The boundary between the Earth's mantle and core is known as the core-mantle boundary (CMB). The temperature at the CMB is estimated to be around 4,500 - 5,500 degrees Celsius (8,132 - 9,932 degrees Fahrenheit).

Here's a breakdown:

* Inner Core: Solid iron and nickel, with temperatures around 5,200 degrees Celsius (9,392 degrees Fahrenheit).

* Outer Core: Liquid iron and nickel, with temperatures around 4,500 degrees Celsius (8,132 degrees Fahrenheit).

* CMB: The transition zone between the liquid outer core and the solid mantle. Temperature is slightly lower than the outer core due to the change in state of matter.

* Mantle: Mostly solid rock, with temperatures increasing with depth, reaching around 3,700 degrees Celsius (6,692 degrees Fahrenheit) at the top of the mantle.

Therefore, the surface temperature closest to the CMB is around 4,500 - 5,500 degrees Celsius (8,132 - 9,932 degrees Fahrenheit).