Mantle Wall Research Articles

Multistage and diverse melt-mantle interaction in dunite-harzburgite channel systems beneath oceanic slow-ultraslow spreading centers: Evidence from the Xigaze ophiolite (Tibet)

High-porosity, melt-focusing channel systems (comprising reactive dunite and its surrounding harzburgite) reflect an essential type of melt extraction within the mantle under mid-ocean ridges, proposed mainly by investigations of ophiolites produced in fast-spreading centers. However, relevant melt-migration processes from mantle wall rocks into dunitic channels under slow-ultraslow spreading centers and their effects are less well understood. We present systematic petrographic observations and whole-rock/mineral compositional analyses of a dunite-harzburgite channel system (∼250 m in width) within the Dazhuka mantle section (∼3–4 km thick) of the Xigaze ophiolite (South Tibet), produced in a Neo-Tethyan slow-ultraslow spreading center. The harzburgites, closely surrounding a dunite-rich zone, show large variations in pyroxene/olivine ratios (0.1–0.6), whole-rock MgO (41.2–45.9 wt%), Al2O3 (0.23–1.87 wt%) and CaO (0.38–2.80 wt%), as well as spinel Cr# (molar Cr3+/(Cr3++Al3+), 0.20–0.73) and pyroxene major and trace elements, almost covering the whole ranges of the Xigaze ophiolitic mantle. The harzburgites closer to the dunite-rich zone are gradually more depleted than those further ones, and finally replaced by the most depleted dunites. Modeling of decompressional melting from a depleted-MORB-mantle (DMM) source shows that the harzburgitic compositions correspond to melt depletion degrees of ∼6.9–19.2%, requiring a ∼30-km solely decompressional distance that is much greater than the sampling size of the channel system. It suggests that the wall-rock harzburgites can be produced by the reaction between high-fluxing, silica-undersaturated melts and the least-depleted harzburgites during the formation of reactive dunite-harzburgite channels. In addition, clinopyroxenes in dunites and some harzburgites show metasomatic enrichments, reflected in higher Na2O (up to 0.32 wt%), TiO2 (up to 0.17 wt%) and concentrations of Zr, Hf, light rare earth elements (LREE) and fluid-mobile elements relative to those in other harzburgites. These distinct signatures suggest the later localized metasomatism by batches of basaltic melts (some are volatile-rich) flowing through the harzburgitic wall rocks into the dunite-rich zone. We therefore propose that the dunite-harzburgite channel system archives at least the formation of reactive channels in deep upwelling asthenosphere during the high-melt-flux stage, and the subsequent metasomatic enrichments by basaltic melts reusing the channels in the shallow asthenosphere. The multistage and diverse melt-mantle interaction styles may characterize the melt extraction processes under oceanic slow-ultraslow spreading centers.

Major-, trace-element and Sr-Nd-Hf isotope geochemistry of diamondiferous dykes from Tonguma and Koidu, Sierra Leone: Highly micaceous kimberlites formed by assimilation of metasomatised lithospheric mantle rocks

The Man Craton in West Africa hosts two clusters of highly micaceous kimberlites at Koidu and Tonguma with mineralogical and geochemical features transitional towards those of cratonic lamproites. To develop a better understanding of the genesis of these peculiar rocks, which have also been documented elsewhere (e.g., South Africa, China, Russia, India), we have undertaken a geochemical and isotopic study of representative fresh samples from Koidu and Tonguma. New mica RbSr dating by isotope dilution confirms the Jurassic age of Koidu (∼145 Ma) and establishes that the Tonguma dykes are marginally younger (∼138 Ma). Bulk-rock analyses of samples from both localities show strong enrichment in K2O (up to 4.4 wt%), but also CaO and CO2 (up to 24 wt% and 18 wt%, respectively), which is entirely consistent with the micaceous nature of these rocks and their variable abundance of calcite. Despite their highly potassic compositions, the incompatible trace element systematics (e.g., Ba/Nb) of these rocks are typical of archetypal kimberlites and unlike those of cratonic lamproites. The Sr-Nd-Hf isotopic composition of the bulk samples range between moderately depleted values (age-corrected 87Sr/86Sr ∼ 0.7032; εNd = +3.8; εHf = +3.9) resembling the common depleted (Prevalent Mantle or PREMA-like) kimberlite component and more geochemically enriched compositions (87Sr/86Sr ∼ 0.7058; εNd = −0.4; εHf = +0.3). The direct relationship between bulk-rock Nd isotopes and average Mg# of magmatic olivine, combined with the previously reported correlation between average Mg# of magmatic and xenocrystic olivine of lithospheric mantle derivation in these kimberlites, suggests that the observed spread in radiogenic isotope compositions stems from the following petrogenetic scenario. Primary kimberlite melts of similar asthenospheric origin with unradiogenic Sr and moderately radiogenic Nd and Hf isotopes interacted with variably metasomatised (i.e., containing phlogopite) and hence geochemically enriched lithospheric mantle wall rocks. Mass balance calculations suggest that the transfer of incompatible elements to the asthenospheric melts was mediated by incongruent melting of metasomatic components in the lithospheric mantle rather than their bulk addition. This study reinforces previous suggestions that interaction between carbonate-rich asthenospheric melts and the sub-continental lithospheric mantle can generate the range of potassic magmas observed within cratons.

Aulacoseira newjerseyana sp. nov. (Bacillariophyta) a new freshwater centric diatom species from the northeastern USA

A new species of Aulacoseira Thwaites is described from sediment deposits of Surprise Lake, an isolated lake located in the New Jersey Highlands. The species is characterized by (1) a low mantle, (2) unevenly distributed mantle areolae of variable size and shape, and (3) a valve face covered by rounded areolae. The new species is most similar to Aulacoseira biseriata (Grunow in Van Heurk) Houk, R. Klee & H. Tanaka. Both taxa possess frustules with mantles that are low, with irregular pervalvar rows or scattered areolae. However, the two species differ in the shape and pattern of the areolae, the shape and structure of the mantle wall, the shape and depth of the ringleist, and other morphological features. In this study, we present light and scanning electron microscopic images of the new species and compare it to the most similar taxa.

A review of sperm storage methods and post-copulatory sexual selection in the Cephalopoda

Abstract In many animal phyla, females have a unique sperm storage organ (SSO). Post-copulatory sexual selection is a powerful driving force of SSO evolution. SSOs are generally considered to have evolved through sexual antagonistic coevolution between male genitalia and the SSO and/or cryptic female choice (CFC). In cephalopods, sperm transfer and fertilization are conducted through complex processes, and sperm storage methods show inter-species variation. In some species, males implant spermatangia superficially under the female skin, and then sperm released from the spermatangia are transferred into a seminal receptacle (SR). Deep-sea cephalopods, which lack a SR, have instead evolved a deep-implanting method by which the spermatangium is embedded deep in the musculature of the mantle wall of the female. In some species, the female stores whole spermatangia within a spermatangium pocket. Because the males of most species do not insert an intromittent organ into the female when transferring sperm, SSO evolution may have been influenced by CFC alone. This review summarizes the sperm storage methods and the mechanisms of post-copulatory sexual selection in cephalopods and it is proposed that these diverse methods evolved as adaptive mechanisms through post-copulatory sexual selection.

Aulacoseira chockii sp. nov., an early freshwater centric diatom from the Eocene bearing a unique morphology

Aulacoseira is a freshwater diatom genus found today in numerous lakes, ponds and rivers worldwide, and inhabiting a wide range of environmental conditions. The genus is one of the oldest diatom lineages known to colonize freshwater environments, dating to the late Cretaceous and radiating over much of the Cenozoic. The purpose of this paper is to describe a new species, Aulacoseira chockii, from an early Eocene locality situated near the Arctic Circle in northern Canada. The exquisitely preserved specimens have allowed for a detailed examination of frustule morphology of this early Cenozoic taxon. Three characteristics clearly separate A. chockii from all other known fossil and modern species in the genus. First, the spines of A. chockii are formed by coalescence of extensions from three to five mantle costae and multiple ribs that originate on the valve face. This type of spine design has never been reported and represents a potentially ancient trait in the Aulacoseiraceae lineage. Second, the valve has multiple sessile rimoportulae that are each connected to the end of a tube or canal that runs parallel to, and inside, the mantle wall. Rimoportulae with this structure are rare, and found primarily on extinct species of Aulacoseira. Third, the mantle striae are sinistrorse, and represent another rarely observed character within the genus. Because frustules of A. chockii possessed only separation spines, existed largely as single cells and not in long filaments, and were found in associated with remains of numerous heliozoans as well as testate euglyphids and sponges, it is believed this species grew in the littoral zone of a shallow waterbody.

Morphology and Mechanical Properties of Fossil Diatom Frustules from Genera of Ellerbeckia and Melosira.

Fossil frustules of Ellerbeckia and Melosira were studied using laboratory-based nano X-ray tomography (nano-XCT), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). Three-dimensional (3D) morphology characterization using nondestructive nano-XCT reveals the continuous connection of fultoportulae, tube processes and protrusions. The study confirms that Ellerbeckia is different from Melosira. Both genera reveal heavily silicified frustules with valve faces linking together and forming cylindrical chains. For this cylindrical architecture of both genera, valve face thickness, mantle wall thickness and copulae thickness change with the cylindrical diameter. Furthermore, EDS reveals that these fossil frustules contain Si and O only, with no other elements in the percentage concentration range. Nanopores with a diameter of approximately 15 nm were detected inside the biosilica of both genera using TEM. In situ micromechanical experiments with uniaxial loading were carried out within the nano-XCT on these fossil frustules to determine the maximal loading force under compression and to describe the fracture behavior. The fracture force of both genera is correlated to the dimension of the fossil frustules. The results from in situ mechanical tests show that the crack initiation starts either at very thin features or at linking structures of the frustules.

Detailed comparative anatomy of the Pinnidae (Mollusca, Bivalvia) reveals further unusual mantle specializations

Pinnids are semi-infaunal bivalves with several unusual morphological features, some of them regarded as adaptations to environments with high concentration of suspended particles. The presence of a waste canal on the mantle wall, for example, helps rejecting surplus particles from the mantle cavity. Despite being such anatomically intriguing bivalves, detailed comparative morphology studies on pinnids are still scarce. To gain further insights into the functional anatomy of pinnids, we have thoroughly investigated the mantle of Pinna carnea Gmelin, 1791 by means of scanning electron and light microscopy, including the application of histochemical staining methods. Additionally, the mantle was analyzed in ten pinnids from the genera Atrina, Pinna, and Streptopinna obtained in museum collections. Comparative morphology revealed a uniform structural organization of the mantle in Pinnidae, but a pronounced variation in pigmentation, presence of commarginal folds, and papillae. Our results also revealed intense secretory activity in the inner mantle fold, inner mantle epithelium, and waste canal, including at least three secretory cell types. Secretions are diverse and comprise mainly acidic mucosubstances on the mantle margin, while the waste canal secretes abundant acidophilic material apparently composed of basic proteins, and also possibly lipid content. Based on these results, we hypothesize that efficient mucociliary transportation is achieved in a hydrophobic environment provided by the waste canal, rapidly agglutinating and removing large amounts of particles from the mantle. Discoid glands secreting protein-rich content were found on the middle mantle fold of Pinna carnea and Streptopinna saccata (Linnaeus, 1758). Surprisingly, these mantle glands have hitherto not been recorded in the Bivalvia, representing, therefore, a new morphological feature in the Pinnidae.

Shape, Size, and Structure Affect Obliquely Striated Muscle Function in Squid.

Hollow, cylindrical body plans, and obliquely striated muscles are characteristic of soft-bodied invertebrates, and both affect the biomechanics of movement in these diverse animals. We highlight two different aspects of functional heterogeneity in obliquely striated muscles, one driven by animal shape and size and the other by the intrinsic mechanical properties of the fibers. First, we show how a hollow, cylindrical shape in the mantle of cephalopod molluscs causes a significant difference in muscle strain (defined as the change in length divided by resting length) across the mantle wall, and describe the implications of such "transmural gradients of strain" for the length-tension relationship of the obliquely striated muscles that power movements in these animals. We show that transmural gradients of strain increase in magnitude as mantle wall proportions change during ontogeny, with the relatively thin mantle walls of newly hatched squid experiencing significantly smaller differences in strain than the thicker mantle walls of adults. Second, we describe how the length-tension relationship of obliquely striated mantle muscles varies with position to accommodate the transmural gradient of strain, with the result that circular muscle fibers near the inner and outer surfaces of the mantle are predicted to produce similar force during mantle contraction. The factors that affect the length-tension relationship in obliquely striated muscles are unknown, and thus we have not yet identified the mechanism(s) responsible for the transmural shift in the length-tension properties of the mantle circular fibers. We have, however, developed a mathematical model that predicts small changes in the oblique striation angle (which varies from 4 to 12° in adult squid) have a significant effect on the shape of the length-tension relationship, with lower angles predicted to result in a broader length-tension curve.

In-situ assimilation of mantle minerals by kimberlitic magmas — Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa)

The lack of consensus on the possible range of initial kimberlite melt compositions and their evolution as they ascend through and interact with mantle and crustal wall rocks, hampers a complete understanding of kimberlite petrogenesis. Attempts to resolve these issues are complicated by the fact that kimberlite rocks are mixtures of magmatic, xenocrystic and antecrystic components and, hence, are not directly representative of their parental melt composition. Furthermore, there is a lack of direct evidence of the assimilation processes that may characterise kimberlitic melts during ascent, which makes understanding their melt evolution difficult.In this contribution we provide novel constraints on the interaction between precursor kimberlite melts and lithospheric mantle wall rocks. We present detailed textural and geochemical data for a carbonate-rich vein assemblage that traverses a garnet wehrlite xenolith [equilibrated at ~1060°C and 43kbar (~140–145km)] from the Bultfontein kimberlite (Kimberley, South Africa). This vein assemblage is dominated by Ca–Mg carbonates, with subordinate oxide minerals, olivine, sulphides, and apatite. Vein phases have highly variable compositions indicating formation under disequilibrium conditions. Primary inclusions in the vein minerals and secondary inclusion trails in host wehrlite minerals contain abundant alkali-bearing phases (e.g., Na–K bearing carbonates, Mg-freudenbergite, Na-bearing apatite and phlogopite). The Sr-isotope composition of vein carbonates overlaps those of groundmass calcite from the Bultfontein kimberlite, as well as perovskite from the other kimberlites in the Kimberley area. Clinopyroxene and garnet in the host wehrlite are resorbed and have Si-rich reaction mantles where in contact with the carbonate-rich veins. Within some veins, the carbonates occur as droplet-like, globular segregations, separated from a similarly shaped Si-rich phase by a thin meniscus of Mg-magnetite. These textures are interpreted to represent immiscibility between carbonate and silicate melts.The preservation of reaction mantles, immiscibility textures and disequilibrium in the vein assemblage, suggests quenching, probably triggered by entrainment and rapid transport toward the Earth's surface in the host kimberlite magma. Based on the Sr-isotope systematics of vein carbonate minerals, and the close temporal relationship between carbonate-rich metasomatism and kimberlite magmatism, we suggest that the carbonate-rich vein assemblage was produced by the interaction between a melt genetically related to the Bultfontein kimberlite and wehrlitic mantle wall rock. If correct, this unique xenolith sample provides a rare snapshot of the assimilation processes that might characterise parental kimberlite melts during their ascent through the lithospheric mantle.

Peridotite and pyroxenite xenoliths from the Muskox kimberlite, northern Slave craton, Canada

We present petrography, mineralogy, and thermobarometry for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite includes 23% coarse peridotite, 9% porphyroclastic peridotite, 60% websterite, and 8% orthopyroxenite. Samples primarily comprise forsteritic olivine (Fo 89–94), enstatite (En 89–94), Cr-diopside, Cr-pyrope garnet, and chromite spinel. Coarse peridotites, porphyroclastic peridotites, and pyroxenites equilibrated at 650–1220 °C and 23–63 kbar (1 kbar = 100 MPa), 1200–1350 °C and 57–70 kbar, and 1030–1230 °C and 50–63 kbar, respectively. The Muskox xenoliths differ from xenoliths in the neighboring and contemporaneous Jericho kimberlite by their higher levels of depletion, the presence of a shallow zone of metasomatism in the spinel peridotite field, a higher proportion of pyroxenites at the base of the mantle column, higher Cr2O3 in all pyroxenite minerals, and weaker deformation in the Muskox mantle. We interpret these contrasts as representing small-scale heterogeneities in the bulk composition of the mantle, as well as the local effects of interaction between metasomatizing fluid and mantle wall rocks. We suggest that asthenosphere-derived pre-kimberlitic melts and fluids percolated less effectively through the less permeable Muskox mantle, resulting in lower degrees of hydrous weakening, strain, and fertilization of the peridotitic mantle. Fluids tended to concentrate and pool in the deep mantle, causing partial melting and formation of abundant pyroxenites.

Description of a new sepioline species, Sepiola boletzkyi sp. nov. (Cephalopoda: Sepiolidae), from the Aegean Sea

A new sepioline species, Sepiola boletzkyi sp. nov. (Cephalopoda: Sepiolidae), is described based on two specimens from the Aegean Sea (eastern Mediterranean). The type specimens are lodged in the Ege University Faculty of Fisheries Museum of Izmir (Turkey). The new species belongs to the Sepiola atlantica group sensu Naef, hence it is compared with the species in this group, namely Sepiola affinis, Sepiola atlantica, Sepiola bursadhaesa, Sepiola intermedia, Sepiola robusta, Sepiola rondeletii, Sepiola steenstrupiana and Sepiola tridens. The male of S. boletzkyi sp. nov. differs from all the others in having the combination of homomorphous ventral arm tips, eight enlarged suckers, subdivided into two groups, in the dorsal row of the distal part of the hectocotylus and a dorsal lobe complementing the copulatory apparatus. In females of S. boletzkyi sp. nov. the bursa copulatrix is large and, unlike all other species in the genus, has a large fungiform process towards the mantle wall. The discovery of S. boletzkyi sp. nov. increases to 12 the number of NE Atlantic-Mediterranean species of Sepiola.

THE ORIGIN OF LARGE IRREGULAR GEM-QUALITY TYPE II DIAMONDS AND THE RARITY OF BLUE TYPE IIB VARIETIES

Type II (N-poor) diamonds do not constitute a single genetic population with a common paragenesis. Rather, several distinct populations (P-, E-, W- and sub-lithospheric) can be recognized, which formed in contrasting geological environments, with very different factors controlling the N-deficient character of these stones. Mantle-derived silicate and oxide inclusions are absent or extremely rare in large, Irregular Type II stones, which are often of exceptional gem quality. Such characteristics set these diamonds apart from N-poor (Type II) stones with “superdeep” inclusions, as well as the peridotitic and eclogitic suites, both of which include stones with non-detectable N. This in turn points to crystallization of the Irregular Type II stones in an environment markedly different from those in which the other three associations formed. Carbon isotopic signatures and syngenetic inclusions link both framesite and Irregular Type II diamonds to the websterite paragenesis. The latter in turn shows strong chemical affinities to the megacryst suite. From a purely petrographic perspective, the large Irregular Type II stones would be classified as megacrysts on the basis of their size (often >10 mm). The megacryst suite formed as pegmatitic veins from small volumes of kimberlitic liquids, injected into fractures within the thermal aureole surrounding the pooled kimberlite magma in the mantle prior to eruption. Crystallization of Irregular Type II diamonds was initiated when small volumes of evolved, residual megacryst magmas became buffered by highly reduced mantle wall rocks. Visually identified graphite inclusions, often rounded and thus inferred to be protogenetic, are relatively common in these diamonds. This observation is consistent with the inferred P-T conditions of crystallization of megacrysts across the graphite-diamond inversion curve, and thus within the lithosphere. Rapid crystallization of framesites from reduced megacryst magmas was probably triggered by stresses in the mantle immediately prior to kimberlite eruption. The websterite diamond suite is thus directly linked to megacryst crystallization, broadly coeval with the kimberlite magmatic event in the mantle. Experimental studies on the crystallization of granite pegmatites, complemented by processes specific to formation of the megacryst suite, provide a framework to account for the absence or extreme rarity of inclusions of other megacryst phases in the Irregular Type II stones. The elevated boron contents of blue Type IIb diamonds are consistent with the model for crystallization from highly fractionated pegmatitic megacryst magmas. Reduction of boron to the metallic state is ascribed to buffering of the megacryst magma by extremely refractory and highly reduced wallrock harzburgites, such as those found at Premier, which is one of the world’s major kimberlite sources of blue diamonds. Formation of fibrous cubic diamonds and the Premier eclogitic diamonds, which are both broadly coeval with eruption of the host kimberlite, can also be linked to the kimberlite magmatic event in the mantle.

Picroilmenites in Yakutian kimberlites: variations and genetic models

Abstract. Major and trace element variations in picroilmenites from Late Devonian kimberlite pipes in Siberia reveal similarities within the region in general, but show individual features for ilmenites from different fields and pipes. Empirical ilmenite thermobarometry (Ashchepkov et al., 2010), as well as common methods of mantle thermobarometry and trace element geochemical modeling, shows long compositional trends for the ilmenites. These are a result of complex processes of polybaric fractionation of protokimberlite melts, accompanied by the interaction with mantle wall rocks and dissolution of previous wall rock and metasomatic associations. Evolution of the parental magmas for the picroilmenites was determined for the three distinct phases of kimberlite activity from Yubileynaya and nearby Aprelskaya pipes, showing heating and an increase of Fe# (Fe# = Fe / (Fe + Mg) a.u.) of mantle peridotite minerals from stage to stage and splitting of the magmatic system in the final stages. High-pressure (5.5–7.0 GPa) Cr-bearing Mg-rich ilmenites (group 1) reflect the conditions of high-temperature metasomatic rocks at the base of the mantle lithosphere. Trace element patterns are enriched to 0.1–10/relative to primitive mantle (PM) and have flattened, spoon-like or S- or W-shaped rare earth element (REE) patterns with Pb > 1. These result from melting and crystallization in melt-feeding channels in the base of the lithosphere, where high-temperature dunites, harzburgites and pyroxenites were formed. Cr-poor ilmenite megacrysts (group 2) trace the high-temperature path of protokimberlites developed as result of fractional crystallization and wall rock assimilation during the creation of the feeder systems prior to the main kimberlite eruption. Inflections in ilmenite compositional trends probably reflect the mantle layering and pulsing melt intrusion during melt migration within the channels. Group 2 ilmenites have inclined REE enriched patterns (10–100)/PM with La / Ybn ~ 10–25, similar to those derived from kimberlites, with high-field-strength elements (HFSE) peaks (typical megacrysts). A series of similar patterns results from polybaric Assimilation + fractional crystallization (AFC) crystallization of protokimberlite melts which also precipitated sulfides (Pb < 1) and mixed with partial melts from garnet peridotites. Relatively low-Ti ilmenites with high-Cr content (group 3) probably crystallized in the metasomatic front under the rising protokimberlite source and represent the product of crystallization of segregated partial melts from metasomatic rocks. Cr-rich ilmenites are typical of veins and veinlets in peridotites crystallized from highly contaminated magma intruded into wall rocks in different levels within the mantle columns. Ilmenites which have the highest trace element contents (1000/PM) have REE patterns similar to those of perovskites. Low Cr contents suggest relatively closed system fractionation which occurred from the base of the lithosphere up to the garnet–spinel transition, according to monomineral thermobarometry for Mir and Dachnaya pipes. Restricted trends were detected for ilmenites from Udachnaya and most other pipes from the Daldyn–Alakit fields and other regions (Nakyn, Upper Muna and Prianabarie), where ilmenite trends extend from the base of the lithosphere mainly up to 4.0 GPa. Interaction of the megacryst forming melts with the mantle lithosphere caused heating and HFSE metasomatism prior to kimberlite eruption.

The length-force behavior and operating length range of squid muscle varies as a function of position in the mantle wall

Hollow cylindrical muscular organs are widespread in animals and are effective in providing support for locomotion and movement, yet are subject to significant non-uniformities in circumferential muscle strain. During contraction of the mantle of squid, the circular muscle fibers along the inner (lumen) surface of the mantle experience circumferential strains 1.3 to 1.6 times greater than fibers along the outer surface of the mantle. This transmural gradient of strain may require the circular muscle fibers near the inner and outer surfaces of the mantle to operate in different regions of the length-tension curve during a given mantle contraction cycle. We tested the hypothesis that circular muscle contractile properties vary transmurally in the mantle of the Atlantic longfin squid, Doryteuthis pealeii. We found that both the length-twitch force and length-tetanic force relationships of the obliquely striated, central mitochondria-poor (CMP) circular muscle fibers varied with radial position in the mantle wall. CMP circular fibers near the inner surface of the mantle produced higher force relative to maximum isometric tetanic force, P0, at all points along the ascending limb of the length-tension curve than CMP circular fibers near the outer surface of the mantle. The mean ± s.d. maximum isometric tetanic stresses at L₀ (the preparation length that produced the maximum isometric tetanic force) of 212 ± 105 and 290 ± 166 kN m(-2) for the fibers from the outer and inner surfaces of the mantle, respectively, did not differ significantly (P=0.29). The mean twitch:tetanus ratios for the outer and inner preparations, 0.60 ± 0.085 and 0.58 ± 0.10, respectively, did not differ significantly (P=0.67). The circular fibers did not exhibit length-dependent changes in contraction kinetics when given a twitch stimulus. As the stimulation frequency increased, L₀ was approximately 1.06 times longer than LTW, the mean preparation length that yielded maximum isometric twitch force. Sonomicrometry experiments revealed that the CMP circular muscle fibers operated in vivo primarily along the ascending limb of the length-tension curve. The CMP fibers functioned routinely over muscle lengths at which force output ranged from only 85% to 40% of P₀, and during escape jets from 100% to 30% of P₀. Our work shows that the functional diversity of obliquely striated muscles is much greater than previously recognized.

The length-force behavior and operating length range of squid muscle vary as a function of position in the mantle wall

The length-force behavior and operating length range of squid muscle vary as a function of position in the mantle wall

Transmural variation in the length‐force relationships and in vivo operating length ranges of the mantle muscles of squid

Recent work has shown that contraction of hollow, cylindrical, muscular organs is accompanied by significant non‐uniformities in circumferential strain across the muscular body wall. In squid, circumferential strain along the inner surface of the mantle wall is 1.3 to 1.6 times greater during the mantle contractions used to power jet locomotion than along the outer surface of the mantle. This transmural gradient of strain may result in the circular muscle fibers from near the inner and outer surfaces of the mantle operating over different regions of the length‐tension curve during the same mantle contraction cycle, potentially generating different instantaneous forces. We used sonomicrometry on swimming squid and isometric tests on bundles of circular fibers to test the hypothesis that circular muscle contractile properties vary transmurally in the mantle in the Atlantic longfin squid, Doryteuthis pealeii. We found that the length‐force relationship of the obliquely striated circular muscle fibers varied significantly with radial position in the mantle wall. Sonomicrometry experiments revealed that the circular fibers operated in vivo primarily along the entire ascending limb of the length‐tension curve. Our work shows that the functional diversity of obliquely striated muscles is much greater than previously thought. Supported by NSF grant IOS‐0950827.

AULACOSEIRA SATSUMAENSIS, A NEW PLIOCENE DIATOM SPECIES WITH TWO MORPHOTYPES FROM KAGOSHIMA, JAPAN

Aulacoseira satsumaensis H. Tanaka sp. nov., is described from a Pliocene freshwater sediment of the Koriyama Formation in Kagoshima Prefecture, Kyushu, Japan. The species is characterized by having different valve morphology between thick wall valves and thin wall valves. The thick wall valves, including the initial cell, have a larger diameter, a shorter mantle, fewer punctae rows and coarser punctae in the rows than the latter. Also, the thick wall morphotype has very thick transverse costae on the inner mantle wall while the thin wall morphotype exhibit thick pervalver costae. Another difference between the two morphotypes is in the location of their rimoportulae. This new species is compared to three other similar Aulacoseira species, A. krammeri R.K. Edgar, Kociolek & S.M. Edgar, reported from Miocene deposits in Oregon, USA, and described as having robust and gracile morphotypes, A. nipponica (Skvortsov) Tuji, originally described from Lake Biwa, Japan and A. sp., reported from a Pliocene deposit of Kyushu, Japan, the same island as A. satsumaensis was found.

Type II diamonds: Flamboyant Megacrysts?

Type II diamonds from Premier are characterised by light carbon isotope-enriched compositions, markedly different from the signatures of the associated Eclogitic (E-type) and Peridotitic (P-type) diamonds at the same locality. This provides strong evidence that these Type II stones represent a third and hitherto unrecognised diamond paragenesis. Their isotopic signature is closely similar to that of rare Orapa diamonds with websterite inclusions, arguing for a common paragenesis. The Orapa websterite garnet inclusions are Ti- and Cr-rich relative to those from other kimberlites, and overlap the compositional field for Orapa garnet megacrysts. These chemical characteristics, coupled with the large size and irregular shape of Type II diamonds, links them to the megacryst suite. This conclusion is supported by evidence which shows that at the low temperature range of megacryst crystallisation, the associated residual liquids, if buffered by mantle wall rocks, at oxygen fugacities between the WM and IW, would be in the diamond P-T-f O2 stability field. The low N concentrations and light cabon isotopic-enriched fingerprint of the Type II stones are consistent with their crystallisation from such evolved magmas. It is demonstrated that in addition to the irregular and often large (megacryst) Type II stones, there are two further suites of N-poor diamonds which can be assigned to either an eclogitic or peridotitic paragenesis on the basis of their inclusions, physical properties and carbon isotopic signatures. Diamonds with complex growth zones and deformation textures can be explained in terms of dynamic mantle processes, linked to formation of the sheared peridotites, within the thermal aureole surrounding the kimberlite magma in which the megacryst suite crystallised. Framesites, fibrous cubic diamonds, and eclogitic diamonds which formed just prior to kimberlite eruption, can also be linked to the processes responsible for crystallisation of the megacryst suite, which is considered to be cognate to the host kimberlite magma.

Structure of the ovary of the articulate brachiopod Hemithyris psittacea (Rhynchonelliformea, Brachiopoda) with special reference to connective tissue pillars

Summary The microscopical anatomy of the ovary of the articulate brachiopod Hemithyris psittacea was investigated by light and transmission electron microscopy. The ovary has a reticulate structure with connective tissue pillars, which pierce it. The pillars have a wine-glass form and consist of two parts—a column that is attached to inner mantle wall and a broader region that is attached to outer mantle wall. The coelomic epithelium of the column contains myofilaments orientated across the gonad cavity. The broader part contains parenchymal tissue with cells that are filled by the expanded profiles of rough endoplasmic reticulum. Globular cells with membrane-free inclusions were found in the mantle wall. The connective tissue pillars may prevent the collapse of gonad during gametogenesis and/or contract to facilitate spawning. It is assumed, that the parenchymal tissue and the globular cells take part in the nutrition of the oocytes and in the storage of the nutrients.

Mechanical specialization of the obliquely striated circular mantle muscle fibres of the long-finned squidDoryteuthis pealeii

The centrally located, mitochondria-poor (CMP) and superficially located, mitochondria-rich (SMR) circular muscle fibres in the mantles of some squids provide one of the few known examples of specialization in an obliquely striated muscle. Little is known of the mechanical properties or of the mechanisms and performance consequences of specialization in these fibres. We combined morphological and physiological approaches to study specialization in the SMR and CMP fibres of the long-finned squid Doryteuthis pealeii. The mean thick filament length was 3.12+/-0.56 mum and 1.78+/-0.27 mum for the SMR and CMP fibres, respectively. The cross-sectional areas of the whole fibre and the core of mitochondria were significantly higher in the SMR fibres, but the area occupied by the myofilaments did not differ between the two fibre types. The area of sarcoplasmic reticulum visible in cross sections was significantly higher in CMP fibres than in SMR fibres. In live bundles of muscle fibres partially isolated from the mantle, mean peak isometric stress during tetanus was significantly greater in SMR [335 mN mm(-2) physiological cross section (pcs)] than in CMP (216 mN mm(-2) pcs) fibres. SMR fibres had a lower average twitch:tetanus ratio (SMR=0.073; CMP=0.18) and a twofold lower unloaded maximum shortening velocity at 20 degrees C (SMR=2.4 L(0) s(-1); CMP=5.1 L(0) s(-1)), where L(0) was the preparation length that yielded the highest tetanic force. The structural differences in the two muscle fibre types play a primary role in determining their mechanical properties, and the significant differences in mechanical properties indicate that squid have two muscle gears. A simple model of the mantle shows that a gradient of strain and strain rate exists across the mantle wall, with fibres adjacent to the outer edge of the mantle experiencing 1.3- to 1.4-fold lower strain and strain rate than fibres adjacent to the inner edge of the mantle. The model also predicts that the CMP fibres generate virtually no power for slow jetting while the SMR fibres are too slow to generate power for the escape jets. The transmural differences in strain and strain rate predicted by the model apply to any cylindrical animal that has circumferentially oriented muscle fibres and an internal body cavity.