Magmatic Groups Research Articles

Nickel isotopic anomalies in troilite from iron meteorites

We measured nickel isotopes, via multicollector inductively coupled plasma mass spectrometry (MC‐ICPMS), in troilite (FeS) from ten iron meteorites from both non‐magmatic (IAB) and magmatic (IIAB, IIIAB, IVA) groups. These are the first reported measurements of 64Ni (the least abundant Ni isotope) in meteoritic sulfides. No excesses of 60Ni from the decay of the short‐lived radionuclide 60Fe (t1/2 = 1.49 My) were found in any of the samples. Resolvable deficits in 60Ni were observed in five samples. These deficits are inconsistent with the effects from 60Fe decay, given the Fe/Ni ratios in these troilites. Also, variations were found in 61Ni and 64Ni in several samples. The variations in Ni isotopic compositions of these samples may be due to the preservation of a component within the troilite that is characterized by the nucleosynthetic signature expected for Ni produced either by a type II supernova or by an AGB star.

Application of the Crosta technique for alteration mapping of granitoidic rocks using ETM+ data: case study from eastern Tauride belt (SE Turkey)

The present study particularly focuses on the determination of alteration products of the granitoid associations by remote sensing techniques. Widespread magmatism has occurred in the Malatya‐Elazığ area of the eastern Tauride belt in south east Turkey as a consequence of plate convergence and continental collision. The development of the magmatism in the study area can be subdivided into two separate phases, the Baskil and the Bilaser Tepe complexes. Based on the petrographic and bulk‐rock geochemical data, the Baskil granitoids are classified as quartz‐diorite, quartz‐monzodiorite and tonalite, whereas the Bilaser Tepe granitoids are classified as granodiorite and dacite‐porphyry. Both the Baskil and Bilaser Tepe granitoids have peraluminous and metaluminous compositions. The Crosta method is found to be very useful for enhancing the altered areas with hydroxyl and iron oxide minerals. The Crosta method is applied to six (1, 2, 3, 4, 5 and 7) and two sets of four (1, 3, 4, 5 and 1, 4, 5, 7) bands of ETM+ data. However, the areas with potassic, phyllic, argillic and propylitic alteration zones are enhanced much better by using six bands of Landsat 7 ETM+ data. The alteration differences of these two magmatic groups are also tested with the Crosta technique using four band combinations. Although no distinct difference is observed on the hydroxyl image, a clear difference is observed between the two magmatites on the iron oxide image. The present study shows that the Crosta technique is not only generating hydroxyl and iron oxide images, but also differentiates between two magmatite groups of different ages and genesis.

Formation of IIAB iron meteorites

Group IIAB is the third largest group of iron meteorites and the second largest group that formed by fractional crystallization; many of these irons formed from the P-rich portion of a magma consisting of two-immiscible liquids. We report neutron-activation data for 78 IIAB irons. These confirm earlier studies showing that the group has the largest known range in Ir concentrations (a factor of 4000) and that slopes are steeply negative on plots of Ir vs. Au or As (or Ni). High negative slopes imply relatively high distribution coefficients for Ir, Au, and As (but, with rare exceptions, remaining less than unity for the latter). IIAB appears to have had the highest S contents of any magmatic group of iron meteorites, consistent with its high contents of other volatile siderophiles, particularly Ga and Ge. Large fractions of trapped melt were present in the IIAB irons with the highest Au and As and lowest Ir contents. As a result, when these irons crystallized, the D Au and D As values can, with moderate accuracy, be estimated to have been roughly 0.53 and 0.46, respectively. These low values imply that the initial nonmetal (S + P) content of the magma was much lower than 170 mg/g, as estimated in earlier studies; our estimate is 75 mg/g. Our results are consistent with an initial P/S ratio of 0.25, similar to the ratio estimated for other magmatic groups. There is little doubt that incompatible S-rich and P-rich metallic liquids were involved during the formation of group IIAB. After 20% crystallization of our assumed starting composition the two-liquid boundary is encountered (at 72 mg/g S and 18 mg/g P). Initially the volume of S-rich liquid is very small, but continued crystallization increased the volume of this phase and decreased its P/S ratio while increasing this ratio in the P-rich liquid. Most crystallization of the IIAB magma would have occurred in the lower, P-rich portion of the core. However, metal was still a liquidus phase at the top of the core and, because both the immiscible liquids would have convected, they may have approached equilibrium throughout the very limited crystallization of the magma recorded in group IIAB. All IIAB irons contain trapped melt, and this melt will have had very different compositions depending on whether the liquid is S-rich (at the outer solid/liquid interface) or P-rich (at the inner interface). The P/S ratio in the melt trapped in the Santa Luzia iron is about 0.6 g/g, consistent with our modeling of Ir–Au and Ir–As trends implying that Santa Luzia formed in the lower, P-rich portion of the core after about 48% crystallization of the magma. Because the liquids were in equilibrium, the point at which immiscibility first occurred is not recorded by a dramatic change in the trends on element–Au diagrams; the main compositional effect is recorded in the P/S ratio of the trapped melt. The high-Au (>0.8 μg/g) irons for which large sections are available all contain skeletal schreibersite implying a relatively high (>0.3 g/g) P/S ratio; none of these irons could have crystallized from the S-rich upper layer of the core.

Isotopic and geochemical evidence of proterozoic episodic crustal reworking within the irumide belt of south-central Africa, the southern metacratonic boundary of an Archaean Bangweulu Craton

Whole-rock geochemistry and Sr–Nd isotopic data for granitoids and volcanic rocks of four main different igneous phases, the Usagaran phase (2.05–1.93 Ga), the Ubendian phase (1.88–1.85 Ga), the Lukamfwa phase (1.65–1.55 Ga) and the Irumide phase (1.05–0.95 Ga), recognised along the southern margin of the Congo Craton in the Bangweulu Block and Irumide Belt of Zambia, demonstrate a long history of crustal recycling of a cryptic Archaean basement complex. The isotopic record indicates that a largely similar crustal source can be assigned to all these magmatic phases, with subtle differences in isotopic and geochemical record reflecting varying distance to orogenic activity during each of these episodes, and varying, but always minor amounts of juvenile mantle input. T DM model ages, ranging between 3.3 and 2.8 Ga for the granitoids, and between 2.9 and 2.4 Ga for volcanic rocks, indicate preponderant Archaean crust within the Bangweulu Block and within the Irumide Belt. The corresponding initial ɛ Nd values vary between −6 and −15 for the granitoids, and between −2 and −7 for the felsic volcanic rocks. Only some of the ca. 1.85 Ga mafic volcanic units record a more juvenile character ( ɛ Nd( T) between −5 and 0 and T DM model ages between 3.2 and 2.4 Ga) but still with a significant old crust input. The geochemistry (major and trace elements) of the various magmatic groups shares many similarities indicating essentially similar crustal sources and conditions of melting. Only the anorogenic, 1.6 Ga, more alkaline granitoids, were generated through a lower degree of partial melting, but still from the same source. Combining all available constraints, we propose that the Irumide Belt corresponds to the recurrently destabilised (at 2, 1.85, 1.6 and 1 Ga) southern boundary of the Bangweulu Block, whose preserved nucleus appears to be an Archaean craton covered by Palaeoproterozoic sediments. There are no juvenile subduction-related rocks in the Irumide Belt s.s., but such rocks are reported to the south, across the Mwembeshi Dislocation Zone (MDZ), in a region referred to as the Southern Irumide Belt. Processes of endo-destabilisation in response to external forces (orogenies acting close or elsewhere), such as that recorded in the Bangweulu Block and the Irumide Belt s.s., correspond to a metacratonic evolution. Within the Irumide Belt itself, tectonic movements were mainly vertical, with horizontal movements restricted mainly to the supracrustal sequences.

Compositional trends among IID irons; their possible formation from the P-rich lower magma in a two-layer core

Group IID is the fifth largest group of iron meteorites and the fourth largest magmatic group (i.e., that formed by fractional crystallization). We report neutron-activation data for 19 (of 21 known) IID irons. These confirm earlier studies showing that the group has a relatively limited range in Ir concentrations, a factor of 5. This limited range is not mainly due to incomplete sampling; Instead, it seems to indicate low solid/liquid distribution coefficients reflecting very low S contents of the parental magma, the same explanation responsible for the limited range in group IVA. Despite this similarity, these two groups have very different volatile patterns. Group IVA has very low abundances of the volatile elements Ga, Sb and Ge whereas in group IID Ga and Sb abundances are the highest known in a magmatic group of iron meteorites and Ge abundances are the second highest (after group IIAB). Group IID appears to be the only large magmatic group having high volatile abundances but low S. In the volatile-depleted groups IVA and IVB it is plausible that S was lost as a volatile from a chondritic precursor material. Because group IID seems to have experienced minimal loss of volatiles, we suggest that S was lost as an early melt having a composition near that of the Fe–FeS eutectic (∼315 mg/g S). When temperatures had risen 400–500 K higher P-rich melts formed, became gravitationally unstable, and drained through the first melt to form an inner core that was parental to the IID irons. As discussed by [Kracher, A., Wasson, J.T., 1982. The role of S in the evolution of the parental cores of the iron meteorites. Geochim. Cosmochim. Acta 46, 2419–2426], it is plausible that a metal-rich inner core and a S-rich outer core could coexist metastably because stratification near the interface permitted only diffusional mixing. The initial liquidus temperature of the inner, P-rich core is estimated to have been ∼1740 K; after >60% crystallization the increase in P and the decrease in temperature may have permitted immiscibility with the S-rich outer core. We have not recognized samples of the outer core.

Silica and pyroxene in IVA irons; possible formation of the IVA magma by impact melting and reduction of L-LL-chondrite materials followed by crystallization and cooling

Group IVA is a large magmatic group of iron meteorites. The mean Δ 17O (=δ 17O − 0.52·δ 18O) of the silicates is ∼+1.2‰, similar to the highest values in L chondrites and the lowest values in LL chondrites; δ 18O values are also in the L/LL range. This strongly suggests that IVA irons formed by melting L-LL parental material, but the mean Ni content of IVA irons (83 mg/g) is much lower than that of a presumed L-LL parent (∼170 mg/g) and the low-Ca pyroxene present in two IVA meteorites is Fs13, much lower than the Fs20-29 values in L and LL chondrites. Thus, formation from L-LL precursors requires extensive addition of metallic Fe, probably produced by reduction of FeS and FeO. Group IVA also has S/Ni, Ga/Ni, and Ge/Ni ratios that are much lower than those in L-LL chondrites or any chondrite group that preserves nebular compositions, implying loss of these volatile elements during asteroidal processing. We suggest that these reduction and loss processes occurred near the surface of the asteroid during impact heating, and resulted partly from reduction by C, and partly from the thermal dissociation of FeS and FeO with loss of O and S. The hot (∼1770 K) low-viscosity melt quickly moved through channels in the porous asteroid to form a core. Two members of the IVA group, São João Nepomuceno (hereafter, SJN) and Steinbach, contain moderate amounts of orthopyroxene and silica, and minor amounts of low-Ca clinopyroxene. Even though SJN formed after ∼26% crystallization and Steinbach formed after ∼77% crystallization of the IVA core, both could have originated within several tens of meters of the core-mantle interface if 99% of the crystallization occurred from the center outwards. Two other members of the group (Gibeon and Bishop Canyon) contain tabular tridymite, which we infer to have initially formed as veins deposited from a cooling SiO-rich vapor. The silicates were clearly introduced into IVA irons after the initial magma crystallized. Because the γ-iron crystals in SJN are typically about 5 cm across, an order of magnitude smaller than in IVA irons that do not contain massive silicates, we infer that the metal was in the γ-iron field when the silicates were injected. The SJN and Steinbach silicate compositions are near the low-Ca-pyroxene/silica eutectic compositions. We suggest that a tectonic event produced a eutectic-like liquid and injected it together with unmelted pyroxene grains into fissures in the solid metal core. Published estimates of IVA metallographic cooling rates range from 20 to 3000 K/Ma, leading to a hypothesized breakup of the core during a major impact followed by scrambling of the core and mantle debris [Haack, H., Scott, E.R.D., Love, S.G., Brearley, A. 1996. Thermal histories of IVA stony-iron and iron meteorites: evidence for asteroid fragmentation and reaccretion. Geochim. Cosmochim. Acta 60, 3103–3113]. This scrambling model is physically implausible and cannot explain the strong correlation of estimated cooling rates with metal composition. Previous workers concluded that the low-Ca clinopyroxene in SJN and Steinbach formed from protopyroxene by quenching at a cooling rate of 10 12 K/Ma, and suggested that this also supported an impact-scrambling model. This implausible spike in cooling rate by a factor of 10 10 can be avoided if the low-Ca clinopyroxene were formed by a late shock event that converted orthopyroxene to clinopyroxene followed by minimal growth in the clinopyroxene field, probably because melt was also produced. We suggest that metallographic cooling-rate estimates (e.g., based on island taenite) giving similar values throughout the metal compositional range are more plausible, and that the IVA parent asteroid can be modeled by monotonic cooling followed by a high-temperature impact event that introduced silicates into the metal and a low-temperature impact event that partially converted orthopyroxene into low-Ca clinopyroxene.

Tungsten isotopic compositions of iron meteorites: Chronological constraints vs. cosmogenic effects

High-precision W isotopic compositions are presented for 35 iron meteorites from 7 magmatic groups (IC, IIAB, IID, IIIAB, IIIF, IVA, and IVB) and 3 non-magmatic groups (IAB, IIICD, and IIE). Small but resolvable isotopic variations are present both within and between iron meteorite groups. Variations in the 182W/184W ratio reflect either time intervals of metal–silicate differentiation, or result from the burnout of W isotopes caused by a prolonged exposure to galactic cosmic rays. Calculated apparent time spans for some groups of magmatic iron meteorites correspond to 8.5 ± 2.1 My (IID), 5.1 ± 2.3 My (IIAB), and 5.3 ± 1.3 My (IVB). These time intervals are significantly longer than those predicated from models of planetesimal accretion. It is shown that cosmogenic effects can account for a large part of the W isotopic variation. No simple relationship exists with exposure ages, compromising any reliable method of correction. After allowance for maximum possible cosmogenic effects, it is found that there is no evidence that any of the magmatic iron meteorites studied here have initial W isotopic compositions that differ from those of Allende CAIs [ε 182W = − 3.47 ± 0.20; [T. Kleine, K. Mezger, H. Palme, E. Scherer and C. Münker, Early core formation in asteroids and late accretion of chondrite parent bodies: evidence from 182Hf–182W in CAIs, metal-rich chondrites and iron meteorites, Geochim. Cosmochim. Acta (in press)]. Cosmogenic corrections cannot yet be made with sufficient accuracy to obtain highly precise ages for iron meteorites. Some of the corrected ages nevertheless require extremely early metal–silicate segregation no later than 1 My after formation of CAIs. Therefore, magmatic iron meteorites appear to provide the best examples yet identified of material derived from the first planetesimals that grew by runaway growth, as modelled in dynamic simulations. Non-magmatic iron meteorites have a more radiogenic W isotopic composition than magmatic ones, even without cosmogenic corrections. This indicates that most of the IAB irons formed between 5 ± 3 and 11 ± 6 My after Allende CAIs. Similarly, the IIE irons formed between 9 ± 4 and 14 ± 5 My after the start of the solar system. Unlike IABs and IIEs, IIICDs do not show any resolvable W isotopic differences relative to Allende CAIs.

Hf–W evidence for rapid differentiation of iron meteorite parent bodies

Abstract New, high-precision W isotope data on iron meteorites are presented that provide important constraints on the timing of silicate–metal segregation in planetesimals. Magmatic iron meteorites all have e 182 W within error or less radiogenic than initial e 182 W estimated by studies of chondritic meteorites. At face value this implies that iron meteorites are as old and older than refractory calcium–aluminium rich inclusions (CAI), which are widely thought to be the oldest solar system objects. Moreover, different meteorites from the same magmatic groups, believed to be derived from the same planetissimal core, display a range of e 182 W. We suggest that the paradoxical e 182 W values more negative than initial Solar System Initial (SSI) are most readily explained as a result of secondary, spallation reactions with cosmic rays during transit between parent body and the earth. This is supported by the most negative e 182 W being found in meteorites with the oldest exposure ages and the magnitude of the effect is shown to be consistent with known nuclear reactions. On the other hand, it is also striking that none of the magmatic iron group meteorites have e 182 W analyses, outside error, more radiogenic than the estimated solar system initial ratio. This suggests that core formation in parent bodies of magmatic iron meteorites occurred ≤ 1.5 Myr after the formation age of CAI [Y. Amelin, A.N. Krot, I.D. Hutcheon, and A.A. Ulyanov, Lead isotopic ages of chondrules and calcium-aluminum inclusions, Science 297, 1678–1683, 2002]. This extremely early metal–silicate differentiation is coeval with the first chondrules [M. Bizzarro, J.A. Baker, and H. Haack, Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions, Nature 431, 275–278, 2004, A.N. Krot, Y. Amelin, P. Cassen, and A. Meibom, Young chondrules in CB chondrites from a giant impact in the early Solar System, Nature 436, 989–992, 2005]. Formation of later chondrules, and hence the parent bodies of some chondritic meteorites, must therefore have occurred in the presence of planetesimals large enough to possess iron cores. We conclude that early planetary accretion and differentiation was sufficiently fast for 26 Al-decay to be an important heat source. Non-magmatic iron meteorites, however, display more radiogenic and varied W isotope signatures. This is in keeping with them being generated later, by impact melting during which the metal (partially) re-equilibrated with the then more radiogenic silicate fraction.

K–Ar and 40Ar/ 39Ar ages of dikes emplaced in the onshore basement of the Santos Basin, Resende area, SE Brazil: implications for the south Atlantic opening and Tertiary reactivation

New K–Ar and 40Ar/ 39Ar data of tholeiitic and alkaline dike swarms from the onshore basement of the Santos Basin (SE Brazil) reveal Mesozoic and Tertiary magmatic pulses. The tholeiitic rocks (basalt, dolerite, and microgabbro) display high TiO 2 contents (average 3.65 wt%) and comprise two magmatic groups. The NW-oriented samples of Group A have (La/Yb) N ratios between 15 and 32.3 and range in age from 192.9±2.2 to 160.9±1.9 Ma. The NNW-NNE Group B samples, with (La/Yb) N ratios between 7 and 16, range from 148.3±3 to 133.9±0.5 Ma. The alkaline rocks (syenite, trachyte, phonolite, alkaline basalts, and lamprophyre) display intermediate–K contents and comprise dikes, plugs, and stocks. Ages of approximately 82 Ma were obtained for the lamprophyre dikes, 70 Ma for the syenite plutons, and 64–59 Ma for felsic dikes. Because Jurassic–Early Cretaceous basic dikes have not been reported in SE Brazil, we might speculate that, during the emplacement of Group A dikes, extensional stresses were active in the region before the opening of the south Atlantic Ocean and coeval with the Karoo magmatism described in South Africa. Group B dikes yield ages compatible with those obtained for Serra Geral and Ponta Grossa magmatism in the Paraná Basin and are directly related to the breakup of western Gondwana. Alkaline magmatism is associated with several tectonic episodes that postdate the opening of the Atlantic Ocean and related to the upwelling of the Trindade plume and the generation of Tertiary basins southeast of Brazil. In the studied region, alkaline magmatism can be subdivided in two episodes: the first one represented by lamprophyre dykes of approximately 82 Ma and the second comprised of felsic alkaline stocks of approximately 70 Ma and associated dikes ranging from 64 to 59 Ma.

Geochemistry and tectonic setting of the Central Loei volcanic rocks, Pak Chom area, Loei, northeastern Thailand

The Central Loei volcanic rocks, as evidenced by those in the Pak Chom area, were formed in the Late Devonian–Early Carboniferous and can be separated into three magmatic groups: transitional tholeiitic basalt, tholeiitic microgabbro and calc-alkalic basalt/andesite on the basis of immobile-element contents and ratios of least altered samples. All the tholeiitic microgabbro possibly occurred as dikes. Chemically, the transitional tholeittic basalt and tholeiitic microgabbro have higher abundances of TiO 2, Ni and Cr relative to the calc-alkalic basalt/andesite at similar values for FeO*/MgO; they also contain higher Ti/Zr but lower Zr/Nb. The transitional tholeiitic basalt has higher concentrations of P 2O 5 and Nb relative to the tholeiitic microgabbro at similar levels of FeO*/MgO, and also has higher ratios of Nb/Y and Ti/V, but lower values for Ti/Zr and Zr/Nb. In terms of chondrite normalized REE and N-MORB normalized patterns, the transitional tholeiitic basalt, tholeiitic basalt and calc-alkalic basalt/andesite are analogous to those from North Atlantic, Southwest Indian Ridge and New Britain Arc. On this basis, the Central Loei volcanic rocks are comprised of MORBs and oceanic island-arc lavas. These arc lavas may have erupted on an oceanic basement in the same ocean basin as those in the Chiang Rai–Chiang Mai volcanic belt.

Mid-ocean ridge and supra-subduction affinities in the Pindos ophiolites (Greece): implications for magma genesis in a forearc setting

The Pindos ophiolitic massif is considered an important key area within the Albanide–Hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units: (1) a lower unit, including an intrusive and a volcanic section; and (2) an Upper Ophiolitic Unit, mainly including mantle harzburgites. Both units share similar metamorphic soles and tectono-sedimentary mélanges at their bases. The intrusive section of the lower unit is composed by an alternation of troctolites with various ultramafic rock-types, including dunites, lherzolites, olivine-websterites, olivine-gabbros, anorthositic gabbros, gabbros and rare gabbronorites. The volcanic and subvolcanic sequence of the lower unit can geochemically be subdivided into three groups of rocks: (1) basalts and basaltic andesites of the lower pillow section showing a clear high-Ti affinity; (2) basaltic andesites of the upper pillow section with high-Ti affinity, but showing many geochemical differences with respect to the first group; (3) very low-Ti (boninitic) basaltic and basaltic andesitic lava flows separating the lower and upper pillow sections, and dykes widespread throughout the Pindos ophiolites. These different magmatic groups originated from fractional crystallization from different primary magmas, which were generated, in turn, from partial melting of mantle sources progressively depleted by previous melt extractions. Group 1 volcanics may have derived from partial melting (ca. 20%) of an undepleted lherzolitic source, while group 2 basaltic rocks may have derived from partial melting (ca. 10%) of a mantle that had previously experienced mid-ocean ridge basalt (MORB) extraction. Finally, the Group 3 boninites may have derived from partial melting (ca. 12–17%) of a mantle peridotite previously depleted by primary melt extraction of Groups 1 and 2 primary melts. In order to explain the coexistence of these geochemically different magma groups, two petrogenetic models formerly proposed for the Albanian ophiolites are discussed.

Calc-alkaline Arc I-type Granitoid Associated with S-type Granite in the Pan-African Belt of Eastern Anti-Atlas (Saghro and Ougnat, South Morocco)

The Sidi Flah and Ougnat inliers are located in the eastern Anti-Atlas antiform between the Anti-Atlas Major Fault (AAMF) and South Atlas Fault (SAF). They consist of many granitoid intrusions emplaced into Neoproterozoic metasedimentary rocks and surmounted by upper Neoproterozoic A-type granites. The Sidi Flah (Saghro) and Ougnat granitoids are part of the Neoproterozoic magmatic activity related to northwards subduction of an oceanic plate beneath the Saghro continental margin. They are post-orogenic I- and S-type granitoids related to the ending of the compressional deformation in this Pan-African belt. A petrographic, geochemical and zircon typology study leads us to subdivide these rocks into three magmatic groups: (1) a medium- to high-K calc-alkaline group formed by quartz diorites and amphibole granodiorites is found in both Sidi Flah and Ougnat inliers; (2) a high-K calc-alkaline group is present in Sidi Flah. These two groups have a (deeper and) hybrid mantle-crust origin; (3) a peraluminous group in Ougnat is linked to the post-collisional setting and has a shallow crustal source. On a primitive mantle-normalized trace-element diagram, almost all of these rocks show a significant Nb depletion relative to K and La, which is typical of the calc-alkaline magmatism from the subduction-zone environment. Absence of structural marks of thrusting upon the West African craton (WAC) of this arc system and the ophiolitic suite in Bou-Azzer, and the presence of Imiter muscovite-bearing granite as part of Pan-African belt do not support the localization of northern limit of WAC at the level of SAF.

Coarse-grained and porphyritic inclusions in phonolitic lavas, Roccamonfina volcano, Italy

The inclusions in the 446–213-ka phonolite lavas of the Roccamonfina volcano (Italy) can be classified as coarse-grained and porphyritic types. Both are wide-varying in composition. The coarse-grained inclusions belong to a cumulate clinopyroxenite–gabbro–monzonitic clinopyroxenite–monzonitic gabbro series, and contain some interstitial, vesicular glass. The porphyritic inclusions show non-cumulate structures, and a coarse-grained, crystalline, vesicular matrix. They correspond to tephrites, phonolitic tephrites, trachybasalts, and basaltic trachyandesites. The glass in the coarse-grained inclusions is similar in composition to the porphyritic inclusions, but in part it deviates compositionally from the latter. These glasses are referred to, respectively, as trapped interstitial liquid from which the cumulates crystallized, and as injected, ‘foreign’ (non-comagmatic) melt. The phonolites vary in composition from tephritic phonolites to phonolites and nepheline phonolites, and do not show any traces of vesicularity. Despite the large eruptive time span, and the broad differentiation, they constitute a coherent magmatic group. Mineralogical and chemical similarities between both inclusion categories, and compositional similarity of the trapped glass to the porphyritic types, suggest that both groups of inclusions are cogenetic. By contrast, deviations of many elements of this glass and of the inclusions from the ideal mixing line inclusions–host phonolites rule out any genetic correlation among these rocks. Least-squares mass-balance calculations reinforce this lack of correlation. This conclusion is also supported by the fact that the phonolites are the most evolved derivatives of the basic and middle terms of the Roccamonfina high-K series, whereas the inclusions appear to constitute a potassic suite compositionally intermediate between both the Roccamonfina high-K and low-K series. Mineralogical and petrologic features suggest that the parental magma of the porphyritic types underwent prolonged fractionation in one or more crustal reservoirs at depths exceeding the stability conditions of plagioclase. The magma then migrated upwards to shallower reservoirs. Fractionation and crystal accumulation formed one (or more) layered crystal mush(es) of coarse-grained rock types, whereas the residual magma migrated toward the surface being stored in one (or more) shallower reservoir(s). When the residual melts of both cumulate and porphyritic layers, still incompletely consolidated, disrupted into coherent fragments and crystals, they were brought to the surface by pulses of phonolitic magma coming from below, and acting during a long time span. The residual melts of cumulate and porphyritic types vesiculated during ascent to the surface, quenching during eruption of the host phonolitic bodies.

Petrographic, Geochemical and Zircon Typology of Neoproterozoic Granitoids: Evidence of Continental Thickening in Saghro Arc (Eastern Anti-Atlas, Morocco)

Saghro is considered as a volcanic arc of the Neoproterozoic. The magmatic activity during this period is characterized by the emplacment of granitic plutons North-East from Bou-Azzer El Graara's suture zone. Sidi Flah-Bouskour buttonhole represents the near part of Saghro to this zone and Ougnat hills the far part. The buttonhole of Sidi Flah-Bouskour is cut by several successive granitic intrusions of Neoproterozoic. The earliest suite, composed of gabbro, quartz diorite and amphibole granodiorite constitute the Pan-African B1 phase related to pre- collision domain, and shows a calc-alkaline chemistry. The second phase includes the biotite granite of Bouskour which belong to the potassic calc-alkali suite emplaced during the Pan- African B2 phase in a syn-collisional environment. The granodioritic and granitic trends show separate evolution in CaO-Na,O-K,O diagram. Quartz diorite and amphibole granodiorite have moderate content of Sr (420-40ppm), Rb (133-88ppm), Cr (177-21ppm), Zr (131-67ppm) and V(470- 30ppm) and hight content of Ba (1156-368ppm). Garnet and biotite have low content of Sr (3lppm), Rb (104ppm), Cr (42ppm), V (93ppm), Zr (93ppm) and Ba (450ppm). REE normalised to MORB show that the Sidi Flah-Bouskour granitoids are enriched in LREE and LILE and relatively depleted in HFSE and Nb.The zircon populations of the quartz diorite and amphibole granodiorite show a strong frequency in the sub-type S17-18 with low @ index and high T index implying a temperature of 750 - 800°C. In addition to the T.E.T, the average points (0, T) placed in the (LA, 1.T) typological diagram permit to classify these rock into the hybrid granites domain which belong to the calc-alkali granodioritic affinity. The biotite granite shows zircon populations with dominance of the S19 sub-type implying high 0 index with apparition of the types P, D and J, and high T index. The average points (0, T) and the T.E.T. permit to classify the biotite granite among the hybrid granites implying a alkali-rich calc-alkali trend. Quartz diorite, amphibole granodiorite and garnet granite are emplaced in Ougnat hill, far separated from the suture zone. Petrographic, geochemical and zircon typology study of these rocks shows existence of two magmatic groups. The quartz diorite and the amphibole granodiorite represents a medium-K calc-alkaline trend and shows alkali enrichment (Bl). CaO-Na,O- KzO relationship shows that quartz diorite and granodiorite evolved separately from that of the garnet granite. Petrographic, mineralogical (zircon typology) and geochemical (overlap of garnet granite REE pattern on calculated crustal partial melt pattern) features of garnet granite are similar to those of crustal anatectic granites (B2). The occurrence of granodiorite very rich in biotite and mantled garnet grains identical to those in the crustal garnet granite indicates processes of assimilation and magma contamination during the emplacement of Ougnat granitoids. Biotite granite emplaced in Sidi Flah-Bouskour, adjacent to Saghro near the suture zone shows strong crustal participation and granitoids in Iknioun show high-K affinities. Away from the suture zone' in Ougnat' garnet granites of crustal origin occur. Isotopic data show variation of ENdi between biotite granite in Sidi Flah-Boukour (ENd= -1.8-2.4) and garnet granite in Ougnat (ENdi= -4.5). This is interpreted to indicate an increase in incorporation of crustal component while moving North-East. The contrast in isotopic data agree well with the spatial zonation observed in Saghro. This zonation might have evolved by crustal thickening toward the axis of Bou-Azzer El Graara's suture which is considered as a paleo-subduction zone during Neoproterozoic according to the model of Meen (1987).

Fractionation trends among IVA iron meteorites: contrasts with IIIAB trends

A neutron-activation study of 48 group-IVA irons shows much lower negative slopes on Ir-Au and Ir-As diagrams than observed in the larger magmatic group IIIAB. This difference seems to reflect the tendency of D Ir, D Au and D As to increase with increasing S content. Contents of S and other volatiles are much lower in IVA irons than IIIAB irons. We show that both groups can be fit with D X values that depend quadratically on S, with initial IVA S contents about 6X lower than those in IIIAB. The IVA scatter fields show a spread in Au or As at constant Ir that appears to reflect variations in the fraction of trapped melt between 0% and 30%. Copper shows an S-shaped trend that may reflect moderate positive and negative changes in D Cu as the magma evolved or, less likely, sampling variations in a broad band reflecting fractionation and trapping of melt. Gibeon, the largest IVA iron with a mass > 30 tons, shows an appreciable range in compositions consistent either with differences in the degree of magma crystallization or with differences in the content of trapped melt. A striking difference between IVA and IIIAB is observed in the Ir/Au ratios in the most Ir-rich irons in the groups; that in IVA is 40% lower than the IIIAB ratio, and lower than those in other iron-meteorite groups. The IVA Ir/Au ratio is about half the ratios in the chondrite groups. We examined three possible explanations of this anomaly: (1) the high-Ir irons contain large amounts of trapped melt; or (2) half of the IVA core (i.e., the first 50% to crystallize) is missing from the terrestrial set of IVA irons; or (3) the IVA magma formed by incomplete melting of the metal in the chondritic precursor material, with the metal that remained in the mantle having high Ir and low Au contents. Plausibility arguments favor the third possibility. The third scenario is the most plausible, but the second cannot be ruled out. We review recent evidence regarding the cooling rates in group IVA. In contrast to recent interpretations, we note several lines of evidence that indicate constant cooling rates independent of composition, as expected if all IVA meteorites were in the same core when cooling between 900 and 650 K occurred.

A metallogenic survey of alkalic rocks of Mt. Somma-Vesuvius volcano

Summary Somma-Vesuvius is an alkaline volcano whose products (pumice, scoria and lava) have alkalineONa2Oa K2OU contents between 6 and 16 wt%, Mg number < 50, SiO2 59‐ 47 wt% and MgO 0‐7.8 wt% (more than 50% of the samples have a content < 2 wt%). Immobile-element ratios (Th/Yb, Ta/Yb, Ce/Yb) indicate a shoshonitic character, while the K2O content (4‐10 wt%) is characteristic of ultrapotassic rocks. The behavior of selected metals is discussed by grouping them on the basis of the stratigraphic sequence and differentiating the volcanic activity between plinian and interplinian (Rolandi et al., 1998; Ayuso et al., 1998). This allows observation of the variation within each formation from 25.000 y. BP to the last historic eruptive cycle (1631‐1944 AD). The main processes to explain the wide distribution of the data presented are fractional crystallization of a mantle-derived magma, magma mixing, and contamination with heterogeneous lower and/or upper crust. Variation diagrams distinguish different behavior for groups of metals: Ag (0.01‐0.2 ppm), Mo (1‐8.8 ppm), W (1.3‐13 ppm), Pb (16‐250 ppm), Sb (0.2‐2.6 ppm), Sc (0.2‐61 ppm), Li (15‐140 ppm) and Be (1‐31 ppm) increase with increasing differentiation and tend to correlate with the incompatible trace elements (Th, Hf, etc). Cu (10‐380 ppm), Au (2‐143 ppb), Co (0.7‐35.1 ppm) and Fe (1.3‐6.2 wt%) decrease towards advanced stage of differentiation. Iron also identifies three magmatic groups. The ratio Fe 3a /Fe 2a ranges between 0.2 and 1.8, and Fe2O3/ (Fe2O3+FeO) ranges between 0.2 and 0.8, giving rise to an oxidized environment; exceptions are in the samples belonging to the interplinian formations: I, II, medieval and 1631‐1994 AD. Fluorine ranges between 0.1 and 0.4 wt% for the complete Mt. Somma-Vesuvius activity, except for the Ottaviano and Avellino plinian (0.8 wt%) events. Chlorine has a wider range, from 0.1 wt% to 1.6 wt%. Mt Somma-Vesuvius has some features similar to those of mineralized alkaline magmatic systems which coincide with the transition between subduction-related

Isotopic signatures and origin of nitrogen in IIE and IVA iron meteorites

Nitrogen concentrations and isotopic signatures have been determined in groups IIE and IVA iron meteorites. Contrary to assumptions made in the literature, the present data show that spallation components significantly modify the N signatures of the metal. All 15N data are corrected for cosmic-ray produced spallation components using 21Ne concentrations measured in aliquots. A production-rate ratio 21Ne/ 15N = 0.80 is obtained, which can reliably be used for this correction, since it is not sensitive to shielding differences. The trapped N signatures in group IVA irons fall into two subgroups IVA (−26) with δ 15N = −26 ± 2 ‰ and IVA (−6) with δ 15N = −6 ± 1.4‰, respectively. Only the latter is close to values reported for metal of L-chondrites. Group IIE irons also define two distinct subgroups IIE (Y) with δ 15N = −7.5 ± 1.5‰ and IIE (O) with δ 15N = −2.3 ± 1‰. Therefore, the earlier proposed subdivision of IIE irons into “young” and “old” subgroups is substantiated by the different trapped N signatures. With regard to a possible relationship with H-chondrites as suggested by oxygen isotopes, only the young IIE subgroup overlaps the range of signatures reported in H-chondrite metal. Seymchan has a distinctly lighter signature (δ 15N = −54‰), consistent with its reclassification as an ungrouped iron. We observe no correlation of either nitrogen concentrations or isotopic signatures with abundances of Ga, Ge, Ir or Ni. The distinct N components in the so-called magmatic group IVA constrain the thermal history of the parent body, as these signatures need to be reconciled with the magmatic history. Implications for the origin of nitrogen components are discussed. Some of the spread in N isotopic data in the literature apparently is due to inclusions.

LAJINHA, UMA INTRUSÃO TARDI-OROGÊNICA E PÓSCOLISIONAL NO EXTREMO OESTE DO COMPLEXO PARAÍBA DO SUL, MINAS GERAIS, ESPÍRITO SANTO, BRASIL

This is an interpretation of geological, petrological and geochemical data of the Lajinha intrusion,which belongs to the Alto Capim Subsuite of the Espírito Santo Magmatic Suite. The Lajinhaintrusion is a plutonic body of medium extensions (48km2) with diapiric structure and magmaticzoning. Two magmatic groups build up this body, one of granitic affinity and the other group ofdioritic affinity. Between this two groups do not exist “commingling” but a chemical interaction (e.g.: “mixing” in the contact region). The acid rocks, that crop out in a ringlike form, can be observedin two distinct litological groups. The main group is coarse-grained, has a great number of big potassicfeldspar-phenocrysts (up to 15cm), small part of matrix and it do not have a preferencial orientation.In some parts exist a flux orientation of the phenocrystals and at the contact with the host rockapears weak intrusive foliation. The other subordinate group is formed by a middle to coarse grainedmatrix, with isotropic apearence and with a very low content of K-feldspar phenocrystals. Theseparation of the two groups may be a result of “filter-pressing” and/or “gravity separation”. Thegranites are constituted principally of quartz, K-feldspar, plagioclase, biotite, accessory and opaqueminerals. Their chemical composition ranges from alkaligranite to granodiorite. The basic rockgroup,which crops out in an oval formed elonged central nuclei, has a middle to coarse granulometry,homogeneous and isotropic apearence over all of the visited outcrops. This rock is built up byamphibole, biotite, plagioclase, small percentage of pyroxene, accessory and opaque minerals. Thechemical composition vary between syeno-diorite, diorite and gabbrodiorite. The contact betweenthe granitc rockgroup and the basic rockgroup is very well defined and shows only metasomatismeffects and not “commingling”. The host of the intrusion are para-rocks like migmatites, bandedgneisses, quartzites, and ortho-rocks like an homogeneous gneisse (Galileia Intrusive Suite; PocraneComplex) and amphibolitic. They are very strong foliated. The contact between the host and theintrusive rocks is normally formed by gradational evolution (transformation) and in some parts ofthe north western contact by faults. From the host towards the intrusion growth up the number ofblasts and the distribuion of fusion effects. Pegmatitic veins cut the mainbody in the same directionas distension fractures (NE-SW). In the northeastern part this fractures are invaded by a fine grainedalkaline basalt of probably cretaceous age. This veins cut the host and the granites of the intrusion,too. The observations make possible the idea of an intrusive process under late-orogeneous to postcollisionalregime. The differentiation trends of the two rock groups are calco-alkaline (trondhjemiticto monzonitic), with peraluminous to metaluminous and alkaline to subalkaline tendency. Thisintrusion make part of the symetric body distribution within the Espírito Santo Magmatic Arc,beginning with late to post-collisional intrusion in the west and east and early to syn-collisionalintrusions in the central region, the later influenced by compressional and transcorrent movements.

Effects of liquid immiscibility on trace element fractionation in magmatic iron meteorites: A case study of group IIIAB

Abstract— Magmatic iron meteorites are generally agreed to represent metal that crystallized in asteroidal cores from a large pool of liquid. Estimates suggest that the metallic liquid contained significant amounts of S and P, both of which are incompatible and exert a strong effect on trace element partitioning. In tandem, S and P are also prone to cause immiscibility between sulfide liquid and P‐rich metal liquid. The liquid immiscibility field occupies ∼70% of the portion of the Fe‐Ni‐S‐P system in which Fe is the first phase to crystallize. In spite of this, previous fractional crystallization models have taken into account only one liquid phase and have encountered significant discrepancies between the meteorite data and model values for the key elements Ni, Ir, Ga, Ge and Au at even moderate degrees of fractionation.For the first time, a model for trace element partitioning between immiscible liquids in the Fe‐Ni‐S‐P system is presented in order to assess the effects on fractionation in magmatic iron meteorite groups. The onset of liquid immiscibility causes a significant change in the enrichment patterns of S and P in both liquids; so elements with contrasting partitioning behavior will show trends deviating clearly from one‐liquid trends. A trend recorded in the solid metal will either be a smooth curve as long as equilibrium is maintained between the two liquids or the trend may diverge into a field limited by two extreme curves depending on the degree of disequilibrium. Bulk initial liquids for most magmatic groups have S/P (wt%) ratios well below 25. In these cases and due to the constitution of the Fe‐Ni‐S‐P system, most of the metal will crystallize from the rapidly decreasing volume of metal liquid and only a subordinate amount from the sulfide liquid. Because of the strong extraction of P into the metal liquid, P will have a much larger influence on trace element partitioning than a low initial P content might suggest.My model calculations suggest that liquid immiscibility played a significant role during the solidification of the IIIAB parent body's core. The two‐liquid model reproduces the IIIAB trends more closely than previous one‐liquid models and can account for: (a) the general widening of the IIIAB trend with increasing Ni and decreasing Ir contents, (b) the occurrence of high‐Ni members that are not strongly depleted in Ir, Ga and Ge; and (c) an upper limit at ∼11 wt% Ni where the metal liquid was almost consumed.

182W and 187Re- 187Os Systematics of Iron Meteorites: Chronology for Melting, Differentiation, and Crystallization in Asteroids

W isotopic compositions (measured by N-TIMS) in five iron meteorite groups are used in conjunction with 187Re- 187Os isotopic data to constrain the timing of metal segregation and crystallization, respectively, in the iron meteorite parent bodies. Fractionation of the parent isotope, 182Hf (half-life = 9 ± 2 Ma), from the daughter, 182W, likely occurred during segregation of metal from silicate in the iron meteorite parent bodies. If W isotopic equilibrium was achieved between silicate and metal during metal segregation, then the W isotopic compositions in the irons should record the timing of this event. Samples were taken from the five largest iron meteorite groups, including the four magmatic groups IIAB, IIIAB, IVA, and IVB and the nonmagmatic IAB-IIICD group. Together these groups comprise approximately 90% (by weight) of iron meteorites classified so far. With the exception of the IVA group, the groups have well-defined 187Re- 187Os isochron ages which are interpreted as the time of crystallization of the metal of their parent bodies. Within each magmatic group, samples that crystallized both early and late in the sequence of each core were analyzed to evaluate possible evolution of the W isotopic composition during crystallization. All the samples examined display very similar ϵ W values within a narrow range of compositions between −5.1 and −3.1. Witzation began, less than a few percent of additional metal was segregated to the cores. Moreover, the average ϵ W values for the five groups have no resolvable difference, ranging from −4.08 ± 0.69 (IVA) to −3.30 ± 0.15 (IAB-IIICD). These data indicate that all of the iron meteorite parent bodies underwent metal segregation within 5 Ma of each other, apparently independent of their composition, size, and time of crystallization of the metal. In all groups, except the IIIAB group, there is a resolvable time interval between the timing of metal segregation, as constrained by the W isotopic data, and the initiation of core crystallization, as given by the Re-Os isotopic systematics.