Melting of subducted slab dictates trace element recycling in global arcs

Temperature-dependent trace element fractionation during melting of subducted slab can explain the composition of arc magmas.

Trace element fractionation between biotite, allanite, and granitic melt

The partitioning of a large suite of trace elements between biotite and water-saturated granitic melt was measured at 2 kbar and 700—800 ˚C. To reach equilibrium and to grow biotite crystals large enough for analysis, runs usually lasted from 30 to 45 days. In every charge, a few trace elements were initially doped at the 0.1—0.5 wt. % level and analyzed by electron microprobe after the run. First-row transition metal ions are highly compatible in biotite with Dbiotite/melt of 17 for Ti, 35 for V, 47 for Co, 174 for Ni, and 5.8 for Zn. A very notable exception is Cu with Dbiotite/melt < 0.9. This is likely one of the reasons why Cu is enriched together with Mo (Dbiotite/melt = 0.29) in porphyry deposits associated with intermediate to felsic plutons, while the other transition metals are not. Both Nb and Ta are mildly compatible in biotite with Dbiotite/melt being larger for Nb (3.69) than for Ta (1.89). Moderate (15—30%) biotite fractionation would be sufficient to reduce the Nb/Ta ratio from the chondritic value to the range observed in the continental crust. Moreover, the strong partitioning of Ti into biotite implies that already modest biotite fractionation suppresses the saturation of Ti-oxide phases and thereby indirectly facilitates the enrichment of Ta over Nb in the residual melt. The heavy alkalis, alkaline earths, and Pb are only mildly fractionated between biotite and melt (Dbiotite/melt = 3.8 for Rb, 0.6 for Cs, 0.6 for Sr, 1.8 for Ba, 0.7 for Pb). The rare earth elements are generally incompatible in biotite, with a minimum for Dbiotite/melt of 0.03–0.06 at Gd, Tb, and Dy, while both the light and heavy rare earths are less incompatible (e.g. Dbiotite/melt = 0.6 for La and 0.3 for Yb). This behavior probably reflects a partitioning into two sites, the K site for the light rare earths and the octahedral Mg site for the heavy rare earths. There is no obvious dependence of the rare earth partition coefficients on tetrahedral Al in the biotite, presumably because charge balancing by cation vacancies is possible. Allanite was found as run product in some experiments. For the light rare earths, Dallanite/melt is very high (e.g. 385 to 963 for Ce and Nd) and appears to increase with decreasing temperatures. However, the rather high solubility of allanite in the melts implies that it likely only crystallizes during the last stages of cooling of most magmas, except if the source magma is unusually enriched in rare earths.

Ultramafic rocks at the Isua supracrustal belt and East Pilbara Terrane are crustal cumulates, not slices of early mantle

&lt;p&gt;The initiation of plate tectonics remains enigmatic, with the proposed onset timing ranging from Hadean to Proterozoic. Recently, many mineralogical, petrological and geochemical studies suggest onset of plate tectonics at ~3 Ga. For example, the geology of East Pilbara Terrane (~3.55 to 2.70 Ga; Australia) is widely interpreted as representing Paleoarchean non-plate tectonics, followed by plate tectonics after a ~3.2 Ga transition. In contrast, Isua supracrustal belt (3.85 to 3.55 Ga; Greenland) has been dominantly interpreted via plate tectonics. There, two ultramafic lenses have been interpreted as depleted mantle slices, emplaced via thrusting in an Eoarchean subduction zone, implying early plate tectonics. We present new petrological and geochemical data of ultramafic samples from the Isua lenses and from the East Pilbara Terrane to explore their origins. Pilbara samples appear to preserve cumulate textures; protolith textures of Isua samples are altered beyond recognition. Samples with low chemical alteration show similar whole-rock chemistry, including up to 5.0 wt.% Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; and up to 0.25 wt.% TiO&lt;sub&gt;2&lt;/sub&gt; that both covary negatively with MgO (37.1 to 47.5 wt.%); these variations suggest cogenetic relationships with local lavas. Flat trace-element fractionation trends parallel those of local lavas in the primitive-mantle normalized spider diagram. Spinel crystals from Pilbara samples yield ~20-60 Mg#, relatively constant Cr# at ~70, and 0.61-4.81 wt.% TiO&lt;sub&gt;2&lt;/sub&gt;. Our data are consistent with crustal cumulate emplacement. In contrast with depleted mantle rocks, our samples have higher whole-rock Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; and TiO&lt;sub&gt;2&lt;/sub&gt;, flat (vs. upward) trace-element fractionation trends from less to more compatible elements, and spinel crystals with higher TiO&lt;sub&gt;2&lt;/sub&gt; and relatively constant (vs. varied) Cr#. Therefore, Isua and Pilbara ultramafic rocks may have similar, non-plate tectonic origins, and the Isua record allows a ~3 Ga onset of plate tectonics.&lt;/p&gt;