Tight bounds on missing late veneer in early Archean peridotite from triple oxygen isotopes (original) (raw)
Related papers
Geoscience Frontiers, 2018
This paper investigates the petrogenesis of the Seqi Ultramafic Complex, which covers a total area of approximately 0.5 km2. The ultramafic rocks are hosted by tonalitic orthogneiss of the ca. 3000 Ma Akia terrane with crosscutting granitoid sheets providing an absolute minimum age of 2978 +/- 8 Ma for the Seqi Ultramafic Complex. The Seqi rocks represent a broad range of olivine-dominated plutonic rocks with varying modal amounts of chromite, orthopyroxene and amphibole, i.e. various types of dunite (s.s.), peridotite (s.l.), as well as chromitite. The Seqi Ultramafic Complex is characterised primarily by re-fractory dunite, with highly forsteritic olivine with core compositions having Mg # ranging from about 91 to 93. The overall high modal contents, as well as the specific compositions, of chromite rule out that these rocks represent a fragment of Earth's mantle. The occurrence of stratiform chromitite bands in peridotite, thin chromite layers in dunite and poikilitic orthopyroxene in peridotite instead supports the interpretation that the Seqi Ultramafic Complex represents the remnant of a fragmented layered complex or a magma conduit, which was subsequently broken up and entrained during the formation of the regional continental crust. Integrating all of the characteristics of the Seqi Ultramafic Complex points to formation of these highly refractory peridotites from an extremely magnesian (Mg# < 80), near-anhydrous magma, as olivine-dominated cumulates with high modal contents of chromite. It is noted that the Seqi cumulates were derived from a mantle source by extreme degrees of partial melting (>40%). This mantle source could potentially represent the precursor for the sub-continental lithospheric mantle (SCLM) in this region, which has previously been shown to be ultra-depleted. The Seqi Ultramafic Complex, as well as similar peridotite bodies in the Fiskefjord region, may thus constitute the earliest cumulates that formed during the large-scale melting event(s), which resulted in the ultra-depleted cratonic keel under the North Atlantic Craton. Hence, a better understanding of such Archaean ultramafic complexes may provide constraints on the geodynamic setting of Earth's first continents and the corresponding SCLM.
Earth and Planetary Science Letters, 2020
The δ 18 O value of seawater is presently buffered by high-temperature hydrothermal alteration and low-temperature weathering of lithospheric rocks. It is much debated whether the δ 18 O of seawater has been buffered to this steady-state value throughout Earth's history or, alternatively, whether it gradually increased towards the present value since the Archean. A third possibility is that the δ 18 O of seawater was buffered at a higher value before continents emerged, and has been buffered at its current value (δ 18 O ∼ −1) since the emergence of continents. In this contribution, we reconstruct the δ 18 O of Archean seawater from triple oxygen isotope (δ 17 O, δ 18 O) variations in >2.7 Ga ultramafic rocks that reacted with fluids that, in turn, had been derived from seawater in the Archean. The samples that were studied are peridotites from the Ivnartivaq complex, an ultramafic lens in the Archean Kuummiut terrane (Rae craton, southeast Greenland) that have unusually low δ 18 O values for peridotites (in olivine, 1.7 ≤ δ 18 O ≤ 4.6 , relative to VSMOW). Bulk rock trace element concentrations, mineral compositions and U-Pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites, that formed by the dehydration of serpentinites ∼2.7 Ga ago. The serpentinite protoliths, in turn, had formed by alteration of ultramafic cumulate rocks at high temperatures (250-450 • C) by fluids that were derived from seawater. Triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower δ 18 O value than that of seawater in the Phanerozoic, but could have been derived from seawater with a value of δ 18 O ∼ −1 , i.e., the modern (ice-free) seawater composition. We conclude that the δ 18 O of seawater had reached its current steady state value by 2.7 Ga ago.
Mesoarchaean peridotite-norite cumulates of SW Greenland - The Miaggoq ultramafic complex
Lithos, 2023
Several studies focused on the ultramafic bodies of the Archaean continental crust in southern Greenland in order to gain information on early Earth petrogenetic, metamorphic and metasomatic processes. This research provides the first petrological dataset of the Miaggoq Ultramafic Complex (~1 km 2) in the Akia terrane, with a minimum age of 2997 ± 15 Ma. It comprises ultramafic (dunite, peridotite) and mafic (orthopyroxenite, norite) rocks along with chromitites and provides a window into Mesoarchaean mantle compositions. Field observations, such as chromitite bands, mineral layering, and orthopyroxenite oikocrysts in peridotites coupled with chemical analysis displaying high abundance of chromites in the dunitic rocks and high forsterite contents (Mg# 91 to 92.5) of the olivines, all point to a layered cumulate origin for the Miaggoq body. Pseudosection calculations along with geothermometry estimations reveal peak metamorphic conditions of 850-1100 • C at pressures of 0.7-1.25 GPa under anhydrous conditions followed by a possible metamorphic overprint at 650-800 • C and 0.7 GPa with relatively dry melting (0.025-0.125 wt% H 2 O). MELTS fractional crystallization coupled with cumulate modelling approximated the compositional trends with conditions on ~3 kbar with 1 wt% H 2 O. This research concludes that the Miaggoq body represents a layered cumulate complex that was derived by large degrees of partial melting of the mantle with possible assimilation (synonymous with contamination) of basalts in the crust. Overall, this study provides complementary data for the Mesoarchaean cumulate bodies of the Akia Terrane and their petrological processes.
Precambrian Research, 2013
We present new whole-rock major, trace and platinum-group element data, as well as Sm-Nd and Lu-Hf isotope data for meta-volcanic rocks from the Mesoarchaean Grædefjord Supracrustal Belt (GSB), located within the Tasiusarsuaq terrane, southern West Greenland. We also present new in-situ zircon U-Pb isotope data (by LA-ICP-MS) for associated felsic rocks. This region has experienced amphibolite to lower granulite facies metamorphism, causing re-equilibration of most mineral phases (including zircon).An intrusive tonalite sheet with a zircon U-Pb age of 2888 ±6.8 Ma, yields a minimum age for the GSB. The Sm-Nd and Lu-Hf isotope data do not provide meaningful isochron ages, but the isotope compositions of the mafic rocks are consistent with the ca. 2970 Ma regional volcanic event, which is documented in previous studies of the Tasiusarsuaq terrane. The major and trace element data suggest a significant crustal contribution in the petrogenesis of andesitic volcanic rocks in the GSB. The trace element variation of these andesitic leucoamphibolites cannot be explained by bulk assimilation-fractional-crystallisation (AFC) processes involving local basement. Rather, the observed patterns require binary mixing between basaltic and felsic end-member magmas with between 50-80% contributions from the latter (depending on the assumed felsic composition). Hf-isotope constraints point to contamination with pre-existing continental crust with an age of ca. 3250 Ma. Basement gneisses of this age were previously described at two localities in the Tasiusarsuaq terrane, which supports the mixing hypothesis. Thus the felsic end-member likely represents melts derived from the local basement.Ultramafic rocks (18.35-22.80 wt.% MgO) in GSB have platinum-group element (PGE) patterns that are similar to magmas derived from high-degree melting of mantle, but they have relatively enriched trace element patterns. We propose that the ultramafic rocks represent arc-related picrites or alternatively were derived by melting of metasomatised sub-continental lithospheric mantle.Overall these new geochemical data from the Mesoarchaean Grædefjord Supracrustal Belt and the petrogenetic mixing model in particular, are similar to observations from modern continental subduction zone environments, which also require large degrees of mixing with felsic basement melts. Therefore, we propose that the metavolcanic rocks formed in a modern-style subduction zone geodynamic setting, which due to the hotter Archaean mantle conditions allowed for substantial amounts of partial melting and magma mixing, rather than assimilating pre-existing continental crust.► Mesoarchaean andesitic meta-volcanic rocks have low initial ɛNd and ɛHf. ► Assimilation-fractional-crystallisation (AFC) cannot explain their trace element variation. ► Mixing of juvenile mafic magmas and re-melted pre-existing crust explains the data. ► Archaean andesites genesis by mixing processes resemble that of modern andesites
Recent geological investigations of the Isua Supracrustal Belt (3.8 Ga), southern West Greenland, have suggested that it is the oldest accretionary complex on earth, defined by an oceanic plate-type stratigraphy and a duplex structure. Plate history from mid-oceanic ridge through plume magmatism to subduction zone has been postulated from analysis of the reconstructed oceanic plate stratigraphy in the accretionary complex. Comparison between field occurrence of greenstones in modern and ancient accretionary complexes reveals that two types of tholeiitic basalt from different tectonic settings, mid-oceanic ridge basalt (MORB) and oceanic island basalt (OIB), occur. This work presents major, trace and rare earth element (REE) compositions of greenstones derived from Isua MORB and OIB, and of extremely rare relict igneous clinopyroxene in Isua MORB. The Isua clinopyroxenes (Cpx) have compositional variations equivalent to those of Cpx in modern MORB; in particular, low TiO 2 and Na 2 O contents. The Isua Cpx show slightly light (L)REE-depleted REE patterns, and the calculated REE pattern of the host magma is in agreement with that of Isua MORB. Analyses of 49 least-altered greenstones carefully selected from approximately 1200 samples indicate that Isua MORB are enriched in Al 2 O 3 , and depleted in TiO 2 , FeO*, Y and Zr at the given MgO content, compared with Isua OIB. In addition, Isua MORB show an LREE-depleted pattern, whereas Isua OIB forms a flat REE pattern. Such differences suggest that the Early Archean mantle had already become heterogeneous, depending on the tectonic environment. Isua MORB are enriched in FeO compared with modern MORB. Comparison of Isua MORB with recent melting experiments shows that the source mantle had 85–87 in Mg# and was enriched in FeO. Potential mantle temperature is estimated to be approximately 1480 ∞ C, indicating that the Early Archean mantle was hotter by at most approximately 150 ∞ C than the modern mantle.