Incompatible element

About: Incompatible element is a research topic. Over the lifetime, 2420 publications have been published within this topic receiving 154052 citations.

Neodymium isotope evidence for ultra-depleted mantle in the early Proterozoic

Abstract: THE episodic extraction of juvenile continental crust from the Earth's mantle over the past 4 Gyr has led to a progressive depletion of incompatible elements in the upper mantle1,2. A knowledge of the degree and uniformity of this mantle depletion throughout Earth history is important for understanding the growth of continents, the evolution of the crust-mantle system and the nature of mantle convection through time. Here we report initial 143Nd/144Nd ratios, for 1.8-Gyr-old mafic volcanics from the Harts Range meta-igneous complex of central Australia which are the highest yet reported for Proterozoic igneous rocks (ɛNd = +6.9 to +8.2 for the least contaminated samples). These ratios far exceed those proposed in models3–5 for the isotopic evolution of the depleted mantle at this time, and imply the existence of a mantle reservoir that had been highly depleted for at least 1 Gyr. This provides strong evidence that major periods of continental growth, such as that in the late Archaean, produced long-lived heterogeneities in the upper mantle.

The Trinity ophiolite (California): the strange association of fertile mantle peridotite with ultra-depleted crustal cumulates

Abstract: This paper addresses the question of the petrological relationships between the mantle section and the crustal section of the Trinity ophiolite. Our conclusions are based on a field survey and on petrographic and electron micro-probe study of about 200 samples. We show that the crustal section of Trinity is more developed and less chaotic than expected on the basis of previous surveys. In the Bear Creek area, we were able to describe a well preserved cumulate sequence about 1,500 m thick. The cumulate pile includes a thick (~800 m) basal part made of ultramafic cumulates (dunites, wehrlites, pyroxenites, etc...) displaying very thin (mm- to cm thick) modal layering. The most salient characterisitc of this basal section is the gradual decrease of the modal abundance of olivine from bottom to top. This paragenetic evolution is correlated with the evolution of mineral chemistry consistent with fractional crystallisation from a common parent melt. Plagioclase appears above this ultramafic sequence, in the upper half of the cumulate section, in a diffuse way at first (plagioclase pyroxenites), becoming increasingly abundant toward the top of the section. Its crystallization always coincides with that of hornblende pseudomorphs on previously crystallized pyroxenes. The layering becomes very irregular at this level and attributable essentially to textural variations. The top of the cumulate sequence is characterized by the abundance of magmatic breccias (pyroxenitic and gabbrodioritic fragments embedded in fine grained diorite). These breccias are cross cut by diabase dykes. The horizontal extent of the Bear Creek "magma chamber" is moderate (2-3 km). The lateral contact with the host peridotites and gabbros is always underlain by a screen of pegmatites reaching several hundred metres in thickness. These pegmatites are made of pyroxenites in the lowermost levels and of diorites in the upper levels. Angular xenoliths of mantle derived lherzolites are frequently observed in the layered ultramafic section, their incorporation being contemporaneous to the crystallization of the cumulates. The field relationships and the lithological succession described above are consistent with the sudden injection of a huge batch of melt (reaching several km3) into the lithosphere (rocks at sub-solidus To) followed by fractional crystallization into the internal part of this magma body. The boniniticandesitic kindred of the parent melt is clearly revealed by the crystallization sequence. This conclusion is corroborated by the extreme depletion of pyroxenes and Cr-spinel in relatively incompatible elements (Ti, Al). The fractional crystallization trend of the Trinity cumulates is identical to the one defined by phenocrysts in present-day high-Ca boninites and is clearly distinct from that of mid-ocean ridge gabbros. The plagioclase composition is buffered around high An% values (90-95%), which is consistent with a low Na content of their parent melt and with H2O saturation at the time of crystallization of this mineral. The various so-called "gabbroic" massifs cropping out in Trinity represent individual intrusions similar to the one we have studied in detail in the Bear Creek area. Two generations of melt migration structures are observed in the mantle section of Trinity: (1) ariegitic-gabbroic segregations in mineralogical and chemical equilibrium with the plagioclase lherzolite and whose injection is contemporaneous with high-To plastic deformation ; (2) pyroxenitic (and, less commonly, dioritic) segregations and dykes post-dating the high-To deformation and characterized by strong mineralogical and chemical disequilibrium with the host plagioclase lherzolite. The parent melts of these second generation segregations and dykes are identical to those of the crustal cumulates. The interaction between the boninitic melts, undersaturated in Al and ultra-depleted in incompatible elements, and the peridotites accounts for extreme mineralogical and geochemical variability of the Trinity mantle. Peridotites, away from reactive dykes, are, as a rule, richer in incompatible elements than the cumulates from the crustal section. The mantle peridotites of Trinity cannot be the source nor the residue of the melt that fed the crustal magma chambers. Accordingly, the mantle-crust complementarity argument that is the basis of the slow spreading mid-ocean ridge model for Trinity (Lherzolite Ophiolite Type), must be reconsidered. A likely tectonic scenario that accounts for our data involves the evolution of a marginal, likely back-arc basin, from its opening to its closure. The ariegitic-gabbroic segregations are the witness of a low degree and shallow (~30 km depth) partial melting event experienced by the cold and relatively fertile Trinity peridotites during the first stage of opening of this basin in a transtensional regime, as suggested by the plastic flow pattern. The injection of the boninitic magma in strong disequilibrium with the lherzolite and feeding the crustal section occurred when one of the margins of the Trinity basin migrated above the zone of melting induced by dehydration of the subducting slab. This event occurred shortly before the definitive closure of the back-arc basin and of the obduction event. Paleomagnetic and geochronological data published so far are consistent with this scenario and with a life time of about 40 Ma for the Trinity basin, which is close to the life time of modern back-arc basins.

Radiogenic Lead Signatures in Au-Rich Volcanic-Hosted Massive Sulfide Ores and Associated Volcanic Rocks of the Early Tertiary Macuchi Island Arc(Western Cordillera of Ecuador)

Abstract: An early Tertiary ensimatic island arc, whose remnants are known as the Macuchi unit, is exposed in the occidental portion of the Western Cordillera of Ecuador. The Macuchi unit consists of two volcaniclastic and volcanic sequences: the Basal Macuchi, of Paleocene age, which is the lower part of the arc pile, and the Main Macuchi, of Eocene age, which is stratigraphically higher. The Main Macuchi hosts the Macuchi, La Plata, and El Patino Au-rich volcanic-hosted massive sulfide (VHMS) deposits. Despite higher contents of SiO2 and lower contents of MgO than rocks of the Basal Macuchi, the least evolved rocks of the Main Macuchi have lower or similar concentrations of incompatible elements and higher Sc/Y, V/Ti, and Ti/Zr ratios. This suggests derivation of the Main Macuchi rocks from a depleted source compared to the Basal Macuchi sequence. The VHMS orebodies are characterized by a mineral assemblage including chalcopyrite, pyrite, low Fe sphalerite, bornite, covellite, digenite, tennantite, and by quartz-sericite and quartz-chlorite alteration. Lead isotope compositions of the volcanic rocks of the two sequences(206Pb/204 Pb = 18.65-19.10, 207Pb/204Pb = 15.53-15.67, and 208Pb/204Pb = 38.20-39.00) define mixing trends between a MORB-type reservoir and an upper crustal source. Volcanic rocks of both sequences display positive correlations of 207Pb/204Pb with MgO, which are compatible with fractional crystallization and assimilation of oceanic crust by a parent magma enriched at its source by radiogenic lead of pelagic sediments. The isotopic compositions of the orebodies overlap largely the compositional field of the Main Macuchi rocks, suggesting derivation of the majority of ore lead from leaching of the Main Macuchi sequence, even though small contributions from the Basal Macuchi cannot be ruled out. Lead isotope compositions are internally homogeneous within each of the three orebodies but vary beyond analytical uncertainties among the deposits (206Pb/204Pb = 18.835 ± 0.009, 207Pb/204Pb = 15.613 ± 0.005, 208Pb/204Pb = 38.535 ± 0.018 for Macuchi; 206Pb/204Pb = 18.859 ± 0.007, 207Pb„Pb = 15.638 ± 0.005, 208Pb/204Pb = 38.643 ± 0.028 for El Patio; 206Pb/204Pb =19.023 ± 0.008, 207Pb/204Pb = 15.660 ±0.003, 208Pb/204 Pb = 38.786 ± 0.015 for La Plata). The isotopic difference between Macuchi (207Pb/204Pb = 15.613 ± 0.005) and El Patino (207Pb/204Pb=15.638 ± 0.005), separated by a few hundreds of meters, suggests fluid homogenization only at a local scale, a possible result of high-level emplacement of magmatic chambers, and consequent reduced size of the convective cells. Consistent covariations between metal ratios and isotopic compositions of the orebodies suggest also that leaching by hydrothermal fluids of different portions of the Main (and Basal?) Macuchi resulted in a distinct metal geochemistry of the orebodies. The highest 207Pb/204Pb value measured in the Eocene Main Macuchi volcanic rocks and associated ores is among the most radiogenic recorded in ensimatic island-arc systems and would require assimilation of unrealistic amounts of pelagic sediments (up to 36 wt %). In agreement with whole-rock geochemistry, it is more likely that this signature results from mixing of radiogenic lead equivalent to assimilation of commonly accepted pelagic sediment amounts (i.e., <10 wt %) with a residual mantle, depleted in incompatible elements such as low radiogenic, MORB-type lead. Melts extracted from a residual source are candidates to be enriched in Au and other chalcophile elements. The strati-graphic association and isotopic affinity of the Au-rich VHMS ore minerals with the rocks of the Main Macuchi sequence suggests, therefore, a possible petrogenetic control on the formation of the VHMS deposits of the Macuchi arc.