1. INTRODUCTIONThe processes that influence differentiation in magma chambers and the rate at which the associated melt-crystal phases separate have important ramifications for volcanic-plutonic connections among silicic igneous rocks. Related to volcanic-plutonic connections among silicic igneous rocks is the identification of cumulate signature in plutons. The subtlety of crystal accumulation signals in silicic igneous rocks has led to their interpretation as representing a true melt composition and being genetically separated from volcanic rocks (Coleman et al. , 2004, Glazner et al. , 2004). While the petrological signature of the cumulate nature of silicic magmas is subtle, it is discernible nonetheless (Bachmann et al. , 2007, Deering & Bachmann, 2010, Gelman et al. , 2014). Knowledge of how melt loss occurs at melt fractions relevant to silicic magma chambers and the associated textural and chemical indicators can facilitate identification of cumulates in plutons. Furthermore, the rate at which the associated melt-crystal phases separate have important ramifications for volcanic hazards. Here, we investigate the Spirit Mountain Batholith (SMB) for chemical and textural evidence of crystallization-differentiation and phase separation by repacking-driven compaction (grain reorganizations).The paper is organized such that we first introduce the geologic setting of the region and of the SMB in particular and provide evidence from previous studies supporting melt loss in the deeper parts of the SMB. Then, results of geochemical analyses are provided, including acquisition of major, minor, and trace elements of bulk rock SMB samples and results from plagioclase composition analyses. Subsequently, textural analyses of selected SMB samples are presented. We identify a near linear unmixing trend in major, minor, and trace element geochemistry defined by samples within a ca. 3 km transect at the base of the exposed batholith and pooled leucogranites near the top of the batholith. The plagioclase compositions suggest that the samples crystallized from the same parental magma and that the magma was less mafic than their bulk rock compositions. We then introduce a trace element model that allows melt and crystal to be lost to estimate relative melt loss (cumulate) or crystal loss (silicic cap) in the SMB. The benefit of this model is that it doesn’t assume a particular separation mechanism; however, it is limited in that it doesn’t provide the range of crystallinities over which melt is lost and doesn’t allow calculation of trapped melt fractions. To accomplish this, we use an unmixing model that treats the analyzed samples as combinations between two different endmembers: melt and crystal at a certain crystallinity. The trapped melt fraction profile is then compared to results from a model of mush compaction based on a crystal repacking rheology to provide order of magnitude timescale estimates for the growth of the silicic cap (melt accumulation layer).

The magmato-tectonic environment(s) of origin for Earth’s earliest crust are enigmatic and fiercely debated. Revealing the composition of the melts from which Hadean (>4.02 Ga) zircons crystallized might clarify conditions of initial crust construction. We calculate model melts using Ti-calibrated zircon/melt partition coefficients (KdZrc(Ti)) and published trace element data for Hadean and Archean zircons. The same treatment is applied to zircons from possible analogue environments (MORB, Iceland, arcs, lunar), to constrain potential petrogenetic similarities and distinctions between the early and modern world. Model melts from oceanic environments (MORB, oceanic arc, Iceland) have higher heavy rare earth element (HREE) contents and shallower middle REE (MREE) to HREE/chondrite (ch) slopes than those from continental arcs and tonalite-trondhjemite-granodiorite suites (TTGs). Hadean and Archean model melts are nearly indistinguishable from one another, both resembling TTGs and continental arcs, with pronounced depletion of HREE and slope reversal in heaviest REE. A limited number of samples > 4.25 Ga yield model melts with broadly similar characteristics to those from younger Hadean and Archean zircons, but with relatively elevated REE (~half order of magnitude) and higher LREE and MREE relative to HREE. Rare earth element patterns of early Earth model melts suggest a common petrogenetic history in the Hadean and Archean, involving garnet +/-amphibole in relatively low-temperature, high-pressure, environments.