Mafic Inclusions Research Articles

Petrogenesis of silicate inclusions in the Weekeroo Station IIE iron meteorite: differentiation, remelting, and dynamic mixing

The Weekeroo Station IIE iron meteorite contains a variety of felsic and mafic inclusions enclosed in an FeNi-metal host. Petrographic, EMP, and SIMS data suggest that the petrogenesis of the silicates was complex, and included differentiation, remelting, FeO-reduction, and dynamic mixing of phases. Differentiation produced a variety of olivine-free inclusion assemblages, ranging from pyroxene + plagioclase + tridymite with peritectic compositions, to coarse orthopyroxene, to plagioclase + tridymite and its glassy equivalent. Individual phases have similar trace-element abundances and patterns, despite large variations in inclusion textures, modes, and bulk compositions, probably as a result of mechanical separation of pre-existing phases in an impact event that dynamically mixed silicates with the metallic host. Trace-element data imply that augite and plagioclase grains in different inclusions crystallized from the same precursor melt, characterized by relatively unfractionated REE abundances of ∼20–30 × CI-chondrites except for a negative Eu anomaly. Such a precursor melt could have been produced by ∼2–5% equilibrium partial melting of an H-chondrite silicate protolith, or by higher degrees of partial melting involving subsequent fractional crystallization. Glass appears to have formed by the remelting of pre-existing plagioclase and orthopyroxene, in a process that involved either disequilibrium or substantial melting of these phases. During remelting, silicate melt reacted with the FeNi-metal host, and FeO was reduced to Fe-metal. Following remelting and metal-silicate mixing, inclusions apparently cooled at different rates in a near-surface setting on the parent body; glass- or pigeonite-bearing inclusions cooled more rapidly (≥2.5°C/hr between 1000–850°C) than pigeonite-free, largely crystalline inclusions. The results of this study point to two likely models for forming IIE iron meteorites, both involving collision between an FeNi-metal impactor and either a differentiated or undifferentiated silicate-rich target of H-chondrite affinity. Each model has difficulties and it is possible that both are required to explain the diverse IIE group.

GEOCHEMISTRY OF THE ULTRAMAFIC XENOLITHS FROM OKI-DOGO ISLAND: IMPLICATIONS FOR THE WEDGE MANTLE EVOLUTION

There are some localities of residual peridotite xenoliths from western edge arcs of the Circum-Pacific Ocean. Abe (1997) and Abe et al. (1998; 1999) show that the peridotite xenoliths from those arcs underwent wedge mantle metasomatism with some influx from subducted slab. We report the petrological and geochemical characteristics of the ultramafic xenoliths in the Cenozoic alkali-olivine basalts of Oki-Dogo island, another locality reported by Abe (1997) and Abe et al. (1998; 1999) in the Japan Sea. Both Group I and II ultramafic xenoliths (classification by Frey and Prinz (1978), equivalent to Wilshire and Shervais (1975) Cr-diopside and Al-augite series, respectively) come from Oki-Dogo island. Preliminary data on the petrology of the ultramafite, and also mafic xenoliths are in Yamaguchi (1964), Aoki (1977) and Takahashi (1978). We present here their mineral chemistry, especially the abundance of trace-elements in clinopyroxenes of Group I peridotite and the metasomatism by Group II ultramafic xenoliths suite. Oki-Dogo Island is located at the northern end of the Southwest Honshu Arc in the Japan Sea. Extensive alkalic volcanism (mugearite, hawaiite, trachy-andesite, trachyte and alkali rhyolite) took place from late Miocene to early Pliocene in this island. Alkali-olivine basalts with a subordinate amount of basanites were extruded from many vents situated inside of the supposed magma chamber. These basalts were monogenetic in most cases and their extrusion started at least 3.6 Ma and lasted at least until 0.8 Ma (Kaneoka et al., 1977). Most of these alkali basalts are fresh and contain 2 to 15 percent normative nepheline. Ultramafic and mafic inclusions are commonly found in most of the monogenetic lava flows. According to Takahashi (1978), those xenoliths are classified into five groups: spinel lherzolite, banded spinel peridotite, banded plagioclase peridotite, gabbro and granulite groups. The rocks of the banded spinel peridotite group are cumulates formed by fractional crystallization of basic magmas in a pressure range between about 10 and 20 kb, and recrystallized at the same depth. Some of the spinel lherzolite group are cumulates from basic magmas in the upper mantle during the partial melting of the preexisting lherzolite. Group I peridotite, which was classified as spinel lherzolite group by Takahashi (1978), has very low-Cr spinel. The Cr# (=Cr/(Cr+Al) atomic ratio) of chromian spinel is less than 0.3. Their Fo content of olivine varies from 90 to 83, while their NiO content is rather constant (0.25 to 0.40 wt%). Clinopyroxene grains in polished thin sections were analyzed in situ for abundances of REE, Ti, Sr, Y, Zr with the secondary ion mass spectrometer (SIMS; Cameca IMS- 3f) at Tokyo Institute of Technology. The chondrite normalized REE patterns of clinopyroxene in Oki-Dogo Group I xenoliths are flat to U-shaped LREE-depleted. HREE contents, however, almost same level, which suggests that those peridotites have the same degree of melt extraction. The evidence from their REE patterns and trace-element contents suggest that the Group I peridotites underwent Fe-rich mantle metasomatism by the alkaline magma. The continental type mantle metasomatism as well as arc type metasomatism suggested by Abe (1997) occurs in mantle wedge.

Metamorphic fluids and multiple lithologies as sources for the gold-quartz vein deposits: Evidence from REE and isotope studies (Experience from the Kolar Gold Field, India)

The auriferous Kolar Schist Belt in the Dharwar craton of southern India is considered to be a late Archean suture between two gneissic terranes. Therefore the famous Kolar gold deposits offers an excellent opportunity to understand the relation between tectonic processes, fluid evolution and gold metallogeny. In Kolar gold occurs as steeply dipping sheeted quartz veins within the sheared komatiitic amphibolite only on the eastern part of the greenstone belt. The gold quartz veins have structural, mineralogical and textural features characteristic of mesothermal, low sulfide and high gold deposits present elsewhere in the world. We discuss here the nature of the fluid, fluid-path, and the processes involved in the formation of gold quartz vein deposits using lanthanide geochemistry, radiogenic and stable isotope data on various representative samples including gold quartz veins, altered and unaltered wall-rock, mineral separates (quartz, pyrite and calcite) and mafic inclusions from the gold quartz veins, and the intrusive pegmatites. The gold quartz vein samples have LREE - enriched HREE - depleted chondrite-normalised patterns without Eu anomaly.

Intracrustal origin of Arenal basaltic andesite in the light of solid–melt interactions and related compositional buffering

The origin of Arenal basaltic andesite can be explained in terms of fractional crystallization of a parental high-alumina basalt (HAB), which assimilates crustal rocks during its storage, ascent and evolution. Contamination of this melt by Tertiary calc-alkalic intrusives (quartz–diorite and granite, with 87 Sr/ 86 Sr ratios ranging 0.70381–0.70397, nearly identical with those of the Arenal lavas) occurs at upper crustal levels, following the interaction of ascending basaltic magma masses with gabbroic–anorthositic layers. Fragments of these layers are found as inclusions within Arenal lavas and tephra and may show reaction rims (1–5 mm thick, consisting of augite, hypersthene, bytownitic–anorthitic plagioclase, and granular titanomagnetite) at the gabbro–lava interface. These reaction rims indicate that complete `assimilation' was prevented since the temperature of the host basaltic magma was not high enough to melt the gabbroic materials (whose mineral phases are nearly identical to the early formed liquidus phases in the differentiating HAB). Olivine gabbros crystallized at pressure of about 5–6 kbar and equilibrated with the parental HAB at pressures of 3–6 kbar (both under anhydrous and hydrous conditions), and temperatures ranging 1000–1100°C. In particular, `deeper' interactions between the mafic inclusions and the hydrous basaltic melt (i.e., with about 3.5 wt.% H 2O) are likely to occur at 5.4 (±0.4) kbar and temperatures approaching 1100°C. The olivine gabbros are thus interpreted as cumulates which represent crystallized portions of earlier Arenal-type basalts. Some of the gabbros have been `mildly' tectonized and recrystallized to give mafic granulites that may exhibit a distinct foliation. Below Arenal volcano a zoned magma chamber evolved prior the last eruptive cycle: three distinct andesitic magma layers were produced by simple AFC of a high-alumina basalt (HAB) with assimilation of Tertiary quartz–dioritic and granitic rocks. Early erupted 1968 tephra and 1969 lavas (which represent the first two layers of the upper part of a zoned magma chamber) were produced by simple AFC, with fractionation of plagioclase, pyroxene and magnetite and concomitant assimilation of quartz–dioritic rocks. Assimilation rates were constant ( r 1=0.33) for a relative mass of magma remaining of 0.77–0.80, respectively. Lavas erupted around 1974 are less differentiated and represent the `primitive andesitic magma type' residing within the middle–lower part of the chamber. These lavas were also produced by simple AFC: assimilation rates and the relative mass of magma remaining increased of about 10%, respectively ( r 1=0.36, and F=0.89). Ba enrichment of the above lavas is related to selective assimilation of Ba from Tertiary granitic rocks. Lava eruption occurred as a dynamic response to the intrusion of a new magma into the old reservoir. This process caused the instability of the zoned magma column inducing syneruptive mixing between portions of two contiguous magma layers (both within the column itself and at lower levels where the new basalt was intruded into the reservoir). Syneruptive mixing (mingling) within the middle–upper part of the chamber involved fractions of earlier gabbroic cumulitic materials (lavas erupted around 1970). On the contrary, within the lower part of the chamber, mixing between the intruded HAB and the residing andesitic melt was followed by simple fractional crystallization (FC) of the hybrid magma layer (lavas erupted in 1978–1980). By that time the original magma chamber was completely evacuated. Lavas erupted in 1982/1984 were thus modelled by means of `open system' AFCRE (i.e., AFC with continuous recharge of a fractionating magma batch during eruption): in this case assimilation rates were r 1=0.33 and F=0.86. Recharge rates are slightly higher than extrusion rates and may reflect differences in density (between extruded and injected magmas), together with dynamic fluctuations of these parameters during eruption. Ba and LREE (La, Ce) enrichments of these lavas can be related to selective assimilation of Tertiary granitic and quartz–dioritic rocks. Calculated contents for Zr, Y and other REE are in acceptable agreement with the observed values. It is concluded that simple AFC occurs between two distinct eruption cycles and is typical of a period of repose or mild and decreasing volcanic activity. On the contrary, magma mixing, eventually followed by fractional crystallization (FC) of the hybrid magma layer, occurs during an ongoing eruption. Open-system AFCRE is only operative when the original magma chamber has been totally replenished by the new basaltic magma, and seems a prelude to the progressive ceasing of a major eruptive cycle.

The role of magma mixing in triggering the current eruption at the Soufriere Hills Volcano, Montserrat, West Indies

The andesite lava currently erupting at the Soufriere Hills volcano, Montserrat, contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite. The andesite phenocrysts have a range of textures and zonation patterns which suggest that non‐uniform reheating of the magma occurred shortly before the current eruption. Reheating resulted in remobilisation of the resident magma and may have induced eruption.

Plumbing the Depths: Pb Isotope Constraints on the Origin of the 3.65 Ga Amîtsoq Gneiss

Archaean crust largely consists of tonalite-trondhjemite-granodiorite (TTG) gneiss provinces with associated greenstone belts. Understanding their genesis is a fundamental task in deciphering crustal evolution through time. The oldest well-exposed major TTG-greenstone belt terrain is the Itsaq gneiss complex of southern West Greenland. This complex consists of the TTG-type Amhsoq gneiss with its highly metamorphosed metasedimentary and mafic inclusions (the Akilia association), and the Isua greenstone belt. The isotope-geochemical signatures of these units, and the conclusions drawn from them, are highly debated issues with far-reaching implications for crustal and mantle evolution. The main controversy centres around the significance of ionprobe U-Pb zircon dates. Zircons from these rocks typically show a wide range (c. 3.60 to 3.87 Ga) of near-concordant U-Pb dates. Initial Nd isotope signatures of individual samples, when corrected for ~47Sm decay by assuming that the oldest U-Pb zircon dates represent the age of rock formation, span a wide range between +5 and 5 epsilon units (Bennett et al., 1993). These Nd isotope data have been interpreted to reflect both extreme mantle isotope heterogeneity and extreme LREE-depletion of early Archaean mantle, implying the existence and persistence of a voluminous crust during early Archaean times (McCulloch and Bennett et al., 1994). Such an interpretation of the Nd isotope data of the Itsaq gneiss complex would require a substantial (irreversible?) change in processes of crustal evolution between midand early Archaean times. Here we challenge the above scenario based on new, previously unpublished and published Pb isotope data for the Amitsoq gneiss. The ancient Amksoq gneiss is characterised by extremely low I.t (= 238U/2~ values and contains the earth's least radiogenic silicate Pb. In addition to previously published and unpublished whole-rock and feldspar Pb isotope data of Am]tsoq gneiss samples, we have specifically included samples in this study which contain zircons with some ion-probe U-Pb dates extending back to >3.8 Ga. Plotting all 84 samples (whole rocks, feldspars, leachates) in a common Pb diagram reveals two major features. Firstly, in spite of the wide area over which the samples were collected, a tight regression line (MSWD = 17.6) corresponding to an age of 3654_+73 Ma is obtained. The second, far more important, feature is that independent of the exact slope of the regression, a very well-defined intercept with plausible terrestrial Pb-isotope evolution curves is obtained. Thus, relative to the depleted mantle evolution curve of Kramers and Tolstikhin (1997), intercept ages of a family of regression lines calculated with sub-sets of the data range only between 3.64 and 3.68 Ga. The scatter around the Pb/Pb regression line is most probably the result of minor initial Pb-isotopic heterogeneity and/or some post-formational metamorphic disturbance. Whilst the slope of the Pb/Pb regression line could, in theory, be spurious if not all samples were genetically related, the really important intercept constraint from the least radiogenic Pb data cannot be discounted by arguing that unrelated rocks have been studied. The only straightforward interpretation of the Amhsoq Pb isotope data is that the magmatic precursors of the Amltsoq gneisses were formed over a short period of time (< 100 Ma), most likely c. 3650 Ma ago. The Pb-isotopic constraints are, and have always been, in obvious conflict with the extended (3.87 to 3.6 Ga) crustal growth history proposed by Nutman et al. (1996) and with the Nd isotope interpretation of Bennett et al. (1993). These authors summarily dismissed the Pb-isotope evidence by stating that these might reflect post-igneous disturbance. However, the only scenario under which the original Pb isotope ratios of 3.87 to 3.6 Ga old rocks could evolve into the observed Pb-isotope array would involve three stages. Firstly, a geological event must be proposed at 3.65 Ga capable of completely

Geologic and geochemical relationships between the contact sublayer, inclusions, and the main mass of the Sudbury Igneous Complex; a case study of the Whistle Mine Embayment

More than half of the giant Ni-Cu-platinum-group element (PGE) sulfide ores of the Sudbury Igneous Complex are associated with a discontinuous unit at the base of the main mass known as the The sublayer is comprised of two fragment-rich members: (1) a metamorphic-textured footwall breccia, and (2) an igneous-textured contact sublayer. The contact sublayer occurs as a thick unit in depressions at the base of the Sudbury Igneous Complex termed embayments; it is associated with disseminated to massive sulfides and contains a range of inclusion types such as diabase, melanorite, olivine melanorite, metamorphosed melanorite, wehrlite, and dunite. We present data for a case study of the Whistle mine embayment and show that the igneous-textured sublayer matrix is geochemically unlike the main mass norites, quartz gabbros, and granophyres. For example, the igneous-textured sublayer matrix in the Whistle mine has La/Sm = 4.0, La/ Nb = 5.1, and Th/Zr = 0.02, and the main mass norites, quartz gabbros, and granophyres have ratios of 5.5 to 7, 2.8 to 4.2, and 0.04 to 0.05, respectively. This matrix contains partially digested hornfels diabase fragments and ghost textures of magnetite remaining from the melting of diabase. The igneous-textured sublayer matrix at the Whistle mine can be modeled with small amounts of assimilation of local country-rock granitoids ( approximately 10%), large degrees of assimilation of diabase that are not derived from the immediate country rocks ( approximately 70%), and small contributions from the main mass magma type ( approximately 20% mafic norite). The amount of diabase assimilation would be too large for traditional assimilation models controlled by the available heat of the mafic magma; even with a superheated magma, 70 percent assimilation is too large. A model is proposed where the melting of the target rocks initially produces a felsic melt sheet which is laden with mafic fragments. These fragments sink to the base of the melt sheet in the crater and are concentrated and further melted to produce the mafic sublayer. There are significant differences in the composition of the igneous-textured sublayer matrix between different embayments which may reflect differing degrees of digestion of compositionally different protolith fragments. The melanorite inclusions in the sublayer at the Whistle mine have ratios of the incompatible trace elements essentially similar to those of the inclusions in the igneous-textured sublayer matrix, but they have similar high incompatible element concentrations (e.g., olivine melanorites have 20-65 ppm Ce in rocks with 15-21 wt % MgO), 1 to 10 percent interstitial sulfide, up to 0.5 percent apatite, 1 to 15 percent biotite, and 1.85 Ga age zircon and baddeleyite. The mafic-ultramafic inclusions are interpreted to be the broken-up remnants of an earlier cumulate formed at depth from a main mass magma; this parental magma has compositional traits which suggest that crystal accumulation took place from a magma which contains a large contribution from the diabase. Olivine compositional data for the mafic inclusions (Fo (sub 71-79) ; 500-3,800 ppm Ni), in the absence of reequilibration, indicate that olivine crystallization both predated and postdated sulfur saturation of the magma. Cr-rich spinels from these rocks confirm that the parental magma was especially Cr rich, and based on the olivine compositional data, had a tholeiitic Mg/Fe ratio.

The effect of microscale pore structure on matrix diffusion—a site-specific study on tonalite

The matrix diffusion of non-sorbing tracers was studied in rocks from the Syyry area, Central Finland (SY1). The effect of alteration and weathering on rock matrix porosity and on the available pore space, which affects diffusivity, are discussed. The main rock type in the crystalline bedrock of Syyry is a slightly foliated, gray tonalite with mica gneiss inclusions as well as minor, more mafic inclusions. The total porosity and the spatial porosity distribution and microstructure of the rocks were investigated using infiltration of carbon- 14-methylmethacrylate, electron microscopy and Hg-porosimetry. The laboratory-scale diffusion experiments were performed using (1) the out-leaching technique of methylmethacrylate (MMA), (2) the through-diffusion of tritiated water (HTO) and chloride in the liquid phase and (3) the through-diffusion of helium in the gas phase. The results of structure investigations indicate that alteration and weathering create micrometric-scale pore apertures, visible as fissures and cracks. These migration pathways dominate in laboratory-scale diffusion experiments and result in high effective porosities and effective diffusivities. Alteration also creates spherical pores, smaller than 1 μm, especially in heavily altered biotite and plagioclase minerals. The altered mineral phases act as quasi dead-end pores hindering the diffusion of non-sorbing tracers. The heterogeneity of the matrix is increased. The heterogeneous spatial pore structure of altered and weathered rock samples causes uncertainty in the diffusion coefficients owing to an assumed homogeneous matrix in Fickian diffusion models.

The dimensions of magmatic inclusions as a constraint on the physical mechanism of mixing

Silicic lavas often bear inclusions of more mafic lava. Several physical processes have been proposed by which these magmas may become mixed. Measurements of the sizes of mafic inclusions in lavas of two calc–alkaline volcanoes are used here to obtain a characteristic length scale for this type of mixing and provide a means to discriminate between different mixing processes. We quantitatively assess the proposal that, in a stratified reservoir, dynamic interactions of rising gas bubbles with the interface between a lower layer of mafic magma and an upper layer of silicic magma lead to the formation of vesiculated blobs of the lower magma in the upper one. This is due to gravitational instability of a foam layer growing at the interface between the two magmas. Our theoretical calculations of this instability predict both the correct order of magnitude for the characteristic size of inclusions and the way in which inclusion size varies with the physical properties of the magmas. This fluid dynamic framework is used to map out different possible flow regimes and place constraints on the fraction of gas in these systems.

Hornblende gabbro sill complex at Onion Valley, California, and a mixing origin for the Sierra Nevada batholith

The steep crest of the Sierra Nevada, California, near Onion Valley, exposes natural cross sections through a mafic intrusive complex that formed as part of the Mesozoic Sierra Nevada batholith. Sheeted sills of hornblende gabbro to hornblende diorite, individually as thick as 1.5 m, form the upper 200 to 300 m of the complex. Thicker, multiply-injected sills, as well as mafic stocks, lie underneath at elevations below 3600 m. Lens-shaped cumulate bodies, as thick as 200 m and more than 700 m broad, lie near the base of the sheeted sill suite. Cumulates are flat-lying, modally layered hornblende gabbro with subsidiary ultramafic olivine hornblendite, plagioclase hornblendite, and late-mobile hornblende-plagioclase pegmatite. Fine grain size, scarce phenocrysts and xenocrysts, and quench mineral textures are evidence that hornblende gabbro sills injected in a largely liquid state and preserve basaltic melt compositions. Most sills reached volatile saturation, as shown by tiny miarolitic cavities that are also widespread in cumulates. Although some sills chilled directly against others, most chilled against septa, millimeters to a few centimeters thick, of medium-grained diorite to granodiorite. Mutually crosscutting relations, as well as chilling, show that the septa were partly molten at the time the sills injected and likely formed the lower portions of an overlying more silicic magma chamber that has since been removed by erosion. Sill compositions range from evolved high-alumina basalt to aluminous andesite with major and trace element abundances similar to those of modern arc magmas. Experimental phase equilibria indicate dissolved water contents near 6 wt% (Sisson and Grove 1993a). The sills show unequivocally that hydrous arc basaltic magmas reached shallow levels in the crust during formation of the largely granodioritic Sierra Nevada batholith. The basaltic magmas appear to have been produced from an enriched mantle source with 87Sr/86Sr ∼0.7065, ɛNd ∼−4.3, 206Pb/204Pb ∼18.6, 207Pb/204Pb ∼15.6, 208Pb/204Pb ∼38.6. Although crystal fractionation contributed to forming the sill suite and the associated cumulates, nearly constant concentrations of Na2O, P2O5, Nb, Zr, and light rare earth elements in the sills indicate that mixing between sill basaltic and more evolved septa magmas was important for producing sills with andesitic compositions. Average Sierran granodiorite major and trace element concentrations are readily reproduced by a simple mixture of average basaltic sill from Onion Valley and average Sierran low-silica granite. This result supports the inference that Sierran granitoids formed chiefly by mixing between crustal and mantle-derived magmas, although in some cases these crustal melts may have been derived by refusion of earlier mafic intrusions near the base of the crust. The common mafic inclusions (enclaves) in Sierran granodiorites bear a superficial resemblance to Onion Valley mafic sills; however, high concentrations of lithophile elements in the inclusions point to extensive chemical exchange between inclusions and their host magmas. The prevalence of hornblende-rich mafic intrusive rocks at Onion Valley, elsewhere in the Sierra Nevada, and in other shallow subduction batholiths stems from two effects of high melt water concentrations (∼4–6 wt% H2O). The hydrous parent basaltic and basaltic andesite magmas had low liquidus temperatures, compared to nearly dry basaltic melts, and thus were chilled less during ascent through the crust and were more capable of ascent as liquids. More importantly, their high water concentrations led to low melt densities, higher than granitoid liquids, but comparable to or less dense than partly solidified granitoid magmas. Thus, the hydrous basaltic and basaltic andesite magmas were neutrally or positively buoyant and were capable of penetrating and rising through partly crystallized granitoids and their partly molten source regions to reach upper crustal emplacement levels. Drier basaltic magmas were probably abundant at depth and contributed heat and mass to granite generation, but were insufficiently buoyant to ascend to shallow levels.

東北日本，月山火山新期噴出物の岩石学的研究

The Gassan volcano is part of the Chokai volcanic zone, and consists of a number of volcanic edifices (Gassan, Ubagatake, Yudonosan, Amamoriyama, Sen-nindake, Yakushidake, Waratahageyama, Shinakurayama, and Ohirayama) which were formed during the Quaternary in northeastern Japan. This study has been focussed on Gassan, Ubagatake, Waratahageyama, Amamoriyama, and Ohamaike, which are defined here as the younger stage lavas of the Gassan volcano. Younger stage lavas of the Gassan volcano are andesites, except in the case of Amamoriyama (dacites). These rocks have some or all of the following features; (1) presence of plagioclase phenocrysts with sieve-textured zones (spongy cores, or dusty cores or zones) and penetrative resorption features, (2) overgrowth of clinopyroxene around orthopyroxene, (3) orthopyroxene reaction rims around olivine, (4) disequilibrium phenocryst assemblages such as Mg-rich olivine and quartz, and (5) existence of mafic inclusions. Mineralogical compositions of host rocks and mafic inclusions have been determined and identified as follows. Cores of plagioclase phenocrysts have a wide range in composition (An 40-90) and show a bimodal distribution (peaks: An 40-60, An 70-90). The Mg/(Mg+Fe) ratios of the cores of clinpyroxene and orthopyroxene phenocrysts have a unimodal distribution (peaks Cpx, 0.72-0.76; Opx, 0.64-0.68) with minor amounts of Mg-rich ones (Cpx, 0.76-0.84; Opx, 0.72-0.76). Mg-rich phenocrysts show Mg-Fe partitioning in equilibrium with Mg-poor olivine phenocrysts. The crystallization temperatures of both clinopyroxene and orthopyroxene phenocrysts estimated by using pyroxene thermometry are about 800°C (Mg-poor pairs) and 1000°C (Mg-rich pairs). These characteristics of phenocrysts strongly suggest that the Gassan younger stage lavas are the products of mixing between silicic (ca. 800°C) and minor amounts of mafic (ca.1000°C) magmas.

Mafic inclusions in the silica-rich rocks of the Tolfa-Ceriti-Manziana volcanic district (Tuscan Province, Central Italy): Chemistry and mineralogy

A chemical and mineralogical study of magmatic inclusions in the silica-rich rocks of the Tolfa-Ceriti-Manziana sector (Tuscan Province, Central Italy) shows that they can be grouped according to their degree of alkalinity. We distinguish: TCM latites (TCML) with an alkaline-potassic affinity; Ceriti latites (CL) with calc-alkaline affinity; and Manziana shoshonites and latites (MS-ML) with an affinity intermediate between alkaline and subalkaline. The latter are hybrid rocks originating from the mixing of two magmas with slightly different geochemical affinities. These magmas may represent liquids derived by partial melting of a heterogeneous mantle, metasomatized by a crustal source. The most alkaline magmas (TCML) are associated with the largest degree of metasomatism which, if more pronounced, could lead to the production of the potassic magmas of the Roman Province.

Small-scale convection at the interface between stratified layers of mafic and silicic magma, Campbell Ridges, NW Palmer Land, Antarctic Peninsula: Syn-magmatic way-up criteria

Experimental data suggest that some igneous mafic inclusions are formed at the interface between underlying mafic magma and more silicic magma in a reservoir by a process of gas exsolution and vesiculation. We report cm-scale plume-like structures exposed over several m 2 at a candidate interface in a large composite pluton from NW Palmer Land, Antarctic Peninsula and suggest that, as well as representing ‘frozen’ mafic inclusion formation, plume-like structures of this type can be used as way-up criteria and may be of particular value in the interpretation of palaeomagnetic data.

Jurassic plutonism and crustal evolution in the central Mojave Desert, California

Middle to Late Jurassic plutonic rocks in the central Mojave Desert represent the continuation of the Sierran arc south of the Garlock fault. Rock types range from calc-alkaline gabbro to quartz monzonite. Chemical and isotopic data indicate that petrologic diversity is attributable to mixing of crustal components with mantle melts. Evidence for magma mixing is scarce in most plutons, but emplacement and injection of plutons into preexisting wallrocks (e.g. pendants of metasedimentary rocks) suggests that assimilation may be locally important. Field and petrographic evidence and major and trace element data indicate that the gabbros do not represent pure liquids but are, at least partly, cumulates. The cumulate nature of the gabbros coupled with field evidence for open-system contamination means that trace element contents of gabbros cannot be used to fingerprint the Jurassic mantle source, nor can isotopic data be unequivocally interpreted to reflect the isotopic composition of the mantle. Correlation of Sr and Nd isotropic composition with bulk composition allows some constraints to be placed on the mantle isotopic signature. Gabbros and mafic inclusions from localities north of Barstow, CA have the most depleted mantle-like isotopic signatures (Sr(i)≈0.705 and ɛNd(t)=≈0 to +1). However, these rocks have likely seen some contamination as well, so the mantle source probably has an even more depleted character. Gabbros with the lowest Sr((i) and highest ɛNd(t) are also characterized by the highest 207Pb/204Pb and 206Pb/204Pb in the entire data set. This may be a feature of the mantle component in the Jurassic arc indicative of minor source contamination with subducted sediment as has been observed in modcrn continental arcs. Locally exposed Precambrian basement and metasedimentary rocks have appropriate Sr, Nd and Pb isotopic signatures for the crustal end members and are possible contaminants. Incorporation of these components through combined anatexis and assimilation can explain the observed spread in isotopic composition. Evidence for a depleted mantle component in these gabbros contrasts with the enriched subcontinental mantle component in Jurassic arc plutons further to the east and suggests there may have been a major mantle lithosphere boundary between the two areas as far back as the Late Jurassic. Crustal boundaries and isotopic provinces defined on the basis of initial isotopic composition (Sr(i)=0.706 isopleth) are difficult to delineate because of the correlation of bulk composition with Sr and Nd isotopic composition and because values may differ depending on the age of the rocks sampled within a given area. Data from plutons intruded into rocks known or inferred to be Precambrian are, however, shifted dramatically (highest Sr(i) and lowest ɛNd(t) toward Precambrian values. The least isotopically evolved rocks (lowest Sr(i) and highest ɛNd(t)) occur within the eugcoclinal belt of the Mojave Desert. This zone has been previously identified as a Precambrian rift zone but more likely represents a zone where mantle magmas have been intruded into isotopically similar crustal rocks of the eugeocline with minor input from old Precambrian crust.

Ophiolitic and metamorphic rocks in the Percy Isles and Shoalwater Bay region, New England Fold Belt, central Queensland

Ophiolitic and metamorphic rocks of the eastern part of the New England Fold Belt in the Shoalwater Bay region and the Percy Isles are grouped in the Marlborough and Shoalwater terranes, respectively. Marlborough terrane units occur on South Island (Percy Isles) and comprise the Northumberland Serpentinite, antigorite serpentinite with rodingite and more silicic dykes and mafic inclusions, the Chase Point Metabasalt, some 800+ metres of pillow lava, and the intervening South Island Shear Zone containing fault‐bounded slices of mafic and ultramafic igneous rocks, schist, and volcaniclastic sedimentary rocks, and zones of melange. The Shoalwater terrane, an ancient subduction complex, consists of the Shoalwater Formation greenschist facies metamorphosed quartz sandstone and mudstone on North East Island and on the mainland at Arthur Point, the Townshend Formation, amphibolite‐grade quartzite, schist and metabasalt on Townshend Island, and the Broome Head Metamorphics on the western side of Shoalwater Bay, u...

G4 北海道中央部, トムラウシ火山の不均質溶岩流から推定したマグマ溜り内での苦鉄質包有物の分布パターン

G4 北海道中央部, トムラウシ火山の不均質溶岩流から推定したマグマ溜り内での苦鉄質包有物の分布パターン

The role of fractional crystallisation, crustal melting and magma mixing in the petrogenesis of rhyolites and mafic inclusion-bearing dacites from the Monte Arci volcanic complex (Sardinia, Italy)

Rhyolitic lavas and mafic inclusion-bearing dacites (MIBD) form the dominant products of the Monte Arci volcanic complex, one of the most active sites of volcanic activity in Sardinia during the Pliocene. The massif is composed of four distinct eruptive episodes (Phase 1: rhyolites; Phase 2: dacites and andesites; Phase 3: quartz-normative trachytes; Phase 4: mafic lavas ranging from subalkaline to mildly alkaline). Monte Arci magmatism has been characterized by open-system behaviour, with both mantle and crustal contributions and magma mixing. Although the mafic products are restricted to the latest stage of activity, mafic inclusions are quite common in many rhyolites and dacites. The mineral assemblages of the inclusions are dominated by plagioclase + orthopyroxene ± augite, with minor olivine, FeTi oxides and variable amounts of residual trapped liquid giving rise to a fine-grained groundmass. They represent blobs of magma entrained in a partly molten state and provide evidence of a basaltic contribution to the petrogenesis of their enclosing lavas, both as parental magmas or as a source of heat for partial melting of crustal rocks. Rhyolites are metaluminous to slightly peraluminous rocks with a wide range of SiO 2 contents (67–75%). They fall into two groups: (1) Mafic inclusion-bearing rhyolites (MIBR). These have initial 87 Sr 86 Sr ratios between 0.70526 and 0.70897 and commonly show relatively high values of Ti, Mg, Fe, Ca, Sr and compatible trace elements (Ni, Cr, V); cordierite-bearing, highly restitic, crustal xenoliths are rare. (2) Inclusion-free rhyolites (IFR). They include both porphyritic lavas (IFR1) and obsidians (IFR2, IFR3), which share low values of Ti, Fe, Mg, Ca, Sr and are strongly depleted in compatible trace elements. IFR1–2 have a marked LREE/HREE fractionation and a narrow range of 87 Sr 86 Sr ratios (0.70885–0.70972), whereas IFR3 have larger Sr isotopic ratios (0.71529–0.71553), low Th, Zr, Hf, LREE contents and low LREE/HREE. Nd isotopic composition is quite uniform for all rhyolite types, with relatively low 143 Nd 144 Nd ratios (0.51216–0.51228, ϵ Nd ranging between −9.3 and −6.9). MIBR appear to be the product of fractionation of subalkaline (tholeiitic) magmas + assimilation of a crustal component moderately enriched in 87Sr and/or with low Sr content. IFR1–2 may be derived from partial melting of moderately 87Sr-rich crustal lithologies with retention of garnet and plagioclase (+K-feldspar) in the restite, although AFC processes (followed by effective separation from the basalts) cannot be excluded. Obsidians IFR3 need a crustal source with higher 87 Sr 86 Sr ; their peculiar geochemical features most probably reflect the complex behaviour during partial melting of accessory minerals carrying REE, Th, Zr, Hf. Among the eruptive products of Phase 2, mafic inclusion-bearing dacites (MIBD) provide evidence of basalt-rhyolite mixing: (a) coexistence of glasses with SiO 2 content ranging from 64 to 72 wt.%; (b) a phenocryst assemblage including phases of basaltic and rhyolitic provenance; and (c) abundant crystal-rich inclusions related to a basaltic intrusion. The extrusion of these lavas has been preceeded by an explosive eruption of rhyolitic tuffs ( 87 Sr 86 Sr initial ratio=0.71097 ). The most satisfactory petrogenetic model for MIBD includes: (1) differentiation of subalkaline (tholeiitic) magma to a silica-poor and Ti-Fe-P rich liquid; (2) partial melting of wall-rocks yielding a rhyolitic magma segregated at the top of the basaltic reservoir; and (3) explosive eruption of the uppermost portion of the magma chamber followed by mixing of rhyolite with the underlying part of the system to form a hybrid dacite.

Isotopic Monitoring of Magma Mingling at Chaos Crags, Lassen Volcanic Park, Ca

Twenty years of high precision bulk geochemical analyses, while broadening our overall understanding as to the role of contributing components, has failed to provide a complete model for magma genesis at convergent margins. In particular, magma chamber processes have yet to be fully understood and quantified. Using Chaos Crags in the Lassen Volcanic National Park as an example, we propose to achieve a quantitative understanding of the mingling process, its effects on bulk chemistry, and its overall importance to the sustained evolution of the southernmost volcanic edifice of the Cascade Range. Chaos Crags is composed of at least two coexisting magmatic components now represented as mafic inclusions (the mafic component) in a silicic host (the differentiated componen O. The presence of host crystals in the inclusions and the coexistence of fresh and resorbed phenocryst phases indicates that phenocryst populations have been exchanged between inclusion-forming and

Nd isotopic evidence for transient, highly depleted mantle reservoirs in the early history of the Earth

3870 Ma to 3760 Ma metadiorites and tonalites, and older mafic inclusions contained within these rocks from southern West Greenland, have a range of initial ε Nd values from +4.5 to −4.5. The extremely positive initial values determined for nine of the fourteen early Archean samples demonstrate that they were derived from a LREE depleted mantle reservoir which had an ε Nd value of ≈ +4 prior to 3800 Ma. These rocks provide the best constrained evidence for the existence of highly LREE fractionated mantle reservoirs in the early Earth. In contrast, previous studies of younger 3700 and 3730 Ma tonalites from the same region show that the latter have lower initial ε Nd values ranging from −4.6 to +2.0. The highest initial ε Nd values determined for 3450 Ma komatiites from western Australia and southern Africa are +2 to +4, and for worldwide ca. 2700 Ma greenstone belts are +2 to +5. These values are well below the values for ε Nd(3450 Ma) (> +8) and ε Nd(2700 Ma) (> +15) that would be expected for the continued evolution of a highly depleted reservoir with Sm/Nd similar to that which produced the ca. 3.8 Ga Greenland metadiorites and tonalites. Thus, if the source region for the oldest Greenland gneisses was the prevalent upper mantle composition, it must have been an ephemeral feature later modified either by mixing with less-depleted mantle or with recycled LREE enriched, negative ε Nd crust. The generation of highly positive ε Nd values by 3.8 Ga requires differentiation of an extremely LREE fractionated reservoir very early in Earth's history. The Archean Nd isotope data may record the isolation, depletion by crustal extraction and subsequent partial rehomogenization of limited portions of the upper mantle, or alternatively may reflect transient large-scale differentiation processes unrelated to crustal extraction such as might occur in a terrestrial magma ocean.

Mixing of stratified liquids by the motion of gas bubbles: application to magma mixing

Mafic magma underlying more silicic magma in a reservoir can exsolve volatiles as it crystallizes, forming bubbles that segregate upwards to the interface with the overlying silicic magma. We describe laboratory experiments designed to investigate how gas bubbles migrate into the more viscous upper layer. In one regime, bubbles move individually across the interface, entraining a small quantity of lower layer fluid and the two layers become progressively stirred into a homogeneous mixture. In a second regime, bubbles form a thin foam layer at the interface which becomes gravitationally unstable, giving rise to two-phase plumes which rise and produce a coarse mixture of the liquids. For a given viscosity ratio, the transition between these regimes occurs when the gas flux is greater than a critical value, which we found to decrease with increasing viscosity ratio. We estimate the critical gas flux for magmatic conditions, and how vesiculation may vary with pressure, degree of crystallization and the composition of volatile components. Mafic inclusions, which are often found in silicic to intermediate lavas, can form in the plume regime; analysis of the instability of the foam predicts wavelengths of between 1 cm and 1 m, which is consistent with the observed sizes of inclusions. Volatiles from the mafic magma may be efficiently transported by this mechanism into the viscous silicic magma as bubble plumes despite low Stokes' velocities for individual vesicles. At small viscosity ratios or low gas fluxes, the bubbling regime can produce hybrid magmas.