations from eleuthera island also document the reciprocal processes associated with freshwater carbonate diagenesis calcium carbonate released by partial dissolution of aragonitic ooids was locally reprecipitated as pore filling low magnesian calcite cement this reciprocity is confirmed by the confinement of freshwater dissolution and cementation to the upper 10 20 cm of the subaerially exposed body of oolitic sand below this case hardened surface oolitic sands remained unconsolidated only very minor amounts of calcite cement were required for effective casae hardening

oriented inclusions of magnetite in clinopyroxene source of stable remanent magnetization in gabbros of the messum complex namibia 1 crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the early cretaceous messum complex namibia rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single domain magnetite consistent with the observed sizes and shape anisotropy of the magnetite inclusions as in previous studies of similar inclusions these are inferred to have formed by exsolution two arrays of inclusions are regularly present in the messum clinopyroxenes inclined at about 74 consistent with formation at about 800 c deduced from optimization of phase boundary orientations virtual geomagnetic poles from these rocks are consistent with reference data confirming that the magnetization is of thermoremanent origin bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal

linear and nonlinear thellier paleointensity behavior of natural minerals 1 we tested single domain sd and multidomain md theories of the acquisition and thermal demagnetization of thermoremanent magnetization trm and partial trm ptrm in thellier paleointensity determination using as samples plagioclase and mafic minerals extracted from matachewan diabase dikes susceptibility hysteresis and coercivity data indicate that the plagioclase contains mainly sd grains of magnetite while the mafic minerals clinopyroxene biotite and actinolite contain mainly md magnetite the extracted minerals were used in simulated thellier experiments with laboratory trm as a proxy for natural remanent magnetization nrm the plagioclase behaved ideally as predicted by neel sd theory giving a linear nrm ptrm arai plot with positive ptrm checks the mafic minerals behaved nonideally with strongly curved arai plots sagging below the ideal sd line in accord with observations for md magnetites and with predictions that in heating to moderate temperatures t more trm is lost than can be regained as ptrm causing convex down arai plots in experiments with ptrm parallel to nrm all ptrm tail checks were zero but ptrm checks increasingly deviated from the arai curve toward the sd line as t increased however in repeat thellier experiments ptrms at all t agreed with the original ptrms the negative ptrm checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original ptrms the inability of ptrm checks to reliably distinguish between md behavior and irreversible alteration could lead to overestimates of paleointensity from straight line fits to the low t unaltered part of the arai curve or underestimates from fits to high t data alternating field cleaning to 15 mt before each thellier step made the arai plot only marginally less curved low temperature demagnetization ltd zero field cycling through the 120 k verwey transition hardly changed the curvature of the arai plot for ptrm parallel to nrm for ptrm perpendicular to nrm ltd led to an inflected arai curve straddling the ideal sd line data of either form would not give reliable paleointensity values no presently available pretreatment renders the paleointensity data of md grains usable sd and nearly sd grains which behave ideally in the thellier experiment remain the only useful recorders of absolute paleointensity

systems tracts and depositional sequences in a carbonate setting a study of continuous outcrops from platform to basin at the scale of seismic lines abstract the application of sequence stratigraphy concepts to carbonates is feasible and this paper presents an example of its successful application the continuity and lateral extent of outcropping sedimentary sections observed at the scale of a seismic line from platform to basin in the southern vercors plateau western alps france confirm the validity of sequence stratigraphy concepts when applied to carbonate depositional systems from the stratal patterns displayed in outcrop we determined that carbonate lowstand systems tracts are major components in the basin even in the deep pelagic realm transgressive and highstand systems tracts are very thin a response to starvation in contrast on the shelf transgressive and highstand systems tracts are the major components carbonate lowstand and shelf margin systems tracts pinch out in the vicinity of the offlap break it clearly appears that the stratal patterns and stratal termination patterns of carbonate systems tracts especially of carbonate lowstand systems tracts are basically similar to those of siliciclastic systems tracts this study documents that carbonate platforms may provide the basin with carbonate clastics during periods of relative sea level falls and lowstand this is one of the most controversial aspects of the application of sequence stratigraphy concepts in carbonate environments a second controversial aspect of the application of sequence stratigraphy concepts to carbonate rocks is the distinction between transgressive and regressive facies cycles and depositional sequences and systems tracts this paper demonstrates that the shoreline position for carbonates which indicates the facies transgression or regression is independent of prograding or retrograding shelf breaks the stratal pattern and facies within systems tracts vary depending on the overall evolution of the accommodation space i e on their position within the transgressive and regressive cycles

calciturbidite composition related to exposure and flooding of a carbonate platform triassic eastern alps the compositional variation of calciturbidites pedata potschen formation deposited adjacent to the dachstein formation carbonate platform in the triassic hallstatt basin was analysed using detailed field measurements and point counting of thin sections the 35 m long section is located in the northern calcareous alps of austria six point count groups were distinguished separating basinal from platform derived input summary statistics cluster and correspondence analysis of the point count data reveals a close relationship between the biota present on the dachstein carbonate platform and the calciturbidite composition the variations in turbidite composition are attributed to fluctuations in sea level and resulting flooding and exposure of the platform which alternately created and destroyed shallow water habitats on the platform top

high production and highstand shedding from deeply submerged carbonate banks northern nicaragua rise abstract because the shallow isolated carbonate banks on the northern nicaragua rise the nicaragua honduras and southern jamaica carbonate shelves and many other modern carbonate banks worldwide are covered by an average of 20 to 30 m of water and by a thin blanket of coarse carbonate sediments other carbonate sedimentologists have considered these banks good examples of incipiently drowned or even drowned carbonate banks however based on recent research on the northern nicaragua rise we can demonstrate that these banks currently are healthy producers of large volumes of periplatform sediments fine aragonite and magnesian calcite which are exported almost entirely to the deep surrounding slopes these sediments which were deposited during the past 9000 10 000 years form peripl tform wedges on the middle and upper slopes which are seen clearly on 3 5 khz seismic profiles radiocarbon ages of the wedge surface sediment range between 230 and 610 ybp and provide clear evidence for the contemporaneous production of sediments on the shallow bank and shelf and their instantaneous export to the upper slopes for the last 5000 years sedimentation rates ranged from 2000 mm ky off pedro bank to 1300 mm ky off jamaica these rates are somewhat lower than but of the same order of magnitude as sedimentation rates on the western leeward slopes of great bahama bank deposition of the metastable carbonate sediments discussed in this paper occurred as soon as sea level rose to flood the bank and shelf which flank walton basin following the last glacial lowstand they com rise a wedge shaped package of transgressive and highstand sediments on the upper and middle slopes of pedro bank and the southern jamaica shelf

reworked neritic fossils in upper mesozoic and cenozoic central pacific deep sea sediments monitor sea level changes upper mesozoic and cenozoic pelagic sediments in the central pacific ocean contain occasional horizons with reworked and displaced fossils produced by organisms that once lived in neritic shallow water environments the flux of neritic fossils was restricted to eight intervals of low eustatic sea level during late mesozoic and cenozoic times they were eroded from shoals along the flanks of volcanic highs which often supported tropical islands and which since then have subsided

fe ti oxide micro inclusions in clinopyroxene of oceanic gabbro phase content orientation relations and petrogenetic implication abstract fe ti oxide micro inclusions in clinopyroxene of oceanic gabbro from the mid atlantic ridge have been studied using electron backscatter diffraction and electron probe microanalyses a first generation of fe ti oxide inclusions occurs as needles or elongated plates lying in the 010 plane of the clinopyroxene host the inclusions show distinct elongation directions following irrational planes either nearly parallel to the c or nearly parallel to the a axis of the clinopyroxene host the habit planes correspond to inclusion host interfaces where densely packed oxygen layers of the inclusion and host phases are coherent across the interface both inclusion types have distinct crystallographic orientation relations to the host which are determined by the nearly parallel alignment of densely packed oxygen layers in the inclusions and the clinopyroxene host based on the angle between the two elongation directions primary formation of the inclusion at 800 to 900 c was inferred initially homogeneous ti bearing magnetite was precipitated together with titanian pargasite lamellae due to reaction of early magmatic clinopyroxene with late magmatic fluid or melt after cooling below 600 c the ti magnetite decomposed into an oriented magnetite ulvospinel intergrowth late stage hydrothermal alteration leads to the corrosion of the magnetite ulvospinel inclusions and partial replacement by ilmenite as well as newly precipitated homogeneous ilmenite plates that are closely associated with actinolite lamellae lying in the 100 plane of the clinopyroxene host