Question: Radiohalos or pleochroic halos are microscopic, spherical shells of discolouration within minerals such as biotite that occur in granite and other igneous rocks, wider interest has been prompted by the claims of a creationist, by Robert V. Gentry, an American young Earth creationist and nuclear physicist, known for his claims that radiohalos provide evidence for a young age of what?

Context paragraphs:
Porphyritic: Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group.  Porphyritic rocks may be aphanites or extrusive, with large crystals or phenocrysts floating in a fine-grained groundmass of non-visible crystals, as in a porphyritic basalt, or phanerites or intrusive, with individual crystals of the groundmass easily distinguished with the eye, but one group of crystals clearly much bigger than the rest, as in a porphyritic granite.  Most types of igneous rocks may display some degree of porphyritic texture.  One main type of rock that has a porphyritic texture are porphyry, though not all porphyritic rocks are porphyries.

Radiohalo: Radiohalos or pleochroic halos are microscopic, spherical shells of discolouration within minerals such as biotite that occur in granite and other igneous rocks.  The shells are zones of radiation damage caused by the inclusion of minute radioactive crystals within the host crystal structure.  The inclusions are typically zircon, apatite, or titanite which can accommodate uranium or thorium within their crystal structures .  One explanation is that the discolouration is caused by alpha particles emitted by the nuclei; the radius of the concentric shells are proportional to the particle's energy .  The phenomenon of radiohalos has been known to geologists since the early part of the 20th century, but wider interest has been prompted by the claims of creationist Robert V. Gentry that radiohalos in biotite are evidence for a young earth .

Provenance (geology): Provenance in geology, is the reconstruction of the origin of sediments.  The Earth is a dynamic planet, and all rocks are subject to transition between the three main rock types: sedimentary, metamorphic, and igneous rocks (the rock cycle).  Rocks exposed to the surface are sooner or later broken down into sediments.  Sediments are expected to be able to provide evidence of the erosion history of their parent source rocks.  The purpose of provenance study is to restore the tectonic, paleo-geographic and paleo-climatic history.

Marcus R. Ross: Marcus R. Ross (born 1976) is an American young earth creationist and vertebrate paleontologist.  Ross was featured in a February 2007 "New York Times" article about the conflict between his young Earth creationist beliefs (which hold the Earth to be only thousands of years old) and his doctoral dissertation (which involved animals extinct for millions of years).  His dissertation was on tracking the diversity, biostratigraphy, and extinction of mosasaurs, an extinct group of marine reptiles whose remains are found in Late Cretaceous period (100–66 Ma) deposits around the world.

Stylolite: Stylolites or styolite (Greek: "stylos", pillar; "lithos", stone) are serrated surfaces within a rock mass at which mineral material has been removed by pressure dissolution, in a process that decreases the total volume of rock.  Insoluble minerals, such as clays, pyrite and oxides, remain within the stylolites and make them visible.  Sometimes host rocks contain no insoluble minerals, in which case stylolites can be recognized by change in texture of the rock.  They occur most commonly in homogeneous rocks, carbonates, cherts, sandstones, but they can be found in certain igneous rocks and ice.  Their size vary from microscopic contacts between two grains (microstylolites) to large structures up to 20 m in length and up to 10 m in amplitude in ice.  Stylolites usually form parallel to bedding, because of overburden pressure, but they can be oblique or even perpendicular to bedding, as a result of tectonic activity.

Robert V. Gentry: Robert V. Gentry (born 1933) is an American young Earth creationist and nuclear physicist, known for his claims that radiohalos provide evidence for a young age of the Earth.  He is a member of the Seventh-day Adventist Church.

Aphanite: Aphanite, or aphanitic as an adjective (from the Greek αφανης, "invisible"), is a name given to certain igneous rocks that are so fine-grained that their component mineral crystals are not detectable by the unaided eye (as opposed to phaneritic igneous rocks, where the minerals are visible to the unaided eye).  This geological texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments.  As a rule, the texture of these rocks is not the same as that of volcanic glass (e.g., obsidian), with volcanic glass being non-crystalline (amorphous), and having a glass-like appearance.

Granite: Granite ( ) is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture.  Granites can be predominantly white, pink, or gray in color, depending on their mineralogy.  The word "granite" comes from the Latin "granum", a grain, in reference to the coarse-grained structure of such a holocrystalline rock.  Strictly speaking, granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term "granite" is used to refer to a wider range of coarse grained igneous rocks containing quartz and feldspar.

Ultramafic rock: Ultramafic (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content).  The Earth's mantle is composed of ultramafic rocks.  Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks.

Igneous petrology: Igneous petrology is the study of igneous rocks — those that are formed from magma.  As a branch of geology, igneous petrology is closely related to volcanology, tectonophysics, and petrology in general.  The modern study of igneous rocks utilizes a number of techniques, some of them developed in the fields of chemistry, physics, or other earth sciences.  Petrography, crystallography, and isotopic studies are common methods used in igneous petrology.

Supporting paragraphs: Radiohalo Robert V. Gentry
Answer: Earth